Inhibition of the effect of serotonin on rat ileal transport by cisapride: evidence in favour of the involvement of 5-HT2 receptors

Involvement of c-Src and protein kinase C delta in the inhibition of Cl(-)/OH- exchange activity in Caco-2 cells by serotonin

Serotonin (5-hydroxytryptamine (5-HT)) is an important neurotransmitter and intercellular messenger regulating various gastrointestinal functions, including electrolyte transport. To date, however, no information is available with respect to its effects on the human intestinal apical anion exchanger Cl(-)/OH- (HCO3-). The present studies were therefore undertaken to examine the direct effects of serotonin on OH- gradient-driven 4,4'-diisothiocyanato-stilbene-2, 2'-disulfonic acid-sensitive 36Cl- uptake utilizing the post-confluent transformed human intestinal epithelial cell line Caco-2. Our results demonstrate that serotonin inhibits Cl(-)/OH- exchange activity in Caco-2 cells via both tyrosine kinase and Ca(2+)-independent protein kinase C delta-mediated pathways involving either 5-HT3 or 5-HT4 receptor subtype. The data consistent with our inference are as follows. (i) The short term treatment of cells with 5-HT (0.1 microM) for 15-60 min significantly decreased Cl(-)/OH- exchange (50-70%, p < 0.05). (ii) The specific agonists for 5-HT3, m-chlorophenylbiguanide, and 5-HT4, 3-(4-allylpiperazin-1-yl)-2-quinoxaline chloronitrile, mimicked the effects of serotonin. (iii) Tropisetron dual inhibitor for both the 5-HT3/4 receptor subtypes significantly blocked the inhibition, whereas specific 5-HT3 (Y-25130) or 5-HT4 receptor (RS39604) antagonist failed to block the inhibitory effects of 5-HT. (iv) The Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl ester) had no effect on the serotonin-induced inhibition. (v) The specific protein kinase C (PKC) inhibitors chelerythrine chloride or calphostin C completely blocked the inhibition by 5-HT. (vi) The specific inhibitor for PKC delta, rottlerin, significantly blocked the inhibition by 5-HT. (vii) The specific tyrosine kinase inhibitor, herbimycin, or Src family kinase inhibitor, PP1, abolished the 5-HT-mediated inhibition of Cl(-)/OH- exchange activity. (viii) 5-HT stimulated tyrosine phosphorylation of c-Src kinase and PKC delta.

Block of the rapid component of the delayed rectifier potassium current by the prokinetic agent cisapride underlies drug-related lengthening of the QT interval

BACKGROUND

Lengthening of the QT interval and torsades de pointes resulting in cardiac arrests and deaths have been noticed during treatment with cisapride, a newly developed gastrointestinal prokinetic agent. The rapid (I[Kr]) and slow (I[Ks]) components of the delayed rectifier current (I[K]) are candidate ionic currents to explain cisapride-related toxicity because of their role in repolarization of cardiac ventricular myocytes. Our objectives were to (1) characterize effects of cisapride on two major time-dependent outward potassium currents involved in the repolarization of cardiac ventricular myocytes, I(Kr) and I(Ks), and (2) determine action potential-prolonging effects of cisapride on isolated hearts.

