In vitro study of the inhibitory effects of dopamine on the rat 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.

Assessment of intestinal peristalsis in vitro

The protocol detailed in this unit is designed to assess intestinal peristaltic motility in the isolated small intestine in vitro and to measure the effects of drugs able to interfere with gut propulsive activity. The procedure is based on Trendelenburg's classic technique, described at the beginning of the 20th century in the isolated guinea pig ileum and, later on, extended to other intestinal preparations from the same animal and other animal species. This unit illustrates the basic procedures for setting up the intestinal preparation, recording peristalsis under near-physiologic conditions, and testing the pharmaco-toxicological effects of drugs and pollutants on the contractile behavior of the gut wall. The protocol allows evaluating the action of drugs affecting sensory and/or motor neurons of the enteric nervous system and how these neurons control the development of the motor program of the gut wall. This model can be exploited to investigate novel compounds undergoing preclinical development and both inhibitors and stimulants of gastrointestinal peristaltic activity, as well as environmental or alimentary pollutants, like xenobiotics and naturally-occurring toxins, endowed with noxious activity with regard to digestive functions.

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

Assessment of gastrointestinal motility using three different assays in vitro

The protocols detailed in this unit are designed to assess the motor activity of different gastric and intestinal muscle preparations in vitro and the effects of drugs that modulate gastrointestinal motility. The preparations described are characterized by different contractile behaviors, consisting of spontaneous (duodenum), neurogenic (ileum), and drug-stimulated (fundus, ileum) motility; these reproduce motility patterns occurring in the gut wall in vivo. These protocols document the variety of factors that can influence the responses of isolated tissues and describe how such tissues can be used for testing substances that affect gut movements. These preparations allow evaluation of direct interactions with the processes that control contractile machinery, as well as indirect effects resulting from the modification of neurotransmitter release from myenteric neurons. These models can be exploited to assay novel compounds undergoing preclinical development or to evaluate the functional toxicity exerted by environmental or alimentary pollutants, like xenobiotics and naturally occurring toxins, as well as the mechanisms underlying these effects.

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.

Influence of aging on gastric emptying of liquids, small intestine transit, and fecal output in rats

The gastric emptying of a liquid meal, the small bowel transit, and the number of pellets and fecal output produced during a 24-h period, were studied in young (three months), adult (12 months) and old (24 months) male Wistar rats. The gastric emptying after 20 min of an intragastrically administered liquid meal containing phenol red and methylcellulose was significantly decreased in old rats. The small bowel transit after 15 and 30 min of the front of a charcoal and Arabic gum containing intragastrically administered meal was similar in the three age groups. The number of pellets and the mass of the feces produced during a 24-h period were significantly decreased with age, while the food intake was similar. The water content of the pellets was similar in the three age groups. These results show decreased gastric emptying of liquids and decreased stool mass in old rats, corresponding with the previously reported age-related changes in colonic smooth muscle contractility. Small intestinal transit was well maintained with age.

Development of cholinergic and inhibitory non-adrenergic non-cholinergic responses in the rat gastric funds

1. Cholinergic contractions and inhibitory non-adrenergic non-cholinergic (NANC) relaxations were studied in longitudinal muscle strips of the gastric funds of 2, 4 and 8 week old rats. 2. Contractions induced by electrical stimulation of the cholinergic neurones and by administration of acetylcholine decreased during development. The potentiating effect of physostigmine was similar in the 3 age groups. 3. Short train stimulation in NANC conditions induced fast relaxations, which were more pronounced in 4 and 8 week than in 2 week old rats. These relaxations were almost completely inhibited by NG-nitro-L-arginine methyl ester (L-NAME) in the 3 age groups. The nitric oxide-induced relaxations did not change during development. 4. Sustained electrical stimulation in NANC conditions induced an initial relaxation, which was almost totally blocked by L-NAME, followed by an almost complete recovery of tone at lower frequencies of stimulation. At higher frequencies of stimulation, the recovery of tone was incomplete or absent. This sustained relaxation was only partially reduced by L-NAME and almost abolished by L-NAME plus alpha-chymotrypsin. The initial relaxations increased during development, while the sustained relaxations remained similar during this period. Vasoactive intestinal polypeptide-induced relaxations were also similar in the 3 age groups. 5. These results show that the sensitivity of the gastric fundus to acetylcholine decreases from 2 weeks to 8 weeks postnatally, while the importance of the nitrergic innervation increases during this period.

