The influence of chloride on the ouabain-sensitive membrane potential and conductance of crayfish giant axons

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Abstract
Resting potential and current-voltage relation were measured in crayfish giant axons bathed in chloride-free and sodium-free solutions with and without ouabain. Chloride-free solution caused a transient depolarization but did not alter the steady-state membrane potential. Utilizing isethionate as an anion substitute, the membrane resistance increased 12.5%. In the absence of extracellular chloride, ouabain (0.5–1 mM) depolarized the axon 6–7 mV. The shape of the current-voltage relation did not change but the curve was shifted along the current axis. These results indicate that ouabain inhibits a steady-state hyperpolarizing electrogenic pump current of approximately 3 μA/cm2. Extracellular sodium removal from axons equilibrated in chloride-free solutions transiently hyperpolarized the membrane 6–7 mV without a change in membrane resistance. The transient hyperpolarization was ouabain and temperature sensitive. The steadystate potential reached in sodium-free and chloride-free solution was not ouabain sensitive. Temperature sensitivity of the steady-state membrane potential was greatly reduced. The transient hyperpolarization produced by extracellular sodium removal was metabolically dirven and may present the expression of a sodium efflux transport current of 7.0–7.5 μA/cm2. Using electrophysiologically measured parameters, sodium and potassium conductance, influx and efflux currents and the coupling ratio for sodium/potassium transport are calculated from a modification of the conductance equation. The sodium/potassium transport coupling ratio for steady-state conditions was estimated at 5:3 (1.67:1).