Effects of Inorganic Cations on Na+Adsorption to the Gill and Body Surface of Rainbow Trout,Oncorhynchus mykiss, in Dilute Solutions

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Abstract
Using the shortest practical exposure time (15 s) to 0.2 mmol∙L−1NaCl solutions containing22Na, 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−1in 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.

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