Inhibitory effect of dopamine on gastric motility in rats

BACKGROUND

There is disagreement with regard to the involvement of dopamine (DA) receptors in gastric motility. The mechanisms of the inhibitory effect of DA on rat gastric motility was investigated in association with acetylcholine (Ach) release in the present study.

METHODS

In vivo vagotomized, splanchnicectomized rats and control rats were used, and gastric movement was determined as the gastric motility index after DA administration. In vitro study of Ach release from the circular muscle strips of the gastric corpus was investigated after administration of domperidone, SCH23390, phentolamine, or propranolol.

RESULTS

In the in vivo study DA inhibited gastric motility in a dose-dependent manner. Vagotomy and splanchnicectomy had no effect on the inhibitory effect of DA. In vitro study DA inhibited [3H]-Ach release in a dose-dependent manner. The inhibitory effect of DA was antagonized by domperidone but not by phentolamine, propranolol, or SCH23390.

CONCLUSIONS

Inhibition of gastric motility by dopamine is independent of extrinsic innervation and seems to be mediated by DA2 receptors in the gastric wall.

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.

Tachykinin receptors involved in the contractile effect of the natural tachykinins in the rat gastric fundus

1. The receptors involved in mammalian tachykinin-induced contractions of longitudinal smooth muscle strips of the rat gastric fundus were characterized pharmacologically. 2. Substance P (SP), neurokinin A, neurokinin B and senktide contracted the strips in a concentration-dependent manner with a potency order of neurokinin A > or = senktide > neurokinin B > substance P. The contractions were not influenced by tetrodotoxin and atropine. 3. L 659877, a NK2B-receptor-preferring antagonist reduced neurokinin A- and neurokinin B-induced contractions (estimated pKB 6.9 and 6.3, respectively) but had less pronounced effects on SP-induced contractions and none on contractions induced by senktide. MEN 10376, an NK2A-receptor-preferring antagonist, reduced the neurokinin A-induced contractions (estimated pKB 5.2), while dactinomycin, reduced the neurokinin A-induced contractions only to a minor extent at 10(-4) M. 4. CP 96345, an NK 1-receptor antagonist, reduced substance P- and neurokinin A-induced responses, but also reduced the contractions induced by KCl and methacholine. RP 67580, another non-peptide NK1-receptor antagonist had no effect on the substance P-, neurokinin A- and neurokinin B-induced contractions up to a concentration of 3 x 10(-6) M. 5. These results suggest that the mammalian tachykinins induce contractions of the longitudinal smooth muscle strip of the rat gastric fundus by direct action at muscular NK2B- and NK3-receptors.

Modulation of non-adrenergic non-cholinergic inhibitory neurotransmission in rat gastric fundus by the alpha 2-adrenoceptor agonist, UK-14,304

