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1 University Children's
Hospital,
Cystic fibrosis (CF) patients show
characteristic defects in epithelial ion transport, such as failure in
cAMP-dependent Cl
secretion. Because the cystic fibrosis transmembrane conductance regulator (CFTR) also functions as a downregulator of epithelial Na+ channels (ENaC), enhanced
Na+ conductance was found in the
airways of CF patients. Here, we examined whether enhanced epithelial
Na+ conductance is also present in
the colonic epithelium of CF patients and examined the underlying
mechanisms. Thus transepithelial voltages were measured, and equivalent
short-circuit currents
(Isc-eq) were
determined by means of a novel type of Ussing chamber. Non-CF tissues
demonstrated cAMP-dependent
Cl secretion that was
absent in biopsies of CF patients. Correspondingly, Isc-eq
was inhibited in non-CF but not in CF epithelia when synthesis of
endogenous prostaglandins was blocked by indomethacin. In the presence of indomethacin, a larger portion of
amiloride-sensitive Isc-eq
was detected in CF tissues, suggesting enhanced ENaC conductance in
colonic mucosa of CF patients. Increase of intracellular cAMP by
forskolin and IBMX inhibited amiloride-sensitive ENaC currents in
non-CF tissues but not in CF biopsies. Therefore, enhanced epithelial
Na+ conductance is present in the
CF colon and is probably due to missing downregulation by CFTR.
cystic fibrosis transmembrane conductance regulator; epithelial transport; amiloride-sensitive epithelial sodium channels; Ussing chamber; transepithelial voltage; colonic sodium absorption
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