Pre- and postsynaptic effects of yohimbine stereoisomers on noradrenergic transmission in the pulmonary artery of the rabbit

Effects of alpha-adrenoceptor antagonists on clonidine-induced inhibition of insulin secretion by isolated pancreatic islets

The effects of clonidine, yohimbine, corynanthine and prazosin on glucose-induced insulin secretion by incubated or perifused mouse pancreatic islets were investigated. Clonidine (0.1 microM) inhibited glucose-induced insulin secretion alone and in the presence of yohimbine (0.1 microM), corynanthine (10 microM) or prazosin (1 microM). In higher concentrations, yohimbine (1-10 microM) antagonized the inhibitory effect of clonidine (0.1 microM) upon glucose-induced insulin secretion by incubated islets and by perifused islets. The results support the view that adrenergic inhibition of insulin secretion is mediated by alpha 2-adrenoceptors on pancreatic beta-cells.

Yohimbine and rauwolscine inhibit 5-hydroxytryptamine-induced contraction of large coronary arteries of calf through blockade of 5 HT2 receptors

5-Hydroxytryptamine (5 HT)-induced contractions were investigated on cocaine-treated strips of bovine large coronary arteries. 1. The alpha 2-adrenoceptor blockers rauwolscine and yohimbine antagonized competitively 5 HT-induced contractions. The estimated equilibrium dissociation constants KB (-log mol/l) were 7.1 for rauwolscine and 7.3 for yohimbine. The affinity of yohimbine for the receptors mediating the response to 5 HT appears to be 10 times higher than for postsynaptic alpha 1-adrenoceptors but 10 times lower than for postsynaptic vascular alpha 2-adrenoceptors. 2. (-)-Noradrenaline and the alpha 2-adrenoceptor-selective agonist B-HT 920 caused maximum contractions amounting to only 20% and 2%, respectively, of the maximum 5 HT effects. Neither 60 mumol/l B-HT 920 nor 1 mumol/l prazosin antagonized the 5 HT effect. 3. Ketanserin was a competitive antagonist (KB = 9.2 (-log mol/l] of the effects of 5 HT. Combinations of rauwolscine or yohimbine with ketanserin antagonized the 5 HT effects as expected from competition of the 4 drugs for a single class of receptor. 4. The evidence is consistent with an interaction of 5 HT, ketanserin, rauwolscine and yohimbine with 5 HT2 receptors. alpha-Adrenoceptors only play a minor role in large coronary arteries and appear not to be involved in the 5 HT-induced contractions. A possible clinical involvement of 5 HT in coronary artery spasm is discussed.

alpha-adrenoreceptor-mediated phasic and tonic activity in rat portal vein in vitro

The effects of alpha 1- and alpha 2-adrenoreceptor agonists on phasic or tonic activity have been examined in rat portal vein in vitro. Noradrenaline (NA) and Phenylephrine (PE) increased phasic activity at low concentrations, these effects were superimposed on a sustained contracture at higher concentrations. The alpha 2-adrenoreceptor agonists UK 14304 or TL 99 increased extracellular electrical activity and associated mechanical phasic activity without inducing sustained contracture. These alpha 2-adrenoreceptor mediated effects were particularly sensitive to changes in Ca2+ concentration. Prazosin was a potent, competitive antagonist of contracture to NA, or PE, but did not antagonise the phasic responses to NA, UK 14304 or TL 99, except at high concentrations. Prazosin was significantly less potent against PE-induced phasic responses than PE-induced contracture. The results suggest that alpha 1-adrenoreceptors are predominantly involved in contracture. Yohimbine was significantly more potent against UK 14304 or TL 99-induced phasic response than against PE-induced responses, however, phasic responses to NA were competitively antagonised only by combination of yohimbine and prazosin. It is concluded that postsynaptic alpha 2-adrenoreceptor mechanisms are involved in the phasic responses to selective alpha 2-adrenoreceptor agonists, and to NA in rat portal vein. The response of contracture is mediated through alpha 1-adrenoreceptors.

Separation of adrenergic and non-adrenergic contractions to field stimulation in the rat vas deferens

Adrenergic and 'non-adrenergic' nerve-induced contractions in rat vas deferens were separated pharmacologically. Responses to single stimuli comprised two components, an alpha-noradrenergic component (IIs), dominant in the epididymal portion, and a 'non-adrenergic' component (Is), dominant in the prostatic portion. Is but not IIs was blocked by nifedipine. A combination of adrenergic blockade and nifedipine virtually abolished all components. After cocaine, a third component (IIIs) emerged which was abolished by either adrenergic blockade or nifedipine. The response to trains of stimuli consisted of 'twitch' and 'secondary' components. This biphasic time course was modified by adrenergic blockade or nifedipine to reveal the time course of the 'non-adrenergic' and adrenergic components, respectively: these did not correspond to the 'twitch' and 'secondary' components. A combination of adrenergic blockade and nifedipine virtually abolished the whole response. Prejunctional alpha 2-adrenoceptor-mediated inhibition of the contractile responses could be blocked by selective alpha 2-adrenoceptor antagonists. The adrenergic contractile response demonstrated this 'feed-back' even on the second pulse at 0.5 Hz. Endogenous inhibition of the 'non-adrenergic' contraction required higher frequencies or enhancement of the extracellular concentration of noradrenaline by blockade of its neuronal uptake. Contractile responses to exogenous noradrenaline were abolished by nifedipine, at a concentration that did not affect the adrenergic (IIs) neurotransmission. These results reinforce the view that part of the motor transmission in rat vas deferens is non-adrenergic and allow the disentanglement of the various postjunctional and prejunctional elements contributing to the complex response to a train of stimuli.

