Beta-adrenergic control of motility in the rat colon. I. Evidence for functional separation of the beta 1- and beta 2-adrenoceptor-mediated inhibition of colon activity

Distribution of adrenergic receptors in the enteric nervous system of the guinea pig, mouse, and rat

Adrenergic receptors in the enteric nervous system (ENS) are important in control of the gastrointestinal tract. Here we describe the distribution of adrenergic receptors in the ENS of the ileum and colon of the guinea pig, rat, and mouse by using single- and double-labelling immunohistochemistry. In the myenteric plexus (MP) of the rat and mouse, alpha2a-adrenergic receptors (alpha2a-AR) were widely distributed on neurons and enteric glial cells. alpha2a-AR mainly colocalized with calretinin in the MP, whereas submucosal alpha2a-AR neurons colocalized with vasoactive intestinal polypeptide (VIP), neuropeptide Y, and calretinin in both species. In the guinea pig ileum, we observed widespread alpha2a-AR immunoreactivity on nerve fibers in the MP and on VIP neurons in the submucosal plexus (SMP). We observed extensive beta1-adrenergic receptor (beta1-AR) expression on neurons and nerve fibers in both the MP and the SMP of all species. Similarly, the beta2-adrenergic receptor (beta2-AR) was expressed on neurons and nerve fibers in the SMP of all species, as well as in the MP of the mouse. In the MP, beta1- and beta2-AR immunoreactivity was localized to several neuronal populations, including calretinin and nitrergic neurons. In the SMP of the guinea pig, beta1- and beta2-AR mainly colocalized with VIP, whereas, in the rat and mouse, beta1- and beta2-AR were distributed among the VIP and calretinin populations. Adrenergic receptors were widely localized on specific neuronal populations in all species studied. The role of glial alpha2a-AR is unknown. These results suggest that sympathetic innervation of the ENS is directed toward both enteric neurons and enteric glia.

Stimulated active potassium secretion in a patient with colonic pseudo-obstruction: a new mechanism of secretory diarrhea

BACKGROUND & AIMS

Secretory diarrhea is caused by inhibition of intestinal active sodium absorption and stimulation of active chloride secretion. The resulting increase in fecal sodium salts causes an isotonic increase in fecal water output. Abnormalities in potassium transport are not known to be a cause of secretory diarrhea. The aim of our report is to describe a patient with secretory diarrhea that was mediated by excess intestinal secretion of potassium.

METHODS

A 78-year-old woman developed colonic pseudo-obstruction, complicated by severe diarrhea and hypokalemia. Her stools were collected quantitatively on 11 occasions and analyzed for electrolyte concentrations. Rectosigmoid potential difference was measured.

RESULTS

The diarrheal fluid had a very high potassium concentration (130-170 mEq/L) and a very low sodium concentration (4-15 mEq/L). Stool potassium losses were as high as 256 mEq/day (normal, 9 mEq/day), and fecal sodium losses were never higher than 13 mEq/day. Potential difference between colonic lumen and a peripheral reference electrode was -14 mV (lumen side negative).

CONCLUSIONS

Fecal potassium salts were the exclusive driving force for severe secretory diarrhea in a patient with colonic pseudo-obstruction. The high fecal output of potassium was due to stimulation of active colonic potassium secretion, possibly because of changes in autonomic nervous system activity and distention of the colon in association with colonic pseudo-obstruction. The extremely low fecal excretion of sodium indicates that active sodium absorption was not inhibited. This case study reveals an ion transport mechanism of secretory diarrhea that has not been previously appreciated.

