Effects of mosapride citrate,a 5-HT4 receptor agonist,on colonic motility in conscious guinea pigs

It is known that 5-HT(4) receptors in the colon of guinea pigs show a distribution similar to that in humans. Thus, we examined the effects of mosapride citrate (mosapride) and cisapride, two 5-HT(4)-receptor agonists, on colonic motility in conscious guinea pigs implanted with force transducers. Mosapride and cisapride administered intragastrically at doses of 3 - 30 mg/kg significantly enhanced the colonic motility. The enhancing effect of mosapride was antagonized by atropine or GR113808, a 5-HT(4)-receptor antagonist, but not by methysergide, a 5-HT(1)- and 5-HT(2)-receptor antagonist; ondansetron, a 5-HT(3)-receptor antagonist; or CP-99994, a tachykinin NK(1)-receptor antagonist. In vitro receptor autoradiography showed that mosapride and cisapride inhibit the specific binding of [(125)I]-SB207710, a selective radioligand of 5-HT(4) receptors, in the colon of guinea pigs. These results suggest that mosapride enhances colonic motility through the 5-HT(4)-receptor activation in guinea pigs and may be useful for treating constipation in patients with colonic motility dysfunction.     

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

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. In the presence of methysergide (60 microM), 5-HT induced relaxation of methacholine (1 microM)-precontracted muscle strips, yielding a monophasic sigmoidal concentration-relaxation curve (pEC50 7.2+/-0.07). Tetrodotoxin (0.3 microM) did not affect the curve to 5-HT, suggesting the inhibitory 5-HT receptor is located on the smooth muscle. Granisetron (0.3 microM) 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. 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 > or = SDZ HTF-919 > 5-MeOT. 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 pA2 10.6). GR 113808 (10 nM) produced a parallel rightward shift of the curve to the selective 5-HT4 receptor agonists R076186 (pA2 8.8). It is concluded that 5-HT induces relaxation of the canine rectum circular muscle through stimulation of a single population of smooth muscle 5-HT4 receptors. For the first time, a nonhuman species was shown to exhibit relaxant 5-HT4 receptors in the large intestine.     

Lysophosphatidic acid presynaptically blocks NO uptake during electric field stimulation-induced relaxation via LPA(1) receptor in cat lower esophageal sphincter

Lysophosphatidic acid (LPA) is a lipid mediator that is involved in many biological responses, such as, in the stimulation/inhibition of proliferation, in cell migration, antiapoptosis, tumor cell invasion, platelet aggregation, vascular remodeling, and neurotransmitter release. In addition, LPA indirectly enhances the contractility of smooth muscle. Furthermore, electric field stimulation (EFS) causes contractions of isolated cat esophageal smooth muscle and relaxations of isolated cat lower esophageal sphincter (LES). To test whether or not LPA enhances postsynaptically-mediated contraction in cat esophageal smooth muscle and LES, both types of muscle strips were stimulated with muscarinic agonists. However, no significant effects were observed, and therefore, to investigate whether LPA is involved in presynaptic signal transduction, cat esophageal smooth muscle and LES were pretreated with LPA and stimulated using EFS. LPA had no effect on EFS-induced contraction in esophageal smooth muscle but the EFSinduced LES relaxation was dose-dependently inhibited by LPA. To identify the LPA receptor subtype that inhibits EFS-induced LES relaxation, we used the specific LPA₁/LPA₃ antagonist Ki16425 and the LPA₃ agonist OMPT. Ki16425 significantly blocked the inhibitory effect of LPA on EFS-induced relaxation, but OMPT did not enhance the effect of LPA. These results suggest that LPA inhibits EFS-induced relaxation in LES via LPA₁ receptor-mediated signaling. It is well known that EFS-induced LES relaxation is related to the release of neurotransmitters, such as, nitric oxide, vasoactive intestinal polypeptide, and calcitonin gene-related peptide. We then investigated whether LPA selectively blocks NO-mediated signaling. Sodium nitroprusside-induced LES relaxation was found to be inhibited in the same manner as EFS-induced LES relaxation by LPA. This result suggests that LPA partially blocks NO uptake by presynaptic pathways, and thus, inhibits LES relaxation.     

