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

The pharmacology of α1-adrenoceptor subtypes

This review examines the functions of α₁-adrenoceptor subtypes, particularly in terms of contraction of smooth muscle. There are 3 subtypes of α₁-adrenoceptor, α_1A- α_1B- and α_1D-adrenoceptors. Evidence is presented that the postulated α_1L-adrenoceptor is simply the native α_1A-adrenoceptor at which prazosin has low potency. In most isolated tissue studies, smooth muscle contractions to exogenous agonists are mediated particularly by α_1A-, with a lesser role for α_1D-adrenoceptors, but α_1B-adrenoceptors are clearly involved in contractions of some tissues, for example, the spleen. However, nerve-evoked responses are the most crucial physiologically, so that these studies of exogenous agonists may overestimate the importance of α_1A-adrenoceptors. The major α₁-adrenoceptors involved in blood pressure control by sympathetic nerves are the α_1D- and the α_1A-adrenoceptors, mediating peripheral vasoconstrictor actions. As noradrenaline has high potency at α_1D-adrenceptors, these receptors mediate the fastest response and seem to be targets for neurally released noradrenaline especially to low frequency stimulation, with α_1A-adrenoceptors being more important at high frequencies of stimulation. This is true in rodent vas deferens and may be true in vasopressor nerves controlling peripheral resistance and tissue blood flow. The α_lA-adrenoceptor may act mainly through Ca²⁺ entry through L-type channels, whereas the α_1D-adrenoceptor may act mainly through T-type channels and exhaustable Ca²⁺ stores. α₁-Adrenoceptors may also act through non-G-protein linked second messenger systems. In many tissues, multiple subtypes of α-adrenoceptor are present, and this may be regarded as the norm rather than exception, although one receptor subtype is usually predominant.

Agonist pharmacology at recombinant α1A - and α1L -adrenoceptors and in lower urinary tract α1 -adrenoceptors

BACKGROUND AND PURPOSE

Two distinct α1 -adrenoceptor phenotypes (α1A and α1L ) have recently been demonstrated to originate from a single α1A -adrenoceptor gene. Here, we examined the agonist profiles of recombinant α1A and α1L phenotypes and of lower urinary tract (LUT) α1 -adrenoceptors.

EXPERIMENTAL APPROACH

A series of drugs (A61603, Ro 115-1240, NS-49 , MK017 and ESR1150) originally developed for stress urinary incontinence (SUI) therapy were used to stimulate recombinant α1A - and α1L -adrenoceptor phenotypes, and their potencies and intrinsic activity estimated from Ca(2+) responses. Agonist-induced contractions were also examined in LUT tissues of rats and humans and in human mesenteric artery and rat tail artery.

KEY RESULTS

All the drugs were potent agonists of the α1A -adrenoceptor compared with the α1L -adrenoceptor phenotype. Among them, Ro 115-1240 was shown to be an α1A -specific partial agonist that produced partial contractions through α1A -adrenoceptors in rat prostate and tail artery, but not in the other LUT tissues and human mesenteric artery. In contrast, P-come 102 showed full agonist activity at α1A - and α1L -adrenoceptors, but was less selective than noradrenaline for α1A -adrenoceptors. Like noradrenaline, P-come 102 was highly potent at inducing contractions in all of the LUT tissues tested. However, the potency and intrinsic activity of P-come 102 were significantly lower than those of noradrenaline in human mesenteric artery.

CONCLUSIONS AND IMPLICATIONS

The α1A - and α1L -adrenoceptor phenotypes and LUT α1 -adrenoceptors were demonstrated to have distinct agonist profiles. As adrenergic contractions in LUT are predominantly mediated through α1L -adrenoceptors, the development of α1L -selective agonists may provide clinically useful drugs for SUI therapy.

Pharmacologically distinct phenotypes of α1B -adrenoceptors: variation in binding and functional affinities for antagonists

BACKGROUND AND PURPOSE

The pharmacological properties of particular receptors have recently been suggested to vary under different conditions. We compared the pharmacological properties of the α1B -adrenoceptor subtype in various tissue preparations and under various conditions.

EXPERIMENTAL APPROACH

[(3) H]-prazosin binding to α1B -adrenoceptors in rat liver (segments, dispersed hepatocytes and homogenates) was assessed and the pharmacological profiles were compared with the functional and binding profiles in rat carotid artery and recombinant α1B -adrenoceptors.

