Remote ischaemic preconditioning suppresses endogenous plasma nitrite during ischaemia-reperfusion: a randomized controlled crossover pilot study

Nitrite and myocardial ischaemia reperfusion injury. Where are we now?

Cardiovascular disease remains the leading cause of death worldwide despite major advances in technology and treatment, with coronary heart disease (CHD) being a key contributor. Following an acute myocardial infarction (AMI), it is imperative that blood flow is rapidly restored to the ischaemic myocardium. However, this restoration is associated with an increased risk of additional complications and further cardiomyocyte death, termed myocardial ischaemia reperfusion injury (IRI). Endogenously produced nitric oxide (NO) plays an important role in protecting the myocardium from IRI. It is well established that NO mediates many of its downstream functions through the 'canonical' NO-sGC-cGMP pathway, which is vital for cardiovascular homeostasis; however, this pathway can become impaired in the face of inadequate delivery of necessary substrates, in particular L-arginine, oxygen and reducing equivalents. Recently, it has been shown that during conditions of ischaemia an alternative pathway for NO generation exists, which has become known as the 'nitrate-nitrite-NO pathway'. This pathway has been reported to improve endothelial dysfunction, protect against myocardial IRI and attenuate infarct size in various experimental models. Furthermore, emerging evidence suggests that nitrite itself provides multi-faceted protection, in an NO-independent fashion, against a myriad of pathophysiologies attributed to IRI. In this review, we explore the existing pre-clinical and clinical evidence for the role of nitrate and nitrite in cardioprotection and discuss the lessons learnt from the clinical trials for nitrite as a perconditioning agent. We also discuss the potential future for nitrite as a pre-conditioning intervention in man.

Grapefruit juice enhances the systolic blood pressure-lowering effects of dietary nitrate-containing beetroot juice

AIMS

Dietary nitrate from sources such as beetroot juice lowers blood pressure (BP) via the nitrate-nitrite-nitric oxide (NO) pathway. However, NO and nitrite are inactivated via reoxidation to nitrate, potentially limiting their activity. Cytochrome P450-3A4 inhibition with troleandomycin prevents nitrite re-oxidation to nitrate in rodent liver. Grapefruit juice contains the CYP3A4 inhibitor furanocoumarin. We therefore hypothesized that grapefruit juice would enhance BP-lowering with beetroot juice by maintaining circulating [nitrite].

METHODS

We performed a randomized, placebo-controlled, 7-hour crossover study in 11 healthy volunteers, attending on 3 occasions, receiving: a 70-mL shot of active beetroot juice (Beet-It) and either (i) 250 mL grapefruit juice (Active Beet+GFJ), or (ii) 250 mL water (Buxton, Active Beet+H2 O); or (iii) Placebo Beet+GFJ.

RESULTS

The addition of grapefruit juice to active beetroot juice lowered systolic BP (SBP): Active Beet+GFJ vs Active Beet+H2 O (P = .02), and pulse pressure, PP (P = .0003). Peak mean differences in SBP and PP were seen at T = 5 hours: -3.3 mmHg (95% confidence interval [CI] -6.43 to -0.15) and at T = 2.5 hours: -4.2 mmHg (95% CI -0.3 to -8.2), respectively. Contrary to the hypothesis, plasma [nitrite] was lower with Active Beet+GFJ vs Active Beet+H2 O (P = .006), as was salivary nitrite production (P = .002) and saliva volume (-0.34 mL/min [95% CI -0.05 to -0.68]). The taste score of Beet+GFJ was 1.4/10 points higher than Beet+H2 O (P = .03).

CONCLUSION

Grapefruit juice enhanced beetroot juice's effect on lowering SBP and PP despite decreasing plasma [nitrite]. Besides suggesting more complex mechanisms, there is potential for maximising the clinical benefit of dietary nitrate and targeting isolated systolic hypertension.

Acute interaction between oral glucose (75 g as Lucozade) and inorganic nitrate: Decreased insulin clearance, but lack of blood pressure-lowering

AIMS

Dietary inorganic nitrate (NO₃ ^- ) lowers peripheral blood pressure (BP) in healthy volunteers, but lacks such effect in individuals with, or at risk of, type 2 diabetes mellitus (T2DM). Whilst this is commonly assumed to be a consequence of chronic hyperglycaemia/hyperinsulinaemia, we hypothesized that acute physiological elevations in plasma [glucose]/[insulin] blunt the haemodynamic responses to NO₃ ^- , a pertinent question for carbohydrate-rich Western diets.

METHODS

We conducted an acute, randomized, placebo-controlled, double-blind, crossover study on the haemodynamic and metabolic effects of potassium nitrate (8 or 24 mmol KNO₃ ) vs. potassium chloride (KCl; placebo) administered 1 hour prior to an oral glucose tolerance test in 33 healthy volunteers.

