Smooth muscle contraction is initiated by a rise in intracellular calcium, leading to activation of smooth muscle myosin light chain kinase (MLCK) via calcium/calmodulin (CaM). Activated MLCK then phosphorylates the regulatory myosin light chains, triggering cross-bridge cycling and contraction. Here, we show that MLCK is a substrate of AMP-activated protein kinase (AMPK). The phosphorylation site in chicken MLCK was identified by mass spectrometry to be located in the CaM-binding domain at Ser(815). Phosphorylation by AMPK desensitized MLCK by increasing the concentration of CaM required for half-maximal activation. In primary cultures of rat aortic smooth muscle cells, vasoconstrictors activated AMPK in a calcium-dependent manner via CaM-dependent protein kinase kinase-beta, a known upstream kinase of AMPK. Indeed, vasoconstrictor-induced AMPK activation was abrogated by the STO-609 CaM-dependent protein kinase kinase-beta inhibitor. Myosin light chain phosphorylation was increased under these conditions, suggesting that contraction would be potentiated by ablation of AMPK. Indeed, in aortic rings from mice in which alpha1, the major catalytic subunit isoform in arterial smooth muscle, had been deleted, KCl- or phenylephrine-induced contraction was increased. The findings suggest that AMPK attenuates contraction by phosphorylating and inactivating MLCK. This might contribute to reduced ATP turnover in the tonic phase of smooth muscle contraction.
