Inhibitory Effect Of Clonidine Research Articles

Inhibition by Central Alpha-2 Adrenergic Mechanism of Thyrotropin-Releasing Hormone-Induced Gastric Acid Secretion in the Rat

Influences of central alpha-2 adrenergic agonists on thyrotropin-releasing hormone (TRH)-stimulated gastric acid secretion were examined in the perfused stomach of anesthetized rats. Clonidine, an alpha-2 adrenergic agonist, given subcutaneously or intracerebroventricularly inhibited the gastric acid secretion stimulated by intracerebroventricular TRH. Intracerebroventricular injection of norepinephrine tended to reduce the acid secretion, while phenylephrine, serotonin and quipazine (serotonin agonist) did not influence the acid secretion. Subcutaneous clonidine enhanced the acid secretion peripherally stimulated by electrical vagus stimulation. The inhibitory effect of clonidine on TRH-induced acid secretion was reversed by yohimbine, an alpha-2 adrenergic antagonist, and phentolamine. In conclusion, the present study suggests that the central alpha-2 adrenergic receptor system participates in the TRH-mediated central nervous system control of gastric acid secretion in anesthetized rats.

Inhibition by central alpha-2 adrenergic mechanism of thyrotropin-releasing hormone-induced gastric acid secretion in the rat.

Influences of central alpha-2 adrenergic agonists on thyrotropin-releasing hormone (TRH)-stimulated gastric acid secretion were examined in the perfused stomach of anesthetized rats. Clonidine, an alpha-2 adrenergic agonist, given subcutaneously or intracerebroventricularly inhibited the gastric acid secretion stimulated by intracerebroventricular TRH. Intracerebroventricular injection of norepinephrine tended to reduce the acid secretion, while phenylephrine, serotonin and quipazine (serotonin agonist) did not influence the acid secretion. Subcutaneous clonidine enhanced the acid secretion peripherally stimulated by electrical vagus stimulation. The inhibitory effect of clonidine on TRH-induced acid secretion was reversed by yohimbine, an alpha-2 adrenergic antagonist, and phentolamine. In conclusion, the present study suggests that the central alpha-2 adrenergic receptor system participates in the TRH-mediated central nervous system control of gastric acid secretion in anesthetized rats.

Changes in sensitivity to the inhibitory effects of adrenergic agonists on intestinal motor activity after chronic sympathetic denervation.

The concentration-effect relationships of adrenergic agonists in inhibiting muscular tone, carbachol-induced contraction of circular muscle strips and nerve-mediated motor activity during the peristaltic reflex have been studied in intact and sympathetically denervated preparations of isolated guinea-pig colon. The order of potencies of adrenergic agonists was different for muscular and nerve-mediated effects, being clonidine greater than noradrenaline greater than methoxamine greater than isoprenaline for the inhibition of peristalsis and isoprenaline greater than noradrenaline greater than methoxamine greater than clonidine for the relaxation of circular muscle. Denervation supersensitivity was specific for the adrenergic agonists and developed both to the muscular and nerve-mediated effects, involving both alpha and beta receptors. The degree of potentiation was similar for noradrenaline and isoprenaline when measured for the muscular effects but was significantly higher for noradrenaline than for isoprenaline or methoxamine when measured for peristalsis inhibition. No potentiation could be observed for papaverine and for the muscular effects of methoxamine and phenylephrine. The increase in potency of noradrenaline ranged from a 26-fold increase for the inhibition of propulsion velocity to a 2.5-fold increase for the inhibition of carbachol-induced contraction. A much narrower range was observed for isoprenaline. Potentiation could also be observed for the inhibitory effect of noradrenaline on acetylcholine release. Clonidine was the most potent agonist against peristaltic reflex and the weakest agonist in relaxing circular muscle. Denervated preparations became subsensitive to the inhibitory effect of clonidine on peristaltic reflex. The potency of clonidine relative to noradrenaline was 488 in intact preparations and only 3.1 in denervated organs.(ABSTRACT TRUNCATED AT 250 WORDS)

α-Adrenoceptor number and function in rat cortex after ethanol and immobilization stress

Alpha-Adrenoceptor number and function were assessed in rat cortex after ethanol and immobilization stress. Rats subjected to immobilization stress for 2 h/day/7 days showed a significant reduction in alpha 2-adrenoceptors in cortex ( [3H]clonidine binding) but no change in alpha 1-adrenoceptors ( [3H]prazosin binding). The inhibitory effect of clonidine (500 nM) on 3H efflux from cortical slices preincubated with [3H]norepinephrine was reversed in the stressed animals. These effects on alpha 2-adrenoceptor number and function were reduced when stress was accompanied by chronic administration of ethanol (35% of calories in a liquid diet) although when given alone ethanol produced small changes in the same direction as those produced by stress. It is suggested that, in its effects on cortical alpha-adrenoceptors, chronic ethanol administration attenuates the effects of severe stress though itself perhaps acting as a mild stressor when given as part of a liquid diet regimen.

