Stimulant effects of 5-hydroxytryptamine on guinea pig stomach preparations in vitro.

5-Hydroxytryptamine (5-HT) contracts and relaxes isolated stomach preparations. This study attempts to characterise receptors involved in the contractile response using electrically stimulated circular muscle strips from guinea pig stomach. Electrically induced contractions were abolished by atropine and tetrodotoxin. 5-HT enhanced contractions in corpus and fundus strips with pEC50% values (-log10 of the concentrations causing a 50% increase in twitch height) of 9.6 and 9.1, respectively. 5-Carboxamidotryptamine and 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), 5-HT1A receptor agonists, and alpha-methyl-5-HT, an agonist at 5-HT2 receptors, reduced contractions. The 5-HT3 receptor agonist, 2-methyl-5-HT, increased contractions. The effect of 2-methyl-5-HT but not of 5-HT was antagonized by the 5-HT3 receptor antagonist, tropisetron (10(-7) M). The 5-HT3 receptor antagonists, tropisetron, MDL 72222 (1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate), grainsetron and ondansetron, did not modify twitch responses at concentrations below 10(-7) M. Renzapride and metoclopramide, agonists at 5-HT4 receptors, increased contractions and this effect was inhibited by the 5-HT4 receptor antagonist SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino) ethyl ester) with a pA2 of 7.4. The effect of 5-HT at a submaximal concentration of 10(-8) M was blocked by SDZ 205-557 (10(-6) M). It is concluded that electrically induced contractions in guinea pig stomach strips are enhanced by activation of 5-HT3- and 5-HT4 receptors and are diminished by 5-HT1 receptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)