Presence of prejunctional D2-dopaminoceptors and α2-adrenoceptors on the cholinergic nerve of the common bile duct of guinea pig

On most adrenergic and cholinergic nerve terminals, prejunctional α-adrenoceptors belonging to the α2-subtype have been identified. Activation of these receptors will decrease the release of norepinephrine. It has been reported that several isolated tissue preparations contain prejunctional dopamine receptors, the stimulation of which inhibits neurotransmission. It has remained uncertain whether the inhibition of cholinergic transmission in the smooth muscle of different tissues is mediated by prejunctional α-adrenoceptors or prejunctional dopaminoceptors. On the basis of these findings we aimed to examine the effects of dopamine and α-adrenergic receptor agonists on twitch response evoked by electrical stimulation of guinea pig common bile duct (CBD). Effects of various doses of bromocriptine and apomorphine, dopamine receptor agonists, on electrically evoked contractile responses of guinea pig CBD were investigated in this study. Bromocriptine and apomorphine produced a concentration -dependent reduction of the twitch heights. IC50 values for bromocriptine and apomorphine were 2.75 ± 0.715 x 10-6 M and 1.69 ± 0.l1 x 10-5 M, respectively. Pretreatment with sulpride, 0.1 and 1.0 µM prevented bromocriptine and apomorphine induced inhibition. PA2 values of sulpiride against bromocriptine and apomorphine were 8.26 and 6.29, respectively. Clonidine, 0.1 -100 µM, produced 49.3 ± 2.5% inhibition of twitch responses. Its effect was partially antagonized by yohimbine (l µM). It is concluded that both dopamine D2 and adrenergic α2 prejunctional receptors are present on the guinea-pig CBD cholinergic nerve and it seems that the role of dopamine D2-receptors is dominant. D2-dopaminoreceptor and α2-adrenoceptor agonists may relieve the biliary spasm.