Alpha 2 adrenergic receptors are located prejunctionally in the Auerbach's plexus of the guinea pig small intestine: direct demonstration by radioligand binding

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

Evidence for mast cell, leukotriene and nitric oxide involvement in the regulation of the adrenoceptor number of inflamed small intestine in guinea pigs

Changes in the populations of neurotransmitter receptors involved in the control of intestinal smooth muscle function have been associated with the altered motility of the inflamed gut. Thus, trinitrobenzenesulphonic acid (TNBS)-induced gut inflammation is accompanied by an increase in alpha- and a decrease in beta-adrenoceptor numbers in guinea pig small intestine. In the present study, we investigated the effects of anti-inflammatory compounds (cyclooxygenase inhibitor indomethacin, lipooxygenase inhibitor MK-886, nitric oxide synthase inhibitor NG-nitro-L-arginine methylester (L-NAME), mast cell stabilizer doxantrazole) on TNBS-induced adrenoceptor changes. Smooth muscle adrenoceptor populations, labelled by subtype-specific radioligands 6 days after TNBS, were significantly different from those of sham-treated controls: alpha 1- and alpha 2-adrenoceptor numbers increased by more than 50%, while beta-adrenoceptor numbers decreased by more than 50%. These changes, associated with severe inflammation as assessed histologically and by myeloperoxidase assay, were prevented by doxantrazole or L-NAME, and only partly by MK-886. In contrast, indomethacin did not prevent these changes. It appears then that: (a) mast cell mediators, nitric oxide and leukotrienes are likely to contribute to TNBS-induced changes in adrenoceptor populations in the guinea pig inflamed intestine; (b) there is no evidence for prostanoid involvement in this process. It was suggested that changes in smooth muscle adrenoceptor populations may be an important mechanism by which gut inflammation alters intestinal motility.

Dopamine-induced inhibition of endogenous acetylcholine release from the isolated ileal synaptosomal preparations of guinea-pig mediated via alpha-adrenoceptors

1. The effect of exogenous dopamine on the release of endogenous acetylcholine (ACh) from isolated ileal synaptosomal guinea-pig preparations was examined by means of high pressure liquid chromatography with electrochemical detection. 2. Release of ACh was induced by substance P or by depolarization with high potassium (50 mM) in a medium containing atropine propranolol and naloxone. 3. Dopamine produced a concentration-dependent inhibition of the evoked ACh release induced by substance P or in samples depolarized by high potassium. This action of dopamine was not reversed by the dopamine receptor antagonists either for the DA2 subtype domperidone, or for the DA1 subtype, SCH23390. Fenoldopam, the agonist of dopamine DA1 receptors, or quinpirole, the agonist of dopamine DA2 receptors, reduced the evoked ACh release, although only in high, non-dopamine-specific concentrations. 4. Failure of guanethidine or desipramine to inhibit this effect of dopamine ruled out mediation by endogenous noradrenaline. 5. Idazoxan and yohimbine reversed this dopamine-induced inhibition at concentration sufficient to abolish the action of clonidine. Influx of (45)Ca stimulated by substance P or high potassium into synaptosomal preparations was attenuated in the presence of dopamine. This inhibition by dopamine was also reversed by idazoxan or yohimbine but not by dopamine receptor antagonists. Moreover, the dopamine-induced inhibitions of both the ACh release and the influx of (45)Ca disappeared in the samples treated with pertussis toxin at a dose sufficient to abolish the action of clonidine. 6. It is concluded that dopamine suppresses the influx of calcium ions into cholinergic nerve terminals via an activation of alpha2-adrenoceptors coupled with a pertussis toxin-sensitive GTP-binding protein, resulting in the decrease of ACh release from ileal synaptosomes of guinea-pigs.

