Pharmacological characterization of the 5-hydroxytryptamine receptor mediating relaxation in the rat isolated ileum

1 The aim of the present study was to investigate a 5-HT4 receptor involvement in the mediation of a 5-HT-induced relaxation response in the rat isolated ileum in vitro. 2 Ileal segments were taken at regular intervals from the ileo-caecal junction to duodenum. 5-HT (1 microM) induced a relaxation or contraction response in segments taken from the terminal ileum: the relaxation decreased and finally disappeared as contractions dominated in the proximal tissues. The 5-HT-induced relaxations were enhanced in the terminal segments and the contractions attenuated in both terminal and proximal segments, in the presence of methysergide (1 microM) and atropine (0.1 microM). 3 In the presence of methysergide (1 microM) and atropine (0.1 microM), a cumulative addition of 5-HT (0.01-1 microM) induced a concentration-dependent relaxation in the terminal (1-20 cm from the ileo-ceacal junction) ileal segments which at higher concentrations of 5-HT (3-30 microM) reverted to contraction. 4 The rank order of potency of indole agonists in inducing a concentration-related relaxation response in tissues of the terminal ileum (pretreated with pargyline (100 microM) and in the presence of methysergide (1 or 100 microM) and atropine (0.1 microM) was 5-hydroxytryptamine (6.97 +/- 0.06), 5-methoxytryptamine (6.50 +/- 0.07), alpha-methyl-5-hydroxytryptamine (5.53 +/- 0.17), 5-carboxamidotryptamine (5.51 +/- 0.12) and 2-methyl-5-hydroxytryptamine (< 5), the pEC50 values (mean +/- s.e.mean) being shown in parentheses. 5 Pretreatment of tissues with pargyline (100 microM) selectively enhanced the potency of 5-methoxytryptamine by a factor of 19 but failed to modify the potency of the other indole agonists. 6 The 5-HT4 receptor antagonists, tropisetron, SDZ 205-557 and GR 113808 antagonized the relaxation response to 5-HT (in the presence of methysergide (1 or 10 microM) and atropine (0.1 microM)) with pKB values (95% CL) of 6.09 (5.94-6.24), 7.0 (6.9-7.09) and 8.95 (8.81-9.1) respectively. Apparent pKB values estimations for tropisetron (1 microM) and GR 113808 (10 nM) using the agonists 5-methoxytryptamine and 5-carboxamidotryptamine were 6.37 +/- 0.31, 5.91 +/- 0.38 and 8.83 +/- 0.11, 8.82 +/- 0.22 respectively. 7 Tropisetron (10 microM), SDZ 205-557 (3 microM) and GR 113808 (10-100 nM) caused an increase in basal tone of the rat terminal ileum when administered in the presence of methysergide and atropine. 8 The relaxation response to 5-HT in the rat terminal ileum was not antagonized by ritanserin (1 microM), ondansetron (1 microM) or N omega-nitro-L-arginine methyl ester (100 microM) and with only a twofold dextral shift of the concentration-response curve by tetrodotoxin (1 microM). 9 It is concluded that the relaxant response to 5-HT in the terminal region of the ileum is mediated directly at the smooth muscle; a ranked indole agonist potency and selective antagonism by 5-HT4 receptor antagonists tropisetron, SDZ 205-557 and GR 113808 indicate a 5-HT4 receptor involvement in the relaxation response.