NO2- uptake and HCO3- excretion in the intestine of the European flounder (Platichthys flesus).

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.