Gastric acid secretion and drinking in the Atlantic cod (Gadus morhua) during acidic or hyperosmotic perfusion of the intestine

A review of causal factors and control measures for bloat in farmed salmonids with a suggested mechanism for the development of the condition

A mechanism for the development of bloat in salmonids is proposed and explained in terms of the physiology of digestion and osmoregulation in fish. Understanding the causal factors for bloat enables control measures to be identified. In farmed salmonids, the chyme produced during the digestion of nutrient-rich pelleted foods that rapidly disintegrate in the stomach will be a potent activator of a negative feedback mechanism (enterogastric control), which slows stomach emptying to protect the small intestine from nutrient overload. In saline environments salmonids continuously drink sea water to replace fluid lost across the gills. Fluid loss is increased during periods of stress caused by factors such as low oxygen levels, elevated temperature or high salinity. When ingestion of nutrient-rich food results in prolonged activation of enterogastric control, slowed stomach emptying leads to decreased absorption of water, thirst and increased drinking. This further exacerbates stomach distention. The proposed mechanism for the development of bloat is supported by on-farm experience where measures to control bloat include reducing food intake, altering the composition of the diet and using appropriate strategies to reduce stress.

Characterization with agonists of the histamine receptors mediating stimulation of gastric acid secretion in the Atlantic cod, Gadus morhua

Gastric acid secretion in response to a number of agonists, active at H1 and/or H2-receptors, were measured in the gastrically and intestinally perfused cod. Histamine (D50 = 15 nmol/kg X h) was the most potent of the agonists tested. The H2-active agonists 4-methylhistamine (4-MeHi), Na-MeHi, and Na,Na-MeHi were about 25 times less potent than histamine in this in vivo model. Na,Na-MeHi was a partial agonist, while 4-MeHi and Na-MeHi elicited maximum responses similar to that of histamine. Dimaprit and impromidine, two highly selective H2-agonists, failed to stimulate acid secretion under basal conditions, and inhibited histamine-stimulated acid secretion. The H1-selective 2-MeHi and 2-pyridylethylamine (2-PEA) were essentially inactive. It is concluded that the cod gastric histamine receptor related to acid secretion is of the H2-class. It is not, however, identical to the mammalian gastric H2-receptor, and should be placed in a separate subgroup.

Effect of 5-HT on basal and stimulated secretions of acid and pepsin and on gastric volume outflow in the in vivo gastrically and intestinally perfused cod, Gadus morhua

Gastric acid and pepsin responses to 5-HT was measured, in cod, during gastric and intestinal perfusions. During basal conditions, both acid and pepsin secretions were stimulated by 0.25 mumol/kg X h of 5-HT. A higher dose, 1 mumol/kg X h inhibited acid secretion and stimulated the output of pepsin both during basal conditions and during stimulation with histamine or carbachol. Histamine and carbachol, when given alone, were powerful acid stimulators but in comparison with 5-HT poor pepsigogues. Most probably due to inhibition of gastric volume secretion, gastric outflow volume decreased during treatment with higher doses of 5-HT. However, when the intestinal perfusion was omitted and water support instead given by the intramuscular route, 5-HT induced a large increase in gastric outflow volume. Our results suggest that 5-HT may be a physiological regulator of acid and pepsin secretion in the fish. The dipsogenic effect seen in the absence of intestinal perfusion indicates that 5-HT may be involved also in the regulation of drinking.

Inhibition of gastric acid secretion by intestinal and parenteral administration of a mixture of L-amino acids in the Atlantic cod, Gadus morhua

The effect on basal gastric acid secretion in the cod of a mixture of L-amino acids (AA) was studied. Acid secretion was measured during a 5 h treatment period when the AA was administered intestinally, i.m. or i.v., and plasma alpha-amino nitrogen following the treatment period. Acid secretion was inhibited, the depression related to the plasma level of AA but unrelated to the administration route. It is suggested that acid secretion is inhibited by absorbed AA acting from the blood side, without involvement of an intestinal mechanism.

