Identification of a selective nonpeptide antagonist of the anaphylatoxin C3a receptor that demonstrates antiinflammatory activity in animal models.

The anaphylatoxin C3a is a potent chemotactic peptide and inflammatory mediator released during complement activation which binds to and activates a G-protein-coupled receptor. Molecular cloning of the C3aR has facilitated studies to identify nonpeptide antagonists of the C3aR. A chemical lead that selectively inhibited the C3aR in a high throughput screen was identified and chemically optimized. The resulting antagonist, N(2)-[(2,2-diphenylethoxy)acetyl]-L-arginine (SB 290157), functioned as a competitive antagonist of (125)I-C3a radioligand binding to rat basophilic leukemia (RBL)-2H3 cells expressing the human C3aR (RBL-C3aR), with an IC(50) of 200 nM. SB 290157 was a functional antagonist, blocking C3a-induced C3aR internalization in a concentration-dependent manner and C3a-induced Ca(2+) mobilization in RBL-C3aR cells and human neutrophils with IC(50)s of 27.7 and 28 nM, respectively. SB 290157 was selective for the C3aR in that it did not antagonize the C5aR or six other chemotactic G protein-coupled receptors. Functional antagonism was not solely limited to the human C3aR; SB 290157 also inhibited C3a-induced Ca(2+) mobilization of RBL-2H3 cells expressing the mouse and guinea pig C3aRS: It potently inhibited C3a-mediated ATP release from guinea pig platelets and inhibited C3a-induced potentiation of the contractile response to field stimulation of perfused rat caudal artery. Furthermore, in animal models, SB 290157, inhibited neutrophil recruitment in a guinea pig LPS-induced airway neutrophilia model and decreased paw edema in a rat adjuvant-induced arthritis model. This selective antagonist may be useful to define the physiological and pathophysiological roles of the C3aR.

Adrenoceptor pharmacology: urogenital applications.

Although the selective alpha1-adrenoceptor antagonists were initially developed as antihypertensive drugs, and they are still utilized for this indication, the alpha1-adrenoceptor blockers are now used extensively for the symptomatic treatment of benign prostatic hyperplasia (BPH). As a result, a number of new drugs in this class have been specifically developed for use in BPH. The utility of alpha1-adrenoceptor antagonists in BPH derives from the observation, made several decades ago, that the irreversible, alpha1- adrenoceptor selective antagonist phenoxybenzamine, blocked the contractile activity of norepinephrine in isolated strips of rat or human prostate. Following the further subclassification of alpha1-adrenoceptors into the alpha1A-, alpha1B- and alpha1D-adrenoceptor subtypes, the relationship between subtype selectivity and efficacy in BPH has been investigated in the hope of developing more selective drugs for the treatment of this disorder. Molecular characterization of the adrenoceptor population in human prostate clearly shows the alpha1A-adrenoceptor subtype to predominate, and highly selective alpha1A-adrenoceptor antagonists have been identified and investigated in BPH. However, controversy remains as to whether prostatic smooth muscle contraction is mediated by the alpha1A-adrenoceptor, or by another novel alpha1-adrenoceptor subtype (not corresponding to any of the three known recombinant alpha1-adrenoceptors), or both. alpha1-Adrenoceptor agonists have been used clinically for the treatment of stress incontinence, acting to increase urethral tone by contracting urethral smooth muscle. Research efforts are ongoing to identify agents of this class having a selective action on urethral versus vascular smooth muscle, in order to produce a greater effect on the urethra without producing dose-limiting increases in blood pressure. Local administration of vascular smooth muscle relaxants, either alone or in combination, has been used for the treatment of erectile dysfunction. An alpha1-adrenoceptor antagonist is often used as one comportent in such mixtures, which act to relax trabecular smooth muscle. The recent demonstration that a systemically administered drug can produce a sufficiently selective action on cavernosal smooth muscle to allow efficacy without producing limiting systemic side effects has renewed interest in the possibility of systemic administration of alpha1-adrenoceptor antagonists for this indication.

Ser165 of transmembrane helix IV is not involved in the interaction of catecholamines with the alpha-2a-adrenoceptor.

