Abnormalities of 5-hydroxytryptamine metabolism in irritable bowel syndrome

Background & Aims: 5-hydroxytryptamine-3 (5-HT3) receptor antagonists improve symptoms in patients with diarrhea-predominant irritable bowel syndrome (D-IBS), 5-HT4 agonists help those with constipation-predominant IBS (C-IBS). These data suggest excess or deficiency in 5-HT in D-IBS or C-IBS, respectively. Mucosal 5-HT-containing enterochromaffin cells (EC) are increased in postinfectious IBS (PI-IBS). Our aim was to define the postprandial release of 5-HT in PI-IBS and C-IBS patients and to relate this to mucosal 5-HT turnover. Methods: Fifteen PI-IBS patients with diarrhea-predominant symptoms, 15 C-IBS patients, and 15 healthy controls underwent serial (platelet-poor) plasma 5-HT measurement for 3 hours after a standard 520-kcal meal. Rectal biopsy specimens were assayed for 5-HT and its metabolite 5-hydroxindoleacetic acid (5-HIAA). Colonic transit was measured using radio-opaque markers. Results: Colonic transit was prolonged in C-IBS patients (mean ± SEM) (49.4 ± 3.8 h) compared with PI-IBS (26.7 ± 4.5) and control patients (34.1 ± 4.5) (P < .02). Release of 5-HT assessed by area under the curve (AUC) of platelet-poor plasma 5-HT from 0 to 180 minutes postprandially was significantly lower in C-IBS patients (2593 ± 309 mmol/L · min) compared with P-IBS (5623 ± 721) and control patients (4822 ± 598) (P < .001). PI-IBS patients showed significantly higher peak postprandial plasma 5-HT values (median, range) (71.7, 43.4-125.3) ng/L compared with C-IBS patients (31.2, 15.2-40.5) and control patients (43.6, 26.7-50.1) (P < .01). Mucosal 5-HT turnover as assessed by mucosal 5-HIAA/5-HT ratio was decreased in both C-IBS and PI-IBS patients, .14 (.01-.6) and .21 (.02-2.5), respectively, compared with control patients 1.12 (.17-3.1) (P < .002). Conclusions: C-IBS patients show impaired postprandial 5-HT release whereas PI-IBS patients have higher peak levels, abnormalities that may be related to their different symptoms.