Indirect evidence for cholinergic inhibition of intestinal bicarbonate absorption in humans

BACKGROUND—The aim of the study was to test the hypothesis that in the fasting state, proximal intestinal HCO3 absorption, which depends on villus Na+/H+ exchanger activity, is tonically inhibited by a cholinergic atropine sensitive mechanism. SUBJECTS—The experiments were performed in 34 healthy volunteers and in eight patients with intestinal villus atrophy. METHODS—HCO3 absorption was measured with a modified triple lumen perfusion technique in the distal duodenum, the most proximal portion of the small intestine. The study was designed to compensate for the inhibitory effects of atropine on intestinal motor activity. RESULTS—Atropine had three effects on HCO3 transport: it reduced HCO3 concentration at the proximal aspiration site, it displaced the relation between HCO3 concentration and HCO3 absorption to the left, and it induced a significant acidification of the perfusate at the distal aspiration site. The magnitude of the stimulatory effect on HCO3 absorption was similar to the difference between patients with intestinal villus atrophy and healthy controls. CONCLUSION—The data suggest that, in the fasting state, duodenal HCO3 absorption, which depends on villus Na+/H+ exchanger activity, may be tonically inhibited by an atropine sensitive cholinergic mechanism. Keywords: small intestine; absorption; cholinergic; muscarinic receptor; villus atrophy; coeliac disease