Acid-Base Balance and Ammonia Loading in the Siberian Sturgeon Acipenser baerii, Exposed to High Concentrations of Ammonia

The ammonia concentration was studied in the blood of the Siberian sturgeon, Acipenser baerii, with and without high concentration in water. In control conditions, the ammonia excretion was mainly due to NH3 diffusion according to the partial pressure gradient. So the distribution of the total ammonia followed the pH gradient between water and blood. After exposure to high concentration of ammonia, the blood concentration increased quickly during the early hours and a metabolic alkalosis appeared. PNH3 was stabilized after less than 24-h exposure, while the total ammonia concentration increased continuously as blood pH returned to initial values. If we take into account the real pH in the branchial boundary layer and not in the water of the circuit to calculate PNH3, we could conclude that the evolution of the blood ammonia concentration could be explained by NH3 diffusion along its partial pressure and that the ammonia exchanges by simple diffusion were dominant. However, whatever the ammonia concentration in water, the fish quickly reached a PNH3 equilibrium with the water and seemed incapable of preventing the invasion of the blood by ammonia. That also explained the initial metabolic alkalosis. But these results did not allow to attribute the ammonia toxicity to NH3.