Inhibitory effect of octopamine on the release of endogenous acetylcholine from isolated myenteric synaptosomes of guinea-pig

1. The effect of octopamine on the release of endogenous acetylcholine (ACh) from isolated ileal synaptosomal preparations of guinea-pigs was examined using high pressure liquid chromatography with electrochemical detection. Release of ACh was induced by substance P or by depolarization with high potassium (50 mmol/L) in medium containing atropine, propranolol and naloxone. 2. Octopamine produced a dose-dependent inhibition of substance P-induced ACh release. A similar inhibitory action of octopamine was found in the samples depolarized by high potassium as a reference. 3. The action of octopamine was not reversed by the dopamine receptor antagonists either for the DA-2 subtype, domperidone, or for the DA-1 subtype, SCH23390, or by haloperidol. However, idazoxan and yohimbine antagonized this octopamine-induced inhibition at concentrations sufficient to abolish the action of clonidine. 4. Failure of guanethidine or nomifensine to inhibit octopamine ruled out mediation by noradrenergic neurotransmitters. 5. Octopamine decreased the influx of [45Ca] stimulated by substance P into synaptosomal preparations and this was reversed by idazoxan or yohimbine at concentrations sufficient to block the action of clonidine. 6. Pertussis toxin abolished the inhibitory action of octopamine at a dose high enough to block the action of clonidine. 7. These results indicate that octopamine suppresses the influx of calcium ions into cholinergic nerve terminals of ileal synaptosomes of guinea-pigs via an activation of alpha 2-adrenoceptors coupled with a pertussis toxin-sensitive GTP-binding protein which results in a decrease of ACh release.

Pharmacological evidence for the presence of a beta-adrenoceptor-like agonist in the amphinoid polychaete Eurythoe complanata

1. Methanolic extracts from the body wall of Eurythoe complanata (ExEc) were tested for biological activity on the isolated rat ileum. 2. ExEc produced either relaxation or relaxation followed by contraction of the rat ileum in a concentration-dependent manner. 3. The predominant relaxation response to ExEc was completely blocked by the beta-adrenoceptor antagonist propranolol and was unaffected by the alpha-adrenoceptor antagonist phenoxybenzamine. 4. The results indicate that the relaxation induced by ExEc is mediated by beta-adrenoceptors. The presence of a myorelaxing substance in E. complanata that selectively activates the beta-adrenoceptors is suggested.

Subtype classification of presynaptic alpha 2-adrenoceptors

1. Presynaptic alpha 2-adrenoceptors in a few tissues have recently been pharmacologically classified in functional studies. 2. Autoreceptors are of alpha 2A-subtype in rabbit occipito-parietal cortex, rat cerebral cortex, vas deferens, submandibular gland, kidney, guinea-pig ileum submucosal arterioles and urethra. 3. Heteroreceptors are of alpha 2A-subtype in rat cerebral cortex, vas deferens, guinea-pig ileum submucosal plexus and Auerbach's plexus. 4. In rat atria autoreceptors have been shown to be of alpha 2B-subtype. 5. Classification is done mainly with alpha 2A-adrenoceptor-selective oxymetazoline, WB 4101 and BRL 44408, and the alpha 2B-adrenoceptor-selective prazosin, AR-C 239, chlorpromazine and BRL 41992. 6. With four alpha 2-adrenoceptor subtypes to consider, a larger number of subtype-selective compounds may have to be characterized in the classification in the many tissues, where presynaptic alpha 2-adrenoceptors are found.

Inverse regulation of alpha- and beta-adrenoceptors during trinitrobenzenesulfonic acid (TNB)-induced inflammation in guinea-pig small intestine

