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

1. Resting membrane potential and the current-voltage relation were measured in crayfish giant axons bathed in various potassium solutions with and without ouabain. 2. Ouabain caused a depolarization of the membrane at each [K]o used but did not affect membrane resistance. 3. The ouabain-sensitive transport current was least (3 microamperemeter/cm2) in 0 mM [K]o and greatest (7 microamperemeter/cm2) in 16.2 and 21.6 mM [K]o. 4. The assumption was made an some indirect evidence presented that axons equilibrated in various potassium solutions maintain constant internal sodium and potassium concentrations for up to 3 h. 5. On the basis of this assumption, the apparent ratio of coupled Na : K transport was calculated. It was found to be least (-1.3/1) in 0 mM [K]o and to approach infinity in 16.2 and 21.6 mM [K]o. 6. 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.