Molecular modeling studies have predicted that the beta-hydroxyl group of the catecholamines interacts with the beta 2-adrenoceptor at the serine residue at position 165 (Ser165) located on transmembrane helix IV; however, this has not been confirmed by site-directed mutagenesis. It has been inferred that this site, which is conserved in all of the nine known alpha- and beta-adrenoceptor subtypes, is also involved in the interaction of catecholamines with the alpha 2a-adrenoceptor. To test the hypothesis that the beta-hydroxyl group of the catecholamines interacts with Ser165 of the alpha 2a-adrenoceptor, we prepared a mutant alpha 2a-adrenoceptor where Ser165 was mutated to alanine. Mutation of Ser165 of the alpha 2a-adrenoceptor to alanine had no effect on the affinity of dopamine (which lacks the beta-hydroxyl group) or either enantiomer of norepinephrine or epinephrine (both of which possess the beta-hydroxyl group), indicating that Ser165 is not involved in the interaction of the catecholamines with the alpha 2a-adrenoceptor. We have previously shown that mutation of Ser90, located in transmembrane helix II, to either alanine or cysteine produces a selective reduction in the affinity of the (-)-enantiomers of the catecholamines for the alpha 2a-adrenoceptor, with no effect on the (+)-enantiomers or the corresponding beta-desoxy analogs. This is consistent with the known stereoselectivity involved in the interactions of catecholamines with the alpha 2a-adrenoceptor. The results of the present investigation indicate that Ser165 is not involved in the interaction of catecholamines with the alpha 2a-adrenoceptor. Because all known alpha-adrenoceptor subtypes have a serine residue at a position corresponding to Ser90 of the alpha 2a-adrenoceptor, it would appear that this site represents an important point for attachment of the beta-hydroxyl group of catecholamines.

The use of alpha-adrenoceptor antagonists in the pharmacological management of benign prostatic hypertrophy: an overview.

Benign prostatic hypertrophy (BPH) produces symptomatic urethral obstruction in a significant percentage of older men. Since the incidence of BPH is age related, the clinical and economic impact of this disease will continue to progress as average lifespan increases. BPH is associated with growth of both glandular and stromal elements of the prostate gland. Glandular hyperplasia can be partially reversed by withdrawal of androgenic tone with androgen receptor antagonists or steroid-5-alpha-reductase inhibitors. However, the reduction in prostatic size produced by these agents has little effect on the dynamic tone induced by nerve mediated contraction of stromal smooth muscle. This tone is mediated by activation of alpha-adrenoceptors. Therefore the alpha-adrenoceptor antagonists represent a useful pharmacological approach to the treatment of BPH. Studies in isolated strips of human prostate show that either exogenous alpha-adrenoceptor agonists or electrical field stimulation will induce contraction. Studies with selective antagonists such as prazosin show that this response is mediated by the alpha 1-adrenoceptor, even though radioligand binding studies show the presence of alpha 1 and alpha 2 adrenoceptor subtypes in approximately equal density. Following the cloning of multiple alpha 1-adrenoceptors, the contractile response in human prostate has been assigned to the alpha 1A adrenoceptor. However, recent data would suggest a functional role for another subtype, which has not yet been cloned, and designated as alpha 1L based on a relatively low affinity for prazosin. Clinical trials have shown efficacy of a variety of alpha-adrenoceptor antagonists in BPH, including non-selective agents such as phenoxybenzamine, as well as a variety of selective alpha 1-adrenoceptor antagonists, most structurally related to prazosin. The agents most commonly employed at the present time include the prazosin analogs terazosin, doxazosin and alfuzosin, as well as the structurally unrelated indoramin and tamsulosin. The design of new alpha 1-antagonists for BPH has concentrated on agents producing preferential blockage of urogenital vis-á-vis vascular alpha 1-adrenoceptors, based either on selectivity for the alpha 1A-adrenoceptor subtype or on functional uroselectivity in animal models. While these newer agents offer the prospect of reducing the incidence of the cardiovascular side effects associated with current therapy their superiority over nonselective alpha 1-adrenoceptor antagonists remains to be demonstrated in the clinical setting.

The alpha 2-adrenoceptors of the human retinoblastoma cell line (Y79) may represent an additional example of the alpha 2C-adrenoceptor.

1. In agreement with the literature, correlation of the ability of a series of agonists and antagonists to displace [3H]-rauwolscine binding shows the alpha 2-adrenoceptors of HT29 cells, NG108-15 cells, OK cells and homogenates of rat sublingual gland to represent four distinct subtypes. 2. [3H]-rauwolscine also bound with high affinity (KD = 0.30 +/- 0.10 mM) to a human retinoblastoma cell line (Y79). Specific binding represents 73% of total binding, and a Bmax of 38 +/- 1 fmol mg-1 protein was determined. 3. Correlation of antagonist affinities against [3H]-rauwolscine with corresponding values in the other four tissue sources showed the Y79 cells to resemble most closely the OK cells, the prototype example of an alpha 2C-adrenoceptor, with a correlation coefficient of 0.90 and a regression slope of 1.01 being obtained for 10 antagonists in these two systems. 4. Comparison of KD values for [3H]-rauwolscine also showed a similarity between the OK cells (0.19 +/- 0.07 nM) and Y79 cells. 5. These data suggest that the human retinoblastoma cell line may represent an additional example of the alpha 2C-adrenoceptor subtype.

