Myosin Light‐chain Kinase Regulates Endothelial Calcium Entry and Endothelium‐dependent Vasodilation

Activation of smooth muscle myosin light‐chain kinase (MLCK) causes contraction. Here we have proven that MLCK controls Ca2+ entry (CE) in endothelial cells (ECs): MLCK antisense oligonucleotides strongly prevented bradykinin (BK)‐ and thapsigargin (TG)‐induced endothelial Ca2+ response, while MLCK sense did not. We also show that the relevant mechanism is not phosphorylation of myosin light‐chain (MLC): MLC phosphorylation by BK required CE, but MLC phosphorylation caused by the phosphatase inhibitor calyculin A did not trigger Ca2+ response. Most important, we provide for the first time strong evidence that, in contrast to its role in smooth muscle cells, activation of MLCK in ECs stimulates the production of important endothelium‐derived vascular relaxing factors: MLCK antisense and MLCK inhibitors abolished BK‐ and TG‐induced nitric oxide production, and MLCK inhibitors substantially inhibited acetylcholine‐stimulated hyperpolarization of smooth muscle cell membrane in rat mesenteric artery. These results indicate that MLCK controls endothelial CE, but not through MLC phosphorylation, and unveils a hitherto unknown physiological function of the enzyme: vasodilation through its action in endothelial cells. The study discovers a counter‐balancing role of MLCK in the regulation of vascular tone.