Language：English   Publishing type：Research paper (scientific journal)   Publisher：Frontiers Media S.A.  

Transmural differences in ventricular myocardium are maintained by electromechanical coupling and mechano-calcium/mechano-electric feedback. In the present study, we experimentally investigated the influence of preload on the force characteristics of subendocardial (Endo) and subepicardial (Epi) single ventricular cardiomyocytes stretched by up to 20% from slack sarcomere length (SL) and analyzed the results with the help of mathematical modeling. Mathematical models of Endo and Epi cells, which accounted for regional heterogeneity in ionic currents, Ca2+ handling, and myofilament contractile mechanisms, showed that a greater slope of the active tension–length relationship observed experimentally in Endo cardiomyocytes could be explained by greater length-dependent Ca2+ activation in Endo cells compared with Epi ones. The models also predicted that greater length dependence of Ca2+ activation in Endo cells compared to Epi ones underlies, via mechano-calcium-electric feedback, the reduction in the transmural gradient in action potential duration (APD) at a higher preload. However, the models were unable to reproduce the experimental data on a decrease of the transmural gradient in the time to peak contraction between Endo and Epi cells at longer end-diastolic SL. We hypothesize that preload-dependent changes in viscosity should be involved alongside the Frank–Starling effects to regulate the transmural gradient in length-dependent changes in the time course of contraction of Endo and Epi cardiomyocytes. Our experimental data and their analysis based on mathematical modeling give reason to believe that mechano-calcium-electric feedback plays a critical role in the modulation of electrophysiological and contractile properties of myocytes across the ventricular wall.

DOI： 10.3389/fphys.2020.00171

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The application of mechanical stimuli to cells often induce increases in intracellular calcium, affecting the regulation of a variety of cell functions. Although the mechanism of mechanotransduction-induced calcium increases has not been fully resolved, the involvement of mechanosensitive ion channels in the plasma membrane and the endoplasmic reticulum has been reported. Here, we demonstrate that voltage-gated L-type calcium channels play a critical role in the mechanosensitive calcium response in H9c2 rat cardiomyocytes. The intracellular calcium level in H9c2 cells increased in a reproducible dose-dependent manner in response to uniaxial stretching. The stretch-activated calcium response (SICR) completely disappeared in calcium-free medium, whereas thapsigargin and cyclopiazonic acid, inhibitors of sarcoendoplasmic reticulum calcium ATPase, partially reduced the SICR. These findings suggest that both calcium influx across the cell membrane and calcium release from the sarcoendoplasmic reticulum are involved in the SICR. Nifedipine, diltiazem, and verapamil, inhibitors of L-type calcium channels, reduced the SICR in a dose-dependent manner. Furthermore, small interfering RNA against the L-type calcium channel α1c subunit diminished the SICR dramatically. Nifedipine also diminished the mechanosensitivity of Langendorff-perfused rat heart. These results suggest that the SICR in H9c2 cardiomyocytes involves the activation of L-type calcium channels and subsequent calcium release from the sarcoendoplasmic reticulum.

DOI： 10.1016/j.ceca.2019.02.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Tokyo  

Myocardial heterogeneity is an attribute of the normal heart. We have developed integrative models of cardiomyocytes from the subendocardial (ENDO) and subepicardial (EPI) ventricular regions that take into account experimental data on specific regional features of intracellular electromechanical coupling in the guinea pig heart. The models adequately simulate experimental data on the differences in the action potential and contraction between the ENDO and EPI cells. The modeling results predict that heterogeneity in the parameters of calcium handling and myofilament mechanics in isolated ENDO and EPI cardiomyocytes are essential to produce the differences in Ca2+ transients and contraction profiles via cooperative mechanisms of mechano-calcium-electric feedback and may further slightly modulate transmural differences in the electrical properties between the cells. Simulation results predict that ENDO cells have greater sensitivity to changes in the mechanical load than EPI cells. These data are important for understanding the behavior of cardiomyocytes in the intact heart.

DOI： 10.1007/s12576-017-0541-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Tokyo  

When a cardiac muscle is held in a stretched position, its [Ca2+] transient increases slowly over several minutes in a process known as stress-induced slow increase in intracellular Ca2+ concentration ([Ca2+]i) (SSC). Transient receptor potential canonical (TRPC) 3 forms a non-selective cation channel regulated by the angiotensin II type 1 receptor (AT1R). In this study, we investigated the role of TRPC3 in the SSC. Isolated mouse ventricular myocytes were electrically stimulated and subjected to sustained stretch. An AT1R blocker, a phospholipase C inhibitor, and a TRPC3 inhibitor suppressed the SSC. These inhibitors also abolished the observed SSC-like slow increase in [Ca2+]i induced by angiotensin II, instead of stretch. Furthermore, the SSC was not observed in TRPC3 knockout mice. Simulation and immunohistochemical studies suggest that sarcolemmal TRPC3 is responsible for the SSC. These results indicate that sarcolemmal TRPC3, regulated by AT1R, causes the SSC.

DOI： 10.1007/s12576-016-0519-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Academic Press  

Mechanical properties of cardiomyocytes from different transmural regions are heterogeneous in the left ventricular wall. The cardiomyocyte mechanical environment affects this heterogeneity because of mechano-electric feedback mechanisms. In the present study, we investigated the effects of the mechanical load (preload and afterload) on transmural differences in contraction of subendocardial (ENDO) and subepicardial (EPI) single cells isolated from the murine left ventricle. Various preloads imposed via axial stretch and afterloads (unloaded and heavy loaded conditions) were applied to the cells using carbon fiber techniques for single myocytes. To simulate experimentally obtained results and to predict mechanisms underlying the cellular response to change in load, our mathematical models of the ENDO and EPI cells were used. Our major findings are the following. Our results show that ENDO and EPI cardiomyocytes have different mechanical responses to changes in preload to the cells. Under auxotonic contractions at low preload (unstretched cells), time to peak contraction (Tmax) and the time constant of [Ca2 +]i transient decay were significantly longer in ENDO cells than in EPI cells. An increase in preload (stretched cells) prolonged Tmax in both cell types
however, the prolongation was greater in EPI cells, resulting in a decrease in the transmural gradient in Tmax at high preload. Comparing unloaded and heavy loaded (isometric) contractions of the cells we found that transmural gradient in the time course of contraction is independent of the loading conditions. Our mathematical cell models were able to reproduce the experimental results on the distinct cellular responses to changes in the mechanical load when we accounted for an ENDO/EPI difference in the parameters of cooperativity of calcium activation of myofilaments.

DOI： 10.1016/j.yjmcc.2017.12.001

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Language：English   Publisher：Elsevier Ltd  

Transient receptor potential (TRP) channels constitute a large family of versatile multi-signal transducers. In particular, TRP canonical (TRPC) channels are known as receptor-operated, non-selective cation channels. TRPC3 and TRPC6, two members in the TRPC family, are highly expressed in the heart, and participate in the pathogenesis of cardiac hypertrophy and heart failure as a pathological response to chronic mechanical stress. In the pathological response, myocardial stretch increases intracellular Ca2+ levels and activates nuclear factor of activated T cells to induce cardiac hypertrophy. Recent studies have revealed that TRPC3 and TRPC6 also contribute to the physiological stretch-induced slow force response (SFR), a slow increase in the Ca2+ transient and twitch force during stretch. In the physiological response, a stretch-induced increase in intracellular Ca2+ mediated by TRPC3 and TRPC6 causes the SFR. We here overview experimental evidence of the involvement of TRPC3 and TRPC6 in cardiac physiology and pathophysiology in response to stretch.

DOI： 10.1016/j.pbiomolbio.2017.06.010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

The electrical and mechanical functions of cardiomyocytes differ in relation to the spatial locations of cells in the ventricular wall. This physiological heterogeneity may change under pathophysiological conditions, providing substrates for arrhythmia and contractile dysfunctions. Previous studies have reported distinctions in the electrophysiological and mechanical responses to ischemia of unloaded subendocardial (ENDO) and subepicardial (EPI) single cardiomyocytes. In this paper, we briefly recapitulated the available experimental data on the ischemia effects on the transmural cellular gradient in the heart ventricles and for the first time evaluated the preload-dependent changes in passive and active forces in ENDO and EPI cardiomyocytes isolated from mouse hearts subjected to simulated ischemia. Combining the results obtained in mechanically loaded contracting cardiomyocytes with data from previous studies, we showed that left ventricular ENDO and EPI cardiomyocytes are different in their mechanical responses to metabolic inhibition. Simulated ischemia showed opposite effects on the stiffness of ENDO and EPI cells and greatly prolonged the time course of contraction in EPI cells than in ENDO cells, thereby changing the normal transmural gradient in the cellular mechanics.

