CYCLIC NUCLEOTIDE PHOSPHODIESTERASE INHIBITORS PREVENT AGGREGATION AND SECRETION OF HUMAN PLATELETS BY RAISING CYCLIC AMP AND REDUCING CYTOPLASMIC FREE CALCIUM MOBILIZATION

Drugs that raise platelet cyclic AMP (cAMP) are potent inhibitors of platelet activation. We have studied the effects of 5 inhibitors of cyclic nucleotide phosphodiesterase (PDE) of different chemical structures (quercetin, Ro 15-2041, HL-725, cilostamide and MY-5445), which are all potent inhibitors of platelet function. The concentrations that inhibit by 50 % crude cAMP-PDE activity (IC₅₀) from human platelets are: 0.06 μM(HL-725), 0.15 μM(Ro 15-2041 ), 0.23 μM(cilostamide), 6.9 μM(MY-5445) and 44.4 μM(quercetin). We measured on the same preparation of washed human platelets loaded with quin2, the aggregation and the increase in intracellular Ca²⁺ ([Ca²⁺]_i) induced by 5 μM ADP alone or in the presence of PDE inhibitors.

PGE₁ (2 nM) potentiates significantly (1.6 to 3.3 fold) the inhibitory effects of PDE inhibitors on [Ca²⁺]_i rises and platelet aggregation. Adrenaline, an inhibitor of adenylate cylase, prevents the effect of PDE inhibitors on ADP-induced [Ca²⁺]_i rise and platelet aggregation. These results suggest that these compounds inhibit [Ca²⁺]_i mobilization and subsequent ADP-induced aggregation through a rise in cAMP, because both effects are potentiated by PGE1 and inhibited by adrenaline. The inhibitor concentrations which potentiate the action of PGE₁, on [ Ca²⁺]_i levels also potentiate the rise in platelet cAMP induced by PGE-<. These results suggest that PDE inhibitors inhibit platelet aggregation Ly raising cAMP levels and subsequently inhibiting [Ca²⁺]_i mobilization.