title: Species differences and the role of sodium-calcium exchange in cardiac muscle relaxation.
abstract: During normal relaxation in rabbit, guinea-pig, and rat ventricular muscle, the Na-Ca exchange system competes with the SR Ca pump, with the former being responsible for about 20-30% of the Ca removal from the cytoplasm. Ca extrusion via Na-Ca exchange is Em-sensitive, whereas Ca uptake by the SR is not. Neither the sarcolemmal Ca-ATPase pump nor mitochondrial Ca uptake appear to contribute significantly to the decline of [Ca]i during relaxation. Furthermore, the diastolic efflux of Ca from cardiac muscle cells appears to be primarily attributable to Na-Ca exchange and not the sarcolemmal Ca-ATPase pump. In rabbit ventricle Ca entry via Na-Ca exchange is favored thermodynamically during much of a normal twitch contraction and Ca extrusion occurs primarily between beats. In rat ventricle Ca efflux via Na-Ca exchange occurs during the contraction and net Ca influx may occur between beats. This fundamental difference in Ca fluxes during the cardiac cycle in rat versus rabbit ventricle may be a simple consequence of the shorter action potential duration and higher aNai in rat ventricle (due to the effects of Em and [Na] and [Ca] gradients on Na-Ca exchange).
authors.affiliation: Division of Biomedical Sciences, University of California, Riverside 92521-0121.
authors.forename: D M
authors.initials: DM
authors.lastname: Bers
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology*; D002352:Carrier Proteins / Q000502:physiology*; D009200:Myocardial Contraction / Q000502:physiology*; D011817:Rabbits; D051381:Rats; D012964:Sodium / Q000502:physiology*; D019831:Sodium-Calcium Exchanger; D013045:Species Specificity
chemical_list: D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D002118:Calcium