Inhibitory effect of dopamine on canine gastric fundus

Dopamine induces inhibitory effects on the circular muscle contractility of mouse distal colon via D1- and D2-like receptors

Dopamine (DA) acts as gut motility modulator, via D1- and D2-like receptors, but its effective role is far from being clear. Since alterations of the dopaminergic system could lead to gastrointestinal dysfunctions, a characterization of the enteric dopaminergic system is mandatory. In this study, we investigated the role of DA and D1- and D2-like receptors in the contractility of the circular muscle of mouse distal colon by organ-bath technique. DA caused relaxation in carbachol-precontracted circular muscle strips, sensitive to domperidone, D2-like receptor antagonist, and mimicked by bromocriptine, D2-like receptor agonist. 7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390), D1-like receptor antagonist, neural toxins, L-NAME (nitric oxide (NO) synthase inhibitor), 2'-deoxy-N₆-methyl adenosine 3',5'-diphosphate diammonium salt (MRS 2179), purinergic P2Y1 antagonist, or adrenergic antagonists were ineffective. DA also reduced the amplitude of neurally evoked cholinergic contractions. The effect was mimicked by (±)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide (SKF-38393), D1-like receptor agonist and antagonized by SCH-23390, MRS 2179, or L-NAME. Western blotting analysis determined the expression of DA receptor proteins in mouse distal colon. Notably, SCH-23390 per se induced an increase in amplitude of spontaneous and neurally evoked cholinergic contractions, unaffected by neural blockers, L-NAME, MRS 2179, muscarinic, adrenergic, or D2-like receptor antagonists. Indeed, SCH-23390-induced effects were antagonized by an adenylyl cyclase blocker. In conclusion, DA inhibits colonic motility in mice via D2- and D1-like receptors, the latter reducing acetylcholine release from enteric neurons, involving nitrergic and purinergic systems. Whether constitutively active D1-like receptors, linked to adenylyl cyclase pathway, are involved in a tonic inhibitory control of colonic contractility is questioned.

In vivo and in vitro evaluation of the effects of domperidone on the gastrointestinal tract of healthy horses

OBJECTIVE

To determine the effects of domperidone on in vivo and in vitro measures of gastrointestinal tract motility and contractility in healthy horses.

SAMPLE

18 adult horses and tissue samples from an additional 26 adult horses.

PROCEDURES

Domperidone or placebo paste was administered to healthy horses in a 2-period crossover study. Gastric emptying was evaluated after oral administration of domperidone paste (1.1 or 5.0 mg/kg) or placebo paste by means of the acetaminophen absorption test in 12 horses. Frequency of defecation, weight of feces produced, fecal moisture, and stomach-to-anus transit time of microspheres were evaluated after administration of domperidone paste (1.1 mg/kg) or placebo paste in 6 horses. The effect of domperidone on smooth muscle contractile activity in samples of duodenum, jejunum, ileum, or colon obtained from 26 horses immediately after euthanasia (for nonsystemic medical problems) was investigated.

RESULTS

Oral administration of 5.0 mg of domperidone/kg increased peak plasma acetaminophen concentration and area under the curve, indicating increased gastric emptying. Administration of 1.1 mg of domperidone/kg had no effect on gastric emptying, transit time, defecation frequency, or amount and moisture of excreted feces. Contractile activities of circular and longitudinal muscle strips from the duodenum, jejunum, ileum, or colon were not altered by domperidone. Dopamine increased contractile activity of longitudinal muscle strips but not that of circular muscle strips from the midjejunum. Domperidone decreased the dopamine-induced contractile activity of midjejunal longitudinal muscle strips.

CONCLUSIONS AND CLINICAL RELEVANCE

The potential beneficial effects of domperidone in horses with ileus need to be evaluated in horses with decreased gastric emptying or adynamic ileus.

Treatment approaches of gastrointestinal dysfunction in Parkinson's disease, therapeutical options and future perspectives

Gastrointestinal (GI) dysfunction is a common but underestimated feature in Parkinson's disease (PD). Out of the multimodal spectrum of treatment options, there currently are only a few pharmacological treatments available to improve gastrointestinal motility and symptoms. Because enteric nervous function is mainly regulated by transmitters different from those involved in the brain, dopamine replacement is not a treatment option in PD patients. This article focuses on the known regulative mechanism of GI function and presents known and upcoming treatment options for GI dysfunction in PD.

