Cystic fibrosis transmembrane conductance regulator activates water conductance in Xenopus oocytes

Abstract

 Multiple properties have been attributed to the cystic fibrosis transmembrane conductance regulator (CFTR), the gene product which is mutated in cystic fibrosis (CF). In this context it has been reported that CFTR transports water. In the present study we demonstrate that expression of wild-type CFTR (wtCFTR) in Xenopus oocytes and then stimulation by 3-isobutyl-1-methylxanthine (IBMX, 1 mmol/l) activates a Cl^–conductance and, in parallel, a water conductance, as measured by a volume increase gravimetrically. In water-injected control oocytes or oocytes expressing a mutant form of CFTR (G551D-CFTR) IBMX had very little effect on Cl^–conductance and no effect on water conductance. Phloretin (350 μmol/l) and p-chloromercuri-benzene sulphonate (pCMBS, 1 mmol/l) inhibited water transport but did not inhibit Cl^–currents when measured in double-electrode voltage-clamp experiments. In contrast, glibenclamide (100 μmol/l) inhibited wtCFTR Cl^–conductance but did not inhibit water conductance in IBMX-stimulated oocytes. Moreover, gravimetric and [¹⁴C]glycerol uptake measurements indicated enhanced glycerol uptake by wtCFTR-expressing oocytes after stimulation with IBMX. Enhanced glycerol uptake could be inhibited by phloretin and pCMBS but not by glibenclamide. Taken together, the data suggest that activation of wtCFTR by an increase of intracellular cAMP is paralleled by the activation of a gylcerol-permeable water conductance. Both water and Cl^–conductive pathways can be inhibited differentially. Thus, water permeation through wtCFTR probably occurs at a site of CFTR which is spatially apart from the domain responsible for Cl^–conductance, or CFTR might be a regulator of an endogenous water channel in oocytes.