A difference in the affinity of some selective alpha 2-adrenoceptor antagonists when compared on isolated vasa deferentia of rat and rabbit

Affinity profile at alpha(1)- and alpha(2)-adrenoceptor subtypes and in vitro cardiovascular actions of (+)-boldine

The present study examines the functional and binding affinities of the aporphine alkaloid, (+)-boldine, at different alpha(1)- and alpha(2)-adrenoceptor subtypes, namely, alpha(1A) (rat vas deferens and kidney) and its L-like state (rabbit spleen), alpha(1B) (guinea pig spleen, mouse spleen and rabbit aorta), alpha(1D) (rat aorta and pulmonary artery), at possible subtypes of prejunctional alpha(2)-adrenoceptors in rat and rabbit vas deferens and rat atrium, alpha(2D) in guinea pig ileum, cloned human alpha(1)-adrenoceptor subtypes A, B and D and alpha(2)-adrenoceptor subtypes A, B and C as well as rat alpha(2D)-adrenoceptors. Additionally, we investigated its Ca(2+) channel antagonism in vascular and cardiac preparations. (+)-Boldine had higher affinity at alpha(1)-adrenoceptor subtype A (pA(2)=7.46, pK(i)=7.21) compared with its L-like state (pA(2)=5.63) or subtype B (pA(2)=5.98- 6.12, pK(i)=5.79) and subtype D (pA(2)=6.18-6.37, pK(i)=6.09). Its affinities at alpha(2)-adrenoceptors in rat and rabbit vas deferens and rat atrium (pA(2)=6.02, 6.36, 6.06, respectively) were identical, but lower at guinea pig ileum alpha(2D)-adrenoceptors (pA(2)=4.38). (+)-Boldine displayed nearly undistinguishable affinity at cloned human alpha(2)-adrenoceptor subtypes A, B and C (pK(i)=6.26, 5.79 and 6.35, respectively), whereas its affinity at rat alpha(2D)-adrenoceptors was low (pK(i)=4.70). In perfused rat kidney, (+)-boldine inhibited K(+)-evoked vasoconstriction at doses 70-fold higher than diltiazem. In guinea pig Langendorff heart, (+)-boldine (10(-5) - 2 x 10(-4) M) was equieffective in increasing coronary flow and in depressing cardiac force, while lower concentrations already depressed heart rate. In papillary muscles from guinea pig, (+)-boldine (10(-6) - 10(-5) M) mainly prolonged the duration of action potential at levels >30% of repolarization. These data reveal that (+)-boldine, except for its moderate selectivity (15 to 25-fold) for alpha(1A)-adrenoceptors, does not discriminate between the alpha(1)-adrenoceptor subtypes B and D and alpha(2)-adrenoceptor subtypes A, B and C, at which the drug consistently displays micromolar affinity. In vascular and cardiac preparations, (+)-boldine, although being at least 50-fold weaker than diltiazem, shows Ca(2+) channel antagonistic properties but no specificity for coronary dilatation relative to cardiodepression.

Rilmenidine reveals differences in the pharmacological characteristics of prejunctional alpha2-adrenoceptors in the guinea-pig, rat and pig

1. The alpha2A and alpha2D-adrenoceptor subtypes are thought to be species homologs most easily differentiated on the basis of the potency of antagonists. In the present study we have compared the effect of rilmenidine with two other selective alpha2-adrenoceptor agonists, UK-14304 (5-bromo-6- [2-imidazolin-2-ylamino]-quinoxaline) and clonidine, against electrically-evoked contractions in five isolated preparations from the rat, guinea-pig and pig, and, where possible, determined the receptor subtype involved. 2. UK-14034, clonidine and rilmenidine produced concentration-dependent inhibition of the electrically-evoked contractions of the rat isolated vas deferens and tail artery and the guinea-pig ileum. These inhibitory effects were reversed by the selective alpha2-adrenoceptor antagonist, RX-811058 (1 microM), except in the rat tail artery preparations where the remaining neurogenic response was inhibited; evidence for the involvement of 'innervated' alpha2-adrenoceptors. Both clonidine and UK-14304 produced concentration-dependent inhibition of responses in the porcine isolated tail artery and urinary bladder but clonidine was markedly less efficacious in these preparations. In contrast, rilmenidine failed to inhibit the neurogenic contractions in either preparation. 3. Although rilmenidine failed to elicit a detectable response in either the porcine isolated tail artery or urinary bladder, it (10 microM and 30 microM, respectively) competitively antagonised the inhibitory effects of UK-14304 with an estimated dissociation constant of (pK(B)) 5.82 and 5.93, respectively. 4. Prazosin (1 microM) failed to alter the effect of UK-14304 against neurogenic contractions in the porcine isolated urinary bladder, while rauwolscine (pK(B) 8.87) was 10 fold more potent than phentolamine (pK(B) 7.56). On the other hand, phentolamine (pK(B) 8.42) was only marginally more potent than rauwolscine (pK 8.05) against clonidine-induced inhibition of electrically-evoked contractions of the guinea-pig isolated ileum. This pharmacological evidence with antagonists supports the presence of alpha2D-adrenoceptors in the rat and guinea-pig and the alpha2A-adrenoceptors in the pig. 5. We have demonstrated that rilmenidine, unlike clonidine and UK-14304, is devoid of any agonist activity at prejunctional alpha2A-adrenoceptors in the pig, but is an efficacious agonist at alpha2D-adrenoceptors in the rat and guinea-pig.

