Pentazocine reduces cholinergic responses in the guinea-pig extrahepatic biliary tract by a non-opiate mechanism.

The mechanical responses of the guinea-pig gallbladder and common bile duct to pentazocine were measured in vitro. At concentrations of 1 microM or higher, pentazocine reduced the responses of the gallbladder and common bile duct to electrical stimulation and to carbachol. This antagonism was not blocked by naloxone and appears to be due to an atropine-like action of pentazocine. At concentrations of 5 microM or higher pentazocine induced gallbladder contraction which was naloxone-insensitive. Both effects were not affected by tetrodotoxin and appear to be due to direct action on the muscle. The antimuscarinic activity of pentazocine should be taken into consideration when this opioid is administered experimentally or clinically.