Effect of thyroid status on K+-stimulated metabolism and 45Ca exchange in rat skeletal muscle.

In the perfused hindlimbs of hypo-, eu-, and hyperthyroid rats, high K+o (20 mM) markedly stimulated glucose uptake, lactate production, and O2-consumption to levels increasing with the thyroid status. At Ca2o+ less than 50 microM, all the responses became transient. In the hypothyroid preparations, caffeine (5 mM) produced no stimulation, but in those obtained from eu- and hyperthyroid rats, it increased all the metabolic parameters as well as K+ release in proportion to thyroid status. In isolated soleus and extensor digitorum longus muscles, both high K+o and caffeine stimulated 45Ca-efflux, with the response increasing in proportion to thyroid status. This stimulation was reduced by 70-100% with dantrolene (10(-5) M), which in earlier studies was shown to suppress the metabolic effects of high K+o and caffeine. High K+o also increased 45Ca-influx but produced no change in muscle calcium contents. The results support the idea that thyroid hormones control substrates availability and energy consumption in skeletal muscle by increasing the amount of sarcoplasmic reticulum and thereby the rate of Ca2+ mobilization into the cytosol.