Human cloned alpha1A-adrenoceptor isoforms display alpha1L-adrenoceptor pharmacology in functional studies

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

Alternative Splicing of G Protein-Coupled Receptors: Relevance to Pain Management

Drugs that target G protein-coupled receptors (GPCRs) represent the primary treatment strategy for patients with acute and chronic pain; however, there is substantial individual variability in both the efficacy and adverse effects associated with these drugs. Variability in drug responses is due, in part, to individuals' diversity in alternative splicing of pain-relevant GPCRs. G protein-coupled receptor alternative splice variants often exhibit distinct tissue distribution patterns, drug-binding properties, and signaling characteristics that may impact disease pathology as well as the extent and direction of analgesic effects. We review the importance of GPCRs and their known splice variants to the management of pain.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Stimulation of α1a adrenergic receptors induces cellular proliferation or antiproliferative hypertrophy dependent solely on agonist concentration

Stimulation of α_1aAdrenergic Receptors (ARs) is known to have anti-proliferative and hypertrophic effects; however, some studies also suggests this receptor can increase cell proliferation. Surprisingly, we find the α_1aAR expressed in rat-1 fibroblasts can produce either phenotype, depending exclusively on agonist concentration. Stimulation of the α_1aAR by high dose phenylephrine (>10⁻⁷ M) induces an antiproliferative, hypertrophic response accompanied by robust and extended p38 activation. Inhibition of p38 with SB203580 prevented the antiproliferative response, while inhibition of Erk or Jnk had no effect. In stark contrast, stimulation of the α_1aAR with low dose phenylephrine (∼10⁻⁸ M) induced an Erk-dependent increase in cellular proliferation. Agonist-induced Erk phosphorylation was preceded by rapid FGFR and EGFR transactivation; however, only EGFR inhibition blocked Erk activation and proliferation. The general matrix metalloprotease inhibitor, GM6001, blocked agonist induced Erk activation within seconds, strongly suggesting EGFR activation involved extracellular triple membrane pass signaling. Erk activation required little Ca²⁺ release and was blocked by PLCβ or PKC inhibition but not by intracellular Ca²⁺ chelation, suggesting Ca²⁺ independent activation of novel PKC isoforms. In contrast, Ca²⁺ release was essential for PI3K/Akt activation, which was acutely maximal at non-proliferative doses of agonist. Remarkably, our data suggests EGFR transactivation leading to Erk induced proliferation has the lowest activation threshold of any α_1aAR response. The ability of α_1aARs to induce proliferation are discussed in light of evidence suggesting antagonistic growth responses reflect native α_1aAR function.

Effects of the noradrenergic neurotoxin DSP-4 on the expression of α1-adrenoceptor subtypes after antidepressant treatment

We have previously reported that chronic imipramine and electroconvulsive treatments increase the α(1A)-adrenoceptor (but not the α(1B) subtype) mRNA level and the receptor density in the rat cerebral cortex. Furthermore, we have also shown that chronic treatment with citalopram does not affect the expression of either the α(1A)- or the α(1B)-adrenoceptor, indicating that the previously observed up-regulation of α(1A)-adrenoceptor may depend on the noradrenergic component of the pharmacological mechanism of action of these antidepressants. Here, we report that previous noradrenergic depletion with DSP-4 (50 mg/kg) (a neurotoxin selective for the noradrenergic nerve terminals) significantly attenuated the increase of α(1A)-adrenoceptor mRNA induced by a 14-day treatment with imipramine (IMI, 20 mg/kg, ip) and abolished the effect of electroconvulsive shock (ECS, 150 mA, 0.5 s) in the prefrontal cortex of the rat brain. The changes in the receptor protein expression (as reflected by its density) that were induced by IMI and ECS treatments were differently modulated by DSP-4 lesioning, and only the ECS-induced increase in α(1A)-adrenoceptor level was abolished. This study provides further evidence corroborating our initial hypothesis that the noradrenergic component of the action of antidepressant agents plays an essential role in the modulation of α(1A)-adrenoceptor in the rat cerebral cortex.

Phenotype pharmacology of lower urinary tract α(1)-adrenoceptors

α(1)-Adrenoceptors are involved in numerous physiological functions, including micturition. However, the pharmacological profile of the α(1)-adrenoceptor subtypes remains controversial. Here, we review the literature regarding α(1)-adrenoceptors in the lower urinary tract from the standpoint of α(1L) phenotype pharmacology. Among three α(1)-adrenoceptor subtypes (α(1A), α(1B) and α(1D)), α(1a)-adrenoceptor mRNA is the most abundantly transcribed in the prostate, urethra and bladder neck of many species, including humans. In prostate homogenates or membrane preparations, α(1A)-adrenoceptors with high affinity for prazosin have been detected as radioligand binding sites. Functional α(1)-adrenoceptors in the prostate, urethra and bladder neck have low affinity for prazosin, suggesting the presence of an atypical α(1)-adrenoceptor phenotype (designated as α(1L)). The α(1L)-adrenoceptor occurs as a distinct binding entity from the α(1A)-adrenoceptor in intact segments of variety of tissues including prostate. Both the α(1L)- and α(1A)-adrenoceptors are specifically absent from Adra1A (α(1a)) gene-knockout mice. Transfection of α(1a)-adrenoceptor cDNA predominantly expresses α(1A)-phenotype in several cultured cell lines. However, in CHO cells, such transfection expresses α(1L)- and α(1A)-phenotypes. Under intact cell conditions, the α(1L)-phenotype is predominant when co-expressed with the receptor interacting protein, CRELD1α. In summary, recent pharmacological studies reveal that two distinct α(1)-adrenoceptor phenotypes (α(1A) and α(1L)) originate from a single Adra1A (α(1a)-adrenoceptor) gene, but adrenergic contractions in the lower urinary tract are predominantly mediated via the α(1L)-adrenoceptor. From the standpoint of phenotype pharmacology, it is likely that phenotype-based subtypes such as the α(1L)-adrenoceptor will become new targets for drug development and pharmacotherapy.

Contractility and pacemaker cells in the prostate gland

PURPOSE

We focused on the current opinion on mechanisms generating stromal tone in the prostate gland.

MATERIALS AND METHODS

We selected the guinea pig as the main species for investigation since its prostate has a high proportion of smooth muscle that undergoes age related changes similar in many respects to that in humans. The main techniques that we used were tension recording and electrophysiology.

RESULTS

We previously reported distinct electrical activity and cell types in the prostate, and speculated on their functional roles. We believe that a specialized group of c-kit immunoreactive prostatic interstitial cells that lie between glandular epithelium and smooth muscle stroma have a role similar to that of gastrointestinal interstitial cells of Cajal, generating the pacemaker signal that manifests as slow wave activity and triggers contraction in smooth muscle cells in guinea pig prostates.

CONCLUSIONS

Since changes in muscle tone are involved in the etiology of age dependent prostate specific conditions such as benign prostatic hyperplasia, knowledge of the electrical properties of the various prostatic cell types and their interactions with each other, with nerves and with the hormonal environment, and how these factors change with age is of considerable medical importance.

Subtypes of functional alpha1-adrenoceptor

In this review, subtypes of functional alpha1-adrenoceptor are discussed. These are cell membrane receptors, belonging to the seven-transmembrane-spanning G-protein-linked family of receptors, which respond to the physiological agonist noradrenaline. alpha1-Adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A fourth alpha1-adrenoceptor, the alpha1L-, represents a functional phenotype of the alpha1A-adrenoceptor. alpha1-Adrenoceptor subtype knock-out mice have refined our knowledge of the functions of alpha-adrenoceptor subtypes, particuarly as subtype-selective agonists and antagonists are not available for all subtypes. alpha1-Adrenoceptors function as stimulatory receptors involved particularly in smooth muscle contraction, especially contraction of vascular smooth muscle, both in local vasoconstriction and in the control of blood pressure and temperature, and contraction of the prostate and bladder neck. Central actions are now being elucidated.