METHODS AND RESULTS

A first set of experiments was performed in isolated guinea pig ventricular myocytes with the whole-cell configuration of the patch-clamp technique. Cells were held at -40 mV while time-dependent outward currents were elicited by depolarizing pulses lasting either 250 ms (I[K250]) or 5000 ms (I[K5000]). Effects of cisapride on the I(Kr) component were assessed by measurement of time-dependent activating currents elicited by short pulses (250 ms; I[K250]) to low depolarizing potentials (-20, -10, and 0 mV). Time-dependent activating currents elicited by long pulses (5000 ms; I[K5000]) to positive potentials (>+30 mV) were recorded to assess effects of the drug on the I(Ks) component. A second set of experiments was conducted in isolated guinea pig hearts buffer-perfused in the Langendorff mode to assess effects of the drug on monophasic action potential duration measured at 90% repolarization (MAPD90). Hearts were exposed to cisapride 100 nmol/L at decremental pacing cycle lengths of 250, 225, 200, 175, and 150 ms to determine reverse frequency-dependent effects of the drug. Overall, 112 myocytes were exposed to seven concentrations of cisapride (10 nmol/L to 10 micromol/L). Cisapride inhibited I(Kr), the major time-dependent outward current elicited by short pulses (I[K250]) to low depolarizing potentials, in a concentration-dependent manner with an IC50 of 15 nmol/L (therapeutic levels, 50 to 200 nmol/L). Conversely, block of I(Ks) by the drug was less potent (estimated IC50 >10 micromol/L). In isolated hearts (n=9 experiments), cisapride 100 nmol/L increased MAPD90 by 23+/-3 (P<.05) at a basic cycle length of 250 ms but by only 7+/-1 ms (P<.05) at a basic cycle length of 150 ms.

CONCLUSIONS

Block of I(Kr) gives an explanation to lengthening of cardiac repolarization observed in isolated guinea pig hearts. Potent block of I(Kr) is also likely to underlie prolongation of the QT interval observed in patients receiving clinically recommended doses of cisapride as well as severe cardiac toxicity (torsades de pointes) observed in patients with increased plasma concentrations of the drug.

Role of the 5-HT receptor in neurogenic inflammation in Fisher 344 rat airways

The increased plasma protein extravasation in the airways of Fisher 344 rats upon stimulation of sensory nerves is in part due to the degranulation of mast cells. In this study, we examined the role of 5-HT and histamine receptors in the capsaicin-induced increase in plasma protein extravasation in Fisher 344 rat airways, using Evans blue as an intravascular marker. We found that only 5-HT2 receptor agonists increased baseline plasma protein extravasation. Furthermore, the 5-HT2 receptor antagonist ketanserin reduced the capsaicin-induced increase in plasma protein extravasation. Combining ketanserin with the tachykinin NK1 receptor antagonist (+/-)-RP 67,580 ((3alphaR,7alphaR)-(7,7-diphenyl-2(1-imino-2-(2-methoxyph enylethyl)-perhydraisoinositol-4-one))) abolished the neurogenic increase in plasma protein extravasation. Finally, using selective receptor agonists and antagonists, we demonstrated that there was no modulation of the capsaicin-induced rise in plasma protein extravasation by stimulation of either histamine receptors or 5-HT1, 5-HT3 and 5-HT4 receptors. We conclude that, in the airways of Fisher 344 rats, the neurogenic increase in plasma protein extravasation is caused by activation of both tachykinin NK1 receptors and 5-HT2 receptors.

5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption in rabbit jejunum

The aim of the present study was to determine the 5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption across rabbit jejunum in vitro. A number of agonists and antagonists were used to characterize the receptors through which serotonin inhibits this absorption. The results show that 2.5x10(-6) M 5-HT inhibits the amino acid absorption by about 20%. The 5-HT receptor agonists, alpha-methyl-5-HT (5-HT2), 2-methyl-5-HT (5-HT3) and zacopride (5-HT4) at concentrations 2.5x10(-6) and 2.5x10(-5) M produced 10-30% inhibition on L-leucine intestinal absorption. 5-carboxyamidotryptamine (5-HT1) did not produce any inhibition. The 5-HT antagonists, GR 113808A (5-HT4) at 2.5x10(-6) M and ritanserin (5-HT2) and ondansetron (5-HT3) at 2.5x10(-5) M completely blocked the effect of 5-HT. However, methiothepin (5-HT1) did not produce any effect on serotonin action in the intestinal absorption of amino acid. It can be concluded that 5-HT2, 5-HT3 and 5-HT4 receptors could mediate inhibition of L-leucine absorption across rabbit jejunum.