Effect of age on the prejunctional alpha-adrenoceptor-mediated feedback in the heart of spontaneously hypertensive rats and normotensive Wistar Kyoto rats. Evidence for a physiological role of prejunctional alpha 2-adrenoceptors

The prejunctional alpha 2-adrenoceptor-mediated feedback in the heart of pithed young and adult spontaneously hypertensive rats (SHR) and corresponding normotensive Wistar Kyoto rats (WKY) was studied. After electrical stimulation of the sympathetic outflow from the spinal cord to the heart, B-HT 920 induced an inhibition of the cardiac response, which was significant at stimulation frequencies up to 1 Hz in young SHR and WKY and up to 2 Hz in the adult animals. Rauwolscine produced a potentiation of the cardiac response to electrical stimulation in SHR, which was significant from 0.2-10 Hz in young SHR and from 0.1-10 Hz in adult SHR. In young WKY, rauwolscine did not potentiate the increase in heart rate to sympathetic nerve stimulation, whereas in adult WKY alpha 2-adrenoceptor blockade by rauwolscine significantly potentiated the cardiac response to electrical stimulation at frequencies in the range of 0.2-10 Hz. In SHR the potentiation of the cardiac response to sympathetic nerve stimulation by rauwolscine was much stronger than in WKY. These results suggest that in adult animals the prejunctional alpha 2-adrenoceptor mediated feedback is more developed than in young rats. In contrast with young WKY, a significant endogenous feedback can be demonstrated in adult WKY. In SHR, however, the physiological role of prejunctional alpha 2-adrenoceptors is much more important.

RX781094, a new potent alpha 2 adrenoceptor antagonist. In vivo and in vitro studies in the rabbit

1. RX781094 [2-(2-(1,4-benzodioxanyl))-2-imidazoline HCl] is a new alpha 2 adrenoceptor selective antagonist. Its effects on cardiovascular response "in vivo" before and after administration of alpha 1 and alpha 2 adrenoceptor agonists have been studied in conscious and anaesthetized rabbits. The affinity of RX781094 for alpha 1 and alpha 2 adrenoceptors has been investigated "in vitro" in radioligand binding studies. 2. Intravenous injection of RX781094 caused an immediate short lived pressor response which was attenuated by alpha 1 and alpha 2 adrenoceptor antagonists; it also caused a prolonged bradycardia. 3. Pretreatment with intravenous RX781094 inhibited the hypotensive response to both intravenous and intracisternal clonidine in a dose dependent manner. 4. RX781094 also showed a dose dependent inhibition of the acute pressor response to the alpha 2 adrenoceptor selective agonist guanabenz. 5. Pressor dose response curves to noradrenaline, a mixed alpha 1/alpha 2 adrenoceptor agonist were shifted to the right after RX781094 whereas those to the alpha 1 adrenoceptor agonist phenylephrine were unchanged. 6. RX781094 was more effective at displacing specifically bound 3H clonidine than 3H prazosin in membrane preparations from rabbit brain. 7. These observations are consistent with a selective alpha 2 adrenoceptor antagonist effect of RX781094 at central and peripheral adrenoceptors in the rabbit.

Binding of yohimbine stereoisomers to alpha-adrenoceptors in rat liver and human platelets

1--Displacement of tritiated prazosin binding to rat liver plasma membranes and tritiated yohimbine human platelet membranes shows that (+)-yohimbine, alloyohimbine and alpha-yohimbine (rauwolscine) are selective alpha 2-adrenoceptor antagonists (KD alpha 1/KD alpha 2:635, 46.6 and 112 respectively) whereas corynanthine is more alpha 1-selective (KD alpha 1/KD alpha 2:0.036). 2--11-Methoxy derivatives of alpha-yohimbine and epi-alpha-yohimbine are very weak alpha-adrenoceptor blockers. 3--It is concluded that the aromatic A ring, the Nb atom, and the carboxymethyl moiety are important for the binding of yohimbine to the alpha-adrenoceptor, the carboxymethyl group being important for the alpha 1/alpha 2 specificity of the molecule.

