Muscarinic receptors in vagal routes to the biliary system in dogs.

Although muscarinic receptor subtypes in the biliary system have been characterized, those in the vagal routes have not. This study was performed to characterize the muscarinic receptors in the vagal routes to the biliary system. The effects of four types of muscarinic antagonists, pirenzepine, AF-DX 116, p-F-HHSiD, and atropine, on excitatory responses in the gallbladder and sphincter of Oddi induced by electrical stimulation of the dorsal motor nucleus of the vagus (DMV) were studied in cervical cord-transected anesthetized dogs. Atropine 10 micrograms/kg and over and p-F-HHSiD 50 micrograms/kg and over significantly decreased the excitatory responses in both effectors, but it required a high dose of AF-DX 116 (1-2 mg/kg) to reduce those responses. Pirenzepine (50-300 micrograms/kg) significantly decreased the excitatory response in the gallbladder, but not that in the sphincter of Oddi. A high dose of atropine (0.5 mg/kg) abolished the gallbladder response, whereas a slight excitatory response remained in the sphincter. With the exception of pirenzepine, there was no significant difference in the effects of the three muscarinic blockers on the gallbladder response and sphincter response after reduction of the mean value of the atropine resistant responses from those of the sphincter responses after the muscarinic blockers. These results indicate that the excitatory vagal routes to the gallbladder include at least M1 receptors and those to the sphincter of Oddi may include M3 receptors.