The Inhibitory Effect of Dopamine on Cat Gastric Smooth Muscle

The inhibitory effect of dopamine has been studied in longitudinal and circular muscle strips of the cat gastric fundus. When tone was raised by transmural electrical stimulation and by administration of methacholine, dopamine concentration-dependently relaxed the strips but the inhibitory effect of dopamine was clearly more pronounced on electrically-induced tone. The effect of dopamine was not influenced by the presence of cocaine or hydrocortisone. The relaxant effect of dopamine, when tone was raised by methacholine, was not influenced by α- and dopamine receptor antagonists but it was significantly reduced by propranolol and ICI 118551 (erythro-dl-1-(7-methylindan-4-yloxy)-3-isopropylaminobutan-2-ol). The inhibitory effect of dopamine on the electrically-induced tone was significantly reduced by phentolamine; domperidone tended to reduce the effect of the lower concentrations of dopamine. In the presence of propranolol, phentolamine and rauwolscine concentration-dependently antagonized the inhibitory effect of dopamine on electrically-induced tone, while prazosin was without influence. These results indicate that the inhibitory effect of dopamine in the cat gastric fundus is mainly due to interaction with postjunctional β-adrenoceptors on the smooth muscle cells and with prejunctional α-adrenoceptors on the intramural cholinergic neurons

Presynaptic modulation of cholinergic neurotransmission in the human proximal stomach

1. This study investigates whether the cholinergic neurones, innervating the human proximal stomach, can be modulated by nitric oxide (NO) or vasoactive intestinal polypeptide (VIP), or via presynaptic muscarinic, alpha(2)- or 5-hydroxytryptamine(4) (5-HT(4)-) receptors. 2. Circular muscle strips, without mucosa, were incubated with [(3)H]-choline to incorporate [(3)H]-acetylcholine into the cholinergic transmitter stores. The basal and electrically-induced release of tritium and [(3)H]-acetylcholine were analysed in a medium containing guanethidine (4 x 10(-6) M), hemicholinium-3 (10(-5) M), physostigmine (10(-5) M) and atropine (10(-6) M). Tissues were stimulated twice for 2 min (S(1) and S(2): 40 V, 1 ms, 4 Hz) and drugs were added before S(2). 3. The NO synthase inhibitor L-N(G)-nitroarginine methyl ester (3 x 10(-4) M) and the NO donor sodium nitroprusside (10(-5) M), as well as VIP (10(-7) M) did not influence the basal release nor the electrically-evoked release. 4. The alpha(2)-adrenoceptor agonist UK-14,304 (10(-5) M) significantly inhibited the electrically-evoked release of [(3)H]-acetylcholine, and this was prevented by the alpha(2)-adrenoceptor antagonist rauwolscine (2 x 10(-6) M). 5. The 5-HT(4)-receptor agonist prucalopride (3 x 10(-7) M) significantly enhanced the electrically-evoked release of [(3)H]-acetylcholine, and the 5-HT(4)-receptor antagonist SB204070 (10(-9) M) prevented this. 6. When atropine (10(-6) M) was omitted from the medium and added before the second stimulation, it significantly increased the release of [(3)H]-acetylcholine. 7. These results suggest that the release of acetylcholine from the cholinergic neurones, innervating the circular muscle in the human proximal stomach, can be inhibited via presynaptic muscarinic auto-receptors and alpha(2)-adrenoceptors, and stimulated via presynaptic 5-HT(4)-receptors. No evidence for modulation by NO or VIP was obtained.

Influence of nitric oxide donors and of the alpha(2)-agonist UK-14,304 on acetylcholine release in the pig gastric fundus

This study in circular muscle strips of the pig gastric fundus aimed to measure the release of acetylcholine directly and to investigate whether NO and alpha(2)-adrenoceptor agonists can modulate acetylcholine release from cholinergic neurones. After incubation of the tissues with [(3)H]-choline, basal and electrically induced release of tritium and [(3)H]-acetylcholine were analyzed in a medium containing physostigmine (10(-5) M) as well as atropine (10(-6) M). The NO synthase inhibitor L-N(G)-nitroarginine methyl ester (3x10(-4) M), and the NO donors sodium nitroprusside (10(-5) M) and 3-morpholino-sydnonimine (10(-5) M) did not influence the basal release nor the electrically evoked release, indicating that NO does not modify [(3)H]-acetylcholine release. The alpha(2)-adrenoceptor agonist UK-14,304 (10(-5) M) significantly inhibited the electrically evoked release of [(3)H]-acetylcholine, and this effect was prevented by the alpha(2)-adrenoceptor antagonist rauwolscine (2x10(-6) M), suggesting that presynaptic alpha(2)-adrenoceptors are present on cholinergic neurones of the pig gastric fundus.

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.

Stimulatory effect of dopamine on acid secretion from the isolated rat stomach

The effect of dopamine (DA) on acid secretion was studied using the everted preparation of isolated rat stomachs. DA concentrations, measured by HPLC-ECD in the rumen, corpus and antrum were 1.06 nmol/mg protein, 0.49 nmol/mg protein and 2.92 nmol/mg protein, respectively. DA stimulated acid secretion at a concentration of 10 nM and elicited the maximum response at 10 microM, which was at a level approximately 1.56-fold that of the spontaneous secretion but only about half that of secretion induced by histamine at a concentration of 0.3 mM. The concentration-dependent stimulation by DA was antagonized by octopamine and SCH 23390. Failure of proglumide and cimetidine to affect this stimulation ruled out the participation of histamine and/or gastrin. Scopolamine and tetrodotoxin completely inhibited the acid secretion induced by low concentrations of DA but inhibited only partially the response induced by high concentrations of DA. The results obtained indicate that DA induces acid secretion via activation of the dopamine D1 receptor, located on the cholinergic neurons and on some nonneuronal cells, in the rat stomach.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Alpha 2-mediated effect of dopamine on the motility of the chicken esophagus

