['Physiological roles of apelin and its specific receptor APJ signaling were investigated in vascular smooth muscle cells (VSMCs). The present study determined whether apelin activates myosin light chain (MLC), a major regulatory event in initiating smooth muscle contraction.', 'To assess MLC activation, we performed Western blot and immunohistochemical studies using an antibody against the phospho-MLC. In VSMCs, apelin induces the phosphorylation of MLC in a concentration-dependent manner with a peak at 2 minutes. Pretreatment of VSMCs with pertussis toxin abolishes the apelin-induced phosphorylation of MLC. Inhibition of protein kinase C (PKC) with GF-109203X markedly attenuated the apelin-induced MLC phosphorylation. In addition, methylisobutyl amiloride, a specific inhibitor of the Na+/H+ exchanger (NHE), and KB-R7943, a potent inhibitor for the reverse mode of the Na+/Ca2+ exchanger (NCX), significantly suppressed the action of apelin. In wild-type mice, apelin phosphorylates MLC in vascular tissue, whereas it had no response in APJ-deficient mice by Western blot and immunohistochemistry. Apelin-induced phosphorylation of MLC was accompanied with myosin phosphatase target subunit phosphorylation.'] Does apelin stimulate myosin light chain phosphorylation in vascular smooth muscle cells?