Expression and function of G-protein-coupled receptorsin the male reproductive tract

This review focuses on the expression and function of muscarinic acetylcholine receptors (mAChRs), α1-adrenoceptors and relaxin receptors in the male reproductive tract. The localization and differential expression of mAChR and α1-adrenoceptor subtypes in specific compartments of the efferent ductules, epididymis, vas deferens, seminal vesicle and prostate of various species indicate a role for these receptors in the modulation of luminal fluid composition and smooth muscle contraction, including effects on male fertility. Furthermore, the activation of mAChRs induces transactivation of the epidermal growth factor receptor (EGFR) and the Sertoli cell proliferation. The relaxin receptors are present in the testis, RXFP1 in elongated spermatids and Sertoli cells from rat, and RXFP2 in Leydig and germ cells from rat and human, suggesting a role for these receptors in the spermatogenic process. The localization of both receptors in the apical portion of epithelial cells and smooth muscle layers of the vas deferens suggests an involvement of these receptors in the contraction and regulation of secretion.

Alpha 1-adrenoceptor pharmacome: alpha 1L-adrenoceptor and alpha 1A-adrenoceptor in the lower urinary tract

Alpha(1)-adrenoceptors are involved in physiological functions such as urinary excretion and ejaculation in the lower urinary tract (LUT). Several alpha(1) antagonists are clinically used for the treatment of urinary obstruction in patients with benign prostatic hyperplasia. At present, three classical alpha(1)-adrenoceptor subtypes (alpha(1A), alpha(1B), and alpha(1D)) have been identified, among which the alpha(1A) and alpha(1D)-adrenoceptor subtypes have been regarded as the main targets of alpha(1) antagonist therapy for LUT symptoms. Prazosin has been used as a prototypic, classical antagonist, to characterize alpha(1)-adrenoceptors pharmacologically, (i.e. all classical alpha(1)-adrenoceptor subtypes show high-affinity for the drug). However, we found that alpha(1)-adrenoceptors in the LUT show atypical low-affinity for prazosin. Therefore, the concept alpha(1L)-receptor, which indicates alpha(1)-adrenoceptor(s) showing low-affinity for prazosin has been introduced. A recent study demonstrated that the alpha(1L)-adrenoceptor is a specific phenotype present in the many intact tissues including human LUT, and that it originates from the ADRA1A gene. Therefore, the alpha(1L)-adrenoceptor in the LUT is now re-defined as alpha(1A(L))-adrenoceptor. The physiological and pharmacological difference between classical alpha(1A(H),) and alpha(1A(L)) which is the native receptor expressed in the LUT is of special interest as it provides fundamental bases for urological alpha(1A)-adrenoceptor blocking pharmacotherapy. Here, we briefly review the alpha(1)-adrenoceptors in the LUT with special reference to phenotype-based (pharmacome) analysis.

Identification of alpha 1L-adrenoceptor in mice and its abolition by alpha 1A-adrenoceptor gene knockout

BACKGROUND AND PURPOSE

The alpha(1L)-adrenoceptor has pharmacological properties that distinguish it from three classical alpha(1)-adrenoceptors (alpha(1A), alpha(1B) and alpha(1D)). The purpose of this was to identify alpha(1L)-adrenoceptors in mice and to examine their relationship to classical alpha(1)-adrenoceptors.

EXPERIMENTAL APPROACH

Radioligand binding and functional bioassay experiments were performed on the cerebral cortex, vas deferens and prostate of wild-type (WT) and alpha(1A)-, alpha(1B)- and alpha(1D)-adrenoceptor gene knockout (AKO, BKO and DKO) mice.

KEY RESULTS

The radioligand [(3)H]-silodosin bound to intact segments of the cerebral cortex, vas deferens and prostate of WT, BKO and DKO but not of AKO mice. The binding sites were composed of two components with high and low affinities for prazosin or RS-17053, indicating the pharmacological profiles of alpha(1A)-adrenoceptors and alpha(1L)-adrenoceptors. In membrane preparations of WT mouse cortex, however, [(3)H]-silodosin bound to a single population of prazosin high-affinity sites, suggesting the presence of alpha(1A)-adrenoceptors alone. In contrast, [(3)H]-prazosin bound to two components having alpha(1A)-adrenoceptor and alpha(1B)-adrenoceptor profiles in intact segments of WT and DKO mouse cortices, but AKO mice lacked alpha(1A)-adrenoceptor profiles and BKO mice lacked alpha(1B)-adrenoceptor profiles. Noradrenaline produced contractions through alpha(1L)-adrenoceptors with low affinity for prazosin in the vas deferens and prostate of WT, BKO and DKO mice. However, the contractions were abolished or markedly attenuated in AKO mice.

CONCLUSIONS AND IMPLICATIONS

alpha(1L)-Adrenoceptors were identified as binding and functional entities in WT, BKO and DKO mice but not in AKO mice, suggesting that the alpha(1L)-adrenoceptor is one phenotype derived from the alpha(1A)-adrenoceptor gene.

Native profiles of alpha(1A)-adrenoceptor phenotypes in rabbit prostate

BACKGROUND AND PURPOSE

alpha(1)-Adrenoceptors in the rabbit prostate have been studied because of their controversial pharmacological profiles in functional and radioligand binding studies. The purpose of the present study is to determine the native profiles of alpha(1)-adrenoceptor phenotypes and to clarify their relationship.

EXPERIMENTAL APPROACH

Binding experiments with [3H]-silodosin and [3H]-prazosin were performed using intact tissue segments and crude membrane preparations of rabbit prostate and the results were compared with alpha(1)-adrenoceptor-mediated prostate contraction.

KEY RESULTS

[3H]-Silodosin at subnanomolar concentrations bound specifically to intact tissue segments of rabbit prostate. However, [3H]-prazosin at the same range of concentrations failed to bind to alpha(1)-adrenoceptors of intact segments. Binding sites of [3H]-silodosin in intact segments were composed of alpha(1L) phenotype with low affinities for prazosin (pKi=7.1), 5-methyurapidil and N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethamine hydrochloride (RS-17053), and alpha(1A)-like phenotype with moderate affinity for prazosin (pKi=8.8) but high affinity for 5-methyurapidil and RS-17053. In contrast, both radioligands bound to a single population of alpha(1)-adrenoceptors in the membrane preparations at the same density with a subnanomolar affinity, showing a typical profile of 'classical' alpha(1A)-adrenoceptors (pKi for prazosin=9.8). The pharmacological profile of alpha(1)-adrenoceptor-mediated prostate contraction was in accord with the alpha(1L) phenotype observed by intact segment binding approach.

CONCLUSIONS AND IMPLICATIONS

Three distinct phenotypes (alpha(1L) and alpha(1A)-like phenotypes in the intact segments and a classical alpha(1A) phenotype in the membranes) with different affinities for prazosin were detected in rabbit prostate. It appears that the three phenotypes are phenotypic subtypes of alpha(1A)-adrenoceptors, but are not genetically different subtypes.

The alpha1A-adrenoceptor gene is required for the alpha1L-adrenoceptor-mediated response in isolated preparations of the mouse prostate

BACKGROUND AND PURPOSE

This study investigated whether deletion of the alpha1A-adrenoceptor gene influences contractile responses of mouse prostate to noradrenaline. Responses of mouse prostate to noradrenaline are known to be mediated by alpha1L-adrenoceptors, which are thought to be a functional phenotype of alpha1A-adrenoceptor.

