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 mmoi r 1 total ammonia (TAmm) at pH7.9 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 NF£4+ to counteract the passive inward diffusion of ammonia. Analysis of blood non-respiratory acid-base status (AH^) revealed an acid load in FW trout and a base load in SW trout following 24 h of exposure. This indicates that active NH 4 + /H + exchange predominates in FW whereas NH4 +/Na+ is the principal exchange utilised in SW under these experimental conditions. The plasma TAmm load incurred during ammonia exposure increased with salinity. Compared to FW trout, plasma TAmm 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 PNH3 gradient because ambient PWH3 was substantially lower at the higher salinities (due to higher pK ' and solubilit