Molecular mechanisms of intestinal chloride channel regulation and potential abnormalities in electrogenic chloride secretion in intestinal epithelium from cystic fibrosis (CF) patients were investigated by a combination of Ussing chamber, vesicle transport and off-cell patch-clamp analysis.

Short circuit current (I_sc) measurements in normal and CF rectal biopsies provided evidence for i) a defect in the cAMP-provoked activation of chloride secretion and a (hyper)expression of cAMP-dependent potassium secretion in all CF patients examined (n=ll); ii) a defect in the carbachol-provoked chloride secretion and a (hyper)expression of carbachol-induced potassium secretion in 6/11 patients; iii) a residual (but still impaired) carbachol-induced chloride secretion in 5/11 CF patients (including 2 sibs). The latter class of CF patients appeared to consist genetically of compound heterozygotes for the major delta-F₅₀₈ deletion, suggesting a correlation between the nature of the mutation in the CF gene and the severity of the chloride secretory defect in CF intestine.

In our search for a regulatory function of GTP-binding (G-) proteins detected previously in the luminal membrane of rat and human intestinal epithelial cells, evidence was found for the presence of a GTP[S]-activatable-and GDP[S]-inhibitable chloride conductance in the apical membrane of rat enterocytes and human colonocytes. In excised patches of human colonocyte membranes, this G-proteine-sensitive chloride conductance was identified further as a novel type of chloride channel (20pS; inwardly rectifying) that was different from the 3 3pS outwardly rectifying chloride channel activatable by cAMP-dependent proteinkinase (PK-A) and voltage depolarization. The possible role of this channel in intestinal chloride secretion and its functioning in CF intestine is under present investigation.