Mini-review. The postsynaptic alpha 2-adrenoreceptor

Differential inhibition of alpha 2-adrenoceptor-mediated pressor responses by (+)- and (-)-verapamil in pithed rats

The inhibitory effects of (+)- and (-)-verapamil on the hypertensive responses brought about by the alpha 2-adrenoceptor agonist B-HT 920 were investigated in pithed normotensive rats. (-)-Verapamil was found to be about 4 times more potent with respect to depressing alpha 2-pressor responses compared with (+)-verapamil. To rule out the effect of 'unspecific' vasodilatation after the administration of the stereoisomers of verapamil, vasopressin was continuously infused into the carotid artery of pithed rats in a separate series of experiments. In the course of this vasopressin infusion, new inhibitory activities of the stereoisomers of verapamil on alpha 2-adrenoceptor-mediated pressor responses were determined. Under these circumstances, the potency ratio of (-)- vs (+)-verapamil was about 7. With the aid of a radioligand binding assay using [3H]clonidine to identify alpha 2-adrenoceptors, low affinities were measured for the stereoisomers of verapamil. A Ki = 6170 nM for (-)-verapamil and a Ki = 41700 nM for (+)-verapamil were calculated. The results indicate that the interaction between alpha 2-adrenoceptor-mediated pressor responses and calcium entry blockers, such as verapamil, is a stereoselective event.

Postjunctional alpha-adrenoceptors in the isolated saphenous vein of the rabbit. Characterization and influence of angiotensin

Experiments on the isolated saphenous vein of the rabbit have been performed in order to determine whether a coexistence of postjunctional alpha 1- and alpha 2-adrenoceptor subtypes can be demonstrated also under in vitro conditions. Prazosin, selective for alpha 1-receptors, and rauwolscine, selective for alpha 2-receptors, were used to antagonize the contractile response of the agonists phenylephrine (alpha 1), B-HT 920 (alpha 2) and noradrenaline (alpha 1/alpha 2). Each of the antagonists was equipotent against all three agonists; the effect of neither antagonist fulfilled the criteria for a competitive antagonism. The preferential alpha 1-receptor antagonist phenoxybenzamine, applied for irreversible blockade of alpha-receptors, reduced the effect of phenylephrine and B-HT 920 to the same degree. Thus, the results obtained with alpha-receptor antagonists cannot be reconciled with the coexistence of alpha 1- and alpha 2-receptors or the existence of only one of them. Contractile responses induced by alpha 1- and alpha 2-receptor agonists, respectively, could be differentiated by the calcium entry blocker nitrendipine. The effect of B-HT 920 was decreased whereas that of phenylephrine was hardly affected. Furthermore, we investigated the influence of angiotensin II on the effects of the alpha-adrenoceptor agonists as well as the antagonists. We observed firstly, that angiotensin, acting postsynaptically, potentiates the contractile response of certain alpha 2-receptor agonists and secondly, that in the presence of angiotensin the characteristics of the receptors as revealed by B-HT 920 are converted to typical alpha 2-adrenoceptors. It is concluded that the postjunctional alpha 2-receptors of the saphenous vein require the blood borne substance angiotensin for their expression.

Effect of adrenalectomy and demedullation of the adrenals on vasopressor responses to stimulation of postjunctional alpha 2-adrenoceptors

After adrenalectomy performed 18-12 h previously, the vasopressor responses induced by stimulation of postjunctional alpha 2-adrenoceptors (agonist: B-HT 920) were attenuated in pithed normotensive rats in contrast to pressor responses mediated by postjunctional alpha 1-adrenoceptors (agonist: cirazoline). Aldosterone and desoxycorticosterone both enhanced the vasopressor responses to the alpha 2-adrenoceptor agonist B-HT 920 in adrenalectomized rats, but did not fully restore the effects. Demedullation of the adrenals also attenuated the vasoconstrictor processes evoked by stimulation of postjunctional alpha 2-adrenoceptors 18-22 h later. This effect was reversed after an infusion with adrenaline. The combination of aldosterone and adrenaline produced full recovery of the increase in diastolic pressure elicited via alpha 2-adrenoceptors in adrenalectomized (18-22 h) animals. The results suggest that circulating adrenaline and aldosterone (or corticosterone) are required for optimal functioning of vascular postjunctional alpha 2-adrenoceptors.

