Ion transport across isolated intestinal segments from the European flounder (Platichthys flesus) was studied with the primary aim of evaluating the mechanisms of nitrite (NO2-) uptake and HCO3- excretion. A double-radiolabelling technique was applied to monitor unidirectional Cl- and Na+ influx. Furthermore, net fluxes of NO2-, HCO3-, Cl-, Na+ and water were recorded. NO2- uptake was inhibited by mucosal application of bumetanide (10(−)4 mol l-1) but not DIDS (10(−)3 mol l-1), suggesting that NO2- is transported across the intestine via the Na+/K+/2Cl- cotransporter rather than via a Cl-/HCO3- exchanger. In addition to transport via the Na+/K+/2Cl- cotransporter, NO2- uptake may also occur through the Na+/Cl- cotransporter and by conductive transport. NO2- and Cl- influx rates seemed to reflect their mucosal concentrations, and NO2- did not influence unidirectional influx or net flux of Cl-. HCO3- efflux was significantly reduced in the presence of 10(−)3 mol l-1 DIDS in the mucosal solution. This may indicate the presence of an apical Cl-/HCO3- exchanger in the intestinal epithelium, which would not comply with the current model of HCO3- excretion in the intestine of marine teleost fish. An alternative model of HCO3- excretion across the intestinal epithelium is proposed.

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