Ascending colon response to feeding: evidence for a 5-hydroxytryptamine-3 mechanism

The role of serotonergic type-3 receptors in proximal human colon is unclear. Our aims were to assess the postprandial volume and emptying of the ascending colon and to explore the role of 5-hydroxytryptamine-3 (5HT3) mechanisms.

In healthy subjects with unprepared colons we evaluated in a randomized trial the effects of the 5HT3 antagonist ondansetron (n = 5) or placebo (n = 5) on ascending colon volume and emptying, using a scintigraphic method.

Base-line ascending colon volumes were similar and were unaltered by ondansetron. After a 1000-kcal liquid meal the placebo group showed a variable change in volume (P = NS versus base line) during the first 25 min (median, -4%; range, -13% to 135%). Increases in volume during this period coincided with ileal emptying of chyme. During a second phase (30-105 min) there was a significant decrease of ascending colon volume (P = 0.02) relative to the early postprandial volume, but the volume was not significantly different from base line. This second phase was associated with transfer of chyme towards the transverse colon. In the ondansetron group there was an initial modest increase in volume (median, 5%; range, -15% to 14%; P = NS versus base line), and the second phase of contraction was inhibited.

The ascending colon response to a meal in health is characterized by a variable initial change in volume, accommodating ileal chyme in some individuals, and a more consistent reduction in volume from 30 to 105 min postprandially. The latter response is inhibited by ondansetron, suggesting partial control of postprandial colonic motor function by 5HT3 mechanisms.