Effects of Intracellular ADP and P_i on the Sodium Pump of Squid Giant Axon

Abstract

THE sodium pumps of human erythrocytes and of the squid giant axon can switch from an Na⁺ : K⁺ exchanging mode of operation to an Na⁺ : Na⁺ exchanging one^1–3. This remarkable property puts severe constraints on any proposed model for the sodium pump⁴. In red blood cells, the mode of operation of the pump is said to depend primarily, though not exclusively, on the intracellular ATP/P_i ratio¹, whereas in squid axon, the behaviour of the pump seems to be governed by the ATP/ADP ratio alone². No experiments have been carried out, however, specifically to test the role of either ADP or P_i in this respect, and it has indeed been envisaged³ that ADP and P_i are equally effective in switching the sodium pump of squid axon from an Na⁺ : K⁺ exchanging mechanism to an Na⁺ : Na⁺ exchanging one. I wish to describe some experiments specifically designed to differentiate the effects of intracellular ADP and P_i on Na⁺ efflux from the squid axon.