INDUCTION OF ORNITHINE-UREA CYCLE IN A FRESH-WATER 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 accumula ted significantly in all the tissues within 7 days of treatment and th e concentration remained high throughout the 4-week period of treatmen t. The activity of enzymes of the ornithine-urea (o-u) cycle were indu ced 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 ammo nia induced the activity of the enzymes of the o-u cycle for its metab olic conversion to urea. This helped the freshwater fish to avoid toxa emia and to tolerate high concentrations of ammonia in the ambient med ium.