1. The influence of the alpha 2-adrenoceptor agonist, UK-14,304, on non-adrenergic non-cholinergic (NANC) relaxation induced by electrical field stimulation was investigated in longitudinal muscle strips of the gastric fundus of reserpinized rats. 2. In tissues where tone was raised by 3 x 10(-7) M prostaglandin F2 alpha (PGF2 alpha), the inhibitory effect of 10(-6) M UK-14,304, on the NANC relaxations induced by short train stimulation (40 V, 1 ms, 20 s) was inversely related to the stimulus frequency (1-4-16 Hz). UK-14,304 (10(-6) M) did not influence relaxations induced by administration of exogenous nitric oxide (NO, 2 x 10(-6) M-10(-4) M). The inhibitory effect of UK-14,304 on the electrically induced relaxations was antagonized by 10(-6) M rauwolscine but not by 10(-6) M prazosin. 3. UK-14,304 (10(-6) M) also reduced the amplitude of the sustained NANC relaxation, induced by electrical field stimulation (40 V, 1 ms, 4 Hz) for 5 min. The effect of UK-14,304 was also antagonized by 10(-6) M rauwolscine but not by 10(-6) M prazosin. UK-14,304 (10(-6) M) did not reduce the relaxation induced by 3 x 10(-9) M vasoactive intestinal polypeptide (VIP). 4. These results suggest that the release of the inhibitory NANC neurotransmitter during short train stimulation, thought to be NO, and during sustained stimulation, thought to be VIP, is inhibited by stimulation of presynaptic alpha 2-adrenoceptors in the rat gastric fundus.

Influence of age on responsiveness of rat gastric fundus to agonists and to stimulation of intrinsic nerves

The contractile effect of methacholine, prostaglandin F2 alpha (PGF2 alpha) and electrical stimulation of cholinergic neurones, and the relaxant effect of nitric oxide (NO), vasoactive intestinal polypeptide (VIP) and electrical stimulation of inhibitory non-adrenergic non-cholinergic (NANC) neurones were studied in longitudinal smooth muscle strips of the gastric fundus of young (3 months), adult (12 months) and old (24 months) male Wistar rats. The contractile responses to methacholine and to electrical stimulation of cholinergic neurones were not significantly different between the three age groups. The responses to PGF2 alpha were significantly more pronounced in young than in adult and old rats. The relaxant response to electrical stimulation of NANC neurones with a cumulative increase in frequency showed a decreased response in old rats at the higher stimulation frequencies. This was mimicked by a decreased response to VIP, suggesting that there is a decrease in muscular sensitivity to VIP rather than an impaired capacity to VIP release with increasing age. The relaxant response to electrical stimulation of NANC neurones with short trains was similar in the three age groups, while the sensitivity to exogenous NO increased with age. The latter might be a compensatory mechanism for a decrease in stimulation-induced NO release with age.

Influence of vasoactive intestinal polypeptide and NG-nitro-L-arginine methyl ester on cholinergic neurotransmission in the rat gastric fundus

The possible modulating effect of vasoactive intestinal polypeptide (VIP) and nitric oxide (NO), on cholinergic neurotransmission was assessed in longitudinal muscle strips of rat gastric fundus. VIP and NO are the putative co-transmitters of the inhibitory non-adrenergic non-cholinergic (NANC) neurones in this tissue. VIP concentration dependently inhibited cholinergic contractions induced by 2-min transmural stimulation, relaxed tissues, the tone of which was continuously raised by transmural stimulation, and shifted to the right the frequency-response curves for contraction induced by transmural stimulation with a cumulative increase of frequency. The same effect was found when contractions were induced with methacholine, suggesting that only functional antagonism at the postsynaptic smooth muscle cell level is involved. On 30-min incubations, 3 x 10(-4) M NG-nitro-L-arginine methyl ester (L-NAME) potentiated cholinergic responses to 20-s transmural stimulation, while not influencing contractions of similar amplitude evoked by methacholine; the cholinergic responses to 2-min transmural stimulation were also not influenced. The potentiating effect of L-NAME was prevented by L-arginine but not D-arginine. These results suggest that endogenous NO released from the inhibitory NANC neurones during short trains of transmural stimulation interferes with cholinergic neurotransmission either by functional antagonism of acetylcholine at the postsynaptic level or by presynaptic inhibition of acetylcholine release.

