Load dependency in force-length relations in isolated single cardiomyocytes

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 μm 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 μm. 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 Ca(2+) transient shape

Effects of Propofol on Left Ventricular Mechanoenergetics in the Excised Cross-circulated Canine Heart

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)-Vo2 (myocardial oxygen consumption per beat) relationship. We obtained 1) the Vo2-PVA relationship of isovolumic contractions with varied LV volumes at a constant Emax, 2) the Vo2-PVA relationship with varied LV volumes at a constant intracoronary concentration of propofol, and 3) the Vo2-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 Vo2-PVA relationship (oxygen cost of PVA) remained unchanged by propofol. The PVA-independent Vo2-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 ß-antagonists—i.e., it exerts negative inotropic effects without changing the oxygen costs of Emax and PVA

High hydrostatic pressure induces slow contraction in mouse cardiomyocytes

Cardiomyocytes are contractile cells that regulate heart contraction. Ca2+ flux via Ca2+ channels activates actomyosin interactions, leading to cardiomyocyte contraction, which is modulated by physical factors (e.g., stretch, shear stress, and hydrostatic pressure). We evaluated the mechanism triggering slow contractions using a high-pressure microscope to characterize changes in cell morphology and intracellular Ca2+ concentration ([Ca2+]i) in mouse cardiomyocytes exposed to high hydrostatic pressures. We found that cardiomyocytes contracted slowly without an acute transient increase in [Ca2+]i, while a myosin ATPase inhibitor interrupted pressure-induced slow contractions. Furthermore, transmission electron microscopy showed that, although the sarcomere length was shortened upon the application of 20 MPa, this pressure did not collapse cellular structures such as the sarcolemma and sarcomeres. Our results suggest that pressure-induced slow contractions in cardiomyocytes are driven by the activation of actomyosin interactions without an acute transient increase in [Ca2+]i

TRPC3 participates in angiotensin II type 1 receptor-dependent stress-induced slow increase in intracellular Ca2+ concentration in mouse cardiomyocytes

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.Fil: Yamaguchi, Yohei. Okayama University; JapónFil: Iribe, Gentaro. Okayama University; JapónFil: Kaneko, Toshiyuki. Asahikawa Medical University; JapónFil: Takahashi, Ken. Okayama University; JapónFil: Numaga-Tomita, Takuro. National Institutes of Natural Sciences; JapónFil: Nishida, Motohiro. National Institutes of Natural Sciences; JapónFil: Birnbaumer, Lutz. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Research Triangle Park; Estados UnidosFil: Naruse, Keiji. Okayama University; Japó