Piboserod (SB 207266), a selective 5-HT4 receptor antagonist, reduces serotonin potentiation of neurally-mediated contractile responses of human detrusor muscle

The aim of this study is to evaluate the potency of piboserod (SB 207266), a selective 5-HT(4) receptor antagonist, at inhibiting the 5-HT(4)-mediated potentiating effect of serotonin (5-HT) on the neurally-mediated contractile responses of human detrusor strips to electrical field stimulations (EFS). Strips of human detrusor muscle were mounted in Krebs-HEPES buffer under a resting tension of 500 mg and EFS (20 Hz, 1 ms duration at 300 mA for 5 s) was applied continuously at 1 min intervals. After stabilization of the EFS-induced contractions, concentration-response curves to 5-HT (0.1 nM-100 microM) were constructed in the absence or presence of 1 or 100 nM of piboserod. The experiments were performed in the presence of methysergide (1 microM) and ondansetron (3 microM) to block 5HT(1)/5HT(2) and 5-HT(3) receptors, respectively. 5-HT potentiated the contractile responses to EFS of human bladder strips in a concentration-dependent manner, with a maximum mean of 60.0+/-19.9% of the basal EFS-evoked contractions. Piboserod did not modify the basal contractions but concentration-dependently antagonized the ability of 5-HT to enhance bladder strip contractions to EFS. In presence of 1 and 100 nM of piboserod, the maximal 5-HT-induced potentiations were reduced to 45.0+/-7.9 and 38.7+/-8.7%, respectively. A mean apparent antagonist dissociation constant value (K(B)) of 0.56+/-0.09 nM was determined. These data show the ability of piboserod to antagonize with high potency the enhancing properties of 5-HT on neurally-mediated contractions of isolated human bladder strips. Therefore, the 5-HT(4) receptor might represent an attractive pharmacological target for the treatment of overactive bladder.

Recent advances in the identification of a 1- and a 2-adrenoceptor subtypes: therapeutic implications

The cloning of multiple subtypes of both alpha1- and alpha2-adrenoceptors has renewed interest in the therapeutic application of agents interacting with these receptors. Effort has primarily been directed towards the design of uroselective alpha1-adrenoceptor antagonists for the treatment of benign prostatic hyperplasia (BPH). Evidence is accumulating for the involvement of a novel alpha1-adrenoceptor, designated as alpha1L-adrenoceptor, in alpha1-adrenoceptor-mediated smooth muscle contraction in prostatic and other urogenital tissues. While several antagonists showing a high degree of uroselectivity in animal models have been identified, their clinical superiority over the currently available alpha1-adrenoceptor antagonists has not yet been demonstrated. It is possible that the interaction with alpha1-adrenoceptors, as yet uncharacterised subtypes, at non-prostatic sites contributes to the therapeutic activity of this drug class in BPH. The alpha1-adrenoceptor subtypes involved in the control of vascular tone are currently being evaluated, and the profile of interaction with the various alpha1-adrenoceptor subtypes may play a key role in the efficacy of cardiovascular drugs such as carvedilol. Alpha2-adrenoceptor agonists are now being employed for a variety of therapeutic applications, most involving actions on receptors within the central nervous system (CNS). These agents are useful in the treatment of hypertension, glaucoma, opiate withdrawal and attention deficit hyperactivity disorder (ADHD), and as analgesics and adjuncts to general anaesthesia. While subtype selectivity has not yet been applied to the design of new alpha2-adrenoceptor agonists for these applications, recent gene mutation/knock-out experiments have identified the alpha2-subtypes involved in some of these actions, and optimisation of a therapeutic profile may be possible. Furthermore, the design of agents combining affinities for multiple adrenoceptor subtypes, or the combination of a specific adrenoceptor affinity profile with another pharmacological action, may offer advantages over molecules selective for an individual adrenoceptor subtype.

Rabbit alpha2-adrenoceptors: both platelets and adipocytes have alpha2A-pharmacology

The recombinant alpha2-adrenoceptors, designated as alpha2a and alpha2d, have highly similar amino acid sequences, but distinct pharmacological properties. It has been suggested that these two receptor subtypes are species orthologs, since the alpha2-adrenoceptors of a given species have pharmacological characteristics corresponding to either the alpha2a- (human, pig) or alpha2d- (rat, mouse, guinea pig, cow) adrenoceptor. Radioligand binding assays in rabbit adipocyte suggest alpha2D-adrenoceptor pharmacology. However, functional studies examining prejunctional alpha2-adrenoceptors in several tissues pharmacologically define the receptor of the rabbit as an alpha2A-adrenoceptor rather than an alpha2D-adrenoceptor. We characterized the alpha2-adrenoceptor of rabbit adipocyte and platelet, comparing the ability of norepinephrine and 13 adrenoceptor antagonists to inhibit the binding of [3H]RX821002 with the affinity of these drugs for the human alpha2a-adrenoceptor or the rat alpha2d-adrenoceptor. Pharmacological characteristics of the adipocyte and platelet receptor were very similar, with an excellent correlation between pK(i) values (r2 = 0.95, slope of regression = 1.01). Drug affinities for both platelet and adipocyte receptors correlated better with the alpha2a-adrenoceptor (r2 = 0.68-0.77) than with the alpha2d-adrenoceptor (r2 = 0.37-0.38). Despite the relatively low affinity of the rabbit adipocyte alpha2-adrenoceptor for yohimbine and rauwolscine, this receptor, as well as the platelet receptor, have alpha2A-adrenoceptor pharmacology. Subtle differences in the alpha2-adrenoceptor binding characteristics of these native rabbit tissues compared with the recombinant human alpha2a-adrenoceptor may result either from minor differences in the sequence of human and rabbit alpha2a-adrenoceptors or from differences in the environment to which native and recombinant receptors are exposed.

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.