KEY RESULTS

In association and saturation-binding experiments with rat liver, binding affinity for [(3) H]-prazosin varied significantly between preparations (KD value approximately ten times higher in segments than in homogenates). The binding profile for various drugs in liver segments also deviated from the representative α1B -adrenoceptor profile observed in liver homogenates and recombinant receptors. L-765,314 and ALS-77, selective antagonists of α1B -adrenoceptors, showed high binding and antagonist affinities in liver homogenates and recombinant α1B -adrenoceptors. However, binding affinities for both ligands in the segments of rat liver and carotid artery were 10 times lower, and the antagonist potencies in α1B -adrenoceptor-mediated contractions of carotid artery were more than 100 times lower than the representative α1B -adrenoceptor profile.

CONCLUSIONS AND IMPLICATIONS

In contrast to the consistent profile of recombinant α1B -adrenoceptors, the pharmacological profile of native α1B -adrenoceptors of rat liver and carotid artery varied markedly under various receptor environments, showing significantly different binding properties between intact tissues and homogenates, and dissociation between functional and binding affinities. In addition to conventional 'subtype' characterization, 'phenotype' pharmacology must be considered in native receptor evaluations in vivo and in future pharmacotherapy.

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.

Re-evaluation of nicotinic acetylcholine receptors in rat brain by a tissue-segment binding assay

Nicotinic acetylcholine receptors (nAChRs) of the cerebral cortex and cerebellum of rats were evaluated by a radioligand binding assay, employing tissue segments, or homogenates as materials. [³H]-epibatidine specifically bound to nAChRs in rat cortex or cerebellum, but the dissociation constants for [³H]-epibatidine differed between segments and homogenates (187 pM for segments and 42 pM for homogenates in the cortex and 160 pM for segments and 84 pM for homogenates in the cerebellum). The abundance of total nAChRs was approximately 310 fmol/mg protein in the segments of cortex and 170 fmol/mg protein in the segments of cerebellum, which were significantly higher than those estimated in the homogenates (115 fmol/mg protein in the homogenates of the cortex and 76 fmol/mg protein in the homogenates of the cerebellum). Most of the [³H]-epibatidine binding sites in the cortex segments (approximately 70% of the population) showed high affinity for nicotine (pK_i = 7.9), dihydro-β-erythroidine, and cytisine, but the binding sites in the cerebellum segments had slightly lower affinity for nicotine (pK_i = 7.1). An upregulation of nAChRs by chronic administration of nicotine was observed in the cortex segments but not in the cerebellum segments with [³H]-epibatidine as a ligand. The upregulation in the cortex was caused by a specific increase in the high-affinity sites for nicotine (probably α4β2). The present study shows that the native environment of nAChRs is important for a precise quantitative as well as qualitative estimation of nAChRs in rat brain.

Influence of tissue integrity on pharmacological phenotypes of muscarinic acetylcholine receptors in the rat cerebral cortex

Distinct pharmacological phenotypes of muscarinic acetylcholine receptors (mAChRs) have been proposed. We compared the pharmacological profiles of mAChRs in intact segments and homogenates of rat cerebral cortex and other tissues by using radioligand binding assays with [(3)H]N-methylscopolamine ([(3)H]NMS). Recombinant M(1) and M(3) mAChRs were also examined. The density of mAChRs detected by [(3)H]NMS binding to rat cerebral cortex segments and homogenates was the same (approximately 1400 fmol/mg tissue protein), but the dissociation constant of [(3)H]NMS was significantly different (1400-1700 pM in segments and 260 pM in homogenates). A wide variation in [(3)H]NMS binding affinity was also observed among the segments of other tissues (ranging from 139 pM in urinary bladder muscle to 1130 pM in the hippocampus). The mAChRs of cerebral cortex were composed of M(1), M(2), M(3), and M(4) subtypes, which showed typical subtype pharmacology in the homogenates. However, in the cortex segments the M(3) subtype showed a low selectivity for M(3) antagonists (darifenacin, solifenacin) and was not distinguished by the M(3) antagonists from the other subtypes. Recombinant M(1) and M(3) mAChRs showed high affinity for [(3)H]NMS and subtype-specific pharmacology for each tested ligand. The present binding study under conditions where tissue integrity was kept demonstrates a wide variation in [(3)H]NMS binding affinity among mAChRs of many rat tissues and the presence of an atypical M(3) phenotype in the cerebral cortex, suggesting that the pharmacological properties of mAChRs are not necessarily constant, rather they may be significantly modified by tissue integrity and tissue type.

