Comparative studies with 5-hydroxytryptamine and its derivatives in isolated, blood-perfused small intestine and ileum strip of the rat.

The mode of actions of 5-hydroxytryptamine (5-HT) and its derivatives, tryptophan (TP), 5-hydroxytryptophan (5-HTP) and 5-hydroxyindole acetic acid (5-HIAA) was studied on the isolated, blood-perfused small intestine and isolated ileum strip of rats. In the isolated, blood-perfused intestinal preparations, 5-HT and 5-HTP injected into the superior mesenteric artery caused a monophasic fast contraction, while TP and 5-HIAA had no effects on the intestine. The contractile responses to 5-HT and 5-HTP were abolished by tetrodotoxin (TTX), hexamethonium (C6) and morphine, but were resistant to blockade of either atropine, methysergide or phentolamine. On the other hand, in the ileum strip preparations, 5-HT contracted the ileum, but its derivatives had no effects on the ileum. TTX, C6, morphine and atropine failed to prevent the contractile response to 5-HT, whereas methysergide effectively antagonized the response. The present results indicate that 5-HT acts by exciting intramural neuronal elements or by directly contracting the smooth muscle of the intestine. 5-HTP seems to act in the same manner as 5-HT.