Influence of fenoldopam and quinpirole in the guinea-pig stomach

1. The influence of the selective DA1-agonist fenoldopam and the selective DA2-agonist quinpirole was investigated in the guinea-pig intact stomach model and in guinea-pig gastric corpus muscle strips. 2. In the intact stomach model, quinpirole induced a relaxation from 10(-6) M on. The relaxation by quinpirole (3 x 10(-5) M) was significantly inhibited by rauwolscine (10(-7) M), yohimbine (10(-7) M) and domperidone (10(-6) M). In the presence of tetrodotoxin, quinpirole (3 x 10(-5) M) induced a contraction. 3. In the same model, fenoldopam induced a relaxation but only at 3 x 10(-5) M. The relaxation by fenoldopam (3 x 10(-5) M) was not inhibited by SCH 23390 (10(-6) M). The relaxant effect of dopamine (3 x 10(-6) M) was significantly inhibited by rauwolscine (10(-7) M), yohimbine (3 x 10(-7) M), haloperidol (10(-6) M) and domperidone (10(-6) M). 4. In circular muscle strips of the gastric corpus, the electrically induced cholinergic contractions were inhibited by dopamine but not consistently influenced by quinpirole or fenoldopam. 5. Dopamine, fenoldopam and quinpirole induced an increase in basal tone of the strips. The contraction by dopamine (10(-5) M) was significantly antagonized by prazosin and methysergide. 6. No evidence was thus found for the presence of DA1-receptors in both guinea-pig stomach models. Equally, no evidence for the presence of DA2-receptors was found when studying quinpirole in the strips. Although the relaxant effect of quinpirole in the intact stomach seems predominantly mediated via alpha 2-adrenoceptors, an involvement of DA2-receptors cannot be excluded.

Excitatory and inhibitory effects of norepinephrine on myenteric neurons of the guinea-pig gastric corpus

The effects of norepinephrine on the electrical and synaptic behaviour of gastric myenteric neurons were investigated in vitro by using conventional intracellular recording methods. Application of norepinephrine (0.1-10 microM) evoked an excitatory effect in 40% of all cells tested. Excitation consisted of a depolarization of the membrane potential associated with increased spike discharge. Phentolamine or prazosin reversibly abolished and (-)phenylephrine mimicked the excitatory norepinephrine response. Yohimbine and clonidine had no effect. Focal electrical stimulation of interganglionic fibre tracts evoked fast excitatory postsynaptic potentials (fEPSPs) in all neurons. Only a minority of these fEPSPs were blocked by norepinephrine. However, fEPSPs evoked by stimulating presumably extrinsic nerves were always totally blocked by norepinephrine. The inhibitory effect of norepinephrine on fEPSPs could be reversed by phentolamine and yohimbine and mimicked by clonidine. Prazosin and phenylephrine had no effect. Isoproterenol and propranolol modified neither the excitatory nor the inhibitory effects. The results indicate that the excitatory effects of norepinephrine on gastric myenteric neurons are mediated by postsynaptic alpha 1 receptors, whereas the inhibitory effects are mediated by presynaptic alpha 2 receptors, which are located presumably on vagal extrinsic nerves. There was no evidence for beta-receptor-mediated effects in gastric myenteric neurons.

Motor activity and neurotransmitter release in the gastric fundus of streptozotocin-diabetic rats

As deficiencies of the cholinergic and non-adrenergic non-cholinergic innervation of the gastrointestinal tract have been described in diabetic rats, we studied the simultaneous release of, and muscular response to, acetylcholine, vasoactive intestinal polypeptide and adenosine-5'-triphosphate in isolated preparations of gastric fundus from control and 8-week streptozotocin-treated diabetic rats. Muscular responses were measured in longitudinal muscle strips prepared from one half of the gastric fundus and release was studied in the other half. The contractile response to acetylcholine and electrical field stimulation was not different in control and diabetic rats. In the presence of atropine, and when tone was increased with prostaglandin F2 alpha, electrical field stimulation, vasoactive intestinal polypeptide and adenosine-5'-triphosphate induced relaxation with a similar response in control and diabetic rats. The basal release of acetylcholine, vasoactive intestinal polypeptide and adenosine-5'-triphosphate was not significantly different in control and diabetic rats. Electrical field stimulation significantly increased the release of the three substances and this increase was tetrodotoxin-sensitive. While the stimulation-induced increase of acetylcholine and vasoactive intestinal polypeptide was not different in control and in diabetic rats, the stimulation-induced release of adenosine-5'-triphosphate increased 3-fold in diabetic compared to control gastric fundus. Desensitization to alpha,beta-methylene adenosine-5'-triphosphate reduced the relaxant response to adenosine-5'-triphosphate and to electrical field stimulation, suggesting a role of adenosine-5'-triphosphate in non-adrenergic non-cholinergic neurotransmission of rat gastric fundus. The reduction of the non-adrenergic non-cholinergic relaxation by alpha,beta-methylene adenosine-5'-triphosphate was greater in diabetic tissues. This, with the increase in stimulation-induced adenosine-5'-triphosphate release, suggests that the purinergic component of the vagal non-adrenergic non-cholinergic response of the stomach may be increased in diabetics.