EXPERIMENTAL APPROACH

Prostate tissues from alpha1A-adrenoceptor knockout mice which were homozygous (alpha1A -/-) and heterozygous (alpha1A +/-) for the disrupted alpha1A-adrenoceptor gene, as well as wild-type (alpha1A +/+) littermates were mounted in glass-isolated organ baths. Electrical field stimulation of nerves and exogenous application of noradrenaline were used to investigate the effects of alpha1A-adrenoceptor disruption on prostate contractility.

KEY RESULTS

Frequency-response curves to electrical field stimulation (0.5 ms pulse duration, 60 V, 0.1-20 Hz) yielded frequency-dependent contractions. At frequencies of 10 and 20 Hz, prostates from alpha1A -/- mice elicited an approximately 30% decreased response compared with prostates from alpha(1A)+/+ mice. Prazosin (0.3 muM) attenuated responses to electrical field stimulation in prostates from alpha1A +/+ and alpha1A +/- mice but not from alpha1A -/- mice. Increasing concentrations of exogenously administered noradrenaline (10 nM-1 mM) produced mean concentration-response curves in prostates from alpha1A +/+ and alpha1A +/- mice, which were not different. Maximum responses to noradrenaline were decreased by approximately 80% in prostates from alpha1A -/- mice compared with alpha1A +/+ mice. Prazosin attenuated responses to noradrenaline in all genotypes.

CONCLUSIONS AND IMPLICATIONS

alpha1L-Adrenoceptor-mediated responses in mouse prostate are abolished in alpha1A -/- mice, demonstrating that the alpha1A-adrenoceptor gene is essential to the manifestation of the prostatic alpha1L-adrenoceptor phenotype. This implies that alpha1L-adrenoceptors are indeed a functional phenotype of alpha1A-adrenoceptor.

Use of Cryopreserved Cells for Enabling Greater Flexibility in Compound Profiling

Measurement of intracellular calcium release following agonist challenge within cells expressing the relevant membrane protein is a commonly used format to derive structure-activity relationship (SAR) data within a compound profiling assay. The Fluorometric Imaging Plate Reader (FLIPR) has become the gold standard for this purpose. FLIPR traditionally uses cells that are maintained in continuous culture for compound profiling of iterative chemistry campaigns. This supply dictates that assays can only be run on 4 of 5 weekdays, or alternative cell culture machinery is required such that plating can occur remotely at the weekend. The data reported here demonstrate that high-quality compound profiling data can be generated from the use of cryopreserved cells and that these cells can also be plated at various densities to generate equivalent data between 24 and 72 h post-plating. Hence, the authors report a method that allows data generation throughout the week and without the requirement of highly automated cell culture or continuous culture. ( Journal of Biomolecular Screening 2008:354-362)

Immunolocalization of alpha(1A)-adrenoceptors in rat and human epididymis

Immunohistochemistry was conducted to analyze the cellular localization of alpha(1A)-adrenoceptors along rat and human epididymis. ADR-A, a polyclonal antibody that recognizes the specific C-terminal region of alpha(1A)-adrenoceptors, immunostained this adrenoceptor subtype in smooth muscle cells surrounding the epididymal tubules and interstitial blood vessels and in subpopulations of epithelial cells from adult rat and human caput and cauda epididymidis. The same cell types from rat epididymidis were immunostained by ADR-1, a polyclonal antibody that recognizes a common region of the three alpha(1)-adrenoceptor subtypes, alpha(1A), alpha(1B), and alpha(1D). Immunostaining with both antibodies was also conducted in adult rat and human vas deferens and seminal vesicle used as positive controls because of the abundance of alpha(1A)-adrenoceptors in these tissues. ADR-A- and ADR-1-positive immunostaining was differentially distributed depending on the antibody, method of tissue fixation (Bouin-fixed and fresh frozen tissues), species (rat and human), tissue (caput and cauda epididymidis), and age (immature and adult rats) analyzed. This is the first report immunolocalizing alpha(1A)-adrenoceptor along rat and human epididymis. The presence of this adrenoceptor subtype in epididymal smooth muscle and epithelial cells indicates their contribution to smooth muscle contractile responses and a possible role in the absorptive and/or secretory activities of the epithelium lining the epididymal duct. Taken together, our results should contribute to a better understanding of the physiological role of alpha(1)-adrenoceptors in the epididymidis and the importance of the sympathetic nervous system for male (in)fertility.

Aminocyclohexylsulfonamides: Discovery of metabolically stable α1a/1d-selective adrenergic receptor antagonists for the treatment of benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS)

Benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS) can be effectively treated by alpha(1) adrenergic receptor antagonists, but these drugs also produce side effects that are related to their subtype non-selective nature. To overcome this limitation, it was hypothesized that an alpha(1a/1d) subtype-selective antagonist would be efficacious while keeping side effects to a minimum. To discover alpha(1a/1d)-selective antagonists and improve metabolic stability of our previously reported compounds, we have designed and synthesized a series of (phenylpiperazinyl)- or (phenylpiperidinyl)-cyclohexylsulfonamides. By incorporating the information obtained from metabolism studies, we were able to discover several compounds that are both alpha(1a/1d) adrenoceptor subtype selective and show increased stability toward human liver microsomal metabolism. The selectivity profile of these compounds provides great improvement over the commercial drug tamsulosin, hence may pave the way to the development of new and efficacious therapeutic agents with reduced side effects.

Multiple GPCR conformations and signalling pathways: implications for antagonist affinity estimates

Antagonist affinity measurements have traditionally been considered important in characterizing the cell-surface receptors present in a particular cell or tissue. A central assumption has been that antagonist affinity is constant for a given receptor-antagonist interaction, regardless of the agonist used to stimulate that receptor or the downstream response that is measured. As a consequence, changes in antagonist affinity values have been taken as initial evidence for the presence of novel receptor subtypes. Emerging evidence suggests, however, that receptors can possess multiple binding sites and the same receptor can show different antagonist affinity measurements under distinct experimental conditions. Here, we discuss several mechanisms by which antagonists have different affinities for the same receptor as a consequence of allosterism, coupling to different G proteins, multiple (but non-interacting) receptor sites, and signal-pathway-dependent pharmacology (where the pharmacology observed varies depending on the signalling pathway measured).

(Phenylpiperidinyl)cyclohexylsulfonamides: Development of α1a/1d-selective adrenergic receptor antagonists for the treatment of benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS)

Although alpha(1) adrenergic receptor blockers can be very effective for the treatment of benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS), their usage is limited by CV-related side-effects that are caused by the subtype non-selective nature of the current drugs. To overcome this problem, it was hypothesized that a alpha(1a/1d) subtype selective antagonist would bring more benefit for the therapy of BPH/LUTS. In developing such selective alpha(1a/1d) ligands, a series of (phenylpiperidinyl)cyclohexylsulfonamides has been synthesized and evaluated for binding to three cloned human alpha(1)-adrenergic receptor subtypes. Many compounds showed equal affinity for both alpha(1a) and alpha(1d) subtypes with good selectivity versus the alpha(1b) subtype.

Pharmacology of α1-adrenoceptor antagonists in the lower urinary tract and central nervous system

The main use of alpha(1)-adrenoceptor (AR) antagonists in urology has been to treat lower urinary tract symptoms (LUTS) in men with benign prostatic obstruction (BPO). The beneficial effects of these agents are primarily assumed to be because of relaxation of prostatic and urethral smooth muscle. The weak correlation between LUTS and prostatic enlargement, outflow obstruction, or both, however, has refocused interest on the role of extraprostatic alpha-ARs in the pathogenesis of LUTS and their treatment. The alpha(1)-ARs present in the bladder, urethra, vas deferens, peripheral ganglia, nerve terminals, and in the central nervous system could all potentially influence LUTS and, when the receptors are blocked, contribute to both the therapeutic and adverse effects of alpha(1)-AR antagonists. The relevance of alpha(1)-AR-subtype selectivity on the clinical usefulness of existing drug therapies has not been firmly established but it seems that blockade of both alpha(1A/L)- and alpha(1D)-ARs is necessary for the optimum balance between clinical efficacy and adverse effects.