The hypothesis has been raised that intestinal motor disturbances induced by inflammation of the digestive tract may reflect alterations in intestinal cell-membrane receptors. This question has been addressed herein for adrenoceptors by performing [3H]prazosin, [3H]rauwolscine and [3H]DHA binding studies on guinea-pig jejunal smooth-muscle membrane preparations from both healthy controls and 3, 6, and 10 days after TNB-induced intestinal inflammation. Each of the adrenoceptor subtype-selective radioligands used bound selectively to a single saturable class of sites, with no significant (p < 0.05) variation of dissociation constant (KD) values along the inflammatory process. In contrast maximal binding capacities (Bmax) for the different radioligands varied moderately but significantly (p < 0.05 and p < 0.01) according to the time after TNB injection. The alpha-adrenoceptors were significantly upregulated, respectively from (mean +/- SE in fmoles/mg of proteins) 27 +/- 3.8 (controls) to 91 +/- 3.1 (day 10) for the alpha 1-subtype, and from 26 +/- 2.7 (controls) to 102 +/- 5.9 (day 10) for the alpha 2-subtype. In contrast beta-adrenoceptors were down-regulated from 384 +/- 34.6 (controls) to 158 +/- 17.2 (day 10). These findings highlighted an apparently inverse pathological regulation of intestinal alpha- and beta- adrenoceptor densities. They suggest that changes in intestinal adrenoceptors, probably resulting from functional denervation of the inflamed bowel tissue, may contribute to the altered intestinal motility observed in inflammatory bowel diseases.

Gastric reflex relaxation by colonic distension

Previous studies in human volunteers have demonstrated an inhibition of gastric motility following painless rectal distension. In the present study we investigated, in anaesthetized rats, the effects of colonic distension on gastric tone and looked at certain aspects of the underlying nervous mechanisms. Changes in gastric volume were monitored continuously by a volumetric method. Colonic distension induced an immediate and pronounced gastric relaxation which lasted throughout the period of distension. The inhibition of gastric tone following colonic distension was abolished by hexamethonium or by bilateral cervical vagotomy. The selective adrenergic alpha 1 blocker, prazosin, the alpha 2 blocker, yohimbine, and the non-selective beta-blocker, propranolol, had no significant effect on gastric relaxation following colonic distension. Likewise, naloxone, an opioid receptor antagonist, did not significantly influence gastric reflex inhibition. It is concluded that colonic distension induced a non-adrenergic inhibition of gastric tone mediated through the vagal nerves. Ganglionic receptors are also suggested to form part of the inhibitory pathway.

Effects of selective adrenergic agonists and antagonists on gastric tone in the rat

The aim of the present in vivo study was to investigate in anaesthetized rats, the effects of selective adrenergic agonists and antagonists on basal gastric tone and phasic contractions by the use of a volumetric method. L-phenylephrine, an alpha-1 agonist, induced hypertension, bradycardia and a significant gastric relaxation. Clonidine, an alpha-2 agonist, caused hypotension, bradycardia and a significant gastric contraction and a reduction of the amplitude of phasic contractions. Salbutamol, a beta-2 agonist, induced a dose-dependent tachycardia and a significant inhibition of gastric tone whereas prenalterol, a beta-1 agonist, induced tachycardia without any significant influence on gastric basal tone. Yohimbine, an alpha-2 blocker, significantly decreased gastric basal tone and reversed the inhibition of phasic contractions induced by clonidine. Prazocine, a selective alpha-1 blocker, and propranolol, a non-selective beta-blocker, had no significant influence on gastric basal tone or phasic contractions. It is concluded that sympathetic inhibition of basal gastric tone in the rat is mediated by alpha-1 and beta-2 adrenergic receptors. Activation of alpha-2 adrenergic receptors significantly increased basal gastric tone and reduced the amplitude of phasic contractions. A blockade of alpha-2 receptors significantly decreased basal gastric tone and restored the amplitude of phasic contractions.

Medetomidine stereoisomers delineate two closely related subtypes of idazoxan (imidazoline) I-receptors in the guinea pig

A number of imidazoline, imidazole and guanidinium compounds and other drugs were compared for their ability to bind to non-adrenergic idazoxan (imidazoline) I-receptors in particulate guinea pig cerebral cortex and ileum smooth muscle fractions. Radioligand binding with [3H]idazoxan was used for the experiments. Computer modelling of the binding data gave dissociation constants for drug binding to both I-receptors and alpha 2-adrenoceptors. Most drugs showed similar affinities for I-receptors in cortex and ileum. However, medetomidine stereoisomers as well as a few other drugs clearly delineated the I-receptors in cortex and ileum as different.