An electrophysiological study of presynaptic alpha-adrenoceptors in the vas deferens of the mouse

1--Effects of clonidine and alpha-adrenoceptor antagonists were studied on sympathetic neuroeffector transmission in the mouse vas deferens. The amplitude of excitatory junction potentials (e.j.ps) was taken as a measure of transmitter release per impulse. 2--At a concentration of 0.5 microM, prazosin abolished depolarizations evoked by iontophoretically applied noradrenaline, but changed neither spontaneous nor nerve stimulation-evoked e.j.ps. 3--Yohimbine 0.1 and 1 microM, rauwolscine 1 microM and corynanthine 1 microM di not change the e.j.p. amplitudes elicited by the first 2-3 pulses in trains of 15 pulses at 3 Hz, but increased the e.j.ps elicited by the subsequent pulses. Corynanthine 1 microM was much less effective than yohimbine 1 microM or rauwolscine 1 microM, and corynanthine 0.1 microM had no effect. 4--Clonidine 0.01 microM reduced the e.j.p. amplitudes evoked by single pulses and its effect was counteracted by yohimbine 1 microM. 5--In vasa deferentia from reserpine-treated mice the e.j.p. trains were changed in much the same way as by yohimbine and rauwolscine. Yohimbine 1 microM did not further increase the e.j.p. amplitudes in these organs, whereas clonidine 0.01 microM caused a marked inhibition. 6--It is concluded that the release of the motor transmitter in the mouse vas deferens is inhibited by activation of presynaptic alpha-adrenoceptors, and that these receptors are normally activated by neurally released noradrenaline.

alpha 2-Adrenoceptor agonist properties of exo- and endo-isomers of 2-amino-6,7,dihydroxybenzonorbornene designed as rigid catecholamines

A series of N-substituted exo- and endo-isomers of 2-amino-6,7-dihydroxybenzonorbornene, designed as rigid catecholamines, have been studied in the pithed rat in-vivo, as vasoconstrictor agents, and as inhibitors of the twitch response in the transmurally stimulated guinea-pig ileum. The exo-isomers examined were vasoconstrictor agonists in the pithed rat and inhibited the twitch response of the ileum. The corresponding endo-isomers were inactive in both preparations. The exo-isomers were less potent than the alpha 2-receptor agonist TL99, but were all directly acting vasoconstrictor agents, since they were still effective in reserpine-pretreated animals. Responses induced by members of the exo-series were selectively antagonized by the alpha 2-receptor antagonist rauwolscine, but were not antagonized by the alpha 1-receptor antagonist, prazosin, or the dopamine-receptor antagonist alpha-flupenthixol. The results demonstrate important conformational requirements for the interaction of catecholamines at presynaptic or postsynaptic alpha 2-receptors, and suggest that a fully extended or anti-conformation of the noradrenaline molecule is involved in alpha 2-receptor-agonist interaction.

Pharmacological characterization of postjunctional alpha-adrenoceptors in isolated feline cerebral and peripheral arteries

The vascular alpha-adrenoceptors in isolated feline cerebral, lingual and mesenteric arteries were characterized and compared. In the middle cerebral artery the relative order of potency for agonists was: clonidine greater than oxymetazoline greater than noradrenaline greater than phenylephrine which indicates that the postjunctional alpha-receptor in this vessel is of alpha 2-type. This view is further supported by the finding that yohimbine, but not prazosin, had a potent, mainly competitive blocking action. In peripheral arteries, clonidine was without effect. In these vessels, the potency difference between phenylephrine and oxymetazoline was more than 40 times less than in cerebral vessels. The pA2-values for prazosin correlated well with pA2-values found for the interaction of this drug with alpha 1-receptors in a variety of other tissues, thus suggesting the occurrence of an alpha 1-receptor in these arteries. However, the pA2-values for yohimbine and rauwolscine correlated well with an alpha 2-receptor, suggesting also the presence of alpha 2-receptors. Schild plots for prazosin and rauwolscine in lingual arteries displayed slopes significantly lower than unity, which also supports the view of a mixture of alpha 1- and alpha 2-receptors in these vessels. However, the Schild plots for the antagonists in mesenteric arteries did not differ significantly from unity, a finding possibly indicating the presence of an alpha-receptor unable to differentiate between substances that in other tissues act preferentially on alpha 1- or alpha 2-receptors.

Functional postsynaptic alpha 2- but not alpha 1-adrenoceptors in dog saphenous vein exposed to phenoxybenzamine

Phenoxybenzamine greatly attenuated phenylephrine-induced contractions of dog saphenous vein in vitro, but had less effect on contractions induced by clonidine. The phenoxybenzamine-resistant responses to clonidine were not affected by prazosin or by corynanthine but were competitively antagonized by yohimbine (pA2 8.2). It is concluded that exposure of saphenous vein to phenoxybenzamine resulted in blockade of alpha 1-adrenoceptors to the extent that there remained a virtually homogeneous population of postsynaptic alpha 2-adrenoceptors. The effects of agents at postsynaptic alpha 2-adrenoceptors can be studied on this preparation without the complications caused by the presence of functional alpha 1-adrenoceptors.