(Arylpiperazinyl)cyclohexylsufonamides: Discovery of α1a/1d-selective adrenergic receptor antagonists for the treatment of Benign Prostatic Hyperplasia/Lower Urinary Tract Symptoms (BPH/LUTS)

Benign Prostatic Hyperplasia/Lower Urinary Tract Symptoms (BPH/LUTS) can be effectively treated by alpha(1)-adrenergic receptor antagonists. Unfortunately, all currently marketed alpha(1) blockers produced CV related side effects that are caused by the subtype non-selective nature of the drugs. To overcome this problem, it was postulated that a alpha(1a/1d) subtype selective antagonist would bring more benefit for the treatment of BPH/LUTS. In developing selective alpha(1a/1d) ligands, (arylpiperazinyl)cyclohexylsulfonamides were synthesized and their binding profiles against three cloned human alpha(1)-adrenergic receptor subtypes were evaluated. Many compounds show equal affinity for both alpha(1a) and alpha(1d) subtypes with good selectivity against the alpha(1b) subtype. They also overcome the problem of dopamine receptor affinity that previous analogues had exhibited.

“Phenotypic” pharmacology: The influence of cellular environment on G protein-coupled receptor antagonist and inverse agonist pharmacology

A central dogma of G protein-coupled receptor (GPCR) pharmacology has been the concept that unlike agonists, antagonist ligands display equivalent affinities for a given receptor, regardless of the cellular environment in which the affinity is assayed. Indeed, the widespread use of antagonist pharmacology in the classification of receptor expression profiles in vivo has relied upon this 'antagonist assumption'. However, emerging evidence suggests that the same gene-product may exhibit different antagonist pharmacological profiles, depending upon the cellular context in which it is expressed-so-called 'phenotypic' profiles. In this commentary, we review the evidence relating to some specific examples, focusing on adrenergic and muscarinic acetylcholine receptor systems, where GPCR antagonist/inverse agonist pharmacology has been demonstrated to be cell- or tissue-dependent, before going on to examine some of the ways in which the cellular environment might modulate receptor pharmacology. In the majority of cases, the cellular factors responsible for generating phenotypic profiles are unknown, but there is substantial evidence that factors, including post-transcriptional modifications, receptor oligomerization and constitutive receptor activity, can influence GPCR pharmacology and these concepts are discussed in relation to antagonist phenotypic profiles. A better molecular understanding of the impact of cell background on GPCR antagonist pharmacology is likely to provide previously unrealized opportunities to achieve greater specificity in new drug discovery candidates.

Alpha1-, alpha2- and beta-adrenoceptors in the urinary bladder, urethra and prostate

1 We have systematically reviewed the presence, functional responses and regulation of alpha(1)-, alpha(2)- and beta-adrenoceptors in the bladder, urethra and prostate, with special emphasis on human tissues and receptor subtypes. 2 Alpha(1)-adrenoceptors are only poorly expressed and play a limited functional role in the detrusor. Alpha(1)-adrenoceptors, particularly their alpha(1A)-subtype, show a more pronounced expression and promote contraction of the bladder neck, urethra and prostate to enhance bladder outlet resistance, particularly in elderly men with enlarged prostates. Alpha(1)-adrenoceptor agonists are important in the treatment of symptoms of benign prostatic hyperplasia, but their beneficial effects may involve receptors within and outside the prostate. 3 Alpha(2)-adrenoceptors, mainly their alpha(2A)-subtype, are expressed in bladder, urethra and prostate. They mediate pre-junctional inhibition of neurotransmitter release and also a weak contractile effect in the urethra of some species, but not humans. Their overall post-junctional function in the lower urinary tract remains largely unclear. 4 Beta-adrenoceptors mediate relaxation of smooth muscle in the bladder, urethra and prostate. The available tools have limited the unequivocal identification of receptor subtypes at the protein and functional levels, but it appears that the beta(3)- and beta(2)-subtypes are important in the human bladder and urethra, respectively. Beta(3)-adrenoceptor agonists are promising drug candidates for the treatment of the overactive bladder. 5 We propose that the overall function of adrenoceptors in the lower urinary tract is to promote urinary continence. Further elucidation of the functional roles of their subtypes will help a better understanding of voiding dysfunction and its treatment.

Signal transduction and regulation: are all alpha1-adrenergic receptor subtypes created equal?

The current manuscript reviews the evidence whether and how subtypes of alpha(1)-adrenergic receptors, i.e. alpha(1A)-, alpha(1B)- and alpha(1D)-adrenergic receptors, differentially couple to signal transduction pathways and exhibit differential susceptibility to regulation. In both regards studies in tissues or cells natively expressing the subtypes are hampered because the relative expression of the subtypes is poorly controlled and the observed effects may be cell-type specific. An alternative approach, i.e. transfection of multiple subtypes into the same host cell line overcomes this limitation, but it often remains unclear whether results in such artificial systems are representative for the physiological situation. The overall evidence suggests that indeed subtype-intrinsic and cell type-specific factors interact to direct alpha(1)-adrenergic receptor signaling and regulation. This may explain why so many apparently controversial findings have been reported from various tissues and cells. One of the few consistent themes is that alpha(1D)-adrenergic receptors signal less effectively upon agonist stimulation than the other subtypes, most likely because they exhibit spontaneous internalization.

α1L-Adrenoceptors mediate contractions of the isolated mouse prostate

The subtype of alpha1-adrenoceptor mediating noradrenaline-induced contractile responses in isolated mouse prostate glands was investigated. Adrenoceptor agonists were able to produce concentration-dependent contractions with the following rank order of potency: adrenaline > or = noradrenaline > or = clonidine = phenylephrine > dopamine > or = isoprenaline. Concentration-response curves to noradrenaline of the prostatic smooth muscle were antagonised by prazosin, N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha, alpha-dimethyl-1H-indole-3-ethanamine (RS-17053), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), tamsulosin and yohimbine with mean antagonist affinity estimates (pA2 or apparent pKB) of 8.12+/-0.10, 6.56+/-0.11, 8.38+/-0.06, 10.14+/-0.19 and 7.38+/-1.36 respectively. Propranolol (1 microM) had no antagonist activity (P = 0.994, n = 6). Yohimbine (0.01, 0.1, 1 microM) had no antagonist activity in the presence of prazosin (0.1 microM) (P > or = 0.059). The results obtained indicate that alpha1-adrenoceptors mediate the contractile response in isolated preparations of the mouse prostate. Furthermore, the particular subtype of alpha1-adrenoceptor mediating the response to exogenously administered noradrenaline corresponds to the alpha1L-subtype, the same subtype as that which has been shown to mediate noradrenaline-induced contractile activity in the human prostate.

Design and Synthesis of anα1a-Adrenergic Receptor Subtype-Selective Antagonist from BE2254

An alpha1a-adrenoceptor-selective antagonist has the potential to be a new benign prostatic hyperplasia drug with reduced side-effects. Modification of the non-selective antagonist BE2254 led to the development of a series of tetralin analogs. Evaluation of these compounds in cloned human alpha1-adrenoceptors resulted in the discovery of an analog that showed selectivity toward the human alpha1a-adrenergic receptor subtype. The compound also showed moderate potency to block human prostate muscle contraction.

