AJP - Cell Physiology, Vol 249, Issue 3 297-C303, Copyright © 1985 by American Physiological Society
Role of myosin phosphorylation in contractility of a platelet aggregate
M. E. Bromberg, R. W. Sevy, J. L. Daniel and L. Salganicoff
The relationship between tension and myosin 20,000-Da light chain
phosphorylation in intact nonmuscle cells was investigated using a
preparation of thrombin-activated, irreversibly aggregated platelets known
as the platelet strip. Steady-state levels of tension generated by the
platelet strip were found to be linearly related to the level of myosin
phosphorylation. This relationship was observed during dose-dependent
relaxation induced by the adenylate cyclase activators prostaglandin (PG)
E1 and PGI2, and during contraction induced by ADP, epinephrine, and the
prostaglandin endoperoxide analogue U-46619, which did not appreciably
alter the basal level of adenosine 3',5'-cyclic monophosphate in the
preparation. The fully relaxed platelet strip, in the absence of external
Ca2+, was associated with a level of 12% light chain phosphorylation, which
increased to 72% on maximal contraction. During both relaxation and
contraction, changes in myosin phosphorylation were also found to precede
or coincide with tension changes. Furthermore, steady-state contraction
induced by ADP was associated with a maintained elevation in the level of
myosin phosphorylation. These results support the concept that myosin
phosphorylation is an important regulatory mechanism for contractility in
platelets.