Pre- and post-synaptic alpha-2 adrenoceptors in the canine nasal mucosa

The alpha-2 adrenoceptor radioligand tritiated yohimbine was used to determine the location of pre- versus postsynaptic alpha-2 adrenoceptors in canine nasal mucosa. Using Scatchard analysis, the maximal binding capacity (Bmax) and an apparent equilibrium dissociation constant (Kd) of the alpha-2 adrenoceptor were measured in seven normal dogs and in seven dogs subjected to a unilateral superior cervical ganglionectomy. The results showed that most alpha-2 adrenoceptors in the canine nasal mucosa were located at postsynaptic membranes and very few were located at presynaptic membranes. After superior cervical ganglionectomy, there were no significant changes of the affinity (Kd) and number (Bmax) of the alpha-2 adrenoceptors in the mucosa. These results suggest that the postsynaptic alpha-2 receptors are located extrasynaptically.

Role of alpha adrenoceptors in opossum internal anal sphincter

The purpose of the present investigation was to examine the role of alpha adrenoceptors in the internal anal sphincter (IAS). Studies wer performed on alpha-chloralose anesthetized opossums. Resting pressure in the IAS (IASP) was recorded using low compliant continuously perfused catheters. The effects of the alpha-1 adrenoceptor agonist phenylephrine and alpha-2 adrenoceptor agonist clonidine and their corresponding selective antagonists, prazosin and yohimbine, respectively, were examined on the resting IASP, and on rectal balloon distension (RBD)-mediated IAS relaxation. Phenylephrine caused a rise in the IASP that was blocked by prazosin and not by yohimbine. Phenylephrine had no effect on IAS relaxation caused by RBD. Clonidine on the other hand caused significant suppression of IAS relaxation in response to RBD, but caused minimal changes in the resting IASP. The suppression of IAS relaxation by clonidine was selectively antagonized by yohimbine but not by prazosin. From these studies we conclude that alpha-2 adrenoceptors exert important neuromodulatory influences on rectoanal inhibitory reflex, while alpha-1 adrenoceptors may exert modulatory effects on the resting IAS tone.

Methoctramine reveals heterogeneity of M2 muscarinic receptors in longitudinal ileal smooth muscle membranes

Direct binding studies on longitudinal ileal and atrial muscarinic receptors revealed that most of the ileal or atrial selective antagonists identified in functional studies did not differentiate between these muscarinic receptors in direct binding studies. Methoctramine, an atrial selective muscarinic receptor antagonist in functional studies was, however, able to partially discriminate between these two receptors in our binding studies. Furthermore the binding data obtained using this compound indicated that longitudinal ileal muscarinic receptors were heterogeneous. The predominant population of ileal muscarinic receptors displayed a similar pharmacology to the cardiac type M2 muscarinic receptor. The minor population of muscarinic receptors identified in binding studies displayed a similar pharmacology to the ileal muscarinic receptor identified in functional studies and were pharmacologically similar to the exocrine gland type M2 muscarinic receptor.

Involvement of alpha-1 and alpha-2 adrenoceptors in the postlaparotomy intestinal motor disturbances in the rat

