CONCENTRATION-DEPENDENT STIMULATION OF INTESTINAL PHASE-III OF MIGRATING MOTOR COMPLEX BY CIRCULATING SEROTONIN IN HUMANS

1. The influence of circulating 5-hydroxytryptamine (serotonin)on smal l intestinal motility was investigated in healthy volunteers. 2. Small intestinal motility was studied by means of a constantly perfused mul ti-channel manometry tube, connected to a computer system, 3. Intraven ous infusions of either 5-hydroxytryptamine at increasing doses or sal ine were given over a period of 4 h, 4. 5-Hydroxytryptamine infusion d ose-dependently increased plasma 5-hydroxytryptamine from approximatel y 2 to 10 and 25 nmol/l respectively, as well as urinary excretions of 5-hydroxytryptamine and 5-hydroxyindole acetic acid, a major 5-hydrox ytryptamine metabolite. 5. The number of phase III of the migrating mo tor complex originating in the small intestine was dose-dependently in creased by 5-hydroxytryptamine and found to correlate to the plasma co ncentration of 5-hydroxytryptamine. The fraction of phase m also incre ased at the expense of phase II activity. In addition, 5-hydroxytrypta mine increased the motility index, propagation velocity of phase m act ivity and the amplitude of contractions during phase II, 6. Whereas th e low dose of 5-hydroxytryptamine (15 nmol . min(-1) . kg(-1)) had no haemodynamic effects, an increase in heart rate by approximately 20 be ats/min, without change in blood pressure, was observed at the higher dose (60 nmol min(-1) . kg(-1)). Respiratory parameters did not change during infusion of 5-hydroxytryptamine at either dose. 7. In conclusi on, elevation of circulating 5-hydroxytryptamine by intravenous infusi on results in more frequent and faster propagating migrating motor com plexes in the human small intestine during the interdigestive period.