Characterization of postsynaptic alpha-adrenoceptors in rat aortic strips and portal veins

Postsynaptic alpha-adrenoceptors in rat isolated aortic strips and portal veins have been examined using a number of agonist and antagonist drugs which have varying selectivity for alpha 1- and alpha 2-adrenoceptors. In both tissues (-)-noradrenaline [-)-NA), (-)-adrenaline [-) Adr) (-)-alpha-methyl noradrenaline [-)-alpha-Me-NA) and (-)-phenylephrine [-)-PE) were full agonists, while clonidine, oxymetazoline and (2-(2,6-dichlorophenyl)-5,6-dihydroimidazo(2,1,b) thiazole (44,549) were partial agonists. Guanfacine was a full agonist in aortic strips but only a partial agonist in portal veins. In aortic strips, pA2 values for prazosin and yohimbine were not significantly different using (-)-NA, (-)-PE or guanfacine as the agonist, suggesting a single population of alpha-adrenoceptors. The order of potency of the antagonists, prazosin = 2-(beta-(4-hydroxyphenyl)-ethylaminomethyl)-tetralone (BE2254) greater than phentolamine greater than yohimbine greater than rauwolscine, is indicative of an alpha 1-type of receptor. In portal veins, the order of potency of the antagonists was prazosin greater than BE2254 greater than phentolamine greater than yohimbine greater than rauwolscine, again indicating an alpha 1-type of receptor. The mean pA2 value for yohimbine was not significantly different in either tissue. However, mean pA2 values for prazosin, BE-2254 and phentolamine were approximately one order of magnitude lower in portal veins than in aortic strips, suggesting that the receptors in the two tissues may not be identical.

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.

Physiological and pharmacological aspects of adrenergic receptor classification

The questions raised are: what is the physiological or pharmacological basis for the differentiation into beta 1- and beta 2-, and alpha 1- and alpha 2-adrenergic receptors?; and do the neurotransmitter norepinephrine and the hormone epinephrine differ in their receptors? On the basis of a preference of beta 2- and alpha 2-adrenergic receptors for epinephrine, the hormone, and of beta 1-and alpha 1-receptors for norepinephrine, the neurotransmitter, it was postulated that the alpha 2- and beta 2-receptors are predominantly epinephrinergic in nature and located extrajunctionally and presynaptically whereas the alpha 1- and beta 1-receptors are predominantly norepinephrinergic in nature and located postsynaptically in the sympathetic terminal junction. The alpha 2- and beta 2-character of the presynaptic receptors matches that of the corresponding extrajunctional receptors. This indicates that a circulating catecholamine, namely epinephrine, is involved in the regulation of adrenergic transmitter release.

Effect of thymoxamine on the human pupil

The specificity of thymoxamine, an alpha-adrenoceptor antagonist was determined from in vitro organ bath studies using isolated preparations of canine saphenous and portal mesenteric veins. It was found that thymoxamine interacts mainly with postjunctional alpha-adrenoceptors. The miotic effects of topical thymoxamine hydrochloride were measured in normal human eyes using infrared pupillography. The effects of five different concentrations of thymoxamine were measured over time. Time-response and concentration-response relationships were calculated. The maximal pupillary response occurs at 60 min after instillation of the drug and has a half-life of 10 hr. The minimum effective concentration of thymoxamine is 0.01%, and the maximum effective concentration is 1.3%. Thymoxamine's properties could make it a clinically useful drug to diagnose angle-closure glaucoma, and also to reverse drug-induced mydriasis.

Relationship of alpha receptor types to hypotension and renal vasodilation caused by alpha blockers in conscious dogs

In conscious instrumented normotensive and two-kidney, one clip Goldblatt hypertensive dogs, we compared the effects of the alpha-receptor blocking agent, urapidil, on blood pressure, renal vascular resistance, heart rate, and plasma renin activity with those of prazosin and phentolamine. Urapidil (2 mg/kg) and prazosin (0.25 mg/kg) decreased blood pressure and renal vascular resistance in both groups of animals, and urapidil caused a small increase in renal blood flow. Heart rate, but not plasma renin activity, was increased at the peak of the hypotension. Phentolamine had no significant effect on any of these parameters. All three agents markedly inhibited the renal vasoconstrictor responses to intraarterially administered phenylephrine and norepinephrine, and thus exhibited an alpha 1-receptor blocking action. Only urapidil significantly antagonized the response to B-HT 933, a selective alpha 2-receptor agonist, indicating that it also interacts at alpha 2-receptor sites. Since both normotensive and hypertensive animals exhibited similar hypotensive responses after both urapidil and prazosin, the degree of alpha-receptor blockade achieved did not reveal greater sympathetic tone in renal hypertension.