The effects of phentolamine, yohimbine and prazosin on laparotomy induced intestinal motor disturbances were studied in anaesthetised fasted rats previously equipped with electrodes, chronically implanted on the duodenum and jejunum. During continuous recording of interdigestive myoelectric activity, laparotomy under thiopental anaesthesia (Nesdonal 40 mg/kg ip) induced a primary phase of total inhibition of spiking activity lasting 26.1 +/- 1.3 min (mean +/- SE) followed by a period of disorganised activity, the first propagated migrating myoelectric complex (MMC) occurring 71.4 +/- 7.9 min after laparotomy. Phentolamine (3 mg/kg), or yohimbine (1 mg/kg) given im before laparotomy decreased by 48 and 49%, respectively, the duration of postsurgical inhibition, with a normal MMC pattern occurring immediately after. In contrast, there was only a shortening of the postlaparotomy initial inhibition of spiking activity after im prazosin (100 micrograms/kg), with a late (50-60 min) recovery of the MMC pattern. These results suggest that the initial inhibition of intestinal motility induced by laparotomy may involve alpha-1 and alpha-2 adrenoceptors, while the disruption of the MMC pattern is mainly caused by the activation of alpha-2 receptors.

Receptor interactions of a series of imidazolines: comparison of the alpha 2-adrenoceptors between the rabbit vas deferens and guinea pig ileum

A series of imidazolines and norepinephrine were used to characterize and differentiate the presynaptic alpha 2-adrenoceptors in the rabbit vas deferens and the guinea pig ileal longitudinal muscle using pharmacological procedures. Based on pEC50-values (the negative log of the 50% effective concentration) for each imidazoline, a rank order of potency of p-aminoclonidine greater than oxymetazoline greater than or equal to clonidine greater than naphazoline greater than phentolamine was obtained in the rabbit vas deferens and an order of p-aminoclonidine greater than clonidine greater than naphazoline greater than oxymetazoline was obtained in the guinea pig ileum. In the rabbit vas deferens, phentolamine, which is generally considered to be a competitive alpha 1- and alpha 2-adrenoceptor antagonist, acted as a full alpha 2-adrenoceptor agonist. The dissociation constants of oxymetazoline and yohimbine were significantly lower in the rabbit vas deferens than in the guinea pig ileum. These results suggest that the presynaptic alpha 2- adrenoceptors in these tissues are different. Furthermore, the pKB-value of yohimbine against norepinephrine was significantly one log unit lower than those obtained using a series of imidazolines. Data from our studies add to increasing evidence of the existence of high and low affinity binding sites on the alpha 2-adrenoceptors in the rabbit vas deferens.

Central alpha 2-adrenergic control of the pattern of small intestinal motility in rats

The effects of central and peripheral administration of alpha 2-adrenoceptor agonists and antagonists on small intestinal motility were examined in conscious rats chronically fitted with electrodes implanted in the duodenojejunal wall and a cannula placed in a cerebral lateral ventricle. In fasted rats, intracerebroventricular or intraperitoneal administration of clonidine (5 micrograms) immediately disrupted the migrating myoelectric complex pattern with a total inhibition of spiking activity during the first hour, followed by a period of irregular spiking activity for 2 h. The inhibition was abolished by previous intramuscular administration of yohimbine (600 micrograms), and the period of irregular activity was suppressed by intracerebroventricular yohimbine (30 micrograms). Naphazoline, an alpha 2-agonist that poorly crosses the blood-brain barrier, only inhibited spiking activity when administered intraperitoneally (1 microgram) and induced only a period of irregular spiking activity when administered intracerebroventricularly at the same dose. In fed rats, intracerebroventricular administration of yohimbine or phentolamine (30 micrograms), and to a lesser extent prazosin, restores a migrating myoelectric complex pattern typical of the fasted state. Peripheral administration of these three antagonists at a dose 20 times higher was ineffective. Finally, both feeding and central administration of alpha 2-agonists disrupt the migrating myoelectric complex pattern. Such pharmacologic data suggest a possible role of central alpha 2-adrenoceptors in the regulation of intestinal motility in rats.