Ability of SK&F 104078 and SK&F 104856 to identify alpha-2 adrenoceptor subtypes in NCB20 cells and guinea pig lung.

Alpha-2 adrenoceptors were characterized in three tissue culture cell lines and in membrane homogenates of guinea pig lung via the ability of a series of alpha adrenoceptor antagonists to inhibit the binding of [3H]clonidine, [3H]UK-14,304 and [3H]rauwolscine. The cells studied included those known to possess receptors of alpha-2A (HT29) and alpha-2B (NG108-15) subtypes as well as the previously uncharacterized NCB20 cells. Correlation of the ability of the antagonists to inhibit [3H]clonidine or [3H]UK-14,304 binding did not identify alpha-2 adrenoceptor subtypes. On the other hand, correlation of antagonist affinities against [3H]rauwolscine binding showed HT29 cells and guinea pig lung to have similar characteristics (r = 0.911) as did NG108-15 and NCB20 cells (r = 0.985). These data suggest subtle differences in the binding of [3H]agonists and [3H]antagonists to the alpha-2 adrenoceptor, resulting in the failure of [3H]clonidine and [3H]UK-14,304 to recognize differences between alpha-2A and alpha-2B receptor subtypes. 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4,5-tetrahydro-3- benzazepine (SK&F 104078) did not differentiate between the alpha-2A and alpha-2B receptor subtypes. However, 2-vinyl-7-chloro-3,4,5,6-tetrahydro-4-methylthieno[(4,3,2ef] [3]benzazepine (SK&F 104856), which has similar selectivity in functional in vitro models, was about 35-fold more potent in displacing [3H]rauwolscine binding to the alpha-2B site. These data provide additional evidence that the functional subclassification of alpha-2 adrenoceptors based on sensitivity to SK&F 104078 and SK&F 104856 subdivides this receptor in a different manner than does the alpha-2A/alpha-2B subclassification scheme.

The water diuretic effect of the alpha-2 adrenoceptor agonist, AGN 190851, is species-dependent.

The effect of the novel alpha-2 adrenoceptor agonist, AGN 190851, was evaluated for its diuretic action in the rat, dog and cynomolgus monkey and its ability to inhibit vasopressin-stimulated cyclic AMP accumulation in rat and dog cortical collecting tubules in vitro. The data indicate that in the rat, AGN 190851 resulted in a dose-dependent water diuresis, which was accompanied by an increase in blood pressure and osmolar clearance. In addition, AGN 190851 resulted in a dose-dependent inhibition of vasopressin-stimulated cyclic AMP accumulation in rat cortical collecting tubules in vitro. In contrast, AGN 190851 was unable to cause either a water diuresis in conscious dogs or inhibit vasopressin-stimulated adenylate cyclase activity in canine tissue in vitro. In the lightly anesthetized cynomolgus monkey, AGN 190851 also failed to alter renal function significantly. Administration of the vasopressin receptor antagonist, SK&F 105494, to either dogs or cynomolgus monkeys demonstrated that antagonism of the vasopressin V2 receptor could result in a brisk water diuresis in both species. The data demonstrate that alpha-2 adrenoceptors can functionally antagonize vasopressin antidiuretic activity in the rat, but not in the dog or cynomolgus monkey.

Distinction of NPY receptors in vitro and in vivo. I. NPY-(18-36) discriminates NPY receptor subtypes in vitro.

