on October 13, 2026
Nitric oxide, a small molecule with a huge function
Nitric oxide, indicated by the abbreviation NO, is a highly diffusible gaseous molecule that performs signaling functions in numerous tissues. Its discovery as a fundamental mediator of vascular function has profoundly changed the understanding of cardiovascular physiology.
The endothelium, the cellular layer that lines the inside of blood vessels, is not merely an anatomical barrier. It is a true endocrine and paracrine organ capable of producing mediators that regulate vascular tone, permeability, cell adhesion, platelet activity, coagulation, and inflammation.
Among these mediators, NO occupies a central position.
Its best-known function is that of a vasodilator: produced by endothelial cells, it rapidly diffuses toward the vascular smooth muscle and induces relaxation of the vessel wall. This activity contributes to the regulation of blood pressure and the distribution of blood flow.
But the action of NO is much broader. The molecule participates in the regulation of platelet activity, interactions between leukocytes and the endothelium, and numerous immune processes. Its function, therefore, is located at the intersection of the cardiovascular system, the immune system, and oxidative metabolism.
From the discovery of EDRF to the biology of NO
The history of cardiovascular NO began with the demonstration that endothelial cells produce a factor capable of causing the relaxation of vascular smooth muscle. This mediator was initially defined as the endothelium-derived relaxing factor, or EDRF.
It was subsequently demonstrated that EDRF corresponded essentially to nitric oxide and that its vasodilating effect depended on the activation of soluble guanylate cyclase, with a consequent increase in the intracellular second messenger cGMP.
This discovery made it possible to understand that the blood vessel possesses a sophisticated regulatory system based on the balance between vasodilating and vasoconstricting signals.
Under physiological conditions, NO represents one of the main mediators that maintain the vessel in a state favorable to vasodilation and homeostasis.
How nitric oxide is produced
The main endogenous pathway for NO production starts with L-arginine and involves a family of enzymes called nitric oxide synthase (NOS).
Three main isoforms are recognized:
· nNOS, or neuronal NOS;
· eNOS, or endothelial NOS;
· iNOS, or inducible NOS.
eNOS is particularly important for cardiovascular physiology. It is located mainly in endothelial cells and responds to mechanical and biochemical stimuli, including shear stress, that is, the force exerted by blood flow on the vascular wall.
The activation of eNOS allows the conversion of L-arginine into L-citrulline and NO. The process requires several cofactors, including tetrahydrobiopterin (BH4).
The proper functioning of eNOS is essential because the same protein can behave differently depending on the redox state of the cell.
When BH4 availability is adequate and the enzymatic system works correctly, NO production prevails. In conditions characterized by oxidative stress and BH4 deficiency, eNOS can undergo so-called uncoupling, promoting the production of reactive oxygen species instead of proper NO synthesis.
This step represents one of the most important biochemical links between oxidative stress and endothelial dysfunction.
The NO-cGMP pathway and vasodilation
NO has a fundamental characteristic: being a small diffusible molecule, it can rapidly cross cell membranes.
From the endothelial cells, it reaches the underlying vascular smooth muscle, where it binds to soluble guanylate cyclase.
The activation of the enzyme increases the production of cyclic guanosine monophosphate (cGMP) from GTP.
The increase in cGMP causes a series of intracellular changes that reduce the concentration of calcium available for smooth muscle contraction. The final result is vascular relaxation and vasodilation.
The signal is subsequently limited by the activity of phosphodiesterases, enzymes that degrade cGMP.
Among these, PDE5 has a particularly well-known role because its inhibition results in the maintenance of higher levels of cGMP in specific vascular beds. This pharmacological principle is the basis for the use of PDE5 inhibitors in erectile dysfunction.
The NO-sGC-cGMP pathway therefore represents one of the fundamental systems through which a signal produced by the endothelium is transformed into a functional response of the vascular wall.
When the endothelium loses the ability to produce NO
In a healthy vessel, there is a dynamic balance between vasodilating and vasoconstricting substances.
Endothelial dysfunction can be described, among other aspects, as a loss of the normal endothelium-dependent vasodilating capacity.
The reduction in NO bioavailability constitutes one of the central phenomena of this process.
The problem does not necessarily consist only in the decrease in the amount of NO produced. It is also important to consider how much NO remains biologically available after being produced.