Pharmacological characterization of the 5-HT receptors mediating contraction and relaxation of canine isolated proximal stomach smooth muscle

1. We aimed to characterize 5-HT receptors mediating contraction and relaxation to 5-HT in dog proximal stomach longitudinal muscle (LM) strips. 2. Of the tryptamine analogues tested, 5-HT was the most potent contractile agent at basal length, while 5-CT was the most potent relaxant of PGF(2alpha)-induced contraction. Neither the contractions to 5-HT, nor the relaxations to 5-CT were influenced by tetrodotoxin, illustrating that action potential propagation is not involved. 3. The 5-HT-induced contraction was antagonized by mesulergine (0.03 to 0.3 microM) and ketanserin (2 - 20 nM), but the antagonism was not of a simple competitive nature, indicating multiple receptor involvement. Ketanserin (3 to 30 nM) and mesulergine (30 nM) competitively antagonized the alpha-Me-5-HT-induced contraction (pK(B): 8.83+/-0.09 and pA(2): 8.25+/-0.06 respectively). These affinity values are in line with literature affinities of ketanserin and mesulergine at 5-HT(2A) receptors in various bioassays. 4. The 5-CT-induced inhibition of PGF(2alpha)-induced contraction was competitively antagonized by mesulergine (pK(B) estimate: 8.52+/-0.12) and by the selective 5-HT(7) receptor antagonist SB-269970 (pK(B) estimate: 9.36+/-0.14). Both pK(B) estimates are in line with literature affinities of these compounds for 5-HT(7) receptors. Mesulergine (30 nM) and SB-269970 (10 nM) shifted the relaxant curve to 5-HT parallel to the right in the presence of ketanserin (0.3 microM) (pA(2) estimates of 8.08+/-0.10 and 8.75+/-0.14 respectively), indicative of 5-HT(7) receptor involvement. 5. It is concluded that 5-HT induces dog proximal stomach (LM) contraction via smooth muscle 5-HT(2A) receptors and relaxation via smooth muscle 5-HT(7) receptors.     

Pharmacological effects of the gastroprokinetic agent mosapride citrate]

Mosapride citrate (mosapride) is a novel gastroprokinetic agent that enhances the gastrointestinal motility by stimulating the 5-hydroxytryptamine4 (5-HT4) receptor. Mosapride dose-dependently enhanced the gastric emptying of a liquid or solid meal in rats with a potency equal to that of cisapride and more potent than that of metoclopramide. In rats, mosapride improved the gastric emptying delayed by gastroduodenal surgical intervention. In the conscious dogs with force transducers implanted chronically, mosapride stimulated antral and duodenal motility with a potency equal to those of cisapride. In isolated guinea-pig ileal longitudinal muscle preparations, mosapride enhanced the electrically stimulated contractions, and the enhancing effect of mosapride was antagonized by a high dose of tropisetron, a 5-HT4-receptor antagonist. In addition, mosapride inhibited [3H]-GR-113808 binding to 5-HT4 receptor sites of guinea-pig ileum and striatum. Mosapride had no affinity for dopamine D2 receptor, whereas metoclopramide and cisapride had a high affinity for dopamine D2 receptor. In isolated guinea-pig papillary muscles, mosapride did not prolong the duration of action potentials, whereas cisapride concentration-relatedly prolonged it. In conclusion, mosapride is a selective and potent 5-HT4 receptor agonist and improves gastrointestinal symptoms in patients with non-ulcer dyspepsia without causing the extrapyraminal syndrome associated with dopamine-D2-receptor blockage and adverse cardiovascular effects such as torsadoes de points.     

The in vitro pharmacological profile of TD-5108, a selective 5-HT4 receptor agonist with high intrinsic activity

