Pharmacological pleiotropism of the human recombinant α_1A-adrenoceptor: implications for α₁-adrenoceptor classification

Abstract

1. Three fully-defined α₁-adrenoceptors (α_1A, α_1B and α_1D) have been established in pharmacological and molecular studies. A fourth α₁-adrenoceptor, the putative α_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 α₁-adrenoceptors is not known.

2. In the present study, binding affinities were estimated by displacement of [³H]-prazosin in membrane homogenates of Chinese hamster ovary (CHO-K1) cells stably expressing the human α_1A-, α_1B- and α_1D-adrenoceptors and were compared with affinity estimates obtained functionally in identical cells by measuring inhibition of noradrenaline (NA)-stimulated accumulation of [³H]-inositol phosphates.

3. For the α_1A-adrenoceptor, binding studies revealed a pharmacological profile typical for the classically defined α_1A-adrenoceptor, such that prazosin, RS-17053, WB 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, WB 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 α_1B- and α_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 α_1A-adrenoceptor in CHO-K1 cells was found with estimates published recently from contractile studies in human lower urinary tract tissues (putative α_1L-adrenoceptor). These data show that upon functional pharmacological analysis, the cloned α_1A-adrenoceptor displays pharmacological recognition properties consistent with those of the putative α_1L-adrenoceptor. Why this profile differs from that obtained in membrane binding, and whether it explains the α_1L-adrenoceptor pharmacology observed in many native tissues, requires further investigation.

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