Abstract
1. 1. Effects of various doses of bromocriptine and apomorphine, dopamine receptor agonists, on electrically evoked contractile responses of guinea pig common bile duct (CBD) were investigated in this study. 2. 2. Bromocriptine and apomorphine produced a concentration-dependent reduction of the twitch heights. IC 50 values for bromocriptine and apomorphine were 2.75±0.715 × 10 −6M, and 1.69±0.11 × 10 −5M, respectively. 3. 3. Pretreatment with sulpiride, 0.1 and 1.0 μM, prevented bromocriptine- and apomorphine-induced inhibition. pA 2 values of sulpiride against bromocriptine and apomorphine were 8.26 and 6.29, respectively. 4. 4. Clonidine, 0.1–100 μM, produced 49.3±2.5% inhibition of twitch responses. Its effect was partially antagonized by 1 μM of yohimbine. 5. 5. It would appear that both dopamine D 2- and α 2-adrenergic presynaptic receptors are present on the guinea-pig CBD cholinergic nerve.
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