Human cloned α1A-adrenoceptor isoforms display α1L-adrenoceptor pharmacology in functional studies

The recombinant alpha(1)-adrenoceptor displays a distinct pharmacological profile ('classical alpha(1)-adrenoceptor') in homogenate binding assays, but displays the properties of the so-called alpha(1L)-adrenoceptor in functional studies in whole cells at 37 degrees C. As three splice variants of the human alpha(1A)-adrenoceptor have been described previously (a(1A-1), alpha(1A-2) and alpha(1A-3)), we have compared their functional pharmacological profiles, when expressed stably in Chinese hamster ovary (CHO-K1) cells (antagonist inhibition of noradrenaline-stimulated [H-3]inositol phosphates accumulation). A fourth, novel isoform (alpha(1A-4)) has also been studied: alpha(1A-4) mRNA predominates in several human tissues including prostate, liver, heart and bladder. In homogenate binding studies, all four isoforms displayed essentially identical affinity profiles, with prazosin (1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2-fuoryl)piperazine), tamsulosin (5-[2-[[2-(2-ethoxyphenoxy)ethyl]-amino]propyl]-2-methoxybenzenesulfonamide), RS-17053 (N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethanamine hydrochloride), WB 4101 ((2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride) and 5-Me-urapidil (5-methyl-6[[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]amino]-1,3-dimethyuracil) all displaying subnanomolar affinities. In functional studies, noradrenaline accelerated [H-3]inositol phosphates production with potencies (p[A](50)) of between 5.8 and 6.6. The affinities of prazosin, RS-17053, WB 4101 and 5-Me-urapidil, at antagonizing responses to noradrenaline, were reduced by approximately 10-fold (cf. binding data), while those for tamsulosin and indoramin (N-[1-[2-(1H-indol-3-yl)ethyl]-4-piperidinyl]benzamide) remained constant or increased, consistent with the previously described alpha(1L)-adrenoceptor. Thus, all four human recombinant alpha(1A)-adrenoceptor isoforms display the pharmacology of the alpha(1L)-adrenoceptor when studied in functional assays, consistent with the hypothesis that the putative alpha(1L)-adrenoceptor represents a functional phenotype of the alpha(1A)-adrenoceptor. (C) 1999 Elsevier Science B.V. All rights reserved.