Central pressor action of neurotensin in conscious rats

The effects of neurotensin upon blood pressure in conscious rats were examined after intracerebroventricular (i.v.t.) or intravenous (i.v.) administration of this peptide. Whereas i.v. injected neurotensin (0.1-2.0 microgram/kg) was depressor, i.v.t. injected neurotensin (1 microgram and above) was pressor. Peripheral depressor responses could not be repeated in the same animal due to tachyphylaxis, but central pressor responses were repeatable without reduction in magnitude, showing that the two effects were separate entities. Thyrotropin-releasing hormone (TRH), which is reported to be a potent neurotensin antagonist, completely abolished the neurotensin depressor response, and attenuated the central pressor action. TRH did not alter the central pressor effect of another peptide, angiotensin II (AII). The potent AII receptor antagonist saralasin, while abolishing the central pressor effect of AII, was completely without effect upon the neurotensin-induced pressor response. These results indicate that i.v.t. injected neurotensin and AII stimulate a rise in blood pressure via different receptors. The alpha-adrenergic antagonists phentolamine, prazosin, or yohimbine (injected i.v.t.) involvement of the sympathetic nervous system in this response. These results are discussed in relation to the central pressor actions of other neuropeptides.

alpha-Adrenoceptor blocking properties of raubasine in pithed rats

1 Raubasine was compared with yohimbine and corynanthine in pithed rats. Antagonist activity at alpha 1-adrenoceptors was evaluated on the pressor response to electrical stimulation of the spinal sympathetic outflow and to phenylephrine administration, both being reduced by raubasine in the dose range 1 to 4 mg/kg. Corynanthine was quantitatively similar, but yohimbine was not only less potent but also in doses of 0.125 to 0.5 mg/kg enhanced the effects of electrical stimulation. 2 Antagonist activity at alpha 2-adrenoceptors was determined against the inhibitory effects of clonidine on tachycardia induced by electrical stimulation of cardiac sympathetic nerves and against the pressor responses to B-HT-933 injection. Raubasine up to 4 mg/kg, like corynanthine, did not affect the pressor responses to B-HT-933 nor did it reduce the inhibitory effect of clonidine. By contrast yohimbine reduced the response to BHT-933 and antagonized clonidine as well as enhancing the tachycardia caused by electrical stimulation. 3 The results indicate that, in vivo, raubasine, like corynanthine, is a selective antagonist at alpha 1-adrenoceptors and that yohimbine is more potent in blocking alpha 2-than alpha 1-adrenoceptors.

alpha 1-adrenoceptor activation can increase heart rate directly or decrease it indirectly through parasympathetic activation

1 The chronotropic effects of alpha- and beta-adrenoceptor agonists were investigated in the pithed rat. 2 The beta-adrenoceptor agonist, isoprenaline, produced only a positive chronotropic response. alpha 1-Adrenoceptor agonists, phenylephrine and amidephrine, produced positive and negative chronotropic effects. Part of the response to phenylephrine was beta-mediated. 3 A positive chronotropic response to amidephrine and phenylephrine was mediated directly through cardiac alpha 1-adrenoceptors and had a different time course from beta-adrenoceptor-mediated responses. 4 A negative chronotropic response to alpha-agonists was potentiated by neostigmine and blocked by atropine, tetrodotoxin or hexamethonium as well as by alpha 1-adrenoceptor antagonists. This may indicate alpha 1-adrenoceptors on preganglionic parasympathetic nerves, stimulation of these receptors causing release of acetylcholine. 5 The alpha 2-adrenoceptor agonist, xylazine, produced a direct negative chronotropic effect on the heart, independent of alpha-adrenoceptors. No evidence was found for functional post-junctional alpha 2-adrenoceptors. At high doses xylazine stimulated cardiac alpha 1-adrenoceptors.

The alpha 1- and alpha 2-adrenoceptor involvement in the central cardiovascular action of clonidine in the conscious renal hypertensive cat

An attempt has been made to characterise the alpha-adrenoceptor subtype (alpha 1 or alpha 2) mediating the hypotensive and bradycardic action of clonidine in conscious renal hypertensive cats. The relatively selective alpha 2-adrenoceptor agonists clonidine, UK-14,304, guanfacine and lofexidine caused significant hypotension and bradycardia when given intracerebroventricularly (i.c.v.). This suggests that alpha 2-adrenoceptors can mediate hypotension and bradycardia. However, both alpha 1- and alpha 2-adrenoceptor antagonists, prazosin, UK-33,274, corynanthine, yohimbine, rauwolscine and RS21361 blocked the hypotensive effect of clonidine. These results suggest an alpha 1-adrenoceptor may also mediate the central hypotensive action of clonidine, or possibly that the central alpha-adrenoceptors in which clonidine acts, may show pharmacological differences to peripheral alpha 1- and alpha 2-subtypes.

[3H]Dihydroergocryptine binding to alpha-adrenergic receptors of human platelets. A reassessment using the selective radioligands [3H]prazosin, [3H]yohimbine, and [3H]rauwolscine