Dual presynaptic effects of 5-hydroxytryptamine on peripheral noradrenergic synapses.

The aim of the present experiments was to study the effects of 5-hydroxytryptamine (5-HT) on the responses to postganglionic stimulation of two models of the peripheral sympathetic nervous system: the isolated nictitating membrane of the cat and the guinea-pig isolated atria. In the nictitating membrane of the cat, 5-HT (0.1 microM) shifted to the left the frequency-response curve to nerve stimulation. This potentiating effect was prevented by 5-HT receptor antagonists (0.1 microM methysergide, 0.1 microM pizotifen and 0.1 microM morphine) and also by the beta-adrenoreceptor blocker propranolol (0.1 microM). The alpha 2-adrenoreceptor antagonist yohimbine (0.1 microM) had no effect on the 5-HT-induced potentiation. In the guinea-pig isolated atria the responses to cardioaccelerans nerve stimulation were diminished by 5-HT (0.1 to 1.0 microM). The shift to the right in the frequency-response curve induced by 5-HT (1.0 microM) was additive to the antagonism caused in the atria by propranolol (0.1 microM). The inhibitory effect of 5-HT on the pacemaker responses to nerve stimulation was prevented by the 5-HT receptor antagonists methysergide (1.0 microM) and pizotifen (1.0 microM) and also by the alpha-adrenoreceptor antagonist phentolamine (0.1 microM). The selective alpha 2-adrenoreceptor agonist clonidine (0.01 microM) reduced to the same extent as 5-HT (1.0 microM) the responses to the guinea-pig atria to nerve stimulation. The inhibitory effect of clonidine was prevented by the alpha-adrenoreceptor blocker phentolamine (0.1 microM) but not by the 5-HT receptor blocker pizotifen (1.0 microM). With the exception of propranolol, which in the atria shifted to the right the concentration-response curve to exogenous noradrenaline (NA), neither 5-HT nor the different antagonists employed modified the sensitivity to NA in the tissues studied. The present observations show that 5-HT can produce a dual effect on the sympathetic neurotransmission. It is proposed that a modification in the overflow of NA in response to nerve stimulation is caused by 5-HT and results from the interaction of 5-HT with specific receptors located on the sympathetic fibres. These presynaptic 5-HT receptors behave as excitatory (cat nictitating membrane) or inhibitory (guinea-pig atria) depending on the tissue studied.

Protection against alloxan-induced diabetes in mice by stimulation of α2-adrenergic receptors

A single intravenous injection of alloxan in mice induced hyperglycemia in a dose dependent fashion. This diabetogenic action of alloxan was prevented by a single intraperitoneal injection of the α 2-adrenergic agonists, i.e. oxymetazoline, clonidine or epinephrine 40 min prior to the injection of alloxan. The α 1-adrenergic agonists, i.e. methoxamine and phenylephrine, and a β-adrenergic agonist, isoproterenol, failed to prevent the diabetogenic action of alloxan. The inhibitory effect of clonidine on alloxan-induced diabetes was antagonized by yohimbine or phentolamine, but not by prazosin. Although α 2-adrenergic agonists caused a transient hyperglycemia at the time of alloxan administration (40 min after the administration of α 2-adrenergic agonists), the plasma glucose level at the time of alloxan injection did not correlate with the anti-diabetogenic effect of α 2-adregernic agents. These results clearly demonstrate that the α 2-adrenergic mechanism which inhibits insulin release from pancreatic B cells prevented the diabetogenic action of alloxan in mice.

Norepinephrine and acetylcholine release in guinea pig thalamus (VL).

