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Recently, it was demonstrated that 5‐HT induces relaxation of human colon circular muscle through activation of 5‐HT4receptors and 5‐HT7receptors. The aim of the current study was to develop a new in vitrobioassay of human colon that would facilitate the pharmacological analysis of 5‐HT responses mediated solely by 5‐HT4receptors.Contracting circular muscle strips with KCl (80 mM) yielded a stable contractile tension and, in contrast to muscarinic cholinoceptor agonists and histamine, a profound reduction of spontaneous contractility. This allowed the establishment of reproducible, fully‐defined, agonist concentration‐response curves by cumulative dosing. Under these conditions, 5‐HT induced a concentration‐dependent relaxation (pEC507.31, Hill slope 0.91).Neither methysergide (10 μM) nor granisetron (1 μM) affected the 5‐HT‐induced relaxation, suggesting that 5‐HT1, 5‐HT2, 5‐HT3, 5‐ht5, 5‐HT6or 5‐HT7receptors are not involved. The lack of effect of tetrodotoxin (0.3 μM) indicated a direct effect of 5‐HT on the smooth muscle.The selective 5‐HT4receptor antagonists GR 113808, GR 125487 and RS 39604 competitively antagonized the 5‐HT‐induced relaxation (pKB9.43, 10.12 and 8.53, respectively). SB 204070 (1 nM) produced a rightward shift (pA210.34) and depression of the 5‐HT curve. These affinity estimates are similar to those previously reported for 5‐HT4receptors.The selective 5‐HT4receptor agonists, prucalopride and R076186, induced relaxations (pEC507.50 and 7.57, respectively), that were blocked by GR 113808 (3 nM), yielding pA2estimates of 9.31 and 9.21, respectively.To summarise, in KCl (80 mM)‐contracted muscle strips, 5‐HT induces relaxation through activation of a homogeneous smooth muscle 5‐HT4receptor population. This new bioassay allows the focused, pharmacological characterization of human colonic 5‐HT4receptors in vitro.

This study aimed to characterize for the first time in vitro5‐HT4receptors in the canine gastrointestinal tract. For this purpose, we used circular muscle strips of the canine isolated rectum.In the presence of methysergide (60 μM), 5‐HT induced relaxation of methacholine (1 μM)‐precontracted muscle strips, yielding a monophasic sigmoidal concentration‐relaxation curve (pEC507.2±0.07).Tetrodotoxin (0.3 μM) did not affect the curve to 5‐HT, suggesting the inhibitory 5‐HT receptor is located on the smooth muscle. Granisetron (0.3 μM) did also not affect the curve to 5‐HT, which excludes the 5‐HT3receptor mediating the relaxation to 5‐HT. The presence of methysergide rules out the involvement of 5‐HT1, 5‐HT2or 5‐HT7receptors.5‐HT, the selective 5‐HT4receptor agonists R076186, prucalopride (R093877) and SDZ HTF‐919 and the 5‐HT4receptor agonists cisapride and 5‐MeOT relaxed the muscle strips with a rank order of potency R076186=5‐HT>cisapride>prucaloprideSDZ HTF‐919>5‐MeOT.The selective 5‐HT4receptor antagonists GR 125487, RS 39604 and GR 113808 competitively antagonized the relaxations to 5‐HT, yielding pKBestimates of 9.7, 7.9 and 9.1, respectively. The selective 5‐HT4receptor antagonist SB 204070 shifted the curve to 5‐HT rightward and depressed the maximal response (apparent pA210.6). GR 113808 (10 nM) produced a parallel rightward shift of the curve to the selective 5‐HT4receptor agonists R076186 (pA28.8).It is concluded that 5‐HT induces relaxation of the canine rectum circular muscle through stimulation of a single population of smooth muscle 5‐HT4receptors. For the first time, a non‐human species was shown to exhibit relaxant 5‐HT4receptors in the large intestine.

Neurotensin(8–13) analogues were biologically stabilised by replacement of the peptide bond between amino acids 8 and 9 by the reduced ψ(CH2-NH) isostere. Diethylenetriaminepentaacetic acid (DTPA) analogues for In-111 labelling and 2-bromo-phenyl-acetyl analogues for radioiodination, showed receptor affinities in the nanomolar range in combination with a biological half life in human plasma up to 275 minutes. Biodistribution studies in male Wistar rats of metabolically stabilised and non-stabilised ¹¹¹In-DTPA-NT(8–13) analogues showed a major clearance from the blood through the kidneys. ¹²⁵I-Labelled neurotensin (8–13) analogues showed accumulation up to 2.2% of the injected dose per g tissue in the liver which might be an important disadvantage when diagnosis of tumours in the gut is aimed. Neurotensin(8–13) analogues labelled with In-111 or I-123 may act as new potential peptidergic radiopharmaceuticals for SPET diagnosis of neurotensin receptor (NTR) positive tumours. Copyright © 1999 John Wiley & Sons, Ltd.

The 5‐hydroxytryptamine (5‐HT) receptor(s) that mediate(s) contraction of the rat ileum longitudinal muscle was studied.

In the guinea pig proximal colon, 5-hydroxytryptamine (5-HT) stimulates neuronal 5-HT1-like receptors to induce relaxations that are mediated by nitric oxide and ATP. In the current study, the effects of cisapride and structural analogs on these 5-HT-induced relaxations were investigated. In the continuous presence of ketanserin (0.3 microM) and tropisetron (3 microM) to block contractions via 5-HT2A, 5-HT3 and 5-HT4 receptors, 5-HT induced relaxations that yielded a biphasic concentration-response curve. Cisapride (0.1-1 microM) enhanced the second phase of the concentration-response curve to 5-HT by about 20% to 40%, whereas from 0.3 microM onwards, it inhibited the first phase. Also in the presence of cisapride (0.3 microM), tetrodotoxin (0.3 microM) abolished the relaxations to 5-HT. Cisapride (0.3 microM) did not affect the concentration-response curves to isoprenaline, nitroglycerin, nitroprusside or exogenous ATP, which demonstrated its specificity. The 5-HT relaxation-enhancing effects of cisapride were not mimicked by phentolamine (1 microM), NAN-190 (0.03 microM), spiperone (1 microM), citalopram (0.3 microM), paroxetine (0.3 microM), pargyline (100 microM) or SDZ 205-557 (0.3 microM). In the presence of the inhibitor of nitric oxide synthesis, NG-nitro-L-arginine (100 microM), cisapride (0.3 microM) still enhanced the remaining relaxations to 5-HT (2-3-fold). However, in the presence of the P2-purinoceptor antagonist suramin (300 microM), cisapride did not enhance the relaxations to 5-HT. In the presence of NG-nitro-L-arginine, the cisapride-enhanced relaxations to 5-HT were inhibited by about 90% by suramin. We conclude that in the guinea pig colon, cisapride selectively facilitates the suramin-sensitive, ATP-mediated part of the relaxation to 5-HT via an unidentified effect on intramural nerves.