Effect of external potassium on the coupled sodium: Potassium transport ratio of axons

Abstract

Resting membrane potential and the current-voltage relation were measured in crayfish giant axons bathed in various potassium solutions with and without ouabain. Ouabain caused a depolarization of the membrane at each [K] _o used but did not affect membrane resistance. The ouabain-sensitive transport current was least (3 μA/cm²) in 0 mM [K] _o and greatest (7 μA/cm²) in 16.2 and 21.6 mM [K] _o . The assumption was made and some indirect evidence presented that axons equilibrated in various potassium solutions maintain constant internal sodium and potassium concentrations for up to 3 h. On the basis of this assumption, the apparent ratio of coupled Na∶K transport was calculated. It was found to be least (−1.3/l) in 0 mM [K] _o and to approach infinity in 16.2 and 21.6 mM [K] _o . The data indicate that the apparent variability of the Na∶K exchange ratio likely represents an intrinsic property of the exchange mechanism and is less likely to be explained by a fixed-ratio coupled Na∶K transport operating in parallel with electro-neutral Na∶Na or K∶K exchange.