The effects of cisapride on elemental content in neonatal mouse small intestine in vivo

The effects of cisapride on elemental content of enterocytes, smooth muscle cells and lamina propria in neonatal mouse jejunum were studied using electron probe X-ray microanalysis. One hour after injection with cisapride (1.7 or 0.017 mg/kg body weight i.p.), Cl content was significantly reduced in villus base, crypt and smooth muscle cells and Na content decreased in muscle cells. No changes were observed in Na or Cl within villus tip cells. Total Ca content did not change significantly in any cell type following treatment with cisapride. These results confirm that cisapride induces net Cl- secretion in neonatal mouse jejunum.

Mucosal protection through active intestinal secretion: neural and paracrine modulation by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is widely distributed within enteroendocrine cells and neurons of the digestive tract. It stimulates active anion secretion in the intestinal epithelium, an effect which promotes the dilution and elimination of luminal pathogens. The intestinal secretory effects of 5-HT appear to be mediated in part by epithelial 5-HT2 like receptors that are linked to phosphatidylinositol turnover. In addition, 5-HT appears to interact with 5-HT3 or 5-HT4 receptors on submucosal neurons to elicit neuronal depolarization and an associated increase in intestinal secretion. Although the precise cellular mechanisms by which 5-HT modulates mucosal ion transport are incompletely understood, it is clear that 5-HT plays an important role as a intestinal secretagogue in certain diarrheal states, in laxation, and in intestinal hypersensitivity reactions.

Evidence that the secretory response of rat intestine to 5-hydroxytryptamine in-vivo involves more than one 5-hydroxytryptamine-receptor subtype

The transintestinal potential difference (PD) across rat mid-small intestine and proximal colon was measured in-vivo. The 5-hydroxytryptamine (5-HT)-induced increase in PD, which reflects a stimulation of electrogenic C1 secretion, was mimicked by both 2-methyl-5-hydroxytryptamine (2-CH3-5-HT), an agonist at 5-HT3 receptors, and 5-methoxytryptamine (5-MT), an agonist that lacks affinity for 5-HT3 receptors. The 5-HT3 antagonist granisetron caused a marked inhibition of the response to 2-CH3-5-HT in both regions, but only produced a small inhibition of the small intestinal response to 5-HT, with a more pronounced effect in the colon. The failure of granisetron to produce a marked antagonism of the 5-HT-induced rise in the transintestinal PD, coupled with the ability of 5-MT to induce a secretory response, indicates that 5-HT3 receptors are not the only ones involved in the stimulation of C1 secretion. The 5-HT2 antagonist ketanserin failed to influence the response to 5-HT in either the small intestine or the colon, but it did inhibit the action of 5-MT, having a much greater effect in the small intestine. In the presence of granisetron however, ketanserin also inhibited the small intestinal response to 5-HT, having only a minimal effect in the colon. This suggests that 5-HT2 receptors can also play a role in the activation of C1 secretion. These observations suggest that both 5-HT2 and 5-HT3 receptors contribute to the stimulation of electrogenic C1 secretion by 5-HT, with 5-HT2 receptors playing a more prominent role in the small intestine and 5-HT3 receptors being more important in the colon.

Efficacy of cisapride therapy in functional dyspepsia

AIM

To assess the efficacy of cisapride therapy in relieving symptoms of functional dyspepsia.

METHODS

After a 2-week placebo run-in period, 61 out of 74 patients were eligible to enter a 4-week double-blind treatment phase, consisting of treatment with cisapride (10 mg) or placebo tablets t.d.s. Gastric emptying was assessed scintigraphically at entry to the study. Patients were stratified before treatment into those with or without active chronic (Helicobacter pylori) gastritis. Patients were also classified retrospectively into those with 'reflux-like' dyspepsia (n = 29) and those with 'motility-like' dyspepsia (n = 32).

