Sodium efflux from voltage clamped squid giant axons.

Abstract

The efflux of radioactive Na was measured from squid [Loligo pealii] axons during simultaneous voltage clamp experiments, to determine the Na efflux associated with a measured voltage clamp current. The extra Na efflux associated with voltage clamp pulses increased linearly with the magnitude of the depolarization .apprx. 40 mV. A 100 mV pulse of sufficient duration to produce all of the Na current increased the rate constant of efflux by .apprx. 10-6. Application of 100 nM tetrodotoxin eliminated the Na current and the extra efflux of radioactive Na. Cooling the axon increased the extra efflux/voltage clamp pulse slightly with a Q10 of 1/1.1. On the same axons cooling increased the integral of the Na current with a Q10 of 1/1.4. Replacing external sodium with Tris, dextrose or Mg-mannitol reduced the extra Na efflux by .apprx. 50%. The inward Na current was replaced with an outward current as expected. Replacing external Na with Li reduced the extra efflux by .apprx. 50% but the Li currents were slightly larger than those in Na. The effect of replacing external Na was not voltage dependent. Cooling reduced the effect so that there was less reduction of efflux on switching to Tris ASW [artificial sea water] in the cold than in the warm. The extra Na efflux into Na-free ASW was approximately the same as the integral of the Na current. Adding external Na produced a deviation from the independence principle such that there was more Na exchange than predicted. Such a deviation from prediction was noted by Hodgkin and Huxley (1952). Using the equations of Hodgkin and Huxley (1952) modified to include the deviation from independence and its temperature dependence, the temperature dependence of the Na efflux associated with action potentials could be predicted; much better agreement was obtained than was possible without these phenomena. This deviation from independence in the Na fluxes was the type expected from some kind of mixing and binding of Na within the membrane phase.