The release of 3H-NE and 3H-ACh from isolated guinea pig thalamic slices was investigated to determine the possible neurotransmitter role of these substances. Thalamic slices preloaded with 3H-NE or 3H-choline were impaled with bipolar electrodes and superfused at 0.2 ml/min with Krebs solution. Electrical stimulation increased the efflux of 3H-NE from these tissues in a current- and frequency-dependent fashion. The release of 3H-NE induced by stimulation of up to 1 mA was inhibited by TTX (10−6 M) or Ca-free medium containing EGTA (10−4 M). The stimulated 3H-NE release was enhanced by yohimbine (10−8 to 10−6 M) and reduced by Clonidine (10−5 M). These findings provide evidence for the neurotransmitter role of NE in this nucleus. The excitatory effect of yohimbine or the inhibitory effect of Clonidine on this 3H-NE release suggests the presence of presynaptic α2-receptors on the noradrenergic nerve terminals. The release of 3H-ACh was slightly increased by electrical stimulation. The stimulated 3H-ACh release was one fourth of the 3H-NE release evoked by the same stimulus parameters. Therefore, innervation of the cholinergic neuron may be less extensive than that of the noradrenergic neuron, in this nucleus.

Effect of islet-activating protein (IAP) on α-adreno-ceptors-mediated contractile responses of rat vas deferens

The influence of IAP on the contractile responses of vas deferens to exogenous norepinephrine (NE) and electric stimulation (ES) and the inhibitory effect of Clonidine on the ES-induced contractile responses were investigated to explore the role of Ni in the presynaptically located α2-receptor regulating the release of NE and the postsynaptically located ctl-receptor on the contractile response to NE.

Dopaminergic and noradrenergic inhibition of hypothalamic self-stimulation: Differentiation of reward and performance effects

Dopaminergic and noradrenergic inhibition of lateral hypothalamic self-stimulation was investigated in a new signalled, discrete-trials shuttle-box paradigm. The differential inhibitory effects of drugs and stimulation frequency reductions within small blocks of trials differentiate reward Inhibition from a variety of performance deficits. They further differentiate among the deficits produced by fatigue, sedation, dyskinesias, akinesia and sensory disruption. Pimozide's selective inhibition of the first response within each block of trials shows that its inhibition of self-stimulation is not due to either an inhibition of reward or to a general performance deficit. Instead, it suggests that pimozide specifically inhibits the initiation of motor responding. Pimozide-induced akinesia appears to be partly reversible by hypothalamic stimulation. Thus the pimozide data do not support a role for dopamine in mediating brain-stimulation reward. Since the inhibitory effects of clonidine were very similar to those of pimozide, it is suggested that clonidine also produces a stimulation-reversible akinesia. Thus the clonidine data do not support a role for noradrenaline in mediating brain-stimulation reward. LU 5-003, which selectively inhibits the presynaptic reuptake of noradrenaline, inhibited self-stimulation in almost exactly the same way as did reducing reward by reducing stimulation frequency. These data do support a primary role for noradrenaline in mediating brain-stimulation reward. However, it is suggested that LU 5-003 inhibits self-stimulation, not by inhibiting reward, but by enhancing reward and making the electrical stimulation superfluous.

HOMOGENEOUS AND NON‐HOMOGENEOUS DISTRIBUTION OF INHIBITORY AND EXCITATORY ADRENOCEPTORS IN THE LONGITUDINAL MUSCLE OF THE GUINEA‐PIG ILEUM

