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The stinging catfish,  Heteropneustes fossilis , can tolerate high concentrations of environmental ammonia. Previously, it was regarded as ureogenic, having a functional ornithine-urea cycle (OUC) that could be up-regulated during ammonia-loading. However, contradictory results indicated that increased urea synthesis and switching to ureotelism could not explain its high ammonia tolerance. Hence, we re-examined the effects of exposure to 30 mmol l –1  NH 4 Cl on its ammonia and urea excretion rates, and its tissue ammonia and urea concentrations. Our results confirmed that  H. fossilis  did not increase urea excretion or accumulation during 6 days of ammonia exposure, and lacked detectable carbamoyl phosphate synthetase I or III activity in its liver. However, we discovered that it could actively excrete ammonia during exposure to 8 mmol l –1  NH 4 Cl. As active ammonia excretion is known to involve Na + /K + -ATPase (Nka) indirectly in several ammonia-tolerant fishes, we also cloned various  nka α -subunit  isoforms from the gills of  H. fossilis , and determined the effects of ammonia exposure on their branchial transcripts levels and protein abundances. Results obtained revealed the presence of five  nka α -subunit  isoforms, with  nka α 1b  having the highest transcript level. Exposure to 30 mmol l –1  NH 4 Cl led to significant increases in the transcript levels of  nka α 1b  (on day 6) and  nka α 1c1  (on day 1 and 3) as compared with the control. In addition, the protein abundances of Nkaα1c1, Nkaα1c2, and total NKAα increased significantly on day 6. Therefore, the high environmental ammonia tolerance of  H. fossilis  is attributable partly to its ability to actively excrete ammonia with the aid of Nka.

frontiers in physiology

The Non-ureogenic Stinging Catfish, Heteropneustes fossilis, Actively Excretes Ammonia With the Help of Na+/K+-ATPase When Exposed to Environmental Ammonia