Unidirectional sodium and potassium fluxes through the sodium channel of squid giant axons.
Unidirectional 22Na-traced sodium influx or 42K-traced potassium efflux across the membranes of voltage-clamped squid giant axons was measured at various membrane potentials under bi-ionic conditions. Tetrodotoxin almost entirely eliminated the extra K+ efflux induced by short repetitive depolarizations in the presence of tetraethylammonium or 3,4-diaminopyridine. A method of determining the voltage dependence of the unidirectional flux through voltage-gated channels is described. This technique was used to obtain the unidirectional flux-voltage relation for the sodium channel in bi-ionic and single-ion conditions. It allows the determination of the unidirectional flux at the zero-current potential which, for influx, was found to be approximately 20% of the value measured 80 mV negative to the zero-current potential. The unidirectional flux ratio under bi-ionic conditions was also measured and the flux ratio exponent found to average 1.15 with an external sodium and an internal potassium solution. A three-barrier, two-site, multi-occupancy model previously obtained for other conditions was found to predict a similar non-unity average for the flux ratio exponent. It is also shown that some single-occupancy models can predict non-unity values for the flux ratio exponent in bi-ionic conditions.
D000818:Animals; D001369:Axons / Q000502:physiology*; D001693:Biological Transport, Active; D049832:Decapodiformes; D007473:Ion Channels / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials; D011188:Potassium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
D007473:Ion Channels; D012964:Sodium; D011188:Potassium