Sidedness of the ATP-Na+-K+ interactions with the Na+ pump in squid axons

Using dialysed squid axons we have been able to control internal and external ionic compositions under conditions in which most of the Na⁺ efflux goes through the Na⁺ pump. We found that (i) internal K⁺ had a strong inhibitory effect on Na⁺ efflux; this effect was antagonized by ATP, with low affinity, and by internal Na⁺, (ii) a reduction in ATP levels from 3 mM to 50 μM greatly increased the apparent affinity for external K⁺, but reduced its effectiveness compared with other monovalent cations, as an activator of Na⁺ efflux, and (iii) the relative effectiveness of different K⁺ congeners as external activator of the Na⁺ efflux, though affected by the ATP concentration, was not affected by the Na⁺/⁺ ratio inside the cells. These results are consistent with the idea that the same conformation of the (Na⁺ + K⁺)-ATPase can be reached by interaction with external K⁺ after phosphorylation and with internal K⁺ before rephosphorylation. They also stress a nonphosphorylating regulatory role of ATP.