Molecular basis for the stereoselective interactions of catecholamines with alpha-adrenoceptors

The catecholamines were found to inhibit the binding of the alpha 2-adrenoceptor agonist, [3H]-clonidine, to the recombinant wild type alpha 2a-adrenoceptor (Table 1) with potencies that are consistent with their functional activity in alpha 2-adrenoceptor test systems [6,7]. Mutation of Ser165 to alanine had no significant effect (less than 2-fold) on the affinity of any of the catecholamines for the alpha 2a-adrenoceptor, and in particular, the ratios of affinities between the corresponding (-)- and (+)-enantiomers of the catecholamines were not altered by the point mutation at Ser165. These findings indicate clearly that Ser165, in contrast to predictions made by molecular modeling, plays little if any role in the binding of the catecholamines in general, and cannot be involved in the attachment of the beta-hydroxyl group to the alpha 2a-adrenoceptor. Mutation of either Ser90 on transmembrane helix II or Ser419 on transmembrane helix VII to alanine produced dramatic and selective reductions in the affinity of the (-)-enantiomers of the catecholamines for the alpha 2a-adrenoceptor, with no changes occurring in affinities of the (+)-enantiomers. Thus, the affinities of (-)-norepinephrine and (-)-epinephrine for the Ser90 and Ser419 mutants of the alpha 2a-adrenoceptor were 35-75 fold lower than their affinities for the wild type receptor (Table 1), suggesting that Ser90 and/or Ser419 are involved in the attachment of the beta-hydroxyl groups of the catecholamines to the receptor. Similarly, the affinity of (+/-)-6-fluoronorepinephrine was reduced by 100-fold for the Ser90 mutant receptor (Table 1). Importantly, the affinities of the (+)-enantiomers of the catecholamines, as well as dopamine and epinine, which are the corresponding analogs of norepinephrine and epinephrine which lack the beta-hydroxyl group, were not affected by mutation of Ser90 or Ser419 to alanine (Table 1). Asn293 in transmembrane helix VI has also been proposed to be involved in the interaction of the beta-hydroxyl group of isoproterenol with the beta 2-adrenoceptor [4]. The alpha 2a-adrenoceptor contains three hydroxyl bearing amino acids at a position corresponding to this site (Thr393-Tyr394-Thr395). These amino acids could theoretically form a hydrogen bond with the beta-hydroxyl group of a catecholamine, and therefore could serve as a potential point of attachment. Simultaneous mutation of all three of these amino acids to Ala-Phe-Ala reduced the affinity of the (-)-enantiomers of the catecholamines by 12-20 fold, which is somewhat less than what was observed for mutation of either Ser90 or Ser419 (Table 1). However, in contrast to mutation of Ser90 or Ser419, which had no effect on the affinity of the (+)-enantiomers, mutation of the three residues in transmembrane helix VI did significantly reduce the affinities of the (+)-enantiomers of the catecholamines by approximately 5- to 9-fold, indicating that mutations at these points of the receptor are not selective for the (-)-enantiomers, and are therefore not likely to be involved in the attachment of the beta-hydroxyl group of the catecholamines.

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.

Cloning, expression, and pharmacological characterization of a novel human histamine receptor

Using a genomics-based reverse pharmacological approach for screening orphan G-protein coupled receptors, we have identified and cloned a novel high-affinity histamine receptor. This receptor, termed AXOR35, is most closely related to the H3 histamine receptor, sharing 37% protein sequence identity. A multiple responsive element/cyclic AMP-responsive element-luciferase reporter assay was used to identify histamine as a ligand for AXOR35. When transfected into human embryonic kidney 293 cells, the AXOR35 receptor showed a strong, dose-dependent calcium mobilization response to histamine and H3 receptor agonists including imetit and immepip. Radioligand binding confirmed that the AXOR35 receptor was a high-affinity histamine receptor. The pharmacology of the AXOR35 receptor was found to closely resemble that of the H3 receptor; the major difference was that (R)-alpha-methylhistamine was a low potency agonist of the AXOR35 receptor. Thioperamide is an antagonist at AXOR 35. Expression of AXOR35 mRNA in human tissues is highest in peripheral blood mononuclear cells and in tissues likely to contain high concentrations of blood cells, such as bone marrow and lung. In situ hybridization analysis of a wide survey of mouse tissues showed that mouse AXOR35 mRNA is selectively expressed in hippocampus. The identification and localization of this new histamine receptor will expand our understanding of the physiological and pathological roles of histamine and may provide additional opportunities for pharmacological modification of these actions.

Activity of N-(phenethyl)phenylethanolamines at beta(1) and beta(2) adrenoceptors: structural modifications can result in selectivity for either subtype

A series of phenylethanolamines bearing a 2-[1-phenylpropyl] substituent on the nitrogen atom was evaluated in vitro for activity at beta(1)- and beta(2)-adrenoceptors. As previously observed, the presence of 3,4-dihydroxy substitution on phenylethanolamine is required for potent activation of both subtypes, whereas the 3,5-dihydroxy analog showed selectivity for the beta(2)-subtype. Replacement by a carboxyl group of the 4-hydroxyl group on the aralkyl nitrogen substituent produced only a small reduction in beta(1) potency (5-fold), whereas beta(2) potency was reduced by more than 100-fold. Hence this structural class includes agonists having either a beta(1), nonselective beta(1)/beta(2) or beta(2) selectivity profile.

Role of neurokinin receptors in the behavioral effect of intravesical antigen infusion in guinea pig bladder

PURPOSE

To characterize a guinea pig behavior model of bladder pain due to intravesical antigen infusion and to determine the role of neurokinin receptor subtypes in mediating this behavior.

MATERIALS AND METHODS

The influence of subtype-selective neurokinin receptor antagonists on increased abdominal licking behavior in response to intravesical antigen infusion in guinea pigs immunized with ovalbumin (OA) was determined.

