Regulation of phosphorylation of nicotinic acetylcholine receptors in mouse BC3H1 myocytes.

By using 32P-labeling methods and performing immunoprecipitations with specific antibodies, we have found that three subunits of the nicotinic acetylcholine receptor are phosphorylated in mouse skeletal muscle cells. In nonstimulated cells, the molar ratios of phosphate estimated in alpha, beta, and delta subunits were 0.02, 0.05, and 0.5, respectively. All three subunits contained predominantly phosphoserine with some phosphothreonine; the beta subunit also contained phosphotyrosine. Incubating cells with agents that stimulate cAMP-dependent pathways (isoproterenol, forskolin, 8-Br-cAMP) increased the phosphorylation of the delta subunit by 50%, but phosphate labeling of the beta subunit was depressed by a third. In contrast, when cells were incubated with the divalent cation ionophores A-23187 or ionomycin, phosphorylation of both the delta and beta subunits increased. The results indicate that acetylcholine receptors are phosphorylated to significant levels in skeletal muscle cells and that cAMP-dependent and Ca2+-dependent pathways exist for controlling the phosphorylation state of the receptor subunits.