α-Adrenoceptor subclassification

Calcium-dependence and antagonism of responses to alpha 1- and alpha 2-adrenoceptor agonists in vascular tissues from hypertensive and normotensive rats

Inhibition by D600 (methoxyverapamil) of responses to an alpha 2-adrenoceptor selective agonist, B-HT 920, (6-allyl-2-amino-5, 6, 7, 8-tetrahydro-4H-thiazolo [4, 5-d] azepin dihydrochloride), an alpha 1-adrenoceptor selective agonist, phenylephrine (PE), and a nonselective agonist, noradrenaline (NA), was studied in isolated preparations of the aortae and carotid arteries obtained from young (5-7 weeks) and old (15-17 weeks) hypertensive (SHR) and normotensive (WKY) rats. Maximum responses of WKY tissues to B-HT 920 were the most sensitive, PE-induced responses the least sensitive and maximum responses to NA were intermediate in their sensitivity to inhibition by D600. Sub-maximal responses to NA and PE were not different in their sensitivity to inhibition by D600, but were less sensitive than the responses to B-HT 20. Sub-maximal responses to PE were significantly more sensitive to D600 inhibition than were the maximal responses to this agonist. NA-induced responses of tissues from older SHR were less sensitive to inhibition by D600 when compared to responses in WKY rats. Responses to B-HT 920, in tissues suspended in calcium-free solutions, showed the largest decline compared to NA- and PE-induced responses. We conclude that responses to B-HT 920 largely utilize extracellular calcium. PE- and NA- induced responses mobilize extracellular calcium to varying degrees depending upon the concentration of the agonist employed to elicit the response.

On the postjunctional alpha-adrenoreceptors in rat cerebral and mesenteric arteries

The contractile response to exogenously applied noradrenaline (NA) was examined in vitro in tubal segments (0.2-0.1 mm in diameter) of rat middle cerebral (MCA), basilar (BA) and mesenteric (MA) arteries. In the MCA, the maximum contractile response to NA (10(-4)M) was considerably smaller than that induced by K+ (124 mM) or 5-hydroxytryptamine (10(-5)M), whereas the inverse relationship was found in the MA. NA usually failed to elicit contraction in the BA even in the presence of propranolol and cocaine. In the MCA, propranolol (3 X 10(-7)M) enhanced the maximum contractile response to NA by approximately 100% without affecting the potency of the agonist. In the MA, propranolol had no effect on the concentration-response relationship for NA. Cocaine (10(-5)M) or 6-hydroxydopamine pretreatment increased the NA sensitivity of the MA by a factor of three, whereas these procedures failed to influence the NA sensitivity of the MCA. A marked stereoselectivity was found in the MCA, as (-)-NA was more than 100 times more potent than (+)-NA as a contractile agent. The order of potency of a series of alpha-adrenoreceptor agonists was (-)-adrenaline greater than oxymetazoline greater than (+/-)-NA approximately (-)-phenylephrine greater than methoxamine in the MCA and (+/-)-NA greater than (-)-phenylephrine in the MA. Clonidine failed to elicit contraction in concentrations lower than 3 X 10(-4)M in both types of artery. Prazosin was between three and four orders of magnitude more potent than rauwolscine in inhibiting NA-induced contractions in the MCA and MA. The pA2 values for, respectively, prazosin and rauwolscine were 9.3 and 5.4 in the MCA and 9.7 and 6.8 in the MA. The slope of the Schild plot deviated significantly from unity only for rauwolscine in the MA (0.64). It is concluded that the contractile response to exogenous NA in the MCA and MA is mediated mainly by stimulation of alpha 1-adrenoreceptors, although a small contribution of postjunctional alpha 2-adrenoreceptors in the MA cannot be excluded. In contrast to the MCA, the BA appears to lack contraction-mediating alpha-adrenoreceptors, indicating regional differences in the alpha-adrenoreceptor distribution in the rat cerebrovascular bed.

The hypotensive activity and side effects of methyldopa, clonidine, and guanfacine

Clonidine (Catapres, Catapresan), guanfacine (Estulic), and methyldopa (Aldomet) are the prototypes of centrally acting antihypertensive drugs. Clonidine and guanfacine are lipophilic drugs that readily penetrate into the brain, where they stimulate alpha-adrenergic receptors in the pontomedullary region. The stimulation of these central alpha-adrenergic receptors has been shown to activate an inhibiting neuron, which causes a reduction of peripheral sympathetic tone and a subsequent fall in arterial blood pressure and heart rate. Both a centrally initiated reduction of vagus reflex activity and the activation of presynaptic alpha 2-adrenergic blocking agents in the heart may contribute to the bradycardia. Studies indicate that methyldopa also penetrates into the brain, where it is converted into alpha-methylnorepinephrine. This amine may stimulate the same central alpha-adrenergic receptors as those activated by clonidine, which will result in a hypotensive effect. Possibly, alpha-methyldopamine might also play a role. Accordingly, the modes of action of clonidine and alpha-methyldopa probably are very similar at a basic level. The central adrenergic receptors probably are located postsynaptically. Their receptor demand corresponds more closely to that of the alpha 2-subtype. Central alpha 1-adrenergic receptors might possibly play a part in the modulation of vagally induced baroreflex bradycardia. A discussion on the pharmacological basis of the side effects of the centrally acting antihypertensives has been limited to those adverse reactions that are somehow related to alpha-adrenergic receptors. Sedation, a common side effect, appears to be mediated by central alpha 2-adrenergic receptors, at least in animal models.(ABSTRACT TRUNCATED AT 250 WORDS)

Treatment with clorgyline and pargyline differentially decreases clonidine-induced hypotension and bradycardia

A study was made of the effects of acute and chronic treatment with monoamine-oxidase (MAO) inhibitors on the peripheral and central cardiovascular response induced by clonidine in anaesthetized normotensive rats. Clonidine (30 nmoles X kg-1 i.v.) produced a biphasic change in mean blood pressure; an initial transient increase was followed by a prolonged hypotensive effect, coinciding with the maximal bradycardia. Twenty-four hours after acute (single) or chronic (daily for 7 days) administration of MAO inhibitors (pargyline 10 mg X kg-1 SC or clorgyline 0.3 mg X kg-1 SC) there was no effect either on the basal cardiovascular parameters or on the initial pressor response induced by clonidine. Chronic but not acute treatment with clorgyline, an inhibitor of type A MAO, greatly decreased the hypotension and bradycardia induced by clonidine for as long as 5 days after its discontinuation. On the other hand, after chronic administration of pargyline (10 mg X kg-1), a preferential type B MAO inhibitor, the hypotension and bradycardia caused by clonidine were differently affected. There was a reduction in the bradycardia up to the third day following the discontinuation of pargyline, whereas the hypotensive response induced by clonidine was only attenuated for 24 h and unaffected with a lower dose of pargyline (0.3 mg X kg-1). It is concluded that chronic administration of the type A MAO inhibitor, clorgyline, attenuates the central responses to clonidine through the reduction in sensitivity of brain alpha-adrenoceptors. Pargyline, that preferentially inhibits type B MAO, reduces only the bradycardia induced by clonidine. This result may indicate a different modulation of the receptors involved in this response to clonidine.

