Figure 1.

TXA₂ receptor-coupled GTPase activation in human platelet membranes is inhibited by 8-Br-cGMP. Platelet membranes were stimulated with 10 μM of the TXA₂ analog U46619 in the absence or presence of varying concentrations of 8-Br-cGMP. Basal GTPase activity (open circle) and U46619-stimulated GTPase activity in buffer-treated membranes (grey circle; +U4) are shown, as is the progressive inhibition of U46619-stimulated GTPase by 8-Br-cGMP (dark circles). The IC₅₀ for inhibition of the TXA₂-stimulated GTPase by 8-Br-cGMP is 75 nM (n = three experiments in triplicate). (Inset) Immunoblot demonstrating the presence of high levels of G kinase in the platelet membranes used for these studies. Lane 1, 0.2 μg of purified G kinase; lane 2, 79 μg of total platelet membrane protein.