Effect of dietary salt load on transepithelial Na+ exchange in freshwater rainbow trout (Oncorhynchus mykiss)

Dietary Na+ loads (0.5­70 mmol kg-1 fish) were almost completely absorbed within 7 h, and branchial Na+ excretion commenced within 1 h. Na+ loads of less than 1 mmol kg-1 were lost through the gills through a significant decrease in Na+ influx with unaltered Na+ efflux rate (compared with Na+ fluxes in unfed fish). At higher salt loads (>18 mmol kg-1), Na+ loss increased as a result of significantly higher Na+ efflux rates, with no further decrease in Na+ influx rate. Tissue Na+ concentrations were unchanged, apart from a significant increase in blood plasma Na+ concentration in fish loaded above 18 mmol kg-1. The results show that branchial Na+ fluxes may be rapidly adjusted in response to prevailing conditions, and possible control mechanisms are discussed.