DOI： 10.1016/j.pbiomolbio.2017.05.011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

Mitochondria are an important source of reactive oxygen species (ROS). Although it has been reported that myocardial stretch increases cellular ROS production by activating nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2), referred to as X-ROS signalling, the involvement of mitochondria in X-ROS is not clear. Mitochondria are organelles that generate adenosine triphosphate (ATP) for cellular energy needs, which are mechanical-load-dependent. Therefore, it would not be surprising if these organelles had mechano-sensitive functions associated with stretch-induced ROS production. In the present study, we investigated the relation between X-ROS and mitochondrial stretch-sensitive responses in isolated mouse cardiac myocytes. The cells were subjected to 10% axial stretch using computer-controlled, piezo-manipulated carbon fibres attached to both cell ends. Cellular ROS production and mitochondrial membrane potential (Δψm) were assessed optically by confocal microscopy. The axial stretch increased ROS production and hyperpolarised Δψm. Treatment with a mitochondrial metabolic uncoupler, carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP), at 0.5 μM did not suppress stretch-induced ROS production, whereas treatment with a respiratory Complex III inhibitor, antimycin A (5 μM), blunted the response. Although NOX inhibition by apocynin abrogated the stretch-induced ROS production, it did not suppress stretch-induced hyperpolarisation of Δψm. These results suggest that stretch causes activation of the respiratory chain to hyperpolarise Δψm, followed by NOX activation, which increases ROS production.

DOI： 10.1016/j.pbiomolbio.2017.05.015

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

Aims Transient receptor potential cation channel subfamily melastatin member 4 (TRPM4), a Ca2+-activated nonselective cation channel abundantly expressed in the heart, has been implicated in conduction block and other arrhythmic propensities associated with cardiac remodelling and injury. The present study aimed to quantitatively evaluate the arrhythmogenic potential of TRPM4.
Methods and results Patch clamp and biochemical analyses were performed using expression system and an immortalized atrial cardiomyocyte cell line (HL-1), and numerical model simulation was employed. After rapid desensitization, robust reactivation of TRPM4 channels required high micromolar concentrations of Ca2+. However, upon evaluation with a newly devised, ionomycin-permeabilized cell-attached (Iono-C/A) recording technique, submicromolar concentrations of Ca2+ (apparent K-d = similar to 500 nM) were enough to activate this channel. Similar submicromolar Ca2+ dependency was also observed with sharp electrode whole-cell recording and in experiments coexpressing TRPM4 and L-type voltage-dependent Ca2+ channels. Numerical simulations using a number of action potential (AP) models (HL-1, Nygren, Luo-Rudy) incorporating the Ca2+-and voltage-dependent gating parameters of TRPM4, as assessed by Iono-C/A recording, indicated that a few-fold increase in TRPM4 activity is sufficient to delay late AP repolarization and further increases (>= six-fold) evoke early afterdepolarization. These model predictions are consistent with electrophysiological data from angiotensin II-treated HL-1 cells in which TRPM4 expression and activity were enhanced.
Conclusions These results collectively indicate that the TRPM4 channel is activated by a physiological range of Ca2+ concentrations and its excessive activity can cause arrhythmic changes. Moreover, these results demonstrate potential utility of the first AP models incorporating TRPM4 gating for in silico assessment of arrhythmogenicity in remodelling cardiac tissue.

DOI： 10.1093/cvr/cvx117

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER SOC CLINICAL INVESTIGATION INC  

Myocardial infarction (MI) results in the generation of dead cells in the infarcted area. These cells are swiftly removed by phagocytes to minimize inflammation and limit expansion of the damaged area. However, the types of cells and molecules responsible for the engulfment of dead cells in the infarcted area remain largely unknown. In this study, we demonstrated that cardiac myofibroblasts, which execute tissue fibrosis by producing extracellular matrix proteins, efficiently engulf dead cells. Furthermore, we identified a population of cardiac myofibroblasts that appears in the heart after MI in humans and mice. We found that these cardiac myofibroblasts secrete milk fat globule-epidermal growth factor 8 (MFG-E8), which promotes apoptotic engulfment, and determined that serum response factor is important for MFG-E8 production in myofibroblasts. Following MFG-E8-mediated engulfment of apoptotic cells, myofibroblasts acquired antiinflammatory properties. MFG-E8 deficiency in mice led to the accumulation of unengulfed dead cells after MI, resulting in exacerbated inflammatory responses and a substantial decrease in survival. Moreover, MFG-E8 administration into infarcted hearts restored cardiac function and morphology. MFG-E8-producing myofibroblasts mainly originated from resident cardiac fibroblasts and cells that underwent endothelial-mesenchymal transition in the heart. Together, our results reveal previously unrecognized roles of myofibroblasts in regulating apoptotic engulfment and a fundamental importance of these cells in recovery from MI.

DOI： 10.1172/JCI83822

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACAD SCIENCES CZECH REPUBLIC, INST PHYSIOLOGY  

Various types of mechanosensitive ion channels, including cationic stretch-activated channels (SACNS) and stretch-activated BKca (SAKca) channels, modulate heart rhythm. Bepridil has been used as an antiarrhythmic drug with multiple pharmacological effects; however, whether it is effective for mechanically induced arrhythmia has not been well investigated. To test the effects of Bepridil on SAKca channels activity, cultured chick embryonic ventricular myocytes were used for single-channel recordings. Bepridil significantly reduced the open probability of the SAKca channel (P-O). Next, to test the effects of bepridil on stretch-induced extrasystoles (SIE), we used an isolated 2-week-old Langendorff-perfused chick heart. The left ventricle (LV) volume was rapidly changed, and the probability of SIE was calculated in the presence and absence of bepridil, and the effect of the drug was compared with that of Gadolinium (Gd3+). Bepridil decreased the probability of SIE despite its suppressive effects on SAKca channel activity. The effects of Gd3+, which blocks both SAKca and SACNS, on the probability of SIE were the same as those of bepridil. Our results suggest that bepridil blocks not only SAKca channels but possibly also blocks SACNS, and thus decreases the stretch-induced cation influx (stabilizing membrane potential) to compensate and override the effects of the decrease in outward SAKca current (destabilizing membrane potential).

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We studied the differences in twitch force of subendocardial and subepicardial cardiomyocytes isolated from mouse left ventricular wall at different preloads using an original single cell stretch method recently developed by us. Then, we used our mathematical models of subendocardial and subepicardial cells to predict underlying cellular mechanisms. Transmural differences in the amplitudes of active tension of subendocardial and subepicardial cardiomyocytes were revealed that could be related to the differences in cooperative end-to-end interaction between the neighboring regulatory units of the thin filament.

DOI： 10.1007/s10517-016-3542-8

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Language：English   Publisher：(一社)日本生理学会  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

OBJECTIVES: Identity of the optimal heart preservation solution remains unknown. Because oxidative stress contributes to contractile failure in the ischaemic/reperfused myocardium and the main characteristic of Celsior is its antioxidant effect, it is important to elucidate the relationship between the inhibitory effect on oxidative stress and cardiac mechano-energetics. We therefore evaluated the efficacy of Celsior from both aspects by comparison with the University of Wisconsin solution (UWS).
METHODS: We used 18 excised cross-circulated canine hearts. Excised hearts were preserved with UWS (n = 6) or Celsior (n = 6) for 3 h at 4 degrees C; the remaining six served as controls. Hearts were then cross-circulated and rewarmed. The end-systolic pressure-volume ratio (LV E-max) and the ventricular pressure-volume area, which is a measure of total mechanical energy, were assessed after reperfusion. Biopsies were taken from the endocardium after excising the heart, before reperfusion, after reperfusion and 4 h after reperfusion to assess the inhibitory effect of each agent on oxidative stress. Endo-myocardial biopsy samples were studied immunohistochemically for expression of 4-hydroxy-2-nonenal (HNE)-modified protein, which is a major lipid peroxidation product.
RESULTS: E-max in the UWS group was significantly smaller than in the control group, whereas the E-max in the Celsior group was preserved. Oxygen cost of E-max in the UWS group was significantly higher than in the Celsior group. Myocardial HNE-modified protein levels increased gradually, both under preservation and after reperfusion in the UWS group. Myocardial HNE-modified protein levels in the Celsior group were lower, mainly before and 4 h after reperfusion compared with the UWS group.
CONCLUSIONS: Celsior may maintain cardiac contractility and conserve oxygen cost by inhibiting oxidative stress.

