COMPARATIVE STUDIES WITH 5-HYDROXYTRYPTAMINE AND ITS DERIVATIVES IN ISOLATED, BLOOD-PERFUSED SMALL INTESTINE AND TLEUM STRIP OF THE RAT

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Abstract
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 but 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 atropine, methysergide or phentolamine. In the ileum strip preparations 5-HT contracted the ileum but its derivatives did not. TTX, C6, morphine and atropine did not prevent the contractile response to 5-HT but methysergide antagonized the response. Apparently 5-HT acts by exciting intramural neuronal elements or by directly contracting the intestinal smooth muscle. 5-HTP seems to act in the same manner as 5-HT.