Effects of dopamine in isolated rat colon strips

The aim of the present work is to investigate the effects of dopamine on isolated rat colon strips, and whether dopamine receptors are involved in these effects. Experiments on spontaneous motility and under potassium contraction were performed with dopamine and isoprenaline, both in the absence and presence of antagonists (distal colon strips, isotonic recording, Tyrode solution, 31 °C, 1 g of resting tension). At higher concentration (10–4mol/L), dopamine abolished spontaneous motility of the rat colon and this effect was not modified by antagonists. In isolated rat colon strips that were depolarized with potassium, dopamine produced concentration-dependent relaxation, without significant differences in reserpinized rats. Preincubation with sulpiride or Sch 23390, dopamine antagonists, did not modify the effects of dopamine. Propranolol shifted the concentration-response curve to the right, though in a noncompetitive manner. Prazosin and yohimbine (α-antagonists) did not modify the response to dopamine. Isoprenaline produced a concentration-dependent relaxant response to the KCl-induced contraction antagonized by propranolol, but not by prazosin, in a noncompetitive manner. In conclusion, dopamine exhibits a relaxant effect on the isolated rat colon, which is not mediated by specific dopamine receptors or α-adrenoceptors but it may be mediated by atypical β-adrenoceptors. Key words: dopamine, isolated rat colon, dopamine receptors.

β3-Adrenoceptors: relaxant function and mRNA detection in smooth muscle cells isolated from the human colon

Functional involvement of β3-adrenoceptors in controlling human gastrointestinal motility has not been unequivocally assessed yet. The direct myogenic contribution of these receptors was examined, by in vitro functional studies and analysis of mRNA expression, on smooth muscle cells separately isolated from taenia coli and circular muscle layers of the human colon. Isoproterenol, a nonselective β-adrenoceptor agonist, relaxed, in a concentration-dependent manner, both human taenia coli and circular colonic smooth muscle cells, although displaying a higher intrinsic activity (65.3 ± 2.3 vs. 55.2 ± 1.4% maximal relaxation) and potency (pEC50: 7.41 ± 0.07 vs. 6.32 ± 0.08) were greater on taenia coli than circular cells. In the presence of the β1-antagonist CGP20712A and of the β2-antagonist ICI 118,551, a 25–30% decrease in isoproterenol intrinsic activity was observed on both cell types and on taenia coli, the nonselective β1/β2-antagonist propranolol produced a rightward shift of the isoproterenol concentration-response curve with mean estimated pKB values (8.12 ± 0.27 at 0.1 µM and 6.45 ± 0.13 at 1 µM) lower than that expected for both β1- and β2-adrenoceptors. CGP12177A and SR 58611A, two β3-adrenoceptor agonists, presented an intrinsic activity comparable to that of isoproterenol in the presence of β1- and β2-antagonists, the former being more potent on taenia coli than on circular smooth muscle cells. β3-Adrenoceptor mRNA was detected by reverse transcription PCR on both cell types. These results strongly suggest a direct functional role of β3-adrenoceptors in the human colon.Key words: adrenoceptors, β3-adrenoceptors, smooth muscle cells, taenia coli, human colon.

Functional and molecular evidence for beta 1-, beta 2- and beta 3- adrenoceptors in human colon

1. Relaxation of carbachol pre-contracted human colonic muscle to (-)-isoprenaline was examined in circular, longitudinal and taenia coli preparations to determine the beta-adrenoceptor subtypes involved. beta 1-, beta 2- and beta 3-Adrenoceptor mRNAs were also measured in colonic muscle and mucosa. 2. (-)-isoprenaline caused relaxation of longitudinal smooth muscle preparations with pEC50-7.39 +/- 0.12, and this response was inhibited by both propranolol (0.1 microM, pKB 8.55 +/- 0.12) and the selective beta 1-antagonist, CGP 20712A (0.1 microM, pKB 8.80 +/- 0.20), while the selective beta 2-antagonist, ICI 118551 (0.1 microM) failed to inhibit isoprenaline relaxation consistently. 3. (-)-Isoprenaline caused relaxation of taenia coli with a pEC50 of 6.70 +/- 0.17. Propranolol (0.1 microM). CGP 20712A (0.1 microM) and ICI 118551 (0.1 microM) inhibited the isoprenaline response with similar low affinities (pKB values 7.93, 7.71 and 7.54, respectively). Carbachol pre-contracted circular smooth muscle preparations failed to relax consistently to isoprenaline and these responses were not characterized. 4. beta 1- and beta 2-Adrenoceptor mRNAs were present in circular longitudinal muscle samples and taenia coli samples, and lower levels were detected in mucosa. beta 3-mRNA was also present in both muscle preparations but was not detected in human colonic mucosa. 5. In summary, beta 1-adrenoceptors are the predominant subtype mediating isoprenaline-induced relaxation of the thin longitudinal smooth muscle of human colon, while beta 1-receptors do not appear to be involved in these responses. However, beta 3-adrenoceptors may play a role in relaxation of the taenia coli as conventional antagonist affinities are low. beta 3-Adrenoceptor mRNA was present in taenia coli and circular/longitudinal smooth muscle but absent from human colonic mucosa.

