Dm. Shasby et al., THROMBIN INHIBITS MYOSIN LIGHT-CHAIN DEPHOSPHORYLATION IN ENDOTHELIAL-CELLS, American journal of physiology. Lung cellular and molecular physiology, 16(2), 1997, pp. 311-319
Histamine and thrombin increase myosin light-chain kinase-mediated pho
sphorylation of myosin light chain (MLC) in human umbilical vein endot
helial cells (HUVEC). The increase in MLC phosphorylation caused by th
rombin persists longer (330 min) than the increase caused by histamine
(<5 min), although both increase cell calcium similarly. We hypothesi
zed that some of the longer duration of the increase in MLC phosphoryl
ation caused by thrombin was because of inhibition of myosin dephospho
rylation by thrombin. Calyculin A, an inhibitor of type 1 and 2A prote
in phosphatases, caused a time-dependent increase in MLC phosphorylati
on in unstimulated HUVEC. As thrombin-stimulated phosphorylation appro
ached its peak at 15 min, calyculin A caused progressively less of an
increase in MLC phosphorylation in thrombin-stimulated HUVEC, and no i
ncrease at the peak of thrombin stimulation. In HUVEC in which cell ca
lcium was maintained at 600 nM, thrombin increased MLC phosphorylation
above the level caused by increased calcium alone at a time coincidin
g with the peak of thrombin stimulation. However, when phosphatase act
ivity was already inhibited with calyculin A, thrombin did not further
increase MLC phosphorylation in cells in which calcium was maintained
at 600 nM calcium. Thrombin increases MLC phosphorylation in HUVEC no
t only by increasing cell calcium but also by inhibiting calyculin A-s
ensitive dephosphorylation of MLC.