Phosphorylation of the Sarcoplasmic Reticulum and Sarcolemma

The ,a-adrenergic actions of catecholamines represent a key control mecha­ nism that regulates the metabolic, electrical, and mechanical performance of the myocardium. Cyclic AMP( cAMP) serves as the intracellular second messenger for these regulatory processes in that ,a-adrenergic activation of a sarcolemmal adenylate cyclase causes intracellular cAMP levels to rise (75). This second messenger appears to exert most of its cellular effects by activating cAMP-dependent protein kinases (cAMP-PKs) (42). These en­ zymes catalyze phosphorylation of phosphorylase kinase and glycogen syn­ thetase, resulting in a marked increase in glycolysis (7), although these metabolic changes are not essential for the positive inotropic response of the well-oxygenated heart to catecholamines (87). Catecholamines and their intracellular messenger cAMP alter several aspects of the excitation-contraction process in the myocardium (26, 27, 87), including increases in the flux of ions across the sarcolemma (SL) (60, 65) and sarcoplasmic reticulum (SR) (26, 80, 83). The present review de­ scribes findings that define the mechanism by which cAMP-PKs react to induce these changes in membrane function.