Effects of thyroid hormones on calcium contents and 45Ca exchange in rat skeletal muscle.

In 4-wk-old rats, pretreatment with L-triiodothyronine (T3) increased calcium content by 100% and the 30-min 45Ca uptake by 64% in the soleus, whereas the extensor digitorum longus (EDL) muscle showed no significant change. The stimulation of 45Ca uptake was resistant to dantrolene and methoxyverapamil (D600) and could not be attributed to altered permeability of the plasma membrane to calcium, but appears to reflect increased net accumulation of calcium in intracellular pools. The stimulating effect of high K0 (20 mM) on 45Ca uptake was more pronounced in soleus than in EDL and could be suppressed by dantrolene and D600. The results indicate that the effects of T3 on calcium content and 45Ca exchange are primarily exerted on muscles containing a large proportion of slow-twitch, oxidative fibers. In soleus muscle from hyperthyroid rats the stimulating effects of high K0 on 45Ca uptake and lactate production were, respectively, 3.4 and 4.5 times larger than in those obtained from controls. These observations further support the earlier proposed idea [C. van Hardeveld and T. Clausen. Am. J. Physiol. 247 (Endocrinol. Metab. 10): E421-E430, 1984] that the metabolic effects of thyroid hormone depend on the availability of cellular as well as extracellular calcium.