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.