Alpha-adrenoceptor-mediated actions of optical isomers and desoxy analogs of catecholimidazoline and norepinephrine in human platelets: In vitro

Abstract

Adrenoceptor-mediated effects of the enantiomers of optically active imidazoline, 2-(3,4,α-trihydroxybenzyl imidazoline (catecholimidazoline; CI), and norepinephrine (NE), and the corresponding desoxy derivatives, 2-(3,4-dihydroxybenzyl)imidazoline (desoxy-CI) and dopamine, have been investigated in human platelets. Differences between responsiveness of platelets from donor to donor were observed in the presence of the isomers and the desoxy analogs of NE and CI. In certain platelet preparations, all compounds gave concentration-dependent stimulatory responses, whereas in other preparations, only R(-)-NE and R(-)-CI were inducers of platelet aggregation and serotonin release. The rank order of stimulatory potencies (ec50; μM) for CI and NE was R(-)-NE (1.3) > R(-)-CI (7.5) > S(+)-NE (19) = S (+)-CI (20) = dopamine (22) > desoxy-CI (>35). Unlike R(-)-CI, both S(+)-CI and desoxy-CI were either agonists or antagonists of human platelet function. In preparations unresponsive to the S(+)-isomers or desoxy analogs, the potencies (EC50) for R(-)-NE and R(-)-CI were 1.7 and 7.7 μM respectively. The corresponding inactive CIs [S(+)-CI and desoxy-CI] were inhibitors of both primary and secondary phases of aggregation and serotonin release responses to R(-)-CI and R(-)-NE, respectively. In contrast, the aggregation responses to ADP, arachidonic acid or U46619 were not blocked by S(+)-CI or desoxy CI. The rank order of inhibitory potencies for selected α-adrenoceptor agents against R(-)-NE was phentolamine > clonidine > desoxy-CI > S(+)-CI. Moreover, the relative inhibitory potencies of phentolamine and desoxy-CI against aggregation responses to R(-)-NE and R(-)-CI, respectively, were the same. These results suggest that (1) the enantiomers and desoxy derivatives of CI and NE mediate their effects in human platelets by an interaction with α-adrenoceptors; (2) catecholamines and imidazolines interact with the same α-adrenoceptors in human platelets; (3) the stereochemical requirements of both chemical classes for stimulatory activity in human platelets adhere to the Easson-Stedman hypothesis in this α2-adrenoceptor system; and (4) desoxy-CI possessed the highest potency as an antagonist of α-adrenoceptors which suggests that the hydroxy group at the benzylic carbon atom of these imidazolines may not be required for maximal binding to adrenoceptors in platelets. © 1986.