RESULTS

At the end of the active treatment phase, there was a similar significant (P < 0.001) reduction in total symptom score from baseline in both cisapride (8.9 +/- 0.5 to 5.8 +/- 0.6) and placebo (9.7 +/- 0.6 to 5.5 +/- 0.6) groups. Scores for heartburn and continual bloating were significantly reduced in the cisapride but not the placebo group; improvement was attributable to patients with normal, rather than delayed, rates of gastric emptying. For continual bloating, significant improvement also occurred in the cisapride subgroup without gastritis, but not in the subgroup with gastritis (mean symptom score reduction 0.48 +/- 0.18, P = 0.03). For global evaluation by the investigator and by the patient, the overall improvement rates were not statistically different between cisapride and placebo groups. In those with normal gastric emptying, however, there was a significant (P = 0.01) improvement in general well-being in the cisapride but not in the placebo group.

CONCLUSIONS

We were unable to show major differences in the short-term efficacy of cisapride and placebo in functional dyspepsia. There were indications, however, of beneficial effects of cisapride over placebo in those with 'reflux-like' dyspepsia, and in those without gastroparesis.

Characterization of the 5-hydroxytryptamine receptor type involved in inhibition of spontaneous activity of human isolated colonic circular muscle

1. Experiments were carried out to characterize pharmacologically the 5-hydroxytryptamine (5-HT) receptor types which mediate inhibition of spontaneous contractions of the intertaenial circular muscle in human isolated colon. 2. 5-HT caused a reproducible concentration-dependent inhibition of spontaneous contractions of the circular muscle of human colon in vitro with a mean EC50 value of 0.2 microM and 95% confidence limits of 0.1-0.5 microM. No evidence for a contractile action of 5-HT was found. Tetrodotoxin (TTX, 1.5 microM) caused a rightward shift of the concentration-response curve of 5-HT with a concentration-ratio of 2.9. 3. The inhibitory response to 5-HT was mimicked by several indoles with the rank order of potency 5-HT > 5-methoxytryptamine = alpha-methyl-5-HT > 5-carboxamidotryptamine >> 2-methyl-5-HT. 5-Hydroxyindalpine was inactive. 4. The substituted benzamides were agonists with the following rank order of potency, 5-HT > renzapride > zacopride > metoclopramide > cisapride. 5. The inhibitory responses to 5-HT were not inhibited by methysergide (10 microM) or methiothepin (1 microM), which are antagonists selective for 5-HT1-like and 5-HT2 receptors, nor by ondansetron (10 microM) which is an antagonist at 5-HT3 receptors. 6. The inhibitory responses induced by 5-HT and 5-methoxytryptamine were competitively antagonized by a weak 5-HT4 receptor antagonist, tropisetron, with pKB values of approximately 6. Tropisetron had no significant effect on the inhibitory response curve produced by isoprenaline (0.01-100 microM). 7. The pharmacological profile of the 5-HT-evoked relaxations of human colon circular muscle are consistent with activation of a 5-HT4-like receptor.

Characterization of 5-hydroxytryptamine receptors mediating mucosal secretion in guinea-pig ileum