RESULTS

Intravesical OA infusion for 30 minutes induced a significantly greater frequency (about 3-fold) of abdominal licking behavior than during either the 30 minutes pre-challenge or post challenge saline infusions. Treatment with IP capsaicin 7 to 10 days before OA challenge abolished the intravesical antigen-induced behavior. IP injection of the NK1 receptor antagonist CP 99994 (10 mg./kg. or 30 mg./kg.), 30 minutes pretreatment, inhibited the increase in the average number of abdominal licks during antigen infusion. The 30 mg./kg., but not the 10 mg./kg. dose increased the percent of animals showing antinociceptive activity (defined as 4 or less abdominal licks during the antigen infusion). The NK2 receptor antagonist SR 48968 reduced the antigen-induced abdominal licking behavior at IP doses of 3 and 10 mg./kg. but was ineffective at 1 mg./kg. The NK3 receptor antagonist SB 235375 (30 mg./kg., IP) did not reduce this behavior.

CONCLUSIONS

These results suggest a role for activation of NK1 and NK2, but not NK3 receptors, by tachykinins released from capsaicin-sensitive nerves, in the increased abdominal licking behavior response of guinea pigs to intravesical antigen infusion.

Drugs targeting adrenergic receptors: does interaction with a specific subtype confer therapeutic advantage?

Many important drugs act via the activation or blockade of adrenergic receptors. Although research has been ongoing in this area for over fifty years, the continual subdivision of the major adrenoceptor classes has provided new opportunities for drug discovery. This review focuses on the recent developments directed towards a few of the many potential targets now available. While there is a rationale to suggest that limiting pharmacological activity to one particular subtype may improve the therapeutic profile for some indications, it is also possible for a drug to have beneficial effects resulting from interactions with multiple adrenergic receptor subtypes.

Adrenoceptor subclassification: an approach to improved cardiovascular therapeutics

The subdivision of alpha adrenoceptors into the alpha 1 and alpha 2 classes was the impetus for the design of the selective alpha 1-adrenoceptor antagonists, which remain useful antihypertensives. alpha 2-Adrenoceptor agonists also have application as antihypertensive drugs, based on their ability to reduce sympathetic outflow. Likewise, subdivision of the beta adrenoceptors has lead to the development of selective beta 1-adrenoceptor antagonists as antihypertensive and selective beta 2 agonists as bronchodilators. In the past decade, both the alpha 1 and alpha 2 adrenoceptors have been further subdivided, each into three subclasses. In addition, there is strong functional evidence to suggest the presence of additional adrenoceptor subtypes, such as the "alpha 1L" adrenoceptor and "beta 4" adrenoceptor. alpha 1A (or alpha 1L)-Adrenoceptor antagonists have been evaluated for benign prostatic hyperplasia (BPH), and selective alpha 1A agonists for stress incontinence. Gene knockout experiments in mice suggest an important role for the alpha 1B adrenoceptor in the control of vascular tone. Hence, selective alpha 1B antagonists may offer a new approach toward hypertension. Although targeting of specific adrenoceptors can be used to optimize the therapeutic profile of a drug, there are also cases where blockade of multiple adrenoceptors is desirable, as with the alpha/beta-adrenoceptor antagonist carvedilol in congestive heart failure. It is possible that combination of affinities for selected adrenoceptor subtypes within a single molecule may be desirable for certain applications.

Characterization of postjunctional alpha-adrenoceptors in the pithed mouse

The adrenoceptor subtypes responsible for the pressor response to alpha1- and alpha2-adrenoceptor agonists have not yet been established, although gene knockout experiments in the mouse have provided evidence for a role of the alpha1B- and alpha2B-adrenoceptor. We have evaluated the blood pressure response to selective activation of postjunctional alpha1- and alpha2-adrenoceptors in the pithed mouse. The pressor response to phenylephrine was sensitive to blockade by terazosin, a selective alpha1-adrenoceptor antagonist, but insensitive to rauwolscine, an antagonist at alpha2-adrenoceptors. Phentolamine, a nonselective alpha-adrenoceptor antagonist, blocked the response to either phenylephrine or the selective alpha2-adrenoceptor agonist B-HT 933, whereas rauwolscine blocked only B-HT 933. A dose of terazosin effective against phenylephrine had no effect on B-HT 933; however, the B-HT 933 response was antagonized when the terazosin dose was increased tenfold. A high dose of doxazosin, an alpha1-adrenoceptor antagonist having no affinity for the alpha2B adrenoceptor, blocked the response to phenylephrine but not B-HT 933. Comparison of the potencies of these antagonists against the pressor response to phenylephrine with their affinities for recombinant alpha1-adrenoceptor subtypes suggests that this response is mediated by either alpha1B- or alpha1D-adrenoceptors. The alpha2B-adrenoceptor subtype is likely to take part in the response to B-HT 933. The ability of certain quinazoline alpha1-adrenoceptor antagonists to block the alpha2B adrenoceptor may contribute to their activity as antihypertensive agents.

Effects of alpha1-adrenoceptor antagonists on agonist and tilt-induced changes in blood pressure: relationships to uroselectivity

We evaluated the uroselectivity of a series of alpha1-adrenoceptor antagonists by comparing their potency against phenylephrine-induced increases in urethral perfusion pressure and diastolic blood pressure in the anesthetized rabbit and pithed rat. In the rabbit, Rec 15/2739 (N-[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]-3-methyl-4-oxo-2-phenyl -4H-1-benzopyran-8-carboxamide) as well as analogs with a chlorine substituent on the methoxyphenyl ring (Rec 15/2869) or this substituent combined with the replacement of the phenyl substituent on the pyran ring by cyclohexyl (Rec 15/3011) were 2-6-fold more potent against the urethral vs. vascular response to phenylephrine. Rec 15/2841 (N-[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]-3-methyl-4-oxo-2-cyc lohexy-4H-1-benzopyran-8-carboxamide) was only 1.5-fold more potent against the urethral response. SL 89.0591 and tamsulosin also showed selectivity for the urethral response (2-2.5-fold), while the quinazolines produced equipotent blockade of urethral and vascular responses (selectivity ratio = 0.9-1.1). The urethral selectivities of Rec 15/2739 and its derivatives were confirmed by evaluation of the response to tilt in sedated, hypovolemic rabbits. Phenylephrine challenge assays did not show any of the antagonists, with the exception of terazosin at 300 microg kg(-1), to be uroselective in the rat (selectivity ratios = 0.2-1.5); potentiation of tilt-induced hypotension in the anesthetized rat showed substantial differences from the rabbit, with Rec 15/2739, but not Rec 15/3011 and Rec 15/2841 showing orthostatic effects equivalent to that observed for prazosin. Hence, Rec 15/2739 was uroselective in the rabbit, but not in the rat, while two of its close structural analogs were highly uroselective in both species. An assay for orthostatic activity in the conscious rat yielded different results, showing prazosin and terazosin, but not Rec 15/2739, to cause a reversal of the pressor response to tilt. Hence, the apparent uroselectivity of an alpha1-adrenoceptor antagonist is both species- and assay-dependent.

