Branchial mechanisms of ion and acid–base regulation in the freshwater rainbow trout, Salmo gairdneri

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Abstract
Blood acid-base balance and branchial fluxes of Na+, Cl-, and acidic equivalents were examined in rainbow trout (Salmo gaidneri) in response to variations in external [NaCl] and following experimental acid or base loads (intravascular infusion of ammonium sulphate, lactic acid, or sodium bicarbonate). NaCl influx, and ammonia excretion covaried with external [NaCl]. Large fluxes of acidic equivalents across the gills were produced by infusion of both ammonium sulphate and sodium bicarbonate, but both treatments had little effect upon Na+ and Cl- uptake. We interpret this result as indicating that apical Na+ -NH4+ and Cl-HCO3- exchange played little role in the branchial clearance of acidic equivalents. Instead, the results are consistent with the notion that acidic equivalents were excreted via diffusion through paracellular channels. A model is presented which suggests that the paracellular channels are the normal route for ionic efflux across the gills and that excretion of acidic equivalents results from modulation of the permselectivity of this pathway.