5-HT3 RECEPTORS DO NOT MEDIATE VAGALLY-INDUCED RELAXATION OR CONTRACTION OF THE ISOLATED STOMACH OF THE GUINEA-PIG

1 We have tested whether 5-HT3 receptors mediate vagally-induced relaxation or contraction of the isolated stomach of the guinea-pig. 2 The antagonists of 5-HT3 receptors, ondansetron (1-10 mu M) or metoclopramide (1-30 mu M) did not inhibit vagally-induced relaxations at low concentrations but partially inhibited them at 30 mu M or 60 mu M, respectively. These higher concentrations of ondansetron and metoclopramide also inhibited relaxations induced by 1,1-dimethyl-4-phenylpiperazinium iodide (30 mu M) but did not affect those induced by glyceryl trinitrate (0.7-1.1 mu M). 3 Desensitization to 5-HT (100 mu M) or 2-Me-5-HT (100 mu M) did not affect relaxations or contractions induced by vagal stimulation. 4 Ondansetron (30 mu M) or metoclopramide (60 mu M) did not inhibit vagally-induced gastric contraction. 5 Thus, 5-HT3 receptors do not mediate vagally-induced relaxation or contraction in the guinea-pig stomach.