Background & aims: The duodenum is a cystic fibrosis transmembrane conductance regulator (CFTR)-expressing epithelium with high bicarbonate secretory capacity. We aimed to define the role of CFTR in human duodenal epithelial bicarbonate secretion in normal (NL) subjects and patients with cystic fibrosis (CF).
Methods: Endoscopic biopsy specimens of the duodenal bulb were obtained from 9 CF patients and 16 volunteers. Tissues were mounted in modified Ussing chambers. Bicarbonate secretion and short-circuit current (Isc) were quantitated under basal conditions and in response to dibutyryl adenosine 3',5'-cyclic monophosphate (db-cAMP), carbachol, and the heat-stable toxin of Escherichia coli (STa). Duodenocytes were also isolated and loaded with the pH-sensitive fluoroprobe BCECF/AM, and intracellular pH (pH(i)) was measured at rest and after intracellular acidification and alkalinization.
Results: Basal HCO(3)(-) secretion and Isc were significantly lower in the CF vs. NL duodenal mucosa. In contrast to NL, db-cAMP failed to alter either HCO(3)(-) or Isc in CF tissues. However, in CF, carbachol resulted in an electroneutral HCO(3)(-) secretion, whereas STa induced electrogenic HCO(3)(-) secretion that was similar to NL. In CF and NL duodenocytes, basal pH(i) and recovery from an acid load were comparable, but pH(i) recovery after an alkaline load in CF duodenocytes was Cl(-) dependent, whereas in NL duodenocytes it was Cl(-) independent.
Conclusions: These findings implicate CFTR in NL duodenal alkaline transport and its absence in CF. Although duodenal bicarbonate secretion is impaired in CF tissues, alternate pathway(s) likely exist that can be activated by carbachol and STa.