Red cell sodium fluxes catalysed by the sodium pump in the absence of K⁺ and ADP

Abstract

In the absence of extracellular Na⁺ or K⁺, the sodium pump catalyses an ouabain-sensitive ‘uncoupled’ Na⁺ efflux^1–4. With red cell ghosts Glynn and Karlish⁵ showed that this Na⁺ efflux is accompanied by ATP hydrolysis and that extracellular sodium ions, at low concentrations, inhibit this efflux as well as the associated ATP hydrolysis. At higher concentrations, extracellular sodium ions restore the hydrolysis of ATP^3,6 but it is not known whether there is an associated increase in Na⁺ efflux and, perhaps, an influx. To answer this question we have used inside-out red cell membrane vesicles which are specially suitable for controlling the composition of the medium at the two membrane surfaces while measuring ²²Na⁺ fluxes in both directions. We report here that the sodium pump can operate in a mode in which influx and efflux of sodium are associated with ATP hydrolysis. This mode is different from the Na–Na exchange described by Garrahan and Glynn⁷, and Glynn and Hoffman⁸, which requires ADP as well as ATP⁹ and is probably associated with ADP–ATP exchange rather than ATP hydrolysis^10,11.