The in vitro pharmacological profile of TD-5108, a novel, selective 5-HT(4) receptor agonist, was compared to that of clinically efficacious gastroprokinetic 5-HT(4) receptor agonists. TD-5108 produced an elevation of cyclic adenosine monophosphate in human embryonic kidney 293 cells expressing the human recombinant 5-HT(4(c)) (h5-HT(4(c))) receptor (pEC(50) = 8.3) and 5-HT(4) receptor-mediated relaxation of the rat esophagus (pEC(50) = 7.9) and contraction of the guinea pig colon (pEC(50) = 7.9). In all in vitro assays, TD-5108 was a high intrinsic activity agonist, unlike tegaserod, mosapride, and cisapride which, in the majority of test systems, had lower intrinsic activity. TD-5108 had high affinity (pK (i) = 7.7) and selectivity (>/=25-fold) for h5-HT(4(c)) receptors over other biogenic amine receptors. TD-5108 was >500-fold selective over other 5-HT receptors (including h5-HT(2B) and h5-HT(3A)) and, at 3 muM, had no effect on human ether-à-go-go-related gene K(+) channels. In conclusion, TD-5108 is a selective 5-HT(4) receptor agonist in vitro. The high intrinsic activity and preferential binding of TD-5108 to 5-HT(4) over other 5-HT receptors may result in an improved clinical profile for the treatment of gastrointestinal disorders of reduced motility.     

Involvement of Nitric Oxide in Fading of 5-Hydroxytryptamine-Induced Vasocontraction

Abstract: This investigation was done to determine whether vascular nitric oxide is involved in the fading of 5-hydroxytryptamine (5-HT)-induced contraction of isolated aortic strips from rats. The 5-HT (100 μM)-induced contractile response showed a gradual decrease in tension after a plateau level had been attained. The degree of the fadeaway was less in strips without endothelium than in those with endothelium. Pretreatment with nitro-L-arginine (100 μM) or haemoglobin (10 μM) suppressed the fadeaway of 5-HT contraction in both types of strips, although the effect was less in strips without endothelium. In contrast to 5-HT contraction, phenylephrine (10 μM) contraction in the presence of endothelium did not begin fading away for at least 1 hr after stimulation. In strips precontracted with 5-HT (100 μM), relaxation induced by sodium nitroprusside (1-50 nM) was significantly higher than in those precontracted with phenylephrine (10 μM). There was no significant difference between the cyclic GMP levels at 1 hr after stimulation with phenylephrine (10 μM) and 5-HT (100 μM). These results suggest that decrease in vascular tone in rat aorta after the plateau of 5-HT contraction has been reached is in part due to nitric oxide derived from vascular endothelial and non-endothelial cells. The vascular tone decreases more quickly after the plateau in the case of 5-HT contraction than of phenylephrine contraction, which may be due to higher sensitivity of 5-HT contraction to nitric oxide.     

Characteristics of the actions by which 5-HT affects electrical and mechanical activities in rabbit jugular vein

BACKGROUND AND PURPOSE

5-HT is known to be a potent vasospasmogenic agonist in various arteries. However, in veins the vasomodulating actions of 5-HT, and the underlying mechanisms, remain to be fully clarified. Here, we characterized the actions by which 5-HT affects electrical and mechanical activities in the rabbit jugular vein.

EXPERIMENTAL APPROACH

Membrane potential and isometric tension were measured in endothelium-intact and -denuded preparations. Localization of 5-HT receptor subtypes was examined immunohistochemically.

KEY RESULTS

5-HT induced a transient then a small, sustained smooth muscle cell hyperpolarization in endothelium-intact strips. In endothelium-denuded strips, 5-HT induced only a sustained hyperpolarization, and this was changed to a depolarization by the selective 5-HT₇ receptor inhibitor SB269970. This depolarization was inhibited by the 5-HT_2A receptor blocker sarpogrelate. 5-HT induced a relaxation of PGF_2α-induced contracted strips that was similar in endothelium-intact and -denuded preparations. The latter relaxation was changed to contraction by SB269970 and this contraction was inhibited by sarpogrelate. Immunoreactive responses against endothelial and smooth muscle 5-HT_2A receptors and smooth muscle 5-HT₇ receptors were identified in the vein. The 5-HT-induced relaxation of the PGF_2α contraction was inhibited by the cAMP-dependent protein kinase inhibitor Rp-cAMPS and by the AC inhibitor SQ22536.

CONCLUSIONS AND IMPLICATIONS

These results indicate that 5-HT activates both smooth muscle 5-HT₇ receptors (to produce relaxation) and smooth muscle 5-HT_2A receptors (to produce contraction) in rabbit jugular vein. We suggest that in this particular vein, the 5-HT_2A receptor-induced depolarization and contraction are masked by the 5-HT₇ receptor-induced responses, possibly via actions mediated by cAMP.     

