Current generated by backward-running electrogenic Na pump in squid giant axons

Abstract

The sodium pump of animal cells is electrogenic^1,2, that is, it normally exports more sodium ions than it imports potassium ions³. In the squid giant axon, the resulting net outward electric current⁴has a density of a few µ A cm⁻², and contributes 1–2 mV to the resting membrane potential^5,6. The pump is driven by the free energy of hydrolysis of ATP, and in some instances^7–10 it has been possible to run the pump backwards and synthesize ATP by lowering the [ATP]/[ADP] · [P_i] ratio and steepening the transmem-brane Na⁺ and K⁺ gradients. Here we have examined the question of whether a backward-running sodium pump conserves its Na⁺/K⁺ > 1 stoichiometry. We demonstrate reversal of the sodium pump of squid giant axon, and find that backward pumping indeed produces a net inward electric current. This current is voltage sensitive. Our observations have mechanistic implications for models of the sodium pump.