Biphasic effect of orthophosphate on the (Na, K)-pump of human red cells

Orthophosphate (P₁) can both stimulate and inhibit the (Na, K)-pump in red cells. At concentrations below 0.5 mmol/1 cells, P_i stimulated the pump, but at higher concentrations P_i was inhibitory. The stimulation was demonstrated in intact cells by preincubation with inosine (which leads to a reduction in cellular P_i concentration), and then by incubating cells in media with various P_i concentrations (which relieved the inhibition caused by inosine). In inosine-treated cells there was an inverse relationship between the hematocrit during measurement of the fluxes and inhibition of the (Na, K)-pump; this was also a reflection of cellular P_i, which was lower in inosine-treated cels at low hematocrit. The stimulation of the (Na, K)-pump by P_i below 0.5 mmol/l cells was an indirect effect due to synthesis of ATP by membrane-bound glycolytic enzymes, which required the appropriate substrates (in addition to P_i). This was shown by studies on inside-out vesicles made from red cell membranes. In the absence of the other substrates, P_i was inhibitory to Na transport in the vesicles. Above 0.5 mmol/l cells P_i was inhibitory to Na transport, both in inside-out vesicles and in intact cells. The mechanism of inhibition, probably a direct effect on the (Na, K)-pump, was not determined, though product inhibition seemed likely. The dependence on P_i of abnormal modes of Na transport by the pump (uncoupled Na efflux and Na/Na exchange) at low P_i concentrations was less than the dependence of normal Na/K exchange. This was attributed to a requirement by the abnormal modes of a lower rate of synthesis of ATP or a lower ATP concentration.