Investigation of the effect of inorganic nitrate on platelet and endothelial function in healthy individuals and in patients with hypercholesterolaemia.
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Ingestion of vegetables rich in inorganic nitrate (NO3-) content has emerged as an effective method, via the formation of a nitrite (NO2-) intermediate, for acutely elevating vascular nitric oxide (NO) levels. As such a number of beneficial effects of NO3- ingestion have been demonstrated including the suggestion that platelet reactivity is reduced. I initially investigated whether inorganic NO3- supplementation might also reduce platelet reactivity in healthy volunteers and have determined the mechanisms involved in the effects seen. I conducted a randomised crossover study in 24 (12 of each sex) healthy subjects assessing the acute effects of potassium nitrate capsules (KNO3, 8 mmol) vs placebo (KCl) control capsule ingestion on platelet reactivity. Inorganic NO3- ingested via supplementation raised circulating NO3- and NO2- levels in both sexes and attenuated ex vivo platelet aggregation responses to adenosine diphosphate (ADP) and, albeit to a lesser extent, collagen but not epinephrine in male but not female volunteers. These inhibitory effects were associated with a reduced platelet P-selectin expression and elevated platelet cyclic guanosine monophosphate (cGMP) levels. In addition, I have shown that NO2- reduction to NO occurs at the level of the erythrocyte and not the platelet. These results demonstrate that inorganic NO3- ingestion, whether via the diet or through supplementation, results in a modest decrease in platelet reactivity in healthy males. I then sought to examine the effects of 6 weeks daily intake of NO3--rich beetroot juice versus a placebo NO3--deplete juice on endothelial and platelet function in a cohort of otherwise healthy non-diabetic untreated hypercholesterolaemics. In this randomised double blind placebo controlled parallel study 69 subjects were recruited. The primary end point was change in endothelial function determined using ultrasound flow-mediated dilatation (FMD). Secondary endpoints included change in pulse wave analysis (PWA), aortic pulse wave velocity (aPWV), platelet P-selectin and platelet monocyte aggregate (PMA) expression and plasma, urine and salivary NO3- and NO2- levels. Baseline characteristics, including lipid levels, were similar between the groups. Dietary NO3- caused an improvement in FMD of ~24% from 4.6%±2.2% to 5.7%±2.6% in the treatment group (p<0.001) not seen in the placebo group (4.5%±1.9% versus 4.3%±1.8% p=0.07). This improvement in FMD was also noted following acute administration of dietary NO3-. Small but significant improvements also occurred in aPWV and PWA augmentation index (p=0.04). The % of platelet monocyte aggregates was significantly reduced in the NO3- limb by 7.6% versus an increase of 10.1% in the placebo group (p=0.004). No adverse effects of dietary NO3- were detected. In this study population, chronic dietary NO3- ingestion improves endothelial function, vascular stiffness and platelet markers of atherogenesis in a cohort of hypercholesterolaemics who are otherwise at increased risk of cardiovascular disease (CVD). This thesis provides strong support for assessment of the potential of dietary NO3- as a primary prevention strategy to prevent atherothrombotic and atherogenic complications in larger cohorts.
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