We studied the possibility of multiple neuropeptide Y (NPY) receptor subtypes. NPY-stimulated Ca2+ mobilization in human erythroleukemia (HEL) cells was used to screen a number of NPY analogues. The potencies of three of these analogues [peptide YY (PYY), [D-Tyr-36]NPY, and NPY-(18-36)] were compared with that of NPY in the following model systems: Ca2+ mobilization and inhibition of adenosine 3',5'-cyclic monophosphate accumulation in HEL cells, potentiation of vasoconstriction in the isolated rabbit ear artery, reduction of cutaneous microvascular perfusion in the rat digit, and inhibition of [3H]serotonin release in rat brain. In each of the five models, PYY was a full agonist that exhibited a similar or slightly higher potency than NPY, whereas [D-Tyr-36]NPY and NPY-(18-36) were partial agonists with lower potencies: NPY-(18-36) had a lower potency and efficacy than [D-Tyr-36]NPY in HEL cells and the rabbit ear artery, but was more effective than [D-Tyr-36]NPY for constricting cutaneous microvasculature and inhibiting serotonin release. Because of its weak partial agonism, we also tested NPY-(18-36) as an antagonist of NPY-stimulated Ca2+ mobilization in HEL cells. NPY-(18-36) shifted the NPY concentration-response curve to the right with a KB affinity value of 297 nM. In summary, [D-Tyr-36]NPY and NPY-(18-36) are partial agonists, the relative potency of which varies between systems. These data demonstrate the presence of multiple NPY receptor subtypes. We propose a modified classification scheme of NPY receptor subtypes.

Effects of neuropeptide Y on the response of isolated blood vessels to norepinephrine and sympathetic field stimulation.

Although neuropeptide Y (NPY) is not a potent vasoconstrictor in many vascular beds, nanomolar concentrations of this peptide can potentiate the response of isolated blood vessels to sympathetic stimulation or exogenous vasoconstrictors. In the isolated perfused rabbit ear artery, NPY produces a concentration-related potentiation of the response to field stimulation. The potency of NPY in inducing potentiation of nerve-mediated vasoconstriction was not dependent on whether the NPY was administered intra- or extraluminally. In these perfused vessels the endothelium was intact. Similar potentiation of the response to field stimulation is also observed in superfused ring segments of rabbit ear artery, provided that the endothelium was not damaged mechanically. The response to field stimulation in the rabbit ear artery is mediated by the alpha-1 adrenoceptor; in contrast, stimulation induced contraction in the canine saphenous vein results from the activation of vascular alpha-2-adrenoceptors. Despite this difference, the effect of NPY in ring segments of canine saphenous vein was similar to that observed in the ear artery. NPY also produced an endothelium-dependent potentiation of the constrictor response to exogenous norepinephrine in superfused segments of both ear artery and saphenous vein. Although the NPY-induced potentiation of stimulation-induced vasoconstriction was not inhibited by indomethacin, suggesting that an arachidonic acid metabolite is not involved, the release of another endothelial-derived vasoconstrictor substance, as yet unidentified, by NPY may explain its ability to potentiate the responses to both nerve stimulation and a variety of exogenous vasoconstrictors.

SK&F 89124, a potent and selective agonist at prejunctional dopamine receptors.

SK&F 89124 (4-[2-(N,N-di-n-propylamino)ethyl]-7-hydroxy-2(3H) indolone) can be considered as a derivative of N,N-di-n-propyldopamine (DPDA) in which the meta-hydroxyl is replaced by a cyclic amide function. SK&F 89124 is at least one order of magnitude more potent than DPDA as an agonist at peripheral inhibitory prejunctional dopamine receptors (DA2 receptors) in the isolated perfused rabbit ear artery. A potent agonist action of SK&F 89124 at the DA2 receptor can also be demonstrated by inhibition of radioactive overflow from prelabelled canine coronary artery or saphenous vein, and in the anesthetized dog as an inhibition of the tachycardia induced by cardioaccelerator nerve stimulation or the increase in hind-limb perfusion pressure induced by stimulation of the lumbar sympathetic chain. SK&F 89124 is a potent inhibitor of the binding of [3H]spiroperidol to D2 receptors in bovine pituitary homogenates. High concentrations of SK&F 89124 do not activate the adenylate cyclase D1 receptor in rat caudate homogenates, nor produce activation of alpha 2-adrenoceptors or H2-histamine receptors in the guinea pig atrium. Although some alpha 1-adrenoceptor mediated vasoconstriction is produced in the rabbit ear artery and rabbit aorta, the concentrations required are several orders of magnitude higher than those active at the DA2 receptor. From these data it is evident that this structural modification can increase both the potency and selectivity of DPDA as a DA2 receptor agonist. The potency and selectivity of SK&F 89124 make this agent a useful tool for determination of the functional role of the DA2 receptor.

In vitro pharmacologic profile of the novel beta-adrenoceptor antagonist and vasodilator, carvedilol.

