No abstract

1 5-HT 4 receptors mediate circular muscle relaxation in both human and canine large intestine, but this phenomenon alone can not explain the improvement in colonic motility induced by selective 5-HT 4 receptor agonists in vivo. We set out to characterize 5-HT 4 receptor-mediated e ects in longitudinal muscle strips of canine and human large intestine. 2 Electrical ®eld stimulation (EFS) was applied providing submaximal isotonic contractions. L-NOARG (0.1 mM) was continuously present in the organ bath to preclude nitric oxide-induced relaxation to EFS. 3 The selective 5-HT 4 receptor agonist prucalopride (0.3 mM) enhanced EFS-evoked contractions, that were antagonized in both preparations by the selective 5-HT 4 receptor antagonist GR 113808 (0.1 mM). The prucalopride-induced increase was present in canine ascending and descending colon, but absent in rectum. Regional di erences in response to prucalopride were not observed in human ascending and sigmoid colon and rectum. Incubation with atropine (1 mM) or tetrodotoxin (0.3 mM) inhibited EFS-induced contractions, which were then una ected by prucalopride (0.3 mM) in both tissues. 4 In the presence of methysergide (3 mM; both tissues) and granisetron (0.3 mM; only human tissues), 5-HT (0.3 mM) enhanced EFS-induced contractions, an e ect that was antagonized by GR 113808 (0.1 mM). In the presence of atropine or tetrodotoxin, EFS-induced contractions were inhibited, leaving 5-HT (0.3 mM) ine ective in both preparations. 5 This study demonstrates for the ®rst time that in human and canine large intestine, 5-HT 4 receptors are located on cholinergic neurones, presumably mediating facilitating release of acetylcholine, resulting in enhanced longitudinal muscle contractility. This study and previous circular muscle strip studies suggest that 5-HT 4 receptor agonism facilitates colonic propulsion via a coordinated combination of inhibition of circumferential resistance and enhancement of longitudinal muscle contractility.

The novel enterokinetic drug prucalopride was tested at various intravenous and oral doses in fasted dogs to assess: (i) the effects on colonic contractile motility patterns; and (ii) the mediation of these effects by 5-hydroxytryptamine (5-HT4) receptors. Colonic motility patterns were assessed in conscious dogs with four chronically implanted strain-gauge force transducers that were sutured on the serosal side of the colon. Prucalopride altered colonic contractile motility patterns in a dose-dependent fashion by stimulating high-amplitude clustered contractions in the proximal colon and by inhibiting contractile activity in the distal colon. Prucalopride was equipotent after oral and intravenous administration, as reflected by the values for the effective dose that induced 50% of maximum effect (95% confidence limits): 0.04 mg kg(-1) p.o. (0.01-0.1 mg kg(-1)) and 0.01 mg kg(-1) i.v. (0.006-0.04 mg kg(-1)). Prucalopride also caused a dose-dependent decrease in the time to the first giant migrating contraction (GMC); at higher doses of prucalopride, the first GMC generally occurred within the first half-hour after treatment. Subcutaneous pretreatment with the 5-HT4 receptor antagonist GR125487 (40 microg kg(-1) bodyweight) completely prevented the effects of orally administered prucalopride (0.31 mg kg(-1) bodyweight). Prucalopride, given orally or intravenously, alters colonic motility in the fasted conscious dog in a dose-dependent fashion. It induces GMCs and causes proximal colon stimulation and distal colon inhibition of contractile motility patterns by stimulating 5-HT4 receptors.

5-HT 4 receptors mediate relaxation of human colon circular muscle. However, after 5-HT 4 receptor blockade (SB 204070 10 nM), 5-HT still induced a relaxation (pEC 50 6.3). 5-HT 4 receptors were suciently blocked, as the curves to 5-HT obtained in the presence of 10 and 100 nM SB 204070 were indistinguishable. This 5-HT-induced relaxation was tetrodotoxin-insensitive, indicative of a smooth muscle relaxant 5-HT receptor. This, and the rank order of potency (5-CT=5-MeOT=5-HT) suggested involvement of 5-HT 1 or 5-HT 7 receptors. Mesulergine, a 5-HT 7 receptor antagonist at nanomolar concentrations, and a 5-HT 1 receptor antagonist at micromolar concentrations, competitively antagonized the 5-HT-induced relaxation (pK B 8.3) and antagonized the relaxation to 5-CT. Methysergide antagonized the 5-HT-induced relaxation (pA 2 7.6). It is concluded that the pro®le of the smooth muscle inhibitory 5-HT receptor resembles that of the 5-HT 7 receptor. These data provide the ®rst evidence for functional human 5-HT 7 receptors. Keywords: 5-HT 7 receptors; 5-HT; human; colon; circular muscle . However, the 5-HT-induced relaxation could not be ascribed to a homogeneous 5-HT 4 receptor population. The selective 5-HT 4 receptor antagonist GR113808 shifted the response curve to 5-HT rightward at nanomolar concentrations, but at higher concentrations, GR113808 did not shift the curve to 5-HT further, which was re¯ected in a decrease in pA 2 estimates with increasing concentrations of antagonist Tam et al., 1995). This suggests that the 5-HT-induced relaxation also involved a non-5-HT 4 receptor mechanism.Interestingly, we found that in the presence of 5-HT 4 receptor blockade (induced by SB 204070), 5-HT was still able to relax the carbachol-contracted tissue and in the current study we describe the characterization of this non-5-HT 4 receptor-mediated relaxation of human colonic circular muscle.