Identification and biochemicai characterization of α_1c adrenoceptor protein

α_1 -adrenoceptors (ARs) comprise a heterogeneous family：two natively expressed subtypes (α_1A and α_1B ) can be distinguished pharmacologically, while three subtypes (α_1b , α_1c and α_1a/d) have been cloned. The relationship between the pharmacologically derfined receptors and the cDNA clones is slill uncertain. Recently, we showed that α_1c AR mRNA expressed in many human tissues , and that the receptor may be the pharmacologically defined α_1A AR (Ref.1 and 2). To obtain the further evidence for the cloned α1_cAR as the pharmacologically defined α_1A AR at protein level, we compared the α_1c AR protein expressed in CHO cells with the pharmacologically defined α_1A AR in native tissues/cells by using photoaffinity labelling and immunological assays. Photoaffinity labeling with 「^^125 I」 aryl azidoprazosin showed the α_1c AR protein as 70 kDa protein. With the anti-peptide antibodies developed against α_1c AR, α_1c AR protein was specificaily identified by Western blotting. Also, the photoaffinity labeled α_1c AR protein was immunoprecipitated by the antibodies. Further examining the tissues/cells known to be expressing the pharmacologically defined α_1A AR, such as human liver and SKNMC cells, we detected α_1c AR protein can be detected in human tissues, where α_1A AR can be identified pharmacologically. These results strongly support the idea that α_1c AR is the pharmacologically defined α_1A AR.