Pre- and post-junctional alpha-adrenoceptor-mediated responses in the rat gastric fundus in-vitro

The effects of alpha 1- and alpha 2-adrenoceptor agonists on smooth muscle tone and on cholinergic excitatory and non-adrenergic, non-cholinergic inhibitory responses to field stimulation have been investigated in the rat gastric fundus in-vitro. None of the alpha-adrenoceptor agonists tested, noradrenaline, phenylephrine, cirazoline, guanoxabenz or UK-14,304 showed any contractile effects at concentrations up to 30 microM. In preparations where tone was raised by barium (0.5-2 mM), the mixed alpha 1- and alpha 2-adrenoceptor agonist noradrenaline (0.01-10 microM), and the selective alpha 1-adrenoceptor agonists cirazoline (0.01-10 microM) and phenylephrine (0.01-10 microM) produced concentration-dependent relaxations which were antagonized by the alpha 1-adrenoceptor antagonist prazosin (0.01-1.0 microM). The selective alpha 2-adrenoceptor agonists UK-14,304 (0.03-30 microM) and guanoxabenz (0.03-30 microM), had no relaxant effects in raised tone. UK-14,304 (0.03-1.0 microM) produced a concentration-dependent inhibition of cholinergic nerve-induced responses which was antagonized by the alpha 2-adrenoceptor antagonist idazoxan (0.03-1.0 microM) but not by prazosin (0.03-1.0 microM). Noradrenaline (0.03-1.0 microM) also produced an inhibition of cholinergic nerve-induced responses which was antagonized by idazoxan (0.03-1.0 microM). A small component of the noradrenaline inhibitory effects was antagonized by prazosin (10%). Cirazoline (0.03-1.0 microM) produced a small inhibition of cholinergic nerve-induced responses which was antagonized by prazosin (0.03-1.0 microM). The prazosin-sensitive components of the inhibitory effects of noradrenaline and cirazoline occurred at concentrations which also produced post-junctional relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the dopamine receptor agonists, fenoldopam and quinpirole, in the rat stomach

The effects of the DA1-receptor agonist, fenoldopam, and the DA2-receptor agonist, quinpirole, were studied with the longitudinal muscle of rat gastric fundus and circular muscle of rat gastric corpus, as there are contrasting reports about the receptors involved in the inhibitory effect of dopamine in these tissues. Quinpirole had no effect on basal tone in the longitudinal muscle of the rat gastric fundus and did not inhibit the sustained contractions induced by electrical field stimulation or by methacholine. Fenoldopam had no effect on the tone increased by methacholine but slightly potentiated the electrically induced contraction at the highest concentrations; it concentration dependently (10(-7)-3 X 10(-5) M) increased the basal tone. The contractile effect of fenoldopam was clearly antagonized by rauwolscine 10(-6) M, yohimbine 10(-6) M and phentolamine 3 X 10(-6) M plus propranolol 10(-5) M. The 5-HT receptor antagonist, methysergide, antagonized the fenoldopam-induced contractions in a non-competitive way. Fenoldopam and quinpirole had no effect on contractions induced in the circular muscle of the rat gastric corpus by methacholine or electrical field stimulation. They induced some contraction at basal tone, at their highest concentrations. As fenoldopam and quinpirole did not mimic the inhibitory effect observed with dopamine in the same models, no evidence was found for the presence of inhibitory dopamine receptors in rat gastric muscle. The contractile effect of fenoldopam in the longitudinal muscle of the fundus is probably due to an interaction with 5-HT 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.

