Organic Nitrates: Direct Antiplatelet Effects and Synergism with Prostacyclin

Abstract

Isosorbide dinitrate inhibits platelet function in vivo at concentrations about 10 times lower than in vitro (10^-7 -10^-6 vs. 10^-6 -10^-5 M). We investigated two possible reasons for this difference. Isosorbide dinitrate and its in vivo longer-lived metabolites, isosorbide-2- and isosorbide-5-mononitrate were incubated for 5 min with human platelet-rich plasma or washed platelets; irreversible aggregation was induced with threshold doses of ADP, adrenaline, collagen, arachidonic acid and thrombin, and thromboxane (TX) B₂ production was measured by radioimmunoassay. Moreover, the concentration of exogenous prostacyclin required to inhibit platelet aggregation by 50% (IC50) after preincubation with isosorbide dinitrate or vehicle was determined. At 10^-7 M, only isosorbide-2-mononitrate inhibited aggregation (-12%, p >0.05) and TX production (-36%, p >0.01) by ADP. At 10^-6 M isosorbide-2-mononitrate inhibited aggregation by adrenaline more than the dinitrate (-41% vs. -25%, p >0.05). In addition, at supra-threshold doses of all the aggregating agents, isosorbide dinitrate decreased IC50 of prostacyclin from 2.7 ± 1.2 to 0.36 ± 0.2 nM. Generation of a platelet-active metabolite and synergism with prostacyclin are new properties of isosorbide dinitrate that may account for antiplatelet effects in vivo.