Abstract
A simple method is described of measuring the potentialin vivo between the blood plasma and external medium of a fish. The potential in the euryhaline flounderPlatichthys flesus averages +19 mV in sea water and −78 mV immediately after transfer to fresh water. The potential is largely dependent on the external concentration of sodium and behaves as a diffusion potential. The relative permeabilities of the gill of the sea water adapted flounder to Na, K. and Cl ions are in the ratio 1∶2.5∶0.03. The permeability to the divalent ions, Ca, Mg and SO4 is very low. The changes of potential account for the apparent “sodium exchange diffusion effect” observed in the sodium fluxes following changes in the external medium and the slight dependence of sodium efflux on the external concentration of potassium in sea water.
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