Glucocorticoid modulation of protein phosphorylation and sarcoplasmic reticulum function in rat myocardium

To decipher the mechanism(s) underlying glucocorticoid action on cardiac co ntractile function, this study investigated the effects of adrenalectomy an d dexamethasone treatment on the contents of sarcoplasmic reticulum (SR) Ca 2+-cycling proteins, their phosphorylation by endogenous Ca2+/calmodulin-de pendent protein kinase II (CaM kinase II), and SR Ca2+ sequestration in the rat myocardium. Cardiac SR vesicles from adrenalectomized rats displayed s ignificantly diminished rates of ATP-energized Ca2+ uptake in vitro compare d with cardiac SR vesicles from control rats; in vivo administration of dex amethasone to adrenalectomized rats prevented the decline in SR function. W estern immunoblotting analysis showed that the relative protein amounts of ryanodine receptor/Ca2+ release channel, Ca2+-ATPase, calsequestrin, and ph ospholamban were neither diminished significantly by adrenalectomy nor elev ated by dexamethasone treatment. However, the relative amount of SR-associa ted CaM kinase II protein was increased 2.5- to 4-fold in dexamethasone-tre ated rats compared with control and adrenalectomized rats. Endogenous CaM k inase II activity, as judged from phosphorylation of ryanodine receptor, Ca 2+-ATPase, and phospholamban protein, was also significantly higher (50-80% increase) in the dexamethasone-treated rats. The stimulatory effect of CaM kinase II activation on Ca2+ uptake activity of SR was significantly depre ssed after adrenalectomy and greatly enhanced after dexamethasone treatment . These findings identify the SR as a major target for glucocorticoid actio ns in the heart and implicate modification of the SR CaM kinase II system a s a component of the mechanisms by which dexamethasone influences SR Ca2+-c ycling and myocardial contraction.