Blockage of clonidine-induced platelet aggregation in rabbits by procainamide

Procainamide was capable of blocking the alpha 2-adrenergic receptor agonist clonidine-induced platelet aggregation, giving an antagonistic index, pA2, of 5.0 +/- 0.6 and half antagonistic concentration, A2, of 10.4 mumol.L-1. Clonidine showed half efficacy concentrations (EC50) of 44, 82, 182, 485, and 662 nmol.L-1, and affinity parameter (pD2) of 7.4, 7.1, 6.7, 6.3, and 6.2 respectively when different concentrations of procainamide were used as blocking reagent. The results indicated that the mechanism of inhibitory effect of procainamide on clonidine-induced platelet aggregation was to competitively antagonize activating alpha 2-receptors and others of clonidine on platelet membrane.