Location of enkephalinase and functional effects of [Leu5]enkephalin and inhibition of enkephalinase in the feline main pancreatic and bile duct sphincters.

1. Morphological studies have demonstrated enkephalinergic nerve fibres in proximity to the sphincter of Oddi, and opiates are known to contract this sphincter. In this study, the flow resistances in the common bile duct and main pancreatic duct sphincters were studied simultaneously in anaesthetized cats using a perfusion technique. 2. Naloxone did not affect the activity of these sphincters under basal conditions, indicating that there is no basal enkephalinergic tone. 3. The response to [Leu5]enkephalin (0.015-15 micrograms/kg), morphine (1 mg/kg) and ketamine (10 mg/kg) was a naloxone-sensitive increased activity in the sphincters with a raised frequency of phasic contractions. The threshold dose for an effect of [Leu5]enkephalin on the sphincter of Oddi was 0.015 microgram/kg and a maximal response was observed at 0.75 microgram/kg. There were no differences in the response of the main pancreatic duct sphincter and the bile duct sphincter to the different drugs. 4. Immunoautoradiographic studies demonstrated enkephalinase in the sphincter++ of Oddi. 5. Acetorphan (3 mg/kg intravenously), which inhibits endogenous enkephalinase both in the peripheral and the central nervous system when administered parenterally, caused a naloxone-sensitive contraction, whereas thiorphan (3-20 mg/kg), an enkephalinase inhibitor that does not easily penetrate the blood-brain barrier, had no effect on the sphincter of Oddi. 6. These results show that endogenous and exogenous opiates influence the function of the feline sphincter of Oddi and that enkephalins may be involved in the physiological control of this sphincter, although not under basal conditions.