For the first time in a marine teleost (the long-horned sculpin; Myoxocephalus octodecimspinosus), the maintenance of blood pH, PCoCo2, [HCO3 and the net movements of NH4+ HCO3 and H+ between the fish and the water have been studied during exposure to ammonia stress induced either by infusion (NH4Cl or NH4HCO3; 5 mmol kg−1) or by external application (NH4Cl; approx. 1 mmoll−1).

Following NH4Cl infusion, a rapid decrease in blood pH (0.36 units) and [HCO3] (2.38 mmoll−1) was observed, and within 1 h about 40% of the ammonia load had been excreted to the water. Analysis of NH4+ and HCO3 transfers revealed that the total ammonia (TAmm) efflux was due to a loss of NH3 and NH4+ in approximately equal proportions when an outwardly directed NH3 diffusion gradient was established.

Infusion of NH4HCO3 induced only small changes in plasma pH, and the rate of net HCO3 excretion was some 90% higher than that of NH4+ over 20 h. These data indicate a predominance of NH3 as the form of ammonia lost. In both infusion experiments, a presumed intracellular buffering of a majority of the ammonia load was noted.

High external TAmm induced an initial uptake of NH4+, but after 4 h of exposure ammonia efflux resumed even though NH3 diffusion gradients were negligible. Thus, in this seawater teleost, a role for the excretion of ammonia in the form of NH4+ is also likely.

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