Evidence for two opposite effects of clonidine on gastric acid secretion in the dog

The effects of clonidine on gastric acid secretion were studied in conscious dogs with both gastric fistulae and Heidenhain pouches. Clonidine infused systemically at graded doses under basal conditions produced a significant increase in acid secretion from both gastric fistulae and Heidenhain pouches. Acid secretion from gastric fistulae submaximally stimulated by pentagastrin was dose-dependently reduced by clonidine while 2-deoxy-D-glucose-induced secretion was completely suppressed. Under these conditions a significant enhancement of secretion from Heidenhain pouches was recorded. An increase in acid secretion from both main stomachs and Heidenhain pouches was observed for clonidine with submaximal doses of bethanechol and histamine as stimulants, though clonidine showed no effect on maximal stimulation by histamine. The stimulant effect of clonidine from gastric fistulae and Heidenhain pouches under basal conditions was fully prevented by cimetidine, while the inhibitory effect of clonidine on acid secretion stimulated by pentagastrin from gastric fistulae was reversed by yohimbine. The present results suggest that clonidine displays two simultaneous yet opposite effects on dog gastric secretion. The inhibitory effect might be mediated through a decrease of vagally released acetylcholine following the activation of alpha 2-adrenoceptors both at central and peripheral sites, while the stimulatory effect probably depends on the histamine-like properties of the drug.

An in vivo study of the alpha-adrenoreceptor subtypes on the renal vasculature of the anaesthetized rabbit

An attempt has been made to classify the subtypes of alpha-adrenoreceptors mediating renal vasoconstriction in vivo using renal nerve stimulation and alpha-adrenoreceptor agonists and antagonists of varying selectivity in anesthetised rabbits. In the first series of experiments prazosin more potently inhibited the phenylephrine than the nerve induced renal vasoconstriction while phentolamine inhibited the response to phenylephrine and nerve stimulation nearly equally. In the second series of experiments low to moderate doses of yohimbine potently inhibited the renal vasoconstriction of clonidine, was less potent on that induced by noradrenaline and did not significantly affect that of phenylephrine. Prazosin was a potent antagonist of phenylephrine induced vasoconstriction but was less potent on nerve stimulation and noradrenaline, and was without effect on clonidine. In the third series of experiments, prazosin partially inhibited the renal vasoconstriction produced by all frequencies of nerve stimulation and that produced by high doses of noradrenaline. The prazosin resistant component of nerve stimulation and exogenous noradrenaline was significantly reduced by the addition of yohimbine. These results have been taken to suggest the presence of both alpha 1- and alpha 2-adrenoreceptors in the renal vasculature of the rabbit which mediate the renal vasoconstriction of exogenous and endogenous noradrenaline. They further show that in the rabbit the alpha 2-adrenoreceptors mediate a much greater proportion of the total renal vasoconstriction compared with the dog, cat or rat in which the alpha 1-adrenoreceptor population appears to be the predominant receptor.

A pharmacologic analysis of the action of platelet-activating factor in the induction of hindpaw edema in the rat

Platelet-activating factor (PAF), a phospholipid product of neutrophils, alveolar macrophages, monocytes, and platelets and an important mediator of inflammatory reactions, was studied for its ability to evoke hindpaw edema in the rat. PAF caused edema, peaking at 1 hr and gradually declining over the next 2 hr. The H1 and H2 antihistamines, mepyramine and cimetidine, the serotonin/histamine antagonist, cyproheptadine, and the serotonin antagonist, methysergide, were ineffective in reducing PAF-induced paw edema. Indomethacin, acetylsalicylic acid, and dexamethasone did not inhibit the peak edematous response but significant reduction was noted with only dexamethasone at 3 hr. Prazosin and propranolol did not prevent PAF-induced edema, whereas, yohimbine, phentolamine, rauwolscine, verapamil and theophylline partially inhibited edema. Clonidine and guanfacine did not induce edema when injected into the rat hindpaw. These results suggest that PAF elicits edema at vascular sites of the rat hindpaw which are partially dependent on extracellular Ca2+ movement, are not due to alpha-1 or alpha-2-adrenoreceptor stimulation, histamine, serotonin, or prostaglandin activity, and demonstrates variable sensitivities to agents blocking Ca2+ entry. Inhibition of specific PAF-sensitive receptors await the discovery of specific PAF antagonists.

Modulation of noradrenaline release in human saphenous vein via presynaptic alpha 2-adrenoceptors

Strips of human saphenous veins were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing cocaine, corticosterone and propranolol. The electrically (6 Hz) evoked overflow of tritium (78% of which was accounted for by unmetabolized [3H]noradrenaline) was abolished by tetrodotoxin or omission of Ca2+ from the superfusion fluid. Unlabelled noradrenaline, alpha-methylnoradrenaline, B-HT 920 and clonidine inhibited the evoked overflow (maximum effect of clonidine lower than that of the other compounds) whereas methoxamine was ineffective. The alpha 2-adrenoceptor antagonists, BDF 6143 and rauwolscine, facilitated the evoked overflow but no effect was obtained with prazosin. Rauwolscine produced a shift to the right of the concentration-response curve of B-HT 920 for its inhibitory effect on evoked outflow and BDF 6143 caused a shift to the right of the corresponding curve of clonidine. It is concluded that the stimulation-evoked release of noradrenaline from the sympathetic nerve fibres of the human saphenous vein is modulated via presynaptic alpha 2-adrenoceptors.

Specific binding of [3H]prazosin to myocardial cells isolated from adult rats

The characteristics of alpha-adrenoceptors in rat myocardium were investigated by specific binding of [3H]prazosin to cells isolated from adult rat heart by perfusion with collagenase and hyaluronidase. The cells were incubated in Krebs-Ringer bicarbonate buffer gassed with 95% O2 and 5% CO2 at 31 degrees with the appropriate concentrations of the different ligands. Non-specific binding was defined by the addition of 10(-5) mole/l. phentolamine. The binding of [3H]prazosin was saturable and reached equilibrium within 15 min. Scatchard analysis showed a straight line giving an apparent dissociation constant, Kd, equal to 155.9 +/- 8.0 pmole/l. and a maximal number of binding sites equal to 76.7 +/- 11.1 fmole/mg protein. Inhibition of specific [3H]prazosin binding by different adrenergic blockers showed the order of potency characteristic of alpha 1-adrenoceptors: prazosin much greater than phentolamine greater than yohimbine much greater than propranolol. Inhibition by adrenergic agonists showed the order of potency: adrenaline greater than noradrenaline = phenylephrine greater than isoprenaline. The same orders of potency were observed in the presence of propranolol. However, propranolol slightly decreased the affinity for noradrenaline and phenylephrine. Hofstee analyses of the inhibition curves showed two binding components for all ordinary alpha-adrenoceptor blockers and agonists including unlabelled prazosin. In contrast, [3H]prazosin showed only one binding component. Both binding components were of the alpha 1-adrenoceptor subtype according to the order of potency of blockers. The different ligands had different affinity ratios for the two binding components giving them different profiles. Trifluoperazine, a phenothiazine compound, also had high affinity for the [3H]prazosin binding sites. This drug, however, apparently detected one class of binding sites only, as interpreted from the Hofstee analysis. Hill analyses of the inhibition data consistently yielded Hill constants, nH, in the range 0.75-0.85 except for [3H]prazosin, where nH = 1.02 and for trifluoperazine, where nH = 1.07. Although the two binding components may serve different functions, it seems impossible at present to relate the negative and the positive inotropic components, respectively, of the alpha-adrenergic inotropic response observed in functional studies only to one or the other binding component.

