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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 [3H]-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 [3H]-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.

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 Ca2+ 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.

α1-Adrenoceptors are involved in numerous physiological functions, including micturition. However, the pharmacological profile of the α1-adrenoceptor subtypes remains controversial. Here, we review the literature regarding α1-adrenoceptors in the lower urinary tract from the standpoint of α1L phenotype pharmacology. Among three α1-adrenoceptor subtypes (α1A, α1B and α1D), α1a-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, α1A-adrenoceptors with high affinity for prazosin have been detected as radioligand binding sites. Functional α1-adrenoceptors in the prostate, urethra and bladder neck have low affinity for prazosin, suggesting the presence of an atypical α1-adrenoceptor phenotype (designated as α1L). The α1L-adrenoceptor occurs as a distinct binding entity from the α1A-adrenoceptor in intact segments of variety of tissues including prostate. Both the α1L- and α1A-adrenoceptors are specifically absent from Adra1A (α1a) gene-knockout mice. Transfection of α1a-adrenoceptor cDNA predominantly expresses α1A-phenotype in several cultured cell lines. However, in CHO cells, such transfection expresses α1L- and α1A-phenotypes. Under intact cell conditions, the α1L-phenotype is predominant when co-expressed with the receptor interacting protein, CRELD1α. In summary, recent pharmacological studies reveal that two distinct α1-adrenoceptor phenotypes (α1A and α1L) originate from a single Adra1A (α1a-adrenoceptor) gene, but adrenergic contractions in the lower urinary tract are predominantly mediated via the α1L-adrenoceptor. From the standpoint of phenotype pharmacology, it is likely that phenotype-based subtypes such as the α1L-adrenoceptor will become new targets for drug development and pharmacotherapy. LINKED ARTICLE This article is commented on by Ventura, pp. 1223–1225 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2011.01663.x

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.[(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.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.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.