EFFECT OF DIETARY SALT LOAD ON TRANSEPITHELIAL NA-WATER RAINBOW-TROUT(ONCORHYNCHUS-MYKISS)( EXCHANGE IN FRESH)

Dietary Na+ loads (0.5-70 mmol kg(-1) fish) were almost completely abs orbed within 7h, 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 (c ompared with Na+ fluxes in unfed fish). At higher salt loads (>18 mmol kg(-1)), Na+ loss increased as a result of significantly higher Na+ e fflux rates, with no further decrease in Na+ influx rate. Tissue Na+ c oncentrations were unchanged, apart from a significant increase in blo od plasma Na+ concentration in fish loaded above 18 mmol kg(-1). The r esults show that branchial Na+ fluxes may be rapidly adjusted in respo nse to prevailing conditions, and possible control mechanisms are disc ussed.