Comparison of metoprolol and carvedilol pharmacology and cardioprotection in rabbit ischemia and reperfusion model

Carvedilol, a selective alpha1 and non-selective beta-adrenoceptor antagonist and antioxidant, has been shown to provide significant cardiac protection in animal models of myocardial ischemia. To further explore the mechanisms contributing to carvedilol cardioprotection efficacy, the effects of carvedilol on hemodynamic variables, infarct size and myeloperoxidase activity (an index of neutrophil accumulation) were compared with a beta1-selective adrenoceptor antagonist, metoprolol. Carvedilol (1 mg/kg) or metoprolol (1 mg/kg or 1 mg/kg + 0.5 mg/kg 90 min later) was given intravenously 5 min before reperfusion. In vehicle-treated rabbits, ischemia (60 min) and reperfusion (180 min) resulted in significant increments in left ventricular end diastolic pressure, large infarcts (59+/-2.6% of area-at-risk) and marked increase in myeloperoxidase activity (0.59+/-0.09 U/100 mg tissue). Carvedilol treatment resulted in sustained reduction of pressure-rate-index and significantly smaller infarcts (22.0+/-2.5%, P < 0.01 vs. vehicle) as well as decreased myeloperoxidase activity (0.186+/-0.056 U/100 mg tissue, P < 0.01 vs. vehicle). The highest dose of metoprolol, 1 mg/kg + 0.5 mg/kg, that resulted in pressure-rate-index comparable to that of 1.0 mg/kg carvedilol, failed to reduce myeloperoxidase activity in the ischemic myocardial tissue, and the infarct size (35+/-3.1%) was significantly larger than in carvedilol-treated animals. Taken together, this study suggests that the superior cardioprotection of carvedilol over metoprolol is not a consequence of hemodynamic variances but possibly the result of the additional pharmacological properties of carvedilol such as the antioxidant and anti-neutrophil effects.

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.

Pharmacological characterization of the uroselective alpha-1 antagonist Rec 15/2739 (SB 216469): role of the alpha-1L adrenoceptor in tissue selectivity, part II

The aim of the present work was to investigate whether or not the uroselectivity of Rec 15/2739 and several other alpha-1 adrenoceptor (alpha1-AR) antagonists observed in the anesthetized dog could be related to selectivity of these compounds for a particular alpha-1 AR subtype. The binding affinity of the tested compounds for canine prostate alpha-1 ARs and their in vitro functional affinity for the alpha-1 ARs of rabbit urethra and prostate correlated with their functional affinity for the alpha-1L AR subtype, but not with the binding affinity for recombinant animal and human alpha-1a, alpha-1b and alpha-1d AR subtypes. Similar results were obtained when the in vivo potency on urethral pressure was correlated with the affinity for the alpha-1 AR subtypes; also in this case alpha-1L AR gave the best correlation. No correlation was obtained by considering the other alpha-1 AR subtypes. The in vivo hypotensive effects observed in dog after i.v. administration of the considered compounds correlated only with the binding affinity for the animal and human alpha-1d subtype. In conclusion, the results shown in the present paper indicate that the potencies of different alpha-1 antagonists against the contractions induced by norepinephrine on tissues of the lower urinary tract of rabbits and dogs are better correlated with their affinity for the putative alpha-1L subtype than for the alpha-1a subtype. Only the compounds showing selectivity for the alpha-1L subtype versus the alpha-1d subtype proved highly selective in vivo for the lower urinary tract versus the vascular tissues.

Pharmacological characterization of the uroselective alpha-1 antagonist Rec 15/2739 (SB 216469): role of the alpha-1L adrenoceptor in tissue selectivity, part I

Alpha adrenoceptor antagonists have been convincingly shown to be beneficial in reducing both subjective and objective indices of urethral obstruction in benign prostatic hyperplasia. Rec 15/2739 (SB 216469) is a novel alpha-1 adrenoceptor (alpha-1 AR) antagonist currently being developed for benign prostatic hyperplasia. When evaluated in radioligand binding assays with expressed animal or human alpha-1 ARs, Rec 15/2739 shows marked to moderate selectivity for the alpha-1a AR subtype. Its affinity for the recombinant alpha-2 AR subtypes or native dopaminergic D2 receptor was about 100-fold lower than that for alpha-1a AR subtype. In canine tissues, Rec 15/2739 was 20-fold more potent as an inhibitor of [3H]prazosin binding to prostate vis-a-vis aorta. Functional studies in isolated rabbit tissues also confirmed the uroselectivity of Rec 15/2739, with substantially higher affinity (Kb = 2-3 nM) being observed in urethra and prostate, compared with ear artery and aorta (Kb = 20-100 nM). The in vitro selectivity observed with Rec 15/2739 was confirmed in vivo in the anesthetized dog, comparing potency against norepinephrine- or hypogastric nerve stimulation-induced urethral contraction with its ability to reduce diastolic blood pressure. In this model, Rec 15/2739 had greater selectivity than any other alpha-1 AR antagonist examined, including terazosin and tamsulosin. Based on the low potency of prazosin and some of its structural analogs in the rabbit and dog lower urinary tract tissues, it appears that norepinephrine contracts these tissues via activation of the alpha-1L AR. Hence this alpha-1 AR subtype, rather than the alpha-1A AR, may mediate the contraction in vivo.

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.

