Effects of adrenalectomy and in vivo administration of dexamethasone on ATP-dependent calcium accumulation by sarcoplasmic reticulum from rat heart.

ATP-dependent Ca2+ accumulation and (Mg2+ + Ca2+)ATPase activities were determined in cardiac sarcoplasmic reticulum (SR) from control, adrenalectomized and adrenalectomized-dexamethasone-treated rats. SR from adrenalectomized rats displayed significantly diminished rates (approximately 40 to 50%, P less than 0.001) of Ca2+ accumulation compared to SR from control rats. The decreased Ca2+ accumulating activity of the membranes from adrenalectomized compared to control rats could be observed at varying Ca2+ concentrations (0.5 to 11.9 microM); kinetic analysis showed that the velocity of Ca2+ transport but not the apparent affinity of the transport system for Ca2+ was altered by adrenalectomy. ATPase activities (with Mg2+ or with Mg2+ + Ca2+) of SR were not altered significantly following adrenalectomy. In vivo administration of dexamethasone to adrenalectomized rats led to a partial (75 to 80%) yet significant (P less than 0.01 to 0.05) restoration of Ca2+ accumulating activity of SR towards normal; dexamethasone-treatment caused decrease in basal (Mg2+)ATPase activity and enhancement in (Mg2+ + Ca2+)ATPase activity. When added in vitro, dexamethasone had no effect on Ca2+ accumulating and ATPase activities of SR from control or adrenalectomized rats. These findings imply an important role for glucocorticoids in the maintenance of membrane Ca2+ transport function, and therefore, normal myocardial contractility.