Effects of Imidazoline and Non-Imidazoline α-Adrenergic Agents on Canine Platelet Aggregation

Aggregatory and antiaggregatory effects of imidazol(in)e and non-imidazol(in)e α-adrenergic agents on canine platelets were examined turbidimetrically in citrated platelet-rich plasma or washed platelet solution. Each α-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 α 2 -adrenoceptor antagonists and/or imidazol(in)e α-adrenergic agents inhibit dose-dependently adrenaline-potentiated aggregation, whereas α 1 -adrenoceptor antagonists, a β-adrenoceptor antagonist and non-imidazol(in)e α-adrenergic agents were no or less effective in inhibiting adrenaline-potentiated aggregation. The α 2 -adrenoceptor agonists did not reduce inhibitory effect of α 2 -adrenoceptor antagonists for adrenaline-potentiated aggregation. The α 2 -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 α 2 -adrenoceptor-blocking agents and/or imidazol(in)e α-adrenergic agents inhibit effectively the adrenaline-potentiated platelet aggregation.