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

Ion transport across isolated intestinal segments from the European flounde r (Platichthys flesus) was studied with the primary aim of evaluating the m echanisms of nitrite (NO2-) uptake and HCO3- excretion. A double-radiolabel ling technique was applied to monitor unidirectional Cl- and Na+ influx. Fu rthermore, net fluxes of NO2-, HCO3-, Cl-, Na+ and water were recorded. NO2 - uptake Has inhibited by mucosal application of bumetanide (10(-4) mol l(- 1)) but not DIDS (10(-3) mol l(-1)), suggesting that NO2- is transported ac ross the intestine via the Na+/K+/2Cl(-) cotransporter rather than via a Cl -/HCO3- exchanger, In addition to transport via the Na+/K+/2Cl(-) cotranspo rter, NO2- uptake may also occur through the Na+/Cl- cotransporter and by c onductive transport. NO2- and Cl- influx rates seemed to reflect their muco sal 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 ma rine teleost fish. An alternative model of HCO3- excretion across the intes tinal epithelium is proposed.