Potential dependence of sodium fluxes across the gills of marine teleosts

Abstract

1. The flux ratio for movement of sodium ions across the gill epithelium of marine teleosts andArtemia salina departs markedly from that predicted for free diffusional fluxes by the Ussing flux ratio equation, yet variation of the sodium efflux with changes in the external solution is often close to that predicted for a free diffusional efflux through a membrane with a uniform potential gradient. 2. Description of the efflux by an equation appropriate for passive diffusion is inconsistent with the deduction that most if not all the branchial fluxes of sodium and chloride pass through the mechanisms of active transport. 3. An examination of active transport of ions treated as a reversible chemical reaction leads to a flux ratio equation which includes the energy used in causing active transport, and which is consistent with thermodynamic descriptions of active transport, providing isotope interactions are not significant. 4. This energy may be readily evaluated from the measured gill potential, flux ratio, and sodium concentrations in blood plasma and sea water. 5. By making a reasonable assumption, equations for the potential dependence of the unidirectional fluxes are derived. 6. The predicted potential dependence of the efflux is similar to that derived from the uniform potential gradient assumption for free diffusion, and as good a fit to the experimental data. 7. The validity of the assumptions made is discussed, with particular reference to imperfect coupling between ion transport and the driving reaction.