1. The receptor subtypes through which 5-hydroxytryptamine (5-HT) increases electrolyte secretion across the mucosa of guinea-pig ileum were studied. 2. Flat sheep preparations of guinea-pig mucosa plus submucosa were placed in Ussing chambers and the short circuit current (ISC), an index of net electrogenic electrolyte transport across the mucosa, was measured under voltage clamp conditions. 3. Low concentrations of 5-HT (10-300 nM) evoked monophasic increases in ISC which were significantly reduced by hyoscine (100 nM), tetrodotoxin (TTX, 300 nM) and the 5-HT2 receptor antagonist, ketanserin (3-300 nM). 4. Higher concentrations of 5-HT (1-10 microM) produced biphasic responses which were reduced by hyoscine (100 nM), TTX (300 nM), ketanserin (3-300 nM) and also by the 5-HT3 receptor antagonists, granisetron (1 microM) and ICS 205-930 (100 nM). 5. 2-Methyl-5-HT (1-100 microM) and alpha-methyl-5-HT (30 nM-30 microM), agonists at 5-HT3 and 5-HT2 receptors respectively, also evoked ISC increases. These responses were reduced by hyoscine (100 nM) and abolished by TTX (300 nM) and the respective receptor antagonists, granisetron (1 microM) and ketanserin (30 nM). 6. The 5-HT4 receptor antagonist, SDZ 205-557 (300 nM) had no effect on the response to 5-HT. 7. The TTX-resistant response to 5-HT was not affected by 5-HT2,3 or 4 receptor antagonists. 8. These results indicate that 5-HT mediates secretion partly by an action on 5-HT3 receptors located on cholinergic and noncholinergic secretomotor neurones, partly by an action on higher affinity'5-HT2-like' receptors predominantly on noncholinergic neurones, and partly by a direct action on the epithelium.

Serotonin-induced short-circuit current in pig jejunum

This paper presents the effects of serotonin (5-HT) on short-circuit current (SCC), sodium and chloride fluxes, and prostaglandin E2 release in pig jejunum, using the Ussing-chamber technique. 5-HT elicited a dose-dependent increase in SCC, yielding an EC50 of 6 +/- 4 microM and EMAX of 77 +/- 8 microA.cm-2 using about 100 microM. Inhibited sodium absorption and stimulated chloride secretion carried part of the 5-HT-induced SCC. 5-HT caused a dose-independent PGE2 release, and indomethacin reduced the SCC-inducing effect of 5-HT by 40%. Octreotide, a long-lasting somatostatin analogue, also reduced 5-HT-induced SCC by about 40%, while tetrodotoxin (TTX) did not alter the effect of 5-HT. In conclusion, 5-HT causes a dose-dependent indomethacin and octreotide-sensitive, and TTX-insensitive increase in SCC, and a chloride secretion and inhibited sodium absorption and an increased release of PGE2 in pig jejunum in vitro.

Short-circuit current responses to 5-hydroxytryptamine in human ileal mucosa are mediated by a 5-HT4 receptor

5-Hydroxytryptamine (5-HT) increases short-circuit current when added to the serosal side of human isolated ileal mucosa; mucosally applied 5-HT was ineffective. Tetrodotoxin reduced both basal short-circuit current and increases in short-circuit current due to electrical field stimulation of mucosal nerves. However, neither tetrodotoxin, ondansetron nor methysergide plus ketanserin affected 5-HT induced increases in short-circuit current. Application of SDZ 205-557 (2-diethylaminoethyl-(2-methoxy-4-amino-5-chloro) benzoate) to the tissue caused a significant increase in the concentration ratio between two successive 5-HT response curves. It is concluded that the effect of 5-HT on short-circuit current of human ileal mucosa appears to be due to stimulation of a 5-HT4 receptor.

Action of cisapride on rat colonic secretion

The stimulating effect of cisapride on the motility of the digestive tract is well known. However, there are only a few studies on the influence of this drug on the absorptive or secretory activity of the colonic mucosa. In the present study, the ability of cisapride to alter the mural transport of water and electrolytes in the colon and its effects on mucus secretion and albumin permeation were studied. The effects of cisapride on the rat colon in vivo were studied under different conditions, by means of an instillation of sodium acetate solution at pH 6.9, which induced absorption of water and electrolytes, and in two models of colonic secretion, one employing the instillation of an acetic acid solution at pH 2.9 and the other, an intravenous infusion of 5-hydroxytryptamine (serotonin) 45 micrograms.kg-1.min-1 together with intracolonic instillation of sodium acetate. Cisapride (i.v.), at a dose of 0.32 mg.kg-1, in rats whose colon was instilled with sodium acetate (pH 6.9) induced an increase in sodium absorption and a reduction in water absorption. Cisapride (i.v.), at doses of 0.32, 0.64 and 1.0 mg.kg-1, inhibited the secretion of water, Na+, Cl-, and mucus and the permeation of albumin induced by acetic acid instillation or by serotonin infusion. It is concluded that the effect of cisapride on the colonic mucosa varies in accordance with the functional mucosal conditions and that this action may be of clinical importance.

