• Open Access

Rho-kinase mediates diphosphorylation of myosin regulatory light chain in cultured uterine, but not vascular smooth muscle cells

Authors

  • Hector N. Aguilar,

    1. Department of Physiology, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
    Search for more papers by this author
  • Curtis N. Tracey,

    1. Department of Physiology, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
    Search for more papers by this author
  • Barbara Zielnik,

    1. Department of Physiology, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
    Search for more papers by this author
  • Bryan F. Mitchell

    Corresponding author
    1. Division of Reproductive Sciences, Department of Obstetrics and Gynaecology, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
    • Department of Physiology, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada
    Search for more papers by this author

Correspondence to: Dr. Bryan F. MITCHELL,220 HMRC, University of Alberta, Edmonton, AB, Canada T6G 2S2.

Tel.: +780-492-8561

Fax: +780-492-1308

E-mail: brymitch@ualberta.ca

Abstract

Phosphorylation of myosin regulatory light chain (RLC) triggers contraction in smooth muscle myocytes. Dephosphorylation of phosphorylated RLC (pRLC) is mediated by myosin RLC phosphatase (MLCP), which is negatively regulated by rho-associated kinase (ROK). We have compared basal and stimulated concentrations of pRLC in myocytes from human coronary artery (hVM), which has a tonic contractile pattern to myocytes from human uterus (hUM), which has a phasic contractile pattern. Our studies reveal fundamental differences between hVM and hUM regarding the mechanisms regulating phosphorylation RLC. Whereas hVM responded to stimulation by phosphorylation of RLC at S19, hUM responded by forming diphosphorylated RLC (at T18 and S19; ppRLC), which, compared to pRLC, causes two to threefold greater activation of myosin ATPase that provides energy to power the contraction. Importantly, the conversion of pRLC to ppRLC is mediated by ROK. In hUM, MLCP has high activity for ppRLC and this is inhibited by ROK through phosphorylation of the substrate targeting subunit (MYPT1) at T853. Inhibitors of ROK significantly reduce contractility in both hVM and hUM. We demonstrated that inhibition of ppRLC in phasic myocytes (hUM) is 100-fold more sensitive to ROK inhibitors than is pRLC in tonic myocytes (hVM). We speculate that these differences in phosphorylation of RLC might reflect evolution of different contractile patterns to perform distinct physiological functions. Furthermore, our data suggest that low concentrations of ROK inhibitors might inhibit uterine contractions with minimal effects on vascular tone, thus posing a novel strategy for prevention or treatment of conditions such as preterm birth.

Ancillary