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.
Konno
F
F
Department of Chemical Pharmacology, Toho University School of Pharmaceutical Sciences, Chiba, Japan.
Kusunoki
M
M
Takayanagi
I
I
D000818:Animals; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects / Q000294:innervation; D007093:Imidazoles / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D009638:Norepinephrine / Q000037:antagonists & inhibitors; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000145:classification / Q000187:drug effects*; D013045:Species Specificity; D014649:Vas Deferens / Q000187:drug effects / Q000294:innervation; D015016:Yohimbine / Q000494:pharmacology
D007093:Imidazoles; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D009638:Norepinephrine