1 The effects of adrenoceptor agonists and antagonists on spontaneous and evoked membrane activities of longitudinal muscle cells from different parts of the guinea-pig ileum were observed, using microelectrode methods.2 Isoprenaline inhibited the generation of spikes in cells in the terminal (0-3 cm from the ileocaecal valve) and proximal (more than 50 cm from the ileocaecal valve) regions of the ileum, with no change on the membrane potential and ionic conductance of the membrane. These actions of isoprenaline were abolished by propranolol.3 Noradrenaline and phenylephrine depolarized the membrane and increased both the spike frequency and ionic conductance of cell membranes of the terminal ileum, whereas noradrenaline and clonidine hyperpolarized the membrane, increased the ionic conductance of the membrane and inhibited the spontaneously generated spikes from cells of the proximal ileum. The excitatory effect of phenylephrine in the cells of the proximal ileum and the inhibitory effect of clonidine on cells of the terminal ileum were less pronounced.4 The excitatory actions of noradrenaline or phenylephrine were antagonized by prazosin and phentolamine, but not by yohimbine, whereas the inhibitory actions of noradrenaline or clonidine were antagonized by yohimbine and phentolamine but not by prazosin.5 The cholinergic e.j.ps evoked by field stimulation to the tissue were not affected by isoprenaline or phenyleprine but were inhibited by noradrenaline and clonidine, in both the terminal and proximal regions of the ileum. These actions of noradrenaline and clonidine were antagonized by yohimbine but not by prazosin.6 The results indicate that in the myenteric plexus and longitudinal muscle tissues of the guinea-pig ileum there are prejunctional inhibitory (alpha(2)), postjunctional inhibitory (alpha(2) and beta) and postjunctional excitatory (alpha(1)) adrenoceptors. The homogeneous distributions of prejunctional alpha(2)- and postjunctional beta-adrenoceptors in the ileum are responsible for inhibitions of cholinergic excitatory junction potentials (e.j.ps) and spontaneous spike activities, respectively. The density of distribution of the postjunctional alpha(1)-adrenoceptors is higher in the terminal than in the proximal regions, and these distributions are reversed in the case of the postjunctional alpha(2)-adrenoceptors. The postjunctional alpha(1)-adrenoceptors are probably responsible for the membrane depolarization and alpha(2)-adrenoceptors for the hyperpolarization induced by catecholamines.

Pharmacological characterization of presynaptic alpha-adrenoceptors in the nucleus tractus solitarii and the cerebral cortex of the rat

The presynaptic α-adrenoceptors mediating inhibition of K +-induced release of [ 3H]noradrenaline (NA) from superfused slices and synaptosomes obtained from the nucleus tractus solitarii (NTS) region were characterized pharmacologically and compared to those in the cerebral cortex. Clonidine concentration-dependently (pD 2 7.80) reduced the K +-induced [ 3H]NA release from slices and synaptosomes from the nucleus tractus solitarii of both normotensive and spontaneously hypertensive rats; 10 −6 M clonidine inhibited the [ 3H]NA release induced by 10 mM K + to 40% of control. This inhibitory effect of clonidine was antagonized by the α 2-adrenoceptor antagonist yohimbine (10 −7 M), but not by the α 1-adrenoceptor antagonist prazosin (10 −7 M). The rank order of apparent affinities of agonists for the presynaptic α-adrenoceptors mediating inhibition of [ 3H]NA release from slices of nucleus tractus solitarii induced by 20 mM K +, was: oxymetazoline > clonidine > adrenaline > α-methylnoradrenaline > noradrenaline > methoxamine. A similar rank order was found for cerebral cortex slices. The rank order of intrinsic activities in both brain regions was found to be: noradrenaline ≈ α-methylnoradrenaline ≈ adrenaline > clonidine > oxymetazoline > methoxamine. The data indicate that presynaptic a-adrenoceptors mediating inhibition of NA release both in the nucleus tractus solitarii and the cerebral cortex belong to the α 2-type. The presynaptic α 2-adrenoceptors in the two brain regions appear to be similar both functionally and pharmacologically. It is suggested that these receptors in the nucleus tractus solitarii may play a role in the hypotensive effect of clonidine.

Presynaptic effect of clonidine antagonized by the tetramine disulphide, benextramine.

1 The presynaptic alpha-adrenoceptor blocking activity of the newly synthesized alpha-adrenoceptor blocking drug, benextramine, was evaluated in the isolated left atrium of the guinea-pig heart. 2 High-voltage stimulation increased the force of contraction of electrically driven atrial strips, presumably by releasing noradrenaline from sympathetic nerve endings. Like phentolamine, benextramine increased the effect of high-voltage stimulation, presumably by blocking presynaptic alpha-adrenoceptors. 3 Clonidine reduced the effect of high-voltage stimulation, presumably by stimulating presynaptic alpha-adrenoceptors. The inhibitory effect of clonidine was antagonized noncompetitively by benextramine and competitively by phentolamine. 4 Combined administration of benextramine and phentolamine only resulted in the competitive phentolamine antagonism. Thus phentolamine protected the presynaptic alpha-adrenoceptors against benextramine blockade.

