Recovery of Siberian sturgeon yearlings after an acute exposure to environmental nitrite: Changes in the plasmatic ionic balance, Na ⁺-K⁺ ATPase activity, and gill histology

We studied the recovery dynamics of physiological and tissue stress indicators after an acute nitrite intoxication episode (9.3 mM NO ₂^-N for 18 h) in Siberian sturgeon yearlings (82±2 g). Nitrite intoxication resulted in a significant accumulation of the toxicant in the skeletal musculature, gills, liver, and blood plasma, provoking a severe methaemoglobinemia and electrolyte unbalance (hyperchloremia and hyperkalaemia). Methaemoglobinemia and plasma Cl^- imbalance were corrected between 24 and 96 h postnitrite intoxication, while plasma K⁺ levels were not recovered until 20 days. The excess of plasma Cl^- might be eliminated via passive diffusion through the gill epithelium due to the large concentration gradient between plasma and water. Recovery of normal K ⁺ levels changed to a hypokalaemia between 96 h and 10 days postexposure to the toxicant. Kidney Na⁺-K⁺ ATPase activities of nitrite-exposed fish during the recovery period indicated the participation of this organ in the elimination of the excess of plasmatic K ⁺ via the urine. At the end of the recovery period, the branchial epithelium (main entrance of the toxicant into the body) still showed important morphological alterations, which did not seem to compromise osmoregulatory performance, as indicated by Na⁺-K⁺ ATPase activities. Branchial and renal Na⁺-K⁺ ATPase activities indicated that the urine was a relevant pathway to actively eliminate the toxicant from the organism. The lower accumulation of nitrite in the liver compared to the rest of studied tissues at the end of the acute exposure period, added to the higher rate of nitrite elimination during the first 6 h of recovery, confirmed its role in the elimination of the toxicant from the organism.