Structure-activity relationships in 1,4-benzodioxan-related compounds. 8.(1) {2-[2-(4-chlorobenzyloxy)phenoxy]ethyl}-[2-(2,6-dimethoxyphenoxy)ethyl]amine (clopenphendioxan) as a tool to highlight the involvement of alpha1D- and alpha1B-adrenoreceptor subtypes in the regulation of human PC-3 prostate cancer cell apoptosis and proliferation

A series of new alpha1-adrenoreceptor antagonists (5-18) was prepared by introducing various substituents (Topliss approach) into the ortho, meta, and para positions of the benzyloxy function of the phendioxan open analogue 4 ("openphendioxan"). All the compounds synthesized were potent antagonists and generally displayed, similarly to 4, the highest affinity values at alpha1D- with respect to alpha1A- and alpha1B-AR subtypes and 5-HT1A subtype. By sulforhodamine B (SRB) assay on human PC-3 prostate cancer cells, the new compounds showed antitumor activity (estimated on the basis of three parameters GI50, TGI, LC50), at low micromolar concentration, with 7 ("clopenphendioxan") exhibiting the highest efficacy. Moreover, this study highlighted for the first time alpha1D- and alpha1B-AR expression in PC3 cells and also demonstrated the involvement of these subtypes in the modulation of apoptosis and cell proliferation. A significant reduction of alpha1D- and alpha1B-AR expression in PC3 cells was associated with the apoptosis induced by 7. This depletion was completely reversed by norepinephrine.

Combined use of alpha-adrenergic and muscarinic antagonists for the treatment of voiding dysfunction

PURPOSE

We provide an overview of the medical literature supporting the combined use of muscarinic and alpha-adrenergic antagonist therapy for the treatment of voiding dysfunction.

MATERIALS AND METHODS

The MEDLINE database (1966 to 2004) of the United States National Library of Medicine was searched for pertinent studies.

RESULTS

Although the mechanism of action of alpha-adrenergic antagonist therapy for voiding dysfunction has traditionally been assumed to be relaxation of the periurethral, prostatic and bladder neck smooth muscle, substantial evidence supports action at extraprostatic sites involved in micturition, including the bladder dome smooth muscle, peripheral ganglia, spinal cord and brain. Likewise the mechanism of action of anticholinergic therapy has been traditionally assumed to be inhibition of the M3 muscarinic receptor subtypes that mediate normal bladder contractions. However, M2 receptor mediates hypertrophied bladder contractions and there is evidence for an M2 component to the suprasacral control of voiding.

CONCLUSIONS

Based on the physiology of alpha-adrenergic and muscarinic receptors the inhibition of each one would be expected to be more beneficial than that of either alone because they would work on 2 components of detrusor function. Patients who would likely benefit from this combination therapy are men with lower urinary tract symptoms, women with urgency/frequency syndrome (overactive bladder), patients with uninhibited bladder contractions due to neurogenic bladder, and patients with pelvic pain and voiding symptoms, ie interstitial cystitis and chronic prostatitis/chronic pelvic pain syndrome.

Correlation between mRNA levels and functional role of alpha1-adrenoceptor subtypes in arteries: evidence of alpha1L as a functional isoform of the alpha1A-adrenoceptor

The mRNA levels for the three alpha1-adrenoceptor subtypes, alpha1A, alpha1B, and alpha1D, were quantified by real-time RT-PCR in arteries from Wistar rats. The alpha1D-adrenoceptor was prominent in both aorta (79.0%) and mesenteric artery (68.7%), alpha1A predominated in tail (61.7%) and small mesenteric artery (73.3%), and both alpha1A- and alpha1D-subtypes were expressed at similar levels in iliac artery. The mRNA levels of the alpha1B-subtype were a minority in all vessels (1.7-11.1%). Concentration-response curves of contraction in response to phenylephrine or relaxation in response to alpha1-adrenoceptor antagonists on maximal sustained contraction induced by phenylephrine were constructed from control vessels and vessels pretreated with 100 micromol/l chloroethylclonidine (CEC) for 30 min. The significant decrease in the phenylephrine potency observed after CEC treatment together with the inhibitory potency displayed by 8-{2-[4-(2-methoxyphenyl)-1-piperazinyl]-8-azaspiro (4,5) decane-7-dionedihydrochloride} (BMY-7378, an alpha1D-adrenoceptor antagonist) confirm the relevant role of alpha1D-adrenoceptors in aorta and iliac and proximal mesenteric arteries. The potency of 5-methylurapidil (an alpha1A-adrenoceptor antagonist) and the changes in the potency of both BMY-7378 and 5-methylurapidil after CEC treatment provided evidence of a mixed population of alpha1A- and alpha1D-adrenoceptors in iliac and distal mesenteric arteries. The low potency of prazosin (pIC50 < 9) as well as the high 5-methylurapidil potency in tail and small mesenteric arteries suggest the main role of alpha1A/alpha1L-adrenoceptors with minor participation of the alpha1D-subtype. The mRNA levels and CEC treatment corroborated this pattern and confirmed that the alpha1L-adrenoceptor could be a functional isoform of the alpha1A-subtype.

Alpha1 adrenoceptor subtypes in human urinary bladder: sex and regional comparison

A detailed study of the presence of alpha1 AR binding sites and alpha1 AR subtype mRNA expression in human urinary bladder areas involved in the micturition (i.e. detrusor, trigone and neck) is reported here, investigating whether or not there are differences between sexes. Results obtained indicated that alpha1 AR proteins were detectable in each bladder area. In both sexes, the detrusor and the neck expressed similar levels of alpha1 ARs: respectively, detrusor: 14.6 +/- 1.2 in men and 13.1 +/- 1.1 fmol/mg prot in women; neck: 16.9 +/- 3.2 in men and 17.5 +/- 4.1 fmol/mg prot in women. In the trigone, significantly higher alpha1ARs were found in women compared to men (20.6 +/- 1.1 vs 11.7 +/- 0.7 fmol/mg prot). Subtype analysis indicated that in women, each area was endowed with mRNA encoding for each alpha1 AR subtype. The men detrusor expressed alpha1a and alpha1d ARs, while in the trigone and the neck, each subtype was present. Since the detrusor muscle hypertrophy is a marker of bladder obstructive outlet, the selective alpha1 AR subtype targeting arouses much interest, as evidence indicates that there are differences in signalling pathways among the subtypes. Furthermore, the significance of the alpha1 ARs coexpression is still unknown; interestingly, recent papers demonstrate that alpha1 AR subtypes could dimerize. Thus, in the human urinary bladder it may be suggested a potential level of alpha1 AR complexity that could have an impact on drug development.

Pharmacology of the lower urinary tract: basis for current and future treatments of urinary incontinence

The lower urinary tract constitutes a functional unit controlled by a complex interplay between the central and peripheral nervous systems and local regulatory factors. In the adult, micturition is controlled by a spinobulbospinal reflex, which is under suprapontine control. Several central nervous system transmitters can modulate voiding, as well as, potentially, drugs affecting voiding; for example, noradrenaline, GABA, or dopamine receptors and mechanisms may be therapeutically useful. Peripherally, lower urinary tract function is dependent on the concerted action of the smooth and striated muscles of the urinary bladder, urethra, and periurethral region. Various neurotransmitters, including acetylcholine, noradrenaline, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in this neural regulation. Muscarinic receptors mediate normal bladder contraction as well as at least the main part of contraction in the overactive bladder. Disorders of micturition can roughly be classified as disturbances of storage or disturbances of emptying. Failure to store urine may lead to various forms of incontinence, the main forms of which are urge and stress incontinence. The etiology and pathophysiology of these disorders remain incompletely known, which is reflected in the fact that current drug treatment includes a relatively small number of more or less well-documented alternatives. Antimuscarinics are the main-stay of pharmacological treatment of the overactive bladder syndrome, which is characterized by urgency, frequency, and urge incontinence. Accepted drug treatments of stress incontinence are currently scarce, but new alternatives are emerging. New targets for control of micturition are being defined, but further research is needed to advance the pharmacological treatment of micturition disorders.

