Thyroxine-induced redistribution of isoenzymes of rabbit ventricular myosin.

We examined ventricular myosin isoenzymes in hearts of normal and thyroxine (T,)treated rabbits by polyacrylamide gel electrophoresis using non-dissociating conditions as described by Hoh et al. (1977).The rabbits received daily injections of T 4 (150 /ig/kg) or saline and were killed at 1 to 17 days.A myosin-enriched fraction was prepared from each ventricle by extraction in 100 mM pyrophosphate.Myofibrils were also prepared from each heart for the measurement of myofibrillar actomyosin and myosin ATPase activity.In normal and thyrotoxic rabbit ventricles, there were three distinct isoenzymes (R-Vi, R-V 2 , and R-V 3 , in decreasing order of mobility) analogous to the myosin isoenzymes Vi, V 2 , and V3 present in rat ventricle.In hearts of normal rabbits, R-V 3 was the predominant species, whereas, in the thyrotoxic hearts, H-V, was the predominant species.The redistribution of isoenzymes was apparent within 2 days of T 4 administration and was complete within 5-7 days of T 4 injections.Associated with the maximum shift in myosin isoenzyme profiles there was an increase of approximately 90% in the myofibrillar myosin Ca 2+ -activated ATPase activity and an increase of approximately 30% in the Mg 2+ -ATPase activity of myofibrillar actomyosin.The Ca 2+ -activated myosin ATPase of the myofibrils was correlated with the percent R-Vi in the preparations.Co-electrophoresis of mixtures of rat and rabbit heart myosins showed four separate bands, resulting from a difference in mobility between R-V2 and V 2 , as well as R-Vi and Vi.Our data indicate that rabbit heart myofibrils have three isoenzymes, some of which may be structurally distinct from rat heart myosin isoenzymes, and that T 4 administration induces preferential synthesis of a preexisting isoenzyme, R-V,.