Cloning of rabbit alpha(1b)-adrenoceptor and pharmacological comparison ofalpha(1a)-, alpha(1b),- and alpha(1d)-adrenoceptors in rabbit

We have isolated a cDNA clone of the rabbit alpha(1b)-adrenoceptor which ha s 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 alpha(1b)-adrenoceptors than to those of rabbit alpha(1a)- and alpha(1b )-adrenoceptors. The pharmacological binding properties of this clone expre ssed in Cos-7 cells showed a characteristic profile as alpha(1b)-adrenocept or; high affinity for prazosin (pK(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-carboxamid e (KMD3213) (8.5), 2-(2,6-dimethoxy-phenoxyethyl)-aminomethyl-1,4-benzodiox ane hydrochloride (WB4101) (8.7), and 8-[2-[4-(2-methoxy-phenyl)-L-piperazi nyl]-8-azaspiro[4,5]decane-7,9-dione dihydrochloride (BMY7378) (7.3). We ha ve compared the levels of mRNA expression of three alpha(1)-adrenoceptor su btypes in rabbit tissues using the competitive reverse transcription/polyme rase chain reaction (RT/PCR) assay. In most rabbit tissues except heart, al pha(1a)-adrenoceptor mRNA was expressed 10 folds more than the other two su btypes. However, binding experiments with [H-3]prazosin and [H-3]KMD3213 in rabbit tissues revealed a poor relationship between binding density and mR NA level. Especially, alpha(1b) binding sites were exclusively predominant in spleen, whereas the alpha(1b) subtype was minor at the mRNA level. These results indicate a high identity of structural and pharmacological profile s of three distinct alpha(1)-adrenoceptor subtypes between rabbit and other species, but there are species differences in their distribution. (C) 2000 Elsevier Science B.V. All rights reserved.