THE JANUS* FACES OF ADRENOCEPTORS: FACTORS CONTROLLING THE COUPLING OF ADRENOCEPTORS TO MULTIPLE SIGNAL TRANSDUCTION PATHWAYS

1. The adrenoceptors (AR) are an important subfamily of rhodopsin-like G-protein-coupled receptors that couple to an increasingly large number of signalling mechanisms. Two important factors that determine the pathways that are used are the C-terminal region of the receptor and the agonist used to activate the receptor. 2. Studies of splice variants of the mouse beta3-AR showed that the C-terminus is a factor controlling the signalling characteristics. Although these receptors differ only at the C-terminus, the beta3b-AR coupled to both Gs and Gi, whereas the beta3a-AR coupled solely to Gs. 3. Examination of four splice variants of the human alpha1A-AR showed that all were able to couple to pertussis toxin-sensitive G-proteins, even though they have radically different C-terminal regions. 4. Comparison of the effects of the beta3-AR ligands CL316243 and SR59230A showed that both can activate the mouse beta3-AR but that SR59230A uses pathways other than cAMP accumulation in 3T3-F442A cells. 5. Examination of a series of alpha1-AR agonists for their ability to activate a number of signalling pathways revealed that A61603 acted as a full agonist in all assays, whereas oxymetazoline was unable to cause cAMP accumulation, suggesting agonist-selective signalling at the human alpha1A-AR.

Effects of androgen manipulation on α1-adrenoceptor subtypes in the rat seminal vesicle

This study analyses possible changes during surgical and chemical castration in the expression and pharmacological characteristics of alpha1-adrenoceptor in adult rat seminal vesicle. Ribonuclease protection assays indicated that alpha1a--was the predominant mRNA, while alpha1b-and alpha1d-adrenoceptor transcripts were detected in lower abundance in this tissue. alpha1a-adrenoceptor mRNA expression presented a complex dependency on androgens, while alpha1b- and alpha1d-adrenoceptor transcripts were both upregulated with surgical and chemical castration, suggesting a negative modulation by androgens. Testosterone treatment reversed the effects caused by surgical castration. Functional studies confirmed the involvement of alpha1A- and alpha1B-adrenoceptor in the seminal vesicle contractile responses, and suggested that alpha1B-induced contractile response was upregulated after castration. Taken together, the results suggest that alpha1-adrenoceptor expression in seminal vesicle is differentially regulated by the androgen status of the rat.

Hepatocytes from alpha1B-adrenoceptor knockout mice reveal compensatory adrenoceptor subtype substitution

1 Alpha1-adrenoceptors (ARs) play an important functional role in the liver; yet little is known about their cellular location. We identified the subtypes present in wild-type (WT) and alpha1B-AR knockout (KO) mice livers at 3 and 4 months of age, and investigated their distribution in hepatocytes. 2 The fluorescent alpha1-AR antagonist quinazolinyl piperazine borate-dipyrromethene (QAPB) was used to visualise hepatic alpha1-ARs and radioligand binding with [3H]-prazosin was used to quantify the alpha1-AR population. 3 QAPB and [3H]-prazosin bound specifically to hepatic alpha1-ARs with nanomolar affinity. The cellular distribution of alpha1-ARs was similar in WT and alpha1B-AR KO hepatocytes; QAPB binding was distributed diffusely throughout the cell with no binding evident on the plasma membrane. Radioligand binding produced Bmax values as follows: 3-month WT - 76+/-3.3 fmol mg(-1); 4-month WT - 50+/-3.1 fmol mg(-1); 3-month alpha1B-AR KO - 7.4+/-0.73 fmol mg(-1); 4-month alpha1B-AR KO - 30+/-2.0 fmol mg(-1). 4 In 3- and 4-month WT liver, all antagonists acted competitively. RS100329 (alpha1A-selective) and BMY7378 (alpha1D-selective) bound with low affinities, indicating the presence of alpha1B-ARs. In 4-month alpha1B-AR KO liver prazosin produced a biphasic curve, whereas RS100329 and BMY7378 produced monophasic curves of high and low affinity, respectively, indicating the presence of alpha1A-ARs. 5 In conclusion, we have made the novel observation that alpha1-ARs can compensate for one another in the absence of the endogenously expressed receptor; yet there appears to be no subtype-specific subcellular location of alpha1-ARs; the WT livers express alpha1B-ARs, while alpha1B-AR KO livers express alpha1A-ARs. This study provides new insights into both hepatocyte and alpha1-AR biology.

Efficacy of Bunazosin Hydrochloride 0.01% as Adjunctive Therapy of Latanoprost or Timolol

PURPOSE

To evaluate the ocular hypotensive response of bunazosin hydrochloride 0.01% administered as adjunctive therapy in patients with glaucoma who were already receiving latanoprost 0.005% or timolol 0.5%.

METHODS

Patients with primary open angle glaucoma who had received latanoprost (n = 60) or timolol (n = 60) for 6 months or longer were enrolled and prospectively randomized to receive additional administration of bunazosin or placebo. One hundred twenty eyes of 120 patients were thus divided into 4 subgroups of 30 patients each. Bunazosin was administered twice daily, and timolol or latanoprost was administered per label. The patients were followed up for 3 months. Responders were defined as having a reduction in intraocular pressure of greater than 2 mm Hg from baseline.

RESULTS

Mean baseline intraocular pressure was 22.3 +/- 3.0 mm Hg in the bunazosin subgroup and 22.3 +/- 3.1 mm Hg in the placebo subgroup of the latanoprost arm, and 22.5 +/- 3.5 mm Hg in the bunazosin subgroup and 22.3 +/- 3.0 mm Hg in the placebo subgroup of the timolol arm. In the bunazosin subgroups of both arms, intraocular pressure was significantly reduced compared with baseline measurements (P < 0.05) with mean intraocular pressure measurement reductions of 2.1 +/- 2.4 mm Hg and 2.8 +/- 2.1 mm Hg in the latanoprost arm and 2.6 +/- 2.1 mm Hg and 2.8 +/- 2.1 mm Hg in the timolol arm at 6 and 12 weeks after the start of the follow-up, respectively. In the latanoprost group, bunazosin provided a further reduction of intraocular pressure (7.7%) at 12 weeks from that initially obtained at 2 weeks (P = 0.0377). In the placebo subgroups of the latanoprost and timolol arms, no significant change was found between at baseline and at any visit after the start of the follow-up. In the latanoprost and timolol arms, there was a significant difference in intraocular pressure and its change between the bunazosin subgroup and placebo subgroup at any visit after 4 weeks from the start of the follow-up (P < 0.01).

CONCLUSION

Bunazosin hydrochloride 0.01% may provide an additional intraocular pressure reduction in patients already receiving latanoprost or timolol. Since adding bunazosin to eyes treated with latanoprost caused a relatively small hypotensive response at 2 weeks and provided a further reduction from 2 weeks to 12 weeks, longer than 4 weeks may be required to evaluate a clinically meaningful response to treatment. Further investigation on more cases and longer follow-up are needed.