Pharmacology of postsynaptic alpha-adrenoreceptors in the mouse anococcygeus muscle

1. The alpha-adrenoreceptor agonists noradrenaline (NA), methoxamine, phenylephrine, naphazoline, oxymetazoline, and xylazine produced contractions of the mouse anococcygeus muscle. All were full agonists. 2. Cocaine (2 microM) increased the pD2 values of NA and phenylephrine. 6-hydroxydopamine pretreatment increased the pD2 values of NA, phenylephrine, and xylazine. After both procedures the order of potency of the agonists was oxymetazoline greater than naphazoline greater than NA greater than phenylephrine greater than methoxamine greater than xylazine. 3. The order of potency of antagonists against all six agonists was prazosin greater than phentolamine greater than yohimbine. However, from differences in the pA2 values and slopes of the Schild plots of the antagonists it was possible to distinguish three distinct groups of agonists: the phenylethylamines; the imidazolines; and the thiazine derivative xylazine. 4. The results suggest that the postsynaptic alpha-adrenoreceptor of the mouse anococcygeus muscle may be broadly classified as alpha 1. However, there may be at least three drug recognition sites on the receptor which interact with agonists of differing chemical structure.

Hypotensive and alpha-adrenergic activities of 2-(2,3,6-trichlorophenylimino) and 2-(2,3-dichloro-6-methylphenylimino) imidazolidine, two potent derivatives of clonidine

1 alpha-Adrenergic activities (hypotension, bradycardia, sedation) and affinities of clonidine and two newly substituted derivatives, 2-(2,3,6-trichlorophenylimino)imidazolidine (I) and 2-(2,3-dichloro-6-methylphenylimino)imidazolidine (II), were determined in various in vitro and in vitro models.2 In anaesthetized normotensive rats, the intravenous -log doses (mol/kg) of clonidine, compound I and II required to decrease mean arterial pressure by 20% were 7.96, 8.39 and 7.79, respectively, and to reduce heart rate by 20% were 7.85, 8.23 and 7.91, respectively. Comparable potencies were obtained in vagotomized rats. Following vertebral arterial infusion of clonidine, compound I and II into anaesthetized cats, the -log doses for 20% diminution in mean arterial pressure were 8.72, 9.17 and 7.92, respectively.3 In pithed normotensive rats, the intravenous -log doses (mol/kg) of clonidine, compound I and II required to elevate diastolic pressure by 50 mmHg were 7.49, 7.80 and 8.04, respectively, whereas 50% of the maximal inhibition of electrical stimulation-induced tachycardia was obtained at -log doses of 8.20, 8.25 and 8.34, respectively.4 In mice, the intraperitoneal -log doses (mol/kg) needed for a prolongation of the hexobarbitone-induced loss of the righting reflex by 100% were 6.51, 6.55 and 6.60 for clonidine, compound I and II, respectively.5 Compounds I and II displayed a higher affinity for alpha(1)- and alpha(2)-adrenoceptors than clonidine, identified in rat cerebral membranes by [(3)H]-prazosin and [(3)H]-clonidine.6 The results show that 2,3,6-trisubstituted derivatives of clonidine are potent alpha-adrenoceptor stimulants. This new class of congeners may have potential for pronounced hypotensive activity following systemic application. It is not likely that among members of this series, hypotensive potency can be separated from sedative activity.

alpha 2-Adrenoceptors in rat resistance vessels

In pithed rats increases in blood pressure were induced by i.v. injections of the alpha 1-agonist methoxamine and the alpha 2-agonists clonidine, oxymetazoline and B-HT 920. The pressor responses were further analyzed by repeated measurements of cardiac output with the thermodilution technique and by calculation of total peripheral vascular resistance. During the pressor phase both vascular resistance and cardiac output were found to be elevated. This indicates that increases in both haemodynamic variables contributed to the pressure rise. Under the assumption that elevated vascular resistance reflected constriction of arterioles and elevated cardiac output constriction of capacitance vessels via increased venous return to the heart, and considering that the magnitude of the increase of both haemodynamic parameters was similar for all three agonists, the results suggest the existence of both alpha 1- and alpha 2-adrenoceptors in resistance as well as in capacitance vessels of rats. For alpha 2-adrenoceptors in resistance vessels this conclusion was supported by the finding that the calcium antagonists verapamil and/or tiapamil virtually abolished the increases of blood pressure and vascular resistance in response to clonidine, oxymetazoline or B-HT 920, but not to methoxamine. The calcium antagonists did not affect the increases in cardiac output, irrespective of which type of alpha-agonist was administered. While the present results support the existence of alpha 2-adrenoceptors in resistance vessels of the rat, they do not allow a firm conclusion as to their occurrence in rat capacitance vessels.

