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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 µmol g −1 ). Na + /K + -ATPase (Nka) is an essential transporter in brain cells, and since NH 4   +  can substitute for K +  to activate Nka, we hypothesized that the brain of  M. albus  expressed multiple forms of Nka α-subunits, some of which might have high K +  specificity. Thus, this study aimed to clone and sequence the  nka α -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 NH 4   +  to activate brain Nka from  M. albus  and  Mus musculus  was also examined by comparing their Na + /K + -ATPase and Na + /NH 4   + -ATPase activities over a range of K + /NH 4   +  concentrations. The full length cDNA coding sequences of three  nkaα  ( nkaα1 ,  nkaα3a  and  nkaα3b ) were identified in the brain of  M. albus , but  nkaα2  expression was undetectable. Exposure to 50 mmol l −1  NH 4 Cl for 1 day or 6 days resulted in significant decreases in the mRNA expression of  nkaα1 ,  nkaα3a  and  nkaα3b . The overall Nka protein abundance also decreased significantly after 6 days of ammonia exposure. For  M. albus , brain Na + /NH 4   + -ATPase activities were significantly lower than the Na + /K + -ATPase activities assayed at various NH 4   + /K +  concentrations. Furthermore, the effectiveness of NH 4   +  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  nkaα 2 expression, (2) high K +  specificity of K +  binding sites of Nkaα1, Nkaα3a and Nkaα3b, and (3) down-regulation of mRNA expression of all three  nkaα  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 .

plos one

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