Induction of ornithine-urea cycle in a freshwater teleost, Heteropneustes fossilis, exposed to high concentrations of ammonium chloride

An ammoniotelic freshwater teleost, Heteropneustes fossilis, tolerated ambient ammonium chloride concentration up to 75 mM. Ammonia accumulated significantly in all the tissues within 7 days of treatment and the concentration remained high throughout the 4-week period of treatment. The activity of enzymes of the ornithine-urea (o-u) cycle were induced within 7 days, and thereafter remained high in both the liver and kidney of the fish. Urea accumulated significantly in various tissues simultaneous with the induction of o-u cycle enzymes. Accumulated ammonia induced the activity of the enzymes of the o-u cycle for its metabolic conversion to urea. This helped the freshwater fish to avoid toxaemia and to tolerate high concentrations of ammonia in the ambient medium.