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.

A comparison of the effects of verapamil and cinnarizine upon responses elicited by selective alpha 1- and alpha 2-adrenoceptor agonists in the autoperfused canine hindlimb

In an attempt to extend the hypothesis that activation of vascular postsynaptic alpha 2-adrenoceptors requires an influx of Ca2+ ions, the effects of 2 calcium entry blocking drugs verapamil and cinnarizine have been examined as inhibitors of the pressor responses to methoxamine and B-HT 920 in autoperfused dog hindlimb preparations. Verapamil (0.1-1 mg i.a.) selectively antagonized responses to B-HT 920 and had little or no effect upon responses to methoxamine, thus supporting this hypothesis. However cinnarizine, over the dose range studied (0.1-1 mg/kg i.a.) produced quantitatively similar inhibitions of the hindlimb responses to B-HT 920 and methoxamine. These results suggest that cinnarizine may have a different site of action to verapamil in resistance vessels of the dog hindlimb.

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.

Comparative aspects of central cardiovascular control with special reference to adrenergic mechanisms

1. The central nervous system probably serves as an overall cardiovascular regulatory centre in all vertebrates. 2. Electrical stimulation of the brain elicits haemodynamic effects in all vertebrate classes, but the quality of the responses may differ between different classes in some cases. 3. In the most "primitive" classes of vertebrates, i.e. fishes, the circulatory regulation may be exerted via mechanisms somewhat different from those in other vertebrates. 4. Amphibians, reptiles, birds and mammals display a similar cardiovascular response pattern to administration of drugs such as clonidine and L-DOPA.

Chronic antidepressant therapy and associated changes in central monoaminergic receptor functioning

Acutely administered antidepressants possess a multiplicity of pharmacological actions. However, the fact that agents possessing similar pharmacological actions are devoid of antidepressant activity, together with the lack of correlation between doses required for acute pharmacological effects and clinical efficacy, suggest that the mechanism(s) of action of antidepressants cannot be directly attributed to the acute pharmacological properties of the drugs. The lag phase in onset of clinical effectiveness emphasizes the importance of adaptive changes following chronic antidepressant administration. A rapidly accelerating trend in attempting to delineate the precise molecular mechanisms of action of antidepressants is the shift in emphasis following chronic antidepressant therapies from alterations in uptake, storage, synthesis and release of neurotransmitters to adaptive changes in receptor functioning. These adaptations occur both pre- and postsynaptically. Examples of the former are alpha 2 and DA presynaptic receptors, both being down-regulated by certain forms of chronic antidepressant therapy. The fact that the NE-coupled adenylate cyclase system in rat brain slices is down-regulated by tricyclics, atypical antidepressants, MAO inhibitors and ECT emphasizes the importance of the system. Electrophysiological and behavioral studies point to the up-regulation of central alpha 1 and 5-HT receptor functioning following long-term antidepressant therapy. In contrast to the beta-adrenoceptor, these findings cannot be correlated with data from radioligand binding studies. In general central alpha 1-adrenoceptor binding remains unaltered. This is also true for 5-HT1 binding whereas cortical 5-HT2 binding is both increased and decreased depending on the type of antidepressant therapy being investigated. The relationship of these adaptive changes to the clinical efficacy of antidepressants in man is not clear since there is generally a lack of good models for studying human central receptor functioning. A review of current data from animal studies would tend to disfavour the view that all forms of antidepressant therapy possess a common mechanism of action. Perhaps multiple intervention sites exist. The introduction and evaluation of agents possessing a specificity of pharmacological action will undoubtedly aid psychotherapeutic research. The knowledge that peptides and 'classical' neurotransmitters can co-exist in the same neurone will undoubtedly generate studies of the significance and importance of the co-transmitter function of peptides in the mechanisms of action of antidepressant therapies.

