Feeding and osmoregulation: dual function of the marine teleost intestine

Abstract

SUMMARY Experiments on Gulf toadfish Opsanus beta demonstrate how feeding impacts osmoregulation in the marine teleost intestine. A high Ca2+diet of pilchards Sardina pilchardus ([Ca2+]=404.2 mmol kg-1) was compared to a low Ca2+ diet of common squid Loligo forbesi ([Ca2+]=1.3 mmol kg-1), as high[Ca2+] has been shown to stimulate intestinal anion exchange. Gastrointestinal fluids and blood plasma were collected over a time course from pre-feeding to 216 h post feeding. Following food intake, monovalent ions were largely absorbed across the intestinal epithelium, leaving a fluid rich in divalent ions, which have a lower osmotic coefficient and effectively reduce osmotic pressure in the lumen to allow for enhanced fluid absorption. Concentrations of Cl- and HCO -3 in fluid along the gastrointestinal tract of fish fed both diets, particularly 1 and 2 days post-feeding, demonstrate that apical Cl-/HCO -3 exchange plays a vital role in postprandial Cl- and water absorption. Postprandial acid-base balance disturbance as indicated by plasma alkalinization was limited or absent,indicating compensation for gastric acid secretion in this teleost fish. Plasma osmolality peaked 12 h post-feeding in toadfish fed squid, but was not accompanied by a significant increase in inorganic ion concentrations. Transient fluid secretion by the gastrointestinal tract was evident from reduced luminal Mg2+ and SO 2-4concentrations for 24-48 h post feeding. Discrepancy between the sum of inorganic osmolytes and measured osmotic pressure was attributed to organic osmolytes, which occurred at high concentrations in the stomach and anterior intestine for up to 24 h post feeding.

Revisiting the effects of crowding and feeding in the gulf toadfish, Opsanus beta: the role of Rhesus glycoproteins in nitrogen metabolism and excretion

“…In the present study, plasma ammonia concentrations were approximately 200moll -1 (Fig.1B). Taking into consideration blood plasma pH (~7.9) and stomach pH [~5.75 (Taylor and Grosell, 2006)], a substantial NH 3 partial pressure gradient exists, favouring NH 3 entry into the lumen of the stomach. In fasted fish, Rhcg1 and Rhcg2 mRNA expression were significantly lower in the stomach of crowded relative to unconfined fish.…”

“…Acidification of the gastric lumen to a pH of 3 (Taylor and Grosell, 2006) and elevation of plasma ammonia tõ 350moll -1 create an even stronger gradient for ammonia secretion into the stomach. Given that piscine Rh proteins function as bidirectional transporters (Nawata et al, 2010a) and that Rhcg1 mRNA expression in the stomach was significantly higher in fed fish, toadfish may manage the post-prandial nitrogen load by increasing ammonia secretion into the stomach.…”

“…Given that piscine Rh proteins function as bidirectional transporters (Nawata et al, 2010a) and that Rhcg1 mRNA expression in the stomach was significantly higher in fed fish, toadfish may manage the post-prandial nitrogen load by increasing ammonia secretion into the stomach. This trend carries over into the intestine, where mild post-prandial acidification [pH ~7.5; (Taylor and Grosell, 2006)] coupled with the elevated circulating ammonia concentration creates an NH 3 partial pressure gradient for the movement of ammonia into the intestine. In keeping with this hypothesis, the relative mRNA expression and presumably protein of all Rh isoforms in the intestine was significantly higher in fed than in fasted fish.…”

Fortification of an Aquafeed with Potassium Chloride Does Not Improve Survival of Juvenile Australian Snapper Pagrus auratus Reared in Potassium Deficient Saline Groundwater

“…Such species have evolved sophisticated mechanisms of hydromineral balance that regulate the major ions in seawater [14] and this regulation is mainly achieved by the cooperative function of the gills, intestine, and kidney [13,[15][16][17]. The gastrointestinal tract (GI tract) of marine fish plays a critical role in osmoregulation via the process of NaCl coupled fluid absorption [18][19][20][21].…”

“…The gastrointestinal tract (GI tract) of marine fish plays a critical role in osmoregulation via the process of NaCl coupled fluid absorption [18][19][20][21]. However, the dual role of the intestine, serving hydromineral functions as well as the digestion and absorption of nutrients, is often in conflict and many of the interactions are not well understood [17][18][19][22][23][24][25]. It is known that consumption of large meals of dry aquafeed immediately increase the load of minerals in the GI tract, drastically changing the ionic balance of the intestine [21,24].…”