DOI： 10.1093/icvts/ivv324

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Soc. of Med. Electronics and Biol. Engineering  

Myocardium contracts against ventricular wall stretch that comes along with ventricular filling. Mitochondria generate required ATP for myocardial contraction. It is well known that mitochondrial ATP production process is one of the sources of reactive oxygen species (ROS). ROS are known as toxic molecules, but also important physiological regulators of intracellular signaling pathways. In the present study, we investigate the relation between myocardial stretch and mitochondrial ROS production, and discuss the role of mitochondria on myocardial response to stretch. Isolated mouse ventricular myocytes were exposed to 10% axial stretch using carbon fiber technique. ROS production was studied using DCF-loaded cells. Axial stretch significantly increased ROS production. Applying 5 μM mitochondrial metabolic uncoupler FCCP blunted the response, indicating mitochondrial ROS production is stretch-sensitive. The present results suggest that stretch enhances electron transport chain to prepare for the more ATP production for the more preloaded, namely, the more energy-consuming contraction.

DOI： 10.11239/jsmbe.52.SY-44

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Language：English   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The previously reported pressure-volume (PV) relationship in frog hearts shows that end-systolic PV relation (ESPVR) is load dependent, whereas ESPVR in canine hearts is load independent. To study intrinsic cardiac mechanics in detail, it is desirable to study mechanics in a single isolated cardiomyocyte that is free from interstitial connective tissue. Previous single cell mechanics studies used a pair of carbon fibers (CF) attached to the upper surface of opposite cell ends to stretch cells. These studies showed that end-systolic force-length (FL) relation (ESFLR) is load independent. However, the range of applicable mechanical load using the conventional technique is limited because of weak cell-CF attachment. Therefore, the behavior of ESFLR in single cells under physiologically possible conditions of greater load is not yet well known. To cover wider loading range, we contrived a new method to hold cell-ends more firmly using two pairs of CF attached to both upper and bottom surfaces of cells. The new method allowed stretching cells to 2.2 mu m or more in end-diastolic sarcomere length. ESFLR virtually behaves in a load independent manner only with end-diastolic sarcomere length less than 1.95 mu m. It exhibited clear load dependency with higher preload, especially with low afterload conditions. Instantaneous cellular elastance curves showed that decreasing afterload enhanced relaxation and slowed time to peak elastance, as previously reported. A simulation study of a mathematical model with detailed description of thin filament activation suggested that velocity dependent thin filament inactivation is crucial for the observed load dependent behaviors and previously reported afterload dependent change in Ca2+ transient shape. (C) 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.pbiomolbio.2014.06.005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Tokai University School of Medicine  

Objective: Although tachycardia is well known to increase cardiac oxygen consumption (Vo2) per min, the relationship between Vo2 for excitation-contraction (E-C) coupling per beat and heart rate change over its full working range still remains controversial. Methods: To elucidate this relationship, we varied heart rate over a reasonably wide range (60-180 beat/min) and studied the relationship between left ventricular (LV) Emax (load-independent contractility index), PVA (pressure-volume area)-independent Vo2, and basal metabolic Vo2 in nine excised, cross-circulated canine hearts. Results: PVA-independent Vo2 per min significantly increased linearly with increasing heart rate while Emax remained unchanged. Basal metabolic Vo2per min was measured under KCl arrest. E-C coupling Vo2 per min obtained by subtracting the constant basal metabolic Vo2 from the PVA-independent Vo2 also significantly increased linearly with increasing heart rate. However, PVA-independent Vo2 per beat significantly decreased with increasing heart rate. In contrast, E-C coupling Vo2 per beat, as well as that normalized to Emax, slightly but significantly increased with increasing heart rate. Conclusion
The E-C coupling energy for myocardial Ca2+ handling increases with heart rate despite constant contractility in the left ventricle of the excised cross-circulated canine heart.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OKAYAMA UNIV MED SCHOOL  

Although propofol is commonly used for general anesthesia, its direct effects on left ventricular (LV) contractility and energetics remain unknown. Accordingly, we studied the effects of intracoronary propofol on excised cross-circulated canine hearts using the framework of the Emax (a contractility index)-PVA (systolic pressure-volume area, a measure of total mechanical energy)-Vo(2) (myocardial oxygen consumption per beat) relationship. We obtained 1) the Vo(2)-PVA relationship of isovolumic contractions with varied LV volumes at a constant Emax, 2) the Vo(2)-PVA relationship with varied LV volumes at a constant intracoronary concentration of propofol, and 3) the Vo(2)-PVA relationship under increased intracoronary concentrations of either propofol or CaCl2 at a constant LV volume to assess the cardiac mechanoenergetic effects of propofol. We found that propofol decreased Emax dose-dependently. The slope of the linear Vo(2)-PVA relationship (oxygen cost of PVA) remained unchanged by propofol. The PVA-independent Vo(2)-Emax relationship (oxygen cost of Emax) was the same for propofol and Ca2+. In conclusion, propofol showed a direct negative inotropic effect on LV. At its clinical concentrations, decreases in contractility by propofol were relatively small. Propofol shows mechanoenergetic effects on the LV that are similar to those of Ca2+ blockers or beta-antagonists-i.e., it exerts negative inotropic effects without changing the oxygen costs of Emax and PVA.

DOI： 10.18926/AMO/49039

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.cardfail.2012.08.116

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPANESE CIRCULATION SOC  

Background: It remains unclear whether sarcolemmal BKCa channels in post-hatch chick ventricular myocytes contribute to stretch-induced extrasystoles (SIE), and whether they are stretch-activated BKCa (SAKCa) channels or a non-stretch-sensitive BKCa variant.
Methods and Results: To determine the role of sarcolemmal BKCa channels in SIE and their stretch sensitivity, an isolated 2-week-old Langendorff-perfused chick heart and mathematical simulation were used. The ventricular wall was rapidly stretched by application of a volume change pulse. As the speed of the stretch increased, the probability of SIE also significantly increased, significantly shortening the delay between SIE and the initiation of the stretch. Application of 100 nmol/L of Grammostola spatulata mechanotoxin 4, a cation-selective stretch-activated channel (SAC) blocker, significantly decreased the probability of SI E. The application of Iberiotoxin, however, a BKCa channel blocker, significantly increased the probability of SIE, suggesting that a K(+) efflux via a sarcolemmal BKCa channel reduces SIE by balancing out stretch-induced cation influx via SACs. The simulation using a cardiomyocyte model combined with a new stretch sensitivity model that considers viscoelastic intracellular force transmission showed that stretch sensitivity in BKCa channels is required to reproduce the present wet experimental results.
Conclusions: Sarcolemmal BKCa channels in post-hatch chick ventricular myocytes are SAKCa channels, and they have a suppressive effect on SIE. (Ciro J 2011; 75: 2552-2558)

DOI： 10.1253/circj.CJ-11-0486

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Language：English   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Cardiac experimental electrophysiology is in need of a well-defined Minimum Information Standard for recording, annotating, and reporting experimental data. As a step towards establishing this, we present a draft standard, called Minimum Information about a Cardiac Electrophysiology Experiment (MICEE). The ultimate goal is to develop a useful tool for cardiac electrophysiologists which facilitates and improves dissemination of the minimum information necessary for reproduction of cardiac electrophysiology research, allowing for easier comparison and utilisation of findings by others. It is hoped that this will enhance the integration of individual results into experimental, computational, and conceptual models. In its present form, this draft is intended for assessment and development by the research community. We invite the reader to join this effort, and, if deemed productive, implement the Minimum Information about a Cardiac Electrophysiology Experiment standard in their own work. (C) 2011 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.pbiomolbio.2011.07.001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

We have previously reported the electrophysiological properties of sarcolemmal stretch-activated BKCa (SAKCA) channels cloned from cultured chick embryonic ventricular myocytes. However, the role of BKCa channels in the electrophysiology of the more mature heart is not clear. We have investigated the effects on the BKCa current of axial stretch in post-hatch ventricular myocytes. Whole-cell currents of ventricular myocytes isolated from 2-week-old chicks were recorded using the patch-clamp technique, while the cells were either held at resting length or stretched to cause a 10% increase in sarcomere length using a pair of carbon fibres attached to opposite ends of the cell. Stretch did not affect whole-cell currents immediately after the stretch was applied. However, sustained stretch for 3 min significantly increased outward currents. This stretch-induced change was reversed by applying 10 nm iberiotoxin, a specific BKCa channel blocker, or a Na+-Ca2+-free environment. These results were reproduced in a computer simulation study. The present study is the first report about the sarcolemmal BKCa current from post-hatch ventricular myocytes. The present results suggest that axial stretch activates BKCa channels via a stretch-induced increase in the cytosolic Na+ concentration followed by an increased Ca2+ influx.