Biochemical pharmacology of paradoxical sleep

The role of noradrenergic cells in the regulation of paradoxical sleep is still controversial, and experimental data have given rise to contradictory interpretations. Early investigations focused primarily on chemical neurotransmissions. However, the process of information transmission between cells involves many other factors, and the cell surface is an important site for transduction of messages into modifications of the activity of postsynaptic cells. alpha-adrenoceptors are believed to play an important role in the control of wakefulness and paradoxical sleep. Experimental evidence suggests that physiological modulation of receptor sensitivity, possibly by specific neuro-modulators, may be a key mechanism in synaptic transmission. In the investigation of the mechanisms involved in paradoxical sleep regulation, lesions of the locus coeruleus have given equivocal results. Collateral inhibition, probably mediated by alpha 2-adrenoceptors, appears to be a powerful mechanism. The exact temporal relationship between noradrenergic cell activation and paradoxical sleep production is not established, but 5-HT appears to be involved. Differences between paradoxical sleep and waking may be related to a physiological modulation of alpha 2-adrenoceptor sensitivity.

Comparison of the actions of centrally and peripherally administered clonidine and guanfacine in the rabbit: investigation of the differences

1 Guanfacine was administered intravenously to rabbits and produced a dose-dependent lowering of blood pressure. 2 Clonidine and guanfacine, administered to rabbits intravenously (30 micrograms/kg and 300 micrograms/kg respectively) and intracisternally (3 micrograms/kg and 12 micrograms/kg respectively) caused a similar degree of hypotension, apparently of central origin. 3 Saliva flow in vivo was estimated. Clonidine (30 micrograms/kg, i.v.) caused a significant decrease in salivation (P less than 0.05) for the first 50 min after injection. Guanfacine caused a significant fall (P less than 0.05) only at 50 and 180 min after injection. 4 Apparent partition coefficients for an octanol/buffer system at pH 7.4 for clonidine and guanfacine were 5.4 and 21.2 respectively. 5 Measurement of guanfacine levels concurrently in both plasma and brain showed that guanfacine had higher brain than plasma levels and that the brain levels were fairly constant over the 3 h measured. Brain:plasma ratios were 2.1:1, 5.3:1 and 13.6:1 after 15, 90 and 180 min respectively. 6 These results suggest that the long duration of action of guanfacine is due to its persistence at its central site of action.

The purported dopamine agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) acts as a nonselective alpha-adrenoceptor agonist in inducing hypertension, hypomotility and hypothermia in the rat

Following peripheral administration the purported dopamine (DA) agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) was shown to increase the diastolic blood pressure of pithed rats and to decrease rat motility and rectal temperature. Dose-effect relationships were established and half-maximal effective doses for the hypertensive and hypothermic response to DPI were calculated to be 4.4 nmol/kg i.v. and 2.0 mumol/kg i.p., respectively. Pretreatment with various antagonists revealed that both alpha 1- and alpha 2-adrenergic mechanisms were responsible for the DPI-induced hypertension, hypomotility and hypothermia, indicating that DPI acts as a non-selective alpha-adrenoceptor agonist. Qualitatively the DPI-induced effects were found to correlate well with those reported for its structural analogue clonidine, thus suggesting a similar mechanism of action for these agents. DA receptor antagonists appeared to lack inhibitory potency towards any of DPI-elicited responses. The results do not therefore support the designation of DPI as a DA receptor agonist.

Postsynaptic alpha 1-and alpha 2-adrenoceptor involvement in the vascular responses to neuronally released and exogenous noradrenaline in the hindlimb of the dog and cat

The contribution of postsynaptic alpha 1-and alpha 2-adrenoceptors to vascular responses was investigated in the blood perfused hindlimbs of pentobarbitone-anaesthetised dogs and chloralose-anaesthetised cats. Responses were obtained to sympathetic nerve stimulation, noradrenaline, phenylephrine and UK-14,304. In the dog and cat hindlimbs, the response to phenylephrine was reduced by the selective alpha 1-adrenoceptor antagonist, prazosin, while that to UK-14,304, in the dog, was reduced by the alpha 2-adrenoceptor antagonist, rauwolscine. Responses to noradrenaline and sympathetic nerve stimulation were only partially blocked by prazosin in both the dog and cat, but were further reduced by the addition of rauwolscine. These results suggest that the hindlimb vasculature of the dog and cat responds to neuronally released, as well as exogenous, noradrenaline by activation of both postsynaptic alpha 1- and alpha 2-adrenoceptors.

