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

Abstract

The ventricles of small mammals express mostly α-myosin heavy chain (α-MHC), a fast isoform, whereas the ventricles of large mammals, including humans, express ∼10% α-MHC on a predominately β-MHC (slow isoform) background. In failing human ventricles, the amount of α-MHC is dramatically reduced, leading to the hypothesis that even small amounts of α-MHC on a predominately β-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 ( fapp) and detachment ( gapp) for myosins composed of 100% α-MHC or β-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% α-MHC or 100% β-MHC were used to measure ATPase activity, isometric force, and the rate constant of force redevelopment ( ktr) in solutions of varying Ca2+concentrations. The rate of ATP utilization was ∼2.5-fold higher in preparations expressing 100% α-MHC compared with those expressing only β-MHC, whereas ktrwas 2-fold faster in the α-MHC myocardium. From these variables, we calculated fappto be approximately threefold higher for α-MHC than β-MHC and gappto be twofold higher in α-MHC. Mathematical modeling of isometric twitches predicted that small increases in α-MHC significantly increased the rate of force development. These results suggest that low-level expression of α-MHC has significant effects on contraction kinetics.