Altered myosin isozyme patterns from pressure-overloaded and thyrotoxic hypertrophied rabbit hearts.

Cardiac hypertrophy, induced by pressure overload, leads to a depression in the rate of force development, velocity of shortening, tension-dependent heat generation, and myosin ATPase activity, whereas cardiac hypertrophy, induced by thyroxine administration, leads to an increase in these parameters. These changes have been attributed, in part, to structural changes in myosin. In this study, we have investigated changes in the relative content of myosin isozymes and differences in primary structure of the isozymes in pressure-overloaded and thyrotoxic cardiac hypertrophy in the rabbit. Three myosin isozymic forms (V1 = fastest, V2 = intermediate, V3 = slowest mobility) were observed in pyrophosphate polyacrylamide gels from normal hearts with the V3 component being the predominant species. In the pressure-overloaded model, the V1 and V2 components disappeared or were present in reduced amounts leaving the V3 more predominant. The most striking difference was the isozymic profile produced in thyrotoxic hearts where the V1 became the predominant component and V2 and V3 the minor components. alpha-Chymotryptic digestion of myosin heavy chains produced characteristic, reproducible peptide patterns for each of the animal models, as did fluorographic analyses of alpha-chymotryptic digests of 14C-iodoacetamide (IAA)-labeled SH1 peptides of myosin. Our results suggest that altered proportions of myosin isozymes may be responsible for altered cardiac performance.