Heat-stable enterotoxin ofEscherichia colistimulates a non-CFTR-mediated duodenal bicarbonate secretory pathway

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Abstract
The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) is an important pathway for duodenal mucosal bicarbonate secretion. Duodenal biopsies from CF patients secrete bicarbonate in response to heat-stable enterotoxin from Escherichia coli (STa) but not cAMP. To explore the mechanism of STa-induced bicarbonate secretion in CF more fully, we examined the role of CFTR in STa-stimulated duodenal bicarbonate secretion in mice. In vivo, the duodenum of CFTR (−/−) or control mice was perfused with forskolin (10−4 M), STa (10−7 M), uroguanylin (10−7 M), 8-bromoguanosine 3′,5′-cGMP (8-Br-cGMP) (10−3 M), genistein (10−6 M) plus STa, or herbimycin A (10−6 M) plus STa. In vitro, duodenal mucosae were voltage-clamped in Ussing chambers, and bicarbonate secretion was measured by pH-stat. The effect of genistein, DIDS (10−4 M), and chloride removal was also studied in vitro. Control, but not CF, mice produced a significant increase in duodenal bicarbonate secretion after perfusion with forskolin, uroguanylin, or 8-Br-cGMP. However, both control and CF animals responded to STa with significant increases in bicarbonate output. Genistein and herbimycin A abolished this response in CF mice but not in controls. In vitro, STa-stimulated bicarbonate secretion in CF tissues was inhibited by genistein, DIDS, and chloride-free conditions, whereas bicarbonate secretion persisted in control mice. In the CF duodenum, STa can stimulate bicarbonate secretion via tyrosine kinase activity resulting in apical Cl−/HCO3− exchange. Further studies elucidating the intracellular mechanisms responsible for such non-CFTR mediated bicarbonate secretion may lead to important therapies for CF.