Which subtype(s) of the alpha-adrenergic receptor occurs on human platelets? Studies of platelet responsiveness to adrenergic compounds and indirect radioligand binding studies addressing this question have yielded contradictory conclusions. These bindings studies employed the ligand [3H]dihydroergocryptine ( [3H]DHE), an alpha-adrenergic antagonist that does not select between alpha 1- and alpha 2-adrenergic receptors and that also binds to other receptor types in some tissues. To determine the subtype of the platelet alpha-adrenergic receptor, we have examined the binding to intact human platelets of [3H]prazosin (alpha 1-selective), [3H]yohimbine (alpha 2-selective), and [3H]rauwolscine (alpha 2-selective), and we have compared the binding of these selective radioligands with that of [3H]DHE. [3H]Yohimbine and [3H]rauwolscine both bound with high affinity (Kd = 2.7 and 4.6 nM, respectively) to an equal number and a single class (Hill coefficient approximately 1.0) of sites ( approximately 300 per platelet), but [3H]yohimbine yielded lower nonspecific binding than did [3H]rauwolscine. In paired experiments, [3H]DHE bound to 1.5 times as many (phentolamine-displaceable) sites as did [3H]yohimbine or [3H]rauwolscine. Unlabeled yohimbine and epinephrine competed for fewer [3H]DHE binding sites than did phentolamine. Thus, in addition to binding to the alpha 2-adrenergic receptors identified by [3H]yohimbine and [3H]rauwolscine, [3H]DHE seems to bind to other sites on human platelets. The nature of these sites is not clear. We found that [3H]prazosin did not identify alpha 1-adrenergic receptors on platelets, and that phenoxybenzamine only inhibited [3H]yohimbine and [3H]DHE binding at higher concentrations than usually observed for alpha 1-adrenergic receptors. We conclude that (1) all alpha-adrenergic sites on human platelets are of the alpha 2 subtype, (2) [3H]DHE may bind to additional, as yet ill-defined, sites in addition to those sites identified by [3H]yohimbine and [3H]rauwolscine, and (3) [3H]yohimbine is the preferred antagonist radioligand for studying the alpha 2-adrenergic receptors on human platelets.

Inhibitory alpha 2-adrenergic mechanism regulating gastric secretion in pylorus-ligated rats

Several alpha-adrenoceptor agonists given intracerebroventricularly or subcutaneously to rats were assessed for their effects on gastric secretion under condition of pylorus ligation. Intracerebroventricular injection of alpha-adrenoceptor agonists reduced gastric secretion, in the following order (relative potency, clonidine = 1): oxymetazoline (100) greater than clonidine (1) greater than methoxamine (0.024) greater than phenylephrine (0.003). The antisecretory effects of oxymetazoline and clonidine given intracerebroventricularly were antagonized with yohimbine administered by the same route. Subcutaneous injection of alpha-adrenoceptor agonists also reduced gastric secretion, in the following order (relative potency, clonidine = 1): clonidine (1) greater than oxymetazoline (0.3), much greater than phenylephrine (0.001) approximately methoxamine (0.0006). Oxymetazoline, when given intracerebroventricularly, was most effective in decreasing the volume and titratable acidity of gastric secretion. Pretreatment with 6-hydroxydopamine intracerebroventricularly (250 micrograms/rat x 2; 72 and 120 h before) reduced the antisecretory effect of clonidine given intracerebroventricularly. Thus, gastric secretion appears to be regulated in an inhibitory manner by alpha 2-, but not by alpha 1- adrenoceptors in the central and peripheral nervous systems, in pylorus-ligated rats.

An examination of the pre- and postsynaptic alpha-adrenoceptors involved in neuroeffector transmission in rabbit aorta and portal vein

1 The alpha-adrenoceptor agonists, clonidine and xylazine, reduced and the alpha-antagonists, yohimbine an rauwolscine, increased the stimulation-evoked tritium overflow from rabbit aorta and portal vein pre-incubated with [3H]-noradrenaline. 2 Based on an order of agonist potency of clonidine greater than xylazine greater than phenylephrine and antagonist potency of rauwolscine = yohimbine greater than prazosin, the presynaptic receptor mediating these effects is of the alpha 2 type. 3 In the aorta, stimulation-evoked contractions were abolished by prazosin (0.1 micrometers) and potentiated by rauwolscine and yohimbine in concentrations that increased the stimulation-evoked overflow tritium. 4 In the portal vein, prazosin was less potent in reducing, and rauwolscine and yohimbine failed to potentiate, the stimulation-evoked contraction. 5 In experiments in which tissues were pre-exposed to phenoxybenzamine (30 nM) to block some of the postsynaptic alpha-receptors, rauwolscine in concentrations that increased stimulation-evoked tritium overflow, reduced the evoked contraction in the portal vein but not in the aorta. 6 It is concluded that presynaptic alpha 2-autoreceptors are present in both tissues and that the postsynaptic alpha-receptors which mediate nerve stimulation-evoked contractions are alpha 1 in the aorta but a mixture of alpha 1 and alpha 2 in the portal vein.

Catecholamines act as alpha 2-adrenoceptors to cause contraction of circular smooth muscle of guinea-pig stomach

Circular smooth muscle strips taken from the body region of the guinea-pig stomach responded to dopamine and noradrenaline with contraction at lower concentrations followed by relaxation at higher concentrations. A beta-adrenoceptor-mediated relaxation response was excluded by propranolol treatment and this allowed the remaining alpha-adrenoceptor involvement with relaxation and contraction to be incisively differentiated in terms of two distinct alpha-adrenoceptor mechanisms. Thus, the relaxation responses to the catecholamines were mimicked by phenylephrine and antagonized by prazosin, phentolamine but not by yohimbine or rauwolscine. In contrast, the catecholamine-induced contractions were mimicked by clonidine and antagonized by yohimbine and phentolamine but not by prazosin. It is therefore concluded that the alpha mechanisms via which dopamine and noradrenaline are able to relax and contract the circular smooth muscle from the body region of guinea-pig stomach are of the alpha 1- and alpha 2-type respectively.

