The cystic fibrosis transmembrane conductance regulator is not a base transporter in isolated duodenal epithelial cells.

Abstract

Duodenal epithelial bicarbonate secretion has previously been shown to be greatly impaired in mice deficient of the cystic fibrosis transmembrane conductance regulator (CFTR). It has been proposed that transmembranal bicarbonate transport occurs through the CFTR channel itself. In the present study, the transport of acid/base equivalents across the plasma membrane of proximal duodenal epithelial cells from CFTR deficient mice was compared with that of cells from normal littermates. Mixed epithelial cells from both villi and crypts were isolated from proximal duodenum and intracellular pH was assessed by cuvette-based fluorescence spectrometry using the pH sensitive dye 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein. The steady state intracellular pH, the acid extrusion rate and the alkaline extrusion rate were unaffected by CFTR deficiency in the presence of CO(2)/HCO(-)(3). Forskolin had no effect on acid extrusion or alkaline extrusion rates. In control experiments without CO(2)/HCO(-)(3), the intrinsic buffering capacities, the steady state intracellular pH and the acid extrusion rates were equivalent in the cells from CFTR deficient mice and normal littermates. The results are consistent with a model where acid/base transport is almost exclusively mediated by the previously described transporters in the murine duodenum (i.e. Na+/H+ exchange, Cl(-)/HCO(-)(3). exchange and Na+:HCO(-)(3). cotransport). There were no evidence for significant CFTR dependent HCO(-)(3). transport in proximal duodenal epithelial cells of mixed villus and crypt origin.