EFFECTS OF INORGANIC CATIONS ON NA+ ADSORPTION TO THE GILL AND BODY-SURFACE OF RAINBOW-TROUT, ONCORHYNCHUS-MYKISS, IN DILUTE-SOLUTIONS

Using the shortest practical exposure time (15 s) to 0.2 mmol.L-1 NaCl solutions containing Na-22, over 80% of the Na+ taken up by rainbow trout, Oncorhynchus mykiss, was located on the gill or body surface (adsorption). Influx to the blood (absorption) was less than 20% of the total taken up, even though Na+ entry into the blood was detected in less than 30 s. Sodium adsorption to the fish surface was strongly reduced by micromolar concentrations of Ca2+ or by acid in the range pH 7-5. At higher NaCl concentrations (2 mmol.L-1 in the medium, Ca2+ effects on adsorption were greatly attenuated. Aluminum had a negligible effect on adsorption at pH 4. It is concluded that adsorption has an important role in presenting Na+ to the apical surface of gill ion transporting cells, since conditions which inhibit the branchial influx of Na+ also reduce Na+ adsorption. The results suggest that adsorption is controlled by ion exchange events at gill surface ligands, and by the effects of other cations on the mobility of Na+ in the bulk water. Adsorption phenomena are likely to influence ionic regulation during normal fluctuations in environmental conditions.