The behaviour of the sodium pump in red cells in the absence of external potassium

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Abstract
When red cells loaded with 24Na are incubated in balanced glucose salt solutions, the removal of external K reduces Na efflux by about 1/3. Ouabain reduces the residual Na efflux by about 1/2. The ouabain-sensitive efflux of Na into K-free solutions is accompanied by an equal ouabain-sensitive influx of Na. If Na in the external K-free salt solution is progressively replaced with choline, both the ouabain-sensitive Na efflux and the ouabain-sensitive Na influx are reduced until, with only 5 mM-Na externally, both become very small. At sodium concentrationsThtermediate between 140 mM and 5 mM, ouabain-sensitive Na influx and ouabain-sensitive Na efflux remain equal within the limits of experimental error. The relation between the magnitude of the ouabain-sensitive Na exchange and the external Na concentration is roughly linear. As the external Na concentration is decreased beyond 5 mM towards zero, ouabain-sensitive Na efflux increases again. In the presence of 5 mM-K, the ouabain-sensitive Na efflux is scarcely affected by replacing most of the external Na with choline. In the presence of ouabain, Na efflux is unaffected by external K and is little affected by replacing external Na with choline. The results suggest that in the absence of external K the ouabain-sensitive transport mechanism catalyses a one-for-one exchange of Na ions across the cell membrane. The relation between this exchange and Ussing''s classical exchange diffusion is discussed. The exchange does not occur in the presence of external K at physiological concentrations. Calculations of Na efflux were based on the specific activities of total cell Na separate experiments confirmed that these were the same as the specific activities of Na lost to Na-free solutions. No evidence of a slowly exchanging Na fraction was found.