Catecholamine binding to the α-adrenergic receptors of hamster adipocytes evidence that guanine nucleotides regulate this binding to the α2-receptor subtype

The binding characteristics of the α-component of (−)-[ 3H]norepinephrine to hamster adipocyte membranes were studied. Binding was rapid, reaching equilibrium in 20 min at 25°C. Dissociation of specific binding by 10 μM phentolamine suggested dissociation from two different sites. The time course of dissociation induced by a 50-fold dilution was unchanged by the addition of norepinephrine, suggesting the absence of cooperative binding sites. [ 3H]norepinephrine binding was saturable, yielding curvilinear Scatchard plots. Computer modeling of these data further supported the existence of two classes of binding sites, one with high affinity ( D = 23 nM) but low binding capacity (96 fmol/mg protein) and one with low affinity ( K D = 400 nM) but high binding capacity (1000 fmol/mg protein). Adrenergic ligands of competed with [ 3H]norepinephrine binding in the following order of potency: (−)-norepinephrine>(−)-epinephrine>>(+)-norepinephrine>(−)-isoproterenol. Displacement by the selective α-adrenergic drugs prazosin, clonidine and yohimbine yielded biphasic curves consistent with binding of [ 3H]norepinephrine to both α 1- (14–22%) and α 2- (78–86%) receptor subtypes. Although Gpp(NH)p failed to alter the binding of [ 3H]dihydroergocryptine, it severely reduced the binding affinity of (−)-epinephrine, (−)-norepinephrine and the selective α 2-agonist, clonidine. The inhibitory effects of clonidine and of the α-component of (−)-epinephrine on the adrenocorticotropin-stimulated cyclic AMP production in the intact adipocyte were closely correlated with their effects on the binding of both [ 3H]norepinephrine and [ 3H]dihydroergocryptine. Conversely, yohimbine but not prazosin markedly antagonised the α-inhibitory effect of norepinephrine on cyclic AMP production. These data led to concluded that [ 3H]norepinephrine can be successfully used to study the entire α-adrenergic receptor population of hamster fat cells and that the predominant α 2 -receptor subtype exists in two different affinity states for agonists, the proportions of which are modulated by guanine nucleotides.

Evidence supporting the existence of presynaptic alpha-adrenoceptors in the regulation of endogenous noradrenaline release upon hepatic sympathetic nerve stimulation in the dog liver in vivo.

The existence and functional significance of presynaptic alpha-adrenoceptors within the liver was investigated in anesthetized dogs. The stimulation-evoked endogenous catecholamine overflow was determined in hepatic venous blood upon hepatic nerve stimulation (12 V, 1-8 Hz, 1 min). Under resting conditions, average plasma catecholamine levels in hepatic venous and aortic blood were 0.064 ng/ml and 0.334 ng/ml, respectively. A frequency-dependent increase (P less than 0.001) was found in the hepatic venous catecholamine overflow, while aortic catecholamine levels did not change significantly at any stimulation frequency tested. The increases in catecholamine overflow were associated with decreases in hepatic arterial vascular conductance (35-66%, P less than 0.001) at all frequencies tested. Yohimbine (0.3 mg/kg, i.v.) potentiated the catecholamine overflow by 110-140% (P less than 0.05) upon stimulation at low frequencies (1-4 Hz). Clonidine (20 micrograms/kg, i.v.) inhibited the catecholamine overflow by 55-76% (P less than 0.05) at frequencies of 1 and 2 Hz, and reduced the hepatic arterial response by 46% (P less than 0.01) at 1 Hz. The pretreatment with yohimbine (0.1 mg/kg, i.v.) abolished the inhibitory effects of clonidine both on the catecholamine overflow and the hepatic arterial responses. The results support the existence of a negative feedback mechanism mediated by presynaptic alpha-adrenoceptors in the local regulation of noradrenaline release from hepatic sympathetic fibers. A functional significance of this process was suggested by an improved correlation found in the presence of clonidine between the catecholamine overflow and the hepatic arterial vascular conductance.