"Vinylogs" and "acetylenylogs" of beta-adrenergic agents

Vinylogous (Groups III and V) and acetylenologous (Group IV) analogs of the classical beta-adrenergic agents--stimulants and blockers--were prepared in order to evaluate the effect of degree of saturation, position of unsaturation and rigidity of the chain linking the aromatic ring and the amino containing functional group on biological activity. Derivatives from Group III, which represent 4-aryl-3-butenyl-2-ol-amine analogs of Group II, retained beta 1-adrenoceptor antagonist activity albeit substantially less potent (50-200-fold) than that possessed by their aryloxy counterparts. Consistent with the SAR for Group II compounds, substitution at position 2 of the aromatic ring yielded the most potent antagonists (5a, 5d, 5g), with KB's ranging from 73-93 nM while 3,4-dichloro substitution (5e) markedly reduced antagonist potency (KB = 2,400 nM). Agonist activity was also noted for 5b and 5d, suggesting that these compounds may be best classified as partial agonists. Representatives from Groups IV and V were inactive as antagonists at the beta 1-adrenoceptor confirming the importance of the spatial relationship between the hydroxyl and the amino nitrogen.

Possible structural and functional relationships between imidazoline receptors and alpha 2-adrenoceptors

Although it is now well established that imidazoline receptors and alpha 2-adrenoceptors are discrete entities with distinct endogenous ligands, the two receptor classes apparently have several common features. While the catecholamines stimulate alpha 2-adrenoceptors but not imidazoline receptors, agmatine, a guanidine analog that may be an endogenous imidazoline receptor ligand, can interact with both I1 and I2 imidazoline receptors as well as alpha 2-adrenoceptors, although, interestingly, other guanidines such as guanabenz are highly selective for alpha 2-adrenoceptors versus I1 receptors. Most I1 receptor agonists such as moxonidine, rilmenidine, and clonidine can also stimulate alpha 2-adrenoceptors, and the same physiological response is produced by activation of central I1 receptors and alpha 2-adrenoceptors, but their anatomical locations differ. The imidazoline idazoxan is an antagonist at I1, I2, and alpha 2-receptors, but minor structural alterations of idazoxan can result in molecules with selectivity for either alpha 2-adrenoceptors or imidazoline receptors. The precise mode of interaction of imidazoline agonists and antagonists with the alpha 2-adrenoceptor is not yet understood, and structures of the imidazoline receptors are still unknown. Nevertheless, the fact that many agents can stimulate or block both receptor classes, combined with the fact that alpha 2-adrenoceptors and I1 receptors can mediate identical physiological responses, suggests that many common structural features may be present.

Vascular alpha-adrenoceptors: from the gene to the human

Adrenoceptors can be subdivided into three major types, the alpha 1-, alpha 2-, and beta-adrenoceptors. Each of these types can be further subdivided into three subtypes, based on pharmacological characteristics. Molecular cloning techniques have supported this subclassification. Recent data now suggest that alpha-adrenoceptor subtypes identified by pharmacological and molecular techniques correspond well, although species orthologs of several adrenoceptor subtypes have been identified. The secondary structure of the adrenoceptors has been elucidated and correlated with their interaction with second messenger molecules. alpha 1-Adrenoceptors, beta-adrenoceptors, and alpha 2-adrenoceptors mediate their actions through stimulation of inositol phosphate release, stimulation of adenylate cyclase, and inhibition of adenylate cyclase, respectively. Site-directed mutagenesis and the preparation of chimeric receptors have located the site of receptor--second messenger interaction to the third intracellular loop for each of these adrenoceptors. While subtypes of each of these classes all interact with the same second messenger, studies with recombinant alpha 2-adrenoceptors show subtype-related differences in receptor--second messenger interaction. Multiple alpha-adrenoceptor subtypes are expressed in vascular smooth muscle and are involved in various aspects of blood vessel function, including contraction, cellular growth, and proliferation. Various physiological factors can selectively influence responses to a particular subtype, and the relative roles of each subtype can vary between vascular beds and along an individual blood vessel as its caliber changes. Functional studies in blood vessels suggest the presence of additional alpha-adrenoceptor subtypes not yet identified via molecular techniques. Optimization of the therapeutic profile of an alpha-adrenoceptor antagonist may be possible via enhancement of selectivity for a particular subtype or by design of a specific profile of affinity for the individual subtypes.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Mediation of the hypotensive action of systemic clonidine in the rat by alpha 2-adrenoceptors

1. During the past few years it has been shown that the sympatholytic effect resulting from localized microinjection of clonidine and other imidazolines into the rostral ventrolateral medulla (RVL) results from activation of 'imidazoline' receptors (I1 receptors) rather than from an alpha 2-adrenoceptor-mediated effect. 2. The relative contributions of these two receptor systems to the hypotensive action of systemically administered clonidine have not been studied. Clonidine has affinity for both I1 and alpha 2-adrenoceptors; guanabenz represents a useful pharmacological tool, since it activates only the alpha 2-adrenoceptor. 3. Antagonists acting at both I1 and alpha 2-adrenoceptors (idazoxan) and at only alpha 2-adrenoceptors (SK&F 86466; 6-chloro-3-methyl-2,3,4,5-tetrahydro-3-benzazepine) are available. Idazoxan (1 mg kg-1, i.v.) and SK&F 86466 (3 mg kg-1, i.v.) produced an equivalent degree of blockade of the pressor response to guanabenz or clonidine in the pithed rat, a response mediated by the alpha 2-adrenoceptor. 4. Guanabenz (30 micrograms kg-1, i.v.) and clonidine (10 micrograms kg-1, i.v.) lowered blood pressure in the chloralose-anaesthetized spontaneously hypertensive rat by 48 +/- 4.6 mmHg and 44 +/- 5.4 mmHg, respectively; this response, for either agonist, was blocked by both idazoxan and SK&F 86466. 5. These data show that the hypotensive effect of intravenously administered clonidine results from activation of central alpha 2-adrenoceptors, with no significant contribution from an I1-mediated effect. Thus clonidine can lower blood pressure by different receptor mechanisms, dependent on the route of administration.

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.