Correlations between pharmacological responses and structure of human lung parenchyma strips

Correlations were sought between responses of human lung parenchyma strip to 5-hydroxytryptamine (5-HT) and (-)-noradrenaline (NA) and the proportions of the three major, potentially contractile components within the strip, namely smooth muscle in airways proximal to alveolar ducts, vascular smooth muscle and contractile cells in alveolar septa. After the isometric measurement of responses to 5-HT or to NA, lung strips were processed for stereological examination at the light microscopic level. On average, approximately 46% of the total volume of the lung strip was tissue and the remainder was air space. Tissue contained blood vessels (16.8%), airways proximal to alveolar ducts (4.8%) and alveolar parenchyma (78.4%). Human lung parenchyma strips relaxed, contracted or failed to respond to 5-HT or NA. Results indicated that these agonists caused simultaneous contraction of blood vessels and relaxation of airways proximal to alveolar ducts. The size and type of responses to 5-HT or NA was significantly correlated with the ratio of the volume of blood vessels and larger airways. Conversely, the proportion of alveolar tissue in lung strips was not significantly correlated with responses to 5-HT or NA.     

Measurement of 5-HT4 receptor-mediated esophageal responses by digital sonomicrometry in the anesthetized rat

INTRODUCTION: In vitro studies have demonstrated a 5-HT4 receptor-mediated relaxation of the pre-contracted rat esophagus. However, it is unclear whether 5-HT4 receptor agonists affect resting esophageal tone in vivo. The activity of 5-HT and several well-established 5-HT4 receptor agonists (tegaserod, BIMU-8, cisapride, renzapride, and mosapride) was investigated in a novel in vivo model designed to measure esophageal relaxation using the technique of digital sonomicrometry. METHODS: Miniature piezo-electric crystals were implanted externally in a longitudinal orientation on the distal esophagus of isoflurane-anesthetized, adult male Sprague-Dawley rats. Measurement of the time for transmission of ultrasonic pulses between the implanted crystals provided a continuous recording of inter-crystal distance and hence esophageal muscle length. RESULTS: Following cumulative intravenous administration, 5-HT (1-100 microg/kg), tegaserod (1-1000 microg/kg), BIMU-8 (3-3000 microg/kg), renzapride (10-3000 microg/kg), cisapride (30-3000 microg/kg), and mosapride (30-10,000 microg/kg) produced a dose-dependent increase in esophageal inter-crystal distance. The mean ED50 values for tegaserod, BIMU-8, renzapride, cisapride, and mosapride were 11, 49, 51, 141, and 1825 microg/kg, respectively. Pre-treatment with the selective 5-HT4 receptor antagonist, piboserod (SB-207266; 1 mg/kg subcutaneously) significantly attenuated the effects of intravenous tegaserod (1-1000 microg/kg). Following cumulative intraduodenal administration (0.03-10 mg/kg), tegaserod and mosapride exhibited a dose-dependent increase in esophageal inter-crystal distance. The doses associated with a 10% increase in muscle length from the resting level were 2.6 and>10 mg/kg for tegaserod and mosapride, respectively. DISCUSSION: In conclusion, dose-dependent, 5-HT4 receptor agonist-mediated increases in longitudinal muscle length in the rat esophagus were observed in vivo using the technique of digital sonomicrometry. This in vivo model of esophageal activity may prove useful in evaluating the activity of novel 5-HT4 receptor agonists.     

Pharmacological properties of a novel gastrointestinal prokinetic benzamide selective for human 5-HT4 receptor versus human 5-HT3 receptor.

