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To characterize the agonist profile of alpha 2-adrenoceptor agonists (imidazoline-like drugs, azoloazepine derivatives, beta-phenethylamines-like drugs) on human platelets, the characteristics of alpha 2-adrenoceptors (KD, Bmax) have been evaluated and the affinity constants measured by displacement technique and computer-assisted analysis of the curves. Furthermore, since alpha 2-adrenoceptor agonists interact with the post-synaptic receptors in a calcium-operated channel, whether the effect of calcium-entry inhibitors (verapamil, nifedipine, diltiazem) is related to a competition with alpha 2-receptors has also been examined. By Scatchard analysis, it was calculated that in human platelets alpha 2-adrenoceptors have KD = 3.45 nM and Bmax = 247 fmol (mg protein)-1. As far as the potency is concerned, imidazoline-like drugs were the most potent agonists in human platelet alpha 2-adrenoceptors (guanabenz IC50 = 8.6 +/- 0.8 X 10(-8), B-HT 920 IC50 = 2.9 +/- 0.3 X 10(-7), (-)-adrenaline IC50 = 3.4 +/- 0.5 X 10(-7)). Among the calcium-entry inhibitors only verapamil antagonized [3H]rauwolscine binding: the effect was stereospecific, (-)-D 600 being more potent than (+)-D 600. Nifedipine and diltiazem did not affect alpha 2-receptor binding. It is concluded that human platelets alpha 2-receptors share the agonist potency profile of other tissues containing alpha 2-receptors (brain, pre-synaptic junction), and that among calcium-entry blockers only verapamil can antagonize alpha 2-agonists. Nifedipine and diltiazem do not appear to interact stereospecifically with alpha 2-adrenoceptors.
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