Thermal Activation of Cardiac Thin Filaments Induces Contraction without Intracellular Ca2+ Changes: Studies with Cardiomyocytes and an In Vitro Motility Assay

Abstract

It is well established that heart beat is triggered by electrical excitation, followed by an increase in [Ca2+]i and ensuing activation of myofilaments in cardiomyocytes. This property is the basis for the regulation of the heart with electric stimulators or chemical compounds that mediate Ca2+ signaling. Recently, we found that microscopic heat pulses induce contractions in isolated rat cardiomyocytes without a rise in [Ca2+]i (Oyama, K. et al., Biochem. Biophys. Res. Commun., 417, 607-612, 2012). Microscopic heat pulses produced by focusing infrared laser beam increase temperature of the extracellular solution near cardiomyocytes. In our experiments, a microscopic heat pulse (ΔT = 5°C for 0.5 sec) induced contractions at a basal temperature of 36°C. When heat pulses were repeatedly applied at 2.5 Hz, oscillatory contractions were observed. In contrast to the electric field stimulation, Ca2+ transients were not detected during the course of contraction. Moreover, we confirmed that skinned cardiomyocytes are thermally activated in the Ca2+-free solution. To elucidate the molecular mechanism of thermal activation, microscopic heat pulses were applied to an in vitro motility assay system utilizing reconstituted thin filaments (rabbit skeletal actin filaments reconstituted with human cardiac tropomyosin and bovine cardiac troponin). Thin filaments started to move during heating in the absence of Ca2+, and most of filaments immediately stopped after switching the laser off, demonstrating that thin filaments can be thermally activated, presumably due to the weakening of the binding affinity of the troponin-tropomyosin complex with actin (Ishiwata, S., Biochim. Biophys. Acta, 534, 350-357, 1978). These results suggest that heat pulses can regulate cardiac contractions by shifting the “on-off” equilibrium in the thin filament toward the “on” state, without accompanying intracellular Ca2+ transients.