Preclinical evaluation of brimonidine

Preclinical studies of brimonidine show that it is a potent alpha 2-adrenoceptor agonist that is 1000-fold more selective for the alpha 2-vs. the alpha 1-adrenoceptor, and is 7-12-fold more alpha 2-selective than clonidine and 23- to 32-fold more alpha 2-selective than apraclonidine (p-aminoclonidine). Brimonidine decreased intraocular pressure (IOP) in various animal models but, unlike apraclonidine, brimonidine was not mydriatic. The site and pharmacology of the IOP response depends on the animal species. In rabbits, the IOP response to brimonidine is mediated by an ocular alpha 2-adrenoceptor while in monkeys, a central nervous system (CNS) 'imidazoline' receptor appears to be involved. Brimonidine decreased IOP by suppressing the rate of aqueous humor flow and enhancing uveoscleral outflow. Topical brimonidine resulted in posterior segment drug levels adequate to activate alpha 2-adrenoceptors, but was not vasoconstrictive in a model designed to assess the vasoactivity of the human retinal microvasculature. Brimonidine protected the rat optic nerve from secondary damage following mechanical injury to the optic nerve and was nontoxic in an array of experiments designed to evaluate ocular and organ toxicity. Taken together, the high alpha 2-adrenoceptor selectivity, ocular hypotensive efficacy, retinal bioavailability and neuroprotective properties make brimonidine an important addition to the field of antiglaucoma agents.

Evidence for functional alpha 2D-adrenoceptors in the rat intestine

1. Peristaltic contractions were induced in segments of rat ileum by raising the intramural pressure. A mean pressure of 4.3 +/- 0.2 cmH2O (n = 112) was required to initiate rhythmic contractions of the longitudinal muscle (preparatory phase) and associated volume expulsions caused by circular muscle contraction. The frequency of peristalsis remained constant over two 15 min periods of stimulation. 2. The alpha 2-adreoceptor agonists, clonidine, UK 14,304, B-HT 920, B-HT 933 and the selective alpha 2A-agonists, oxymetazoline and guanfacine, caused concentration-dependent inhibition of peristaltic contractions. The potency order and IC50 values (nM) were: clonidine (2.81) > or = oxymetazoline (4.23) > or = UK 14,304 (4.48) > or = guanfacine (5.51) > B-HT 920 (78.72) > B-HT 933 (442.48). 3. High concentrations of clonidine, amounting to more than 10 and 100 times the IC50 value in the peristaltic reflex (30 and 300 nM respectively), failed to inhibit the cholinergic contractile response to transmural electrical stimulation over the range of 2.5 to 40 Hz. 4. alpha 2-Adrenoceptor antagonists were used to determine the subtype of presynaptic alpha 2-adrenoceptor in rat ileum. All the antagonists tested caused parallel shifts to the right of the concentration-response regression line to clonidine and none, except ARC 239, influenced the rate of peristalsis. However, ARC 239 significantly decreased the frequency of control period peristaltic contractions. The order of affinity of the antagonists against clonidine (pKB values in parentheses) was RX 821002 (8.99) > phentolamine (8.07) > BRL 44408 (7.43) > or = rauwolscine (7.41) > or = yohimbine (7.28) > prazosin (5.86) > or = ARC 239 (5.74). 5. These results, when compared with binding and functional data from various other tissues and cell lines, are consistent with the presence of presynaptic alpha 2D-adrenoceptors in rat ileum. Further evidence is provided that this subtype of alpha 2-adrenoceptor is probably located proximal to the final cholinergic neurones in the reflex arc.

