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

Aggregatory and antiaggregatory effects of imidazo[< in > e and non-imidazo l < in > e alpha-adrenergic agents on canine platelets were examined turbid imetrically in citrated platelet-rich plasma or washed platelet solution. E ach alpha-adrenoceptor agonist alone did not induce aggregation, but adrena line and noradrenaline potentiated dose-dependently aggregation stimulated by ADP, collagen or thrombin. Small potentiation of ADP- or collagen-stimul ated aggregation was also observed in response to oxymetazoline. The alpha( 2)-adrenoceptor antagonists and/or imidazol < in > e alpha-adrenergic agent s inhibit dose-dependently adrenaline-potentiated aggregation, whereas alph a(1)-adrenoceptor antagonists, a beta-adrenoceptor antagonist and non-imida zol < in > e alpha-adrenergic agents were no or less effective in inhibitin g adrenaline-potentiated aggregation. The alpha(2)-adrenoceptor agonists di d not reduce inhibitory effect of alpha(2)-adrenoceptor antagonists for adr enaline-potentiated aggregation. The alpha(2)-adrenoceptor antagonists and/ or imidazo[< in > es were no or less effective in inhibiting aggregation in duced by ADP or thrombin alone. These results demonstrated that alpha(2)-ad renoceptor-blocking agents and/or imidazol < in > e alpha-adrenergic agents inhibit effectively the adrenaline-potentiated platelet aggregation.