DIRECT ACTION OF 4-BETA-PHORBOL-12,13-DIBUTYRATE (PDBU) ON NICOTINIC ACETYLCHOLINE-RECEPTOR CHANNEL INDEPENDENT OF PROTEIN-KINASE-C ACTIVATION

Torpedo acetylcholine receptor (AChR) has a protein kinase C (PKC) phosphorylation site, which modulates channel properties, on the alpha and delta subunit. The effect of a potent PKC activator, PDBu on AChR expressed into Xenopus oocytes was examined by whole cell voltage clamp recordings. The pretreatment with 4-beta-PDBu reversely accelerated desensitization of ACh-elicited membrane currents and the same effect was shown by co-application of 4-beta-PDBu and ACh without pre-incubation. Treatment with the inactive stereoisomer of phorbol ester, 4-alpha-PDBu also demonstrated an acceleration of desensitization. Furthermore, 4-beta-PDBu enhanced the rate of desensitization in mutant AChR deleting PKC phosphorylation sites on the alpha and delta subunit. These results indicate that phorbol ester directly acts on the AChR channel independent of PKC activation.