Nitrite uptake and bicarbonate excretion by flounder intestine

(NO2−) is a potential contaminant in aquatic environments that receive nitrogenous waste and in recirculated aquaculture systems. Several studies have examined the toxicity and physiological effects of NO2− in fish (for reviews, see Lewis and Morris, 1986; Eddy and Williams, 1987; Jensen, 1995, 1996). Most investigations have concerned freshwater teleosts, because NO2− is generally more toxic to freshwater than to marine species. However, we have shown that a marine teleost, the European flounder Platichthys flesus, accumulates NO2− in the plasma during exposure to 1 mmol l−1 ambient NO2− and that this accumulation is associated with an increased blood methaemoglobin content (haemoglobin not available for oxygen transport) (M. Grosell and F. B. Jensen, in preparation). Whereas freshwater fish take up NO2− primarily across the gills, marine fish may be expected to have significant intestinal uptake as a result of the high rate of drinking required for osmoregulation (Jensen, 1995). Indeed, in vivo studies have demonstrated that the European flounder has high NO2− concentrations in the intestinal fluids during exposure to elevated ambient NO2− concentrations (M. Grosell and F. B. Jensen, in preparation). Furthermore, by using in situ perfusion of the intestine (M. Grosell, G. Deboeck, O. Johannsson and C. M. Wood, in preparation), we have demonstrated that intestinal NO2− uptake accounts for approximately two-thirds of the total whole-body uptake (M. Grosell and F. B. Jensen, in preparation). Having established that the intestine is the primary site of NO2− uptake in the European flounder, we set out to investigate the mechanism of NO2− transport across the intestinal epithelium. In freshwater fish, it has been suggested that the branchial Cl−/HCO3− exchanger is the carrier mediating the apparent active uptake of NO2− (Bath and Eddy, 1980), but no attempts have been made to document this. In the present study, insight into transport mechanisms was obtained by using preparations of freshly isolated intestinal segments, a modification of the technique described by M. Grosell, G. Deboeck, O. Johannsson and C. M. Wood (in preparation), which allowed us to manipulate the composition of both the apical (mucosal) and basolateral (serosal) media. To evaluate the possible uptake of NO2− via an intestinal Cl−/HCO3− exchanger, we applied 4,4′-diisothiocyanostilbene2,2′-disulphonic acid (DIDS), a well-known blocker of Cl−/HCO3− exchange in erythrocytes and other cells 2103 The Journal of Experimental Biology 202, 2103–2110 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JEB2156