Effect of alkalinity (pH 10) on ureogenesis in the air-breathing walking catfish, Clarias batrachus

Exposure of fish to alkaline conditions inhibits the rate of ammonia excretion, leading to ammonia accumulation and toxicity. The purpose of this study was to determine the role of ureogenesis via the urea cycle, to avoid the accumulation of ammonia to a toxic level during chronic exposure to alkaline conditions, for the air-breathing walking catfish, Clarias batrachus, where a full complement of urea cycle enzyme activity has been documented. The walking catfish can survive in water with a pH up to 10. At a pH of 10 the ammonia excretion rate by the walking catfish decreased by approximately 75% within 6 h. Although there was a gradual improvement of ammonia excretion rate by the alkaline-exposed fish, the rate remained 50% lower, even after 7 days. This decrease of ammonia excretion was accompanied by a significant accumulation of ammonia in plasma and body tissues (except in the brain). Urea-N excretion for alkaline-exposed fish increased 2.5-fold within the first day, which was maintained until day 3 and was then followed by a slight decrease to maintain a 2-fold increase in the urea-N excretion rate, even after 7 days. There was also a higher accumulation of urea in plasma and other body tissues (liver, kidney, muscle and brain). The activity of glutamine synthetase and three enzymes operating in the urea cycle (carbamyl phosphate synthetase, argininosuccinate synthetase, argininosuccinate lyase) increased significantly in hepatic and extra-hepatic tissue, such as the kidney and muscle in C. batrachus, during exposure to alkaline water. A significant increase in plasma lactate concentration noticed during alkaline exposure possibly helped in the maintenance of the acid-base balance. It is apparent that the stimulation of ureogenesis via the induced urea cycle is one of the major physiological strategies adopted by the walking catfish (C. batrachus) during chronic exposure to alkaline water, to avoid the in vivo accumulation of ammonia to a toxic level in body tissues and for the maintenance of pH homeostasis.