Benzamide action at alpha 2-adrenoceptors modifies catecholamine-induced contraction and relaxation of circular smooth muscle from guinea-pig stomach

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.

Biospectra Analysis: Model Proteome Characterizations for Linking Molecular Structure and Biological Response

Establishing quantitative relationships between molecular structure and broad biological effects has been a long-standing goal in drug discovery. Evaluation of the capacity of molecules to modulate protein functions is a prerequisite for understanding the relationship between molecular structure and in vivo biological response. A particular challenge in these investigations is to derive quantitative measurements of a molecule's functional activity pattern across different proteins. Herein we describe an operationally simple probabilistic structure-activity relationship (SAR) approach, termed biospectra analysis, for identifying agonist and antagonist effect profiles of medicinal agents by using pattern similarity between biological activity spectra (biospectra) of molecules as the determinant. Accordingly, in vitro binding data (percent inhibition values of molecules determined at single high drug concentration in a battery of assays representing a cross section of the proteome) are useful for identifying functional effect profile similarity between medicinal agents. To illustrate this finding, the relationship between biospectra similarity of 24 molecules, identified by hierarchical clustering of a 1567 molecule dataset as being most closely aligned with the neurotransmitter dopamine, and their agonist or antagonist properties was probed. Distinguishing the results described in this study from those obtained with affinity-based methods, the observed association between biospectra and biological response profile similarity remains intact even upon removal of putative drug targets from the dataset (four dopaminergic [D1/D2/D3/D4] and two adrenergic [alpha1 and alpha2] receptors). These findings indicate that biospectra analysis provides an unbiased new tool for forecasting structure-response relationships and for translating broad biological effect information into chemical structure design.

Mechanism for the gastrokinetic action of domperidone. In vitro studies in guinea pigs

To test the hypothesis that domperidone stimulates gastric muscle contraction by antagonizing the inhibitory effects of dopamine on postsynaptic cholinergic neurons in the myenteric plexus, the effects of dopamine on circular muscle from the body of the guinea pig stomach were examined. Dopamine inhibited circular muscle contraction evoked by electric field stimulation in a dose-related manner. The threshold dose was 10(-6) mol/L and half-maximal inhibition occurred at 10(-5) mol/L. Preincubation of muscle contraction with atropine or tetrodotoxin abolished the contractile response to electric field stimulation, indicating mediation via a cholinergic pathway. The adrenergic antagonists phentolamine and propranolol and the DA1 antagonist SCH 23390 were ineffective in antagonizing the action of dopamine. In contrast, the DA2 antagonist domperidone blocked the inhibitory effect of dopamine on electric field stimulation-mediated contractions. Schild analysis showed a Ki of 3 x 10(-8) mol/L and a slope of unity. In addition, it was shown that dopamine inhibited veratridine-evoked release of [3H]acetylcholine from the gastric myenteric plexus in a dose-related manner (median effective dose, 5.2 x 10(-5) mol/L). Tetrodotoxin abolished [3H]acetylcholine release evoked by veratridine, but hexamethonium had no effect. Domperidone, but not SCH 23390, antagonized the inhibitory action of dopamine. Pretreatment with pertussis toxin blocked the action of dopamine to inhibit evoked release of [3H]acetylcholine. These observations indicate that dopamine inhibits gastric muscle contraction evoked by electric field stimulation by inhibiting cholinergic transmission. This is mediated by DA2 receptors located on the postganglionic cholinergic neurons, and the pathway involves a pertussis toxin-sensitive G protein. The DA2-receptor antagonist domperidone antagonizes the inhibitory effect of dopamine, resulting in stimulation of gastric muscle contraction. This provides a mechanism for the gastrokinetic effect of domperidone.

Receptors involved in mechanical responses to catecholamines in the circular muscle of guinea-pig stomach treated with meclofenamate

1. In circular muscle strips of the fundus and corpus of guinea-pig stomach, mechanical responses to catecholamines were studied mainly in the presence of a prostaglandin biosynthesis inhibitor, meclofenamate. 2. Normal preparations developed considerable muscle tone, and adrenaline (10-100 microM) in the presence of 3-5 microM propranolol produced a multiphasic response, generally consisting of transient relaxation and contraction, followed by slow relaxation and then contraction. Responses to phenylephrine were similar to those of adrenaline. 3. Meclofenamate (0.3 microM) nearly abolished the muscle tone and under this condition, both adrenaline and phenylephrine produced a simple contraction. This response was strongly inhibited by prazosin, but only weakly by yohimbine. 4. When muscle tone was maintained by prostaglandin E2 (10 nM) in the presence of meclofenamate, phenylephrine (30 microM) produced transient relaxation followed by slow contraction in most preparations. These were strongly inhibited by prazosin. Adrenaline produced a similar response, but the relaxation was only partially reduced by prazosin. The remaining relaxation was more dominant in the middle fundic region and this was considered to be mediated through beta-adrenoceptors. 5. It is concluded that in the circular muscle of the fundic region of guinea-pig stomach, endogenous prostaglandins are involved in maintaining muscle tone and in modifying the response to catecholamines and that both contraction and relaxation are mediated by alpha 1-adrenoceptors.