Binding properties of gastrointestinal prokinetic benzamides for both cloned human 5-hydroxytryptamine (5-HT)3 receptors and cloned human 5-HT4 receptors were examined and pharmacological properties of YM-53389{(+)-(S)-2-chloro-5-methoxy-4-[5-(2-piperidylmethyl)-1,2, 4-oxadiazol-3-yl]aniline monohydrochloride} were characterised in animals. Cisapride, renzapride and zacopride inhibited specific binding of [3H]ramosetron to cloned human 5-HT3 receptors, with Ki values of 684, 7.64 and 0.38 n m, respectively. YM-53389, however, slightly replaced that (Ki>10,000 n m). YM-53389, cisapride, renzapride and zacopride replaced specific binding of [3H]GR 113808 to cloned human 5-HT4 receptors, with Ki values of 54.6, 41.5, 115 and 373 n m, respectively. The potency for inhibitory effect of YM-53389 on 5-HT3 receptor-mediated contraction in the guinea-pig isolated colon was very low with pIC50 of 4.7. YM-53389 exerted 5-HT4 receptor-mediated relaxation in the carbachol-precontracted rat isolated oesophagus with pEC50 of 6.3. In mice, YM-53389 at 10 and 30 mg kg-1, s.c. significantly shortened whole gut transit time, in contrast to cisapride, renzapride and zacopride which were reported to delay that. YM-53389 had no significant effect on upper gastrointestinal propulsion at doses up to 30 mg kg-1, s.c. Based on these results, YM-53389 may surpass existing benzamides in facilitating lower intestinal propulsion and benefit patients with gastrointestinal disorders associated with impair of intestinal propulsion, such as constipation, based on the selective interaction with human 5-HT4 receptors vs human 5-HT3 receptors.     

Investigation into the 5-hydroxytryptamine-induced atropine-resistant neurogenic contraction of guinea-pig proximal colon.

1. The aim of this study was to characterize the receptors mediating the atropine-resistant neurogenic contraction to 5-hydroxytryptamine (5-HT) in the longitudinal muscle of the guinea-pig proximal colon and to determine the type of tachykinin receptors involved in the contractile response to 5-HT by the use of selective antagonists. 2. In the presence of atropine (0.3 microM), guanethidine (5 microM), hexamethonium (100 microM), ketanserin (0.1 microM) and indomethacin (3 microM), 5-HT (0.01-3 microM) produced concentration-dependent neurogenic contractions of colonic strips and at 0.3 microM produced a maximal effect (pEC50 = 7.39 +/- 0.09, n = 18). The 5-HT4 receptor stimulant, 5-methoxytryptamine (5-MeOT, 0.03-10 microM) also produced neurogenic contractions with similar maximum effect to those of 5-HT (pEC50 = 6.89 +/- 0.16). 3. The 5-HT4 receptor antagonist, DAU 6285 (3 microM) shifted the concentration-response curves to both 5-HT and 5-MeOT to the right without significant depression of the maximum, but the 5-HT1/5-HT2 receptor antagonist, metitepine (0.1 microM) and the 5-HT3 receptor antagonist, ondansetron (0.3 microM) had no effect on the control curves to 5-HT and 5-MeOT. 4. The selective NK1 receptor antagonist, FK 888 (1 microM) markedly attenuated the contractions to 5-HT and 5-MeOT. In contrast, the selective NK2 receptor antagonist, SR 48968 (10 nM) and the selective NK3 receptor antagonist, SR 142801 (10 nM) had no effect on the contractions to 5-HT and 5-MeOT. 5. These results indicate that the 5-HT-induced atropine-resistant neurogenic contraction of guinea-pig proximal colon is due to activation of 5-HT4 receptors, presumably located on excitatory motor neurones, innervating the longitudinal muscle. The contraction evoked by activation of the 5-HT4 receptors is mediated primarily via NK1 receptors but not NK2 or NK3, suggesting that the 5-HT4 receptor-mediated contraction is evoked indirectly via tachykinin release from tachykinin-releasing excitatory neurones.     

4-氨基吡啶对鼠胃肠道功能的影响

目的观察钾通道阻滞剂4氨基吡啶(4AP)对于鼠胃肠道功能的影响。方法水溶性炭末法检测小鼠胃肠蠕动;多通道生理记录仪记录大鼠胃肠纵形肌条张力参数;胃酸分泌实验检测大鼠胃酸分泌。结果4AP(5mg·kg-1,ig)抑制小鼠胃肠推进,4AP(5mmol·L-1)能增加大鼠十二指肠最大收缩力和最小舒张力,使收缩幅度减小、频率降低;4AP(2.5mg·kg-1,ip)促进大鼠胃酸分泌。结论4AP对鼠类胃肠运动功能有抑制作用,对大鼠胃酸分泌功能有促进作用。     

Prokinetic benzamides stimulate peristaltic activity in the isolated guinea pig ileum by activation of 5-HT4 receptors.

