P1592A cardioprotective effect of dietary nitrate mediated by red blood cells

Background Reduced bioavailability of nitric oxide (NO) is a key factor behind coronary artery disease and ischemic heart disease. Besides being produced from L-arginine by NO synthase, reduction of nitrate/nitrite to NO is also an important pathway for NO generation, especially during hypoxic/ischemic conditions. Existing evidences suggest that dietary nitrate improves endothelial function and ischemic tolerance via a NO-dependent mechanism. Recent data indicate that red blood cells (RBCs) are an important source of NO bioactivity that protects the heart from ischemia-reperfusion injury. It is unknown whether dietary nitrate exerts cardioprotection by increasing export of RBC NO bioactivity. Purpose To investigate whether dietary nitrate protects the heart against ischemia-reperfusion injury via a mechanism mediated by RBCs. Methods Patients with mild hypertension (systolic blood pressure >130, ≤159 mmHg) were randomly assigned to three groups after a 2-week run-in period on a diet low in nitrate: group 1 received nitrate-rich vegetables (green leafy, ∼150 g/day) plus capsules with placebo salt (KCl), group 2 received low nitrate vegetables (cherry tomato, sweet corn, capsicum, carrot, ∼150 g/day) and capsules with nitrate salt (KNO3, 300mg) and group 3 received low nitrate vegetables and placebo capsules for 5 weeks. The nitrate content of the pills and nitrate-rich vegetables were precisely matched. As a pre-specified substudy, RBCs were collected blindly from 48 subjects before (baseline) and after the 5-week treatment (follow-up). The RBCs were given to isolated Langendorff-perfused rat hearts at the onset of ischemia with and without the soluble guanylyl cyclase (sGC) inhibitor (1H-[1,2,4] Oxadiazolo[4,3-a]quinoxalin-1-one, ODQ). The hearts were subjected to 25 min global ischemia following 60 min reperfusion. Left ventricular developed pressure (LVDP) was recorded as an indicator of cardiac function. Results The recovery in LVDP during reperfusion following global ischemia in hearts given RBCs collected at baseline was similar in the three groups (Fig. A). Of note, post-ischemic recovery of LVDP was significantly improved by administration of RBCs from patients randomized to high nitrate vegetables or nitrate capsule compared to the group randomized to low nitrate and placebo (Fig. B). There was no difference in LVDP recovery between the groups receiving high nitrate vegetables or nitrate capsule (Fig. B). The nitrate-induced improvement in post-ischemic cardiac recovery was totally abolished by the sGC inhibitor ODQ (Fig. C), indicating that the protective effect induced by RBCs from subjects given nitrate is NO-sGC dependent. Conclusion Dietary nitrate in the form of leafy vegetables induces protection against myocardial ischemia-reperfusion injury via a mechanism involving the NO-sGC signaling pathway in RBCs.