MUSCARINIC AUTOINHIBITION AND MODULATORY ROLE OF PROTEIN-KINASE-C IN ACETYLCHOLINE-RELEASE FROM THE MYENTERIC PLEXUS OF GUINEA-PIG ILEUM

The modulatory role of protein kinase C on phospholipase A(w), activat ion of which had been suggested to result in acetylcholine release fro m cholinergic neurons, was studied in longitudinal muscle preparations with the myenteric plexus of guinea pig ileum. The relationship of mu scarinic autoinhibition to the modulation was also examined. Phorbol-1 2,13-dibutyrate (PDBu), an activator of protein kinase C, dose-depende ntly increased spontaneous and electrical field stimulation-induced ac etylcholine releases from the preparation. The inhibitors of protein k inase C, staurosporine and calphostin C, inhibited the stimulatory eff ects of PDBu, but neither inhibitor affected spontaneous or electrical held stimulation-induced acetylcholine release in the absence of PDBu . On the other hand, atropine significantly increased electrical field stimulation-induced release by blocking a muscarinic autoinhibitory m echanism. Under the autoinhibition blocked condition, U73122, an inhib itor of phospholipase C, and staurosporine significantly inhibited the effect of atropine on electrical held stimulation-induced release. An inhibitor of phospholipase A(2), mepacrine, inhibited PDBu-induced ac etylcholine release and also inhibited the effect of atropine on elect rical field stimulation-induced release. An activator of phospholipase A(2), melittin, and a product of the phospholipase, arachidonic acid, increased the spontaneous and electrical field stimulation-induced re leases. These results suggest that the phospholipase C - protein kinas e C system modulates acetylcholine release from cholinergic neurons by activating phospholipase A(2) in the myenteric plexus of guinea pig i leum, and the activation of muscarinic autoreceptor may negatively mod ulate acetylcholine release at a point upstream of the system.