Preferential blockade of postsynaptic α-adrenoceptors by BE 2254

The effects of 2-[β-(4-hydroxyphenyl)-ethyl-aminomethyl]-tetralone (BE 2254) on pre- and postsynaptic α-adrenoceptors were studied in isolated rabbit hearts strips of rabbit pulmonary arteries. The KCl (80 mM)-evoked output of endogenous noradrenaline from perfused hearts was significantly increased by BE 2254 (10 −6−10 −6 M). The inhibitory effect of clonidine (10 −7 M) on KCl-evoked noradrenaline output was antagonized by BE 2254 (10 −6 and 10 −5 M) indicating that the compound causes a blockade of presynaptic α-adrenoceptors. In superfused pulmonary arteries, BE 2254 produced a parallel shift to the right of the concentration-response curve for noradrenaline: increasing effect on tension, pA 2 = 8.75. The electrically (2 Hz) evoked 3H overflow from arteries preincubated with 3H-noradrenaline was increased by BE 2254 (3.2 × 10 −8−3.2 × 10 −7 M). Considerably lower concentrations were sufficient to cause an inhibition of the electrically evoked increase in tension: 3.2 × 10 −9 M BE 2254 caused a 45% decrease. These results suggest that BE 2254, in spite of its rather high affinity to presynaptic α-adrenoceptors, preferentially blocks postsynaptic α-adrenoceptors.

COMPARISON OF PRE- AND POSTSYNAPTIC α-ADRENOCEPTOR BLOCKING EFFECTS OF E-643 IN THE ISOLATED VAS DEFERENS OF THE RAT

AbstractEffectiveness of E-643, a newly developed α-blocker, and four α-antagonists in blocking pre- and postsynaptic α-adrenoceptors were compared in the isolated rat vas deferens. The inhibitory effect of Clonidine on the field-stimulated twitch response was antagonized in the presence of the α-antagonists. The order of affinity (pA2) for presynaptic α-receptors, as assessed from parallel shift of the dose-response curve to Clonidine, was: phentolamine>yohimbine>tolazoline>E-643≥prazosin. At concentrations from 10−8 to 10−6 M, neither E-643 nor prazosin had any effect on the twitch which had been depressed by the treatment with Clonidine, whereas phentolamine, yohimbine and tolazoline partially reversed it. Contractile effects of cumulative concentrations of noradrenaline were also antagonized by α-antagonists. The order of affinity (pA2) for postsynaptic α-receptors was: E-643≥prazosin>phentolamine>yohimbine> tolazoline. Selectivity for pre- versus postsynaptic α-receptors was assessed by comparing KB values for pre- and postsynaptic α-receptors. The order of selectivity for the presynaptic α-receptors was : yohimbine>tolazoline>phentolamine>>prazosin≥E-643. It is concluded that E-643 is a potent and highly selective postsynaptic α-blocker.

Inhibition of neurosympathetic cerebroarterial constriction by clonidine in cats

In cats anaesthetized with chloralose the cervical sympathetic nerves were separated from the vagosympathetic trunk and severed. Bilateral preganglionic electrical stimulation of the efferent sympathetic stumps induced rate-dependent decrements in cerebrospinal fluid pressure measured in the cerebral lateral ventricle. Intravenous injection of 3, 10 and 30 μg/kg clonidine caused a dose-related shift in threshold and inhibition of the pressure decrease particularly at lower rates of stimulation. Intravenous infusion of 3.75 μg/kg · min yohimbine for 30 min reduced the inhibitory effect of 30 μg/kg clonidine i.v. Intravenous injection of 5 mg/kg phentolamine reduced the depressor effect at all rates of electrical sympathetic stimulation. In another group of cats craniotomy was performed and the diameter of pial arteries (0.1–0.2 mm) was measured with an eyepiece micrometer under the microscope. Bilateral electrical stimulation of the sympathetic nerves decrease the artery diameter by 16%. This effect was diminished by 30 μg/kg clonidine i.v. Because of the characteristic shift of the frequency-response curve caused by clonidine and the antagonistic effect of yohimbine it is assumed that clonidine inhibits sympathetic neurotransmission in pial arteries by activation of prejunctional α-adrenoceptors. The inhibitory effect of clonidine in neurosympathetic transmission might explain the efficacy of the drug in the treatment of migraine.

