Myosin from failing and non-failing human ventricles exhibit similar contractile properties.

In non-failing human myocardium, V1 myosin comprises a small amount (<10%) of the total myosin content, whereas end-stage failing hearts contain nearly 100% V3 myosin. It has been suggested that this shift in V1 myosin isoform content may contribute to the contractile deficit in human myocardial failure. To test this hypothesis, myosin was isolated from human failing and non-failing ventricles, and non-failing atria. Performance was assessed in in vitro motility and isometric force assays. Consistent with prior reports, a small amount of V1 myosin was present in both non-failing (6.2 +/- 1.0%) and failing (3.5 +/- 1.4%) ventricular tissues. No difference in isometric force or unloaded shortening velocity was observed for failing and non-failing ventricular myosin irrespective of myosin isoform content. Atrial tissue expressing predominantly V1 myosin (66.7 +/- 4.1%) generated half the force but greater velocity compared with ventricular tissue, expressing predominantly V3 myosin. In additional experiments, rabbit cardiac myosin was used in a calcium regulated assay system to determine if V1 and V3 isoforms differentially affect thin filament activation. Half-maximal calcium activation was similar for the two cardiac isoforms. A 1:9 mixture of V1/V3 myosin, simulating isoform composition in non-failing human myocardium, was indistinguishable from 100% V3 myosin (simulating the failing state) with regard to velocity of shortening and average force. These data suggest that the myosin isoform shift reported in human myocardial failure does not significantly contribute to the contractile deficit of this disease.

Cardiac troponin T isoforms demonstrate similar effects on mechanical performance in a regulated contractile system.

Alteration of troponin T (TnT) isoform expression has been reported in human and animal models of myocardial failure. The two adult beef cardiac TnT isoforms (TnT(3) and TnT(4)) were isolated for comparative functional analysis. Thin filaments were reconstituted containing pure populations of the isoforms. The in vitro motility assay was used to directly compare the effect of the two TnT isoforms on force and unloaded shortening as a function of free calcium. We found no significant differences between the two isoforms in terms of calcium sensitivity, cooperativity, or maximal activation (velocity and force) as assessed in a fully calcium-regulated system. Activation by myosin strong binding was similar for thin filaments containing either of the two TnT isoforms. Whereas maximally activated velocity and cooperativity was depressed at pH 6.5, no difference between thin filaments containing the two isoforms was detected. From the small magnitude of the TnT isoform shifts detected in myocardial failure and the lack of significant mechanical effect detected in the motility assay, variable TnT isoform expression is unlikely to be any functional significance in heart failure.