Blockade of central alpha 2-adrenoceptors enhances the carotid occlusion response in dogs

Intravertebral artery injection of yohimbine at total doses of 10, 50, 100 and 200 micrograms caused dose-related enhancement of the systemic pressor response to bilateral carotid artery occlusion (BCO) in dogs; the response was depressed by 5000 micrograms yohimbine. Basal blood pressure was increased by 100 and 200 micrograms yohimbine, and decreased by 5000 micrograms. Intravertebral artery injection of rauwolscine, 5 micrograms enhanced the BCO response but had no effect on basal blood pressure. Corynanthine, 200 or 500 micrograms was ineffective. The pressor response to intravenously administered noradrenaline was not affected by any of the drugs. It is concluded that enhancement of the BCO response by yohimbine and rauwolscine is caused by blockade of central alpha 2-adrenoceptors.

Pharmacological characterization of the alpha adrenoceptors of the dog basilar artery

The effects of alpha adrenoceptor agonists and antagonists on the postsynaptic alpha receptors were examined in the dog basilar, mesenteric and renal arteries and the type of alpha adrenoceptors present was characterized. In the basilar artery, noradrenaline, clonidine and phenylephrine produced almost the same maximal contraction, the pD2 values being 6.51 +/- 0.11, 5.49 +/- 0.16 and 5.65 +/- 0.13, respectively. Yohimbine (1-3 x 10(-8) M) inhibited the contractile responses to noradrenaline and clonidine competitively and the response to phenylephrine noncompetitively. Corynanthine (10(-6) M) had no effect on such contractile responses. In the mesenteric and renal arteries, the maximal responses to noradrenaline and phenylephrine were markedly greater than those to clonidine. Yohimbine (10(-7) - 10(-5) M) and corynanthine (10(-7) - 10(-5) M) both antagonized noradrenaline competitively in these vessels. In the basilar, mesenteric and renal arteries preloaded with 3H-noradrenaline, 3H-efflux induced by electrical transmural stimulation was attenuated by clonidine (10(-10) - 10(-7) M), while phenylephrine (10(-10) - 10(-8) M) was without effect. Yohimbine at considerably lower concentrations than corynanthine increased the 3H-efflux elicited by electrical stimulation. These results indicate that presynaptic and postsynaptic alpha receptors of the dog basilar artery are largely alpha 2 in contrast to those of peripheral arteries.

Binding characteristics of [3H]guanfacine to rat brain alpha-adrenoceptors. Comparison with [3H]clonidine

The tritium-labeled alpha-adrenoceptor agonist and antihypertensive drug guanfacine, N-amidino-2-(2,6-dichlorophenyl)-acetamide (sp. act. 24.2 Ci/mmole) was employed for a direct identification and characterization of alpha-adrenoceptors in rat brain membranes. Its usefulness as a radioligand was studied in comparison with [3H]clonidine (sp. act. 26.7 Ci/mmole). The nonspecific binding of [3H]guanfacine to rat cerebral membranes was considerably more pronounced than that observed for [3H]clonidine. The specific binding of [3H]guanfacine (0.1 - 20 nM) and [3H]clonidine (0.1 - 20 nM) as defined as the excess over blanks containing (-)-norepinephrine (10 microM) was saturable. Scatchard analyses of these binding data indicated single populations of binding sites for both ligands. KC values of 3.9 ([3H]guanfacine) and 3.7 nM ([3H]clonidine) were calculated. Maximal number of specific binding sites amounted to 220 and 195 fmole/mg protein for [3H]guanfacine and [3H]guanfacine and [3H]clonidine, respectively. In case unlabeled guanfacine (1 microM) was used to characterize the specific bonding of [3H] guanfacine, KD value and maximal number of binding sites were about twice as high as determined in the presence of excess (-)-norepinephrine. The rate of association of both radioligands was rapid. Binding reached equilibrium by about 10-15 min of incubation. Half-maximal binding was attained at approximately 1-2 min. The rates of dissociation were biphasic. A rapid and a slow component were identified. The specific binding sites of [3H] guanfacine in rat brain possess the general characteristics of alpha 2-adrenoceptors. Selective antagonists of alpha 2-adrenoceptors, like yohimbine and rauwolscine strongly interfered with this binding. However, preferential blocking agents of alpha 1-adrenoceptors, such as prazosin and corynanthine, were weak competitors. The relative potency of agonists and antagonists in displacing [3H]guanfacine was identical to their effectiveness in competing for [3H]clonidine specific binding sites. It is concluded that [3H]guanfacine labels the same alpha 2-adrenoceptor population in rat brain as [3H]clonidine. However, [3H]guanfacine seems not as suitable as [3H]clonidine for routine use in the direct identification of alpha 2-adrenoceptors in view of its relatively high nonspecific binding.