The pharmacologic profile of the novel beta-adrenoceptor antagonist/vasodilator, carvedilol, has been investigated in vitro. Carvedilol produced competitive antagonism of the beta 1-adrenoceptor mediated positive chronotropic response to isoproterenol in guinea pig atria, and the beta 2-adrenoceptor mediated relaxation to isoproterenol in carbachol (1 mumol/l) precontracted guinea pig trachea, with a dissociation constant (KB) for beta 1-adrenoceptors of 0.8 nmol/l and beta 2-adrenoceptors of 1.3 nmol/l. At slightly higher concentrations, carvedilol produced competitive inhibition of the alpha 1-adrenoceptor mediated contractile response to norepinephrine in rabbit aorta with a KB of 11 nmol/l. Carvedilol had no significant effect on the contractile response to angiotensin II in rabbit aorta at concentrations up to 10 mumol/l, thus demonstrating the lack of nonspecific vasodilator actions in arteries. In canine saphenous vein, carvedilol produced noncompetitive blockade of alpha 2-adrenoceptor mediated vasoconstriction, indicative of some additional activity. In estrogen-primed rat uterus precontracted by depolarization with KCl (70 mmol/l), carvedilol produced concentration-dependent relaxation (IC50 of 7.6 mumol/l), consistent with the notion that carvedilol may be a calcium channel antagonist. Support for this hypothesis was obtained in KCl (70 mmol/l) depolarized rabbit aorta where carvediol (10 mumol/l) produced a 10-fold parallel rightward shift in the concentration-response curve to calcium chloride. These studies demonstrate that carvedilol is a potent beta 1-, beta 2- and alpha 1-adrenoceptor antagonist, and a moderately potent calcium channel antagonist. These multiple activities of carvedilol may contribute to the antihypertensive activity of the compound.

Comparison of the alpha-adrenoceptor characteristics in human and canine prostate.

Alpha adrenoceptors, mediating contraction, have been shown to be present in strips of hypertrophic prostate surgically removed from patients with benign prostatic hypertrophy (BPH), providing a rational explanation for the demonstrated effectiveness of alpha antagonists in the symptomatic treatment of this disease. Inasmuch as the dog develops spontaneous and hormonally induced prostatic enlargement, studies were performed to compare the adrenoceptor characteristics of canine and human prostate to determine whether the dog represents a useful model to search for more effective alpha-adrenolytic therapy for human BPH. Norepinephrine produces contraction in isolated strips of canine prostate although it is only one-tenth as potent as previously reported in human tissue. In contrast, several selective alpha 1-adrenoceptor agonists are potent contractile agents in canine prostate, but are nearly inactive in the human tissue. This difference may be a consequence of their partial agonist character. The potency of selective alpha-adrenoceptor antagonists in blocking the norepinephrine-induced contractile response in both canine and human tissue is consistent with an action of norepinephrine on the alpha 1 adrenoceptor. The receptor dissociation constants for these antagonists are similar in prostatic tissue from dogs and humans, and the values in canine tissue correlate well with those obtained in the rabbit ear artery, a standard model for vascular alpha 1 adrenoceptors. Hence the dog may represent a useful model for studies of the potential responsiveness of human prostate to adrenergic agents.

Synthesis and dopaminergic activity of some halogenated mono- and dihydroxylated 2-aminotetralins.

In a series of 7,8-dihydroxy-1-phenyltetrahydro-3-benzazepine dopamine receptor agonists introduction of a chloro or fluoro substituent into the 6-position increases dopaminergic potency. Also, in this series replacement of the 7-hydroxyl group with a halogen results in inversion of activity from dopamine receptor agonist to antagonist. The present study was aimed at exploring the possibility that the structure-activity observations in the 3-benzazepine series of dopaminergic agents might be extrapolated to another class of dopamine receptor agonists, the 2-aminotetralins. Thus, a series of chloro- and fluoro-substituted mono- and dihydroxylated 2-aminotetralins was prepared and evaluated for dopaminergic properties in D-1 and D-2 receptor-related tests. Introduction of a chloro substituent into the 8-position of the prototype of this series, i.e. 2-amino-6,7-dihydroxytetralin (ADTN), resulted in a compound with a high degree of selectivity for the D-1 subpopulation of dopamine receptors; it was equally or more potent than ADTN in the D-1 receptor-related tests with greatly decreased effectiveness in the tests involving D-2 receptors. A similar effect was observed with 8-fluoro-ADTN; however, the N-(4-hydroxyphenethyl)-N-propyl derivative 4g of the 8-chloro-substituted ADTN showed marked D-2 binding affinity. Conversely, introduction of a chloro substituent into the 5-position of ADTN markedly decreased D-1 receptor affinity and efficacy. This effect was not seen with the related 5-fluoro derivative, suggesting D-1 receptors are more sensitive to bulk in the 5-position of ADTN than are the D-2 receptors. Replacement of either the 6- or 7-hydroxyl groups of ADTN with a chloro or fluoro substituent, in contrast, did not parallel the response seen in the benzazepine series (i.e., the compounds uniformly demonstrated less receptor affinity and did not have dopamine receptor antagonist activity); however, the decrease in agonist potency was less marked in the case of 2-amino-6-fluoro-7-hydroxytetralins than in the chlorinated monohydroxyaminotetralins. Thus, a parallelism in structure-activity relationships in the benzazepine and aminotetralin series of dopamine receptor agonists was not observed. The differences may reflect altered modes of receptor binding in the two series.

