Prostacyclin analogues reduce ADP-ribosylation of the alpha-subunit of the regulatory Gs-protein and diminish adenosine (A2) responsiveness of platelets.

Prostacyclin and adenosine activate adenylate cyclase in human platelet membranes and inhibit platelet aggregation. Results are presented which show that prolonged incubation of platelets with iloprost (a stable prostacyclin analogue) results in a reduction in the capacity for adenylate cyclase activation by the adenosine analogue 5'-(N-ethyl)-carboxamidoadenosine (NECA), NaF, guanyl-5'-yl imidodiphosphate or GTP. However, iloprost pretreatment resulted in no change in the binding of [3H]-NECA to platelet membranes. These results contrast with those obtained after pretreatment with 2-chloroadenosine which revealed no change in NaF or guanyl-5'-yl imidodiphosphate sensitivity of adenylate cyclase. Pretreatment with 2-chloroadenosine resulted in reduced NECA-dependent adenylate cyclase activation, and loss of [3H]-NECA binding sites. The heterologous desensitization of adenosine A2-receptors by iloprost is accompanied by a loss (greater than 80%) of a 45 kDa protein from the plasma membrane, as revealed by [32P]-ADP-ribosylation in the presence of cholera toxin. It is proposed that this example of heterologous desensitization is mediated by elimination of Gs alpha, a subunit of the stimulatory guanyl nucleotide-binding regulatory protein.