Kinetic evidence indicating separate stimulatory and inhibitory prostaglandin receptors on platelet membranes.

Progress curves of cyclic AMP production by human platelet lysates in the presence of 10 microM GppNp were upward curving, reaching a steady-state rate after about 20 min. Increasing concentrations of prostaglandin E1 reduced the lag time required to reach steady-state and increased the steady-state rate in a dose-dependent manner. At 0.3 microM PGE1 the progress curve was linear for at least 40 min. At higher concentrations of PGE1 the initial rate of cyclic AMP formation was linear and similar to that obtained at 1.0 microM PGE, however, the progress curve showed a downward curvature after several minutes, reaching a new steady-state rate after about 10 min. The extent of downward curvature was dose-dependent, and at 10 microM PGE1 the final steady-state rate had almost returned to that observed in the presence of GppNp alone. Analysis of initial and final steady-state rates as a function of prostaglandin concentration revealed two apparently saturable processes that were interpreted as binding to a stimulatory receptor (EC50 = 130 nM), followed by binding to a lower affinity inhibitory receptor (EC50 = 1200 nM) that showed a slow response to receptor occupancy. Qualitatively similar results were obtained with PGD2 and the stable PGI2 analog ZK36374.