Thyroxine-induced redistribution of isoenzymes of rabbit ventricular myosin.

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Abstract
Ventricular myosin isoenzymes were examined in hearts of normal and thyroxine (T4)-treated rabbits by polyacrylamide gel electrophoresis using non-dissociating conditions. The rabbits received daily injections of T4 (150 .mu.g/kg) or saline and were killed at 1-17 days. A myosin-enriched fraction was prepared from each ventricle by extraction in 100 mM pyrophosphate. Myofibrils were prepared from each heart for the measurement of myofibrillar actomyosin and myosin ATPase activity. In normal and thyrotoxic rabbit ventricles, there were 3 distinct isoenzymes (R-V1, R-V2 and R-V3, in decreasing order of mobility) analogous to the myosin isoenzymes V1, V2 and V3 present in rat ventricle. In hearts of normal rabbits, R-V3 was the predominant species, while in the thyrotoxic hearts, R-V1 was the predominant species. The redistribution of isoenzymes was apparent within 2 days of T4 administration and was complete within 5-7 days of T4 injections. Associated with the maximum shift in myosin isoenzyme profiles there was an increase of .apprx. 90% in the myofibrillar myosin Ca2+-activated ATPase activity and an increase of .apprx. 30% in the Mg2+-ATPase activity of myofibrillar actomyosin. The Ca2+-activated myosin ATPase of the myofibrils was correlated with the percent R-V1 in the preparations. Co-electrophoresis of mixtures of rat and rabbit heart myosins showed 4 separate bands, resulting from a difference in mobility between R-V2 and V2, as well as R-V1 and V1. Rabbit heart myofibrils have 3 isoenzymes, some of which may be structurally distinct from rat heart myosin isoenzymes, and T4 administration induces preferential synthesis of a preexisting isoenzyme, R-V1.