Responses of a Stenohaline Freshwater Teleost (catostomus Commersoni) to Hypersaline Exposure Ii. Transepithelial Flux of Sodium, Chloride and 'acidic Equivalents'

The effects of exposure to SOOmosmoll" sodium chloride on the transepithelial movement of sodium, chloride and 'acidic equivalents' was examined in the stenohaline freshwater teleost Catostomus commersoni (Lacfipede), the white sucker. The transepithelial potential (TEP) was negative in control fish acclimated to soft freshwater ([Ca] <0-lmmoll~) but positive in control fish acclimated to hardwater ([Ca] > l-OmmolF). Using permeability coefficients calculated from measured unidirectional effluxes of sodium and chloride for both groups above, the Goldman equation predicts the observed change in polarity of the TEP. During saline exposure the ability of external calcium to influence the TEP was greatly attenuated, and the TEP remained positive throughout 96 h exposure to either hard or soft saline water. As a consequence of the reduced (and reversed) chemical potential and of the prevailing TEP, the electrochemical difference for sodium was directed out from the fish while that for chloride was directed inwards. Thus, the passive movement of sodium and chloride in opposite directions could potentially account for the previously reported decrease in plasma strong ion difference (SID), and therefore the prevailing acid-base status. Unfortunately, the existence of possible exchange diffusion processes for sodium and chloride observed in salineexposed fish prevented a more detailed examination of this hypothesis. Since the change in plasma SID occurred gradually over a 96-h period, there was no measurable change in the net flux of 'acidic equivalents' which could have been associated with the active and passive transbranchial movement of sodium and chloride. The significance (or lack of it) of the Na + /H + -NH 4 + and C r / H C O 3 " OH~ exchange pumps to systemic acid-base balance is discussed.