Cisapride and a structural analogue, R 76,186, are 5-hydroxytryptamine4 (5-HT4) receptor agonists on the guinea-pig colon ascendens

The purpose of this study was to investigate whether the effects of cisapride and its close structural analogue R 76,186 on the isolated guinea-pig colon ascendens, are mediated through 5-HT4 receptors. Both cisapride and R 76,186 induced contractions in a concentration-dependent fashion, giving monophasic concentration-response curves (cisapride: EC50 = 1.1 x 10(-7) M, maximum effect = 40.3% of methacholine-induced (3 x 10(-7) M) contractions; R 76,186: EC50 = 2.4 x 10(-8) M, maximum effect = 52.1%). Blockade of either 5-HT2 or 5-HT3 receptors did not affect the responses to cisapride. However, tropisetron (in 5-HT4 receptor-blocking concentrations), and DAU 6285 and SDZ 205-557, two novel selective 5-HT4 receptor antagonists, depressed the concentration-response curve to cisapride (to about 50%), and the curve to R 76,186 was shifted to the right. The apparent pA2 values were 6.6 (tropisetron), 6.3 (DAU 6285), and 7.5 (SDZ 205-557). However, none of these antagonisms was purely competitive as higher concentrations of these antagonists depressed the curve to R 76,186. Desensitization of the 5-HT4 receptor with 5-methoxytryptamine (5-MeOT) inhibited the responses to cisapride, and abolished those to R 76,186. The contractions to cisapride and R 76,186 were sensitive to mutual antagonism, depressing their concentration-response curves.

CONCLUSIONS

Both cisapride and R 76,186 mediate their contractile effects in the guinea-pig colon ascendens through agonism at the 5-HT4 receptor, though cisapride also uses a non-5-HT mechanism. R 76,186 is a selective and potent 5-HT4 receptor agonist.

Receptors implicated in the actions of serotonin on chicken ileum longitudinal smooth muscle

The presence of serotonin (5-HT) in the chicken gastrointestinal tract has been previously reported, but its motor effects have been poorly described. The aims of this work were: A) to define the effects of 5-HT on chicken longitudinal ileum; B) to explore the mechanisms by which such effects occur and C) to identify the subtype(s) of 5-HT-ergic receptors implicated. The motor responses to 5-HT were assayed in vitro using ileal strips taken from male White Leghorn chickens 7-9 week old. 5-HT elicited ileal contraction (EC50 9.6 x 10(-8) M), which was markedly decreased in the presence of tetrodotoxin (TTX). Repeated exposure of the tissue to supramaximal concentrations of 5-HT did not however lead to desensitization. Atropine (10(-6) M), ketanserin (10(-5) M), methysergide (10(-5) M) and methiothepine (10(-6) M) attenuated the response to 5-HT. Ketanserin was an effective inhibitor of the residual response to 5-HT obtained even in the presence of TTX. Several serotonergic agonists were assayed to further analyse the type of receptors involved in the response to 5-HT. 5-methoxytryptamine (5-MOT), a mixed 5-HT1, 5-HT2 and 5-HT4 agonist, reproduced all the effects of 5-HT. 8-OH-DPAT, a selective 5-HT1A agonist, trifluoromethylphenylpiperazine, a mixed 5-HT1B/C agonist, and m-chlorophenylbiguanide, a 5-HT3 agonist, did not induce any consistent contractile effects. Sumatriptan, a 5-HT1D agonist, exerted a slight agonistic effect which was blocked by methiothepine and decreased by TTX but not by atropine. Cisapride, a 5-HT4 partial agonist in mammals, decreased the effects of both 5-HT and 5-MOT. These results indicate that chicken ileum contains 5-HT1 receptors similar to the 5-HT1D mammalian subtype but not the 5-HT1A, 5-HT1B, 5-HT1C or 5-HT3 subtypes. 5-HT2 receptors are also present and would appear to be located on smooth muscle.

