Intestinal HCO3- secretion in marine teleost fish: evidence for an apical rather than a basolateral Cl-/HCO3- exchanger

Intestinal fluid was collected from 11 marine teleost fish from the Baltic sea and the Pacific ocean. The anterior, mid and posterior segments of the intestine contained 33-110 mM of HCO3- equivalents (with exception of the A tlantic cod which contained only 5-15 mM). Considering literature values of transepithelial potentials and concentration gradients, these high levels of HCO3- equivalents are probably the result of active HCO3- transport. Pos sible HCO3- transport mechanisms were studied in the Pacific sanddab (Citha richthys sordidus) in vitro. Measurements of net secretion of HCO3- equival ents across the intestinal epithelium revealed mucosal DIDS sensitivity (10 (-4) M) and Cl--dependence of the HCO3- equivalent net flux, but no serosal DIDS (10(-4) M) sensitivity. Net Na+ uptake was abolished in the absence o f Cl-, but some Cl- uptake persisted in the absence of Na+, at a rate simil ar to that of net HCO3- secretion. Anterior, mid and posterior segments of the intestine performed similarly. These observations support the presence of an apical rather than a basolateral Cl-/HCO3- exchanger and thus contras t the currently accepted model for intestinal HCO3- secretion. This apical Cl-/HCO3- exchanger alone, however, is not sufficient for maintaining the o bserved HCO3- equivalents gradient in vivo. We suggest a coupling of cytoso lic carbonic anhydrase, a basolateral proton pump and the apical Cl-/HCO3- exchanger to explain the intestinal HCO3- transport.