Inorganic nitrate ingestion improves vascular compliance but does not alter flow-mediated dilatation in healthy volunteers

Abstract

Ingestion of inorganic nitrate elevates blood and tissue levels of nitrite via bioconversion in the entero-salivary circulation. Nitrite is converted to NO in the circulation, and it is this phenomenon that is thought to underlie the beneficial effects of inorganic nitrate in humans. Our previous studies have demonstrated that oral ingestion of inorganic nitrate decreases blood pressure and inhibits the transient endothelial dysfunction caused by ischaemia-reperfusion injury in healthy volunteers. However, whether inorganic nitrate might improve endothelial function per se in the absence of a pathogenic stimulus and whether this might contribute to the blood pressure lowering effects is yet unknown. We conducted a randomised, double-blind, crossover study in 14 healthy volunteers to determine the effects of oral inorganic nitrate (8 mmol KNO(3)) vs. placebo (8 mmol KCl) on endothelial function, measured by flow-mediated dilatation (FMD) of the brachial artery, prior to and 3h following capsule ingestion. In addition, blood pressure (BP) was measured and aortic pulse wave velocity (aPWV) determined. Finally, blood, saliva and urine samples were collected for chemiluminescence analysis of [nitrite] and [nitrate] prior to and 3h following interventions. Inorganic nitrate supplementation had no effect on endothelial function in healthy volunteers (6.9±1.1% pre- to 7.1±1.1% post-KNO(3)). Despite this, there was a significant elevation of plasma [nitrite] (0.4±0.1 μM pre- to 0.7±0.2 μM post-KNO(3), p<0.001). In addition these changes in [nitrite] were associated with a decrease in systolic BP (116.9±3.8mm Hg pre- vs. 112.1±3.4 mm Hg post-KNO(3), p<0.05) and aPWV (6.5±0.1 m/s pre- to 6.2±0.1 post-KNO(3), p<0.01). In contrast KCl capsules had no effect on any of the parameters measured. These findings demonstrate that although inorganic nitrate ingestion does not alter endothelial function per se, it does appear to improve blood flow, in combination with a reduction in blood pressure. It is likely that these changes are due to the intra-vascular production of NO.

Fig. 1

Inorganic nitrate supplementation elevates NO_x in healthy volunteers. The effects of inorganic nitrate supplementation (KNO₃, 8 mmol) or placebo (KCl, 8 mmol) on circulating plasma (a) [nitrate] and (b) [nitrite]; salivary (c) [nitrate] and (d) [nitrite]; and urinary (e) [nitrate] and (f) [nitrite]. Significance shown for Bonferroni post hoc tests between groups as ^∗∗p < 0.01 and ^∗∗∗p < 0.001 following 1-way ANOVA. Data are expressed as mean ± SEM (n = 14).

Fig. 2

Inorganic nitrate does not alter endothelial function in healthy volunteers. The effects of inorganic nitrate supplementation (KNO₃, 8 mmol) or placebo (KCl, 8 mmol) on FMD. No significance found for comparisons between groups using Bonferroni post hoc tests, following 1-way ANOVA. Data are expressed as mean ± SEM (n = 13).

Fig. 3

Inorganic nitrate reduces SBP and aPWV in healthy volunteers. The effects of inorganic nitrate supplementation (KNO₃, 8 mmol) or placebo (KCl, 8 mmol) on (a) SBP, (b) DBP, (c) HR and (d) aPWV. Significance shown for Bonferroni post hoc tests between groups as ^∗p < 0.05, ^∗∗p < 0.01 following 1-way ANOVA. Data are expressed as mean ± SEM (n = 14).