The effect of hypothyroidism on sarcoplasmic reticulum in fast-twitch muscle of the rat.

The effects of hypothyroidism on the Ca2+-transport capabilities of fast-twitch muscle (m. gastrocnemius) of the rat were studied in whole-muscle homogenate and isolated sarcoplasmic reticulum. Hypothyroidism did not affect the percentage recovery and the vesicle composition of the sarcoplasmic reticulum fraction, the total lipid and phospholipid-to-protein ratios and the protein composition (both qualitative and quantitative). Also the Ca2+-loading capacity of purified sarcoplasmic reticulum, in the presence of oxalate, and the Ca2+ and pH dependence of both the uptake reaction and the coupled ATPase activity were unchanged. However, the homogenate Ca2+-loading capacity and the Ca2+-uptake activity were depressed, as was the yield of purified sarcoplasmic reticulum. The results indicate a 31% reduction of the entire sarcoplasmic reticulum membrane system per volume of muscle. Ca2+/ATP coupling ratios, determined in purified sarcoplasmic reticulum vesicles by measurement of initial rates of net Ca2+ uptake and Ca2+-Mg2+-dependent hydrolysis of ATP, were found to be 1.48 +/- 0.06 and 2.08 +/- 0.05 in the euthyroid and hypothyroid groups, respectively. Identical values were obtained with a recently described Ca2+-pulse method (Meltzer, S. and Berman, M.C. (1984) Anal. Biochem. 138, 458-464), i.e., 1.53 +/- 0.06 and 2.01 +/- 0.03 in the euthyroid and hypothyroid groups, respectively. Passive Ca2+ efflux from sarcoplasmic reticulum was the same in both groups (30 nmol/mg per min), as was the fraction of vesicles that did not show net uptake of Ca2+ (less than 10%), which makes it unlikely that these parameters provide an explanation for the differences in the coupling ratio. The energy of activation of the (Ca2+ + Mg2+)-ATPase was increased in hypothyroidism, which may point to changes in the phospholipid environment of the enzyme. Physiological concentrations of T3 and T4 had no effect on the (Ca2+ + Mg2+)-ATPase in vitro, but all observed changes in the hypothyroid state could be reversed within 14 days by administration of T3 to hypothyroid animals. Approximate calculations indicate that the observed changes in the sarcoplasmic reticulum as a result of thyroid-hormone depletion may contribute significantly to the decrease in relaxation rate and the decrease in energy consumption during contraction.