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

Abstract The recombinant α 1A -adrenoceptor displays a distinct pharmacological profile (`classical α 1A -adrenoceptor') in homogenate binding assays, but displays the properties of the so-called α 1L -adrenoceptor in functional studies in whole cells at 37°C. As three splice variants of the human α 1A -adrenoceptor have been described previously (α 1A-1 , α 1A-2 and α 1A-3 ), we have compared their functional pharmacological profiles, when expressed stably in Chinese hamster ovary (CHO-K1) cells (antagonist inhibition of noradrenaline-stimulated [ 3 H ]inositol phosphates accumulation). A fourth, novel isoform (α 1A-4 ) has also been studied: α 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-furoyl)piperazine), tamsulosin (5-[2-[[2-(2-ethoxyphenoxy)ethyl]-amino]propyl]-2-methoxybenzenesulfonamide), RS-17053 ( N -[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-α,α-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 [ 3 H ]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 α 1L -adrenoceptor. Thus, all four human recombinant α 1A -adrenoceptor isoforms display the pharmacology of the α 1L -adrenoceptor when studied in functional assays, consistent with the hypothesis that the putative α 1L -adrenoceptor represents a functional phenotype of the α 1A -adrenoceptor.