| The specific effects of the TXA(,2)/PGH(,2) analogues, U46619 (9,11-dideoxy,9(alpha)-11(alpha)-methanoepoxy-PGF(,2(alpha))), and 9,11aza-PGH(,2), on human platelet shape change, myosin light chain phosphorylation, serotonin release, fibrinogen receptor exposure and platelet aggregation were measured and compared with binding of ('3)H-U46619 to platelets. Shape change and myosin light chain phosphorylation were found to be saturable and dose dependent, having EC(,50) values of 0.036 +/- 0.004 (mu)M and 0.057 +/- 0.021 (mu)M, respectively (mean +/- SEM). These two effects were competitively inhibited by specific antagonists of TXA(,2)/PGH(,2) receptors (BM13177 and I-PTA-OH) indicating that they are receptor mediated. Binding of ('3)H-U46619 showed two components. Occupancy of high affinity binding sites (K(,d) = 0.041 +/- 0.009 (mu)M, B(,max) = 19.4 +/- 5.2 fmoles/10('7) platelets, with 1166 +/- 310 sites/platelet; n = 12) correlated with platelet shape change and myosin and light chain phosphorylation. A second component with an apparent K(,d) of 1.46 +/- 0.47 (mu)M (n = 12), may represent a second, low-affinity site. Mean EC(,50) values for U46619-induced serotonin release, platelet aggregation and fibrinogen receptor exposure were 0.54 +/- 0.128 (mu)M, 1.31 +/- 0.34 (mu)M and 0.53 +/- 0.2 (mu)M, 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 U46619. Fibrinogen receptor exposure and platelet aggregation caused by U46619 appeared to be events mediated by the release of ADP from platelet dense granules. |
