Receptor interactions of a series of imidazolines: comparison of the alpha 2-adrenoceptors between the rabbit vas deferens and guinea pig ileum.

A series of imidazolines and norepinephrine were used to characterize and differentiate the presynaptic alpha 2-adrenoceptors in the rabbit vas deferens and the guinea pig ileal 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 greater than oxymetazoline greater than or equal to clonidine greater than naphazoline greater than phentolamine was obtained in the rabbit vas deferens and an order of p-aminoclonidine greater than clonidine greater than naphazoline greater than oxymetazoline was obtained in the guinea pig ileum. In the rabbit vas deferens, phentolamine, which is generally considered to be a competitive alpha 1- and alpha 2-adrenoceptor antagonist, acted as a full alpha 2-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 alpha 2- 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 alpha 2-adrenoceptors in the rabbit vas deferens.