Sodium-Calcium Exchange: Calcium Regulation at the Sarcolemma

Abstract

Regulation of intracellular Ca is of paramount importance in maintaining the contractile, electrophysiological, and metabolic integrity of the myocardium. Myocardial cells regulate cytoplasmic Ca by multiple transport pathways in the sarcolemma and intracellular organelles. The sarcolemma can transport Ca by Na-Ca exchange, an ATP-dependent Ca pump, and a voltage-sensitive Ca channel. In addition, the sarcoplasmic reticulum and the mitochondria both possess Ca uptake and release mechanisms. Because of this complexity, it is often difficult to unequivocably assign to a specific transport pathway a Na or Ca flux or a current measured in intact tissue. In myocardial tissue, Na-Ca exchange is usually studied by introducing large interventions such as altering the external Na or Ca concentration. One must assume that these interventions have no effect on other Na or Ca transport mechanisms. In addition, as internal Na and Ca concentrations adjust in response to the change in the extracellular milieu, cellular metabolic and electrophysiologic properties may change. Thus there is often uncertainty in identifying and quantitating Na-Ca exchange in myocardial tissue.