Mechanisms of and defences against ammonia toxicity in two air-breathing fishes with high ammonia tolerance

Abstract

This study examined the effects of MSO or MK801 against acute ammonia toxicity in two air-breathing fishes, Clarias gariepinus and Monopterus albus. MSO reduced mortality rate and prolonged the time to death in both fishes injected with a lethal ammonia dose by affecting GDH and ALT activities from the brain and suppressing the rate of ammonia build up therein. MK801 injection before a lethal dose of ammonia had partial protective effect on M. albus but not C. gariepinus, suggesting the activation of NMDA receptors in ammonia toxicity in M. albus. C. gariepinus can actively excrete ammonia, mainly through the kidneys to defend against ammonia toxicity. M. albus has high tolerance of ammonia in the brain, partly due to the high potassium specificity of sodium-potassium ATPase as compared to the mouse. There are two isoforms of sodium-potassium ATPase in the brain of M. albus and their regulations and functions might differ.