Ca transport during contraction and relaxation in mammalian ventricular muscle.

Abstract

During relaxation of cardiac muscle four Ca transport systems can compete to remove Ca from the myoplasm. These are 1) the SR Ca-ATPase, 2) the sarcolemmal Na/Ca exchange, 3) the sarcolemmal Ca-ATPase, and 4) the mitochondrial Ca uniporter. Isolated ventricular myocytes loaded with the intracellular fluorescent Ca indicator indo-1 were used to study [Ca]i decline during relaxation. By selective inhibition of the various Ca transporters above the dynamic interaction of these systems during relaxation was evaluated. Quantitatively the SR Ca-ATPase and Na/Ca exchange are clearly the most important (accounting for > 95% of Ca removal). However, the balance of Ca fluxes between these systems vary in a species dependent manner. For example, the SR is much more strongly dominant in rat ventricular myocytes, where ~ 92% of Ca removal is via SR Ca-ATPase and only 7% via Na/Ca exchange during a twitch. In other species (rabbit, ferret, cat, and guinea-pig) the balance is more in the range of 70–75% SR Ca-ATPase and 25–30% Na/Ca exchange. Ferret ventricular myocytes also exhibit a unusually strong sarcolemmal Ca-ATPase. During the normal steady state cardiac contraction-relaxation cycle the same amount of Ca must leave the cell as enters over a cardiac cycle. This implies that 25–30% of the Ca required to activate contraction must enter the cell at each cardiac cycle. Experiments using voltage clamp to measure both Ca current and Na/Ca exchange current demonstrate that this amount of Ca may be supplied by the L-type Ca current.