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 Atlantic 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. Possible HCO3- transport mechanisms were studied in the Pacific sanddab (Citharichthys sordidus) in vitro. Measurements of net secretion of HCO3- equivalents 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 of Cl-, but some Cl- uptake persisted in the absence of Na+, at a rate similar 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 contrast the currently accepted model for intestinal HCO3- secretion. This apical Cl-/HCO3- exchanger alone, however, is not sufficient for maintaining the observed HCO3- equivalents gradient in vivo. We suggest a coupling of cytosolic carbonic anhydrase, a basolateral proton pump and the apical Cl-/HCO3- exchanger to explain the intestinal HCO3- transport.