Thrombin inhibits myosin light chain dephosphorylation in endothelial cells. Shasby, D. Michael, Troy Stevens, Dana Ries, Alan B. Moy, Jay M. Kamath, Anant M. Kamath, and Sandra S. Shasby. Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, IA (1) and the Department of Pharmacology, University of South Alabama Mobile, AL (2). This work was conducted in part during Dr. A.B. MoyOs tenure as a Physician Scientist of the American Heart Association. The work was also supported by NIH grant HL 33540
APStracts 3:0228L, 1996.
Histamine and thrombin increase myosin light chain kinase (MLCK) mediated phosphorylation of myosin light chain (MLC) in human umbilical vein endothelial (HUVE) cells. The increase in MLC phosphorylation caused by thrombin persists longer (330 minutes) than the increase caused by histamine (< 5 minutes), although both increase cell calcium similarly. We hypothesized that some of longer duration of the increase in MLC phosphorylation caused by thrombin was due to inhibition of myosin dephosphorylation by thrombin. Calyculin A, an inhibitor of type 1 and 2A protein phosphatases, caused a time dependent increase in MLC phosphorylation in unstimulated HUVE cells. As thrombin stimulated phosphorylation approached its peak at 15 minutes, calyculin A caused progressively less of an increase in MLC phosphorylation in thrombin stimulated HUVE cells, and no increase at the peak of thrombin stimulation. In HUVE cells in which cell calcium was maintained at 600 nM thrombin increased MLC phosphorylation above the level caused by increased calcium, alone, at a time coinciding with the peak of thrombin stimulation. However, when phosphatase activity 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 HUVE cells not only by increasing cell calcium, but also by inhibiting calyculin A sensitive dephosphorylation of MLC. 

Received 8 May 1996; accepted in final form 1 October 1996.
APS Manuscript Number L137-6.
Article publication pending Am. J. Physiol. (Lung Cell. Mol.
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996