Pharmacological pleiotropism of the human recombinant alpha(1A)-adrenoceptor: implications for alpha(1)-adrenoceptor classification

1 Three fully-defined al-adrenoceptors (alpha(1A), alpha(1B) and alpha(1D)) have been established in pharmacological and molecular studies. A fourth alpha(1)-adrenoceptor, the putative alpha(1L)-adrenoceptor, has been defined in functional but not molecular studies, and has been proposed to mediate contraction of human lower urinary tract tissues; its relationship to the three fully characterized alpha(1)-adrenoceptors is not known. 2 In the present study, binding affinities were estimated by displacement of [H-3]-prazosin in membrane homogenates of Chinese hamster ovary (CHO-KI) cells stably expressing the human alpha(1A)-, alpha(1B)- and alpha(1D)-adrenoceptors and were compared with affinity estimates obtained functionally in identical cells by measuring inhibition of noradrenaline (NA)-stimulated accumulation of [H-3]-inositol phosphates. 3 For the alpha(1A)-adrenoceptor, binding studies revealed a pharmacological profile typical for the classically defined alpha(1A)-adrenoceptor, such that prazosin, RS-17053, WE 4101, 5-methylurapidil, Rec 15/2739 and S-niguldipine all displayed subnanomolar affinity. A different profile of affinity estimates was obtained in inositol phosphates accumulation studies: prazosin, WE 4101, 5-methylurapidil, RS-17053 and S-niguldipine showed 10 to 40 fold lower affinity than in membrane binding. However, affinity estimates were not 'frameshifted', as tamsulosin, indoramin and Rec 15/2739 yielded similar, high affinity estimates in binding and functional assays. 4 In contrast, results from human alpha(1B)- and alpha(1D)-adrenoceptors expressed in CHO-K1 cells gave antagonist affinity profiles in binding and functional assays that were essentially identical. 5 A concordance of affinity estimates from the functional (inositol phosphates accumulation) studies of the alpha(1A)-adrenoceptor in CHO-KI cells was found with estimates published recently from contractile studies in human lower urinary tract tissues (putative alpha(1L)-adrenoceptor). These data show that upon functional pharmacological analysis, the cloned alpha(1A)-adrenoceptor displays pharmacological recognition properties consistent with those of the putative alpha(1L)-adrenoceptor. Why this profile differs from that obtained in membrane binding, and whether it explains the alpha(1L)-adrenoceptor pharmacology observed in many native tissues, requires further investigation.