Presynaptic muscarinic and alpha-adrenoceptor-mediated regulation of GABA release from myenteric neurones of the guinea-pig small intestine

The effects of cholinomimetic and sympathomimetic drugs on the release of [3H]-gamma-aminobutyric acid ([3H]-GABA) evoked by high K+ from the isolated small intestine of the guinea-pig were investigated, in the presence of tetrodotoxin. Acetylcholine and oxotremorine, at concentrations ranging from 10(-9) to 10(-6) M inhibited the evoked release of [3H]-GABA in a concentration-dependent manner, while nicotine was without effect. Scopolamine and pirenzepine inhibited the effect of oxotremorine, while hexamethonium had no effect. The IC50 values for scopolamine and pirenzepine of the oxotremorine (3 X 10(-8) M)-induced inhibition were 1.02 X 10(-9) M and 9.78 X 10(-10) M, respectively. Noradrenaline, but not isoprenaline inhibited the evoked release of [3H]-GABA. Clonidine (10(-10)-10(-6) M) reduced the evoked release of [3H]-GABA in a concentration-dependent manner, but phenylephrine had no effect. The inhibitory effect of clonidine was antagonized by yohimbine but not by prazosin. These findings provide evidence for the localization of M1-muscarinic and alpha 2-adrenoceptors on GABAergic nerve terminals and their involvement in the presynaptic control of the release of GABA from the guinea-pig small intestine.

Noradrenaline depletion increases noradrenaline-induced antinociception in mice

Mice were treated with N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4), which causes severe noradrenaline (NA) depletions in brain regions and the spinal cord, or vehicle i.p. They were tested 14 days later for antinociception induced by intrathecal injections of different doses of NA. A potentiation of the NA effect upon pain sensitivity was observed, with both an increase in the magnitude and duration of the antinociceptive responses. Upon biochemical analysis of spinal cords, it was found that DSP4-treated mice had a 80% depletion of NA, whereas dopamine and 5-hydroxytryptamine were unaffected. Radioligand binding of [3H]clonidine in membranes prepared from spinal cord, showed no differences in density of alpha 2-adrenoceptors, but the affinity had been increased, probably explaining the supersensitivity.

Role of adenosine 3',5'-cyclic monophosphate in adrenoceptor-mediated control of 3H-noradrenaline secretion in guinea-pig ileum myenteric nerve terminals

Secretion of 3H-noradrenaline was evoked by electrical field stimulation (1 Hz, 300 shocks) in guinea-pig ileum myenteric plexus. The role of cyclic nucleotides in the presynaptic receptor-mediated control of 3H-noradrenaline secretion was studied. The secretion of 3H-noradrenaline was maximally enhanced to the same extent, viz. 300-400% of control, by two analogues of cyclic AMP (8-Br cyclic AMP and dibutyryl cyclic AMP), by an adenylate cyclase activator (forskolin) and by three structurally different inhibitors of phosphodiesterase (3-isobutyl-1-methylxanthine, SQ 20,006 and Ro 20-1724), but not altered by two analogues of cyclic GMP (8-Br cyclic GMP and dibutyryl cyclic GMP). Added separately an alpha 2-adrenoceptor antagonist (yohimbine) and a beta-adrenoceptor agonist (isoprenaline) enhanced the 3H-noradrenaline secretion. Yohimbine, but not isoprenaline, increased additively the 'maximal enhancement' of the 3H-noradrenaline secretion caused by 3-isobutyl-1-methylxanthine. These results suggest that neuronal cyclic AMP may be involved in beta- but not in alpha-adrenoceptor-mediated control of 3H-noradrenaline secretion in guinea-pig ileum myenteric nerve terminals.

Ultrastructural evidence for beta-endorphin-like immunoreactivity in the nervous system of the rat duodenum

Using a modified immunoelectron microscopic staining technique the opioid peptide beta-endorphin could be demonstrated within large dense-core vesicles (LDVs) which are localized in presumptive axons and dendrites of the duodenal myenteric plexus of the rat duodenum. Pre-absorption tests with synthetic beta-endorphin (10(-6)M, final concentration) abolished the beta-endorphin immunoreactivity, resulting in only beta-endorphin unstained vesicles. To demonstrate more clearly electron-dense beta-endorphin material within vesicles the ultrathin sections were not counterstained with heavy metal salts. These sections revealed that the beta-endorphin material is localized as 'cores' with a mean diameter of 35-50 nm within LDVs corresponding to cores of LDVs, which are 70-90 nm in mean diameter.

