Regulation of myosin synthesis by thyroid hormone: relative change in the .alpha.- and .beta.-myosin heavy chain mRNA levels in rabbit heart

Abstract

The expression of mRNA for 2 cardiac myosins was examined in the ventricles of hypo- and hyperthyroid rabbits by means of cloned cDNA [complementary DNA] sequences corresponding to the mRNA of the .alpha.- and .beta.-myosin H chains (HC). The temporal change in the relative levels of the .alpha.- and .beta.-HC mRNA after 3,5,3''-triiodothyronine (T3) treatment of hypothyroid rabbits was determined by nuclease S1 mapping. In the hypothyroid state, only HC.beta.-mRNA was expressed in the ventricles. The HC.alpha.-mRNA was first detectable 4 h after administration of T3 (200 .mu.g/kg) to hypothyroid animals. By 12 h, HC.alpha.-mRNA represented 20% of total myosin mRNA, increasing to 50% by 24 h and to about 90% by 72 h. The relationship between the relative mRNA levels and relative synthesis rates of the myosin HC was evaluated in 5-6-wk-old normal and thyrotoxic rabbits. Myosin synthesis rates were determined by labeling of protein in vivo with [3H]leucine. The V1 (HC.alpha.) and V3 (HC.beta.) isomyosins were separated by affinity chromatography with monoclonal antibodies, and the HC were isolated electrophoretically. In a normal euthyroid group of animals and in animals 12 and 24 h after administration of 200 .mu.g of 3,5,3'',5''-tetraiodothyronine/kg, the relative mRNA levels and relative synthesis rates of the .alpha.- and .beta.-HC were not significantly different. Thyroid hormone causes a rapid accumulation of HC.alpha.-mRNA and loss of HC.beta.-mRNA and, in normal and thyrotoxic rabbits, the relative synthesis rates of HC.alpha. and HC.beta. reflect the relative abundance of the .alpha.- and .beta.-HC mRNA. Apparently thyroid hormone regulation of cardiac myosin synthesis occurs pretranslationally.