High-frequency EF enhances MEK phosphorylation and MEK–cRaf complex formation in microvascular endothelial cells. (a) Cell exposure to high-frequency EF resulted in significantly higher protein levels of phosphorylated MEK, while the total levels of MEK remained unchanged with EF exposure both in the absence or presence of MEK inhibitor U0126. (b) Cell exposure to high-frequency EF significantly increased protein levels and phosphorylation of MEK-cRaf complex, when compared with low-frequency or no-EF groups (n = 7, p < 0.001). This was consistent with low levels of free (unbound) MEK in the high-frequency EF group, when compared with those in low-frequency and no-EF groups (n = 7, p < 0.05). Free MEK levels in the low-frequency group were significantly larger than the values in the high-frequency and no-EF groups (n = 7, p < 0.05). (c) In contrast to the EF effects observed in the absence of U0126 in (b), cell exposure to high-frequency EF in the presence of MEK inhibitor resulted in significantly reduced protein levels and phosphorylation of MEK-cRaf complex, as well as increased free MEK levels (n = 7, p < 0.05), when compared with low-frequency or no-EF groups (n = 7, p < 0.01).