Typical and atypical neuroleptics are potent antagonists at alpha 1-adrenoceptors of the dorsal lateral geniculate nucleus. An electrophysiological study

The present electrophysiology studies examined the actions of neuroleptics at central alpha 1-adrenoceptors in the rat. In single-cell recording experiments, typical and atypical neuroleptics, when administered systemically or locally (iontophoresis and pressure ejection), were found to be potent antagonists of activating alpha 1-adrenoceptor responses in the dorsal lateral geniculate nucleus (dLGN). Doses of neuroleptics effective as antagonists at alpha 1-adrenoceptors had very weak effects at muscarinic receptors in the dLGN. Since doses of neuroleptics employed in the present study were within the clinical range, it appears likely that central alpha 1-adrenoceptors would be blocked during a neuroleptic therapy in humans.

Presynaptic activity of the imidazolidine derivative ST 587, a highly selective alpha 1-adrenoceptor agonist

St 587 increased the blood pressure of pithed rats. When the antagonists prazosin (alpha 1) and rauwolscine (alpha 2) were used the drug was shown to be a more selective alpha 1-adrenoceptor agonist than methoxamine. St 587 inhibited the tachycardia elicited by sympathetic stimulation in pithed rats. This effect was presynaptic and due to stimulation of alpha 1-adrenoceptors, as revealed by the different antagonism exerted by prazosin and yohimbine (alpha 2), respectively. The existence of presynaptic alpha 1-adrenoceptors is discussed.

Norepinephrine constricts the canine coronary bed via postsynaptic alpha 2-adrenoceptors

The effect of alpha 2-blockade (0.3 mg/kg i.v. rauwolscine) and alpha 1-blockade (1.2 mg/kg i.v. prazosin) on coronary constrictions induced by intracoronary injections of azepexole (B-HT 933, alpha 2-agonist, 0.1-10 microgram/kg), phenylephrine (0.3-3 microgram/kg) and norepinephrine (0.001-0.1 microgram/kg) were studied in dog hearts perfused in situ under beta-blockade. Constrictions by azepexole (antagonized by rauwolscine, yet resistant to prazosin and methysergide) demonstrated coronary alpha 2-adrenoceptors. Norepinephrine-induced constrictions were more attenuated (22-fold) by alpha 2-blockade than by alpha 1-blockade (2.6-fold) and thus were mediated mainly by activation of postsynaptic alpha 2-receptors.

Evidence that the purported dopaminergic agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) may reduce rat striatal dopamine turnover by an alpha 2-adrenergic mechanism

The potent alpha-adrenergic agonist DPI, which has also been claimed to be a selective dopaminergic agonist, was shown to reduce rat striatal dopamine (DA) synthesis, DA utilization and DA metabolism following intraperitoneal administration (25 mumol/kg). An analytical procedure for the determination of DPI was developed and its application showed that DPI did not penetrate into the brain in substantial amounts. The possibility of a direct stimulatory action upon striatal presynaptic DA receptors was excluded by the demonstration that DPI lacked effectiveness both in the gamma-butyrolactone model and following intrastriatal administration. The selective alpha-adrenergic agonists phenylephrine (alpha 1) and tramazoline (alpha 2) decreased and increased DA metabolism, respectively, the yohimbine-induced increase being antagonized by DPI. The carbon-bridge analogue (3,4-dihydroxybenzyl)-2-imidazoline (DHBI) had about the same activity as DPI, whereas the potential DPI metabolite (4-hydroxy-3-methoxyphenylimino)-2-imidazolidine (HMPI) was without effect upon striatal DA metabolism. The results are discussed in relation to the remarkable resemblance with the literature data concerning clonidine. It is concluded that the DPI-elicited attenuation of striatal DA turnover is, in all likelihood, the result of a stimulation of alpha 2-adrenoceptors possibly located within the central nervous system. The results cast some doubt on the designation of DPI as a selective DA-inhibitory receptor agonist.

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.

