Role of cyclic AMP in regulating cardiac muscle contractility: Novel pharmacological approaches to modulating cyclic AMP degradation by phosphodiesterase

Abstract

Abstract The role of cyclic AMP in modulating the contractile function of cardiac muscle has been the subject of intensive investigation for more than two decades. These studies have shown that the positive inotropic response to cyclic AMP involves multiple components, including adenylate cyclase, the enzyme that catalyzes the conversion of ATP to cyclic AMP, activation of cyclic AMP‐dependent protein kinases, which are responsible for phosphorylating key intracellular proteins, and phosphodiesterase, the enzyme that hydrolyzes cyclic AMP, thereby terminating the response. Several reports have shown, however, that increases in intracellular cyclic AMP are not always accompanied by increases in myocardial contractility, suggesting that cyclic AMP may be compartmentalized within cardiac cells and that only certain compartments are involved in modulating contractility. In addition, although cardiac muscle contains only one form of adenylate cyclase, multiple forms of phosphodiesterase have been identified in ventricular muscle, which vary in their substrate specificity and their response to allosteric effectors such as calmodulin. Recent studies have also shown that these various forms of phosphodiesterase can be located in different regions within the cardiac cell and that only certain forms of the enzyme are involved in modulating the inotropic response to cyclic AMP. This review summarizes the current state of knowledge regarding the involvement of cyclic AMP with cardiac contractile function and also explores several controversial aspects of this involvement.