The effects of 5-hydroxytryptamine on large intestinal motility and blood flow in the cat

The effects of 5-HT on proximal colonic and rectal motility and total colonic blood flow were studied in anaesthetized cats and compared with those evoked by pelvic nerve stimulation. Vasodilator responses, consistently elicited by low doses (5-10 micrograms/min close i.a.) and intermediate doses (10-50 micrograms/min) of 5-HT were invariably abolished by hexamethonium indicating a nerve mediated response. Simultaneously to the vasodilator response both cholinergic and non-cholinergic hexamethonium sensitive non-adrenergic motility responses were evoked in the proximal colon. Furthermore non-cholinergic, non-adrenergic inhibitory neurons were excited. In the rectum excitation of such inhibitory neurons was the most consistent finding. The mechanisms behind the nerve mediated vascular and motility responses seem to differ as only the vascular effects were blocked by dihydroergotamine. The nerve mediated responses were in many respects similar to those evoked by pelvic nerve stimulation indicating that 5-HT exerts its effects on the same instrinsic neurons as the pelvic nerves. At intermediate and high doses (50-100 micrograms/min) the effects exerted on the instrinsic reflex arcs were modulated by direct effects on the smooth muscle.