Subtype determination of presynaptic alpha 2-autoreceptors in the rabbit pulmonary artery and human saphenous vein

The pharmacological properties of the presynaptic a2-autoreceptors mediating inhibition of noradrenaline release were investigated in human saphenous vein and rabbit pulmonary artery. Segments of these blood vessels were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing uptake1 and uptake2 blockers. The potencies of a2-adrenoceptor antagonists in facilitating (pEC40) the electrically (2 Hz) evoked tritium overflow were determined. The order of potency and potency ratios of a2-adrenoceptor antagonists obtained in our experiments were compared with the corresponding order of affinity and affinity ratios from radioligand binding studies in tissues and cells expressing only one of the alpha 2-adrenoceptor subtypes. In the rabbit pulmonary artery, oxymetazoline was a highly potent agonist at presynaptic a2-adrenoceptors, as reflected by its ability to inhibit at low concentrations the electrically evoked tritium overflow. However, in the human saphenous vein oxymetazoline behaved as a partial agonist, which, in interaction experiments with the a2-adrenoceptor agonist B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine), exhibited high potency in antagonizing the inhibitory effect of the latter drug on tritium overflow. Prazosin given alone at concentrations up to 1 mumol/l did not affect tritium overflow. The data obtained with oxymetazoline and prazosin make it very improbable that the a2-autoreceptors on the sympathetic nerves in both tissues are of the a2B- or a2C-subtype. In both blood vessels, rauwolscine given alone was highly potent in facilitating the electrically evoked overflow. In agreement with this, rauwolscine exhibited high potency in antagonizing the inhibitory effect of oxymetazoline on tritium overflow in the rabbit pulmonary artery and of B-HT 920 in the human saphenous vein. The ratio phentolamine/rauwolscine calculated from their potencies in increasing the electrically evoked tritium overflow was also used to discriminate between the various a2-adrenoceptor subtypes. Comparison of this potency ratio with the corresponding affinity ratios for a2-adrenergic binding sites on HT 29 cells, human platelets, bovine pineal gland, rat submaxillary gland, and cell lines transfected with the human a2 genes indicates that in the rabbit pulmonary artery and human saphenous vein the pharmacological characteristics of the autoreceptors conform best to those of a2A-adrenoceptors. Finally, in both blood vessels the potencies of the antagonists BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), rauwolscine, corynanthine, phentolamine, idazoxan, SKF 104078 (6-chloro-9-[(3-methyl-2-butenyl) oxyl]-3-methyl-1H-2,3,4,5-tetrahydro-3-benzazepine), and/or tolazoline in facilitating evoked noradrenaline release was determined. The potencies of these drugs which can discriminate between a2A- and a2D-adrenoceptors (but not between these and a2B/2C-adrenoceptors) were correlated significantly with their affinities for a2A, but not a2D, sites in radioligand binding studies. In conclusion, the present results suggest that the sympathetic nerves of the human saphenous vein and rabbit pulmonary artery are endowed with a2-autoreceptors of the a2A subtype.

Subclassification of presynaptic alpha 2-adrenoceptors: alpha 2A-autoreceptors in rabbit atria and kidney

The study was devised to classify, by means of antagonist affinities, the presynaptic alpha 2-autoreceptors of rabbit atria and kidney in terms of alpha 2A, alpha 2B, alpha 2C and alpha 2D. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the left atrium and slices of the kidney cortex were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. In one series of experiments, tissue pieces were stimulated by relatively long pulse trains (1 or 2 min) leading to alpha 2-autoinhibition. All 11 (atria) or 10 (kidney) antagonists increased the evoked overflow of tritium. pEC30% values (concentrations causing 30% increase) were interpolated from concentration-response curves. In a second series of experiments, tissue pieces were stimulated by brief pulse trains (0.4 s) that did not lead to alpha 2-autoinhibition, and concentration-inhibition curves of the alpha 2-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) were determined. Most of the 11 (atria) or 8 (kidney) antagonists shifted the concentration-inhibition curve of UK 14,304 to the right. pKd values of the antagonists were calculated from the shifts. pEC30% values correlated with pKd values, both in atria (r = 0.728) and in the kidney (r = 0.930). pEC30% values in atria correlated with pEC30% values in the kidney (r = 0.988) and pKd values in atria correlated with pKd values in kidney (r = 0.923). It is concluded that the alpha 2-autoreceptors in atria and the kidney are the same. Comparison with antagonist affinities for prototypic native alpha 2 binding sites, alpha 2 binding sites in cells transfected with alpha 2 subtype genes, and previously classified presynaptic alpha 2-adrenoceptors--all taken from the literature--indicates that both autoreceptors are alpha 2A. This conclusion is reached with either of the two independent estimates of autoreceptor affinity, pEC30% and pKd. The results are compatible with the hypothesis that at least the majority of alpha 2-autoreceptors belong to the alpha 2A/D branch of the alpha 2-adrenoceptor tree, across mammalian or at least rodent and lagomorph species.

