Incorporation of the Terminal Phosphate of ATP into Membranal Protein of Rabbit Cardiac Sarcoplasmic Reticulum: CORRELATION WITH ACTIVE CALCIUM TRANSPORT AND STUDY OF THE EFFECTS OF CYCLIC AMP

Abstract

The transfer of the terminal phosphate of ATP to a material from the sarcoplasmic reticulum of cardiac muscle which can be precipitated by trichloroacetic 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 10^-7 and 10^-5M. The time course of both phenomena was similar. These and other observations relating Ca²⁺-dependent phosphorylation of the membrane of the sarcoplasmic reticulum with the activesequestering 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.