Clonidine-induced desensitization differentiates presynaptic alpha-2 adrenoceptors of the guinea-pig ileum and rat vas deferens.

Clonidine-induced inhibitions of twitch responses of coaxially stimulated guinea-pig ileum and transmurally stimulated rat vas deferens were antagonized by yohimbine in a competitive manner. In contrast to rat vas deferens, clonidine caused desensitization of presynaptic alpha 2-adrenoceptors on cholinergic neurons of guinea-pig ileum. Cross desensitization was observed between clonidine and oxymetazoline. No cross desensitization between clonidine and morphine was observed and indicates that presynaptic alpha 2-adrenoceptors are involved in clonidine-induced desensitization of the presynaptic site in the guinea-pig ileum.