Cardiovascular actions of a new selective postjunctional alpha-adrenoceptor antagonist, SK&F 104856, in normotensive and hypertensive dogs

1. SK&F 104856 (2-vinyl-7-chloro-3,4,5,6-tetrahydro-4- methylthieno[4,3,2ef][3]benzazepine) is a novel postjunctional alpha 1- and alpha 2-adrenoceptor antagonist. 2. SK&F 104856 as well as prazosin and SK&F 86466 reduced blood pressure in the anaesthetized normotensive dog. 3. SK&F 86466 and rauwolscine but not SK&F 104856 or prazosin, produced a marked increase in myocardial contractility which corresponds with their ability to block prejunctional alpha 2-adrenoceptors. 4. Intravenous or oral administration of SK&F 104856 resulted in dose-dependent antihypertensive responses in 1-kidney, 1-clip (1-K, 1-C) Goldblatt hypertensive dogs with baseline blood pressure of approximately 140 mmHg. At 0.1 and 1 mg kg-1, i.v., mean arterial blood pressure fell by 11 +/- 5 and 23 +/- 5 mmHg, respectively. At 3 and 10 mg kg-1, p.o., blood pressure fell by 9 +/- 3 and 22 +/- 5 mmHg, respectively. At 10 mg kg-1, p.o., the antihypertensive effect of SK&F 104856 was still evident at 4 h. 5. The data indicate that SK&F 104856 shows selectivity in vivo for postjunctional versus prejunctional alpha-adrenoceptors and is a potent and long-acting antihypertensive agent in 1-K, 1-C Goldblatt hypertensive dogs.

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.

Additional evidence for functional subclassification of alpha-2 adrenoceptors based on a new selective antagonist, SK&F 104856

SK&F 104856 (2-vinyl-7-chloro-3,4,5,6-tetrahydro-4-methyl-thieno[4,3,2ef][3] benzazepine) shows a similar selectivity profile to the previously reported alpha adrenoceptor antagonist, SK&F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4,5-tetrahydro-3- benzazepine), having the ability to block alpha-1 and postjunctional alpha-2 adrenoceptors, although having little or no activity at most prejunctional alpha-2 adrenoceptors. SK&F 104856 is more potent than SK&F 104078, and lacks the 5-hydroxytryptamine receptor antagonist activity associated with the earlier compound. The postjunctional vs. prejunctional selectivity of SK&F 104856 at alpha-2 adrenoceptors in the same tissue preparation was demonstrated in both canine and human saphenous vein. Concentrations substantially higher than those required to block postjunctional alpha-2 adrenoceptor-agonist induced vasoconstriction had no effect on the ability of norepinephrine, acting on prejunctional alpha-2 adrenoceptors, to inhibit stimulation evoked transmitter overflow in the human tissue, and only a small effect in the canine vein. As observed with SK&F 104078, SK&F 104856 has some prejunctional alpha-2 adrenoceptor antagonist activity in the rat vas deferens, although the receptor dissociation constant is nearly 50-fold higher than that at the postjunctional alpha-2 adrenoceptor in the canine saphenous vein. The results obtained with SK&F 104856 provide additional evidence to support the premise that alpha-2 adrenoceptors can be functionally differentiated. Because SK&F 104856 can selectively antagonize certain alpha-2 adrenoceptor-mediated responses, this agent may be a useful tool to evaluate the functional roles of the multiple alpha-2 adrenoceptor subtypes that have been identified in biochemical and molecular studies.

Lack of a pharmacological distinction between alpha-1 adrenoceptors mediating intracellular calcium-dependent and independent contractions to sympathetic nerve stimulation in the perfused rat caudal artery

The contractile response of the rat caudal artery to field stimulation occurred in two phases; an initial rapidly developing contraction (phasic) and a subsequent more slowly developing contraction (tonic). Phenoxybenzamine, an irreversible antagonist, and prazosin, a competitive antagonist at the alpha-1 adrenoceptor, nonselectively inhibited both contractile phases. Ryanodine, an inhibitor of intracellular calcium release, selectively inhibited the phasic component, whereas nifedipine, a membrane calcium channel blocker, selectively inhibited the tonic component. These data demonstrate that both contractile phases are mediated by alpha-1 adrenoceptors and the phases are distinguished by the calcium source mobilized; intracellular stores of calcium are mobilized during the phasic component and extracellular calcium is mobilized during the tonic component. Inasmuch as alpha-1 adrenoceptors on nonvascular smooth muscle may be divided into alpha-1A and alpha-1B subtypes based on ability to open membrane calcium channels or ability to stimulate intracellular calcium release, respectively, we characterized the alpha-1 adrenoceptor mediating the contraction to determine whether a specific subtype was coupled to each phase. WB4101, a marginally alpha-1A adrenoceptor selective antagonist, 5-methyl-urapidil, a highly alpha-1A adrenoceptor selective antagonist and chloroethylclonidine, a highly alpha-1B adrenoceptor selective antagonist, all inhibited both contractile phases. These findings indicate that endogenous norepinephrine contracts the rat caudal artery via alpha-1 adrenoceptors that mobilize both the release of intracellular calcium and the influx of calcium through membrane channels.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of alpha adrenoceptor subtypes in sympathetic control of the acral-cutaneous microcirculation

In pithed and anesthetized rats, laser-Doppler flowmetry was used to evaluate the role of alpha-1 and alpha-2 adrenoceptors in mediating sympathetic responses in acral regions of the cutaneous circulation. The intravenous administration of the selective alpha-1 agonist, phenylephrine, was a more potent vasopressor agent than BH-T 933 (a selective alpha-2 adrenoceptor agonist) in pithed rats. However, BH-T 933 was more potent and more efficacious than phenylephrine in reducing cutaneous microvascular perfusion (CP). BH-T 933 also caused a greater increase in cutaneous microvascular resistance. Neural and humoral sympathetic effects on CP were characterized with selective alpha-1 and alpha-2 adrenoceptor antagonists (prazosin and rauwolscine, respectively). It was found that frequency-related reductions in CP elicited by sciatic nerve stimulation were antagonized by prazosin, but not by rauwolscine. In fact, rauwolscine enhanced neurally evoked reductions in CP at the highest stimulation frequencies. However, both prazosin and rauwolscine antagonized reductions in CP elicited by electrical stimulation of the thoracolumbar outflow (sympathoadrenal activation). Ganglionic stimulation (intravenous 1,1-dimethyl-4-phenylpiperazinium) also caused a profound, transient reduction in CP that was abolished by rauwolscine, but was not significantly altered by prazosin. In contrast, 1,1-dimethyl-4-phenylpiperazinium-induced increases in mean arterial pressure were reduced by prazosin, but not by rauwolscine. In ketamine-anesthetized rats, rauwolscine caused a dose-related increase in CP without altering mean arterial pressure, whereas prazosin lowered mean arterial pressure but did not alter CP. We conclude that acral regions of the cutaneous vasculature are more sensitive to alpha-2 vis-a-vis alpha-1 adrenoceptor-mediated vasoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathetic regulation of cutaneous circulation in the rat

