ABSTRACT
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.
