Ammonia and Acid-Base Balance During High Ammonia Exposure in a Marine Teleost (Myoxocephalus Octodecimspinosus)

Abstract

For the first time in a marine teleost (the long-horned sculpin; Myoxocephalus octodecimspinosus), the maintenance of blood pH, PCo_Co2, [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 (T_Amm) 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 T_Amm 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.