The internalization of [3H]iloprost, a prostacyclin analogue, was studied in human platelets by binding studies. After incubation with [3H]iloprost at 37 °C, addition of unlabelled ligand at either 37 °C or 4 °C caused dissociation of 74% and 52% of the bound ligand respectively, suggesting that a portion had been internalized. The percentage of [3H]iloprost bound at equilibrium to the surface (evaluated by acid treatment) at either 37 °C or 4 °C was markedly different (80% versus 25%). Internalization was dependent on time and on the ligand nature and concentration. Energy-depleting agents (dinitrophenol and 2-deoxyglucose) completely inhibited internalization, whereas probenecid (inhibitor of organic anion transporters) did not affect it significantly. Subcellular fractionation indicated that, at 4 °C or in the absence of ligand, most of the receptor was present in membrane fractions (pellet at 27000 or 105000 g), whereas, when platelets were preincubated at 37 °C with iloprost, the receptor was found mainly in the cytosolic fraction. In platelets preincubated with iloprost at 4 °C, two classes of binding sites were present, whereas after preincubation at 37 °C only the lower-affinity sites were detected. After exposure to the agonist, iloprost-induced inhibition of platelet aggregation and activation of adenylate cyclase and cAMP production were significantly lower. Taken together, these data demonstrate that human platelets can internalize a high-affinity binding site for iloprost, presumably the prostacyclin receptor.
Present address: Rotta Farmaceutici, Monza (MI), Italy.