Characterization of the prejunctional muscarinic receptors mediating inhibition of evoked release of endogenous noradrenaline in rabbit isolated vas deferens

The aim of the present study was to characterize the prejunctional modulation of evoked release of endogenous noradrenaline in rabbit vas deferens by the use of muscarinic receptor agonists and subtype-preferring antagonists. Vasa deferentia of the rabbit were stimulated electrically by trains of 120 pulses delivered at 4 Hz or trains of 30 pulses at 1 Hz. The inhibition by muscarinic agonists of the stimulation-evoked overflow of endogenous noradrenaline in the absence and presence of antagonists was used to determine affinity constants for antagonists. These values were compared with those observed at putative M1 receptors inhibiting neurogenic twitch contractions in the rabbit vas deferens and with affinity data obtained at M1(m1)-M4(m5) receptors in functional studies and binding experiments. The evoked overflow of noradrenaline from sympathetic nerves was enhanced by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) and indomethacin, indicating a tonic inhibition by endogenous A1 and P2 purinoceptor agonists and prostanoids, respectively. The stimulation-evoked overflow at 4 Hz was not sensitive to inhibition by the muscarinic agonists methacholine or 4-(4-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343). In contrast, at a stimulation frequency of 1 Hz the evoked noradrenaline release was decreased by muscarinic agonists (EC50): arecaidine propargyl ester (0.062 microM), 4-Cl-McN-A-343 (0.32 microM), 4-(4-fluorophenylcarbamoyloxy)-2-butynyl-N-methyl-pyrrolidinium tosylate (4-F-PyMcN+; 0.48 microM) and methacholine (0.86 microM). The affinity constants of most of the muscarinic antagonists [atropine: pKB = 9.47; (R)-trihexyphenidyl: pKB = 9.18; pirenzepine: pA2 = 7.68; methoctramine: pKB = 6.90] are consistent with estimates of these antagonists at M1(m1) receptors determined in various functional and binding studies. The high antagonistic potency of pirenzepine and (R)-trihexyphenidyl and the agonistic activity of 4-F-PyMcN+ argue for the involvement of M1, and against that of M2 and M3 receptors in the inhibition of evoked noradrenaline overflow. However, the high apparent pKB of 8.30 for himbacine is not in accordance with an M1 receptor; by contrast, it would be compatible with the presence of M2 or M4 receptors. The potencies of the tested muscarinic agonists and antagonists largely agree with those obtained for the inhibition of neurogenic twitch responses (0.05 Hz) in the rabbit vas deferens.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic alpha 2-autoreceptors in brain cortex: alpha 2D in the rat and alpha 2A in the rabbit

Presynaptic alpha 2-autoreceptors in rat and rabbit brain cortex were compared by means of antagonists and agonists. Brain cortex slices were preincubated with [3H]-noradrenaline and then superfused and stimulated by 3 (rat) or 4 (rabbit) pulses at a frequency of 100 Hz. The alpha 2-adrenoceptor agonist bromoxidine (UK 14304) reduced the electrically evoked overflow of tritium with EC50 values of 4.5 nmol/l in the rat and 0.7 nmol/l in the rabbit. The antagonists phentolamine, 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazo le (BRL 44408), rauwolscine, 1,2-dimethyl-2,3,9,13b-tetrahydro-1H-dibenzo(c,f)imidazo(1,5-a)aze pine (BRL 41992), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4, 5-tetrahydro-3-benzazepine (SKF 104078), imiloxan, prazosin and corynanthine did not per se increase the evoked overflow of tritium but shifted the concentration-inhibition curve of bromoxidine to the right in a manner compatible with competitive antagonism. Up to 4 concentrations of each antagonist were used to determine its dissociation constant KD. The KD values correlated only weakly between the rat and the rabbit. Dissociation constants KA of bromoxidine were calculated from equieffective concentrations in unpretreated brain slices and slices in which part of the alpha 2-adrenoceptors had been irreversibly blocked by phenoxybenzamine. The KA value was 123 nmol/l in the rat and 7.2 nmol/l in the rabbit. The results confirm the species difference between rat and rabbit brain presynaptic alpha 2-autoreceptors. Comparison with data from the literature indicates that the rat brain autoreceptors can be equated with the alpha 2D subtype as defined by radioligand binding, whereas the rabbit brain autoreceptors conform to the alpha 2A subtype. For example, the antagonist affinities for the rat autoreceptors correlate with their binding affinities for the gene product of alpha 2-RG20, the putative rat alpha 2D-adrenoceptor gene (r = 0.97; P < 0.01), but not with their binding affinities for the gene product of alpha 2-C10, the putative human alpha 2A-adrenoceptor gene. Conversely, the rabbit autoreceptors correlate with the alpha 2-C10 (r = 0.98; P < 0.001) but not with the alpha 2-RG20 gene product. Since presynaptic alpha 2-autoreceptors are also alpha 2D in rat submaxillary gland and perhaps vas deferens and alpha 2A in rabbit pulmonary artery, the possibility arises that the majority of alpha 2-autoreceptors generally are alpha 2D in the rat and alpha 2A in the rabbit. Moreover, receptors of the alpha 2A/D group generally may be the main mammalian alpha 2-autoreceptors.

