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 μ m ) 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 μ m ) and atropine (0.1 μ m ). 3 In the presence of methysergide (1 μ m ) and atropine (0.1 μ m ), a cumulative addition of 5‐HT (0.01‐1 μ m ) 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 μ m ) 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 μ m ) and in the presence of methysergide (1 or 100 μ m ) and atropine (0.1 μ m )) was 5‐hydroxytryptamine (6.97 ± 0.06), 5‐methoxytryptamine (6.50 ± 0.07), α‐methyl‐5‐hydroxytryptamine (5.53 ± 0.17), 5‐carboxamidotryptamine (5.51 ± 0.12) and 2‐methyl‐5‐hydroxytryptamine (< 5), the pEC 50 values (mean ± s.e.mean) being shown in parentheses. 5 Pretreatment of tissues with pargyline (100 μ m ) 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‐HT 4 receptor antagonists, tropisetron, SDZ 205–557 and GR 113808 antagonized the relaxation response to 5‐HT (in the presence of methysergide (1 or 10 μ m ) and atropine (0.1 μ m )) with p K B 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 p K B values estimations for tropisetron (1 μ m ) 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 μ m ), SDZ 205–557 (3 μ m ) 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 μ m ), ondansetron (1 μ m ) or Nω‐nitro‐ l ‐arginine methyl ester (100 μ m ) and with only a twofold dextral shift of the concentration‐response curve by tetrodotoxin (1 μ m ). 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‐HT 4 receptor antagonists tropisetron, SDZ 205–557 and GR 113808 indicate a 5‐HT 4 receptor involvement in the relaxation response.