Effect of external potassium on the coupled sodium: Potassium transport ratio of axons
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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.
Key wordsPotassium Coupled Na∶K transport Axon Procambarus clarkii
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