Fig. 1. Carbachol-induced protein tyrosine phosphorylation and extracellular signal-regulated kinase (ERK) activation in PC12 cells. ( A ) The cells were stimulated with 1 mM carbachol for the indicated times. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis and Western blot analysis with anti-phosphotyrosine antibody were performed. Carbachol induced rapid and transient increases in tyrosine phosphorylation of several proteins. The molecular mass markers, in kilodaltons, are indicated on the left. ( B ) By using anti-ERK antibody, 44- and 42-kd phosphoproteins were identified as active forms of ERK1 and ERK2. The upper two arrows on the left indicate ERK1 ( 
, active forms of ERK1) and the lower two arrows indicate ERK2 ( 
, active forms of ERK2). ( C ) The density of bands of activated ERKs was measured by the densitometer (○ ERK1;• ERK2). Data represent the mean ±SD from three experiments. * P < 0.05 versus 0 min;** P < 0.0001 versus 0 min.
