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

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 nka?-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?-subunit isoforms, with nka?1b having the highest transcript level. Exposure to 30 mmol l-1 NH4Cl led to significant increases in the transcript levels of nka?1b (on day 6) and nka?1c1 (on day 1 and 3) as compared with the control. In addition, the protein abundances of Nka?1c1, Nka?1c2, and total NKA? 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.

SUBMITTER: Chew SF

PROVIDER: S-EPMC6987325 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature

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

Chew Shit F SF Tan Stephanie Z L SZL Ip Sabrina C Y SCY Pang Caryn Z CZ Hiong Kum C KC Ip Yuen K YK

Frontiers in physiology 20200122

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 ammoni ...[more]

PMID: 32038295

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Project description:The air-breathing singhi catfish (Heteropneustes fossilis) is frequently being challenged by different environmental insults such as hyper-ammonia, dehydration and osmotic stresses in their natural habitats throughout the year. The present study investigated the effect of hyperosmotic stress, due to exposure to hypertonic environment (300 mM mannitol) for 14 days, on gluconeogenesis in this catfish. In situ exposure to hypertonic environment led to significant stimulation of gluconeogenic fluxes from the perfused liver after 7 days of exposure, followed by further increase after 14 days in presence of three different potential gluconeogenic substrates (lactate, pyruvate and glutamate). Environmental hypertonicity also caused a significant increase of activities of key gluconeogenic enzymes, namely phosphoenolpyruvate carboxykinase, fructose 1, 6-bisphosphatase and glucose 6-phosphatase by about 2-6 fold in liver, and 3-6 fold in kidney tissues. This was accompanied by more abundance of enzyme proteins by about 1.8-3.7 fold and mRNAs by about 2.2-5.2 fold in both the tissues with a maximum increase after 14 days of exposure. Hence, the increase in activities of key gluconeogenic enzymes under hypertonic stress appeared to be as a result of transcriptional regulation of genes. Immunocytochemical analysis further confirmed the tissue specific localized expression of these enzymes in both the tissues with the possibility of expressing more in the same localized places. The induction of gluconeogenesis during exposure to environmental hypertonicity possibly occurs as a consequence of changes in hydration status/cell volume of different cell types. Thus, these adaptational strategies related to gluconeogenesis that are observed in this catfish under hypertonic stress probably help in maintaining glucose homeostasis and also for a proper energy supply to support metabolic demands mainly for ion transport and other altered metabolic processes under various environmental hypertonic stress-related insults.

Project description:The air-breathing singhi catfish (Heteropneustes fossilis) is frequently being challenged by bacterial contaminants, and different environmental insults like osmotic, hyper-ammonia, dehydration and oxidative stresses in its natural habitats throughout the year. The main objectives of the present investigation were to determine (a) the possible induction of inducible nitric oxide synthase (iNOS) gene with enhanced production of nitric oxide (NO) by intra-peritoneal injection of lipopolysaccharide (LPS) (a bacterial endotoxin), and (b) to determine the effects of hepatic cell volume changes due to anisotonicity or by infusion of certain metabolites, stress hormones and by induction of oxidative stress on production of NO from the iNOS-induced perfused liver of singhi catfish. Intra-peritoneal injection of LPS led to induction of iNOS gene and localized tissue specific expression of iNOS enzyme with more production and accumulation of NO in different tissues of singhi catfish. Further, changes of hydration status/cell volume, caused either by anisotonicity or by infusion of certain metabolites such as glutamine plus glycine and adenosine, affected the NO production from the perfused liver of iNOS-induced singhi catfish. In general, increase of hydration status/cell swelling due to hypotonicity caused decrease, and decrease of hydration status/cell shrinkage due to hypertonicity caused increase of NO efflux from the perfused liver, thus suggesting that changes in hydration status/cell volume of hepatic cells serve as a potent modulator for regulating the NO production. Significant increase of NO efflux from the perfused liver was also observed while infusing the liver with stress hormones like epinephrine and norepinephrine, accompanied with decrease of hydration status/cell volume of hepatic cells. Further, oxidative stress, caused due to infusion of t-butyl hydroperoxide and hydrogen peroxide separately, in the perfused liver of singhi catfish, resulted in significant increase of NO efflux accompanied with decrease of hydration status/cell volume of hepatic cells. However, the reasons for these cell volume-sensitive changes of NO efflux from the liver of singhi catfish are not fully understood with the available data. Nonetheless, enhanced or decreased production of NO from the perfused liver under osmotic stress, in presence of stress hormones and oxidative stress reflected its potential role in cellular homeostasis and also for better adaptations under environmental challenges. This is the first report of osmosensitive and oxidative stress-induced changes of NO production and efflux from the liver of any teleosts. Further, the level of expression of iNOS in this singhi catfish could also serve as an important indicator to determine the pathological status of the external environment.

Dataset Information

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

Publications

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

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