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

The sodium pump of animal cells is electrogenic 1,2 , that is, it normally exports more sodium ions than it imports potassium ions 3 . In the squid giant axon, the resulting net outward electric current 4 has 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.