DOI： 10.1113/expphysiol.2009.051896

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：LIPPINCOTT WILLIAMS & WILKINS  

We investigate acute effects of axial stretch, applied by carbon fibers (CFs), on diastolic Ca2+ spark rate in rat isolated cardiomyocytes. CFs were attached either to both cell ends (to maximize the stretched region), or to the center and one end of the cell (to compare responses in stretched and nonstretched half-cells). Sarcomere length was increased by 8.01 +/- 0.94% in the stretched cell fraction, and time series of XY confocal images were recorded to monitor diastolic Ca2+ spark frequency and dynamics. Whole-cell stretch causes an acute increase of Ca2+ spark rate (to 130.7 +/- 6.4%) within 5 seconds, followed by a return to near background levels (to 104.4 +/- 5.1%) within 1 minute of sustained distension. Spark rate increased only in the stretched cell region, without significant differences in spark amplitude, time to peak, and decay time constants of sparks in stretched and nonstretched areas. Block of stretch-activated ion channels (2 mu mol/L GsMTx-4), perfusion with Na+/Ca2+-free solution, and block of nitric oxide synthesis (1 mmol/L L-NAME) all had no effect on the stretch-induced acute increase in Ca2+ spark rate. Conversely, interference with cytoskeletal integrity (2 hours of 10 mu mol/L colchicine) abolished the response. Subsequent electron microscopic tomography confirmed the close approximation of microtubules with the T-tubular-sarcoplasmic reticulum complex (to within approximate to 10(-8)m). In conclusion, axial stretch of rat cardiomyocytes acutely and transiently increases sarcoplasmic reticulum Ca2+ spark rate via a mechanism that is independent of sarcolemmal stretch-activated ion channels, nitric oxide synthesis, or availability of extracellular calcium but that requires cytoskeletal integrity. The potential of microtubule-mediated modulation of ryanodine receptor function warrants further investigation. (Circ Res. 2009; 104: 787-795.)

DOI： 10.1161/CIRCRESAHA.108.193334

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Language：English   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Cardiac cellular calcium (Ca2+) handling is the well-investigated mediator of excitation-contraction coupling, the process that translates cardiac electrical activation into mechanical events. The reverse-effects of mechanical stimulation on cardiomyocyte Ca2+ handling-are much less well understood, in particular during the inter-beat period, called 'diastole'. We have investigated the effects of diastolic length changes, applied axially using a pair of carbon fibres attached to opposite ends of Guinea pig isolated ventricular myocytes, on the availability of Ca2+ in the main cellular stores (the sarcoplasmic reticulum; SR), by studying the rest-decay of SR Ca2+ content [Ca2+](SR), and the reloading of the SR after prior depletion of Ca2+ from the cell.
Cells were loaded with Fura-2 AM (an indicator of the cytosolic 'free' Ca2+ concentration, [Ca2+](i)), and preconditioned by field-stimulation (2Hz) at 37 degrees C, while [Ca2+](i) transients and sarcomere length (SL) were recorded simultaneously. After reaching a steady state in the behaviour of observed parameters, stimulation was interrupted for between 5 and 60 s, while cells were either held at resting length, or stretched (controlled to cause a 10% increase in SL, to aid inter-individual comparison). Thereafter, each cell was returned to its original resting length, followed by swift administration of 10mM of caffeine (in Na+/Ca2+ -free solution), which causes the release of Ca2+ from the SR (caffeine), but largely prevents extrusion of Ca2+ from the cytosol to the cell exterior (Na+/Ca2+-free solution). By comparing the [Ca2+](i) in cells exposed/not exposed to diastolic stretch of different duration, we assessed the rest-decay dynamics of [Ca2+](SR). To assess SR reloading after initial Ca2+ depletion, the same stretch protocol was implemented after prior emptying of the cell by application of 10 mM of caffeine in normal Tyrode solution (which causes Ca2+ to be released from the SR and extruded from the cell via the Na+/Ca2+ exchanger; NCX).
Axial stretch enhanced the rate of both rest-decay and reloading of [Ca2+](SR). Application of 40 mu M streptomycin, a blocker of stretch-activated ion channels, did not affect the stretch-induced increase in SR reloading. This behaviour was reproduced in a computer simulation study, using a modified version of the 2006 Iribe-Kohl-Noble model of single cardiac myocyte Ca2+ handling, suggesting that stretch increases both Ca2+ leak from the SR and Ca2+ influx via the sarcolemma. This may have important implications for the mobilisation of Ca2+ in stretched cells, and could contribute to the regional 'matching' of individual cardiomyocyte contractility to dynamic, and regionally varying, changes in mechanical loads, such as diastolic pre-load of cardiac tissue. (c) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.pbiomolbio.2008.02.012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSIOLOGICAL SOC  

We developed a dynamic force- length ( FL) control system for single intact cardiomyocytes that uses a pair of compliant, computer- controlled, and piezo translator ( PZT)- positioned carbon fibers ( CF). CF are attached to opposite cell ends to afford dynamic and bidirectional control of the cell's mechanical environment. PZT and CF tip positions, as well as sarcomere length ( SL), are simultaneously monitored in real time, and passive/ active forces are calculated from CF bending. Cell force and length were dynamically adjusted by corresponding changes in PZT position, to achieve isometric, isotonic, or work- loop style contractions. Functionality of the technique was assessed by studying FL behavior of guinea pig intact cardiomyocytes. End- diastolic and end- systolic FL relations, obtained with varying preload and/ or afterloads, were near linear, independent of the mode of contraction, and overlapping for the range of end- diastolic SLs tested ( 1.85 - 2.05 mu m). Instantaneous elastance curves, obtained from FL relation curves, showed an afterload- dependent decrease in time to peak elastance and slowed relaxation with both increased preload and afterload. The ability of the present system to independently and dynamically control preload, afterload, and transition between end- diastolic and endsystolic FL coordinates provides a valuable extension to the range of tools available for the study of single cardiomyocyte mechanics, to foster its interrelation with whole heart pathophysiology.

DOI： 10.1152/ajpheart.00909.2006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOCIETY  

We hypothesize that slow inactivation of Ca2+/calmodulin-dependent kinase II (CaMKII) and its modulatory effect on sarcoplasmic reticulum (SR) Ca2+ handling are important for various interval-force (I-F) relations, in particular for the beat interval dependency in transient alternans during the decay of post-extrasystolic potentiation. We have developed a mathematical model of a single cardiomyocyte to integrate various I-F relations, including alternans, by incorporating a conceptual CaMKII kinetics model into the SR Ca2+ handling model. Our model integrates I-F relations, such as the beat interval-dependent twitch force duration, restitution and potentiation, positive staircase phenomenon and alternans. We found that CaMKII affects more or less all I-F relations, and it is a key factor for integration of the various I-F relations in our model. Alternans arises, in the model, out of a steep relation between SR Ca2+ load and release, owing to SR, load-dependent changes in the releasability of Ca2+ via the ryanodine receptor. Beat interval-dependent CaMKII activity, owing to its kinetic properties and amplifying effect on SR. Ca2+ load dependency of Ca2+ release, replicated the beat interval dependency of alternans, as observed experimentally. Additionally, our model enabled reproduction of the effects of various interventions on alternans, such as the slowing or accelerating of Ca2+ release and/or uptake. We conclude that a slow time-dependent factor, represented in the model by CaMKII, is important for the integration of I-F relations, including alternans, and that our model offers a useful tool for further analysis of the roles of integrative Ca2+ handling in myocardial I-F relation.