Comparison of central and peripheral alpha 1-adrenoceptors

A series of alpha-adrenergic agonists and antagonists having diverse chemical structure was examined for both central and peripheral alpha 1-adrenoceptor activity. The agonists tested included several novel aminotetralin derivatives which were potent and selective alpha 1-agonists. Peripheral alpha 1-activity was determined in the isolated rabbit ear artery; central alpha 1-receptor affinity was measured as the ability to inhibit 3H-WB 4101 binding to rat brain homogenates. In the agonist series, an excellent correlation between peripheral alpha 1-activity and central alpha 1-affinity was obtained, providing that partial agonists were excluded. Likewise, the receptor dissociation constant for blockade of the peripheral alpha 1-adrenoceptor correlated well with affinity for the central receptor for all of the alpha-antagonists. These data support the conclusion that central and peripheral alpha 1-adrenoceptors are similar or identical.

Differential effects of prazosin and rauwolsin on the release of prostaglandins I2 and E2 from the perfused mesenteric arterial bed of the rabbit following nerve stimulation

Sympathetic nerve stimulation of the perfused mesenteric arterial bed of the rabbit, in vitro, increases the secretion of prostaglandin (PG)I2 and PGE2. Prazosin (4.8 X 10(-6) M), an alpha 1 adrenergic receptor antagonist, inhibited this increase in release of PGI2 but not of PGE2 whereas rauwolsin (10(-7) M), and alpha 2 adrenergic receptor antagonist, inhibited the increase in release of PGE2 but not PGI2. Prazosin (10(-6) M) completely blocked the vasoconstrictor response to nerve stimulation, and to norepinephrine and phenylephrine administration, suggesting there to be little of an alpha 2 adrenergic receptor component in this response. It is concluded that the increase in PGI2 release allows the activation of alpha1 adrenergic receptors and is therefore post-junctional in origin, whereas the increase in PGE2 release following the activation of alpha 2 adrenergic receptors and may be pre- and/or post-junctional in origin.

Impairment by nifedipine of vasopressor responses to stimulation of postsynaptic alpha 2-adrenoreceptors in ganglion--blocked rabbits. Further evidence for the selective inhibition of postsynaptic alpha 2-adrenoreceptor-induced pressor responses by calcium antagonists

1 After ganglionic blockade and bilateral vagotomy, vasopressor responses induced by activation of postsynaptic alpha 1- and alpha 2-adrenoreceptors were elicited in the intact circulatory system of rabbits. 2 The hypertensive effects of the selective stimulating agents methoxamine (alpha 1-agonist) and B-HT 920 (alpha 2-agonist) were effectively antagonized by the adrenoreceptor antagonists prazosin and yohimbine, respectively. These findings confirm the existence of two types of postsynaptic alpha-adrenoreceptors (alpha 1- and alpha 2-type) in vascular smooth muscle of rabbits. 3 The calcium antagonistic drug nifedipine did not affect the maximal increase in diastolic pressure brought about by methoxamine, whereas it strongly inhibited the hypertensive effects of B-HT 920. 4 It is concluded that this confirmation of the selective inhibition of postjunctional alpha 2-adrenoreceptor-mediated vasopressor responses by a calcium antagonistic drug, such as nifedipine, indicates that this activity constitutes a general phenomenon. This finding supports the hypothesis that an influx of extracellular calcium is necessary for the vasoconstriction mediated by postsynaptic alpha 2-adrenoreceptors.

Involvement of alpha 1- and alpha 2-adrenoceptors in the vasoconstriction caused by ergometrine

In pithed normotensive rats, the pressor effects evoked by i.v. serotonin (5-HT) and ergometrine were analysed using the relatively selective alpha 1-, alpha 2- and serotonin antagonists prazosin, yohimbine methysergide, respectively. The pressor response to ergometrine was reduced by both prazosin and yohimbine but only moderately affected by methysergide. On the other hand, the rise in diastolic pressure brought about by serotonin was strongly depressed by methysergide but not influenced by the blockade of either alpha 1- or alpha 2-adrenoceptors. The calcium antagonist nifedipine shifted the dose-pressor response curve of ergometrine to the right in a dose-related, non-parallel manner. The maximum pressor response was diminished by nifedipine. In contrast, the rise in pressure provoked by serotonin was not affected by nifedipine. The results thus show that in the pithed rat the vasoconstrictor response to ergometrine is mediated by alpha-adrenoceptors (both alpha 1 and alpha 2) rather than by serotonin receptors. These findings question the role of serotonin receptors in the vasoconstriction induced by ergometrine.

Evidence for more than one type of post-junctional alpha-adrenoceptor

The concept of two types of alpha-adrenoceptor, alpha 1 located on smooth muscle and mediating contraction and alpha 2 located on nerve terminals and mediating inhibition of transmitter release, has broken down. In vivo it has been shown that post-junctional receptors, with characteristics closely related to those of the alpha 2-adrenoceptors at nerve terminals, can mediate pressor responses and are, "post-junctional alpha 2-adrenoceptors". Several differences among agonists in vitro have superficial similarities to the in vivo alpha 1/alpha 2 system but do not correspond precisely and seem to point to a subdivision of post-junctional alpha 1-adrenoceptors. A preliminary hypothesis is: in vivo alpha 1 is rapid in onset, short-lived, utilises internal Ca2+, prefers alkalosis and responds to short-term stimuli such as short bursts of nerve impulses or bolus injections of catecholamines; alpha 2 is slower in onset, longer-lived, utilises external Ca2+, prefers acidosis and responds to more prolonged stimuli such as circulating catecholamines; in vitro these categories of response occur but antagonists fail to define an alpha 1/alpha 2 split, suggesting that some critical factor is missing in vitro. The implications of these trends in alpha-adrenoceptor classification are discussed in relation to current pharmacological and biochemical methods for receptor typing, to the possible physiological actions and roles of such receptors and to structure/activity relationships among agonists and antagonists.