In-vitro study of the enkephalinergic hypothesis for non-adrenergic, non-cholinergic innervation in the cat stomach

It has been suggested that enkephalins are involved in the gastric relaxation induced by stimulation of the non-adrenergic, non-cholinergic vagal fibres in the cat stomach. Experiments were therefore performed on strips of cat stomach. With longitudinal and circular gastric fundus and corpus strips from reserpinized cats, non-adrenergic, non-cholinergic relaxatory responses could be elicited by transmural electrical stimulation in Tyrode solution containing atropine and 5-hydroxytryptamine. Morphine, leu-enkephalin and met-enkephalin did not influence the tone of the strips or the relaxation evoked by stimulation at 8 Hz, and neither did the opioid antagonist, naloxone. These results do not support the enkephalinergic hypothesis for the non-adrenergic, non-cholinergic vagal fibres in the cat stomach.

The effects of various pharmacological agents on the metoclopramide-induced increase in cholinergic-mediated contractions of rat isolated forestomach

In longitudinal muscle strips of rat forestomach, metoclopramide (Mcp) increased the height of cholinergic-mediated contractions evoked by electrical field stimulation, probably by facilitating the release of neuronal acetylcholine. This response to Mcp was not prevented by drugs which blocked the synthesis of prostanoids or the actions of nicotine, morphine, noradrenaline, histamine and substance P. An involvement of dopamine in the mechanism of the response to Mcp was also excluded. Tachyphylaxis with 5-hydroxytryptamine (5HT) increased the electrically-evoked contractions and prevented the response to Mcp. Of the 5HT antagonists tested, only high concentrations of methysergide or phenylbiguanide reduced the ability of Mcp to increase the cholinergic-mediated contractions. These experiments are discussed in relation to the possibility that Mcp may increase neuronal ACh release in the gut by affecting 5HT synthesis or by acting on 5HT receptors.

Differences in dopamine- and noradrenaline-induced amylase release from the rat parotid gland

The effects of dopamine and noradrenaline on amylase secretion from rat parotid gland were studied in a batch incubation system. Dopamine effectively caused amylase secretion but the concentration-response curve was shifted to the right in reserpinized animals, suggesting a minor indirect component in the action of dopamine. The noradrenaline-evoked secretion was the same in reserpinized glands as in the controls. Bromocriptine, a dopamine receptor agonist, was without effect on amylase secretion. The dopamine receptor antagonists pimozide, chlorpromazine, haloperidol and droperidol as well as SKF 38393 all effectively inhibited dopamine-induced amylase secretion without affecting the enzyme release caused by noradrenaline. The D-2 antagonist sulpiride was without effect on both dopamine- and noradrenaline-stimulated secretion. The dopamine-induced secretion was also significantly blocked by the non-selective beta-blocker propranolol as well as by beta 1- and beta 2-selective antagonists. Several alpha-antagonists were all partial blockers of dopamine-stimulated amylase secretion. In contrast, the noradrenaline-evoked amylase release was exclusively abolished by propranolol and beta 1-selective antagonists. The results suggest that the dopamine- and noradrenaline-induced amylase secretion are activated via different receptor systems, and that dopamine stimulation is mainly a postsynaptic D-1 effect and only to a minor extent due to presynaptic interaction.