DOI： 10.1098/rsta.2006.1758

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

Background. Better protective effects of Celsior on cardiac function than the other conventional solutions have been reported in acute experiments and in clinical trials for at-risk patients. However, no study has yet precisely elucidated how these preservation solutions affect cardiac mechanoenergetics. Therefore, we evaluated the effects of St. Thomas' Hospital solution No. 2, University of Wisconsin solution, and Celsior on left ventricular contractility (E-max: end-systolic pressure-volume ratio) and oxygen consumption.
Methods. We used 32 canine excised cross-circulated hearts. Twenty-three hearts served as donor hearts after hypothermic ischemia. with one of the three solutions, and the remaining 9 served as controls. After arrest with each solution, the hearts were preserved for 4 hours at VC. Then, we measured left ventricular pressure, volume, and oxygen consumption to obtain E-max and the relation between ventricular pressure-volume area (a measure of total mechanical energy) and oxygen consumption. We also evaluated the oxygen cost of E-max by changing E-max with calcium administration.
Results. Celsior did not significantly affect E-max (6-3 +/- 2.4 in control versus 5.3 +/- 1.3 mm Hg (.) mL(-1) (.) 100 g with Celsior) nor the oxygen cost of E-max (1.2 +/- 0.6 versus 1.6 +/- +/- 0.5 mL O-2 (.) mL (.) mm Hg-1 (.) beat(-1 .) 100 g(-2), respectively). In contrast, St. Thomas' Hospital and University of Wisconsin solutions significantly decreased E-max (4.5 +/- 1.1 and 3.5 +/- 0.9 mm Hg (.) mL(-1) (.) 100 g, respectively) and increased the oxygen cost of E-max (2.5 +/- 0.8 and 2.4 +/- 0.9 mL O-2 (.) mL (.) mm Hg-1 (.) beat(-1 .) 100 g(-2), respectively) compared with control and Celsior-preserved hearts. The slope and intercept of the oxygen consumption versus pressure-volume area relation showed no significant difference among the four groups.
Conclusions. Celsior showed better protective effects on cardiac mechanoenergetics than St. Thomas' Hospital and University of Wisconsin solutions in the acute phase of heart transplantation.

DOI： 10.1016/j.athoracsur.2005.07.067

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSIOLOGICAL SOC  

We previously found the frequency distribution of the left ventricular (LV) effective afterload elastance (E-a) of arrhythmic beats to be nonnormal or non-Gaussian in contrast to the normal distribution of the LV end-systolic elastance (E-max) in canine in situ LVs during electrically induced atrial fibrillation (AF). These two mechanical variables determine the total mechanical energy [systolic pressure-volume area (PVA)] generated by LV contraction when the LV end-diastolic volume is given on a per-beat basis. PVA and Emax are the two key determinants of the LV O-2 consumption per beat. In the present study, we analyzed the frequency distribution of PVA during AF by its chi(2), significance level, skewness, and kurtosis and compared them with those of other major cardiodynamic variables including Ea and Emax. We assumed the volume intercept (V-0) of the end-systolic pressure-volume relation needed for Emax determination to be stable during arrhythmia. We found that PVA distributed much more normally than E-a and slightly more so than Emax during AF. We compared the chi(2), significance level, skewness, and kurtosis of all the complex terms of the PVA formula. We found that the complexity of the PVA formula attenuated the effect of the considerably nonnormal distribution of E-a on the distribution of PVA along the central limit theorem. We conclude that mean (SD) of PVA can reliably characterize the distribution of PVA of arrhythmic beats during AF, at least in canine hearts.

DOI： 10.1152/ajpheart.00584.2004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

Left ventricular (LV) O-2 consumption (Vo(2)) per minute is measurable for both regular and arrhythmic beats. LV Vo(2) per beat can then be obtained as Vo(2) per minute divided by heart rate per minute for regular beats, but not for arrhythmic beats. We have established that Vo(2) of a regular stable beat is predictable by Vo(2) = a PVA + b E-max + c, where PVA is the systolic pressure-volume area as a measure of the total mechanical energy of an individual contraction and Em,. is the end-systolic maximum elastance as an index of ventricular contractility of the contraction. Furthermore, a is the O-2 cost of PVA, b is the O-2 cost of E-max, and c is the basal metabolic Vo(2) per beat. We considered it theoretically reasonable to expect that the same formula could also predict LV Vo(2) of individual arrhythmic beats from their respective PVA and E-max with the same a, b, and c. We therefore applied this formula to the PVA - E-max data of individual arrhythmic beats under electrically induced atrial fibrillation (AF) in six canine in situ hearts. We found that the predicted Vo(2) of individual arrhythmic beats highly correlated linearly with either their Vo(2) (r = 0.96 +/- 0.01) or E-max (0.97 +/- 0.03) while both also highly correlated linearly with each other (0.88 +/- 0.04). This suggests that the above formula may be used to predict LV Vo(2) of absolute arrhythmic beats from their E-max and PVA under AF.

DOI： 10.2170/jjphysiol.R2099

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

We have previously found that the postextrasystolic (PES) potentiation (PESP) of the left ventricular (LV) contractility (Emax) decays typically in transient alternans even in the normally ejecting canine heart. This contradicted the general expectation that arterial pressure (AP) and LV pressure (LVP) usually decay exponentially during PESP. We hypothesized this contradiction to be due to the different cardiodynamic behaviors of AP and LVP from LV Emax during PESP. We tested this hypothesis by measuring AP, LVP, LV volume, Emax, effective arterial elastance (Ea) as an index of afterload, and pulse pressure (PP) during PESP in eight anesthetized open-chest dogs by using the conductance catheter system. We changed Ea by changing the total peripheral resistance (TPR) with methoxamine hydrochloride (iv) and repeated the measurements. Although the Emax alternans patterns during PESP were comparable between the normal and high afterloads, LVP and PP were slightly potentiated and alternated under the normal afterload, whereas LVP and PP were obviously potentiated and alternated under the high afterload. We also simulated the effects of Ea/Emax on the transient alternans of AP and LVP on a computer. Despite the same alternans pattern of Emax, a higher Ea/Emax, which is typical in heart failure, caused a larger PP alternans, whereas a lower Ea/Emax, which is typical in normal hearts, almost eliminated it. These results suggest that a transient alternans of LV contractility during PESP could be overlooked when AP and LVP are monitored in in situ normal hearts.

DOI： 10.2170/jjphysiol.54.373

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER-VERLAG  

We examined the sarcomere length-dependence of the spacing of the hexagonal lattice of the myofilaments and the mass transfer of myosin cross-bridges during contraction of right ventricular papillary muscle of the rat. The lattice spacing and mass transfer were measured by using X-ray diffraction, and the sarcomere length was monitored by laser diffraction at the same time. Although the lattice spacing and the sarcomere length were inversely related, their relationship was not exactly isovolumic. The cell volume decreased by about 15% when the sarcomere length was shortened from 2.3 mum to 1.8 mum. Twitch tension increased with sarcomere length (the Frank-Starling law). At the peak tension, the ratio of the intensity of the (1,0) equatorial reflection to that of the (1,1) reflection was smaller when the tension was greater, showing that the larger tension at a longer sarcomere length accompanies a larger amount of mass transfer of cross-bridges from the thick to the thin filament. The result suggests that the Frank-Starling law is due to an increase in the number of myosin heads attached to actin, not in the average force produced by each head.

DOI： 10.1007/s00424-004-1243-z

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

We have reported that the postextrasystolic (PES) potentiation of left ventricular (LV) contractility usually decays in alternans at heart rates above 80-100 beats/min in the canine excised, cross-circulated heart. We examined whether the PES contractility would also decay in alternans even in the canine in situ heart presumably more physiological than the excised heart. In anesthetized, ventilated, and open-chest mongrel dogs, we measured LV pressure and volume with a micromanometer and a conductance catheter cannulated into the LV and obtained LV end-systolic maximum elastance (E-max) as the reasonably load-independent contractility index. We inserted an extrasystole followed by a compensatory pause into steady-state regular beats at heart rates above 90 beats/min and analyzed the PES decay pattern of E-max. We found that E-max potentiated in the first PES beat decayed in alternans within 5-6 PES beats. This indicates that PES contractility also decays in alternans in the normal canine in situ heart.

DOI： 10.2170/jjphysiol.53.313

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

The recirculation fraction of intramyocardial Ca2+ (RF) has conventionally been obtained from the monotonic decay of postextrasystolic potentiation (PESP). The used assumption is that the decay is exponential. However, we have found that PESP usually decays in alternans even at spontaneous heart rates (>100 beats/min) in excised, cross-circulated canine heart preparations under normal coronary perfusion and normothermia. We have already devised a means of extracting the exponential decay component for RF calculation by subtracting the oscillatory component from the alternans PESP decay by a curve-fitting method. Using mathematics, we assessed the possible error in estimated RF when an exponential curve was naively fit to the alternans PESP decay. We obtained results showing that the exponential assumption may considerably underestimate RF even when the alternans is trivial with the oscillatory component of only 10% of the exponential component.