Dopamine receptor agonists: 3-allyl-6-chloro-2,3,4,5-tetrahydro- 1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-diol and a series of related 3-benzazepines.

The N-allyl derivative (SK&F 85174) of 6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-dio l (SK&F 82526) not only retains the exceptional D-1 agonist potency of its parent but also displays reasonably potent D-2 agonist activity, as measured by a dopamine-sensitive adenylate cyclase test and a rabbit ear artery assay, respectively. Several additional N-substituted compounds were prepared to explore the D-2/D-1 agonist relationship. The N-methyl analogue retained good D-2 agonist potency, but this substitution converted D-1 agonist activity into antagonist activity. Most other N-substituents sharply decreased D-2 agonist potency including the N-n-propyl group. This observation was surprising since the introduction of mono- or di-N-n-propyl substituent(s) is commonly linked with retention or enhancement of D-2 agonist potency in other series of dopamine agonists. The N-(2-hydroxyethyl) analogue retains about one-fourth the D-2 potency of SK&F 85174. Several synthetic methods were used to prepare these compounds. N-Allylation of a trimethoxybenzazepine followed by cleavage of the methyl ethers with boron tribromide was the preferred method. Other methods used were direct alkylation of the trihydroxy secondary amine, i.e., SK&F 82526, and an acylation-amide reduction-cleavage method.

Dopamine receptor agonist activity of some 5-(2-aminoethyl)carbostyril derivatives.

The potency of beta-adrenoreceptor agonists, e.g., isoproterenol, is strikingly increased by substitution of the meta catecholic hydroxyl group with the NH group of a carbostyril system. To explore the possibility that comparable potency enhancement might occur upon similar modification of the catechol ring of dopamine, a series of 5-(2-aminoethyl)carbostyril derivatives was prepared and examined for D-1 and D-2 dopamine receptor-stimulating activity. Only the parent compound, 5-(2-aminoethyl)-8-hydroxycarbostyril (2), produced measurable activation of dopamine-sensitive adenylate cyclase (29% at a concentration of 10 microM). Some of the compounds, however, did produce significant activity in tests, namely displacement of [3H]spiroperidol binding from bovine pituitary homogenate and an isolated perfused rabbit ear artery preparation, that measure interaction with D-2 receptors. Potency of the carbostyrils was enhanced by 8-hydroxylation and by appropriate substitution of the amino group of the ethylamine side chain. The most potent member of the series was 8-hydroxy-5-[2-[[2-(4-hydroxyphenyl)ethyl]-n-propylamino]ethyl] carbostyril (16b). This compound was about 3 times more effective than dopamine in the D-2 receptor tests. Clearly, the results of this study indicate that potency of dopamine receptor agonists is not increased by carbostyril replacement of the m-hydroxyl as is noted with the beta-adrenergic receptor agonists.

In vitro characterization of the alpha-adrenoceptors in human prostate.

The alpha-adrenoceptors of the human prostate gland were characterized in vitro by the use of antagonists selective for alpha 1- and alpha 2-adrenoceptor subtypes. The contractile response induced by norepinephrine in this tissue could be antagonized by prazosin, a selective alpha 1-antagonist, with a receptor dissociation constant (KB) of 4.0 +/- 0.9 nM. The selective alpha 2-antagonists rauwolscine and SK&F 86466 were less potent antagonists of this response, with KB values of 1020 +/- 400 nM and 2400 +/- 800 nM, respectively. The irreversible alpha-antagonists benextramine and phenoxybenzamine, both of which preferentially inactivate the alpha 1-subtype, produced marked depression of norepinephrine-induced contraction. These data would suggest that the alpha-receptors on prostatic smooth muscle are predominantly of the alpha 1-subtype.