Determination of rate constants for turnover of myosin isoforms in rat myocardium: implications for in vivo contractile kinetics

The ventricles of small mammals express mostly [alpha]-myosin heavy chain ([alpha]-MHC), a fast isoform, whereas the ventricles of large mammals, including humans, express ~10% [alpha]-MHC on a predominately [beta]-MHC (slow isoform) background. In failing human ventricles, the amount of [alpha]-MHC is dramatically reduced, leading to the hypothesis that even small amounts of [alpha]-MHC on a predominately [beta]-MHC background confer significantly higher rates of force development in healthy ventricles. To test this hypothesis, it is necessary to determine the fundamental rate constants of cross-bridge attachment ([f.sub.app]) and detachment ([g.sub.app]) for myosins composed of 100% [alpha]-MHC or [beta]-MHC, which can then be used to calculate twitch time courses for muscles expressing variable ratios of MHC isoforms. In the present study, rat skinned trabeculae expressing either 100% [alpha]-MHC or 100% [beta]-MHC were used to measure ATPase activity, isometric force, and the rate constant of force redevelopment ([k.sub.tr]) in solutions of varying [Ca.sup.2+] concentrations. The rate of ATP utilization was ~2.5-fold higher in preparations expressing 100% [alpha]-MHC compared with those expressing only [beta]-MHC, whereas [k.sub.tr] was 2-fold faster in the [alpha]-MHC myocardium. From these variables, we calculated [f.sub.app] to be approximately threefold higher for [alpha]-MHC than [beta]-MHC and [g.sub.app] to be twofold higher in [alpha]-MHC. Mathematical modeling of isometric twitches predicted that small increases in [alpha]-MHC significantly increased the rate of force development. These results suggest that low-level expression of [alpha]-MHC has significant effects on contraction kinetics. [alpha]-myosin heavy chain; rate constants of cross-bridge attachment and detachment; rate of rise of force