Properties and Expression of Na+/K+-ATPase alpha -Subunit Isoforms in the Brain of the Swamp Eel, Monopterus albus, Which Has Unusually High Brain Ammonia Tolerance: e84298

The swamp eel, Monopterus albus, can survive in high concentrations of ammonia (>75 mmol l-1) and accumulate ammonia to high concentrations in its brain (4.5 mu mol g-1). Na+/K+-ATPase (Nka) is an essential transporter in brain cells, and since NH4+ can substitute for K+ to activate Nka, we hypothesized that the brain of M. albus expressed multiple forms of Nka alpha -subunits, some of which might have high K+ specificity. Thus, this study aimed to clone and sequence the nka alpha -subunits from the brain of M. albus, and to determine the effects of ammonia exposure on their mRNA expression and overall protein abundance. The effectiveness of NH4+ to activate brain Nka from M. albus and Mus musculus was also examined by comparing their Na+/K+-ATPase and Na+/NH4+-ATPase activities over a range of K+/NH4+ concentrations. The full length cDNA coding sequences of three nka alpha (nka alpha 1, nka alpha 3a and nka alpha 3b) were identified in the brain of M. albus, but nka alpha 2 expression was undetectable. Exposure to 50 mmol l-1 NH4Cl for 1 day or 6 days resulted in significant decreases in the mRNA expression of nka alpha 1, nka alpha 3a and nka alpha 3b. The overall Nka protein abundance also decreased significantly after 6 days of ammonia exposure. For M. albus, brain Na+/NH4+-ATPase activities were significantly lower than the Na+/K+-ATPase activities assayed at various NH4+/K+ concentrations. Furthermore, the effectiveness of NH4+ to activate Nka from the brain of M. albus was significantly lower than that from the brain of M. musculus, which is ammonia-sensitive. Hence, the (1) lack of nka alpha 2 expression, (2) high K+ specificity of K+ binding sites of Nka alpha 1, Nka alpha 3a and Nka alpha 3b, and (3) down-regulation of mRNA expression of all three nka alpha isoforms and the overall Nka protein abundance in response to ammonia exposure might be some of the contributing factors to the high brain ammonia tolerance in M. albus.