Clonidine inhibits electrodermal potentials induced by preganglionic stimulation

The electrodermal potential (EDP) recorded with surface electrodes between the palm and the shaven back of the right forepaw of anaesthetized and vagotomized cats was taken as a measure of the activity of cholinergic-sympathetic sudomotor nerves. EDPs were induced by preganglionic electrical stimulation of the stellate ganglion for 2 s with trains of DC pulses (2 ms duration, 0.5-128 Hz) at regular intervals of 60 s. The EDPs amounted to 12 mV and increased little with stimulation rate (14 mV). The i.v. injection of 30 micrograms/kg of the alpha 2-adrenergic agonist clonidine did not change the EDPs significantly. A consistent result was obtained in cats pretreated i.p. with 5 mg/kg reserpine 18 h beforehand for depletion of catecholamines. Three hours after the i.v. injection of 3 mg/kg guanethidine, clonidine (30 micrograms/kg i.v.) induced significant reduction of EDPs in the lower range of the stimulation rate but did not affect those at 16 and 32 Hz. Partial blockade of ganglionic nicotinic receptors by i.v. infusion of 0.08-0.3 mg/kg per min hexamethonium diminished EDPs (1 Hz) by 30-50%. Under these conditions the i.v. injection (30 micrograms/kg) or topical application of 1 microgram clonidine to the right stellate ganglion inhibited EDPs at all rates of stimulation. The inhibitory effects of clonidine could be antagonized by 200 micrograms/kg yohimbine i.v. Partial ganglionic blockade by i.v. infusion of the depolarizing blocker suxamethonium (0.2-0.4 mg/kg per min) decreased EDPs. However, the topical application of 1 microgram clonidine to the stellate ganglion during infusion of suxamethonium caused no further decrease.(ABSTRACT TRUNCATED AT 250 WORDS)

Clonidine inhibits nicotinic effects in ganglia of the cholinergic-sympathetic system

The electrodermal potential (EDP) recorded with surface electrodes between the palm and the shaven back of the forepaws of anaesthetized cats was taken as a measure of sudomotor activity in response to submaximal activation of cholinergic-sympathetic efferents with the nicotinic ganglionic stimulant DMPP. The intravenous injection of 30 micrograms/kg clonidine inhibited EDPs in despinalized cats and in vagotomized cats with chronic denervation of the stellate ganglion. In vagotomized cats clonidine (30 micrograms/kg i.v.) shifted the dose-response curve of DMPP (4-256 micrograms/kg i.v.) to the right. An inhibitory effect was also observed in despinalized cats upon topical application of 0.1 and 1 microgram clonidine to the stellate ganglion. In all preparations the inhibitory effect of clonidine could be antagonized by intravenous injection of 200 micrograms/kg yohimbine. As all substances used were also active when injected during arrest of blood flow to the foreleg by means of a tourniquet, actions at the level of the sweat gland could be excluded. The results lead to the conclusion that the activation of postsynaptic somadendritic alpha 2-adrenoceptors in sudomotor ganglia of the cholinergic-sympathetic nervous system inhibits the effects of nicotinic ganglionic stimulation. It is assumed that this action is due to the known ganglionic hyperpolarization induced by alpha 2-adrenoceptor stimulants.

Facilitating interaction between rauwolscine and angiotensin in the mesenteric artery of the rabbit

The interaction between rauwolscine and angiotensin II was investigated in the isolated mesenteric artery of the rabbit. Rauwolscine, known as an antagonist at alpha 2-adrenoceptors, did not induce contraction itself but interacted with angiotensin to produce a facilitated response of the vascular tissue. In the presence of rauwolscine, the contractile response of the tissue to angiotensin was markedly enhanced. The degree of facilitation appeared to be dependent on the rauwolscine concentration used rather than that of angiotensin. Moreover, rauwolscine induced a concentration-dependent increase in tension (pD2 = 6.8) in the presence of even subcontractile concentrations of angiotensin (10(-10) mol/l). This effect was not attributable to an indirect action involving presynaptic catecholamines, as revealed by the use of tissue strips from animals pretreated with reserpine or after chemical sympathectomy. Furthermore, an interaction via the prostaglandin system was excluded by negative results obtained with indomethacin. The 'agonistic effect' of rauwolscine was significantly attenuated by phentolamine (alpha 1/alpha 2) but not by prazosin (alpha 1) or phenoxybenzamine when applied for only a short time. The alpha 2-antagonist BDF 6143 behaved like rauwolscine whereas the alpha 1-antagonist corynanthine, a stereoisomer of rauwolscine, did not. The results indicate that the 'rauwolscine effect' is mediated by a receptor with alpha 2-characteristics. In general, angiotensin appears to interfere with some process which determines the expression of a drug's intrinsic effect.

Alpha 1-adrenoceptor-mediated vasoconstriction in vivo to enantiomers of SK & F 89748-A

The pressor activity of the 1-enantiomer of SK & F 89748-A, 1,2,3,4-tetrahydro-8-methoxy-5-(methylthio)-2-naphthalenamine, in pithed normotensive rats was found comparable with that of 1-phenylephrine. The d-enantiomer was half as potent. The log dose-pressor effect curves for d- and 1-SK & F 89748-A were not influenced by reserpine treatment (2 X 5 mg/kg i.p., -48 and -24 h), were virtually unaffected by yohimbine (1 mg/kg i.v., -15 min) but were markedly shifted to the right by prazosin (0.1 mg/kg i.v., -15 min) and phentolamine (1 mg/kg i.v., -15 min). Similar observations were made for the 1-enantiomer in pithed cats. It is concluded that d- and 1-SK & F 89748-A are potent, directly acting highly selective agonists of (vascular) postjunctional alpha 1-adrenoceptors. Potency and selectivity were equally pronounced for both enantiomers. The currently available selective agonists of alpha 1-adrenoceptors, including the optical isomers of SK & F 89748-A, cannot distinguish between alpha 1- and alpha 2-adrenoceptors. This conclusion is based on binding affinity since these affinities are linearly correlated as shown by radioligand displacement experiments.

An investigation into the selectivity of a novel series of benzoquinolizines for alpha 2-adrenoceptors in vivo

The potencies and selectivities of a novel series of benzoquinolizines for the alpha 2-adrenoceptor have been investigated in the rat in comparison with yohimbine and indoramin. Peripheral postjunctional alpha 2- and alpha 1-adrenoceptor blockade was measured as the reversal of B-HT 933 and methoxamine-induced pressor responses, respectively, in the pithed rat. Peripheral prejunctional alpha 2-adrenoceptor blockade was measured as the reversal of B-HT 933-induced inhibition of an electrically evoked tachycardia in the pithed rat. Central alpha 2-adrenoceptor blockade was measured as a reversal of the hypotension induced in anaesthetized rats by central (i.c.v.) administration of clonidine. Wy 25309, Wy 26392, Wy 26703 and yohimbine (0.3-3 mg kg-1 i.v.) evoked dose-dependent shifts to the right of the dose-response curves to B-HT 933 whilst having minimal effects on the methoxamine dose-response curve. The selectivity for alpha 2-adrenoceptors increased with the dose of antagonist administered. In general, the order of selectivity was Wy 25309 greater than Wy 26392 greater than Wy 26703 greater than yohimbine. Indoramin (1 mg kg-1 i.v.) shifted the methoxamine pressor dose-response curve to the right without affecting the B-HT 933 dose-response curves, confirming its selective alpha 1-antagonist activity. Peripheral administration of all three benzoquinolizines (1-100 micrograms kg-1 i.v.) led to a dose-dependent reversal of the hypotension evoked by central administration of clonidine (500 ng i.c.v.). The reversal was incomplete, higher doses causing a further decrease in blood pressure. A similar degree of hypotension induced by the ganglion blocking agent chlorisondamine (1 mg kg- I i.v.) was not reversed by the benzoquinolizines. 9 It is concluded that Wy 25309, Wy 26392 and Wy 26703 are selective alpha 2-adrenoceptor antagonists which readily penetrate the CNS.

