Eating lowers defecation threshold in pigs through cholinergic pathways

The effect of atropine on defecation threshold was compared to placebo pre-and postprandially in four 20- to 30-kg pigs. Stepwise balloon distention was performed 10 cm from the anal verge with a 5-cm latex balloon. Volume was increased in steps of 10 ml up to 200 ml of air or until the balloon was defecated (defecation threshold). Dependent measures were balloon volume, rectal pressure, rectal compliance, and an index of distention-induced contractile activity. Under placebo conditions, the volume and pressure to elicit defecation were significantly lower after feeding ( p < 0.05). The distention-induced contractile activity significantly increased near the defecation threshold, but pre- and postprandial conditions were not different. No differences were seen between pre- and postprandial rectal compliance curves. Atropine abolished the postprandial decrease in defecation threshold, but did not affect rectal compliance. The increase in contractile activity at defecation threshold seen with placebo was abolished by atropine. These results show that eating lowers the defecation threshold in terms of distention volume and rectal pressure, and demonstrate that these changes are mediated through cholinergic pathways.