Inhibition of gastric acid secretion inthe Atlantic cod, Gadus morhua, by sulphated and desulphated gastrin, caerulein, and CCK-octapeptide

Gastric acid secretory effects of gastrin/CCK-like peptides have been assayed in cods rendered "spontaneously" secreting by a continuous intestinal perfusion with diluted (33%) seawater. A high dose of pentagastrin induced a weak stimulation (31%) of acid output, while gastrin 17-II, caerulein and CCK8 were inhibitory. Caerulein was the most potent peptide, with an estimated D50 for inhibition of 0.013 nmol/kg.h. Although displaying lower potencies, also the desulphated forms of gastrin-17, caerulein and CCK8 were inhibitory. The results may be explained by release of an endogenous inhibitor, or by interaction with endogenous "codfish gastrin". In the latter case two alternatives are considered: Either gastrin 17, CCK8, and caerulein possess lower efficacies than "codfish gastrin" and therefore act as partial agonists. Alternatively, "codfish gastrin" is itself an inhibitory principle (gastron), the effect of which is mimicked by gastrin 17, caerulein and CCK8. The actions of gastrin and the gastrin-like peptides in the cod indicate a structure-activity relationship different from previously described systems, both mammalian and submammalian.

Effects of exogenous angiotensin II in the Atlantic cod, Gadus morhua. Observations on gastric acid secretion, gastric sham drinking and gastric mucosal plasma flow (14C-aniline clearance)

Cods were equipped with cannulae for drainage of the stomach and for the separate perfusion of the stomach (pure seawater containing phenol red as a volume marker) and intestine (diluted seawater). Acidity of the gastric effluence was titrated, its volume calculated from the phenol red concentration. Gastric mucosal plasma flow (MPF) was estimated by gastric 14C-aniline clearance. I.m. injection of angiotensin II (AII) depressed basal acid secretion in a dose-dependent fashion. Also the MPF was reduced, but relatively less than the secretory depression. Therefore, the AII-induced secretory inhibition could not be explained by restrained mucosal blood flow. Perfusion of the intestine with diluted seawater, or a continuous i.m. infusion of 0.6% NaCl both rendered the fishes non-drinking. A high dose of AII (150 micrograms/kg . h) induced drinking in intestinally perfused cod while lower doses (15, 50 micrograms/kg . h) did not. In i.m. saline-injected cod, all three doses were dipsogenic. The results suggest that 0.6% saline infusion induces a permanent satiety and that intestinal perfusion in addition induces a preabsorptive satiety. The preabsorptive satiety appears more resistant to the dipsogenic action of AII than the permanent one.

Stimulation of gastric acid secretion and suppression of VIP-like immunoreactivity by bombesin in the Atlantic codfish, Gadus morhua

Cods were equipped with cannulae for drainage of the stomach and for separate perfusion of the stomach (pure sea-water) and intestine (diluted sea-water). Acidity and volume of gastric effluence were measured. Plasma immunoreactive gastrin and vasoactive intestinal polypeptide (VIP) were assayed in some experiments. The high rate of "basal" acid secretion was further elevated by i.m. administration of bombesin, but not by pentagastrin. Exogenous VIP inhibited acid secretion. Following 5 h of bombesin infusion, plasma gastrin-IR was unaffected while VIP-IR was depressed compared to saline-treated controls. The possibility that bombesin stimulates acid secretion by inhibiting VIP-release is discussed.

Effect of vagotomy and glucose administration on gastric acid secretion in the Atlantic cod, Gadus morhua

Cods were prepared for measurement of gastric acid secretion. Cannulae were implanted for drainage of the stomach and for separate perfusion of the stomach and intestine. Fishes, intestinally perfused with diluted sea-water (33% SW) exhibited high rates of acid secretion. This "basal" secretion was abolished (96% inhibition) by bilateral vagotomy. Perfusion of the stomach with isosmotic glucose (5.5%) diminished the amount of titratable acid in the gastric effluence, and induced oral ingestion of water. Only part of the decrease could be explained by neutralization by ingested sea-water. Perfusion of the intestine with 5.5% glucose, containing sea-water to raise the osmolality to that of 50% SW, depressed gastric acid secretion compared to 50% SW-perfused controls, without significantly affecting oral intake of water. I.m. administration of glucose, increasing plasma glucose to the same level as intraluminal administration, did not affect acid secretion or oral water ingestion. The study demonstrates the existence of a potential intestinal mechanism for inhibition of gastric acid secretion, and indicates that vagal activity is independent of the blood glucose level in the cod.