5-HT receptor types in the rat ileum longitudinal muscle: focus on 5-HT2 receptors mediating contraction.

The 5-hydroxytryptamine (5-HT) receptor(s) that mediate(s) contraction of the rat ileum longitudinal muscle was studied. 5-HT and alpha-methyl-5-HT equipotently induced contractions, whereas 5-methoxytryptamine and 2-methyl-5-HT (partial agonist) were less potent; this rank order of potency suggests involvement of a 5-HT2 receptor. Neither tetrodotoxin nor atropine affected the contraction to 5-HT, suggesting a smooth muscle localization of these 5-HT2 receptors. The presence of either a selective 5-HT2B (SB 204741), 5-HT3 (granisetron) or 5-HT4 (SB 204070) antagonist, slightly affected the contractions to 5-HT. Thus, they were also included in the organ bath solution in all subsequent experiments in order to pharmacologically isolate the main contractile component. Using (if possible) 5-HT2A receptor-selective concentrations, ketanserin, ritanserin, metergoline, spiperone, mianserin, methiothepin, mesulergine, methysergide and cisapride all inhibited the contractions to 5-HT, causing a depression of the curve to 5-HT (i.e. surmountable antagonism was not observed with any of the above agents). Comparison of the affinities of these compounds for the various 5-HT2 receptor subtypes revealed that the receptor involved in the contractions to 5-HT most closely resembles the 5-HT2A receptor. However, cinanserin at a concentration expected to inhibit 5-HT2A receptor-mediated effects, failed to affect the contractions to 5-HT. It is thus concluded that on the longitudinal smooth muscle of the rat ileum, at least a part of the contraction to 5-HT is mediated by 5-HT receptors resembling the 5-HT2A receptor subtype.