Sodium and calcium movements in dog red blood cells.

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Abstract
Determinants of 45Ca influx, 45Ca efflux and 22Na efflux were examined in dog red blood cells. 45Ca influx was strongly influenced by the Na concentration on either side of the membrane, being stimulated by intracellular Na and inhibited by extracellular Na. A saturation curve was obtained when Ca influx was plotted as a function of medium Ca concentration. The maximum Ca influx was a function of pH (increasing with greater alkalinity) and cell volume (increasing with cell swelling). Quinidine strongly inhibited Ca influx. Efflux of 45Ca was stimulated by increasing concentrations of extracellular Na. 22Na efflux was stimulated by either Ca or Na in the medium, and the effects of the 2 ions were mutually exclusive rather than additive. Quinidine inhibited Ca-activated 22Na efflux. The results were considered in terms of a model for Ca-Na exchange; the system showed many features of such a coupled ion transport system. The stoichiometric ratio between Ca influx and Ca-dependent Na efflux was highly variable under different experimental conditions. Because the Ca fluxes may reflect a combination of ATP-dependent, outward transport and Na-linked passive movements, the true stoichiometry of an exchanger may not be ascertainable in the absence of a specific Ca pump inhibitor. The meaning of these observations for Ca-dependent volume regulation by dog red blood cells was discused.