Substituted benzamides such as metoclopramide, cisapride, zacopride, renzapride or BRL 20627, stimulate intestinal motility in various species. As they are antagonists at 5-HT3 and agonists at 5-HT4 receptors and as both mechanisms could potentially contribute to their gastrointestinal prokinetic effect, the underlying mechanism is unclear. To clarify this, the effect of some substituted benzamides on gut motility was investigated in the isolated guinea pig ileum using the Trendelenburg technique, in which pressure-induced peristaltic contractions are measured. All benzamides stimulated the peristaltic reflex with the rank order of potency: renzapride greater than cisapride greater than BRL 20627 greater than (+/-)-zacopride greater than metoclopramide. ICS 205-930, granisetron and 2-methyl-5-HT did not change the peristaltic response. 5-HT and 5-methoxytryptamine potently mimicked the effect of the benzamides. The effect of 5-HT was not blocked by ICS 205-930 (10(-7) M). These results indicate that the Trendelenburg preparation is suitable for the investigation of intestinal prokinetic effects of the substituted benzamides. Furthermore, the results suggest that the intestinal effect of benzamides results from activation of 5-HT4 receptors rather than from blockade of 5-HT3 receptors.     

Pharmacological characterization of 5-hydroxytryptamine-induced contraction in the chicken gastrointestinal tract

5-Hydroxytryptamine (5-HT) receptor subtypes involved in 5-HT-induced contraction of the chicken gastrointestinal tract were characterized pharmacologically using subtype-selective agonists and antagonists. The proventriculus (area of stomach adjacent to the oesophagus) and ileum are examined. 5-HT applied cumulatively caused sustained contraction of the proventriculus that was not decreased by tetrodotoxin, atropine or l-nitro-arginine methylester (l-NAME). alpha-Methyl-5-HT showed the same potency as that of 5-HT, indicating the involvement of the 5-HT(2) receptor. (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane (DOI), 5-methoxytryptamine and 1-(3-chlorophenyl)piperazine hydrochloride (mCPP) were potent, and 2-methyl-5-HT, 5-carboxamidotryptamine, BW723C86 and 6-chloro-2-(1-piperazinyl)pyrazine hydrochloride (MK212) were moderate, but (+/-)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT), [endo-N-8-methyl-8-azabicyclo-(3,2,1)oct-3-yl]-2,3-dihydro-(1-methyl)ethyl-2-oxo-1H-benzimidazol-1-carboxamide (BIMU-8) and cisapride were weak agonists. Correlation of pEC(50) values of these agonists with documented pEC(50) values for 5-HT(2C) receptor was higher than 5-HT(2A) and 5-HT(2B). Cinanserin, ketanserin, methiothepin, methysergide, mianserin, (8-[5-(2,4-dimethoxy-5-(4-trifluoromethylphenylsulphonamido)phenyl-5-oxopentyl)-1,3,8-triazaspiro[4,5]decane-2,4-dione hydrochloride (RS102221), N-(1-methyl-1H-indolyl-5-yl)-N'-(3-methyl-5-isothiazolyl)urea (SB204741), spiperone and N-desmethylclozapine concentration-dependently inhibited the contractile responses to 5-HT. Correlation of pK(b)/pA(2) of antagonists with documented pK(i) for 5-HT(2C) receptor was highest among 5-HT(2) receptor subtypes. In the methysergide- and ketanserin-treated proventriculus, 5-HT, 2-methyl-5-HT and cisapride did not enhance the electrical field stimulation (5 Hz)-induced cholinergic contractions. 5-HT applied non-cumulatively caused transient contraction of ileum, and the responses were partly decreased by atropine or tetrodotoxin. 5-Methoxytryptamine, alpha-methyl-5-HT, 5-carboxamidotryptamine, L692,247 and DOI were potent agonists. However, 2-methyl-5-HT, cisapride, BW723C86, 8-OH-DPAT and 5-nonyloxytryptamine, mCPP and MK212 were less effective. Ketanserin and methysergide decreased the 5-HT-induced ileal contraction, but neither GR113808 nor SB269970 inhibited the responses. In conclusion, 5-HT causes contraction of the proventriculus via 5-HT(2C)-like receptors present on smooth muscle. 5-HT also causes contraction of the ileum, but the underlying mechanisms are complex, involving neural and smooth muscle components, and both 5-HT(1)- and 5-HT(2)-like receptors. Neural 5-HT receptors similar to 5-HT(3)/5-HT(4) receptors were not found in the chicken proventriculus and ileum.     

