Open‐circuit sodium and chloride fluxes across isolated opercular epithelia from the teleost Fundulus heteroclitus.

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Abstract
1. The Na+ and Cl‐ fluxes across opercular epithelia from sea water‐adapted Fundulus heteroclitus were measured in vitro under open‐circuit conditions while bathed on the mucosa with sea water and the serosa with Ringer solution. 2. The mean predicted Na+ flux ratio was 0.94 +/‐ 0.08 and the observed ratio was 1.14 +/‐ 0.12 (n = 15; mean +/‐ S.E. of mean). The difference in these means was not significant (P greater than 0.20). The mean predicted Cl‐ flux ratio was 11.4 +/‐ 0.9 and the mean observed ratio was 1.38 +/‐ 0.27 (n = 10). The difference in these means was significant (P less than 0.001). 3. Ouabain, at 10(‐6) M in the serosal solution, produced a significant (P less than 0.01) reduction in the Na+ efflux while having no significant (P greater than 0.40) effect on the Na+ influx. The agreement between the predicted (1.70 +/‐ 0.14) and observed (1.72 +/‐ 0.18) Na+ flux ratios after ouabain treatment suggested that this effect could be completely attributed to the depolarization of the epithelium secondary to ATPase inhibition. 4. beta‐adrenergic activation by isoprenaline stimulated the Cl‐ efflux 24.2% and alpha‐adrenergic activation by noradrenaline inhibited the Cl‐ efflux 66.5%. These changes occurred oppositely to those predicted by the changes in the electrical gradient produced by these agents, while the changes in the Cl‐ influxes corresponded to the electrical changes. Short‐circuit experiments confirmed these effects on the Cl‐ efflux and the lack of effects on the Cl‐ influx. 5. The results suggested that Na+ was near theromodynamic equilibrium and that the unidirectional fluxes were passive. The effects of alpha‐ and beta‐adrenergic activation suggested that the active Cl‐ secretion may be antagonistically regulated by catecholamines.