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Abstract
Presynaptic inhibitory effects of noradrenaline, clonidine and fentanyl on twitch contractions evoked by transmural electrical stimulation were investigated in guinea-pig common bile duct (CBD). Antagonism of the response to fentanyl by naloxone and to noradrenaline and clonidine by yohimbine confirmed the involvement of presynaptic opiate and α2-adrenoceptors respectively.
Clonidine caused desensitization of presynaptic α2-adrenoceptors. No cross desensitization between clonidine and fentanyl was observed. Furthermore, partial agonistic activity of clonidine at presynaptic α2-adrenoceptors was also observed by the antagonism of noradrenaline response in this preparation.