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

Alpha1-adrenoceptors in the rat cerebral cortex: new insights into the characterization of alpha1L- and alpha1D-adrenoceptors

Among the three alpha(1)-adrenoceptor subtypes (alpha(1A), alpha(1B) and alpha(1D)) a peculiar intracellular localization and poor coupling to membrane signals of cloned alpha(1D)-adrenoceptor have been reported. In addition, the alpha(1L)-adrenoceptor (low affinity for prazosin), a functional phenotype of alpha(1A), has been described. The purpose of this work was to analyze the expression, cellular localization and coupling to membrane signalling (inositol phosphate accumulation) of alpha(1)-adrenoceptor subtypes in a native tissue, the rat cerebral cortex. mRNA for the three subtypes was quantified by real-time RT-PCR (alpha(1D)>alpha(1B)>>alpha(1A)). alpha(1)-Adrenoceptors were also detected by immunoblotting, revealing alpha(1A)- and alpha(1B)-adrenoceptors to be predominantly expressed in the membrane fraction and the alpha(1D)-adrenoceptor to be localized in the cytosolic fraction. Competitive radioligand binding studies revealed the presence of alpha(1D)-adrenoceptor in tissue homogenates, whereas only alpha(1A)- and alpha(1B)-subtypes were detected in membranes. The proportion of alpha(1A)-adrenoceptor increased after treatment with noradrenaline, suggesting differences in agonist-mediated trafficking. Saturation experiments detected high- and low (alpha(1A/L))-prazosin binding sites, the latter of which disappeared on incubation with GppNHp. The alpha(1A/L)-adrenoceptor was heavily implicated in the inositol phosphate response, while the alpha(1D)-subtype did not play a relevant role. These results suggest that the predominant cytosolic localization of alpha(1D)-adrenoceptor lies behind its poor coupling to membrane signalling such as inositol phosphate pathway. The fact that the alpha(1L)-adrenoceptor detected in radioligand binding studies disappeared in the presence of GppNHp implies that it represents a conformational state of the alpha(1A)-adrenoceptor coupled to G-protein.

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.

Identification of the alpha1L-adrenoceptor in rat cerebral cortex and possible relationship between alpha1L- and alpha1A-adrenoceptors

BACKGROUND AND PURPOSE

In addition to alpha1A, alpha1B and alpha1D-adrenoceptors (ARs), putative alpha1L-ARs with a low affinity for prazosin have been proposed. The purpose of the present study was to identify the alpha1A-AR and clarify its pharmacological profile using a radioligand binding assay.

EXPERIMENTAL APPROACH

Binding experiments with [3H]-silodosin and [3H]-prazosin were performed in intact tissue segments and crude membrane preparations of rat cerebral cortex. Intact tissue binding assays were also conducted in rat tail artery.

KEY RESULTS

[3H]-silodosin at subnanomolar concentrations specifically bound to intact tissue segments and membrane preparations of rat cerebral cortex at the same density (approximately 150 fmol mg(-1) total tissue protein). The binding sites in intact segments consisted of alpha1A and alpha1L-ARs that had different affinities for prazosin, while the binding sites in membranes showed an alpha1A-AR-like profile having single high affinity for prazosin. [3H]-prazosin also bound at subnanomolar concentrations to alpha1A and alpha1B-ARs but not alpha1L-ARs in cerebral cortex; the binding densities being approximately 200 and 290 fmol mg(-1) protein in the segments and the membranes, respectively. In the segments of tail artery, [3H]-silodosin only recognized alpha1A-ARs, whereas [3H]-prazosin bound to alpha1A and alpha1B-ARs.

CONCLUSIONS AND IMPLICATIONS

The present study clearly reveals the presence of alpha1L-ARs as a pharmacologically distinct entity from alpha1A and alpha1B-ARs in intact tissue segments of rat cerebral cortex but not tail artery. However, the alpha1L-ARs disappeared after tissue homogenization, suggesting their decomposition and/or their pharmacological profile changes to that of alpha1A-ARs.

“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.

Identification of alpha-1L and alpha-1A adrenoceptors in human prostate by tissue segment binding

PURPOSE

Silodosin (KMD-3213 or [(-)-1-(3-hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2trifluoroethoxy)phenoxy]ethyl}amino)propyl]-2,3-dihydro-1H-indole-7-carboxamide]) (Kissei Pharmaceutical Co., Ltd., Matsumoto, Japan) is a selective antagonist for alpha-1A and alpha-1L adrenoceptors. Using this tritiated ligand the 2 alpha-1 adrenoceptors were examined in binding studies with intact tissue segments and membrane preparations of human prostate, and compared with functionally identified alpha-1 adrenoceptor.

MATERIALS AND METHODS

Binding assays with tissue segments and membrane preparations of human prostate samples were performed using [3H]-silodosin and binding affinities for various drugs were estimated. In functional experiments antagonist affinities were evaluated from the inhibitory potency against the contractile response to noradrenaline.

RESULTS

[3H]-silodosin bound to intact segments and membrane preparations of human prostate with subnanomolar affinity. [3H]-silodosin binding sites in intact segments were divided into 2 distinct components with different affinities for prazosin and RS-17053 (N-[2(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha, alpha-dimethyl1H-indole-3-ethanamine hydrochloride) (Research Biochemicals International, Natick, Massachusetts), while binding in membrane preparations showed single high affinity for these drugs. [3H]-silodosin binding sites also showed high affinity for silodosin and tamsulosin but low sensitivity to BMY 7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-8-azaspiro(4.5)decane-7,9-dione) (Research Biochemicals International) in intact segments and in membrane preparations. In functional experiments silodosin and tamsulosin potently inhibited the contractile response to noradrenaline but prazosin, RS-17053 and BMY 7378 showed low antagonistic affinity.

