Na-Ca exchange in cardiac tissues

Abstract

Our awareness of the importance of Na-Ca exchange in cardiac muscle has progressed from early observations of Na-Ca antagonism in the activation of contractile force. This was followed by demonstrations of actual Na-Ca ion countertransport across cell membranes and later functional studies in which manipulation of intracellular and extracellular Na and Ca concentrations has permitted a better characterization of the exchange process and its contribution to contractile force. The recent development of vesicle preparations from cardiac sarcolemmal membranes has, despite some drawbacks, produced useful information on the electrogenicity of the exchange mechanisms and on the relative affinity of the exchange carrier compared to the ATPase-driven Ca pump. These studies confirmed earlier estimates of the approximate exchange ratio of the Na-Ca countertransport system and have demonstrated its large maximum transport rate capabilities. The application of ion-sensitive microelectrodes in recent years has enabled measurements of the actual ion-activity gradients across the sarcolemmal membrane. These activity gradients together with the membrane potential control the rate and direction of the Na-Ca exchange. Despite the wide range of techniques employed to tackle the problem, the exchange ratio of Na to Ca movement is still in some doubt, with most estimates ranging between 5:2 and 4:1.