The intestine of marine teleosts osmoregulates in part due to Cl-/HCO3- exchange.

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“Despite early reports of high HCO3 and CO3 concentrations in the gut fluid of marine teleosts, intestinal anion exchange was largely overlooked until recently. HCO3 − secretion occurs in the intestine of marine teleosts via Cl−/HCO3 − exchange across the apical membrane and contributes up to 50–70% to Cl−/fluid absorption. It is well documented that marine fish must drink seawater to combat diffusive water loss to a hyperosmotic environment. As imbibed seawater passes through the gut, Na+ and Cl− absorption occurs via Na+:Cl− and Na+:K+:2Cl− cotransporters, in addition to apical Cl−/HCO3− exchange. Water follows this salt absorption, leaving behind Mg2+ and SO4 − at concentrations often more than three times those of seawater. In contrast, HCO3 − is present in the gut of marine teleosts at values up to 50 times seawater levels as a result of apical Cl−/HCO3 – exchange within the intestine. Bicarbonate secretion may also play a role in calcium homeostasis, inhibiting intestinal Ca2+ absorption by precipitating CaCO3 which is subsequently excreted. Carbonate precipitation concomitantly promotes water absorption, lowering osmolality by removing Ca2+and CO3 2− from solution.” (Taylor et al. 2006:523)

Journal article
Evolutionary aspects of intestinal bicarbonate secretion in fishVolume 143, Issue 4April 1, 2006
Taylor, J. R.; Grosell, M.

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