Deletion of Enigma Homologue from the Z-disc slows tension development kinetics in mouse myocardium

Enigma Homologue (ENH) is a component of the Z-disc, a structure that anchors actin filaments in the contractile unit of muscle, the sarcomere. Cardiac-specific ablation of ENH protein expression causes contractile dysfunction that ultimately culminates in dilated cardiomyopathy. However, whether ENH is involved in the regulation of myocardial contractility is unknown. To determine if ENH is required for the mechanical activity of cardiac muscle, we analyze muscle mechanics of isolated trabeculae from the hearts of ENH+/+ and ENH−/− mice. We detected no differences in steady-state mechanical properties but show that when muscle fibers are allowed to relax and then are restretched, the rate at which tension redevelops is depressed in ENH−/− mouse myocardium relative to that in ENH+/+ myocardium. SDS-PAGE analysis demonstrated that the expression of β-myosin heavy chain is increased in ENH−/− mouse myocardium, which could partially, but not completely, account for the depression in tension redevelopment kinetics. Using top-down proteomics analysis, we found that the expression of other thin/thick filament regulatory proteins is unaltered, although the phosphorylation of a cardiac troponin T isoform, cardiac troponin I, and myosin regulatory light chain is decreased in ENH−/− mouse myocardium. Nevertheless, these alterations are very small and thus insufficient to explain slowed tension redevelopment kinetics in ENH−/− mouse myocardium. These data suggest that the ENH protein influences tension redevelopment kinetics in mouse myocardium, possibly by affecting cross-bridge cycling kinetics. Previous studies also indicate that ablation of specific Z-disc proteins in myocardium slows contraction kinetics, which could also be a contributing factor in this study.

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 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 ktr was 2-fold faster in the α-MHC myocardium. From these variables, we calculated fapp to be approximately threefold higher for α-MHC than β-MHC and gapp to 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.