DIFFERENTIAL-EFFECTS OF THYROID-HORMONE ON THE EXPRESSION OF SARCOPLASMIC-RETICULUM CA2+-ATPASE ISOFORMS IN RAT SKELETAL-MUSCLE FIBERS

Thyroid hormone increased the percentage of fibers expressing fast-type sarcoplasmic reticulum Ca2+-ATPase in the slow rat soleus muscle from 17% in the hypothyroid to 100% in the hyperthyroid state. This was accompanied by a 12-fold increase in the fast-type Ca2+-ATPase protein content of soleus muscle homogenates, suggesting that also the amount of this protein per muscle fiber was increased. In contrast to the fast-type isoform, a decrease in the percentage of fibers expressing slow-type Ca2+-ATPase from 100% to 70% was observed in the transition from the hypothyroid to the hyperthyroid stare. Slow-type Ca2+-ATPase protein levels in muscle homogenates however did not decrease on the same trajectory, but were even elevated in the euthyroid state. In the fast extensor digitorum longus muscle qualitatively similar changes in Ca2+-ATPase isoform expression were observed. The results suggest a dual action of thyroid hormone: 1. increasing slow-type Ca2+-ATPase expression in individual fibers 2. decreasing the fraction of slow-type Ca2+-ATPase expressing fibers. (C) 1994 Academic Press, Inc.