Glutamine synthetase in tilapia gastrointestinal tract: zonation, cDNA and induction by cortisol

Abstract

Glutamine synthetase, an enzyme generally associated with ammonia detoxication in the vertebrate brain and with hepatic nitrogen turnover in mammals, shows substantial activities in the gastrointestinal tract of teleostean fishes. Enzyme activity is highest in the central area of the stomach and reveals a distinct distribution pattern in stomach and along the intestine of tilapia (Oreochromis niloticus), rainbow trout (Oncorhynchus mykiss) and copper rockfish (Sebastes caurinus). In all three species, intestinal activity peaks in the distal region of the intestine. The brain contains the highest titre of the enzyme (46 U g(-1) in tilapia brain versus 15 U g(-1) in tilapia stomach), but because of the relative mass of the stomach, the largest glutamine synthetase pool in tilapia body appears to be localized in the stomach. Activities in white and red muscle are very modest at 0.1% of the brain. Independent of distribution, peak activities of glutamine synthetase in selected areas of tilapia stomach and intestine are significantly (two- to fourfold) increased after a 5-day treatment with an intraperitoneal cortisol deposit. Cortisol also increases glutamine synthetase activity in tilapia liver, white and red muscle, while activities in brain remain unaffected. We cloned and sequenced the predominant transcript of tilapia stomach glutamine synthetase (about 1.9 kb), encoding a 371-amino acid peptide. The open reading frame shows considerable identity with glutamine synthetase in toadfish (92% at peptide level, 87% at nucleotide level), but possesses a longer 3'-untranslated region than the toadfish. The tilapia glutamine synthetase mRNA contains a remnant of a putative mitochondrial leader sequence, but without a conserved second site for initiation of translation. We also find evidence for additional transcripts of glutamine synthetase in tilapia, suggesting multiple genes. Finally, we present evidence for similar abundance of glutamine synthetase transcripts in all regions of rockfish intestine. The physiological significance of the presence of glutamine synthetase in teleostean intestine is discussed.

Postprandial changes in plasma free amino acid levels obtained simultaneously from the hepatic portal vein and the dorsal aorta in rainbow trout (Oncorhynchus mykiss)

“…To what degree ammonia and urea might be generated in the lumen of the intestine during digestion, and pass directly into the hepatic portal blood is unclear. However, Mommsen et al discovered a high activity of glutamine synthetase in tilapia gastrointestinal tract and have suggested its role is in ammonia detoxification (Mommsen et al, 2003). Our data concur with the need for such a role, but suggest that if present in Atlantic salmon, the detoxification role is partial and hepatic metabolism in combination with gill excretion are additional components of such a protective detoxification system.…”

Dietary nitrogen and fish welfare

“…Therefore, this pathway may assume a special importance in fish exposed to exogenous ammonia, prolonged starvation and osmotic challenge. Supporting the idea that glutamine formation is linked to removal and detoxification of ammonia is the observation that ammonia exposure increases abundance of glutamine synthetase transcripts in intestinal tissue (Anderson et al 2002) and glutamine synthetase activity is increased by cortisol (Mommsen et al 2003). Augmented cortisol levels are usually associated with increased nitrogen turnover in fish and are known to increase the availability of amino acids through activation of endogenous proteolysis (Mommsen et al 1999).…”

Intestinal anion exchange in marine fish osmoregulation

“…However, it is likely that some CO 2 diffuses from the epithelial cells to the extracellular fluid, perhaps indicating that the metabolic rate may be even higher than five-to eightfold that of mass-specific whole animal rates. In addition to the osmoregulatory role of the intestine, other functions including digestion, nutrient absorption, endocrine activity and barrier functions (Mommsen et al, 2003) are conceivably energetically costly and may explain the need for high abundance of mitochondria. The hydration of the metabolic waste product, CO 2 , and the subsequent exchange of HCO 3 -for Cl -effectively exchanges a gas that exerts limited osmotic pressure with a main osmolyte, Cl -, which in turn provides the osmotic driving force for cellular water uptake.…”