Na-Ca Exchange in Cardiac Sarcolemmal Vesicles
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Every second of a heart cell’s existence is characterized by large fluctuations in the cytoplasmic concentration of calcium ions. Accordingly, heart cells exhibit a very sophisticated mechanism for the regulation of their cytoplasmic calcium concentration ([Ca2+]j), a system which involves the coordinated interaction of Ca2+ channels and pumps in the plasma membrane and intracellular organelles as well as Ca-binding proteins in both the cytoplasm and, of course, the myofibrillar apparatus itself. One element of this complex mechanism for regulating [Ca]j is the Na-Ca exchange system, a carrier-mediated transport process in which the movement of Ca2+ in one direction across the membrane is directly coupled to the movement of Na+ in the opposite direction. It is a component of the cardiac plasma membrane (sarcolemma) and mediates Ca2+ movements in either direction between the cell cytoplasm and the extracellular fluid. As discussed in detail elsewhere (1,2), its primary physiological role is probably to serve as a Ca2+ pump, i.e. to effect the net efflux of Ca2+ from the myocardial cell, although it seems likely that under some circumstances it can bring about the net entry of Ca2+ as well.
KeywordsVesicle Preparation Squid Axon Exchange Carrier Native Vesicle Redox Reagent
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