Inhibition by alpha 2-adrenoceptor agonists of contractions of rabbit isolated colon elicited by pelvic nerve stimulation

1. Segments of rabbit distal colon were set up for isotonic recording and the extrinsic pelvic parasympathetic nerves were stimulated for 30 s periods at 2 Hz. 2. Atropine (100 nM) abolished acetylcholine-induced contractions, but only partly reduced responses to nerve stimulation. 3. Clonidine and related compounds, UK14819, UK14304, UK15121, UK11957 and UK42620, inhibited nerve stimulation-evoked contractions at concentrations which had no effect on the response to exogenous acetylcholine, suggesting that the compounds inhibited transmitter release. 4. The imidazolidine compounds UK14819, UK14304 and UK15121 had three to five times the potency of clonidine. Another imidazolidine, UK11957, was about three times less potent than clonidine and appeared to have lower intrinsic activity. The morpholinocatechol UK42620 was about 100 times less potent than clonidine. Phenylephrine was about 10,000 times less potent than clonidine. 5. Idazoxan (1-10 microM) and yohimbine (300 nM-3 microM) reversed the depressant effects of clonidine and the UK compounds, indicating that the effects were exerted on alpha 2-adrenoceptors. This was confirmed by the finding that the slope of Schild plot of idazoxan against UK14304 was close to unity with the pA2 value = 7.14. 6. Atropine-resistant neurogenic contractions were completely abolished by UK14819 (100 nM) and this effect was completely reversed by idazoxan (10 microM). 7. Depending on other (especially centrally mediated) effects, the more potent inhibitors of neurogenic colonic motor activity, with their higher alpha 2-adrenergic agonist activity, may have advantages over clonidine as antidiarrhoeal drugs.

Pharmacological characterization of presynaptic alpha 2-autoreceptors in rat submaxillary gland and heart atrium

1. The pharmacological properties of presynaptic alpha 2-autoreceptors were studied in rat isolated submaxillary glands and atria. Tissue pieces were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine, and stimulated electrically. In one series of experiments, pEC30 values of 12 alpha-adrenoceptor antagonists were determined, i.e., negative logarithms of concentrations that increased the electrically evoked overflow of tritium by 30%. In another series, pKD values of 9 alpha-adrenoceptor antagonists against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), and of 3 antagonists against the release-inhibiting effect of methoxamine, were determined. 2. In submaxillary glands, the pEC30 values of the antagonists correlated well with their pKD values against UK 14304 (r = 0.93). The same was true for atria (r = 0.92). 3. In submaxillary glands, the pKD values of 3 antagonists against UK14304 were very similar to their pKD values against methoxamine, with a maximal difference of 0.4. The same was true for atria where the maximal difference was 0.3. 4. The pEC30 values obtained in submaxillary glands correlated significantly with those obtained in atria (r = 0.81). The same was true for the pKD values (r = 0.79). However, the pEC30 and pKD values also indicated consistent differences between the two tissues. 5. It is concluded that the sites of action of the imidazoline UK 14304 (alpha 2-selective), the phenylethylamine noradrenaline, and the phenylethylamine methoxamine (alpha 1-selective) are exclusively alpha 2-adrenoceptors. There is no indication for presynaptic alpha 1-adrenoceptors or for an effect of UK 14304 mediated by presynaptic imidazoline receptors.The 02-autoreceptor population in the submaxillary gland differs from that in the atrium.6. Comparison with studies from the literature indicates that the submaxillary autoreceptors are closely similar to the a2D radioligand binding site found in the bovine pineal gland and probably the rat submaxillary gland. The atrial autoreceptors also conform best to this site, but the agreement is more limited; the atrial autoreceptors may represent a type related to, but distinct from, the a2D site, or a mixture of different types.