CONCLUSIONS

The current binding studies in human prostate samples clearly show that alpha-1L and alpha-1A adrenoceptors coexist as pharmacologically distinct entities in intact tissues but not in crude membrane preparations. Also, alpha-1 adrenoceptors involved in the contractile response to noradrenaline are the alpha-1L subtype.

Can smooth muscle represent a useful target for the treatment of rapid ejaculation?

Rapid ejaculation is probably the most common form of male sexual dysfunction. Current research into the treatment of the condition has focused on centrally acting or topical desensitizing agents; however, no treatment has yet been approved. An alternative approach could be to develop drugs that act directly upon the target organ itself and our increasing knowledge of the molecular biology of the accessory sex organs makes this a realistic possibility. This review analyzes the information in the literature that would support such a hypothesis. Particular emphasis has been placed on articles that have investigated smooth muscle cell relaxation. A critical review of the literature has revealed that there are potentially a myriad of targets through which rapid ejaculation can be treated.

Identification of α-1L Adrenoceptor in Rabbit Ear Artery

The alpha-1L adrenoceptor (AR) was identified in rabbit ear artery by both functional and ligand binding studies. In functional studies using arterial rings, the contractile response to NS-49 [(R)-(-)-3'-(2-amino-1-hydroxyethyl)-4'-fluorometh-anesulfonanilide hydrochloride] (alpha-1A and alpha-1L AR-selective agonist) was competitively antagonized with low affinities by prazosin, RS-17053 [N-[2-(2-cyclopropylmethoxyphenoxy) ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethamine hydrochloride], and 5-methylurapidil but with high affinities by tamsulosin and KMD-3213 [(-)-1-(3-hydroxypropyl)-5-[(2R)-2-([2-[(2,2,2-trifluoroethoxy)phenoxy]ethyl]amino)propyl]-2,3-dihydro-1H-indole-7-carboxamide]. In contrast, the response to noradrenaline (nonselective alpha-1 AR agonist) was inhibited noncompetitively by these antagonists (except 5-methylurapidil) with Schild slopes different from unity. These results suggest that the response to NS-49 was mediated predominantly via alpha-1L ARs, whereas the response to noradrenaline was produced through two distinct alpha-1 AR subtypes (presumably alpha-1B and alpha-1L ARs). In binding studies with intact segments of rabbit ear artery, [3H]KMD-3213 bound with high affinity (pKD=9.7) to alpha-1 ARs, which were subdivided by prazosin, RS-17053, and 5-methylurapidil into two subtypes (alpha-1A and alpha-1L ARs). In contrast, [3H]prazosin binding sites in ear artery segments (pKD = 9.8) were identified as alpha-1A and alpha-1B ARs. In conventional binding studies using isolated rabbit ear artery microsomal membranes, [3H]KMD-3213 binding sites were identified as alpha-1A ARs with high affinities for prazosin, RS-17053, and 5-methylurapidil. Our study indicates that an alpha-1L AR having a unique pharmacological profile coexists with alpha-1A and alpha-1B ARs in rabbit ear artery and can be identified either functionally or by binding studies using intact tissues but not microsomal membrane preparations.

Expression of Rho-kinase and its functional role in the contractile activity of the mouse vas deferens

The effects of two Rho-kinase inhibitors, Y-27632 and fasudil, were investigated on the contractions produced by electrical field stimulation (EFS, 40 V, 1 mS, 2, 4, 8 and 16 Hz, for 20 s), KCl (30 - 60 mm), phenylephrine (Phe) (10-5 - 10-4 m), adenosine-3', 5'-triphosphate (ATP) (10-4 - 10-3 m) and alpha,beta-methylene ATP (10-5 m). EFS produced frequency-dependent reproducible contractile activity, which was almost abolished by guanethidine (10-5 m, for 1 h). This contraction consisted of two components (a phasic initial contraction followed by a tonic one), and it was inhibited by Y-27632 and fasudil (both at 10-5 m). However, these inhibitors had no effect on resting tension of the tissue. Contractions elicited by KCl (30 - 60 mm) were insensitive to guanethidine (10-5 m, for 1 h), but suppressed by Y-27632 (10-5 m) and fasudil (10-5 m). In addition, the contractions induced by Phe (an alpha1-adrenoceptor agonist) and ATP (a purinergic agent) were inhibited significantly by Y-27632 (10-5 m). Phasic contractions evoked by the selective P2X purinoceptor agonist alpha,beta-methylene ATP were also suppressed by Y-27632 (10-5 m). Western blot analysis revealed that the mouse vas deferens expresses Rho-kinase (ROKalpha, ROCK-2 isoform) protein with a molecular weight of approximately 160 kDa. As a positive control, the presence of this protein was also shown in homogenates of smooth muscle from the rat mesenteric artery. In conclusion, Rho-kinase protein is expressed in the mouse vas deferens, and it mediates neurogenic contractile activity as well as the contractions induced by KCl, Phe, ATP and alpha,beta-methylene ATP. Owing to the suppressive effects of Rho-kinase inhibitors on the contractile activity of the vas deferens, the possibility that these compounds might impair ejaculation must be taken into account when considering them as potential agents in the treatment of erectile dysfunction.

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.