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.

Structure-Activity Relationship Studies of a Number of α1 -Adrenoceptor Antagonists and Antiarrhythmic Agents

Arylpiperazines represent one of the most studied classes of α1 -adrenoceptor (α1 -AR) antagonists. Currently, α1 -AR antagonists are useful in the treatment of benign prostatic hyperplasia, lower urinary tract symptoms or cardiac arrhythmia. The activity of various derivatives of 1-[3-(4-arylpiperazin-1-yl)propyl]pyrrolidin-2-one as α1 -adrenergic receptor antagonists and antiarrhythmic (AA) agents was described using the qualitative inverse Structure Activity Relationship (SAR) model. The three-dimensional structures of the pyrrolidin-2-one derivatives in the basic form were obtained using AM1 semi-empirical quantum chemical calculations. All the molecules were geometry-optimized until the root-mean-square (RMS) gradient value was smaller than 10(-6) a.u. Single-point energy (SPE) calculations were performed at the DFT/B3LYP level of theory using the 6-31G** basis set. The main focus of this inverse SAR study is to find which features cause enhancing of antiarrhythmic properties between subtly different types of activity (α1 -adrenoreceptor antagonists and antiarrhythmic activities). Our SAR study involves the charge distribution in the plane of the pharmacophore model for α1 -AR. Suitable maps of the electrostatic potential were plotted based on the electronic and nuclear charge distribution obtained from the energy calculations. The results of this modelling study indicate that if the terminal arylpiperazine moiety is surrounded by regions of negative electrostatic potential, then the antiarrhythmic activity is blocked.

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.

Visualization and tissue distribution of alpha1L-adrenoceptor in human prostate by the fluorescently labeled ligand Alexa-488-silodosin

PURPOSE

Although alpha(1L)-adrenoceptor is recognized as a target of alpha(1) antagonist therapy for benign prostatic hyperplasia, the most common techniques, such as immunohistochemistry and in situ hybridization, are not applicable to examine alpha(1L)-AR vs alpha(1A)-AR tissue distribution because alpha(1L)-AR is now considered another phenotype sharing the alpha(1A)-AR gene and protein molecule. We labeled the alpha(1A) and alpha(1L)-adrenoceptor selective antagonist silodosin (Kissei Pharmaceutical, Matsumoto, Japan) with the fluorophore Alexa Fluor(R) 488 (Alexa-488-silodosin) to visualize alpha(1L)-AR expression.

MATERIALS AND METHODS

Radioligand binding and functional bioassay experiments were done to assess alpha(1)-AR expression in Chinese hamster ovary cells and human prostate tissues. Confocal imaging was subsequently performed.

RESULTS

Although Alexa-488-silodosin had about 10 times lower affinity for all alpha(1)-AR subtypes than silodosin in binding and functional studies, it had high selectivity to alpha(1A) and alpha(1L)-ARs. Confocal imaging revealed clear localization of fluorescence on the membrane of Chinese hamster ovary cells expressing alpha(1A)-AR but not alpha(1B)-and alpha(1D)-ARs, and in the muscle layer of the human prostate. The fluorescent signal in Chinese hamster ovary cells disappeared in the presence of 3 nM prazosin but fluorescence was observed in the human prostate even in the presence of 100 nM prazosin.

CONCLUSIONS

Alexa-488-silodosin is a powerful fluorescent probe with high selectivity to alpha(1A) and alpha(1L)-ARs. Thus, Alexa-488-silodosin successfully visualizes the site of alpha(1L)-ARs in the muscle layer of the human prostate without losing its distinct pharmacological profile.

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.

Expression of distinct alpha 1-adrenoceptor phenotypes in the iris of pigmented and albino rabbits

BACKGROUND AND PURPOSE

The expression of multiple pharmacological phenotypes including alpha(1L)-adrenoceptor has recently been reported for alpha(1)-adrenoceptors. The purpose of the present study was to identify alpha(1)-adrenoceptor phenotypes in the irises of pigmented and albino rabbits.

EXPERIMENTAL APPROACH

Radioligand binding and functional bioassay experiments were performed in segments or strips of iris of pigmented and albino rabbits, and their pharmacological profiles were compared.