The mechanism of action of dopamine to inhibit field stimulation-induced contractions of guinea pig stomach strips

Field stimulation (0.125-10 Hz) of longitudinal smooth muscle strips taken from the fundus of guinea pig stomach caused frequency related contraction responses (atropine-sensitive) associated with relaxation at the higher frequencies (both responses tetrodotoxin-sensitive). Dopamine, noradrenaline, adrenaline and apomorphine antagonised the contraction responses at all frequencies; the concentration-response curves were steep and the use of higher concentrations was precluded by changes in base line tension per se. That noradrenaline was approximately 10 fold more potent than dopamine, that the dopamine response was not antagonised by the dopamine antagonists haloperidol, domperidone or (-)sulpiride, but was mimicked by the alpha 2-agonist guanfacine and partially antagonised by the alpha 2-adrenoceptor antagonist yohimbine (which could also antagonise the inhibitory actions of guanfacine) would indicate that a component of dopamine's action to reduce cholinergic activity is effected via alpha 2-adrenoceptor mechanisms. A failure of phenylephrine to mimic, or prazosin to attenuate the agonist inhibitory responses would not indicate an additional alpha 1-adrenoceptor involvement. The persistence of the dopamine response following disruption of noradrenergic function by reserpine or inhibition of catecholamine reuptake processes (desmethylimipramine plus corticosterone, GBR 13098) would indicate a direct action of dopamine. It is concluded that the ability of dopamine to reduce cholinergic induced contractions in longitudinal smooth muscle of the stomach does not reflect a dopamine receptor or alpha 1-adrenoceptor stimulation but may involve an action on alpha 2-adrenoceptors.

Pharmacological characterization of the postjunctional beta-adrenoceptors in the rat gastric fundus

In order to characterize the postjunctional beta-adrenoceptors in the rat gastric fundus, we studied the influence of beta-agonists and beta-antagonists on methacholine-contracted fundus strips. The mixed beta-agonist isopropylnoradrenaline and the beta 2-selective agonist fenoterol had a concentration-dependent relaxing effect and at higher concentrations completely inhibited the methacholine-induced tone. The reputedly beta 1-selective agonist prenalterol only produced about 50% inhibition and another reputedly beta 1-selective agonist, tazolol, had almost no relaxing effect. The beta-antagonists propranolol (beta 1 + beta 2), practolol (beta 1), H35/25 (beta 2) and ICI 118,551 (beta 2) all shifted the concentration-response curves for isopropylnoradrenaline and fenoterol in a parallel way to the right, but the slope of the Schild plot was not significantly different from 1 only for the antagonism of isopropylnoradrenaline by H35/25. The relaxing effect of prenalterol was only clearly antagonized by ICI 118,551. The results suggest that postjunctional beta 1- and beta 2-adrenoceptors are present in the rat gastric fundus.

Inhibitory effect of dopamine on canine gastric fundus

The mechanism of the inhibitory effect of dopamine on canine stomach fundus was studied in longitudinal and circular muscle fundus strips, contracted by transmural electrical stimulation or by methacholine. Results obtained for longitudinal and circular strips were similar. Dopamine (1 X 10(-6)-1 X 10(-4) M) concentration-dependently inhibited frequency-response curves to electrical stimulation; these concentrations did not change the resting tone of the strips. Dopamine (1 X 10(-4) M), tested on contractions of similar amplitude induced in the same strips by electrical stimulation at 0.5 Hz and by methacholine, inhibited the electrically induced contractions but had little influence on the contractions induced by methacholine. The inhibition of the electrically induced contractions by dopamine 1 X 10(-4) M was not influenced by the presence of cocaine 3 X 10(-5) M or hydrocortisone 3 X 10(-5) M. The alpha 1- and alpha 2-adrenoceptor antagonist phentolamine and the alpha 2-adrenoceptor antagonist rauwolscine markedly antagonized the inhibitory effect of dopamine on the response to electrical stimulation at 0.5 Hz. The alpha 1-adrenoceptor antagonist prazosin and the dopamine receptor antagonists haloperidol and domperidone had no effect. The dopamine receptor antagonist metoclopramide decreased the inhibitory effect of dopamine but had a similar effect on the inhibition caused by noradrenaline. These results indicate that the inhibitory effect of dopamine in the dog gastric fundus is mainly mediated by an interaction with alpha 2-adrenoceptors on the intramural cholinergic neurons; this effect is largely direct since it was not influenced by cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)