Mechanisms of and defense against acute ammonia toxicity in the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis

The objective of this study was to elucidate the mechanisms of acute ammonia toxicity in the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis, and to examine how this turtle defended against a sublethal dose of NH4Cl injected into its peritoneal cavity. The ammonia and glutamine contents in the brains of turtles that succumbed within 3 h to an intraperitoneal injection with a lethal dose (12.5 μmol g−1 turtle) of NH4Cl were 21 and 4.4 μmol g−1, respectively. Since the brain glutamine content increased to 8 μmol g−1 at hour 6 and recovered thereafter in turtles injected with a sub-lethal dose of NH4Cl (7.5 μmol g−1 turtle), it can be concluded that increased glutamine synthesis and accumulation was not the major cause of acute ammonia toxicity in P. sinensis. Indeed, the administration of l-methionine S-sulfoximine (MSO; 82 μg g−1 turtle), a glutamine synthetase (GS) inhibitor, prior to the injection of a lethal dose of NH4Cl had no significant effect on the mortality rate. Although the prior administration of MSO led to an extension of the time to death, it was apparently a result of its effects on glutamate dehydrogenase and glutamate formation, instead of glutamine synthesis and accumulation, in the brain. By contrast, a prior injection with MK801 (1.6 μg g−1 turtle), a NMDA receptor antagonist, reduced the 24 h mortality of turtles injected with a lethal dose of NH4Cl by 50%. Thus, acute ammonia toxicity in P. sinensis was probably a result of glutamate dysfunction and the activation of NMDA receptors. NMDA receptor activation could also be exacerbated through membrane depolarization caused by the extraordinarily high level of ammonia (21 μmol g−1 brain) in the brain of turtles that succumbed to a lethal dose of NH4Cl. One hour after the injection with a sub-lethal dose of NH4Cl, the brain of P. sinensis exhibited an extraordinarily high tolerance of ammonia (16 μmol g−1 brain). The transient nature of ammonia accumulation indicates that P. sinensis could ameliorate ammonia toxicity through the suppression of endogenous ammonia production and/or the excretion of exogenous ammonia. Despite being ureogenic and ureotelic, only a small fraction of the exogenous ammonia was detoxified to urea. A major portion of ammonia was excreted unchanged, resulting in an apparent ammonotely in the experimental turtles. Since there were increases in total essential free amino acid contents in the brain, liver and muscle, it can be deduced that a suppression of amino acid catabolism had occurred, reducing the production of endogenous ammonia and hence alleviating the possibility of ammonia intoxication.