Calcium transport ATPase of cardiac sarcoplasmic reticulum in experimental hyperthyroidism

Abstract

When compared with euthyroid controls, sarcoplasmic reticulum isolated from the hearts of hyperthyroid rats had increased rates of Ca2+ uptake (101 .+-. 3.6 vs. 70 .+-. 2.1 nmol/(mg.cntdot.min), P < 0.01) and Ca2+ ATPase (53 .+-. 3.9 vs. 37 .+-. 2.0 nmol Pi per mg.cntdot.min, P < 0.01). To gain further insight into the mechanisms responsible for these differences, the kinetics of the cardiac Ca2+-transport ATPase were studied. A phosphoprotein intermediate (EP) is formed during ATP hydrolysis by cardiac microsomes. The steady-state levels of this intermediate were higher for microsomes from hyperthyroid rats (1.7 .+-. 0.2 vs. 1.4 .+-. 0.1 nmol/mg). The Ca2+ concentration required for half-maximal stimulation of EP formation was 3.8 .+-. 0.1 .mu.M for hyperthyroid microsomes and 5.0 .+-. 0.2 .mu.M for euthyroid microsomes. The apparent Km for MgATP, pH optimum, and turnover number of the transport ATPase (calculated as the ratio of ATPase activity to EP levels) were unchanged by hyperthyroidism. The enhancement of both Ca2+ uptake and EP formation by T4 [thyroxine] administration depended on protein synthesis because it was prevented by concomitant injection of actinomycin D, cycloheximide, or puromycin. This may indicate a preferential synthesis of Ca2+-Mg2+-ATPase in hyperthyroid rats. The passive permeability of microsomal vesicles to Ca2+ was enhanced in the preparations from hyperthyroid animals (7.8 .+-. 0.4 vs. 4.9 .+-. 0.3 nmol Ca2+ per mg.cntdot.min in controls, P < 0.05). The possibility was examined that these permeability changes were associated with altered lipid composition of the sarcoplasmic reticulum. Phospholipid content of cardiac microsomes from hyperthyroid rats was increased (0.56 vs. 0.42 mg/mg protein). The fatty acid composition of the phospholipids was also changed mainly due to an increase in palmitate and stearate coupled with decreases in arachidonate and linoleate levels. These results may partly explain the altered Ca2+ transport function of the sarcoplasmic reticulum in experimental hyperthyroidism.