Subclassification of the presynaptic alpha 2-autoreceptors in rabbit brain cortex

1. alpha 2-Adrenoceptor binding sites have been subclassified into alpha 2A sites of which a main characteristic is very low affinity for prazosin, and alpha 2B sites with relatively high affinity for prazosin. The presynaptic alpha 2-autoreceptors in rabbit brain cortex were studied in order to classify them in terms of alpha 2A and alpha 2B. Release of [3H]-noradrenaline in cortical slices was elicited by trains of 4 pulses delivered at 100 Hz. 2. Clonidine caused concentration-dependent inhibition of the stimulation-evoked overflow of tritium, with an EC50 of 7.5 nM and a maximal inhibition by 96%. 3. The following alpha-adrenoceptor antagonists shifted the concentration-response curve of clonidine to the right (antagonist-receptor dissociation constants KD in brackets): yohimbine (14 nM), 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazo le (BRL 44408; 15 nM) and 1,2-dimethyl-2,3,9,13betetrahydro-1H-dibenzo[c,f]imidazo[1,5-a]aze pine (BRL 41992; 630 nM). Prazosin 1 microM and 2-[2-[4-(o-methoxyphenyl)piperazine-1-yl]-ethyl]-4,4-dimethyl-1,3 (2H,4H)-isoquinolinedione (AR-C 239) 1 microM failed to antagonize the effect of clonidine. Higher concentrations of prazosin and AR-C 239 greatly accelerated the basal efflux of tritium. 4. The method used permits the functional determination of antagonist affinities undistorted by endogenous alpha 2-autoinhibition. A comparison with affinities derived from radioligand binding experiments indicates that the presynaptic alpha 2-autoreceptors in rabbit brain cortex are markedly different from the alpha 2B-subtype and probably belong to the prazosin-insensitive alpha 2A-subtype.

Alpha 2-adrenergic binding sites in the medulla oblongata of tree shrews demonstrated by in vitro autoradiography: species related differences in comparison to the rat

Alpha 2-adrenergic binding sites in the medulla oblongata of tree shrews and rats were detected and quantified by in vitro-autoradiography with the alpha 2-antagonist 3H-rauwolscine (3H-RAUW). The autoradiographic pattern of the radioligand binding in the tree shrew medulla oblongata resembles that which has been described by others for the human myelencephalon. This pattern coincides well with the occurrence of catecholaminergic structures detected by immunocytochemistry with antibodies against phenylethanolamine-N-methyltransferase and tyrosine hydroxylase. In contrast to the rat, where only the nucleus tractus solitarii and the nucleus dorsalis nervi vagi were labeled, five discrete nuclei specifically bound 3H-RAUW in tree shrews. The highest number of binding sites was detected in the nucleus dorsalis nervi vagi (nX; Bmax: 333 fmoles/mg) and the nucleus tractus solitarii (NTS; 311 fmoles/mg), followed by the nucleus nervi hypoglossi (nXII; 297 fmoles/mg), the nucleus reticularis parvocellularis (FRS; 230 fmoles/mg), and the area of the catecholamine cell groups A1 and C1 (area C1; 202 fmoles/mg). Maximal binding in the two labeled nuclei of the rat was 158 fmoles/mg. The discrete nuclei of the two species also showed different affinities for 3H-RAUW with Kd ranging from 0.17 to 0.83 nM in tree shrews and 1.80 to 1.95 nM in rats. Competition experiments revealed that the radioligand bound specifically to alpha 2-binding sites. In the tree shrew, nX, nXII and the area C1, also have a relatively high affinity for the alpha 1-antagonist prazosin which is a quality of the adrenoceptor subtype alpha 2B. Furthermore, in the area C1, 3H-RAUW binding was inhibited by the dopamine antagonist haloperidol. There are thus species related as well as regional differences with respect to the number, the affinity, and the pharmacological properties of alpha 2-binding sites in the medulla oblongata. In tree shrews, alpha 2-adrenoceptors can be autoradiographically quantified in regions which are not labeled in the rat, although former data predicted the existence of such receptors, e.g., in the area of the adrenaline cell group C1.

Assessment of imiloxan as a selective alpha 2B-adrenoceptor antagonist

1. The alpha 2-adrenoceptor binding sites of rabbit spleen and rat kidney, labelled with [3H]-rauwolscine, were characterized using a range of subtype selective ligands. 2. In rabbit spleen, the alpha-2-adrenoceptor binding sites displayed high affinity for oxymetazoline and WB 4101 and low affinity for prazosin and chlorpromazine suggesting the presence of an alpha 2A subtype. 3. There was evidence for heterogeneity of the alpha 2-adrenoceptor binding sites present in rabbit spleen. The results obtained with oxymetazoline and WB 4101 indicated that at least 75% of the [3H]-rauwolscine binding sites in this preparation displayed a pharmacology consistent with the presence of an alpha 2A subtype. 4. In rat kidney, the alpha 2-adrenoceptor binding sites displayed high affinity for prazosin and chlorpromazine and low affinity for oxymetazoline and WB 4101 suggesting the presence of an alpha 2B subtype. 5. The inclusion of guanylylimidodiphosphate (Gpp(NH)p, 0.1 mM) did not modify the pharmacology of the alpha 2-adrenoceptor binding sites present in the two preparations. Furthermore, when the two membrane preparations were combined, the resultant pharmacology was still consistent with the presence of two receptors that retained the characteristics of the alpha 2A and alpha 2B subtypes. 6. Imiloxan was identified as a selective alpha 2B ligand while benoxathian displayed a high degree of selectivity for the alpha 2A-adrenoceptor binding site. The selectivity of imiloxan for the alpha 2B-adrenoceptor binding site, coupled with its specificity for alpha 2-adrenoceptors, should make it a valuable tool in the classification of alpha 2-adrenoceptor subtypes.

