Involvement of alpha 2-adrenoceptors in the activation of kininogenesis by clonidine.
The effects of clonidine and alpha 2-adrenoceptor blocking agents, yohimbine and tolazoline on kininogen and prekallikrein levels in plasma and on blood pressure were investigated in male Wistar rats. Clonidine (0.5 mg/kg) decreased concentration of kininogen. Yohimbine (3 mg/kg) and tolazoline (5 mg/kg) counteracted this effect of clonidine and increased the kininogen level in plasma. Clonidine did not change the level of prekallikrein, however in rats receiving alpha 2-adrenoceptor blockers alone or together with clonidine kallikrein utilization was reduced, which indicated indirectly that the level of prekallikrein in plasma augmented. Yohimbine and tolazoline in the same doses (3 mg/kg and 5 mg/kg resp.) increased the blood pressure and counteracted the hypotensive action of clonidine, which indicated the presynaptic alpha 2-adrenoceptor blocking effect of the doses used. The data indicate that clonidine-induced activation of kininogenesis as well as its hypotensive effect are counteracted by yohimbine and tolazoline in doses blocking specifically presynaptic alpha 2-adrenoceptors. It can be suggested that alpha 2-adrenoceptors are involved in the kininogenesis-activating effect of clonidine.