COMPARATIVE ASPECTS OF NA+ K+ PUMP-MEDIATED UNCOUPLED NA+ EFFLUX IN RED-BLOOD-CELLS AND KIDNEY PROTEOLIPOSOMES/

Ouabain-sensitive uncoupled Na+ efflux has been studied in human, pig, and rat red cells and in vesicles containing reconstituted kidney Na/K+ pumps obtained from these same species. The red cells from the dif ferent species gave qualitatively similar results; the uncoupled Na+ e fflux was 15-30% of the Na+/K+ exchange rate, and this flux was inhibi ted at 5 mM extracellular Na+ (Na-o(+)). At higher levels of Na-o(+) t here was a monotonic increase in the Na+ efflux. As has previously bee n observed in human red cells, the uncoupled efflux from pig red cells consists of Na+ and anion cotransport, suggesting that anion cotransp ort may be a general characteristic of uncoupled Na+ efflux in red cel ls. The uncoupled Nat efflux carried out by pig and rat kidney Na+/Kpumps differs from the red cell activity in that it represents no more than 2-4% of the Na+/K+ exchange rate and that 5 mM Na-o(+) does not inhibit this efflux. Furthermore, the efflux does not appear to be dep endent on anion cotransport. Vesicles containing human kidney Na+/K+ p umps differ from vesicles derived from pig or rat kidneys in that the Na+ efflux is not inhibited or stimulated by N-a+ present on the oppos ite side; it thus appears that the Na+,K+-ATPase in these vesicles may be incapable of Na+/Na+ exchange. These results indicate that the liga nd and kinetic properties of the uncoupled Na+ efflux mode of red cell s are markedly different from kidney-derived Na+/K+ pumps reconstitute d into proteoliposomes. The basis for these differences may be inheren t in the Na+/K+ pumps themselves or represent differences between the two types of preparations studied.