Chloride Transport in the Cystic Fibrosis Enterocyte
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 (Isc) 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.
KeywordsCystic Fibrosis Apical Membrane Chloride Channel Ussing Chamber Chloride Secretion
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