Desensitization of platelets to iloprost. Loss of specific binding sites and heterologous desensitization of adenylate cyclase.

The binding characteristics of [3H]prostacyclin and [3H]iloprost ([3H]5-[(E)-(1S,5S,6R,7R)-7-hydroxy-6-[(E)-(3S,4RS) -3-hydroxy-4-methyl-1-octen-6-inyl]-bicyclo[3.3.0]octan-3-yl idene] -pentanoic acid) and platelet adenylate cyclase activities were investigated in platelet-rich plasma preincubated with iloprost. The exposure of platelets to 0.1 microM iloprost (12 h, 20 degrees C) caused a significant loss of iloprost binding sites (P less than 0.01) without causing changes in binding affinity. This loss of specific [3H]iloprost binding was time- and dose-dependent. The reduction of iloprost receptor density was accompanied by an impaired responsiveness of platelet adenylate cyclase to iloprost, prostaglandin D2 and forskolin. In contrast, basal adenylate cyclase activity was not affected by iloprost pretreatment. The diminished response of the enzyme to GTP and NaF pointed to an involvement of the stimulatory guanyl nucleotide-binding protein (Gs) in iloprost-induced heterologous desensitization. Consequently, [32P]NAD+ and cholera toxin were used for the direct labelling of Gs. Platelet membranes desensitized to iloprost incorporated less label into the 45 kD subunit of Gs. These data suggest that the site of action of iloprost for heterologous desensitization of human platelet adenylate cyclase is located on Gs.