Comparative effects of RX 781094, mianserin, yohimbine, rauwolscine and prazosin in reversing clonidine inhibition of MIX-stimulated lipolysis in hamster isolated white fat cells

Different alpha-adrenergic receptor antagonists: RX 781094, yohimbine and rauwolscine (selective alpha 2-adrenergic antagonists); mianserin (tetracyclic anti-depressant, antagonist at alpha 2-presynaptic autoreceptors) and prazosin (selective alpha 1-adrenergic antagonist) were used at concentrations ranging from 10(-7) to 10(-4) M, to reverse clonidine inhibition of MIX-stimulated lipolysis in the hamster fat cell. In this adipose tissue (like human but unlike rat) there co-exist prolipolytic beta-adrenoceptors and antilipolytic alpha-adrenoceptors. Although no effects were observed with prazosin, RX 781094 was ten times more potent than yohimbine or its isomer rauwolscine in reversing clonidine inhibition of the MIX-stimulated lipolysis. Mianserin was an effective blocker only from a concentration of 10(-4) M, consistent with its relative lack of specificity for alpha 2-adrenoceptors cited elsewhere. Overall these results confirm the utility of this model for testing compounds presumed to act at alpha 2-adrenoceptors.

Identification of vascular postsynaptic alpha 1- and alpha 2-adrenoceptors in man

We studied postsynaptic alpha-adrenoceptors in human blood vessels by measuring the influence on forearm blood flow induced by intra-arterial infusions of selective alpha 1- and alpha 2-adrenoceptor agonists (methoxamine, B-HT 933, clonidine and guanfacine) and antagonists (doxazosin and yohimbine). The studies were done in healthy volunteers, and forearm blood flow was measured by plethysmography. All agonists produced a significant and dose-dependent vasoconstriction. The effect of B-HT 933 was completely abolished by the concomitant infusion of yohimbine, whereas it was hardly influenced by doxazosin. The effect of methoxamine was prevented by doxazosin and little influenced by yohimbine. The vasoconstriction by clonidine and guanfacine was partially prevented by both doxazosin and yohimbine. The single intra-arterial infusion of yohimbine, as well as doxazosin, resulted in vasodilation. These findings provide strong evidence for the existence of postsynaptic alpha 1- as well as alpha 2-adrenoceptors, both mediating vasoconstriction and contributing to basal vascular tone. The (patho-)physiological significance of this subdivision of alpha-adrenoceptors remains to be elucidated.

Adrenoceptors in the control of human nasal mucosal blood flow

The blood flow in the human nasal mucosa is controlled by the functional state of the resistance vessels. This blood flow was studied by means of the 133Xe washout method. Using topical application of drugs that stimulate or block adrenoceptors, including clinical doses of nasal decongestants, we classified the adrenoceptors of the resistance vessels in the mucosa. The results suggest that stimulation of alpha2-adrenoceptors causes a reduction of nasal mucosal blood flow; stimulation of alpha1- and beta2-adrenoceptors seems to have no significant effects. Drugs that selectively stimulate alpha2-adrenoceptors, as well as those with preference for alpha1-adrenoceptors, cause nasal decongestion. To avoid the negative effects of a nasal mucosal blood flow reduction, it is suggested that a nasal decongestant with an alpha1-adrenoceptor stimulating effect should be preferable to a drug acting mainly on alpha2-adrenoceptors, provided that the decongestive effects are equal.

Characterization of the alpha-adrenoceptors in the female rabbit urethra

A radioligand binding technique was used to evaluate the proportions of alpha 1- and alpha 2-adrenoceptors in crude membrane preparations obtained from the female rabbit bladder base and urethra. In addition, urethral rings were studied in vitro in an attempt to determine if alpha 1- and/or alpha 2-adrenoceptors are located postjunctionally in the urethral smooth muscle. Studies of the inhibition of [3H]-dihydroergocryptine binding by the selective alpha 1-adrenoceptor antagonist prazosin or the selective alpha 2-adrenoceptor antagonist rauwolscine revealed the alpha-adrenoceptor population to consist of approximately 25% alpha 1-adrenoceptors and 75% alpha 2-adrenoceptors. These proportions were confirmed in saturation studies with [3H]-prazosin and [3H]-rauwolscine. The sum of alpha 1- and alpha 2-adrenoceptors labelled by these selective alpha 1- and alpha 2-adrenoceptor antagonists was about equal to the number labelled by the non-selective alpha-adrenoceptor antagonist [3H]-dihydroergocryptine. Noradrenaline, as well as the selective alpha 1-adrenoceptor agonist phenylephrine and the selective alpha 2-adrenoceptor agonist clonidine, induced contractions of urethral ring preparations. Prazosin blocked contractions induced by phenylephrine to a greater extent than contractions induced by clonidine. The opposite was true for the inhibitory effect of rauwolscine. In addition to showing that both alpha 1- and alpha 2-adrenoceptor binding sites exist in membrane preparations of the rabbit bladder base and urethra, the results reveal the presence of both adrenoceptor subtypes postjunctionally in the rabbit urethra; and both mediate contraction of the smooth muscle.

Evaluation of monoaminergic receptors in the genetically epilepsy prone rat

The intensity of sound-induced convulsions in the genetically epilepsy-prone rat (GEPR) was reduced in a dose related fashion by intracerebroventricular administration of dobutamine, (beta 1 agonist), terbutaline (beta 2 agonist) or phenylephrine (alpha 1 agonist). BHT-920 (alpha 2 agonist) did not cause a dose-related decrease in sound-induced convulsion intensity. Binding studies showed that whole brain alpha and beta receptor densities (Bmax) were normal while the Kd was increased for the beta ligand in GEPR brain.

Alpha 1/alpha 2-adrenoceptor agonist selectivity of mono- and dihydroxy-2-N,N-DI-n-propylaminotetralins

The pressor activities and the identity of the postjunctional alpha-adrenoceptors involved were determined for a series of congeneric mono- and dihydroxy-substituted 2-N,N-di-n-propylaminotetralins and N,N-di-n-propyldopamine (DPDA) following i.v. administration to pithed normotensive rats. The affinity for alpha 1- and alpha 2-adrenoceptor-like binding sites was obtained from radioligand displacement studies. The 5- and 7-OH substituted tetralins as well as DPDA were reasonably potent and about equieffective pressor agents. The 6-OH congener had almost no vasoconstrictor effects whereas the 8-OH positional isomer occupied an intermediate position. The 5,6- and 6,7-di-OH analogs very effectively raised the diastolic pressure of pithed rats. On account of the inhibition exerted by prazosin (0.1 mg/kg) and yohimbine (1 mg/kg) the 5- and 7-OH isomers as well as DPDA can be classified as mixed alpha 1/alpha 2-adrenoceptor agonists, the alpha 1-adrenoceptor-stimulating potency being more pronounced especially for the 5-OH congener. In addition, a significant contribution of serotonin receptors to the pressor responses to the 8-OH compound was detected. Similarly, alpha 2-adrenoceptors were mainly responsible for the vasoconstriction caused by the 6,7-di-OH isomer, whereas the 5,6-di-OH congener very selectively stimulated this alpha 2-type receptor in the lower dose range and alpha 1-adrenoceptor stimulation predominated at higher doses of this agonist. The 6,7-di-OH compound failed to activate vascular postjunctional beta 2-adrenoceptors. The results indicate that the alpha 1/alpha 2-adrenoceptor agonist selectivity depends on the position(s) and the number of hydroxy groups present as well as on the alkyl substitution at the amino function. 2-N,N-Di-n-propylamino-6,7-dihydroxytetralin may be a more suitable alpha 2-adrenoceptor selective agonist than M-7.