The purpose of the present study was to characterize, in vivo, the function and origin of peripheral alpha-adrenergic mechanisms in acral regions of the cutaneous microvasculature. Laser-Doppler flowmetry was used to continuously monitor changes in local cutaneous microvascular perfusion (CP) measured at the plantar surface of the terminal phalange in the rat. In ketamine-anesthetized rats, the intravenous administration of phentolamine (a nonselective alpha-adrenoceptor antagonist) elicited a dose-dependent (0.01-1.0 mg/kg, i.v.) increase in CP, a decrease in mean arterial pressure (MAP) and a marked reduction in cutaneous vascular resistance (CVR). These results demonstrate a high degree of endogenous alpha-adrenergic tone in the cutaneous microvasculature of the rat. In pithed rats, cutaneous vasoconstriction could be evoked by ganglionic stimulation with DMPP or by electrical stimulation of the peripheral cut end of the sciatic nerve trunk. DMPP and sciatic nerve responses were not altered by propranolol or atropine, but were inhibited by phentolamine. In addition, the effects of DMPP on CP were abolished by bilateral adrenal demedullation, but were unaltered by hindlimb denervation. These results suggest that the predominant vasoconstrictor tone in the cutaneous vasculature is mediated by a humoral action of circulating catecholamines at postjunctional alpha-adrenoceptors. The adrenal medulla appears to be the origin of this humoral tone. Postganglionic sympathetic nerves play an insignificant role in mediating cutaneous vasoconstriction elicited by ganglionic stimulation.

Evidence for heterogeneity of prejunctional alpha-2-adrenoceptors

The interactions between SK&F 104078 and several selective alpha 2-adrenoceptor agonists at pre- and postjunctional alpha 2-adrenoceptors were investigated in order to assess the previously reported selectivity of SK&F 104078 for postjunctional alpha 2-adrenoceptors and to determine whether or not SK&F 104078 could uncover subtypes of alpha 2-adrenoceptors located prejunctionally as has also been suggested. The alpha 2-adrenoceptor agonists, UK 14,304, xylazine, B-HT 933, B-HT 920, clonidine and M-7, produced concentration-dependent prejunctional alpha 2-adrenoceptor-mediated inhibition of neurogenic responses in the guinea pig atrium and rat vas deferens and produced postjunctional alpha 2-adrenoceptor-mediated contraction of the canine saphenous vein. The alpha 2-adrenoceptor antagonist, rauwolscine, blocked all the agonists at both pre- and postjunctional alpha 2-adrenoceptors without demonstrating preference for any agonist or any synaptic location of alpha 2-adrenoceptors. In marked contrast, SK&F 104078 produced equivalent antagonism of all agonists in the canine saphenous vein but had no significant effect against the same agonists in the guinea pig atrium, suggesting a high degree of selectivity for postjunctional alpha 2-adrenoceptors in these test systems, consistent with our previous observations. In the rat vas deferens, however, SK&F 104078 significantly antagonized the prejunctional alpha 2-adrenoceptor-mediated effects of clonidine and M-7 but did not block the responses to UK 14,304, xylazine, B-HT 933 and B-HT 920. These results indicate that the prejunctional alpha 2-adrenoceptor antagonist effects of SK&F 104078 are tissue and agonist dependent, and that there may be at least two subtypes of prejunctional alpha 2-adrenoceptors that can be discriminated with SK&F 104078 but not with rauwolscine. Both subtypes of prejunctional alpha 2-adrenoceptors may be present in the rat vas deferens, while only the SK&F-104078-insensitive subtype is present in the guinea pig atrium.

Metabolic regulation by alpha 1- and alpha 2-adrenoceptors

The role of alpha-adrenoceptors in the mediation of autonomic function, particularly in the control of the cardiovascular system, is widely known. However, alpha-adrenoceptors are also important in the regulation of a variety of metabolic processes that occur in the body either through direct action or by stimulation of the release of other mediators that control metabolic function. Thus, alpha 2-adrenoceptor activation by circulating or neuronally released catecholamines inhibits the release of insulin from pancreatic islet beta-cells and, by inhibiting this response, alpha 2-adrenoceptor antagonists have been shown to have an antihyperglycemic effect. The alpha-adrenoceptor-mediated regulation of the release of pituitary hormones is indirect, with alpha-adrenoceptors being located on peptidergic neurons in the hypothalamus that secrete releasing hormones into the hypophysial portal system to regulate the secretion of hormones from the anterior pituitary gland. Thus, the increase in cortisol secretion from the adrenal glands following a meal is produced, at least in part, by an alpha 1-adrenoceptor-mediated increase in vasopressin and CRF-41 secretion from neurons on the hypothalamus that stimulate the release of adrenocorticotrophic hormone secretion from the pituitary gland, which subsequently stimulates the synthesis and release of cortisol from the adrenal medulla. In addition to metabolic regulation by alpha 1- and alpha 2-adrenoceptors within the endocrine system, alpha-adrenoceptors are also a component of the system that regulates certain aspects of metabolism within autonomic effector cells, such as the control of smooth muscle cell division and growth during periods of continued alpha-adrenoceptor activation as a result of activation of second messenger systems.