Smooth muscle 5-HT2A receptors mediating contraction of porcine isolated proximal stomach strips

1. The aim of this study was to characterize the 5-HT receptors involved in the 5-HT-induced contraction of longitudinal muscle (LM) strips of porcine proximal stomach. This was done in a classical organ bath set-up for isotonic measurement. 2. The concentration-contraction curve to 5-HT was not modified by 5-HT(3) and 5-HT(4) receptor antagonism. Methysergide, ketanserin and mesulergine antagonized the curve to 5-HT. Concomitantly, increasing concentrations of ketanserin and mesulergine progressively revealed a biphasic nature of the 5-HT curve. Ketanserin antagonized the low-affinity receptor while it did not modify the high-affinity receptor. 3. Tetrodotoxin did not influence the concentration-contraction curve to 5-HT neither in the absence nor presence of ketanserin, indicating that nerves are not involved. 4. Ketanserin competitively antagonized the monophasic concentration-response curve to alpha-Methyl-5-HT, yielding a Schild slope that was not significantly different from unity. After constraining the Schild slope to unity, a pK(B) estimate of 8.23+/-0.90 was obtained. This affinity estimate of ketanserin closely approximates previously reported affinities at 5-HT(2A) receptors. 5. In the presence of ketanserin (0.1 microM; exposing the high-affinity receptor), a wide range of 5-HT receptor antagonists covering all 5-HT receptors known, was tested. Only methysergide and ritanserin inhibited the response to 5-HT, thus expressing affinity for the high-affinity receptor. This did not reveal the identity of the receptor involved. 6 It can be concluded that 5-HT induces pig proximal stomach (LM) contraction via 5-HT(2A) receptors located on smooth muscle. A ketanserin-insensitive phase of contractions could not be characterized between the actually known classes of 5-HT receptors with the pharmacological tools that were used.     

New insights into the treatment of gastroesophageal reflux disease

Gastroesophageal reflux disease (GERD) is a chronic relapsing disorder commonly encountered in the primary care setting. Considerable uncertainty still exists with respect to basic questions concerning pathophysiological factors and the cause of extraoesophageal symptoms in some patients with GERD. The numerous receptors located in the muscular layer at the site of the lower oesophageal sphincter (LES) are under neurological control. These receptors are of interest in the investigation of both LES dysfunction and new potential pharmacological approaches for GERD. Therapies for GERD, based on various pathophysiological mechanisms, with attention to either drug inhibition of acid secretion, protection of the oesophageal mucosa, influencing salivary composition or improving oesophageal clearance, have been reviewed. Alternative approaches to the more traditional treatments for gastroesophageal reflux disease are also being pursued. In particular, pharmacological efforts towards modification of intrinsic and extrinsic neurological controls of oesophageal motility, aimed at improving LES function, are discussed.     

Heat stress alters G-protein coupled receptor-mediated function and endothelium-dependent relaxation in rat mesenteric artery

Heat stress has been demonstrated to have strong cardiovascular effects. However, the underlying mechanism-mediated cardiovascular effects are still not fully understood. The present study was designed to examine if heat stress alters vascular G-protein coupled receptor-mediated vasomotion and endothelium function in rat mesenteric artery. Rats were divided into two groups, heat stress rats and control. The G-protein coupled receptors of endothelin type B (ETB) receptor-, endothelin type A (ETA) receptor-, 5-hydroxytryptamine (5-HT) receptor-, calcitonin gene-related peptide (CGRP) receptor-, alpha-adrenoceptor-mediated vosoactivity and endothelium-dependent relaxation on rat mesenteric artery ring segments were monitored by a myograph system. The plasma level of CGRP was determined by radioimmunological assay. Compared with control arterial segments, the contractile response curves of sarafotoxin 6c, a selective ETB receptor agonist and 5-HT in the arterial segments from heat stress rats were shifted towards left. An increased maximum contraction (Emax) induced by sarafotoxin 6c, but not 5-HT, was seen in the arterial segments from heat stress rats. CGRP-induced relaxation in endothelium-denuded arterial segments from heat stress rats was enhanced. The relaxation in endothelium-intact arterial segments induced by acetylcholine was significantly decreased in heat stress rats. In addition, the plasma concentration of CGRP was increased in heat stress rats. The endothelium-dependent relaxation was characterized and shown there was a decrease in nitric oxide and endothelium-derived hyperpolarizing factor-mediated relaxation in the arterial segments from heat stress rats. In conclusion, heat stress induces an enhanced vascular endothelin ETB-, 5-HT-receptors-mediated contraction, an enhanced CGRP-receptor-induced relaxation and damage to endothelium-dependent relaxation.     