KEY RESULTS

[(3)H]-silodosin at subnanomolar concentrations bound to intact segments of iris of pigmented and albino rabbits at similar densities (approximately 240 fmol x mg(-1) protein). The binding sites in the iris of a pigmented rabbit were composed of a single component showing extremely low affinities for prazosin, hydrochloride [N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethamine hydrochloride (RS-17053)] and 5-methylurapidil, while two components with high and low affinities for prazosin, RS-17053 and 5-methylurapidil were identified in irises from albino rabbits. In contrast, specific binding sites for [(3)H]-prazosin were not clearly detected because a high proportion of non-specific binding and/or low affinity for prazosin occurred. Contractile responses of iris dilator muscle to noradrenaline were antagonized by the above ligands, and their antagonist affinities were consistent with the binding estimates at low-affinity sites identified in both strains of rabbits.

CONCLUSIONS AND IMPLICATIONS

A typical alpha(1L) phenotype with extremely low affinity for prazosin is exclusively expressed in the iris of pigmented rabbits, while two distinct phenotypes (alpha(1A) and alpha(1L)) with high and moderate affinities for prazosin are co-expressed in the iris of albino rabbits. This suggests that a significant difference in the expression of phenotypes of the alpha(1)-adrenoceptor occurs in the irises between the two strains of rabbits.

Pharmacological evaluation of ocular beta-adrenoceptors in rabbit by tissue segment binding method

AIMS

This study evaluates ocular (iris, ciliary body and ciliary process) and nonocular (atria and lung) beta-adrenoceptors in rabbit to characterize the plasma membrane beta-adrenoceptors and binding affinities of beta-adrenoceptor antagonists.

MAIN METHODS

The tissue segment binding method with a hydrophilic radioligand (-)-4-[3-t-butylamino-2-hydroxypropoxy]-[5,7-(3)H]benzimidazol-2-one ([(3)H]-CGP12177) was employed.

KEY FINDINGS

Specific and saturable binding of [(3)H]-CGP12177 to intact tissue segments was detected by using (+/-)-propranolol to define nonspecific binding, showing a single population of plasma membrane binding sites with high affinity. Competition experiments with selective beta(1)- and beta(2)-adrenoceptor antagonists revealed a single population of beta(2)-adrenoceptors in ocular tissues and of beta(1)-adrenoceptors in atria, but mixed populations of beta(1)- and beta(2)-adrenoceptors in 70% and 30%, respectively, in lung. A competition curve for timolol was biphasic in lung and its binding affinity for beta(2)-adrenoceptors was approximately 158-fold higher than for beta(1)-adrenoceptors, indicating the beta(2)-selectivity of timolol. In contrast, competition curves for stereoisomers of befunolol, carteolol, and propranolol were monophasic in all tissues. The (-)-enantiomers of these antagonists were more potent than corresponding (+)-enantiomers in displacing from [(3)H]-CGP12177 binding, and the isomeric potency ratios of befunolol and carteolol were less than those of propranolol.

SIGNIFICANCE

This study with tissue segment binding method suggests that the binding affinity of (-)-enantiomers of beta-adrenoceptor antagonists for plasma membrane beta-adrenoceptors (beta(1)-adrenoceptors of atria, beta(2)-adrenoceptors of ocular tissues, and mixed beta(1)-/beta(2)-adrenoceptors of lung) is higher than that of corresponding (+)-enantiomers and their stereoselectivity is different between beta-adrenoceptor antagonists.

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 alpha1L-adrenoceptor is an alternative phenotype of the alpha1A-adrenoceptor

Despite over two decades of research, the molecular identity of the alpha1L-adrenoceptor phenotype has remained elusive. In this issue of the BJP, Gray et al. (2008) provide persuasive evidence that the in vivo alpha1L-adrenoceptor phenotype requires the expression of the alpha1A-adrenoceptor gene. They have shown that in mice lacking the functional alpha1A-adrenoceptor gene, alpha1L-mediated responses to noradrenaline in prostate smooth muscle are substantially attenuated. These findings support earlier evidence that the alpha(1L)-adrenoceptor profile represents a functional phenotype of the alpha(1A)-adrenoceptor gene product, but additional cell background-dependent factors must act in concert with the alpha(1A)-adrenoceptor protein to determine whether an alpha(1L)- or a classical alpha(1A)-adrenoceptor profile is expressed. The challenge remains to establish the nature of these cellular factors and the mechanism(s) by which they influence G-protein-coupled receptor pharmacology.