Cloning of rabbit α 1b-adrenoceptor and pharmacological comparison of α 1a-, α 1b- and α 1d-adrenoceptors in rabbit

Abstract

We have isolated a cDNA clone of the rabbit α 1b-adrenoceptor which has an open reading frame of 1557 nucleotides encoding a protein of 518 amino acids. The sequence shows higher identity to those of hamster, human, and rat α 1b-adrenoceptors than to those of rabbit α 1a- and α 1d-adrenoceptors. The pharmacological binding properties of this clone expressed in Cos-7 cells showed a characteristic profile as α 1b-adrenoceptor; high affinity for prazosin (p K i=10.3), relatively high affinity for tamsulosin (9.5) and low affinity for (−)-( R)-1-(3-hydroxypropyl)-5-[2-[[2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl]amino]propyl]indoline-7-carboxamide (KMD3213) (8.5), 2-(2,6-dimethoxy-phenoxyethyl)-aminomethyl-1,4-benzodioxane hydrochloride (WB4101) (8.7), and 8-[2-[4-(2-methoxy-phenyl)- l-piperazinyl]-8-azaspiro[4,5]decane-7,9-dione dihydrochloride (BMY7378) (7.3). We have compared the levels of mRNA expression of three α 1-adrenoceptor subtypes in rabbit tissues using the competitive reverse transcription/polymerase chain reaction (RT/PCR) assay. In most rabbit tissues except heart, α 1a-adrenoceptor mRNA was expressed 10 folds more than the other two subtypes. However, binding experiments with [ 3H]prazosin and [ 3H]KMD3213 in rabbit tissues revealed a poor relationship between binding density and mRNA level. Especially, α 1b binding sites were exclusively predominant in spleen, whereas the α 1b subtype was minor at the mRNA level. These results indicate a high identity of structural and pharmacological profiles of three distinct α 1-adrenoceptor subtypes between rabbit and other species, but there are species differences in their distribution.