Subsensitivity of enteric cholinergic neurones to alpha 2-adrenoceptor agonists after chronic sympathetic denervation

The concentration-effect relationships of noradrenaline, dopamine and clonidine in inhibiting resting and stimulated acetylcholine output have been studied in intact and in sympathetically denervated preparations of guinea pig isolated distal colon. The order of potencies for the inhibition of resting acetylcholine release in intact preparations was clonidine greater than dopamine greater than noradrenaline while the order of intrinsic activities was noradrenaline greater than dopamine greater than clonidine. Sympathetic denervation was able to modify the potency of either clonidine, dopamine and noradrenaline. Noradrenaline was 6 times more potent in inhibiting resting acetylcholine release in denervated than in intact preparations, while clonidine and dopamine underwent a 18-fold and a 11-fold decrease in potency after denervation. The potency of clonidine relative to noradrenaline was 110 in intact preparations and only 1.2 in denervated organs. The intrinsic activities of noradrenaline, dopamine and clonidine were almost unchanged in denervated organs. A dose-dependent facilitatory effect of yohimbine on both the resting acetylcholine output and the peristaltic reflex could be observed in intact but not in sympathetically denervated preparations at concentrations ranging from 2.5 X 10(-8) M to 2.5 X 10(-7) M. Yohimbine was able to counteract the inhibitory effect of dopamine and to remove the inhibitory effect of periarterial nerve stimulation on both acetylcholine release and the peristaltic reflex. Our results are consistent with the existence of a tonic physiological modulation of enteric cholinergic neurones by postganglionic sympathetic fibres. The order of potencies of adrenoceptor agonists and the antagonism by yohimbine is consistent with such a modulation being entirely carried out through alpha 2-heteroceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral receptor populations involved in the regulation of gastrointestinal motility and the pharmacological actions of metoclopramide-like drugs

This minireview is concerned with a re-examination of the locus of action and the possible peripheral mechanisms involved in the gastrointestinal (GI) stimulant effects of metoclopramide. Such a re-evaluation is opportune given the increasing use of this drug in the therapy of certain GI tract disorders. To provide an orientation on this subject the location in the GI tract and function of several relevant receptor types have been reviewed. In the past metoclopramide has been reported to enhance contractions of a variety of GI preparations to electrical stimulation, acetylcholine, carbachol and ganglion stimulants, to inhibit responses to alpha 2-adrenoreceptor agonists and 5-hydroxytryptamine, as well as blocking those to dopamine. Also in such preparations metoclopramide facilitates the release of acetylcholine to transmural stimulation. One important question is whether this effect is mediated via a specific prejunctional receptor. In this respect 2 suggestions have been made. Firstly that the drug may act as a preferential, prejunctional muscarinic antagonist thus inhibiting the negative feedback inhibition of acetylcholine release and secondly that metoclopramide may be a prejunctional agonist (partial) at 5-hydroxy-tryptamine receptors. Although the latter possibility appears most tenable at present, the involvement of a specific receptor remains to be confirmed. The important finding that dopamine receptors are probably not involved in the local stimulant effects of metoclopramide has important implications for future research orientated towards the discovery of a new generation of GI drugs lacking the side effects associated with central dopamine receptor blockade. Several compounds (cinitapride, BRL 20627A and cisapride) are now in the early stages of clinical evaluation.

