The effect of sodium, calcium and metabolic inhibitors on calcium efflux from goldfish heart ventricles.

Abstract

45Ca efflux and tissue Ca content were examined in goldfish ventricles under conditions known to affect cellular Ca movements. EGTA [ethylene glycol-bis (.beta.-aminoethylether)-N,N''-tetraacetic acid] or Ca-EGTA was added to the washout solutions in sufficient concentration (10 mM) to avoid retardation of the apparent tissue 45Ca efflux by extracellular 45Ca binding or backflux. After a variable initial increase, the cellular Ca content usually stabilizes within 60 min when ventricles are immersed in Li- or K-substituted saline containing 1.8 mM Cao (under these conditions the internal Ca2+ concentration is below 10-5 M). 45Ca efflux is maximally activated by external concentrations of Ca2+ as low as 10-6 M, in both Na-containing and Na-free saline. 45Ca efflux decreases in Na-free solutions. It is reactivated by Na-saline. The effect of different external Na concentration on 45Ca efflux is comparable at external Ca2+ concentrations between 10-6 M and 2 .times. 10-3 M. Reactivation of Ca efflux after Nao readmission is inhibited by metabolic poisoning, or with 10 mM-caffeine. Loading with 45Ca at very low external Ca2+ concentration prevents the inhibition of Ca efflux in Na-free medium. Caffeine (10 mM) produces contractions of about equal size when K-depolarized preparations are immersed in either Na- or Li-saline. At the same time there is a similar increase in 45Ca efflux with and without Nao. In the virtual absence of .**GRAPHIC**. (10-5 M-Ca, 10-2 M-EGTA) and .**GRAPHIC**. the residual 45Ca efflux is reversibly inhibited by cyanide (2 mM). The results are roughly compatible with the general concept of ATP-dependent Na-Ca exchange in internal Ca2+ homeostasis. This hypothesis should probably be modified to account for the fact that under physiological concentrations .**GRAPHIC**. and .**GRAPHIC**. do not compete for activating 45Ca efflux. Metabolic products may be involved in Nao- and Cao-dependent Ca efflux. It is not excluded that a Na-independent active mechanism co-operates with Na-Ca exchange in Ca extrusion.