Myosin synthesis and enzymatic properties during thyroxine intoxication in rabbits.

Hyperthyroidism has been induced to rabbits by a daily subcutaneous injection of D-L thyroxine (0.3 mg/kg). The ventricular myosin Ca2+ ATPase was increased after one week and reached + 100 % after two weeks while the K+ EDTA ATPase remained normal. During the same period, the heart weight, and the myosin incorporation of 3H-lysine after a four hour period of labelling had increased in parallel. The enhancement of both the Ca2+ ATPase and the labelling of myosin correlate, which suggests the appearance of a new molecule during the course of intoxication. This biochemical abnormality was unchanged after treatment with propranolol.