Differential inhibition of platelet aggregation and calcium mobilization by nitroglycerin and stabilized nitric oxide

We compared the mechanisms of the antiplatelet effects of nitroglycerin (NTG) and stabilized nitric oxide (NO). Stabilized NO was in the form of S-nitrosothiols [S-nitroso-albumin (S-NO-Alb) and S-nitrosocaptopril (S-NO-Cap)] or heme-NO [sodium nitro-prusside (SNP)]. The molecular structure of S-NO-Cap was confirmed by mass spectrometry. NTG, SNP, S-NO-Alb, and S-NO-Cap inhibited ADP-induced platelet aggregation dose dependently. The inhibitory IC50 value was 109 microM for NTG, 0.98 microM for SNP, 2.99 microM for S-NO-Alb, and 2.5 microM for S-NO-Cap. NTG (200 microM) released 15.4 microM nitrite anion into platelet-rich plasma (PRP) after 60-min incubation, to which platelets contributed 5.4 microM. On the other hand, SNP and S-NO-Cap released undetectable amounts of NO2- when incubated in either PRP or platelet-poor plasma (PPP). The platelet cytosolic calcium ion (Ca2+) concentration was measured fluorometrically in Fura-2-loaded gel-filtered platelets. Thrombin-induced Ca2+ mobilization was significantly inhibited by 10 microM NTG, SNP, S-NO-Alb, and S-NO-Cap, whereas resting Ca2+ was unaltered. Ca2+ mobilization was inhibited 28.6% by NTG, 91.9% by SNP, 90.0% by S-NO-Alb, and 92.7% by S-NO-Cap. These results demonstrate that NTG is an exogenous donor of NO, but releases it only slowly. On the other hand, SNP and S-nitrosothiols inhibited platelet aggregation by the action of stabilized NO incorporated in their structure and did not release NO. NTG and stabilized NO shared a common mechanism of antiplatelet activity, which involved inhibition of calcium mobilization.