An interaction of endogenous and exogenous opiates in the guinea-pig isolated ileum

In the myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum an interaction of exogenously applied opiates with posttetanic inhibition was studied. The electric tetanic stimulation (30 Hz) evoking the release of endogenous opioids resulted in an inhibition of neurogenic cholinergic twitch contractions (0.1 Hz) of the same preparation. This posttetanic inhibition was largely attenuated or even reversed into potentiation if tested in the presence of ketocyclazocine (40 nmol/l) or pentazocine (0.3 and 3 mumol/l) and also methionine-enkephalin (0.2 and 2 mumol/l) or morphine (1 mumol/l) while it was not affected in the presence of leucine-enkephalin (2 mumol/l) or D-Ala2-D-Leu5-enkephalin (40 and 400 nmol/l). An involvement of kappa- and mu-opiate receptors in the action of the endogenous mediator(s) of posttetanic inhibition is suggested.