Effect of yohimbine and its diastereoisomers on clonidine-induced depression of exploration in the rat

The effects of yohimbine, rauwolscine and corynanthine on clonidine-induced depression of exploration (ambulation and rearing) in the rat were investigated. Yohimbine and rauwolscine antagonized clonidine-induced hypoactivity in a dose-range of 0.3 - 3 mg/kg i.p. By contrast, corynanthine was found to lack such an effect up to ten times higher doses. These results are in agreement with the reported differential affinity of these drugs for the alpha 2-adrenoceptors respectively and suggest the involvement of alpha 2-receptors in the mediation of depressant effect of clonidine. The antagonism of clonidine-induced hypoactivity appears, therefore, to provide a suitable in vivo measure of preferential alpha 2-adrenoceptor blocking activity of drugs.

Identification of alpha adrenoceptor subtypes in dog arteries by (3H) yohimbine and (3H) prazosin

Binding of the alpha adrenergic antagonists (3H)prazosin and (3H) yohimbine to membranes of dog arteries exhibit the characteristics expected of alpha adrenoceptors. Binding of both ligands is saturable with dissociation constants of 0.19nM and 1.15nM for (3H)prazosin and (3H)yohimbine respectively. A series of catecholamines inhibit binding of both ligands with a potency in the order epinephrine greater than norepinephrine much greater than isoproterenol, corresponding with the activity of these agents at alpha adrenoceptors in blood vessels. Competition for binding in both instances is stereoselective. l-Phenylephrine has similar potencies in inhibiting (3H)prazosin and (3H)yohimbine specific binding whilst the imidazoline related partial alpha adrenergic agonists clonidine and guanfacine are more potent in inhibiting (3H) yohimbine specific binding. The affinity of prazosin for the (3H)yohimbine binding site is approximately 2500 times less than for the (3H)prazosin site whilst yohimbine is approximately 150 times more potent in inhibiting (3H)yohimbine than (3H)prazosin specific binding. Non-selective alpha adrenergic antagonists have similar affinities for both binding sites. The concentration of (3H)yohimbine binding sites in different arteries vary about two fold whilst for (3H)prazosin in the variation was about three fold. These results indicate that there are two discrete noradrenergic binding sites in the major arteries of dog which have binding properties expected of alpha 1 and alpha 2 adrenoceptors.

A comparison of peripheral pre- and postsynaptic alpha 2-adrenoreceptors using meta-substituted imidazolidines

1 The cardiac presynaptic activity, as derived from the inhibition of tachycardia to electrical stimulation of the cardiac sympathetic nerve in pithed, normotensive rats of 8 meta-substituted phenyl(imino)imidazolidines (2, 3- and 2,5- derivation) was determined. 2 The agonistic activity of the imidazolidines was measured with respect to vascular alpha 1- and alpha 2-adrenoreceptors. Accordingly, the increase in diastolic pressure of pithed, normotensive rats to intravenous administration of the imidazolidines was evaluated after pretreatment with 5% w/v glucose solution, yohimbine (1 mg/kg), prazosin (0.1 mg/kg) and the combination of both alpha-adrenoreceptor antagonists. 3 With respect to pre- and postsynaptic alpha 2-adrenoreceptors the 2,5-substituted derivatives were generally less active than the corresponding 2,3-substituted isomers. The steric bulk of the substituent at the 5-position of the imidazolidine molecule appeared to govern the activity with respect to cardiac presynaptic alpha 2-adrenoreceptors, whereas no such clear relationship could be derived for the activity on vascular postsynaptic alpha 2-adrenoreceptors. This indicates that there exists a close resemblance between pre- and post-synaptically located alpha 2-adrenoreceptors. However, these receptor sites are not identical.

Pre- and postsynaptic alpha-adrenoceptor blocking activity of raubasine in the rat vas deferens

1 The actions of raubasine, yohimbine and corynanthine at pre- and postsynaptic alpha-adrenoceptors were studied in the rat vas deferens. 2 Low frequency electrical stimulation of the isolated vas deferens of the rat produced regular contractions that were inhibited by low concentrations of clonidine. This inhibition was presynaptic in origin and involved alpha-adrenoceptors. 3 Presynaptic alpha-adrenoceptor antagonist activity was assessed by studying the effect of increasing antagonist concentrations on cumulative clonidine dose-response curves on the stimulated vas deferens. 4 Postsynaptic alpha-adrenoceptor antagonist activity in the isolated vas deferens was assessed by comparing control cumulative noradrenaline dose-response curves in the absence and in the presence of increasing concentrations of antagonists. 5 The results indicate that raubasine and corynanthine preferentially block postsynaptic alpha-adrenoceptors. Yohimbine is more potent in blocking pre- than postsynaptic alpha-adrenoceptors. The ration of the pre/postsynaptic potency declines in the order yohimbine less than raubasine less than corynanthine.