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.

Abnormalities of innervation of internal anal sphincter in fecal incontinence

Physiological and histological studies have shown that the internal and sphincter is abnormal in idiopathic fecal incontinence. We have recently demonstrated that the in vitro contractile response of the internal anal sphincter to the sympathetic neurotransmitter noradrenaline is decreased in incontinence. In this study we have further defined this reduced sensitivity and provided more information about the intrinsic innervation in both the normal and the incontinent sphincter muscle. Muscle strips from 12 incontinent patients undergoing post and repair and from 11 controls undergoing rectal excision for low rectal carcinoma were studied. Responses to noradrenaline were recorded initially alone and then in the presence of phentolamine, an alpha-adrenoceptor blocker. In the presence of phentolamine, noradrenaline caused relaxation: there was no significant difference in the relaxation-response curves and the EC50 was the same in the two groups. These results demonstrate that the previously documented reduced sensitivity to noradrenaline is due to an altered sensitivity of the alpha-adrenoceptors. Electrical field stimulation produced relaxations in all muscle strips, but only in the controls was the magnitude of the relaxation significantly increased in the presence of phentolamine. This indicates that there is an alpha-adrenergic excitatory component of the response to electrical field stimulation of the intramural nerves, which was present in tissues from control patients but which was absent in tissues from patients with idiopathic fecal incontinence.

Review article: adrenergic control of motor and secretory function in the gastrointestinal tract

The role that the sympathetic nervous system plays in modulating physiological processes in the gastrointestinal tract is becoming clearer. It is now known that motor, secretomotor and vasomotor activity are all modulated independently by the system. Adrenoreceptor stimulation appears to reduce intestinal contraction (except at sphincters), both via alpha-receptors which inhibit neurotransmitter release and also by a direct beta-receptor mediated action on smooth muscle. There is also evidence for tonic activity in the beta-adrenergic pathway, since beta-antagonists tend to increase contraction pressures. In animals alpha-receptor-mediated pathways modulate fluid and electrolyte absorption, and alpha-adrenergic agonists enhance net absorption and reduce net secretion. In man there is also evidence for a beta-adrenergic pathway which controls secretomotor function. Carbohydrate absorption appears to be dependent on activity in a beta-adrenergic pathway, although this may be an indirect effect of changes in motor function. The time course of changes of both secretomotor and motor activity, induced by modulating sympathetic or adrenergic input, differ from the vascular changes indicating that the effects occur independently of each other. The gastrointestinal response to stressors is mediated, in part at least, by the sympathetic nervous system. Differences between individuals are likely to prove important. Since the sympathetic nervous system regulates gastrointestinal function both in the basal state and under stressful conditions, it will have effects on pathophysiological responses. Modification of such responses is likely to ameliorate symptoms, as has already been found for alpha-2-adrenergic agonists which have an antidiarrhoeal action.