Inhibition by D 600 of norepinephrine- and clonidine-induced responses of the aortae from normotensive (WKY) and spontaneously hypertensive rats (SHR)

Comparison was made of the inhibition by D 600 of the norepinephrine (NE)- and clonidine-induced responses of aortic strips from normotensive (WKY) and spontaneously hypertensive rats (SHR). Clonidine-induced responses of aortae from both strains of rats were markedly more susceptible to inhibition by D 600 than those of NE. Omission of calcium from the physiological solution resulted in a greater reduction in response to clonidine. Aortae from SHR were less susceptible to inhibition by D 600 but showed no difference from WKY rats in their responses in nominally calcium-free physiological solutions. Our results clearly indicate a greater dependence on extracellular calcium by clonidine-induced responses of the aortae relative to those of NE.

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.

Cardiovascular effects of clonidine in an avian species, Larus argentatus

Blood pressure and heart rate were recorded in the sea gull, Larus argentatus, under light pentibarbitone anaesthesia. Clonidine 10(-7) and 10(-8) mol . kg-1 (27 and 2.7 microgram . kg-1) i.v. produced a biphasic effect on blood pressure, a brief initial increase being followed by a prolonged hypotensive response. There was an immediate reduction in heart rate rate which persisted throughout the hypotensive phase. After spinal transection at the level of C4, clonidine administration elicited hypertension and bradycardia. Bilateral vagotomy abolished the effect of clonidine on heart rate but did not alter the blood pressure response. Vagotomy in combination with spinal transection abolished the effect of clonidine on heart rate but the hypertensive response was accentuated. Yohimbine 10(-7) or 10(-6) mol . kg-1 (0.039 or 0.39 mg . kg-1) given 5 min after clonidine 10(-7) mol.kg-1 (27 microgram . kg-1) effectively antagonized the cardiovascular effects of clonidine, while prazosin 10(-7) or 10(-6) mol . kg (0.042 or 0.42 mg . kg-1) had no such effect. We conclude that clonidine acts in the central nervous system of the sea gull to produce a lowering of blood pressure and heart rate. These effects are mediated by central inhibition of sympathetic activity and, in the case of the heart rate, mostly by central activation of vagal activity to the heart. This central action of clonidine involves activation of alpha-adrenoceptors which are blocked by yohimbine but not by prazosin and therefore may belong to the alpha 2 subtype.

alpha 2-Adrenoceptor agonistic effect of B-HT 920 in isolated perfused hindquarters of rats

In isolated perfused hindquarters of rats pretreated with reserpine the alpha 2-adrenoceptor selective agonist B-HT 920 decreased the flow dose-dependently. This effect was strongly antagonized by yohimbine and less by prazosin, and the reverse potency was demonstrated for methoxamine. With increasing concentrations of B-HT 920, a half maximal effect was achieved with 10(-6.64) M. This value is discussed wih respect to the antagonistic potency of B-HT 920 at alpha 1-adrenoceptors with a pA2 of 4.9.

Organic and inorganic calcium antagonists reduce vasoconstriction in vivo mediated by postsynaptic alpha 2-adrenoceptors

The influence of various calcium antagonists and divalent metal cations on the pressor responses induced by the selective alpha 1-adrenoceptor agonist methoxamine and the selective alpha 2-adrenoceptor stimulating agent B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]-azepine) was studied in pithed rats. 1. The calcium antagonists verapamil, D 600 and nifedipine, when given intraarterially (i.a.) in doses up to 1 mg/kg did not influence the pressor effects of methoxamine. Only higher amounts of these calcium antagonistic drugs (1 - 3 mg/kg i.a.) somewhat reduced this pressor response. 2. The vasoconstriction due to B-HT 920, as reflected by the increase in diastolic pressure, was markedly inhibited by verapamil, D 600 and nifedipine in a dose-dependent manner. In low doses a parallel displacement to the right was observed, whereas in higher amounts the shift was non-parallel. 3. The divalent cations MN2+, Ni2+ and Co2+ (0.05 - 0.15 mmol/kg i.a.) hardly affected the pressor effect of methoxamine, whereas B-HT 920-induced vasoconstriction was highly sensitive to these metal ions. La3+ and Mg2+ were ineffective. 4. The calcium antagonists verapamil, D 600 and nifedipine displayed only minor affinities for [3H]prazosin (alpha 1) as well as [3H]clonidine (alpha 2) binding sites of rat brain membranes. 5. It is concluded that an influx of extracellular Ca2+ is necessary for the vasoconstriction in vivo initiated by stimulation of vascular postsynaptic alpha 2-adrenoceptors. On the other hand, vasopressor responses to alpha 1-adrenoceptor stimulation are not directly dependent on a transmembrane influx of calcium ions.