The influence of cytoplasmic sodium concentration on the stoichiometry of the sodium pump.

Using inside-out vesicles of human red cell membranes, the effects of cytoplasmic Na+ in the range 0-5 mM on ATP-dependent 22Na+ influx (normal efflux) and 86Rb+ efflux (normal influx) were tested. The sodium pump stoichiometry, i.e. the ratio of net 22Na+ influx:86Rb+ efflux was reduced markedly when the cytoplasmic Na+ was reduced to less than 1 mM. Reduction in cytoplasmic Na+ concentration was associated also with a decreased sensitivity of the pump to effects of extracellular Rb+. Thus, extracellular (intravesicular) Rb+ stimulation observed at high ATP concentration and inhibition observed at low ATP concentration were not observed when the cytoplasmic (extravesicular) Na+ concentration was reduced to less than or equal to 0.2 mM. It is suggested that at low cytoplasmic Na+, the pump can operate with less than maximal sites filled with Na+ ions. Under this condition, it is likely that an enzymic step associated with either the ion translocation step or the enzyme's conformational transition becomes rate-limiting.