Effect of 5‐Hydroxytryptamine on Gastrointestinal Contractile Activity in Conscious Dogs

The effect of intravenous infusion of 5‐hydroxytryptamine (5‐HT) on gastrointestinal contractile activity was studied in conscious dogs during the digestive and interdigestive states. It was found that 30 to 300 μg/kg per hour of 5‐HT induced phasic contractions mainly in the gastric antrum, duodenum, jejunum, and mid‐intestine. However, when doses greater than 100 μg/kg per hour were used, a stimulatory effect was seen in the ileum and colon. The dose‐dependent stimulatory effect of 5‐HT on gastrointestinal motor activity was observed in both the frequency and amplitude of contractions. Atropine‐resistant 5‐HT‐induced contractions in the stomach were completely inhibited by methysergide. This fact strongly suggests the existence of 5‐HT receptor on the smooth muscle. The stimulatory effect of 5‐HT given intravenously quickly disappeared with the cessation of infusion. No significant side effects such as nausea, vomiting, diarrhea, or other symptoms suggesting abdominal pain were observed during or after the intravenous infusion of 5‐HT in doses between 10 and 300 μg/kg per hour in any of the dogs.