Actions of serotonin antagonists on cholera-toxininduced intestinal fluid secretion

The effects of several 5-hydroxytryptamine (5-HT) receptor antagonists were tested in rats in vivo on the intestinal fluid secretion evoked by cholera toxin. Five receptor antagonists were used, namely 2-bromolysergic acid diethylamine (2-bromo-LSD), granisetron, ketanserin, methysergide and ondansetron. The drugs were used in doses that inhibited the arterial hypertension and/or bradycardia evoked by 5-HT given i.v. Granisetron and ondansetron markedly diminished cholera-toxin-evoked secretion, whereas ketanserin was without any effect. Methysergide also diminished cholera-toxin-induced fluid secretion particularly when the drug was given as an i.v. infusion. The results are considered in relation to the pathophysiology of cholera secretion and to the current views of receptor subtypes for 5-HT. It is proposed that the receptor involved is a 5-HT3 receptor, possibly also a receptor of the 5-HT1 type. Results from experiments in which 5-HT (20 mM) was placed in the intestinal lumen to evoke an intestinal secretion suggest that the 5-HT3 receptor is located in the villus tissue. It was also demonstrated that zimeldine, an inhibitor of presynaptic 5-HT reuptake, diminished choleraic secretion, an effect that may be ascribed to a 5-HT tachyphylaxis caused by an accumulation of 5-HT in a synaptic cleft.

Involvement of non-classical 5-HT receptor in serotonin and cisapride induced secretion in hen colon

1. In hen colon 5-HT induces a tetrodotoxin-resistant, bumetanide-sensitive, chloride secretion, positively coupled with adenylate cyclase activity. 2. The 5-HT receptor mediating this response seems non-classical since it cannot be blocked by 5-HT1-like, 5-HT2 or 5-HT3 antagonists. 3. Effects are presented of new putative 5-HT agonists and antagonists on short circuit current and cord conductance in the hen colon, using the Ussing chamber technique. 4. The substituted benzamides, cisapride and BRL 24924, induced a dose-dependent short circuit currents but both with less potency than 5-HT. 5. Cisapride mediated this dose-dependent bumetanide sensitive response mainly by release of acetylcholine, since atropine reduced cisapride response by 70%. 6. Neither BRL 24924, 5-HTP-DP, ketanserin, ICS 205-930, prazosin, yohimbine, atropine nor piroxicam, covering the 5-HT1P, 5-HT2P, 5-HT2, 5-HT3, 5-HT4, adrenergic and muscarinic receptor types and the prostaglandin synthesis, altered 5-HT induced increases in short circuit current and cord conductance. 7. Results suggest (a) cisapride mediates it's response mainly by releasing acetylcholine, which then stimulates muscarinic receptors to release 5-HT. (b) Involvement of a non-classical 5-HT receptor subtype in 5-HT induced chloride secretion in hen colon.

Investigation of the 5-hydroxytryptamine receptor mechanism mediating the short-circuit current response in rat colon