Autoinhibition of noradrenaline release from the rat heart as a function of the biophase concentration. Effects of exogenous alpha-adrenoceptor agonists, cocaine, and perfusion rate

Rat isolated perfused hearts with the right sympathetic nerves intact were loaded with 3H-(-)-noradrenaline. The nerves were stimulated with trains of 180 pulses at 3 Hz and at 10 min intervals. The overflow of 3H-noradrenaline and 3H-metabolites was determined by liquid scintillation spectrometry. Clonidine (IC50 17 nM), oxymetazoline (IC50 63 nM), and alpha-methylnoradrenaline (apparent IC50 35 nM, determined in the presence of cocaine and propranolol) decreased the stimulation-evoked overflow of 3H-noradrenaline by 26, 49, and 78%, respectively, but not methoxamine up to 100 microM (propranolol present). Oxymetazoline and alpha-methyl-noradrenaline did not cause desensitization of the presynaptic adrenoceptors when present at their IC80 for 33 min. At a perfusion rate of 7 ml/min, yohimbine 1 microM enhanced the stimulation-evoked 3H-noradrenaline overflow by 26% in the absence, and by 58% in the presence of cocaine. Phentolamine 1 microM increased it by 69% when the neuronal reuptake was blocked. The increase by the antagonists faded with successive period of nerve stimulations, and was positively correlated with the biophase concentration of noradrenaline as reflected by the amount of 3H-noradrenaline released into the perfusate per nerve stimulation. At a perfusion rate of 1.8 ml/min (neuronal reuptake blocked), yohimbine 1 microM increased the overflow by 127%. The results indicate that the alpha 2-adrenoceptor-mediated autoinhibition in the rat perfused heart depends on the clearance of transmitter from the biophase via neuronal reuptake and diffusion into the vascular space. Reduction of either elimination pathway enhances the biophase concentration of noradrenaline, thus increasing the autoinhibition of release.

Alpha-adrenoceptors: recent development and some comparative aspects

On anatomical and functional bases, alpha-adrenoceptors have been divided into pre- and postsynaptic alpha-adrenoceptors. Recently, alpha-adrenoceptors have been classified as alpha 1 and alpha 2 according to their pharmacological responses, irrespective of their anatomical location. The presynaptic alpha-adrenoceptors, which have been recognized as alpha 2, determine the frequency of the nerve impulses travelling along the axon and also the amount of transmitter released per nerve impulse from the varicose terminal. Postsynaptic alpha-adrenoceptors have been recognized in various tissues including smooth muscle, pancreatic islets, fat cells, platelets and other tissues. Both alpha 1- and alpha 2-adrenoceptors have been located postsynaptically. alpha-Adrenoceptors have been found also in the central nervous system. Generally, they fall into the same categories (alpha 1 and alpha 2) as the peripheral alpha-adrenoceptors. A new class of drugs, the so called calcium blockers, inhibit the postsynaptic response to alpha 2 stimulation but not the alpha 1-mediated response, indicating that the alpha 2-adrenoceptors are dependent on Ca2+ ions for their function. In the most primitive group of vertebrates, the fishes, alpha-adrenoceptors seem to be different in as much as they do not respond to many of the classical drugs employed to distinguish between alpha-adrenoceptors in mammals. In reptiles and amphibians alpha 2-adrenoceptors have been shown to exist. These receptors are involved in the regulation of melanocytes. In the most advanced non-mammalian vertebrates (birds) both peripheral and central alpha-adrenoceptors seem to be qualitatively similar to the mammalian types.

Pharmacological characterization of postjunctional alpha-adrenoceptors in isolated human omental arteries and veins

The alpha-adrenoceptors in human omental arteries and veins were characterized and compared. In the arteries both prazosin (pA2 9.48) and rauwolscine (pA2 7.19) displaced the noradrenaline (NA) concentration-response (cr) curve towards higher concentrations without reduction of maximum. Neither clonidine, nor oxymetazoline had any consistent contractile effects. Phenylephrine had a lower potency than NA, but a similar intrinsic activity. In the veins, both prazosin (pA2 9.72) and rauwolscine (pA2 8.11) displaced the NA cr-curve towards higher concentrations, but also significantly depressed maximum. Clonidine and oxymetazoline contracted veins from 3 out of 7 and 4 out of 6 patients, respectively. Their pD2-values were similar to that of NA, but their intrinsic activities were significantly lower. NA was more potent than phenylephrine in these vessels, and there was no significant difference in intrinsic activity. The results suggest that in human omental arteries, the postjunctional alpha-adrenoceptors are mainly of the alpha 1-type, even if a small population of alpha 2-adrenoceptors cannot be excluded. In omental veins, there seems to be a functionally important population of postjunctional alpha 2-adrenoceptors occurring together with a population of alpha 1-adrenoceptors.

The effects of alpha-methylnoradrenaline on protein and electrolyte secretion by rat submandibular and parotid glands

alpha-Methylnoradrenaline (alpha-mNA) is a potent secretagogue for the parotid and submandibular glands of rats. With regard to the parotid glands, alpha-mNA activates mainly beta-adrenoceptors. In the submandibular glands, alpha-mNA activates alpha-adrenoceptors at higher doses whereas at relatively lower doses it activates beta-adrenoceptors. alpha-mNA may not stimulate the specific alpha 2-adrenoceptors of the salivary glands of rats.

Effects of yohimbine stereoisomers on contractions of rat aortic strips produced by agonists with different selectivity for alpha 1- and alpha 2-adrenoceptors

Three stereoisomers of yohimbine (corynanthine, rauwolscine and yohimbine) have been used to characterize alpha-adrenoceptors in rat aortic strips. pA2 values for each antagonist were calculated using 3 different agonists ((-)-noradrenaline, (-)-phenylephrine and guanfacine) which possess varying affinities for alpha 1 and alpha 2 receptors. Mean pA2 values were not significantly different irrespective of the agonist used and the order of the potency was corynanthine greater than yohimbine greater than rauwolscine. The results are consistent with the presence of alpha 1-adrenoceptors in rat aorta.

