Factors affecting the relative magnitudes of the sodium: potassium and sodium: sodium exchanges catalysed by the sodium pump

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Abstract
The effects of external K on Na : K exchange and Na : Na exchange in human red cells were estimated from measurements of ouabain-sensitive K influx and ouabain-sensitive Na influx in media containing different concentrations of K. As the external K concentration is increased from zero to 5 mM, Na : Na exchange [long dash]as judged by ouabain-sensitive Na influx[long dash]is progressively suppressed, and Na : K exchange[long dash]as judged by ouabain-sensitive K influx[long dash]is progressively increased. Both exchanges are half-maximal between 1 and 2 mM-K, and at 5 mM-K Na : Na exchange becomes very small as Na : K exchange approaches a maximum. Experiments were carried out, mainly on resealed ghosts, to determine what factors affect the magnitude of the Na : Na exchange in K-free solutions. Na : Na exchange does not occur in the absence of ATP. Ghosts containing high concentrations of Na, no K and high concentrations of ATP show no ouabain-sensitive loss of Na into K-free solutions. The ability to carry out Na : Na exchange can be restored by replacing most of the internal Na with K or by preparing the cells so that they contain much more orthophosphate (Pi) than ATP. Ghosts containing Na in low concentration, K in high concentration and with a low [ATP]/([ADP].[Pj]) ratio show a greater ouabain-sensitive loss of Na into K-free media than into media containing K; i.e. external K reduces ouabain-sensitive Na efflux. The effect of Pi is not the result of competitive inhibition of the transport ATPase since Pj at the concentrations used does not inhibit ATPase activity in fragmented ghosts.