Activation of protein kinase C in platelets by epinephrine and A23187:
correlation with fibrinogen binding
M Saitoh, EW Salzman, M Smith and JA Ware
Charles A. Dana Research Institute, Harvard Medical School, Boston, MA.
Activation of protein kinase C (PKC), as revealed by phosphorylation of a
47 kd protein (p47), occurs in platelets stimulated by some agonists (eg,
thrombin or phorbol esters). It is not known if activation of PKC occurs
with pairs of agonists, such as epinephrine and A23187, that do not
individually phosphorylate p47, nor is it known what role the concentration
of cytoplasmic Ca++ ([Ca++]i) plays in these events. We stimulated
aequorin-loaded platelets with subaggregating concentrations of epinephrine
and A23187, neither of which by itself phosphorylated p47. The combination
of agonists resulted in p47 phosphorylation, an increase in platelet-bound
fibrinogen, and aggregation, but only if the concentration of each agonist
was sufficient to increase [Ca++]i if it was added separately.
Subaggregating concentrations of A23187 alone released platelet fibrinogen
and increased platelet membrane binding of [3H]-phorbol dibutyrate, but
these were not enhanced by epinephrine. Epinephrine and A23187 did not
increase production of diacylglycerol. Thus, epinephrine and A23187
together activate PKC by a mechanism that does not require phospholipase C
or enhanced binding of PKC to the plasma membrane; PKC activation in turn
is correlated with enhanced platelet fibrinogen binding and aggregation.
These events require an initial elevation of [Ca++]i above a threshold.
Volume 74,
Issue 6,
pp. 2001-2006,
11/01/1989
Copyright © 1989 by The American Society of Hematology