Beta adrenergic influence on esophageal and colonic motility in man

Gastrointestinal (GI) motility is centrally controlled through the sympathetic and parasympathetic nerves, sympathetic effects being partly mediated by beta adrenoceptors. Although beta adrenoceptor agonists and antagonists are widely used for different disorders, little is known about the influence of these agents on GI motility. The present study was initiated to investigate whether there is a physiological, beta adrenergic influence on human GI motility and to describe the effects of selective beta adrenoceptor stimulation on motility in the proximal and distal parts of the GI tract. Esophageal peristalsis was measured in healthy subjects using electronic catheters. Distal colonic motility was measured with an open-tipped, water-perfused catheter in the sigmoid colon and from an air-filled balloon in the rectum in healthy subjects and in patients with the irritable bowel syndrome (IBS). In one study, colonic motility was stimulated with continuous infusion of the octapeptide of cholecystokinin (CCK-OP). Esophagus: Peristaltic amplitude was increased in the distal smooth muscle part of the esophageal body after infusion of both the nonselective beta blocker propranolol and the beta-1 selective blocker metoprolol. After infusion of the beta-1 agonist prenalterol and the beta-2 selective agonist terbutaline, a profound decrease in esophageal peristaltic amplitude was seen. Pretreatment with metoprolol selectively blocked the response to a moderate dose of prenalterol but did not block the response to terbutaline. The latter response was blocked by propranolol. Peristaltic velocity in the proximal part of the esophagus was decreased by beta-1 stimulation and in the distal part by beta-2 stimulation. Distal colon: In healthy subjects the sigmoid motility index showed a dose-dependent increase after metoprolol and propranolol, respectively. The increase was more marked after propranolol infusion. Terbutaline decreased the sigmoid motility index both in healthy subjects and in patients with the IBS. Furthermore, the rectal motility index was decreased in the group of healthy subjects. The effects of prenalterol on rectal and sigmoid motility did not differ from those of placebo. The IBS patient group showed larger intraindividual variations in sigmoid motility from day to day and also lower rectal motility indices than the healthy subjects. Infusion of CCK-OP increased the sigmoid motility index compared to non-stimulated conditions. No effects on CCK-OP stimulated motility were seen after either terbutaline, prenalterol or placebo.(ABSTRACT TRUNCATED AT 400 WORDS)

Alpha-2 receptors in the gastrointestinal system: a new therapeutic approach

Alpha-2 receptor activation mediates the inhibition of a number of gastrointestinal functions including gastric and intestinal secretions. Alpha-2 receptors are located in the brain and presynaptically on cholinergic nerve terminals; activation of either inhibits vagus nerve activity. Intestinal secretions are inhibited by postsynaptic alpha-2 receptors located on intestinal epithelial cells. Agents which selectively activate alpha-2 receptors in the gut may therefore be beneficial in treating gastric ulcers and diarrheal states. Two such agents which activate alpha-2 receptors in the gut are WHR-1370A [1-n-butoxy-3-(2,6-dimethylphenylcarbamoyl) guanidine hydrochloride] and lidamidine. WHR-1370A is a potent gastric antisecretory and antiulcer agent which inhibits the release of acetylcholine from the vagus nerve. WHR-1370A's activity is blocked by yohimbine. Lidamidine is a clinically effective antidiarrheal agent. Lidamidine's response is partially inhibited by yohimbine in animal diarrheal models. Alpha-2 agonists represent a new class of drugs which have a promising future in the treatment of gastrointestinal disorders.

Influence of sympathectomy on alpha 2 adrenoceptor binding sites in canine blood vessels

The effects of denervation of alpha 2 adrenoceptor binding sites were examined in canine arteries and veins. Denervation of the lower abdominal aorta, renal and femoral arteries and femoral veins marked reduced vessel norepinephrine concentrations. Denervation had little effect on the concentration of alpha 2 adrenoceptor binding sites or the affinity of (3H)yohimbine for these sites. The apparent lack of any significant reduction in receptor binding sites suggests that the majority of these sites are located on smooth muscle cells of blood vessels. The failure of any appreciable rise in receptor concentration following denervation is consistent with the hypothesis from functional studies that postsynaptic alpha 2 adrenoceptors on blood vessels are located extra-synaptically and hence not influenced by neurally released norepinephrine.