Pharmacological characterization of 5-HT4 receptors mediating relaxation of canine isolated rectum circular smooth muscle

1 This study aimed to characterize for the first time in vitro 5-HT4 receptors in the canine gastrointestinal tract. For this purpose, we used circular muscle strips of the canine isolated rectum. 2 In the presence of methysergide (60 mu M), 5-HT induced relaxation of methacholine (1 mu M)precontracted muscle strips, yielding a monophasic sigmoidal concentration-relaxation curve (pEC(50) 7.2+/-0.07). 3 Tetrodotoxin (0.3 mu M) did not affect the curve to 5-HT, suggesting the inhibitory 5-HT receptor is located on the smooth muscle. Granisetron (0.3 mu M) did also not affect the curve to 5-HT, which excludes the 5-HT3 receptor mediating the relaxation to 5-HT. The presence of methysergide rules out the involvement of 5-HT1, 5-HT2 or 5-HT7 receptors. 4 5-HT, the selective 5-HT4 receptor agonists R076186, prucalopride (R093877) and SDZ HTF-919 and the 5-HT4 receptor agonists cisapride and 5-MeOT relaxed the muscle strips with a rank order of potency R076186 = 5-HT > cisapride > prucalopride greater than or equal to SDZ HTF-919 > 5-MeOT. 5 The selective 5-HT4 receptor antagonists GR 125487, RS 39604 and GR 113808 competitively antagonized the relaxations to 5-HT, yielding pK(B) estimates of 9.7, 7.9 and 9.1, respectively. The selective 5-HT4 receptor antagonist SB 204070 shifted the curve to 5-HT rightward and depressed the maximal response (apparent pA(2) 10.6). GR 113808 (10 nM) produced a parallel rightward shift of the curve to the selective 5-HT4 receptor agonists R076186 (pA(2) 8.8). 6 It is concluded that 5-HT induces relaxation of the canine rectum circular muscle through stimulation of a single population of smooth muscle 5-HT4 receptors. For the first time, a nonhuman species was shown to exhibit relaxant 5-HT4 receptors in the large intestine.