Studies on the mechanism of neuropeptide Y induced potentiation of neurogenic vasoconstriction in the isolated rabbit ear artery

Neuropeptide Y (NPY) has been shown to potentiate the response of the isolated rabbit ear artery to field stimulation, provided that the vascular endothelium is intact. This report describes efforts to elucidate the mechanism by which NPY produces this effect. The response of the perfused rabbit ear artery to brief intermittent field stimulation was significantly enhanced by NPY (100 nmol/L). The administration of indomethacin (10 mumol/L) to inhibit cyclooxygenase did not affect the ability of NPY to potentiate neurogenic vasoconstriction. Blockade of calcium channels with nifedipine (1 mumol/L) produced a partial attenuation of the NPY effect. Our studies therefore suggest that an arachidonic acid metabolite is not involved in NPY induced potentiation of vascular neuroeffector transmission in the rabbit ear artery. On the other hand, the translocation of extracellular calcium may play a role in this process.

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.

Cardiovascular effects of SK&F 104078, a novel alpha-adrenoceptor antagonist, in normotensive and hypertensive rats

SK&F 104078 is a novel alpha-adrenoceptor antagonist derived from the 3-benzazepine alpha 2-adrenoceptor antagonist SK&F 86466. SK&F 104078 will block both alpha 1- and vascular postjunctional alpha 2-adrenoceptors but does not block most prejunctional alpha 2-adrenoceptors. Intravenous (i.v.) administration of SK&F 104078 decreased blood pressure (BP) in both spontaneously hypertensive and DOCA-salt hypertensive rats. SK&F 104078 potentiated the hypotensive response to tilt in anesthetized spontaneously hypertensive rats (SHR). Although SK&F 104078 had no effect on BP in normotensive rats, the tilt-induced decrease in BP in these animals was also potentiated. In this regard, SK&F 104078 resembled the selective alpha 1-adrenoceptor antagonist prazosin, rather than the alpha 2-adrenoceptor antagonists rauwolscine or SK&F 86466. Oral administration of SK&F 104078 had no significant effect on BP in SHR unless extremely high doses were administered. This was consistent with low plasma concentrations of SK&F 104078 observed after oral administration. After i.v. administration, the clearance of SK&F 104078 from plasma was 123 ml/min/kg, the steady-state volume of distribution was 17 L/kg, and the fraction excreted unchanged in urine was less than 1%. The low oral bioavailability of SK&F 104078 did not appear to be due to high first-pass oxidative metabolism, since pretreatment of SHR with the suicide substrate inhibitor of cytochrome P-450, 1-aminobenzotriazole (ABT), did not result in increased oral efficacy. SK&F 101253, a close structural analogue of SK&F 104078, was an effective antihypertensive when administered orally. Comparison of the stability of SK&F 101253 and SK&F 104078 in acid media showed SK&F 104078, but not SK&F 101253, to be rapidly degraded.(ABSTRACT TRUNCATED AT 250 WORDS)

The pharmacology of carvedilol

Carvedilol is a potent antihypertensive agent with a dual mechanism of action. At relatively low concentrations it is a competitive beta-adrenoceptor antagonist and a vasodilator, whereas at higher concentrations it is also a calcium channel antagonist. The antihypertensive activity of carvedilol is characterized by a decrease in peripheral vascular resistance, resulting from the vasodilator activity of the compound, with no reflex tachycardia, as a result of beta-adrenoceptor blockade. The antihypertensive activity of carvedilol is associated with an apparent "renal sparing" effect in that the reduction in mean arterial blood pressure does not compromise renal blood flow or urinary sodium excretion. Studies on the mechanism of action of carvedilol indicate that the compound is a potent competitive antagonist of beta 1- and beta 2-adrenoceptors with a dissociation constant (KB) of 0.9 nM at both beta-adrenoceptor subtypes. Carvedilol is also a potent alpha 1-adrenoceptor antagonist (KB = 11 nM), which accounts for most, if not all, of the vasodilating response produced by the compound. At concentrations above 1 microM, carvedilol is a calcium channel antagonist. This activity can be demonstrated in vivo at doses that represent the higher end of the antihypertensive dose-response curve. Although the calcium-channel blocking activity of carvedilol may not contribute to the antihypertensive activity of the compound, it may play a prominent role in certain peripheral vascular beds, such as the cutaneous circulation, where marked increases in blood flow are observed. The data indicate that carvedilol is an antihypertensive agent that is both a beta-adrenoceptor antagonist and a vasodilator.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of alpha-1 and alpha-2 adrenoceptors in the sympathetic control of the proximal urethra

In the pithed rat, postjunctional alpha-1 and alpha-2 adrenoceptors mediate increases in proximal urethral perfusion pressure (UPP). The present study examined the role of alpha-1 and alpha-2 adrenoceptors in sympathetic control of the isolated in situ proximal urethra of the rat. An endogenous alpha adrenergic constriction was demonstrated in the proximal urethra by eliciting dose-related and phentolamine-sensitive increases in UPP after the systemic administration of tyramine. Transient dose-related and hexamethonium-sensitive increases in UPP were also elicited by ganglionic stimulation with 1,1-dimethyl-4-phenylpiperazinium (DMPP). Based on the relative sensitivity of the DMPP response to standard autonomic blockers, it was determined that constriction is the predominant autonomic response in the proximal urethra and this response is mediated by alpha adrenoceptor mechanisms. Prazosin, a selective alpha-1 adrenoceptor antagonist, substantially reduced (72%) the steady-state increases in UPP elicited by discrete low frequency electrical stimulation of the spinal hypogastric outflow. In contrast, selective alpha-2 adrenoceptor blockade with rauwolscine significantly increased (28%) the UPP response to hypogastric stimulation. Prazosin also abolished the increase in urethral tone elicited by tyramine, whereas rauwolscine had no effect on the tyramine. A different response profile was observed for prazosin and rauwolscine when the steady-state increase in UPP was evoked by simultaneously stimulating the midthoracic and hypogastric spinal outflow. Under these conditions of sympathoadrenal activation, UPP was reduced by both prazosin and rauwolscine (63 and 50%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)