Adenosine 3′:5′-Monophosphate-dependent Protein Kinase-catalyzed Phosphorylation Reaction and Its Relationship to Calcium Transport in Cardiac Sarcoplasmic Reticulum

Abstract A rapid, manyfold increase in phosphorylation of cardiac microsomes consisting primarily of sarcoplasmic reticulum was seen when these membranes were incubated in the presence of a bovine cardiac adenosine 3':5'-monophosphate (cyclic AMP)-dependent protein kinase (protein kinase) and cyclic AMP. Over 85% of the 32P associated with membrane protein under similar conditions was identified as phosphoserine and phosphothreonine. A less marked increase in phosphoprotein formation was observed when cardiac microsomes were incubated in 1 µm cyclic AMP in the absence of added protein kinase. This could be attributed to the presence of an endogenous protein kinase. When cardiac microsomes were incubated with protein kinase alone, phosphorylation also was enhanced, finally reaching the level seen with cyclic AMP and added protein kinase. The increased phosphorylation induced by protein kinase alone was attributable to the presence of an adenylate cyclase in the microsomal preparation. Epinephrine could be shown to stimulate both adenylate cyclase and phosphorylation of cardiac microsomes. Protein kinases prepared from both bovine cardiac or rabbit fast skeletal muscle catalyzed the formation of microsomal phosphoprotein. The extent of phosphoprotein formation correlated closely with the increment in the stimulation of the rate of calcium uptake by cardiac microsomes when concentrations of either protein kinase were varied, and the relationship between phosphoprotein formation and stimulation of calcium transport was independent of the source of the protein kinase. These data suggest the existence of a functional relationship between cardiac microsomal phosphorylation and an increased rate of calcium transport.