Potentiation of platelet responsiveness to nitric oxide by angiotensin-(1-7) is associated with suppression of superoxide release

Abstract

Recently we showed that angiotensin (Ang) II potentiates platelet aggregation, while Ang-(1-7) potentiates the anti-aggregatory action of the nitric oxide (NO) donor sodium nitroprusside (SNP), and may therefore counteract platelet NO resistance that accompanies cardiovascular disease and is associated with increased levels of superoxide (O(2)(-)). In the current study, we investigated whether the effect of Ang-(1-7) on platelet NO responsiveness is associated with the modulation of O(2)(-) release and is mediated by a specific Ang-(1-7) receptor. In whole blood, SNP (10 micromol/L) inhibited ADP (2.5 micromol/L)-induced platelet aggregation by 21 +/- 8% (p < 0.02), measured via extent of aggregation. Ang-(1-7) did not directly affect platelet aggregation, but potentiated the inhibitory action of SNP. This effect of Ang-(1-7) was bimodal, with maximal increase in SNP-induced inhibition of aggregation by incremental 18 +/- 2% (2-fold, on average; p<0.01) at 10-100 nmol/L Ang-(1-7) (Cmax), and was abolished at higher concentrations of Ang-(1-7). The Ang-(1-7) receptor antagonist D-ala7-Ang-(1-7) (1 micromol/L) completely eliminated the potentiating effects of Ang-(1-7). Platelet aggregation was accompanied by O(2)(-) release (assessed via lucigenin-derived chemiluminescence). SNP suppressed this O(2)(-) release, and Ang-(1-7) at Cmax augmented (by incremental 23 +/- 8%, p<0.03) the effect of SNP. In order to examine possible association of Ang-(1-7) receptor with platelets, we performed aggregation experiments in platelet-rich plasma. However, in these experiments Ang-(1-7) did not potentiate the anti-aggregatory action of SNP. Furthermore, in isolated polymorphonuclear leukocytes (PMN), a major cellular source of O(2)(-) in blood, Ang-(1-7) did not modify O(2)(-) release (after stimulation with fMLP, PMA or ADP), either in the absence or presence of SNP. Hence, Ang-(1-7) effects occurred only in whole blood. In conclusion, Ang-(1-7) potentiates the anti-aggregatory effects of NO donor, presumably via a specific Ang-(1-7) receptor. This potentiation is associated with the suppression of O(2)(-) release during aggregation and arises via an interaction between platelets and PMN.