Spotlight commentary: Nitric oxide, vascular dysfunction and emerging therapeutic insights

Endothelial dysfunction is a hallmark of cardiometabolic disease. Central to this process is nitric oxide (NO), whose decreased synthesis, enhanced degradation or impaired signalling shifts vascular tone toward vasoconstriction, inflammation and thrombosis. Over the past decade, a series of human studies and randomized trials, including several published in the BJCP, have clarified where NO-modulating strategies offer clinical relevance and where therapeutic optimism should be tempered. This spotlight commentary synthesizes key mechanisms with demonstrated translational impact and highlights interventions supported by contemporary clinical evidence, primarily from the last 5 to 10 years. In clinical settings, three recurring mechanisms diminish NO bioactivity: oxidative degradation leading to peroxynitrite formation; impaired substrate and cofactor availability for endothelial nitric oxide synthase (eNOS), including dysregulation of the L-arginine/asymmetric dimethylarginine (ADMA) axis and heightened arginase activity; and pro-inflammatory signalling that disrupts endothelial phenotype and vasomotor control. Vascular inflammation is increasingly recognized as a modifiable determinant of endothelial function. In a BJCP placebo-controlled study, short-term colchicine therapy improved beta-adrenoceptor-mediated vasodilation in men with essential hypertension, indicating that anti-inflammatory intervention can restore adrenergic responsiveness.1 Complementary Mendelian randomization studies published in BJCP provide causal evidence linking IL6 and IL6R signalling to cardiometabolic disease risk, highlighting the broader immunological drivers of endothelial dysfunction and nitric oxide dysregulation.2, 3 These insights align with a recently proposed conceptual framework in MedComm, which describes eNOS uncoupling, oxidative stress and inflammatory signalling as key disruptors of endothelial function.4 Rather than bypassing eNOS, dietary nitrate enters a complementary nitrate–nitrite–NO pathway, which interconverts with NOS-derived species via the enterosalivary circulation. In hypoxic and acidic environments, nitrite is reduced to NO, allowing localized bioactivation where oxygen tension is lowest. Recent BJCP trials have explored its clinical utility. In a randomized crossover trial, O'Gallagher et al.5 investigated the interaction between grapefruit juice and beetroot-derived nitrate. Although the original hypothesis centred on CYP3A4 inhibition limiting nitrite oxidation, the study found that grapefruit juice amplified the systolic blood pressure-lowering effect, even though plasma nitrite levels were reduced. This suggests that grapefruit juice influenced nitrate metabolism upstream, likely by altering oral nitrate reduction, without directly enhancing nitrite levels, and that the haemodynamic effects may involve nitric oxide species other than nitrite. In the BJCP-published VaSera study, Mills et al. reported that dietary nitrate reduced central aortic systolic blood pressure in patients at risk of or with type 2 diabetes, independent of concurrent antihypertensive treatments.6 A subsequent analysis from the same trial reported that 6 months of dietary nitrate supplementation slowed progression of carotid intima-media thickness compared with nitrate-depleted controls, underscoring structural benefit beyond hemodynamic changes.7 Importantly, the oral microbiome is integral to nitrate bioactivation. Antiseptic mouthwashes can reduce salivary nitrate-reducing bacteria, blunting blood pressure responses and altering microbial ecology.8 Meta-analyses reinforce a modest, consistent systolic blood pressure reduction, though diastolic effects remain inconsistent.9, 10 In clinical practice, the use of nitrate-rich vegetables and standardized beetroot products in appropriate patients, together with consideration of interfering agents such as antiseptic mouthwashes, represents a pragmatic strategy. L-citrulline supplementation more reliably raises systemic L-arginine levels than oral L-arginine due to superior intestinal absorption and avoidance of hepatic first-pass metabolism. It can also favourably influence the L-arginine/asymmetric dimethylarginine (ADMA) ratio, a biomarker of NO availability. Recent BJCP evidence has shown modest improvements in endothelial function and arterial stiffness in selected populations.11 These supplements are best considered adjunctive therapies in patients with suspected impairments in arginine metabolism, such as elevated ADMA levels, increased arginase activity or low baseline L-arginine. Their use should ideally be guided by biochemical markers, particularly the arginine/ADMA ratio, rather than nonspecific endpoints like ambulatory blood pressure.11 Not all interventions targeting NO pathways have succeeded. For instance, combining antioxidants with NO donors has failed to improve vasodilatory effects. A recent BJCP study by He and Parker12 found that vitamin C did not augment nitroglycerin-mediated vasodilation, and following a second infusion, paradoxically reduced vasodilation, even in individuals with ALDH2 polymorphism implicated in GTN bioactivation. Phosphodiesterase-5 inhibitors (PDE5i), which prolong cyclic GMP signalling remain established treatments for pulmonary arterial hypertension and erectile dysfunction. Observational studies suggest possible cardioprotective effects with long-term use, though randomized vascular outcome trials are still needed.13 Expanding beyond NO-centric strategies, Bergougnan et al. conducted a BJCP published, randomized controlled trial in 54 patients with type 2 diabetes, showing that a G-protein biased sphingosine-1-phosphate receptor-1 (S1P1) agonist significantly improved flow-mediated dilation (FMD). While variability may limit FMD's broader interpretability, this study suggests that, with rigorous methodology and significant sample size, FMD can still yield meaningful insights into endothelial-targeted interventions.14 Accurately quantifying endothelial function remains a clinical challenge. FMD is widely used but can be limited by variability, particularly between laboratories. However, as shown by Holder et al. in a BJCP-published reference study,15 strict adherence to methodological guidelines substantially improves FMD reproducibility and interpretive reliability. When properly implemented, FMD remains a valuable endpoint for detecting changes in endothelial function in prospective intervention trials. A pragmatic therapeutic synthesis can be proposed. First, inflammation should be addressed in patients with elevated inflammatory tone, as this may restore adrenergic vasodilation and NO bioavailability.1-3 Second, dietary nitrate is an accessible and generally safe intervention with a modest but consistent systolic effect. Its clinical use should take into account microbiome dependent nitrate to nitrite conversion and potential interactions, such as those affecting salivary nitrate reduction.5-10 Third, L-citrulline, preferable to L-arginine, due to superior bioavailability, may benefit individuals with impaired L-arginine metabolism, such as elevated ADMA levels or heightened arginase activity.11 The use of such supplements should ideally be guided by biochemical indices (e.g., L-arginine/ADMA ratio) or validated clinical metrics. Finally, combining antioxidants with NO donors is not supported by clinical evidence: in a BJCP-published study, vitamin C failed to enhance (and even reduced!) nitroglycerin-mediated vasodilation following repeated infusion.12 These interventions are summarized in Table 1, which outlines their mechanisms of action and reported clinical effects. Personalized NO-modulating strategies are increasingly feasible. Integrating inflammatory biomarkers, such as high-sensitivity C-reactive protein (hs-CRP), genetic markers of IL-6 signalling and biochemical indices such as L-arginine/ADMA balance may help identify individuals more likely to respond to targeted interventions.2, 3, 11 Combination approaches, such as nitrate supplementation alongside standard cardiovascular therapies (ACE inhibitors and statins) and anti-inflammatory agents, should be evaluated in well-powered trials using clinically robust endpoints. Preserving oral nitrate-reducing flora may also emerge as a cost-effective means of enhancing nitrate efficacy.8 Viewed through the lens of contemporary human research, strategies to modulate nitric oxide bioavailability represent promising adjuncts in cardiovascular risk management. A nuanced understanding of mechanistic pathways, individual variability and evidence-based interventions is essential to translating this promise into meaningful clinical benefit. The authors declare no conflict of interest.