DOI： 10.2170/jjphysiol.53.89

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KLUWER ACADEMIC/PLENUM PUBL  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

Mean levels of left ventricular rhythm and contractility averaged over arrhythmic beats would characterize the average cardiac performance during atrial fibrillation (AF). However, no consensus exists on the minimal number of beats for their reliable mean values. We analyzed their basic statistics to find out such a minimal beat number in canine hearts. We produced AF by electrically stimulating the atrium and measured left ventricular arrhythmic beat interval (RR) and peak isovolumic pressure (LVP). From these, we calculated instantaneous heart rate (HR=60,000/RR), contractility (E-max=LVP/isovolumic volume above unstressed volume), and beat interval ratio (RR1/RR2). We found that all their frequency distributions during AF were variably nonnormal with skewness and kurtosis. Their means standard deviations alone cannot represent their nonnormal distributions. A 90% reduction of variances of E-max and RR1/RR2 required a moving average of 15 and 24, respectively, arrhythmic beats on the average, whereas that of RR and HR required 60 beats on the average. These results indicate that a statistical characterization of arrhythmic cardiodynamic variables facilitates better understanding of cardiac performance during AF.

DOI： 10.2170/jjphysiol.52.41

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

Myocardial Ca2+ handling in excitation-contraction coupling is the second primary determinant of energy or O-2 demand in a working heart. The intracellular and extracellular routes remove myocardial Ca2+ that was released into the sarcoplasma with different Ca2+ : ATP stoichiometries. The intracellular route is twice as economical as the extracellular route. Therefore the fraction of total Ca2+ removed via the sarcoplasmic reticulum, i.e., the recirculation fraction of intracellular Ca2+ (RF), determines the economy of myocardial Ca2+ handling. RF has conventionally been estimated as the exponential decay rate of postextrasystolic potentiation (PESP). However, we have found that PESP usually decays in alternans, but not exponentially in the canine left ventricle beating above 100 beats/min. We have succeeded in estimating RF from the exponential decay component of an alternans PESP. We previously found that the Frank-Starling mechanism or varied ventricular preload did not affect the economy of myocardial Ca2+ handling. Then, to account for this important finding, we hypothesized that the Frank-Starling mechanism would not affect RF at a constant heart rate. We tested this hypothesis and found its supportive evidence in 11 canine left ventricles. We conclude that RF at a constant heart rate would remain constant, independent of the Frank-Starling mechanism.

DOI： 10.2170/jjphysiol.51.733

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

We have aimed to assess total Ca2+ handling in excitation-contraction coupling in a beating left ventricle (LV). Our newly developed integrative analysis method utilizes the internal Ca2+ recirculation fraction (RF), O-2 consumption (Vo(2)) for Ca2+ handling, and O-2 cost of Emax (contractility index) of the LV. We have obtained the O-2 cost of Emax from Vo(2) measured at different contractility levels, and have combined the cost with RF calculated from the beat-constant of the exponential decay component of the postextrasystolic potentiation. Our method calculates the unknown total Ca2+ handling from the RF and the "Ca2+ handling Vo(2)". The calculated total Ca2+ handling fell between 30 and 110 mu mol/kg, depending on contractility and pathological conditions. The present method also enable's reasonable assessment of futile Ca2+ cycling and of the Ca2+ reactivity of Emax. Our method seems useful to better understanding of the pathophysiology of total Ca2+ handling in a beating heart.

DOI： 10.1142/S0219519401000180

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

In our previous studies, we calculated the internal Ca2+ recirculation fraction (RF) after obtaining the beat decay constant (tau (e)) of the monoexponential component in the postextrasystolic potentiation (PESP) of the alternans decay by curve fitting. However, this method sometimes suffers from the sensitive variation of tau (e) with small noises in the measured contractilities of the 5th and 6th postextrasystolic (PES) beats in the tail of the exponential component. We now succeeded in preventing this problem by a new method to calculate RF without obtaining tau (e). The equation for the calculation in the new method expresses an alternans decay of PESP as a recurrence formula of PESP. It can calculate RF directly from the contractilities of the 1st through the 4th PES beats without any fitting procedure. To evaluate the reliability of the new method, we calculated RF from the alternans decay of PESP of the left ventricle (LV) of the canine excised cross-circulated heart preparation by both the original fitting and the new method. Although there was no significant difference in the mean value of the obtained RF between these two methods, the variance of RF was smaller with the new method than with the original method. Thus the new method proved useful and more reliable than the original fitting method.

DOI： 10.2170/jjphysiol.51.143

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSIOLOGICAL SOC  

We discovered that the coupling beat interval from a slow to a tachycardiac pacing period considerably affected the pattern of the beat-to-beat alternation of the tachycardia-induced sustained contractile alternans. We analyzed the relationship between the coupling interval and the pattern and amplitude of the alternans in the isovolumic left ventricle of canine blood-perfused hearts. The alternans pattern and amplitude varied transiently over the first 30-50 beats and became gradually stable over the first minute in all 12 hearts. We discovered that stable alternans, even under the same tachycardiac pacing, had three different strong-weak beat patterns depending on the coupling interval. A relatively short coupling interval produced a representative sustained alternans of the strong and weak beats. A relatively long coupling interval produced a similar sustained alternans but in a reversed order of even-and odd-numbered beats counted from the coupling interval. However, sustained alternans disappeared after 1-3 specific coupling intervals. We conclude that ventricular pacing rate does not solely determine the pattern and amplitude of sustained contractile alternans induced by tachycardia.

DOI： 10.1152/ajpheart.2001.280.3.H1368

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL RESEARCH COUNCIL CANADA  

We assessed total Ca2+ handling (transport, flux) in excitation-contraction (E-C) coupling in a beating left ventricle (LV). We developed a new integrative analysis method that utilizes the internal Ca2+ recirculation fraction (RF), O-2 consumption ((V) over dot(O2)) for Ca2+ handling, and O-2 cost of E-max (contractility index) of the LV. We obtained the RF from the beat constant of the exponential decay component of the postextrasystolic potentiation, and the O-2 cost of E-max from (V) over dot(O2) measured at different E-max. Our equation calculated the unknown total Ca2+ handling, futile Ca2+ cycling, and Ca2+ reactivity of E-max from the RF and Ca2+ handling (V) over dot(O2). The calculated total Ca2+ handling fell between 30 and 110 mu mol/kg, depending on E-max and pathological conditions. Our method also allowed an assessment of futile Ca2+ cycling and Ca2+ reactivity of E-max in a beating LV. These data are not available using conventional methods. Our method can be used to better understand the pathophysiology of total Ca2+ handling in a beating heart.

DOI： 10.1139/cjpp-79-1-87

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

Whether 2,3-butanedione monoxime (BDM, less than or equal to5 mmol/l) suppresses primarily crossbridge cycling or total Ca2+ handling in the blood-perfused whole heart remains controversial. Although BDM seems to suppress primarily total Ca2+ handling in canine hearts, more evidence is lacking. We therefore analyzed the cardiac mechanoenergetics, namely, E-max (contractility), PVA (total mechanical energy), and O-2 consumption of canine BDM-treated hearts by our recently developed integrative method to assess myocardial total Ca2+ handling. This method additionally required the internal Ca2+ recirculation fraction. We obtained this from the beat constant of the exponential decay component of the postextrasystolic potentiation. Our analysis indicated significant decreases in both internal Ca2+ recirculation fraction and total Ca2+ handling in the BDM-treated heart, but virtually no change in the reactivity of E-max to total Ca2+ handling. This result corroborates the view that BDM suppresses primarily total Ca2+ handling rather than crossbridge cycling in the canine blood-perfused heart.

DOI： 10.2170/jjphysiol.50.543

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSIOLOGICAL SOC  

In isolated, blood-perfused canine hearts, postextrasystolic potentiation (PESP) decays monotonically after a noncompensatory pause following a spontaneous extrasystole (ES). The monotonic PESP decay yields myocardial internal Ca2+ recirculation fraction (RF). We have found that after a compensatory pause (CP), PESP decays in alternans, consisting of an exponential and a sinusoidal decay component. We have proposed that this exponential component also yields RF. In the present study, we examined the reliability of this alternative method by widely changing the ES coupling interval (ESI), CP, and heart rate in the canine excised, cross-circulated left ventricle. We found that all PESP decays consisted of the sum of an exponential and a sinusoidal decay component of variable magnitudes whether a CP existed or not. Their decay constants as well as the calculated RF were independent of the ESI and CP. This confirmed the utility of our alternative RF determination method regardless of the ESI, CP, and heart rate. Direct experimental evidence of Ca2+ dynamics supportive of this alternative method, however, remains to be obtained.

