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Figure 3 Representative Western blot showing the effect of menadione (vitamin K) on the activation and phosphorylation of the mitogen-activated protein kinases (MAPK) in neonatal rat cardiac myocytes and fibroblasts. Cells were exposed to different concentrations of menadione (50, 100, 200 µM) for a period of 30 min before protein extraction and analysis. Erk = extracellular signal-regulated kinases; JNK = c-jun NH₂ terminal kinase.