Impact of temperature on cross-bridge cycling kinetics in rat myocardium

The dependence of contractile properties on intracellular calcium in cardiac tissue is a highly cooperative process. Here, the temperature and calcium dependence of contractile and energetical properties in permeabilized cardiac trabeculae from rat were studied to provide novel insights into the underlying kinetic processes. Myofilament Ca2+ sensitivity significantly increased with temperature between 15 and 25 degrees C, whereas its steepness was independent of temperature. A direct proportionality between active tension and Ca2+-activated rate of ATP hydrolysis was observed; the slope of this relationship (tension cost) was highly temperature dependent. The rate of tension i-edevelopment following a quick release-restretch manoeuvre (k(tr),) depended in a complex manner on the level of contractile activation and on temperature. At saturating calcium levels, the temperature dependence (Q(10)) of ktr and Ca2+-activated ATP hydrolysis rate were similar (Q10 similar to 3.5), and significantly higher than the Q10 for maximum tension (T-max; Q(10) similar to 1.3) or tension cost (Q(10) similar to 2.5). In contrast, at a low level of contractile activation (similar to 5% of T-max), the Q(10) of k(tr) was similar to that of tension cost, and significantly lower than the Q(10) of Ca2+-activated ATP hydrolysis at that level of contractile activation. Our results are consistent with the hypothesis that at high levels of contractile activation, the rates of tension redevelopment and Ca2+-activated ATP hydrolysis are determined by both apparent cross-bridge attachment and detachment rates, while at low levels, ktr is limited by cross-bridge detachment rate. Tension cost, on the other hand, is determined solely by cross-bridge detachment kinetics at all temperatures and levels of contractile activation.