Estimation of pA2 values at presynaptic alpha 2-autoreceptors in rabbit and rat brain cortex in the absence of autoinhibition

An attempt was made to determine pA2 values of antagonists at the presynaptic, release-inhibiting alpha 2-autoreceptors of rabbit and rat brain cortex under conditions when there was very little released noradrenaline in the autoreceptor biophase and, hence, pA2 values were not distorted by endogenous autoinhibition. Cortex slices were preincubated with 3H-noradrenaline and then superfused and stimulated by trains of 4 pulses delivered at 100 Hz or, in a few cases, by trains of 36 pulses at 3 Hz. The alpha-adrenoceptor agonists clonidine, noradrenaline, and alpha-methylnoradrenaline concentration-dependently decreased the stimulation-evoked overflow of tritium. The alpha-adrenoceptor antagonists yohimbine, rauwolscine and idazoxan did not increase the overflow of tritium elicited by 4 pulses/100 Hz in rabbit brain slices and increased it only slightly in rat brain slices. In contrast, the antagonists increased markedly the overflow at 36 pulses/3 Hz. All antagonists caused parallel shifts to the right of the concentration-response curves of clonidine, noradrenaline, and alpha-methylnoradrenaline. pA2 values were calculated either from linear regression of log [agonist concentration ratio - 1] on log [antagonist concentration] or from sigmoid curve fitting. The slopes of the linear regression lines were close to unity, and the pA2 values calculated by the two methods agreed well. There was no consistent preferential antagonism of any antagonist to any agonist. pA2 values determined with stimulation by 4 pulses/100 Hz were by 0.53-0.80 log units higher than those determined with stimulation by 36 pulses/3 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of selective antagonists (CH-38083 and idazoxan) to distinguish between prejunctional and postjunctional alpha 2-adrenoreceptors

1. The prejunctional alpha 2-adrenoreceptor antagonist activity of CH-38083 (7,8-(methylenedioxi)-14-alpha-hydroxyalloberbane HCl), idazoxan and prazosin was determined against B-HT 920 on the tachycardia induced by cardiac nerve stimulation in pithed rats. Antagonism of cirazoline and B-HT 920 pressor responses was used to assess postjunctional alpha 1- and alpha 2-adrenoreceptor antagonist activities. 2. CH-38083 was more potent than idazoxan in blocking pre- and postjunctional alpha 2-adrenoreceptor sites. There was no difference between the activities of the two selective alpha 2-adrenoreceptor blocking agents on pre- and postjunctional alpha 2-adrenoreceptors. 3. These data classify CH-38083 as a potent and highly selective alpha 2-adrenoreceptor antagonist in vivo and further support evidence of the homogeneity of pre- and postjunctional alpha 2-adrenoreceptors.

Pharmacological analysis of postjunctional alpha-adrenoceptors mediating contractions to (-)-noradrenaline in the rabbit isolated lateral saphenous vein can be explained by interacting responses to simultaneous activation of alpha 1- and alpha 2-adrenoceptors

1. The pharmacological characteristics of the alpha-adrenoceptor population in the rabbit isolated saphenous vein has been examined with (-)-noradrenaline (NA), as principal agonist, and a number of antagonists with selectivity for either alpha 1- or alpha 2-adrenoceptors. 2. The rank order of potency of various agonists is consistent with a population of alpha 2-adrenoceptors; UK-14304 greater than (-)-noradrenaline = (-)-adrenaline greater than B-HT 920 = cirazoline greater than phenylephrine greater than amidephrine, but the rank order of pA2 values for the antagonists against (-)-noradrenaline: BDF-6143 greater than rauwolscine = prazosin greater than CH-38083 = YM-12617 greater than Wy-26703 = phentolamine greater than corynanthine, is indicative of a mixed population of alpha 1- and alpha 2-adrenoceptors or, alternatively, a new subtype with characteristics of both the alpha 1- and alpha 2-subtypes. 3. Further evidence for two discrete populations of alpha-adrenoceptors is provided by, (a) the potent but non-competitive effect of prazosin against (-)-noradrenaline, (b) the presence of a component of the contractions elicited by NA and phenylephrine which is resistant to the selective alpha 2-adrenoceptor antagonists rauwolscine and CH-38083: these responses were inhibited by the selective alpha 1-adrenoceptor antagonists prazosin and YM-12617, but not by the selective alpha 2-adrenoceptor antagonist BDF-6143 and, (c) the relative potency of the yohimbine diastereoisomers rauwolscine and corynanthine against NA, phenylephrine and UK-14304. 4. In spite of the overwhelming evidence for a population of postjunctional alpha 2-adrenoceptors, prazosin was similarly effective against all agonists and failed to discriminate between those with putative selectivity for alpha 1- and alpha 2-adrenoceptors. This suggests an interaction of the effects of agonists at the two alpha-adrenoceptor subtypes. 5. An attempt has been made to reconcile a number of paradoxical observations with regard to the identification of postjunctional alpha 2-adrenoceptors in vitro, and it is suggested that in many of the isolated blood vessels presently available for examination both subtypes reside on the same smooth muscle cell. The pharmacological consequences of multiple subtypes of receptors mediating the same response is considered.