Evidence for a physiological role of presynaptic alpha-adrenoceptors: modulation of noradrenaline release in the pithed rabbit

Rabbits were pithed and the preganglionic nerves at T 8 were stimulated continuously at a frequency of 3 Hz. 3H-noradrenaline was infused to reach a steady-state plasma level, from which the noradrenaline plasma clearance was calculated. The plasma level of endogenous noradrenaline was also determined and the rate of noradrenaline release into the plasma was then derived. The noradrenaline plasma clearance was decreased by guanethidine (7.5 mg/kg), desipramine (1 mg/kg), yohimbine (1 mg/kg) and rauwolscine (1 mg/kg). It was unaffected by corynanthine (1 mg/kg), prazosin (0.1 mg/kg), alpha-methylnoradrenaline (2 micrograms/kg per min) and clonidine (1 micrograms/kg per min). The electrical stimulation resulted in an increase in blood pressure without an increase in heart rate. Both adrenaline and noradrenaline were detected in the plasma. It is likely that the noradrenaline was of neuronal origin as guanethidine decreased its plasma level. The alpha 2-adrenoceptor-selective blocking drugs yohimbine and rauwolscine increased the noradrenaline release rate and only slightly decreased blood pressure. On the other hand, the alpha 1-adrenoceptor-selective blocking drugs corynanthine and prazosin had no effect on the noradrenaline release rate and decreased blood pressure more markedly. The alpha 2-adrenoceptor-selective agonists alpha-methylnoradrenaline and clonidine both decreased the noradrenaline release rate. This effect was blocked by yohimbine, and for the case of clonidine, not blocked by corynanthine. Plasma adrenaline levels were increased by yohimbine and rauwolscine, but not by corynanthine and prazosin. Clonidine reduced the plasma adrenaline level.(ABSTRACT TRUNCATED AT 250 WORDS)

A regional difference in the distribution of postsynaptic alpha-adrenoceptor subtypes in canine veins

The responsiveness of helical venous strips isolated from fifteen different sites in the body of dogs to relatively selective alpha 1- and alpha 2-adrenoceptor agonists was studied, as well as to a non-selective alpha-adrenoceptor agonist. Longitudinal strips of the portal and mesenteric veins and the inferior vena cava between the liver and the renal vein (segment C) were also investigated. All veins contracted to noradrenaline or phenylephrine whereas only seven veins responded significantly to clonidine: the saphenous, cephalic, jugular and femoral veins and longitudinal strips of the portal and mesenteric veins and the segment C of the inferior vena cava. The brachiocephalic, azygos, pulmonary and splenic veins and the superior vena cava and the supradiaphragmatic portion (segment A) and the infrarenal portion (segment D) of the inferior vena cava responded little to clonidine. Unlike the longitudinal strips, the helical strips of the portal and mesenteric veins and the segment C of the inferior vena cava did not respond to clonidine. According to the relative sensitivities to phenylephrine and clonidine, those veins which responded to clonidine could be divided into three groups. (1) The veins in which the sensitivity to phenylephrine was higher than to clonidine: longitudinal strips of the portal vein and segment C of the inferior vena cava, (2) the veins whose sensitivity to phenylephrine was lower than to clonidine: the saphenous, cephalic, femoral and external jugular veins, (3) the vein whose sensitivity to the two agonists was comparable: longitudinal strips of the mesenteric vein. Subtype characteristics were further analyzed in the saphenous vein and in the portal vein using prazosin, phentolamine and yohimbine as antagonists. Analysis of Schild plots to noradrenaline suggested that a mixed population of alpha-adrenoceptor subtypes might be present in the saphenous vein, whereas a rather homogeneous population of a single subtype might occur in the portal vein. The results of the antagonism experiment against phenylephrine and clonidine suggested that contractions of the saphenous vein are mediated by both alpha 1- and alpha 2-adrenoceptors whereas contractions of the portal vein are exerted mainly through alpha 1-adrenoceptors. The results suggest that there may be a distinct regional difference with respect to postsynaptic alpha- adrenoceptor subtypes in the canine venous system.

Differential effect of calcium entry blockers on alpha 1-adrenoceptor-mediated vasoconstriction in vivo

The effects of the calcium entry blockers nifedipine, (-)-verapamil and the dihydropyridine derivative PY 108-068 were evaluated on the increase in diastolic pressure of pithed normotensive rats caused by the selective alpha 1-adrenoceptor agonists cirazoline, (-)-phenylephrine, (+/-)-erythro-methoxamine, (-)-amidephrine and St 587 [(2-chloro-5-trifluoromethylphenylimino)-2-imidazolidine] as well as by the mixed alpha 1/alpha 2-adrenoceptor agonists clonidine and DPI [(3,4-dihydroxyphenylimino)-2-imidazolidine]. The calcium entry inhibitors (up to 3 mg/kg) caused 3- to 5-fold, parallel rightward shifts of the log dose-pressor effect curves to cirazoline, (-)-phenylephrine, (+/-)-erythro-methoxamine and (-)-amidephrine accompanied by only a slight depression of the maximal pressor response. In contrast, the calcium entry inhibitors produced a dose-dependent profound depression of both maximum and slope of the log dose-pressor response curves to St 587 and clonidine. For DPI about 10- and 100-fold parallel displacements to the right without reduction of the maximum were found following treatment with 1 and 3 mg/kg of nifedipine, respectively. Infusion of vasopressin to counteract the vasodilatory action produced by the calcium entry inhibitors did not significantly change the pattern of interference observed under the conditions of decreased baseline diastolic pressure. The results indicate that alpha 1-adrenoceptor-mediated vasoconstriction in the pithed normotensive rat, which is characterized by its sensitivity to blockade by prazosin and its relative insensitivity to antagonism by yohimbine or rauwolscine, can be subdivided into two distinct processes which are differentially influenced by blockade of calcium entry.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective stimulation of vascular postjunctional alpha-adrenoreceptors by (-)-amidephrine in rats and cats

The vasopressor and chronotropic responses of (-)-amidephrine and the receptor types involved were studied in pithed rats of different strains and in pithed cats. The increase in diastolic pressure of pithed rats after i.v. administration of (-)-amidephrine was not influenced by pretreatment with propranolol (1 mg/kg, i.v.), reserpine (2 x 5mg/kg in 48 h i.p.) or yohimbine (1 mg/kg, i.v.), but was strongly antagonized by prazosin (0.1 mg/kg, i.v.). In pithed cats, the pressor responses were antagonized by prazosin (1 mg/kg, i.v.) but much less so by yohimbine (1 mg/kg, i.v.). (-)-Amidephrine elicited minor positive chronotropic responses in pithed rats and pithed cats. This tachycardia was not influenced by propranolol (1 mg/kg, i.v.) but was abolished by prazosin (0.1--1.0 mg/kg). The results show that (-)-amidephrine acts as a selective agonist at vascular postjunctional alpha 1-adrenoreceptors in pithed rats and pithed cats. The positive chronotropic effects are attributable to stimulation of alpha 1-adrenoreceptors in the heart.