DOI： 10.1152/ajpheart.2000.279.1.H225

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSIOLOGICAL SOC  

Postischemic myocardial stunning halved left ventricular contractility [end-systolic maximum elastance(E-max)] and doubled the O-2 cost of E-max in excised cross-circulated canine heart. We hypothesized that this increased Oil cost derived from energy-wasteful myocardial Ca2+ handling consisting of a decreased internal Ca2+ recirculation, some futile Ca2+ cycling, and a depressed Ca2+ reactivity of E-max. We first calculated the internal Ca2+ recirculation fraction (RF) from the exponential decay component of postextrasystolic potentiation. Stunning significantly accelerated the decay and decreased RF from 0.63 to 0.43 on average. We then combined the decreased RF with the halved E-max and its doubled O-2 cost and analyzed total Ca2+ handling using our recently developed integrative method. We found a decreased total Ca2+ transport and a considerable shift of the relation between futile Ca2+ cycling and Ca2+ reactivity in an energy-wasteful direction in the stunned heart. These changes in total Ca2+ handling reasonably account for the doubled O-2 cost of E-max in stunning, supporting the hypothesis.

DOI： 10.1152/ajpheart.2000.278.5.H1464

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CENTER ACADEMIC PUBL JAPAN  

Caffeine causes a considerable Ca waste for positive inotropism in myocardium by complex pharmacological mechanisms. However, no quantitative study has yet characterized the mechanoenergetics of caffeine, particularly its O-2 cost Of contractility in the E-max-PVA-VO2 framework. Here, E-max is an index of ventricular contractility, PVA is a measure of total mechanical energy generated by ventricular contraction, and VO2 is O-2 consumption of ventricular contraction. The E-max-PVA-VO2 framework proved to be powerful in cardiac mechanoenergetics. We therefore studied the effects of intracoronary caffeine at concentrations lower than 1 mmol/l on left ventricular (LV) E-max and VO2 for excitation-contraction (E-C) coupling in the excised cross-circulated canine heart. We enhanced LV E-max by intracoronary infusion of caffeine after beta-blockade with propranolol and compared this effect with that of calcium. We obtained the relation between LV VO2 and PVA with E-max as a parameter. We then calculated the VO2 for the E-C coupling by subtracting VO2 under KCI arrest from the PVA-independent (or zero-PVA) VO2 and the O-2 cost of E-max as the slope of the E-C coupling VO2-E-max relation. We found that this cost was 40% greater on average for caffeine than for calcium. This result, for the first time, characterized integratively cardiac mechanoenergetics of the O-2 wasting effect of the complex inotropic mechanisms of intracoronary caffeine at concentrations lower than 1 mmol/l in a beating whole heart.

DOI： 10.2170/jjphysiol.50.257

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：LIPPINCOTT WILLIAMS & WILKINS  

Objective: To measure the hepatic venous oxygen saturation in patients after cardiac surgery and to compare the effects of olprinone (OLP), a newly synthesized phosphodiesterase III inhibitor, with those of milrinone (MIL) and amrinone (AMR) on hepatosplanchnic oxygen dynamics. Phosphodiesterase III inhibitors are used to improve the hemodynamic state after cardiac surgery. However, the effect of these agents on the hepatosplanchnic circulation has not been investigated thoroughly.
Design: Prospective, randomized study.
Setting: University hospital intensive care unit (ICU).
Patients: Twenty-nine patients undergoing elective cardiac surgery.
Measurements and Main Results: In each patient, a 7.5-Fr oximeter catheter was placed in the hepatic vein via the right femoral vein. Catheterization was completed before admission to the ICU, and the study was performed 8 to 24 hrs after surgery, after obtaining stable systemic hemodynamics in the ICU. The patients were assigned randomly to three groups, and they received one of three drugs for 2 hrs (OLP group, 0.3 mu g/kg/min of OLP; MIL group, 0.5 mu g/kg/min of MIL; AMR group, 10 mu g/kg/min of AMR). The authors did not change the patient's hemodynamic interventions, including catecholamines and vasodilators, throughout the study period. Arterial and hepatic venous blood gas data and hemodynamic data (via a pulmonary artery catheter) were obtained before and after drug infusion. Using these data, the authors calculated systemic oxygen delivery and consumption, the systemic oxygen extraction ratio and the hepatosplanchnic oxygen extraction ratio, and the change in hepatosplanchnic blood flow using Pick's equation.
Although the increases in cardiac index were not significantly different among the three groups, hepatic venous oxygen saturation increased significantly only in the CLP group (from 47.1% +/- 2.6% to 57.0% +/- 1.5% in the OLP group, from 48.4% +/- 2.3% to 50.9% +/- 2.6% in the MIL group, and from 49.8% +/- 3.6% to 50.8% +/- 1.7% in the AMR group). The calculated hepatosplanchnic blood flow change was significantly larger in the OLP group than in the other groups (30.1% +/- 5.7% in the OLP group, 9.3% +/- 5.1% in the MIL group, and 2.6% +/- 6.5% in the AMR group).
Conclusions: These results suggest that OLP enhances hepatosplanchnic blood flow and thus may be beneficial in protecting the hepatosplanchnic organs after cardiac surgery.

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Language：Japanese   Publishing type：Research paper (scientific journal)  

An 18-year old female with mental retardation and unexamined complex congenital heart disease received dental care under general anesthesia. Anesthesia was induced and maintained successfully without any significant hemodynamic changes with inhalation of nitrous oxide, oxygen (FIO2 0.25-0.3) and sevoflurane after a heavy premedication (morphine 10 mg, scopolamine 0.3 mg and midazolam 5 mg i.m.). After induction of anesthesia, cardiac anomaly was diagnosed by transesophageal echocardiography as TGA, VSD, PS, and operation was completed without any problem. Two points are considered important in this case
first, to appropriately estimate preoperative cardiac function and second, to adequately manage anesthesia to avoid any hemodynamic fluctuation.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MUNKSGAARD INT PUBL LTD  

Background: Although several vasodilators are used to control vascular resistance during cardiac surgery, their effects on splanchnic circulation during extracorporeal circulation are unknown. We designed the present noninvasive study to evaluate the effect of prostaglandin E1 and nitroglycerin on portal venous flow during extracorporeal circulation using transesophageal echography.
Methods: We included 26 patients undergoing cardiac surgery with moderate hypothermic extracorporeal circulation in this study. After obtaining hemodynamic stability under extracorporeal circulation, we measured portal venous diameter, mean flow velocity and the velocity time integral using transesophageal echography and calculated portal venous flow The patients were assigned to two groups where either prostaglandin E1 (N=13) or nitroglycerin (N=13) was administered intravenously to maintain perfusion pressure at the level of 70 mmHg. We measured the same parameters 20 and 40 min following administration of the drug.
Results: Visualization of the portal vein was obtained by transesophageal echography in anesthetized patients. Calculated portal venous flow significantly increased in the prostaglandin E1 group, while it did not alter in We nitroglycerin group.
Conclusion: The present results indicate that transesophageal echography may be a feasible tool to assess portal venous flow, and that prostaglandin E1 may improve the blood distribution to the splanchnic area and the liver during hypothermic extracorporeal circulation.

DOI： 10.1034/j.1399-6576.1999.430506.x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER VERLAG  

We analyzed total Ca handling of the left ventricle (LV) in the mildly failing heart preparation induced by a temporary intracoronary Ca overloading intervention in eight excised and cross-circulated canine hearts. This Ca intervention consisted of interruption of coronary blood perfusion by Ca-free oxygenated Tyrode perfusion for 10 min followed by high-Ca (16 mmol/l) oxygenated Tyrode perfusion for 5 min. This intervention decreased the LV contractility index, E-max (end-systolic maximum elastance), by 40% after restoration of the blood cross-circulation. We expected a Ca overload or paradox failing heart resembling the postischemic stunned heart and being characterized by an increased O-2 cost of E-max. However, LV O-2 consumption under mechanically unloading conditions decreased by 30% from control without increasing the O-2 cost Of E-max. To Obtain a mechanistic view of this failing heart, we investigated cardiac total Ca handling by our integrative analysis method. In this method, we obtained the internal Ca recirculation fraction (RF) from the decay beat constant of the postextrasystolic potentiation following each sporadic spontaneous extrasystole in these failing LVs. We combined the RF with the decreased E-max and the unchanged O-2 cost of E-max in our recently developed formula of total Ca handling. We found that these failing LVs had a slightly but significantly increased RF accompanied by either a slightly increased futile Ca cycling or a slightly decreased Ca reactivity of E-max, or both. Any of these three possible changes can account for the unchanged O-2 cost of E-max. This result indicates that the present mildly failing heart has not yet fallen into a typical Ca overload or paradox by the temporary Ca overloading intervention.