In vitro inhibition of intestinal motility by phenylethanolaminotetralines: evidence of atypical beta-adrenoceptors in rat colon

1. The new compounds phenylethanolaminotetralines (PEAT), unlike the reference beta-adrenoceptor agonists isoprenaline (Iso), ritodrine (Ri) and salbutamol (Sal), produced half-maximal inhibition of spontaneous motility of rat isolated proximal colon at substantially lower concentrations (EC50 2.7-30 nM) than those inducing beta 2-adrenoceptor-mediated responses (relaxation of guinea-pig isolated trachea and rat uterus) and had virtually no chronotropic action (EC50 greater than 3 x 10(5) M) on the guinea-pig isolated atrium (a beta 1-adrenoceptor-mediated response). 2. The nonselective beta-adrenoceptor antagonists alprenolol and propranolol prevented the inhibition of rat colon motility by the PEAT with low and different potencies (pA2 values around 7.5 and 6.5 respectively). Conversely alprenolol and propranolol had a higher and similar potency (pA2 values around 9.0) in preventing typical beta 1 or beta 2-responses (increase in atrial frequency by Iso or tracheal relaxation by Ri or Sal). 3. The selective beta-adrenoceptor antagonists CGP 20712A (beta 1) and ICI 118,551 (beta 2) either alone or in combination, did not prevent rat colon motility inhibition by the representative PEAT SR 58611A, which was also fully resistant to alpha-adrenoceptor, acetylcholine, dopamine, histamine, opioid and 5-hydroxytryptamine antagonists. 4. These results indicate that the PEAT are a new class of beta-adrenoceptor agonists and suggest that their preferential intestinal action may be accounted for by selectivity for atypical beta-adrenoceptors, abundant in the rat colon and distinct from the currently recognized beta 1 and beta 2 subtypes.

Single doses of ritodrine delay orocaecal transit in patients with irritable bowel syndrome

The lactulose hydrogen breath test was used to assess the effect of a single dose of the beta 2-adrenoceptor agonist ritodrine on orocaecal transit time in 11 patients (three men) with irritable bowel syndrome. Transit time (median values, range) was significantly longer (P less than 0.01) after ritodrine than after placebo (120, 50-200 vs 75, 40-100 min). Median heart rate was similar before treatments whereas the maximal increase in heart rate was significantly greater (P less than 0.01) after ritodrine than after placebo.

Central and peripheral beta-adrenergic influences on reticulo-rumen and upper-gut myoelectrical activity in sheep

The effects of intravenous (i.v.) and intracerebroventricular (i.c.v.) administration of beta-adrenoceptor agonists were evaluated on the reticulo-rumen and upper-gut myoelectrical activity in six ewes chronically fitted with intraparietal electrodes and a cannula in a lateral ventricle of the brain. Intravenous infusion of the beta 1 agonist dobutamine (30 micrograms/kg/min for 15 min) reduced the frequency of reticulo-ruminal and abomasal contractions and stimulated duodeno-jejunal motility, inducing a Phase III on the jejunum. These effects were reproduced by i.c.v. dobutamine at a dose of 10 micrograms/kg. Intravenous infusion of the beta 2 agonist ritodrine (15 micrograms/kg/min for 15 min) selectively inhibited antral and duodenal motility. Ritodrine i.c.v. (15 micrograms/kg) did not affect forestomach or gastrointestinal motility. The mixed beta 1, beta 2 agonist isoprenaline infused i.v. (0.6 micrograms/kg/min for 15 min) reproduced the effects of i.v. dobutamine, except at the antro-duodenal level which was strongly inhibited. The effects of i.v. dobutamine were antagonized by i.v. or i.c.v. acebutolol, a specific beta 1 antagonist. The effects of i.v. ritodrine were blocked by i.v. but not i.c.v. administration of propranolol, a mixed beta 1, beta 2 antagonist. These data indicate that the stimulation of central beta 1 adrenoceptors inhibits forestomach and antral motility and stimulates duodeno-jejunal motility. Stimulation of peripheral beta 2 adrenoceptors selectively inhibits duodeno-jejunal motility.

