Search

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.

Background Inorganic nitrate has been shown to exert beneficial cardiovascular effects, which are thought to be mediated via sequential reduction of nitrate to nitrite and nitric oxide (NO). We have previously reported that hearts from type 2 diabetic db/db mice have impaired cardiac ischemic tolerance and that this effect involves reduced export of NO-like bioactivity from red blood cells (RBCs). It remains unknown whether nitrate supplementation may affect cardiac ischemic tolerance in diabetes through interference with RBC function. Purpose To test the hypothesis that dietary nitrate supplementation improves cardiac ischemic tolerance of hearts via an effect mediated through RBCs in type 2 diabetes. Methods Type 2 diabetic (db/db) and wild type (WT) mice on nitrate-free chow were treated with vehicle or nitrate (1 mM) in the drinking water for 4 weeks. Hearts were isolated and perfused using the Langendorff technique. After 30 min stabilization, the hearts were subjected to 40 min global ischemia followed by 60 min reperfusion. In protocol 1, isolated hearts from db/db and WT mice given vehicle or nitrate were perfused with buffer. In protocol 2, only hearts from untreated WT mice were used. Washed RBCs from WT or db/db mice treated with vehicle or nitrate were administered to WT 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). In both protocols post-ischemic recovery of cardiac function was evaluated by determination of left ventricular developed pressure (LVDP). Results In Protocol 1, post-ischemic recovery of LVDP was impaired in hearts from db/db mice in comparison with hearts from WT mice (Fig. A). Dietary nitrate restored the ischemic tolerance of hearts from db/db mice but did not affect post-ischemic recovery of hearts from WT mice (Fig. A). In Protocol 2, administration of RBCs collected from vehicle-treated db/db mice significantly impaired post-ischemic recovery of hearts from WT mice (Fig. B). Notably, administration of RBCs from nitrate-treated db/db mice completely reversed the impairment of post-ischemic cardiac function induced by diabetic RBCs (Fig. B). Interestingly, post-ischemic cardiac function did not differ between hearts given RBCs from nitrate-treated db/db and WT mice (Fig. B). The protective effect of RBCs from nitrate-treated mice was abolished by pre-incubation of the RBCs with ODQ, an inhibitor of soluble guanylate cyclase (sGC) (Fig. C). By contrast, pretreatment of isolated WT hearts with ODQ failed to block the protective effect of RBCs from nitrate-treated mice (Fig C) indicating that sGC in the RBC but not in the heart is critical for nitrate-induced cardiac protection. Conclusion Dietary nitrate restores cardiac ischemic tolerance in db/db mice and protects the heart against ischemia–reperfusion injury via an RBC NO-sGC pathway.