DOI： 10.1007/BF02481741

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PubMed

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER VERLAG  

We have reported that the postextrasystolic potentiation (PESP) decays in alternans or monotonically, respectively, depending on whether the first postextrasystolic beat interval has a compensatory pause or not, in the canine left ventricle. To get better mechanistic insight into the alternans PESP decay. we hypothesized that the myocardial mechanical restitution and potentiation could partly account for both types of PESP decay, To test this hypothesis, we simulated PESP decay on a computer using a documented equation combining myocardial mechanical restitution and potentiation. We changed the first postextrasystolic beat interval after a fixed extrasystolic beat interval without changing regular and other postextrasystolic beat intervals. The simulated PESP decayed in alternans or monotonically as a function only of the first postextrasystolic beat interval. Thus, the myocardial mechanical restitution and potentiation could partly account for both alternans and monotonic decay of PESP. We conclude that myocardial mechanical restitution and potentiation may partly underlie the initial two alternating beats, the first beat being the most potentiated and the second beat being the most depressed, of alternans PESP decay in the canine heart.

DOI： 10.1007/BF02481747

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PubMed

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Publisher：Elsevier BV  

DOI： 10.1016/j.bpj.2020.11.633

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Language：English   Publisher：(一社)日本生理学会  

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Publisher：Elsevier BV  

DOI： 10.1016/j.bpj.2019.11.2345

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J-GLOBAL

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Language：English   Publisher：(一社)日本生理学会  

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Language：English   Publisher：(一社)日本生物物理学会  

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Language：English   Publisher：(一社)日本生理学会  

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Language：English   Publisher：(一社)日本生理学会  

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J-GLOBAL

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Language：Japanese   Publisher：(一社)日本生理学会  

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Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

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Language：English   Publisher：(一社)日本生理学会  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：CELL PRESS  

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Language：Japanese   Publisher：Japanese Society for Medical and Biological Engineering  

<p>When myocardium is held in a stretched position, its [Ca²⁺]_i and twitch force slowly increase over several minutes (slow force response to stretch: SFR). We have reported that TRPC3, non-selective cation channel, contributes to the SFR. Although their intracellular localization is important for the cellular function, it still remains unclear. To predict the localization of TRPC3, we investigated the changes in [Ca²⁺]_SR during SFR in single isolated mouse cardiomyocytes in combination with mathematical model simulation study. Fura-4F-loaded cells were electrically stimulated at 1 Hz. A pair of carbon fibers was attached to cell ends to apply axial stretch. [Ca²⁺]_SR was estimated by the caffeine-induced changes in [Ca²⁺]_i before and after stretch (300 s). The stretch significantly increased the [Ca²⁺]_SR, while C36I-A, TRPC3 blocker, diminished the increase in [Ca²⁺]_SR. The results were reproduced only by the model with sarcolemmal stretch-activated cation channels, suggesting TRPC3 channels are located on sarcolemma.</p>

DOI： 10.11239/jsmbe.54Annual.S395

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Language：English   Publisher：IEEE Computer Society  

Electrical and mechanical properties of myocardium vary transmurally in the left ventricular wall. Regional differences in the mechanical environment of cardiomyocytes may potentially contribute to this heterogeneity due to mechano-electric feedback. In the present study, we investigate transmural differences in active and passive tensions at different preloads between the subepicardial (EPI) and subendocardial (ENDO) cells isolated from mouse left ventricle, using our recently developed single cell stretch method where each cell end was held by a pair of carbon fibers to apply various extent of preload to the cells. To predict underlying mechanisms of the transmural differences, we used our electromechanical EPI and ENDO cell models to simulate experimentally obtained results. Wet experiments showed that both passive and active tensions at different preloads are higher in ENDO cardiomyocytes, indicating higher stiffness and contractility in ENDO cells compared to EPI cells. Our mathematical models reproduce experimental results, suggesting differences in the kinetics of cross bridges and calcium-troponin C complexes in ENDO and EPI models may essentially contribute to the differences in mechanical properties between the cells.

DOI： 10.1109/CIC.2015.7411073

Scopus

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Language：Japanese   Publisher：Japanese Society for Medical and Biological Engineering  

The heart is a mechanically functioning organ which consists of cardiomyocytes. Although it has been revealed that mechano-electric coupling plays an important role in cardiac physiology, it is difficult to investigate direct link between subcellular mechano-sensitive responses and tissue or organ functions in wet experiments. To integrate experimentally obtained findings in micro and macro study, we have developed a mathematical cardiac myocyte model based on Iribe-Kohl-Noble model. To describe mechano-sensitive properties, we implemented stretch activated channels (SACs), viscoelastic property of cytoskeleton or cytosolic structures, and shortening-rate dependency in troponin I (TnI) inactivation. Viscoelastic property in our model gave stretch-rate dependency in SACs activation, which reasonably reproduced mechanically induced ectopic beat after quick stretch and delayed increase in developed force after slow stretch. Shortening-rate dependent TnI inactivation reproduced afterload dependency in muscle mechanics previously reported. Our modeling study suggests advantages of computational biology in macro-micro integration in cardiac physiology.

DOI： 10.11239/jsmbe.53.S139_02

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Language：English   Publisher：(一社)日本生理学会  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：CELL PRESS  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

It is important to give a cardiomycyte mechanical load for developing cardiomyocite physiology study. Therefore a gripper which grasps an isolated cardiomyocyte is required. However at present, there is no device grasping the cell easily without electrical and magnetical noises. In our previous research,a pneumatic gripper which has a fluid chamber and composed of polymer materials has been developed. Gripper's drive properties were clarified by the last year. At a result,it was found that the gripper cannot maintain appropriate cardiomyocyte grasping because of creep phenomena. Therefore in this research,the gripper which is made in nylon material and have fluid chambers of the bellows structure is developed. As an experimental result,in comparison with the previous gripper,the quantity of creep is restrained from 107μm to 34μm. Namely the device can realize accurate positioning as the gripper for cadiomyocyte. Actually,the gripper grasps a small piece of the rubber which is a glass beads.

DOI： 10.1299/jsmemdt.2014.14.33

CiNii Research

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J-GLOBAL

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Azelnidipine and amlodipine are dihydropyridine-type Ca2+ channel blockers for the treatment of hypertension. Although these drugs have high vasoselectivity and small negative inotropic effects in vivo, little is known regarding their direct effects on cellular contractility without humoral regulation or the additive effects of these drugs with other antihypertensive drugs on myocardial contractility. To investigate the effects of Ca2+ channel blockers on single cell mechanics, mouse cardiomyocytes were enzymatically isolated, and a pair of carbon fibers was attached to opposite cell-ends to stretch the cells. Cells were paced at 4 Hz superfused in normal Tyrode solution at 37 C. Cell length and active/passive force calculated from carbon fiber bending were recorded in 6 different preload conditions. Slopes of end-systolic force length relation curves (maximum elastance) were measured as an index of contractility before and after drugs were administered. Azelniclipine at 10 nM and 100 nM did not change maximum elastance, while amlodipine at 100 nM did decrease maximum elastance. The combination of RNH-6270 (active form of angiotensin II receptor blocker, olmesartan, 10 nM) and either amlodipine (10 nM) or azelnidipine (10 nM) did not affect maximum elastance. Although both amlodipine and azelnidipine can be used safely at therapeutically relevant concentrations even in combination with olmesartan, the present results suggest that azelnidipine has a less negative inotropic action compared to amlodipine. (C) 2013 Elsevier B.V. All rights reserved,

DOI： 10.1016/j.ejphar.2013.05.030

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Language：English   Publisher：Japanese Society for Medical and Biological Engineering  

DOI： 10.11239/jsmbe.51.M-32

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：LIPPINCOTT WILLIAMS & WILKINS  

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Language：Japanese   Publisher：(公社)日本生体医工学会  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：OXFORD UNIV PRESS  

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J-GLOBAL

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：CELL PRESS  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：WILEY-BLACKWELL PUBLISHING, INC  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：OXFORD UNIV PRESS  

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J-GLOBAL

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：SPRINGER TOKYO  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：BIOPHYSICAL SOCIETY  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：BIOPHYSICAL SOCIETY  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：BIOPHYSICAL SOCIETY  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：BIOPHYSICAL SOCIETY  

Web of Science

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：LIPPINCOTT WILLIAMS & WILKINS  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：LIPPINCOTT WILLIAMS & WILKINS  

Web of Science

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Language：English   Publisher：Japanese Circulation Society  

CiNii Books

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Language：English   Publisher：Japanese Circulation Society  

CiNii Books

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Language：Japanese  

J-GLOBAL

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：FEDERATION AMER SOC EXP BIOL  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：FEDERATION AMER SOC EXP BIOL  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：LIPPINCOTT WILLIAMS & WILKINS  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：LIPPINCOTT-RAVEN PUBL  

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