Signal transduction involved in the platelet adenylate cyclase sensitization associated with PGH2/TxA2 receptor desensitization.

The exposure of human platelets to prostaglandin H2 analogue (PGH2, U46619) induces homologous desensitization and a concomitant adenylate cyclase (AC) sensitization. We demonstrate the involvement of phospholipase C (PLC) in this enzyme sensitization. Pre-incubation of platelets with neomycin, a PLC activity inhibitor, prevented AC sensitization but not PGH2/thromboxane (Tx)A2 receptor desensitization. PGH2/TxA2 receptor desensitization, although necessary, is not sufficient to induce AC sensitization, since neomycin, which prevents AC sensitization, failed to prevent receptor desensitization. Inositol phosphate formation, determined in parallel, was also inhibited. Interestingly, no guanylate cyclase sensitization was noted, suggesting a specific relationship between PGH2/TxA2 receptor desensitization and AC sensitization. In addition, using alkaline phosphatase, a dephosphorylating enzyme, and the tyrosine kinase inhibitor erbstatin, we examined the role of phosphorylation-dephosphorylation on AC sensitization. Effectively, alkaline phosphatase, which has no effect by itself, enhances the cAMP production triggered by prostacyclin in control but not in desensitized platelets. In contrast, erbstatin failed to modify this synthesis, indicating the non-involvement of tyrosine kinase pathway in this process. Our results indicate that the AC sensitization was mediated by PLC and also suggest the participation of other mechanisms, including phosphorylation-dephosphorylation processes. This specific enzyme sensitization may be relevant for the in vivo modulation of platelet activation, in different thrombotic diseases with an increased TxA2 generation.