Colon-specific contractile responses to tetrodotoxin in the isolated mouse gastrointestinal tract

1?Tetrodotoxin (TTX) is a useful pharmacological tool for distinguishing neural and myogenic responses of isolated visceral organs to drugs. Although TTX does not generally affect smooth muscle tonus, in this study, we have found that TTX causes contraction of the mouse colon. The aim of this study was to characterize this TTX-induced contraction in the mouse gastrointestinal tract. 2?Longitudinal and circular muscle strips from the stomach and small intestine were less sensitive to TTX. However, TTX contracted both smooth muscle strips from the proximal colon and distal colon. 3?Pretreatment with TTX, Nω -nitro-L-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and apamin inhibited the TTX-induced contraction. L-NAME, ODQ or apamin itself caused contraction in the colon but not in the gastric and small intestinal strips. Region dependency of L-NAME, ODQ and apamin-induced contraction correlated with that of TTX-induced contraction. 4?L-arginine but not D-arginine inhibited contractility of the colonic strips without affecting the contractility of muscle strips from other regions. Sodium nitroprusside caused strong relaxation of the colonic strips. 5?1,1-dimethyl-4-phenylpiperazinium (DMPP) caused relaxation of proximal and distal colons, which was significantly decreased by L-NAME or apamin. 6?In conclusion, among mouse gastrointestinal preparations, TTX induces contraction of colonic strips preferentially through blockade of potent tonic inhibitory neural outflow, which involves nitrergic and apamin-sensitive pathways. Colon-specific responses to L-arginine, L-NAME, ODQ and apamin support the hypothesis that there is a continuous suppression of colonic motility by enteric inhibitory neurons.     

The 5-HT4 receptor agonist, tegaserod, is a potent 5-HT2B receptor antagonist in vitro and in vivo

1 Tegaserod (Zelnorm) is a potent 5-hydroxytryptamine4 (5-HT4) receptor agonist with clinical efficacy in disorders associated with reduced gastrointestinal motility and transit. The present study investigated the interaction of tegaserod with 5-HT2 receptors, and compared its potency in this respect to its 5-HT4 receptor agonist activity. 2 Tegaserod had significant binding affinity for human recombinant 5-HT2A, 5-HT2B and 5-HT2C receptors (pKi=7.5, 8.4 and 7.0, respectively). The 5-HT2B receptor-binding affinity of tegaserod was identical to that at human recombinant 5-HT4(c) receptors (mean pKi=8.4) in human embryonic kidney-293 (HEK-293) cells stably transfected with the human 5-HT4(c) receptor. 3 Tegaserod (0.1-3 microm) inhibited 5-HT-mediated contraction of the rat isolated stomach fundus potently (pA2=8.3), consistent with 5-HT(2B) receptor antagonist activity. Tegaserod produced, with similar potency, an elevation of adenosine 3',5' cyclic monophosphate in HEK-293 cells stably transfected with the human 5-HT4(c) receptor (mean pEC50=8.6), as well as 5-HT4) receptor-mediated relaxation of the rat isolated oesophagus (mean pEC50=8.2) and contraction of the guinea-pig isolated colon (mean pEC50=8.3). 4 Following subcutaneous administration, tegaserod (0.3 or 1 mg kg(-1)) inhibited contractions of the stomach fundus in anaesthetized rats in response to intravenous dosing of alpha-methyl 5-HT (0.03 mg kg(-1)) and BW 723C86 (0.3 mg kg(-1)), selective 5-HT2B receptor agonists. At similar doses, tegaserod (1 and 3 mg kg(-1) subcutaneously) evoked a 5-HT4 receptor-mediated increase in colonic transit in conscious guinea-pigs. 5 The data from this study indicate that tegaserod antagonizes 5-HT2B receptors at concentrations similar to those that activate 5-HT4 receptors. It remains to be determined whether this 5-HT2B receptor antagonist activity of tegaserod contributes to its clinical profile.