Autoregulation of phosphorylation of the nicotinic acetylcholine receptor

We have investigated the regulation of phosphorylation of the nicotinic ACh receptor (nAChR) in rat myotubes by the agonist carbamylcholine. Treatment of primary rat myotube cultures with carbamylcholine resulted in a 100% increase in phosphorylation of the nAChR γ- (52 kDa) subunit and a 30% increase in phosphorylation of the nAChR δ- (62 kDa) and δ'- (66 kDa) subunits. These responses to carbamylcholine were dose dependent, with a half-maximal response occurring at 10 μM and a maximum response achieved within 2 min. Pretreatment of myotubes with d-tubocurare, but not with atropine, inhibited carbamylcholine-stimulated phosphorylation of the nAChR. Preincubation with open-channel blockers of the nAChR also inhibited phosphorylation of the nAChR induced by carbamylcholine. Depletion of extracellular calcium from myotube cultures prevented carbamylcholine- stimulated increases in nAChR phosphorylation whereas application of a calcium ionophore mimicked the effect of carbamylcholine on nAChR phosphorylation. Pretreatment of myotubes with TTX did not inhibit carbamylcholine-stimulated nAChR phosphorylation and potassium depolarization of myotubes had no effect on nAChR phosphorylation. Carbamylcholine increased nAChR phosphorylation to the same extent and with the same time course and subunit specificity as that induced by phorbol esters. However, chronic treatment of myotubes with phorbol esters that eliminated any subsequent phorbol ester-stimulated nAChR phosphorylation did not diminish the increase in nAChR phosphorylation induced by carbamylcholine. The calmodulin antagonist W7 was similarly unable to inhibit carbamylcholine-stimulated nAChR phosphorylation. These results suggest that the nAChR is a substrate for an uncharacterized protein kinase in situ, and that activity of this protein kinase is stimulated by calcium ions that permeate through the activated nAChR ion channel. The data reveal a novel signal transduction pathway whereby the nAChR may autoregulate its own functional activity.