抄録
A series of imidazolines and norepinephrine were used to characterize and differentiate the presynaptic alpha2-adrenoceptors in the rabbit vas deferens and the guinea pig Heal longitudinal muscle using pharmacological procedures. Based on pEC50-values (the negative log of the 50% effective concentration) for each imidazoline, a rank order of potency of p-aminoclonidine>oxymetazoline≥clonidine>naphazoline>phentolamine was obtained in the rabbit vas deferens and an order of p-aminoclonidine>clonidine>naphazoline>oxymetazoline was obtained in the guinea pig ileum. In the rabbit vas deferens, phentolamine, which is generally considered to be a competitive alpha1- and alpha2-adrenoceptor antagonist, acted as a full alpha2-adrenoceptor agonist. The dissociation constants of oxymetazoline and yohimbine were significantly lower in the rabbit vas deferens than in the guinea pig ileum. These results suggest that the presynaptic alpha2-adrenoceptors in these tissues are different. Furthermore, the pKB-value of yohimbine against norepinephrine was significantly one log unit lower than those obtained using a series of imidazolines. Data from our studies add to increasing evidence of the existence of high and low affinity binding sites on the alpha2-adrenoceptors in the rabbit vas deferens.
