AND STUDY OF THE EFFECTS OF CYCLIC AMP

The transfer of the terminal phosphate of ATP to a material from the sarcoplasmic reticulum of cardiac muscle which can be precipitated by trichloro-acetic acid was studied, and the relationship of this biochemical event to active Ca2 + transport was examined. A component of the phosphoryl transfer reaction was stimulated specifically by Ca2 +. Both the phosphorylation of the reticulum and the active sequestering of Ca2+ were proportional to the Ca2 + concentration between Kh7 and 10-°M. The time course of both phenomena was similar. These and other observations relating Ca2 +-dependent phosphory-lation of the membrane of the sarcoplasmic reticulum with the active sequestering of Ca2+ suggest that the phosphoryl transfer reaction may represent the formation of a carrier system which facilitates the inward flux of Ca2+ against a concentration gradient. Cyclic AMP, theophylline, or the combination of these two agents did not influence the rate or extent of the phosphorylation of the sarcoplasmic reticulum. These results have led us to question the hypothesis that the cardiac sarcoplasmic reticulum is the site of action of cyclic AMP in its role as a mediator of cardiac inotropic effects. KEY WORDS cyclic AMP protein phosphorylation sarcoplasmic reticulum protein kinase cardiac contractility calcium uptake phosphoryl transfer • The sacroplasmic reticulum of cardiac and skeletal muscle is important in the regulation of calcium ion concentration within these muscle cells (1, 2), because it can concentrate Ca2+ within its tubular structure and thus influence the contraction-relaxation events of the cells. Recently, a considerable effort has been directed towards the elucida-tion of the mechanism by which Ca2 + is transported across the membrane of the sarcoplasmic reticulum. Hasselbach and Makinose (3) reported that the hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and inorganic phosphat