Rational pharmacotherapy of gastrointestinal motility disorders

Nervous control of gastrointestinal motility is extremely complex, is regulated by the enteric system, the "brain of the gut", and modulated by extrinsic nerves. This system with its multiplicity of transmitters and receptors does not always allow a clear interpretation of experimental data, especially with compounds lacking specificity. In this review the complex situation is described particularly in relation to receptor populations (cholinergic, adrenergic, dopamine, histamine, 5-hydroxytryptamine, opioid, gamma-aminobutyric acid (GABA), prostanoid and dihydropyridine receptors), therapeutic aspects of drugs and their usefulness in children. Newer principles with known drugs and promising new compounds with a more appropriate kinetic or fewer side-effects, deriving from distinct pharmacological groups, as candidates for the treatment of gastrointestinal disorders are considered e.g. anticholinergics (prifinium or actilonium bromide), adrenergic alpha 2-agonists (clonidine, lidamidine) for diarrhoea in diabetic neuropathy, adrenergic beta-blockers for shortening postoperative ileus (propranolol), dopamine receptor antagonists (metoclopramide, domperidone, alizapride) and another prokinetic substance (cisapride) which may be useful for a number of applications as gastro-oesophageal reflux, gastro-paresis, intestinal pseudo-obstruction, cystic fibrosis and constipation, morphine derivatives (e.g. loperamide) for intractable diarrhoea and calcium antagonists (e.g. nifedipine) for achalasia. Increasing experience in digestive tract pharmacology and reliable clinical studies will furthermore be the basis for a more specific and better tolerated therapy of gastrointestinal motility disorders in adults and children.

Muscarinic receptor interaction with full and partial beta-adrenoceptor agonists in the rat colon strip

beta-Adrenoceptor agonists inhibit contractile activity in isolated colon strips. In order to demonstrate that beta-adrenoceptors are located at different functional levels within the colon wall, increasing concentrations of muscarinic agonists were used to interact functionally with the beta-adrenoceptor-mediated inhibition of spontaneous colon activity. The effects of the full agonists isoprenaline and terbutaline and of the partial agonist prenalterol were functionally antagonized by carbachol (0.03 and 0.3 mumol/l) and bethanechol (1.3 and 30 mumol/l). This functional antagonism was parallelled by an increase in baseline tension and spontaneous contractile activity of the isolated colon strip. At lower concentrations of carbachol (0.003 and 0.01 mumol/l) or bethanechol (0.03 and 0.3 mumol/l) no effect on the contractile status of the smooth muscle or on the pD2-values of the full agonists was seen. However, at these lower concentrations of muscarinic agonists a marked decrease in the maximal inhibitory response to the partial beta-adrenoceptor agonist prenalterol was demonstrated. The inhibitory response to prenalterol showed a biphasic concentration-response curve. The muscarinic antagonist atropine produced an increase in the maximal response of the high potency component of the concentration-response curve for prenalterol and an increase in the sensitivity to isoprenaline. These results demonstrate the presence of a high cholinergic tone in the colon preparation of a magnitude that clearly reduces the sensitivity to beta-adrenoceptor agonists. The different responses to full and partial beta-adrenoceptor agonists in the presence of increasing concentrations of muscarinic agonists may indicate that beta-adrenoceptors are located on two different functional units within the colon wall.

