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

Abstract

The effects of hypothyroidism on the Ca 2+-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 Ca 2+-loading capacity of purified sarcoplasmic reticulum, in the presence of oxalate, and the Ca 2+ and pH dependence of both the uptake reaction and the coupled ATPase activity were unchanged. However, the homogenate Ca 2+-loading capacity and the Ca 2+-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. Ca 2+/ATP coupling ratios, determined in purified sarcoplasmic reticulum vesicles by measurement of initial rates of net Ca 2+ uptake and Ca 2+-Mg 2+-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 Ca 2+-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 Ca 2+ 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 Ca 2+ (< 10%), which makes it unlikely that these parameters provide an explanation for the differences in the coupling ratio. The energy of activation of the (Ca 2+ + Mg 2+)-ATPase was increased in hypothyroidism, which may point to changes in the phospholipid environment of the enzyme. Physiological concentrations of T 3 and T 4 had no effect on the (Ca 2+ + Mg 2+)-ATPase in vitro, but all observed changes in the hypothyroid state could be reversed within 14 days by administration of T 3 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.