Abstract
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 NH4Cl 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 NH4Cl. As active ammonia excretion is known to involve Na+/K+-ATPase (Nka) indirectly in several ammonia-tolerant fishes, we also cloned various nkaα-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 nkaα-subunit isoforms, with nkaα1b having the highest transcript level. Exposure to 30 mmol l–1 NH4Cl led to significant increases in the transcript levels of nkaα1b (on day 6) and nkaα1c1 (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.
Highlights
In fishes, proteins ingested would be first catabolized to amino acids and further broken down through deamination or transamination with the eventual production of ammonia
The daily urea excretion rate of H. fossilis kept in fresh water was relatively low, ranging from 1.24 to 1.76 μmol urea−N g−1, and it remained statistically unchanged during 6 days of exposure to 30 mmol l−1 NH4Cl (Figure 1)
P. schlosseri does not depend on increased urea synthesis to detoxify ammonia when it is in a terrestrial environment (Ip et al, 1993, 2001b), high ammonia environment (Peng et al, 1998; Randall et al, 1999), or alkaline medium (Chew et al, 2003a)
Summary
Proteins ingested would be first catabolized to amino acids and further broken down through deamination or transamination with the eventual production of ammonia. The production of ammonia can occur in many tissues (Walton and Cowey, 1977), the liver is the main organ where it is produced (Pequin and Serfaty, 1963). Acidic environmental conditions augment ammonia excretion, as NH3 diffusing across the branchial epithelium is converted into NH4+ and trapped in the external environment. An increase in the pH of the environment would impede the diffusion of NH3 into the external medium leading to the accumulation of endogenous ammonia in the body. When fishes emerge from water, there could be a reduction in the excretion of ammonia due to a lack of water to flush the gills. Ammonia concentrations in the ambient water could increase due to the decomposition of organic matters or the addition of fertilizers, and impede ammonia excretion in fishes
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