Neuropeptide Y inhibits excitatory enteric neurons supplying the circular muscle of the guinea pig small intestine

The effect of neuropeptide Y on the motor activity of the circular and longitudinal muscle of the guinea pig small intestine was investigated. Neuropeptide Y (0.3-30 nM) inhibited, in a concentration-dependent manner, the ascending enteric reflex contraction of the circular muscle; both the cholinergic and noncholinergic component of the ascending enteric reflex were suppressed. Neuropeptide Y also blocked nerve-mediated phasic contractions of the circular muscle, induced by dimethylphenylpiperazinium or ceruletide, and abolished the peristaltic movements of the small intestine. Nerve-mediated cholinergic and noncholinergic contractions of the longitudinal muscle, induced by electrical field stimulation, were only slightly depressed by neuropeptide Y. Further analysis showed that neuropeptide Y did not act directly on intestinal muscle but interrupted excitatory pathways of the enteric nervous system. The effect of neuropeptide Y did not involve activation of alpha-adrenoceptors, opioid receptors, or P1 purinoceptors or the release of endogenous factors stimulating these receptors. These findings suggest a possible physiologic role of neuropeptide Y as a nonnoradrenergic inhibitory transmitter involved in the enteric nervous control of intestinal circular muscle activity.

Autoradiographic location of beta-adrenoceptor subtypes in cat colon smooth muscle

In order to localize beta-adrenoceptors 125I-(-)pindolol (IPIN) was used in binding to sections from cat colon. The binding characteristics for IPIN to beta-adrenoceptors on colon sections were estimated by demonstrating reversible binding in the presence of isoprenaline and by steroselective binding to the isomers of propranolol. The binding of IPIN to both beta 1- and beta 2-adrenoceptors was shown by biphasic displacement curves in the presence of the selective beta-adrenoceptor compounds betaxolol, ICI 118.551 and procaterol. The colon sections were found to contain proportions of beta 1-adrenoceptors (30-50%) and beta 2-adrenoceptors (50-70%). In the autoradiographic studies, 100% of the developed grains after exposure of IPIN to the photographic emulsion were displaced by 50 microM of isoprenaline. By microscopic counting at autoradiographic grains, 30-40% of the grains were found in the circular smooth muscle, while 60-70% of the grains were found in the longitudinal smooth muscle. A concentration of 2 nM ICI 118.551 completely displaced all grains in the circular smooth muscle and partly displaced those found in the longitudinal smooth muscle. A high concentration of ICI 118.551 (1 microM) displaced all grains above background from the smooth muscle. It is concluded that the circular smooth muscle only contains beta 2-adrenoceptors, while longitudinal smooth muscle may contain a proportion of beta 1-adrenoceptors. Whether such a location of beta adrenoceptors can be related to the beta 1-adrenoceptor-mediated inhibition of colon motility can not be clarified from these studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased beta I-adrenoceptor density after 6-hydroxydopamine pretreatment in rat colon and lung

Estimation of beta-adrenoceptor-binding sites with 125I-(-)-pindolol in rat colon show a proportion of 30% beta I-adrenoceptors and 70% beta 2-adrenoceptors. Studies on the isolated colon strip have revealed a neuronal beta-adrenoceptor involved in the inhibitory response of colon motility to beta-adrenoceptor stimulation. In order to further characterize the beta-adrenoceptors in the colon, acute and chronic treatments with 6-hydroxydopamine were made. Both acute pretreatment of rats with 6-hydroxydopamine for 8 and 24 h (one intravenous injection) and chronic treatment for 3 days (implanted osmotic mini-pumps), reduced the noradrenaline tissue content by 90%, and successively increased the beta-adrenoceptor-binding sites from 14.2 to 21.7 fmol mg-I P-I in colon and from 158 to 240 fmol mg-I P-I in lung membranes. Displacement of the radiolabelled ligand by the selective beta-adrenoceptor antagonists, pafenolol and ICI 118.551 showed that the density of beta I-adrenoceptor binding sites was more than doubled, whereas the density of beta 2-adrenoceptor-binding sites was only marginally increased by chronic treatment with 6-hydroxydopamine. Thus sympathetic denervation by 6-hydroxydopamine treatment produced a selective increase in beta I-adrenoceptors in the rat colon. These results may indicate that stimulation of beta I-adrenoceptors in both colon and lung have a neuronal linkage.