TRANSBRANCHIAL AMMONIA GRADIENTS AND ACID-BASE RESPONSES TO HIGH EXTERNAL AMMONIA CONCENTRATION IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) ACCLIMATED TO DIFFERENT SALINITIES

Transbranchial ammonia gradients and blood acid-base status have been examined in rainbow trout acclimated to fresh water (FW), 33% sea water (33%SW) and sea water(SW) and exposed to 1.0 mmoll-1 total ammonia (T(Amm)) at pH 7.9 for 24 h. At all three salinities trout maintained large negative (inwardly directed) NH3 and NH4+ gradients throughout the exposure, presumably by active excretion of NH4+ to counteract the passive inward diffusion of ammonia. Analysis of blood non-respiratory acid-base status (DELTA-H(m)+ revealed an acid load in FW trout and a base load in SW trout following 24 h of exposure. This indicates that active NH4+/H+ exchange predominates in FW whereas NH4+/Na+ is the principal exchange utilised in SW under these experimental conditions. The plasma T(Amm) load incurred during ammonia exposure increased with salinity. Compared to FW trout, plasma T(Amm) values were 34 and 73 % higher in the 33 % SW and SW trout, respectively, after 24 h. This cannot be explained by differences in the prevailing transbranchial PNH, gradient because ambient P(NH3) was substantially lower at the higher salinities (due to higher pK' and solubility values). We interpret the difference between FW and SW trout as an increased permeability to NH4+ in fish acclimated to the higher-salinity environments. Transbranchial diffusion of NH4+ is, therefore, probably more important as a route for ammonia excretion in SW than in FW trout, especially considering the favourable transepithelial potentials normally found in SW teleosts. In addition, increased NH4+ permeability implies that the toxicity of ammonia will be greater in seawater than in freshwater teleosts and should not simply be measured as a function of the unionised ammonia concentration when considering seawater-adapted species.