Receptors mediating the contractile effect of serotonin (5-HT) in the isolated common bile duct of guinea-pig.

The contractile effect of 5-HT in the isolated common bile ducts of guinea-pigs was studied. 5-HT, administered non-cumulatively, evoked contractions which were concentration-dependent. Responses due to the low concentrations of 5-HT were antagonised significantly by ketanserin (10(-7) M and 10(-6) M) and methysergide (10(-6)M and 10(-5) M) whereas those induced by the higher concentrations of 5-HT remained unchanged. Atropine (3 x 10(-8) and 10(-7) M) and ICS 205-930 (10(-7) M and 10(-6) in contrast inhibited the contractions elicited by high concentrations of 5-HT without altering significantly the responses due to the lower ones. The results led us to conclude that 5-HT evoked contractions at low concentrations are predominantly mediated by 5-HT2 receptors whereas those at high concentrations are dependent on acetylcholine release via the stimulation of 5-HT3 receptors.