Effects of yohimbine, rauwolscine and corynanthine on contractions and calcium fluxes induced by depolarization and prostaglandin F2 alpha in rat aorta

The effects of the selective alpha 2-adrenoceptor antagonists yohimbine and its stereo-isomer rauwolscine and the selective alpha 1-adrenoceptor antagonist corynanthine (a third yohimbine stereoisomer) on contractions induced in rat aorta by depolarization and prostaglandin F2 alpha (PGF2 alpha) have been compared. In calcium-free solution, depolarization with 100 mM K+ failed to produce a contraction of rat aorta but PGF2 alpha (3 microM) stimulated a contraction equal to about 23% of maximal elicited in normal physiological solution. Yohimbine had no significant effect on depolarization-induced contractions except at concentrations greater than 30 microM. Rauwolscine and corynanthine (1 to 100 microM) depressed depolarization-induced contractions in a concentration-dependent manner, but the characteristics of inhibition were not identical. Contractions induced by PGF2 alpha (3 microM) were depressed in a concentration-dependent manner by rauwolscine (3 to 100 microM) but were unaffected by yohimbine or corynanthine. Depolarization-stimulated 45Ca influx was depressed by rauwolscine and corynanthine to about the same extent as were the contractions; while rauwolscine (100 microM) completely inhibited PGF2 alpha-stimulated 45Ca influx, it also depressed part of the PGF2 alpha-stimulated contraction dependent on intracellular calcium. Rauwolscine (100 microM) partly inhibited PGF2 alpha-stimulated release of 45Ca from aortic smooth muscle in calcium-free solution. It is concluded that the yohimbine structure possesses a calcium entry blocking action as well as a depressant action on contractions not dependent on calcium entry. The predominant effect depends on the structural configuration and the nature of the stimulating agent.

Characterization of alpha adrenoceptors in pial arteries of the bovine brain

When attempting to characterize the nature of adrenoceptors in bovine pial arteries, we found specific 3H-yohimbine binding was saturable, reversible and of high affinity (KD = 18.3 +/- 1.2 nM) with a Bmax of 687 +/- 27 fmol/mg protein (N = 4). On the other hand, there was no specific 3H-prazosin binding in these tissues. Scatchard and Hill plot analyses of specific 3H-yohimbine binding indicated one class of binding sites. From kinetic analyses of the data, association and dissociation rate constants of 1.6 +/- 0.3 X 10(7) M-1min-1 and 0.51 +/- 0.04 min-1, respectively, were calculated (N = 3). The dissociation constant from the equation KD = K-1/K+1 was 35.7 +/- 7.6 nM, such being in good agreement with the KD value estimated from Scatchard plots. Specific binding of 3H-yohimbine was displaced effectively by alpha 2 adrenergic agents and less effectively by alpha 1 adrenergic agents or beta adrenergic agents. Ki values for adrenergic drugs of 3H-yohimbine binding were as follows: yohimbine, 25 nM; clonidine, 260 nM; methoxamine, 6.8 microM; propranolol, 8.7 microM; prazosin, 21 microM; phenylephrine, 22 microM; noradrenaline, 27 microM; adrenaline, 66 microM; isoproterenol, 3,300 microM. These results indicate that alpha adrenoceptors in the bovine cerebral arteries can be classified as the alpha 2 subtype.

(-)-Amidephrine, a selective agonist for alpha 1-adrenoceptors

The alpha-adrenoceptor activity of (-)-amidephrine in the isolated vas deferens of the rat was characterized. In this preparation (-)-amidephrine showed a selective activity on alpha 1-adrenoceptors, and failed to exert any effect on alpha 2-adrenoceptors. The alpha 1-adrenoceptor antagonists prazosin (pA2 8.19 and 8.48) and E-643 (pA2 8.36 and 8.25) inhibited the agonist activity of (-)-amidephrine and (-)-phenylephrine. The pA2 values obtained did not differ significantly, indicating that the two agonists exert their effect on the same receptor. The slopes of Schild plots were not significantly different from 1, suggesting that the inhibition was truly competitive in nature.

Further functional in vitro comparison of pre- and postsynaptic dopamine receptors in the rabbit caudate nucleus

Slices of the rabbit caudate nucleus were preincubated with 3H-dopamine or 3H-choline and then superfused and stimulated electrically. DiPr-5,6-ADTN reduced the stimulation-evoked overflow of tritium over the same concentration range, independently of whether slices had been preincubated with 3H-dopamine or 3H-choline, and the same was true for apomorphine, NPA and pergolide. Three other putative dopamine receptor agonists, namely 3-PPP, DPI and SKF 38393, failed to decrease the evoked overflow of tritium. Each of six antagonists--(-)-sulpiride, (+)-sulpiride, CGP 11109 A, cis-flupentixol, domperidone and corynanthine--increased the evoked overflow over the same concentration range in experiments with 3H-dopamine and in those with 3H-choline. For each of these antagonists except cis-flupentixol, and also for chlorpromazine, haloperidol and rauwolscine, the pA2 values against apomorphine obtained in 3H-dopamine and in 3H-choline experiments were closely similar. The antagonist effect of cis-flupentixol against apomorphine was not purely competitive. (-)-Sulpiride was a more potent antagonist than (+)-sulpiride, and cis-flupentixol was more potent than trans-flupentixol. This study supplements a previous one in which (+/-)-sulpiride, metoclopramide and molindone were used as antagonists. It is a functional in vitro approach to receptor characterization, as opposed to radioligand binding studies or in vivo investigations. The results show that a large number of dopamine receptor agonists and antagonists are unable to distinguish between the presynaptic, release-inhibiting dopamine autoreceptors and those postsynaptic dopamine receptors which, when activated, depress the release of acetylcholine.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of alpha 2-adrenergic receptors in human myometrium by [3H]rauwolscine binding

Human myometrium contains alpha 3-adrenergic receptors which can be identified by binding of the alpha 2-adrenergic antagonist [3H]rauwolscine. Scatchard analysis of saturation binding data on myometrial membranes revealed that [3H]rauwolscine bound to a single class of noncooperative sites (262 +/- 89 fmol/mg of membrane protein) with high affinity (i.e., with an equilibrium dissociation constant of 5.3 +/- 2.2 nM). The alpha 2-adrenergic nature of these sites was derived from the order of potencies and stereospecificity of alpha-adrenergic agonists and antagonists to compete with [3H]rauwolscine binding.

A study of stimulation-evoked activation of alpha 2-adrenoceptors in the rat isolated vas deferens

Experiments were performed with prostatic and epididymal segments of rat vas deferens to determine whether alpha 2-adrenoceptors in this organ were activated during field stimulation of sympathetic terminals with short trains of pulses applied at low frequencies. In prostatic segments the magnitude of twitches evoked by trains of ten field pulses at 1 or 2 Hz declined after 2-3 s of stimulation. In contrast, facilitation of twitches and fusion of contractions occurred when similar stimulation was applied to epididymal segments. In prostatic segments from rats treated with reserpine (2.5 mg/kg i.p.) 24 h previously, there was no decline in the magnitude of twitches produced by successive impulses in a train of stimulating pulses. In epididymal segments from reserpine-treated rats facilitation of twitches in response to successive impulses in each train still occurred. In prostatic segments cocaine (5 and 10 mumol/l) enhanced twitch fade with stimulation at 1 and 2 Hz without altering the time for onset of this effect. In epididymal segments cocaine led to enhancement and prolongation of contractile responses. In prostatic segments yohimbine (0.01-0.06 mumol/l) reduced or reversed the effect of cocaine in enhancing twitch fade. In preparations where the reversal of the effect of cocaine by yohimbine was incomplete, subsequent addition of phentolamine (1 mumol/l) produced complete reversal. In epididymal segments yohimbine (0.01 mumol/l) produced a further enhancement in the twitch responses to stimulation at 1 and 2 Hz. Subsequent addition of phentolamine (1 mumol/l) reversed the facilitatory effects of cocaine and yohimbine. Propranolol (10 mumol/l) was without effect upon responses to stimulation in either segment of rat vasa deferentia. These experiments indicate that noradrenaline, released by short trains of impulses applied at low frequency to hypogastric nerve terminals activates prejunctional alpha 2-adrenoceptors in the prostatic segment of the vas deferens. In the epididymal portion the effects arising from activation of prejunctional alpha 2-adrenoceptors are outweighed by the consequences of activation of extrajunctional alpha 1-adrenoceptors located on longitudinally arranged muscle.