Investigations on the mode of action of a new antihypertensive drug, urapidil, in the isolated rat vas deferens

Urapidil (6-{3-[4-(o-methoxyphenyl)-1-piperazinyl]-propylamino}-1,3-dimethyluracil), phentolamine (both 10 −8–10 −5 M), yohimbine (10 −7 M), and clonidine (10 −6–10 −5 M) competitively antagonized the effect of noradrenaline on postsynaptic α-adrenoceptors in organ bath-suspended rat vas deferens, yielding pA 2 values of 7.02, 7.64, 6.48 and 5.81, respectively, whereas xylazine (BAY 1470) was ineffective as an antagonist. In contrast to clonidine and xylazine, urapidil did not exert α-adrenoceptor agonist properties at the postsynaptic level in this preparation. Stimulation of presynaptic α-adrenoceptors by urapidil, clonidine and xylazine was assessed by inhibition of twitch contractions of the superfused rat vas deferens in response to intramural nerve stimulation. Inhibition of contraction caused by these drugs was reversed by phentolamine and yohimbine, indicating that they were presynaptic in origin. The presynaptic α-adrenoceptor antagonist activity of urapidil and phentolamine was quantitatively determined by studying the effects of increasing concentrations of these compounds on clonidine dose-response curves in the vas deferens after electrical stimulation. The inhibitory effect of clonidine could be competitively blocked by phentolamine (10 −7–10 −6 M), whereas urapidil (10 −5−3 × 10 −4 M) elicited partial agonism on presynaptic α-adrenoceptors which was apparent from the dose-response curves revealing competitive dualism. The pA 2 value for phentolamine and urapidil against clonidine was 7.97 and 5.11, respectively. These results indicate that, with respect to pre- and postsynaptic α-adrenoceptors, the action of urapidil is complex but suggests a primary presynaptic stimulatory site of action at doses similar to those of clinidine, but with an apparent intrinsic activity of 0.52. The α-adrenoceptor blocking potency of urapidil at the postsynaptic site was 4–8 times weaker than that of phentolamine, whereas antagonism at the presynaptic level was negligible.

Interactions between clonidine and alpha-adrenoceptor blocking drugs on the tachycardic response to stimulation of the cardiac nerve in dogs.

In pentobarbital-treated dogs clonidine (10 micrograms/kg) reduced the increase in heart rate caused by electrical stimulation of the cardiac nerve (1-10 Hz). We studied the actions of six alpha-adrenoceptor blocking drugs. Yohimbine (0.3 mg/kg) and phentolamine (1 mg/kg) potentiated the effects of nerve stimulation and antagonized the inhibitory effects of clonidine. Piperoxan (1 mg/kg) increased the response to nerve stimulation but antagonized the effects of clonidine only at the lowest frequency of stimulation. Thymoxamine (1 mg/kg) and prazosin at high doses (1 mg/kg) also antagonized the effects of clonidine but failed to increase the positive chronotropic response to stimulation of the cardiac nerve. AR-C239, a new and potent alpha-adrenoceptor blocking agent, changed neither the response to nerve stimulation nor the inhibitory effect of clonidine. The effects of all these drugs were observed at doses which reduced or reversed the pressor response to adrenaline. Therefore, our results afford further evidence for a dissimilarity between postsynaptic and presynaptic alpha-adrenoceptors in the dog. In addition, they show that the failure of an alpha-adrenoceptor blocking compound to increase the response to nerve stimulation does not necessarily indicate a lack of presynaptic alpha-adrenoceptor blockade.

The inhibitory effects of presynaptic alpha-adrenoceptor agonists on contractions of guinea-pig ileum and mouse vas deferens in the morphine-dependent and withdrawn states produced in vitro.

1 Isolated ilea from guinea-pigs implanted with morphine pellets were stimulated coaxially, either with or without morphine present in the bath fluid, and the longitudinal contractions recorded. 2 In the absence of morphine the inhibitory effects of the presynaptic alpha-adrenoceptor agonists, clonidine and oxymetazoline were much reduced and the dose-response curve was flat. This state of 'withdrawal' was readily reversed by morphine and levorphanol but not its inactive (+)-isomer, dextrophan. 3 The kappa-agonists, ketazocine and ethylketazocine, also restored the effects of clonidine as did the opioid peptides Tyr-D-Ala-Gly-Phe-D-Leu, acting preferentially on delta-receptors, and Tyr-D-Ala-Gly-MePhe-Met(O)-ol, acting mainly on micro-receptors. 4 The inhibitory effects of adrenaline and adenosine 3',5'-diphosphate were reduced at low but not at high concentrations. 5 In contrast, the inhibitory effect of clonidine on the electrically evoked contractions of vasa deferentia from mice implanted with morphine pellets was not abolished by the lack of morphine in the bath fluid or by addition of naloxone. 6 A possible explanation is suggested for the loss of the inhibitory effects of presynaptic alpha-adrenoceptor agonists in the withdrawn state of the dependent ileum.