AJP - Heart and Circulatory Physiology, Vol 253, Issue 5 1035-H1043, Copyright © 1987 by American Physiological Society
Receptor-mediated effects of a PGH2 analogue (U 46619) on human platelets
T. A. Morinelli, S. Niewiarowski, J. L. Daniel and J. B. Smith
Department of Physiology, Temple University School of Medicine, Philadelphia, Pennsylvania 19140.
The specific effects of U 46619 (9,11-dideoxy,9 alpha-11
alpha-methanoepoxyprostaglandin F2 alpha), thromboxane A2-prostaglandin H2
(TxA2/PGH2) analogue, on human platelet shape change, myosin light-chain
phosphorylation, serotonin release, fibrinogen receptor exposure, and
platelet aggregation were measured and compared with binding of [3H]U 46619
to platelets. Shape change and myosin light-chain phosphorylation were
found to be saturable and dose dependent, having effective concentration
producing 50% of the maximum response (EC50) values of 0.035 +/- 0.005 and
0.057 +/- 0.021 microM, respectively (mean +/- SE). These two effects were
competitively inhibited by specific antagonists of TxA2/PGH2 receptors (BM
13177, PTA-OH, and 1.PTA-OH) indicating that they are receptor mediated.
Binding of [3H]U 46619 showed two components. Occupancy of high-affinity
binding sites [dissociation constant (Kd) = 0.041 +/- 0.009 microM, maximum
binding site (Bmax) = 19.4 +/- 5.3 fmol/10(7) platelets, with 1,166 +/- 310
sites/platelet; n = 12] correlated with platelet shape change and myosin
light-chain phosphorylation. We propose that a second component with an
apparent Kd of 1.46 +/- 0.47 microM (n = 12) represents a second,
low-affinity site. Mean EC50 values for U 46619-induced serotonin release,
platelet aggregation, and fibrinogen receptor exposure were 0.54 +/- 0.13.
1.31 +/- 0.34 and 0.53 +/- 0.21 microM, respectively. Therefore, the
platelet release reaction was not directly correlated with occupancy of
high-affinity receptors but could be related to the second binding
component of U 46619. Fibrinogen receptor exposure and platelet aggregation
caused by U 46619 appeared to be events mediated by the release of
adenosine diphosphate from platelet-dense granules.
