Effects of imidazoline and non-imidazoline alpha-adrenergic agents on canine platelet aggregation

Aggregatory and antiaggregatory effects of imidazol(in)e and non-imidazol(in)e alpha-adrenergic agents on canine platelets were examined turbidimetrically in citrated platelet-rich plasma or washed platelet solution. Each alpha-adrenoceptor agonist alone did not induce aggregation, but adrenaline and noradrenaline potentiated dose-dependently aggregation stimulated by ADP, collagen or thrombin. Small potentiation of ADP- or collagen-stimulated aggregation was also observed in response to oxymetazoline. The alpha2-adrenoceptor antagonists and/or imidazol(in)e alpha-adrenergic agents inhibit dose-dependently adrenaline-potentiated aggregation, whereas alpha1-adrenoceptor antagonists, a beta-adrenoceptor antagonist and non-imidazol(in)e alpha-adrenergic agents were no or less effective in inhibiting adrenaline-potentiated aggregation. The alpha2-adrenoceptor agonists did not reduce inhibitory effect of alpha2-adrenoceptor antagonists for adrenaline-potentiated aggregation. The alpha2-adrenoceptor antagonists and/or imidazol(in)es were no or less effective in inhibiting aggregation induced by ADP or thrombin alone. These results demonstrated that alpha2-adrenoceptor-blocking agents and/or imidazol(in)e alpha-adrenergic agents inhibit effectively the adrenaline-potentiated platelet aggregation.