RESULTS

Compared to placebo, there were no significant differences in systolic or diastolic BP (P = 0.27 and P = 0.30 on ANOVA, respectively) with KNO₃ , nor in pulse wave velocity or central systolic BP (P = 0.99 and P = 0.54 on ANOVA, respectively). Whilst there were significant elevations from baseline for plasma [glucose] and [C-peptide], no differences between interventions were observed. A significant increase in plasma [insulin] was observed with KNO₃ vs. KCl (n = 33; P = 0.014 on ANOVA) with the effect driven by the high-dose cohort (24 mmol, n = 13; P < 0.001 on ANOVA; at T = 0.75 h mean difference 210.4 pmol/L (95% CI 28.5 to 392.3), P = 0.012).

CONCLUSIONS

In healthy adults, acute physiological elevations of plasma [glucose] and [insulin] result in a lack of BP-lowering with dietary nitrate. The increase in plasma [insulin] without a corresponding change in [C-peptide] or [glucose] suggests that high-dose NO₃ ^- decreases insulin clearance. A likely mechanism is via NO-dependent inhibition of insulin-degrading enzyme.

Cardiac effects of 6 months' dietary nitrate and spironolactone in patients with hypertension and with/at risk of type 2 diabetes, in the factorial design, double-blind, randomized controlled VaSera trial

AIMS

The aims of the present study were to explore whether a long-term intervention with dietary nitrate [(NO₃ ^- ), a potential tolerance-free source of beneficial vasoactive nitric oxide] and spironolactone (to oppose aldosterone's potential deleterious cardiovascular effects) improve cardiac structure/function, independently of blood pressure (BP), in patients with/at risk of type 2 diabetes (a population at risk of heart failure).

METHODS

A subsample of participants in our double-blind, randomized, factorial-design intervention (VaSera) trial of active beetroot juice as a nitrate source (≤11.2 mmol) or placebo (nitrate depleted) beetroot juice, and either ≤50 mg spironolactone or ≤16 mg doxazosin (control), had transthoracic cardiac ultrasounds at baseline (n = 105), and at 3 months and 6 months (n = 87) after the start of the intervention. Analysis was by modified intent-to-treat.

RESULTS

Nitrate-containing juice (n = 40) decreased left ventricular (LV) end-diastolic volume {-6.3 [95% confidence interval (CI) -11.1, -1.6] ml} and end-systolic volume [-3.2 (95% CI -5.9, -0.5) ml], and increased end-diastolic mass/volume ratio [+0.04 (95% CI 0.00, 0.07)], relative to placebo juice (n = 47). Spironolactone (n = 44) reduced relative wall thickness compared with doxazosin (n = 43) [-0.01 (95% CI -0.02, -0.00)]. Although spironolactone reduced LV mass index relative to baseline [-1.48 (95% CI -2.08, -0.88) g m^-2.7 ], there was no difference vs. doxazosin [-0.85 (95% CI -1.76, 0.05) g m^-2.7 ]. Spironolactone also decreased the E/A ratio [-0.12 (95% CI -0.19, -0.04)] and increased S' (a tissue-Doppler systolic function index) by 0.52 (95% CI 0.05, 1.0) cm s^-1 . BP did not differ between the juices, or between the drugs.

CONCLUSIONS

Six months' dietary nitrate decreased LV volumes ~5%, representing new, sustained, BP-independent benefits on cardiac structure, extending mechanisms characterized in preclinical models of heart failure. Spironolactone's effects on cardiac remodelling and systolic-diastolic function, although confirmatory, were independent of BP.

Remote ischaemic preconditioning suppresses endogenous plasma nitrite during ischaemia-reperfusion: a randomized controlled crossover pilot study

AIM

The aim of this article is to test the hypothesis that remote ischaemic preconditioning (RIPC) increases circulating endogenous local and systemic plasma (nitrite) during RIPC and ischaemia-reperfusion (IR) as a potential protective mechanism against ischaemia-reperfusion injury (IRI).

METHODS

Six healthy male volunteers (mean age 29.5 ± 7.6 years) were randomized in a crossover study to initially receive either RIPC (4 × 5 min cycles) to the left arm, or no RIPC (control), both followed by an ischaemia-reperfusion (IR) sequence (20 min cuff inflation to 200 mmHg, 20 min reperfusion) to the right arm. The volunteers returned at least 7 days later for the alternate intervention. The primary outcome was the effect of RIPC vs. control on local and systemic plasma (nitrite).

RESULTS

RIPC did not significantly change plasma (nitrite) in either the left or the right arm during the RIPC sequence. However, compared to control, RIPC decreased plasma (nitrite) during the subsequent IR sequence by ~26% (from 118 ± 9 to 87 ± 5 nmol l^-1 ) locally in the left arm (P = 0.008) overall, with an independent effect of -58.70 nmol l^-1 (95% confidence intervals -116.1 to -1.33) at 15 min reperfusion, and by ~24% (from 109 ± 9 to 83 ± 7 nmol l^-1 ) systemically in the right arm (P = 0.03).

CONCLUSIONS

RIPC had no effect on plasma (nitrite) during the RIPC sequence, but instead decreased plasma (nitrite) by ~25% during IR. This would likely counteract the protective mechanisms of RIPC, and contribute to RIPC's lack of efficacy, as observed in recent clinical trials. A combined approach of RIPC with nitrite administration may be required.