A functional CFTR protein is required for mouse intestinal cAMP‐, cGMP‐ and Ca2+‐dependent HCO3− secretion

Abstract

Most segments of the gastrointestinal tract secrete HCO 3−, but the molecular nature of the secretory mechanisms has not been identified. We had previously speculated that the regulator for intestinal electrogenic HCO 3− secretion is the cystic fibrosis transmembrane regulator (CFTR) channel. To prove this hypothesis, we have now measured HCO 3− secretion by pH‐stat titration, and recorded the electrical parameters of in vitro duodenum, jejunum and ileum of mice deficient in the gene for the CFTR protein (‘CF‐mice’) and their normal littermates. Basal HCO 3− secretory rates were reduced in all small intestinal segments of CF mice. Forskolin, PGE 2, 8‐bromo‐cAMP and VIP (cAMP‐dependent agonists), heat‐stable enterotoxin of Escherichia coli (STa), guanylin and 8‐bromo‐cGMP (cGMP‐dependent agonists) and carbachol (Ca 2+ dependent) stimulated both the short‐circuit current (ISC) and the HCO 3− secretory rate (J HCO 3‐) in all intestinal segments in normal mice, whereas none of these agonists had any effect on J HCO 3‐ in the intestine of CF mice. To investigate whether Cl−–HCO 3− exchangers, which have been implicated in mediating the response to some of these agonists in the intestine, were similarly active in the small intestine of normal and CF mice, we studied CF gradient‐driven 36 Cl− uptake into brush‐border membrane (BBM) vesicles isolated from normal and CF mouse small intestine. Both the time course and the peak value for 4,4’‐diisothiocyanostilbene‐2’,2‐disulphonic acid (DIDS)‐inhibited 36 Cl− uptake was similar in normal and CF mice BBM vesicles. In summary, the results demonstrate that the presence of the CFTR channel is necessary for agonist‐induced stimulation of electrogenic HCO 3− secretion in all segments of the small intestine, and all three intracellular signal transduction pathways stimulate HCO 3− secretion exclusively via activation of the CFTR channel.