1. 5-Hydroxytryptamine (5-HT) stimulated an increase in short-circuit current (SCC) in rat isolated colonic mucosa with an EC50 value of approximately 4 microM. The purpose of the present study was to investigate the 5-HT receptor mechanism(s) involved in this response. 2. The relatively selective 5-HT receptor agonists 5-carboxamidotryptamine (5-CT) and alpha-methyl-5-HT stimulated SCC and were 6 to 8 times less potent than 5-HT. 2-Methyl-5-HT was inactive both as an agonist and an antagonist. 3. The following compounds produced no significant inhibition of the SCC response to 5-HT: ketanserin (1 microM), methysergide (1 microM), methiothepin (0.3 microM), GR38032F (0.3 microM), tetrodotoxin (0.3 microM) and sulpiride (1 microM). 4. Both metoclopramide (3 and 10 microM) and cisapride (0.1 and 1 microM) inhibited the SCC responses to 5-HT in a concentration-related manner, and the higher doses similarly inhibited the responses to 5-CT. With both agonists the inhibitory effects of metoclopramide and cisapride were insurmountable. However, these inhibitory actions appeared to be selective since neither metoclopramide nor cisapride affected the basal SCC or the SCC response to prostaglandin E2. 5. The SCC responses to 5-HT and 5-methoxytryptamine were selectively inhibited by ICS205-930 at 3 microM, and respective pKB values of 6.0 and 6.6 were calculated. 6. It is concluded that 5-HT stimulates an SCC response in rat colon via a receptor mechanism that cannot be clearly identified as 5-HT1-like, 5-HT2 or 5-HT3. This receptor is selectively antagonized by ICS 205-930 and by the benzamides, metoclopramide and cisapride. The 5-HT receptor in rat colon therefore exhibits some of the properties associated with the so-called 5-HT4 receptor.

Stimulatory effects of 5-hydroxytryptamine on fluid secretion and transmural potential difference in rat small intestine are mediated by different receptor subtypes

The rise in transmural potential difference (PD) and the fluid secretion induced by 5-hydroxytryptamine (5-HT) were measured in rat small intestine in-vivo. Both cisapride and ketanserin abolished the 5-HT-induced rise in systolic blood pressure mediated by 5-HT2 receptors. Cisapride inhibited the 5-HT-induced increases in the transintestinal PD, but over the same dose range it had no effect on the fluid secretion induced by 5-HT. In contrast, ketanserin caused a dose-dependent reduction in 5-HT-induced fluid secretion at doses that failed to influence the rise in PD. It is concluded that different receptors are responsible for the effects of 5-HT on fluid secretion and electrical activity in the rat small intestine.

Cisapride. A preliminary review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use as a prokinetic agent in gastrointestinal motility disorders

Cisapride, a substituted piperidinyl benzamide chemically related to metoclopramide, is an orally administered prokinetic agent which facilitates or restores motility throughout the length of the gastrointestinal tract. Its novel mechanism of action is thought to involve enhancement of acetylcholine release in the myenteric plexus of the gut. Because of its specificity cisapride is devoid of central depressant or antidopaminergic effects; side effects such as diarrhoea or loose stools, which occur infrequently, are related to its primary pharmacological action. Evidence exists from comparisons with placebo in initial trials to establish the efficacy of cisapride in improving healing rates and symptoms in patients with reflux oesophagitis, in alleviating symptoms in patients with non-ulcer dyspepsia, and in accelerating gastric emptying in gastroparesis. There are less conclusive data regarding the efficacy of cisapride in relieving symptoms in patients with gastroparesis, although preliminary results support a role for cisapride in certain groups such as diabetics. Limited data suggest that patients with chronic constipation due to underlying motility disorders may benefit from cisapride. Unfortunately, there is a paucity of trials comparing the efficacy of cisapride with other therapeutic agents. Thus, the relative position of cisapride in therapy cannot be defined at present. Should future results support preliminary evidence of comparable efficacy to metoclopramide, domperidone and ranitidine (in oesophagitis), cisapride with its favourable tolerability profile should claim a prominent position in the therapy of patients with a variety of gastrointestinal motility disorders.

Receptor regulation of ion transport in the intestinal epithelium

The active transport of ions by the intestinal epithelium is regulated by a number of enteric neurotransmitters, hormones and other substances. Our knowledge of the receptors mediating the actions of these substances is generally fragmentary. This review summarizes current knowledge on the location and functional characteristics of transmitter receptors regulating transport function in the small intestine, highlighting recent research on cholinergic and bradykinin receptors.