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)

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.

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.

Mechanical and electrical responses to alpha-adrenoceptor activation in the circular muscle of guinea-pig stomach

1 In the circular muscle of the corpus region of the guinea-pig stomach, the effects of catecholamines on mechanical activity were studied with simultaneous recording of membrane potential by use of intracellular microelectrodes. In order to investigate responses mediated through alpha-adrenoceptors, the beta-adrenoceptors were blocked by propranolol (10(-6) M) in most experiments. 2 Adrenaline (less than 10(-6) M) produced a monophasic contraction with little change in membrane potential. At higher concentrations (greater than 10(-5) M), adrenaline usually produced an early transient contraction followed by a slow tonic contraction. During the early phase, the membrane was hyperpolarized and slow waves were reduced in amplitude. 3 The phasic contractions superimposed on the late slow phase of the contractile response were higher in frequency and larger in amplitude than in controls before adrenaline application. These changes were associated with an increase in the amplitude and frequency of slow waves, and with a spike-like component appearing on the top of each slow wave. 4 Adrenaline-induced changes in mechanical and electrical activity were inhibited by prazosin, but largely unaffected by yohimbine. In some preparations, there was a transient weak inhibition of phasic contraction before muscle tone was increased by adrenaline, and this became more pronounced in the presence of yohimbine. 5 Phenylephrine, selective for alpha 1-receptors, produced responses very similar to those of adrenaline, whereas agonists selective for alpha 2-receptors, clonidine and B-HT 920, produced only a small slow contraction without any clear change in membrane potential. 6 It is concluded that in the circular muscle of guinea-pig stomach, the contractile response and changes in the electrical slow wave activity produced by adrenaline are both mediated mainly through the activation of alpha-receptors.

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.

Antidepressant versus neuroleptic activities of sulpiride isomers on four animal models of depression

The atypical neuroleptic sulpiride is also prescribed for depression because of its activating effect. However, such an effect does not necessarily imply an action identical to that of classical antidepressants, and a laboratory comparison of the neuroleptic and antidepressant activities of sulpiride may contribute to a better definition of its psychotherapeutic profile. Sulpiride isomers were studied in the rat in four behavioural models of depression which are thought to be influenced by neuroleptics in different ways. Desipramine (imipramine) and haloperidol were employed in each test as a standard antidepressant and neuroleptic, respectively. The four tests were: 1) prevention of apomorphine-induced sedation: 2) antagonism of apomorphine-induced hypothermia; 3) behavioural despair (swim test); 4) learned helplessness ( FR2 lever pressing escape). Desipramine ameliorated behaviour in all tests; haloperidol ameliorated the response to test 1, influenced that to test 2 in a neuroleptic-like way and worsened the responses to tests 3 and 4. (-)-Sulpiride worked in a similar way to haloperidol in all tests. (+)-Sulpiride significantly and dose-dependently ameliorated the responses to test 3 and was inactive in the others. No conclusion was drawn from test 1 owing to its lack of specificity; the results of the remaining tests indicated a neuroleptic profile of (-)-sulpiride and suggested a potential "antidepressant" activity of (+)-sulpiride which merits further investigation.

Differences in the reactions of isomeric ortho- and para-aminophenols with hemoglobin

The metabolites of phenacetin, 2-hydroxyphenetidine and 4-nitrosophenetol, rapidly produced ferrihemoglobin both in vivo (dogs) and in vitro. At low concns, 2-hydroxyphenetidine was superior to 4-nitrosophenetol in ferrihemoglobin formation. The kinetics of ferrihemoglobin formation by 2-hydroxyphenetidine in solutions of purified human hemoglobin was biphasic and exhibited an unusual dose response. Similar to p-aminophenols, 2-hydroxyphenetidine was oxidized by oxyhemoglobin, and the oxidation product(s) were reduced by ferrohemoglobin with the formation of ferrihemoglobin. In addition, these oxidation products condensed to 2-amino-7-ethoxy-3H-phenoxazine-3-one (u.v., i.r., 1H-NMR and mass spectroscopy). This metabolite produced ferrihemoglobin by itself and was responsible for the slow phase of ferrihemoglobin formation observed with 2-hydroxyphenetidine. This condensation reaction, which was also observed with 2-aminophenol, prevented thioether formation of the transient o-quinonimines with the cysteine residues of hemoglobin and reduced glutathione as observed with 4-aminophenol and 4-dimethylaminophenol. Phenoxazone formation, which depends on the square of the o-quinonimine concn, was negligible at micromolar concns. At similar concns addition reactions to thiols prevailed also with 2-hydroxyphenetidine and 2-aminophenol. Other electrophilic reactions, e.g. with primary amino groups of amino acids, were insignificant. These dose-dependent differences in the reactions of isomeric aminophenols may explain the low nephrotoxicity of those o-aminophenols capable of forming phenoxazones when given in a single dose. This self-detoxication of some o-quinonimines, however, should not function during long-term exposure to repetitive low doses of such o-aminophenols.