Selective irreversible binding of chloroethylclonidine at alpha 1- and alpha 2-adrenoceptor subtypes

We have determined the alkylating effects and affinity of chloroethylclonidine at alpha 1- and alpha 2-adrenoceptor subtypes in saturation and competition radioligand binding studies. Treatment with chloroethylclonidine (10 microM, for 30 min at 37 degrees, with subsequent washout) abolished [3H]prazosin binding to alpha 1B-adrenoceptors in rat spleen almost completely and reduced specific binding in rat kidney and cerebral cortex by a percentage comparable to the known alpha 1B-adrenoceptor content of these tissues. Chloroethylclonidine treatment also markedly reduced [3H]rauwolscine binding to human platelet and kidney membranes but did not affect [3H]rauwolscine binding to rat kidney. Similar chloroethylclonidine treatment (10 microM, 20 min at 37 degrees) reduced the number of detectable alpha 2-adrenoceptors in cell lines transfected with the alpha 2-C10 or alpha 2-C4 gene but not in those transfected with alpha 2-C2 adrenoceptors. In concentration-response experiments, higher chloroethylclonidine concentrations were required for inactivation of human platelet alpha 2A-adrenoceptors, compared with rat spleen alpha 1B-adrenoceptors, and a smaller maximal inactivation was achieved. The lack of inactivation of rat alpha 1A- and alpha 2B- and human alpha 2-C2-adrenoceptors was not due to a lack of chloroethylclonidine binding, because the affinity of chloroethylclonidine at these subtypes, as determined in competition binding experiments, was at least as high as the apparent affinity at the alkylated subtypes. alpha 2A-Adrenoceptor alkylation by chloroethylclonidine treatment was functionally relevant, because it significantly reduced alpha 2A-adrenoceptor-mediated Ca2+ elevations in HEL cells. We conclude that chloroethylclonidine binds to all major alpha-adrenoceptor subtypes and irreversibly inactivates not only alpha 1B-adrenoceptors but also alpha 2A- and alpha 2C-adrenoceptors, whereas alpha 1A- and alpha 2B-adrenoceptors are relatively resistant to its alkylating action, although they can bind chloroethylclonidine.