Investigation of alpha 1-adrenoceptor subtypes in canine aorta, using alkylating agents

The ability of putative selective irreversible ligands SZL-49 (1-(4-amino-6,7-dimethoxy-2,5-diene-2-carbonyl)) and CEC (chlorethylclonidine), for alpha 1A and alpha 1B adrenoceptor subtypes, respectively, to affect alpha 1-adrenoceptors of canine aorta microsomal membranes was investigated. These membranes contain an apparently homogeneous population of [3H]prazosin binding sites. SZL-49, like phenoxybenzamine, abolished all binding of [3H]prazosin. CEC abolished 75% of the prazosin binding sites under the most stringent conditions we applied. However, the remaining 25% of binding sites was identical in affinity for prazosin with control membranes, and competition studies of other subtype-selective ligands revealed unchanged ability to complete against CEC-sensitive and -insensitive sites. We concluded that SZL-49 and CEC are not alpha 1A- and alpha 1B-adrenoceptor selective under in vitro conditions. Our data led to the hypothesis that canine aortic membranes contain exclusively alpha 1B-adrenoceptors but that current tools for identifying alpha 1-adrenoceptor subtypes proved inadequate in vitro in this study.