Some pharmacological properties of Wy 27127 a more selective alpha 2:alpha 1-adrenoceptor antagonist than idazoxan in vitro

Wy 27127 and idazoxan were approximately equipotent as antagonists at alpha 2-adrenoceptors as estimated by their ability to block clonidine-induced inhibition of electrically-evoked contractions of the rat isolated vas deferens. Idazoxan was seven times as potent as Wy 27127, as an antagonist at alpha 1-adrenoceptors as indicated by blockade of methoxamine-induced contractions of the rat isolated anococcygeus muscle. Thus, the alpha 2:alpha 1 selectivity ratio, as calculated from these tests was 407 for Wy 27127 and 76 for idazoxan. Wy 27127 and idazoxan were equipotent in enhancing stimulation-evoked overflow of tritium from rabbit isolated pulmonary arteries preloaded with [3H]-noradrenaline as expected for alpha 2-adrenoceptor antagonists. At higher concentrations both compounds reduced the stimulation-evoked contraction of the pulmonary artery but idazoxan was 15 times as potent as Wy 27127 in this respect. Neither compound had marked antagonist actions at 5-hydroxytryptamine (D), muscarinic, presynaptic dopamine or histamine (H1) receptors or at beta 1-adrenoceptors. Thus, idazoxan and Wy 27127 were equipotent alpha 2-adrenoceptor antagonists in vitro, however, the alpha 2:alpha 1 selectivity of Wy 27127 was considerably greater than that of idazoxan by virtue of weaker alpha 1-adrenoceptor antagonism.

Differences between central and peripheral rat alpha-adrenoceptors revealed using binuclear ligands

We have used two homologous series of binuclear ligands, diacridines and diquinolines, and the radioligand receptor assay to compare the topology of alpha 1- and alpha 2-adrenoceptors in rat cerebral cortex and kidney membranes. While the chain length-dependence of affinity of the diacridines, as well as that of the diquinolines, for the alpha 1-adrenoceptors of these central and peripheral tissues are similar, we find marked differences in affinity profiles for interaction with central and peripheral alpha 2-adrenoceptors. In the context of our previously proposed model for the binding of diacridines and diquinolines to alpha-adrenoceptors the results suggest that the surface features of central and peripheral alpha 2-adrenoceptors differ in the area surrounding the noradrenaline binding site. This difference may prove to be of therapeutic relevance.

Comparison of potency of alpha 2-adrenoceptor antagonists in vitro: evidence for heterogeneity of alpha 2-adrenoceptors

A comparison has been made of affinity of alpha-adrenoceptor antagonists for alpha 2 binding sites in radioligand binding assays, and functional antagonist activity at pre- and postjunctional alpha 2-adrenoceptors in various in vitro preparations. The antagonists displaced [3H]-rauwolscine from rat brain and rabbit spleen membranes but there were substantial differences in rank order and absolute potency in the two tissues. pA2 values for yohimbine, phentolamine and Wy26703 against the selective alpha 2 agonist UK-14,304 were determined in the rat left atrium, rat and rabbit vas deferens and rabbit saphenous vein preparations. The pA2 values varied substantially between the tissues, differing by two orders of magnitude in the case of Wy26703. Yohimbine was more potent in rabbit preparations while Wy26703 was markedly more potent in all the rat preparations. Yohimbine and Wy26703 were compared in the dog saphenous vein preparation where pre- and postjunctional alpha 2 antagonist activity can be compared in the same tissue. As in the rabbit preparations, yohimbine was more potent than Wy26703 at both sites but the absolute potencies were different. It is concluded that alpha 2-adrenoceptors are a heterogeneous population, different subgroups being more apparent between species rather than between tissue types or location.