Dopamine (DA), apomorphine and B-HT 933 produced dose related contractions on isolated longitudinal strips of chicken esophagus, whereas phenylephrine elicited no effect. DA induced contractions of myogenic origin, these contractions were insensitive to DA antagonists and were partially suppressed by yohimbine, which suggested an alpha 2-adrenergic implication in this DA effect. This hypothesis was further investigated by performing binding experiments, in which B-HT 933 displaced the binding of [3H]DA to esophageal homogenates. The results suggest the participation of an alpha 2-adrenergic receptor in the contractile response elicited by DA in the isolated chicken esophagus.

Effect of adrenergic agents on urethral pressure and urethral compliance measurements in dog proximal urethra

The effects of adrenergic drugs on urethral compliance and urethral closure pressure at the proximal urethra of dogs were investigated using a new probe. The probe, with a balloon at the tip, was equipped with two generating and two detecting electrodes. The probe was inserted into the proximal urethra, and saline was then infused at a constant rate into the balloon with an infusion pump. The pressure and cross-sectional area of the urethra were recorded and a correlation curve of these parameters was drawn. The regression (of the form y = ax + b) was determined from the curve. Urethral compliance and closure pressures were obtained from the values of "a" and "-b/a" respectively, and the maximum cross-sectional area was measured when no further distension of the urethra was obtained. Urethral compliance decreased after the administration of phenylephrine, but increased after the administration of prazosin and labetalol. The urethral closure pressure decreased after the administration of dopamine and haloperidol. There was no change in either urethral compliance or closure pressure after the administration of clonidine, yohimbine, isoproterenol, or propranolol. The results indicate that adrenergic drugs work on urethral compliance, and that the alpha-adrenergic receptor of the proximal urethra is of the alpha-1 type. The changes in the urethral pressure after dopamine administration seems to be a response mediated by dopamine receptors.

The bindings of 3H-prazosin and 3H-yohimbine to alpha adrenoceptors in the guinea-pig stomach

Alpha adrenoceptor subtypes have been investigated by radioligand binding study in guinea-pig stomach using 3H-prazosin and 3H-yohimbine. The specific 3H-prazosin binding to guinea-pig stomach was saturable and of high affinity (KD = 1.4 nM) with a Bmax of 33 fmol/mg protein. Specific 3H-yohimbine binding to the tissue was also saturable and of high affinity (KD = 25.5 nM) with a Bmax of 150 fmol/mg protein. Adrenergic drugs competed for 3H-prazosin binding in order of prazosin greater than phentolamine greater than methoxamine greater than norepinephrine greater than clonidine greater than epinephrine greater than yohimbine. These drugs competed for 3H-yohimbine binding in order of yohimbine greater than phentolamine greater than clonidine greater than epinephrine greater than norepinephrine greater than prazosin greater than greater than prazosin greater than methoxamine. We also examined whether dopamine receptors exist in guinea-pig stomach, using radioligand binding study. Specific binding of 3H-spiperone, 3H-apomorphine, 3H-dopamine and 3H-domperidone was not detectable in the stomach. Dopaminergic drugs such as dopamine, haloperidol, domperidone and sulpiride competed for 3H-prazosin binding in order of haloperidol greater than domperidone greater than dopamine greater than sulpiride. Metoclopramide, sulpiride and dopamine competed for 3H-yohimbine binding in order of metoclopramide greater than sulpiride greater than dopamine. These results suggest that guinea-pig stomach has alpha 1 and alpha 2 adrenoceptors and has no specific dopamine receptors. It is also suggested that some dopamine receptor antagonists such as domperidone, haloperidol, sulpiride and metoclopramide have antagonistic actions on alpha adrenoceptors.

Effects of chemical sympathectomy on dopamine and noradrenaline content of the dog gastrointestinal tract

The content of dopamine and noradrenaline in the mucosa-submucosa and muscular layers of different gastrointestinal areas of the dog, and its modification by 6-hydroxydopamine or pargyline plus 6-hydroxydopamine was studied by means of high pressure liquid chromatography with electrochemical detection. The amounts of dopamine and noradrenaline show a cephalocaudal increase but their physiological levels were rather low when compared with those reported in other tissues. This finding was consistent with the sparse noradrenergic innervation classically described for the gut with histochemical methodologies. On the basis that noradrenergic neurones are considered more susceptible to 6-hydroxydopamine than dopaminergic neurones, a difference abolished by previous treatment with pargyline, our findings did not suggest the existence of dopaminergic neurones in the gastrointestinal tract of the dog. In the muscular layer of the duodenum, jejunum and ileum it was observed that catecholamine depletion by both treatments was smaller than that obtained in the mucosa-submucosa. Due to the resistance to both kinds of chemical sympathectomy exhibited by the dopamine content, the existence of dopamine-containing enterochromaffin cells is proposed in the mucosa-submucosa of different portions of the stomach and small intestine.