Effect of cyproterone acetate on alpha1-adrenoceptor subtypes in rat vas deferens

Gonadal hormones regulate the expression of alpha1-adrenoceptor subtypes in several tissues. The present study was carried out to determine whether or not cyproterone acetate, an anti-androgenic agent, regulates the alpha1-adrenoceptor subtypes that mediate contractions of the rat vas deferens in response to noradrenaline. The actions of subtype selective alpha1-antagonists were investigated in vas deferens from control and cyproterone acetate-treated rats (10 mg/day, sc, for 7 days). Prazosin (pA2 approximately 9.5), phentolamine (pA2 approximately 8.3) and yohimbine (pA2 approximately 6.7) presented competitive antagonism consistent with activation of alpha1-adrenoceptors in vas deferens from both control and treated rats. The pA2 values estimated for WB 4101 ( approximately 9.5), benoxathian ( approximately 9.7), 5-methylurapidil (approximately 8.5), indoramin ( approximately 8.7) and BMY 7378 ( approximately 6.8) indicate that alpha1A-adrenoceptors are involved in the contractions of the vas deferens from control and cyproterone acetate-treated rats. Treatment of the vas deferens from control rats with the alpha1B/alpha1D-adrenoceptor alkylating agent chloroethylclonidine had no effect on noradrenaline contractions, supporting the involvement of the alpha1A-subtype. However, this agent partially inhibited the contractions of vas deferens from cyproterone acetate-treated rats, suggesting involvement of multiple receptor subtypes. To further investigate this, the actions of WB 4101 and chloroethylclonidine were reevaluated in the vas deferens from rats treated with cyproterone acetate for 14 days. In these organs WB 4101 presented complex antagonism characterized by a Schild plot with a slope different from unity (0.65 0.05). After treatment with chloroethylclonidine, the complex antagonism presented by WB 4101 was converted into classical competitive antagonism, consistent with participation of alpha1A-adrenoceptors as well as alpha1B-adrenoceptors. These results suggest that cyproterone acetate induces plasticity in the alpha1-adrenoceptor subtypes involved in the contractions of the vas deferens.

Effects of castration on alpha 1-adrenoceptor subtypes in the rat aorta

The expression of alpha 1-adrenoceptor subtypes in several tissues is regulated by gonadal hormones. In this study, we investigated whether castration regulates the alpha 1-adrenoceptor subtypes mediating the contractions of the aorta from male rats to noradrenaline. Noradrenaline induced similar concentration-dependent contractions in the aorta from control and castrated rats. Treatment of the aorta from both control and castrated rats with the alpha 1B/alpha 1D-adrenoceptor alkylating agent chloroethylclonidine resulted in approximately 1600-fold rightward shift in the concentration-response curves to noradrenaline. The pA2 values found for WB 4101, benoxathian (alpha 1A-selective) and BMY 7378 (alpha 1D-selective) indicate that alpha 1D-adrenoceptors are involved in the contractions of the aorta from control and castrated rats to noradrenaline. However, there was a 15-fold difference between the pKB estimated through the lowest effective concentrations of the alpha 1A-adrenoceptor selective antagonist 5-methyl-urapidil in the aorta from control and castrated rats. The pKB estimated in aorta from control rats is consistent with the interaction with alpha 1D-adrenoceptors (7.58 +/- 0.06), while that calculated in organs from control rats is consistent with alpha 1A-adrenoceptors (8.76 +/- 0.09). These results suggest that castration induces plasticity in the alpha 1-adrenoceptor subtypes involved in the contractions of the aorta to noradrenaline.

Prazosin-Related Compounds. Effect of Transforming the Piperazinylquinazoline Moiety into an Aminomethyltetrahydroacridine System on the Affinity for α1-Adrenoreceptors

In a search for structurally new alpha(1)-adrenoreceptor (alpha(1)-AR) antagonists, prazosin (1)-related compounds 2-11 were synthesized and their affinity profiles were assessed by functional experiments in isolated rat vas deferens (alpha(1A)), spleen (alpha(1B)), and aorta (alpha(1D)) and by binding assays in CHO cells expressing human cloned alpha(1)-AR subtypes. Transformation of the piperazinylquinazoline moiety of 1 into an aminomethyltetrahydroacridine system afforded compound 2, endowed with reduced affinity, in particular for the alpha(1A)-AR subtype. Then, to investigate the optimal features of the tricyclic moiety, the aliphatic ring of 2 was modified by synthesizing the lower and higher homologues 3 and 4. An analysis of the pharmacological profile, together with a molecular modeling study, indicated the tetrahydroacridine moiety as the most promising skeleton for alpha(1)-antagonism. Compounds 5-8, where the replacement of the furoyl group of 2 with a benzoyl moiety afforded the possibility to evaluate the effect of the substituent trifluoromethyl on receptor binding, resulted, except for 7, in a rather surprising selectivity toward alpha(1B)-AR, in particular vs the alpha(1A) subtype. Also the insertion of the 2,6-dimethoxyphenoxyethyl function of WB 4101 on the tetrahydroacridine skeleton of 2, and/or the replacement of the aromatic amino function with a hydroxy group, affording derivatives 9-11, resulted in alpha(1B)-AR selectivity also vs the alpha(1D) subtype. On the basis of these results, the tetrahydroacridine moiety emerged as a promising tool for the characterization of the alpha(1)-AR, owing to the receptor subtype selectivity achieved by an appropriate modification of the lateral substituents.

Effects of castration and of testosterone replacement on alpha(1)-adrenoceptor subtypes in the rat vas deferens

The contractions of the rat vas deferens in response to noradrenaline are mediated through alpha(1A)-adrenoceptors. We observed participation of alpha(1B)-adrenoceptors in these contractions after castration. We now investigated the time course of this plasticity and the effects of testosterone by determining the actions of competitive antagonists on noradrenaline-induced contractions after 7, 14, 21 and 30 days of castration. BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride) antagonised noradrenaline-induced contractions in control and castrated rats with low pA(2) values (approximately = 6.8). In control vas deferens, WB 4101 (2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride) had a slope in the Schild plot no different from 1.0, while slopes lower than 1.0 (approximately 0.6) were observed for vas deferens from castrated rats. Chloroethylclonidine was ineffective in the control vas while it inhibited noradrenaline-induced contractions in vasa from castrated rats and converted the complex antagonism by WB 4101 into simple competitive antagonism. Treatment of castrated rats with testosterone prevented the effects of castration. The results suggest that alpha(1B)-adrenoceptors are detectable in vas deferens from at least the 7th through the 30th day after castration and that testosterone prevents this plasticity.

Functional characterization of alpha-adrenoceptors mediating pupillary dilation in rats

Previously, we reported that the alpha(1A)-adrenoceptor, but not the alpha(1D)-adrenoceptor, mediates pupillary dilation elicited by sympathetic nerve stimulation in rats. This study was undertaken to further characterize the alpha-adrenoceptor subtypes mediating pupillary dilation in response to both neural and agonist activation. Pupillary dilator response curves were generated by intravenous injection of norepinephrine in pentobarbital-anesthetized rats. Involvement of alpha(1)-adrenoceptors was established as mydriatic responses were inhibited by systemic administration of nonselective alpha-adrenoceptor antagonists, phentolamine (0.3-3 mg/kg) and phenoxybenzamine (0.03-0.3 mg/kg), as well as by the selective alpha(1)-adrenoceptor antagonist, prazosin (0.3 mg/kg). The alpha(2)-adrenoceptor antagonist, rauwolscine (0.5 mg/kg), was without antagonistic effects. alpha(1A)-Adrenoceptor selective antagonists, 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB-4101; 0.1-1 mg/kg) and 5-methylurapidil (0.1-1 mg/kg), the alpha(1B)-adrenoceptor selective antagonist, 4-amino-2-[4-[1-(benzyloxycarbonyl)-2(S)- [[(1,1-dimethylethyl)amino]carbonyl]-piperazinyl]-6,7-dimethoxyquinazoline (L-765314; 0.3-1 mg/kg), as well as the alpha(1D)-adrenoceptor selective antagonist, 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY-7378; 1 mg/kg), were used to delineate the adrenoceptor subtypes involved. Mydriatic responses to norepinephrine were significantly antagonized by intravenous administration of both WB-4101 and 5-methylurapidil, but neither by L-765314 nor by BMY-7378. L-765314 (0.3-3 mg/kg, i.v.) was also ineffective in inhibiting the mydriasis evoked by cervical sympathetic nerve stimulation. These results suggest that alpha(1B)-adrenoceptors do not mediate sympathetic mydriasis in rats, and that the alpha(1A)-adrenoceptor is the exclusive subtype mediating mydriatic responses in this species.