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.

Characterisation of postjunctional alpha-adrenoceptors in isolated human femoral veins and arteries

In order to characterise the pharmacological properties of postjunctional alpha-adrenoceptors, both the contractile effects of alpha-adrenoceptor agonists and the blocking potencies of selective alpha-adrenoceptor antagonists were studied in isolated human femoral veins and arteries. The veins were more sensitive to noradrenaline than the arteries. Guanfacine had a higher intrinsic activity in veins than in arteries, whereas the reverse was true for phenylephrine. The antagonists rauwolscine and yohimbine were more potent against noradrenaline in the veins than in arteries, while corynanthine was equally potent in either tissue. They antagonised the noradrenaline response in a competitive manner. Prazosin proved to be the most potent competitive antagonist in arteries, while in veins it exerted weak and non-competitive antagonism. The results suggest that the alpha-adrenoceptor population at the postjunctional site differs between human femoral veins and arteries. The veins seem to contain more alpha 2- than alpha 1-adrenoceptors postjunctionally, whereas in the arteries the alpha 1-subtype prevails. The results indicate the possibility of influencing selectively adrenergic reactions in the capacitance and resistance vessels.

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.

Neuropharmacological studies in rodents on the action of RX 781094, a new selective alpha 2-adrenoceptor antagonist

Several neuropharmacological effects of RX 781094, a new selective alpha 2-adrenoceptor antagonist, have been investigated in rodents. In rats, RX 781094 (0.1-1.0 mg kg-1, i.v.) produced a rapid dose-related reversal of cortical EEG synchronisation and behavioural sedation, induced by clonidine or the more selective alpha 2-adrenoceptor agonist, guanoxabenz. The alpha 2-adrenoceptor antagonists yohimbine and mianserin were also effective in blocking guanoxabenz-induced EEG synchronisation but had a lower potency than did RX 781094. In specificity experiments, RX 781094 (1.0 mg kg-1, i.v.) failed to antagonise the EEG synchronisation and pronounced behavioural sedation induced by the CNS depressant sodium pentobarbitone (15 mg kg-1, i.v.). In mice, pretreatment (i.v. or p.o.) with RX 781094 inhibited in a dose-dependent way both guanoxabenz-induced behavioural hypoactivity and clonidine-induced hypothermia. By itself, RX 781094 had no effect on the temperature of normal mice. In sleep-waking studies in rats, RX 781094 (0.1 and 1.0 mg kg-1, i.v.) had no measurable stimulant or depressant effect on the CNS, in contrast to (+)-amphetamine (1.0 mg kg-1, i.v.) which elicited marked CNS stimulation. These results support the conclusion that RX 781094 is a potent antagonist at central alpha 2-adrenoceptors.

Radiation inactivation of alpha 1-adrenoceptors

Radiation inactivation of alpha 1-adrenoceptors in rat cerebral cortex membranes has been performed with 10 MeV electrons from a linear accelerator at temperatures less than or equal to -100 degrees C. Alpha 1-adrenoceptor inactivation was monitored with [ 3H ]-prazosin and [( 125I ]-2-(beta-4-hydroxylphenyl)ethylaminomethyl)tetralone [( 125I ]-HEAT). Saturation analysis of irradiated membranes with both ligands indicated that a decrease in alpha-adrenoceptor density occurred with increasing radiation dose. The dissociation constants of [ 3H ]-prazosin and [ 125I ]-HEAT were not markedly changed by the irradiation. Application of the target volume theory gave molecular weights of 91,500 +/- 1,700 (S.D.) (D37: 19,6 +/- 0.36 Mrad) with [ 125I ]-HEAT as ligand, and 77,000 +/- 18,000 (S.D.) (D37: 23.3 +/- 4.6 Mrad) with [ 3H ]-prazosin, respectively, when an empirical temperature correction factor of 2.8 was used. [ 3H ]-flunitrazepam-labelled benzodiazepine receptor target size was used as an internal control. The molecular weight of the alpha 1-adrenoceptors, corrected for this internal control, was 85,000 +/- 1.600 [( 125I ]-HEAT) and 71,500 +/- 17,000 [( 3H ]-prazosin).

Different alpha-adrenoceptors modulate the release of 5-hydroxytryptamine and noradrenaline in rat cortex

1 The potassium-evoked release of [3H]-noradrenaline from slices of rat occipital cortex and the potassium-evoked release of [3H]-5-hydroxytryptamine from slices of rat frontal cortex were measured using a superfusion system. 2 The rank order of potency for a number of alpha-adrenoceptor agonists was different for the two neuronal systems, clonidine and azepexole being the most potent inhibitors of noradrenaline release and methoxamine and phenylephrine being the most potent against 5-hydroxytryptamine release. 3 The rank order of potency for a series of alpha-adrenoceptor antagonists in reversing the inhibition of noradrenaline release produced by clonidine was: phentolamine greater than rauwolscine = yohimbine = corynanthine much greater than WB4101, whereas against methoxamine-inhibition of 5-hydroxytryptamine release the rank order of potency was: WB4101 greater than phentolamine greater than corynanthine greater than yohimbine greater than rauwolscine. 4 The results suggest that the alpha-adrenoceptors which modulate potassium-evoked 5-hydroxytryptamine release are not identical with the alpha 2-adrenoceptors which modulate potassium-evoked 5-hydroxytryptamine release are not identical with the alpha 2-adrenoceptors located on noradrenergic nerve terminals and may more closely resemble alpha 1-than alpha 2-adrenoceptors.

The determination of presynaptic pA2 values of yohimbine and phentolamine on the perfused rat heart under conditions of negligible autoinhibition

1 Rat isolated perfused hearts with the right sympathetic nerves attached were loaded with [3H]-(-)-noradrenaline. The nerves were stimulated with up to 40 trains of 10 pulses every min at 1 Hz, and the evoked increases of [3H-]noradrenaline overflow into the perfusate, of right atrial tension development and ventricular beating frequency were measured. 2 Oxymetazoline inhibited the evoked transmitter overflow (IC50: 10 nM) and decreased the postsynaptic responses in a concentration-dependent manner. It behaved as a full against in abolishing the evoked transmitter overflow. 3 Yohimbine up to 1 microM neither enhanced the evoked [3H]-noradrenaline overflow nor the postsynaptic parameters. Phentolamine (1 microM) caused a transient, minor (less than 30%) increase in [3H]-noradrenaline overflow. 4 Yohimbine (0.03-1.0 microM) and phentolamine (0.1-5.0 microM) shifted to the right the concentration-response curve of oxymetazoline for the inhibition of [3H]-noradrenaline overflow in response to nerve stimulation without depressing the maxima. The pA2 values were 7.82 and 7.52, respectively. 5 Yohimbine (0.1 microM) also antagonized the decrease induced by oxymetazoline in the postsynaptic responses to nerve stimulation. 6 The results confirm the existence of presynaptic inhibitory alpha 2-adrenoceptors at the adrenergic nerve fibres of the rat heart in vitro. Under the stimulation and perfusion conditions selected, the released endogenous transmitter apparently does not activate a negative feedback mechanism, thus permitting the determination of pA2 values.