Contractile actions of imidazoline alpha-adrenoceptor agonists and effects of noncompetitive alpha1-adrenoceptor antagonists in human vas deferens

The contractile actions of imidazoline alpha-adrenoceptor agonists were investigated in human vas deferens longitudinal and circular muscle. The effects of phenoxybenzamine were studied in comparison to dibenamine and SZL-49 (4-amino-6,7-dimethoxy-2-quinazolinyl-4-(2-bicyclo[2,2,2]octa-2,5-dienylcarbonyl-2-piperazine), an alkylating prazosin analogue that discriminates between alpha(1H)- and alpha(1L)-adrenoceptor subtypes. The imidazoline alpha-adrenoceptor agonist, A-61603 (N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulfonamide hydrobromide), was a potent agonist (pD(2); longitudinal muscle 6.9, circular muscle 6.4) and cirazoline a partial agonist (pD(2); longitudinal muscle 6.1, circular muscle 5.1). Oxymetazoline was less effective, indanidine and clonidine were ineffective. SZL-49 produced a differential inhibition of contractions evoked by A-61603 in circular (alpha(1H)) compared to longitudinal (alpha(1L)) muscle and phenoxybenzamine had the opposite effect. Dibenamine inhibited the contractions comparably in both muscle types and analyses of its partial alkylation of receptors yielded identical estimates of equilibrium dissociation constant (pK(d)) for A-61603 in longitudinal (5.82) and circular (5.84) muscle. Receptor occupancy-response relationships revealed that whilst the muscle types are not different in receptor reserves for A-61603, contraction to the potent imidazoline is more efficiently coupled in longitudinal than in circular muscle. This underlies the markedly different responsiveness of the muscle types to cirazoline or oxymetazoline (alpha-adrenoceptor agonists with lower efficacies relative to A-61603). The differential inhibitory actions of phenoxybenzamine and SZL-49 are discussed.

Enhancement of alpha-adrenoceptor-mediated responses in prostate of testosterone-treated rat

The present study was undertaken to investigate the effect of testosterone on the alpha-adrenoceptor-mediated contractile responses in ventral lobe of rat prostate. Contractile responses to various alpha-adrenoceptor agonists (phenylephrine, A61603 (N-[5-(4,5-dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl] methanesulphonamide), clonidine, guanfacine, ST587 ((2,3-dihydro-benzo[1,4]dioxin-2-ylmethyl)-[2-(2,6-dimethoxy-phenoxy)-ethyl]-amine) and xylazine) were tested in prostate strips obtained from control and testosterone (3 mg/kg, s.c. 5 days a week for 15 days-10 doses total)-treated rats. Dose-response curves for alpha-adrenoceptor agonists in testosterone-treated animals showed a leftward shift, indicating increased sensitivity of tissue to alpha-adrenoceptor agonists. To find the mechanism of increased sensitivity, K(A) value and receptor reserve of phenylephrine were estimated. Neither the K(A) value nor the receptor reserve of phenylephrine was altered in testosterone-treated rats. The concentration-occupancy curve for A61603 was shifted leftward and the K(A) value for A61603 decreased about four-fold. The K(B) value of 2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane (WB4101) was not altered, however, the K(B) value for prazosin was decreased approximately 5.5-fold. These findings indicate that the testosterone-mediated increase in sensitivity of prostate to alpha-adrenoceptor agonists is due to alterations in the alpha(1)-adrenoceptor pool.

Enhancement of apomorphine-induced penile erection in the rat by a selective alpha(1D)-adrenoceptor antagonist

1. Effects of A-322312 (alpha(1B)-adrenoceptor (AR) antagonist), A-119637 (alpha(1D)-AR antagonist), prazosin (non-selective alpha(1)-AR antagonist), and yohimbine (alpha(2)-AR antagonist) were studied in rat corpus cavernosum (CC) and cavernous artery (Acc) preparations. Effects of intracavernous (i.c.) or intraperitoneal (i.p.) administration of alpha(1)-AR antagonists on apomorphine-induced erections were investigated. 2. A-119637 attenuated electrically induced contractions in isolated CC (-logIC(50); 8.12+/-0.15), and relaxed noradrenaline (NA)-contracted preparations by more than 90% at 10(-7) M. At the same concentration, the -logEC(50) value for NA in Acc was altered from 6.79+/-0.07 to 4.86+/-0.13. In the CC and Acc, prazosin similarly inhibited contractile responses. 3. Inhibitory effects of A-322312 (10(-7) M) in electrically activated CC were 32.3+/-5.1%, whereas no effect on concentration-response curves for NA was observed in the Acc. Yohimbine (10(-8) M and 10(-7) M), enhanced electrically-induced contractions in isolated CC by 20 to 50%. At 10(-6) M, inhibitory effects of yohimbine were obtained. 4. A-119637 (0.3 micromol kg(-1), i.p.) tripled the number of erections, and produced a 6 fold increase in the duration of apomorphine-induced erectile responses. A-322312, prazosin, or yohimbine did not enhance erections induced by apomorphine. None of the alpha(1)-AR antagonists significantly increased ICP upon i.c. administration. Decreases in blood pressure were seen with A-119637 and prazosin. 5. The present findings show that there is a functional predominance of the alpha(1D)-AR subtype in the rat erectile tissue, and that blockade of this receptor facilitates rat penile erection induced by a suboptimal dose of apomorphine.

Discrimination by SZL49 between contractions evoked by noradrenaline in longitudinal and circular muscle of human vas deferens

The effects of irreversible alpha1-adrenoceptor antagonists, SZL-49 (an alkylating analogue of prazosin), dibenamine and benextramine on contractions to noradrenaline (NA) in longitudinal and circular muscle of human epididymal vas deferens were investigated. Competitive alpha1-adrenoceptor antagonists were also used to further characterize the alpha1-adrenoceptor subtype stimulated by NA in longitudinal and circular muscle. NA evoked concentration-dependent contractions of both muscle types (pD2; 5.4 and 5.2 respectively). The contraction of circular muscle was comparatively more sensitive than that of longitudinal muscle to pretreatment with SZL-49. In contrast, dibenamine or benextramine produced comparable effects in both muscle types. The relationship between receptor occupancy and contraction in either longitudinal or circular muscle was nonlinear, with half-maximal response requiring similar receptor occupancy (longitudinal muscle 14%, circular muscle 16%). Maximal response in both muscle types occurred with little or no receptor reserve (<10%). The competitive alpha1-adrenoceptor antagonists produced dextral shifts of the dose-response curves to NA in longitudinal and circular muscle. The inhibitory potencies, estimated from the apparent pKB values were significantly different in longitudinal and circular muscle respectively for either WB 4101 (pKB, 8.6 and 9.5) or RS-17053 (pKB, 7.1 and 9.0) but not for Rec 15/2739 (pKB, 9.2 and 9.8) or HV 723 (pKB, 8.3 and 8.4). In conclusion, the potency profile of the competitive alpha1-adrenoceptor antagonists and the lack of different receptor reserves for NA in the muscle types suggest that the discriminatory effects of SZL-49 is primarily due to a predominance of the alpha1L-adrenoceptor subtype in longitudinal muscle and alpha1A-subtype in circular muscle.