Oxidative stress can indeed promote the reaction of NO with the superoxide radical, generating peroxynitrite (ONOO−). This reaction removes NO from the vasodilating system and simultaneously produces a highly reactive species capable of modifying proteins, lipids, and other cellular structures.
Thus, a possible vicious cycle is created:
oxidative stress → reduced NO bioavailability → endothelial dysfunction → greater redox alteration → further loss of vascular function.
The importance of L-arginine and endogenous inhibitors
NO synthesis depends on substrate availability and the efficiency of eNOS.
Among the factors that can interfere with this pathway are reduced L-arginine availability, alterations in the cellular transport of the amino acid, and the presence of endogenous molecules capable of interfering with NOS.
Particular attention has been paid to ADMA, asymmetric dimethylarginine, which can act as an endogenous inhibitor of NO synthesis.
To this are added reduced BH4 availability, increased reactive oxygen species, guanylate cyclase alterations, and increased vasoconstrictor mediators.
The overall result is a loss of the physiological balance that keeps the vessel adequately responsive to vasodilating stimuli.
NO and hypertension: a bidirectional relationship
Hypertension represents one of the contexts in which the relationship between NO and endothelial function takes on particular importance.
A significant role is played by angiotensin II (Ang-II), one of the main vasoconstrictor effectors of the renin-angiotensin system.
The activation of the AT1 receptor promotes vasoconstriction, oxidative stress, and a series of intracellular signals that can compromise endothelial function.
Ang-II can also increase the production of reactive oxygen species, promoting BH4 consumption and contributing to eNOS uncoupling.
A functional relationship is therefore created between two apparently opposing systems:
Ang-II → vasoconstriction and oxidative stress
versus
NO → vasodilation and protection of vascular homeostasis.
The balance between these pathways is particularly important in the regulation of vascular tone. The literature considers the loss of NO signaling as an early and relevant element in the pathophysiology of hypertension, although the causal relationship between hypertension and endothelial dysfunction is complex and bidirectional.
NO and inflammation: a double-edged relationship
One of the most interesting aspects of nitric oxide is that it cannot simply be classified as an anti-inflammatory or pro-inflammatory molecule.
Its effect depends on the amount produced, the cell type involved, the duration of the stimulus, and above all, the redox context.
Physiological NO production by the endothelium contributes to the maintenance of vascular homeostasis and can limit some processes involved in the progression of atherosclerosis.
NO can indeed influence the adhesion of inflammatory cells to the endothelium, the proliferation of smooth muscle cells, and platelet activity.
The situation is different during an intense immune response.
The iNOS isoform, inducible during inflammatory processes, can produce very high amounts of NO. In the presence of high concentrations of superoxide, NO can be rapidly converted into peroxynitrite.
Therefore, the same molecular system can have different consequences:
Physiological NO → signaling, vasodilation, and homeostasis
NO under conditions of intense oxidative stress → formation of reactive nitrogen species and possible oxidative damage.
It is precisely this duality that explains why NO should be considered above all as a biological regulator, rather than simply as a “good” or “bad” molecule.
Dietary nitrate as an alternative pathway to NO
NO physiology does not depend exclusively on the L-arginine-eNOS pathway.
There is also a second pathway, defined as nitrate-nitrite-NO, through which dietary nitrate can be progressively converted into nitrite and subsequently into NO.
This pathway is particularly interesting because it involves the oral microbiota and becomes complementary to endothelial enzymatic synthesis.
Foods rich in nitrates, especially certain vegetables, have therefore been studied as potential nutritional tools capable of increasing NO availability.
Early reviews of human studies highlighted a possible acute reduction in blood pressure after the intake of dietary nitrates and favorable signals regarding endothelial function.
Subsequently, numerous randomized trials have explored this phenomenon in greater depth.
A meta-analysis of 47 randomized studies, including 1,101 participants, found an average reduction in systolic and diastolic blood pressure with repeated administration of inorganic nitrate, although it highlighted heterogeneity and non-uniform certainty of evidence. No convincing effects emerged, however, on blood pressure measured over 24 hours.
A meta-analysis specifically conducted in people with hypertension also found, in the available studies on nitrate-rich beetroot juice, a reduction in systolic blood pressure of approximately 5 mmHg, without a statistically significant reduction in diastolic blood pressure.
These results are interesting, but do not allow dietary nitrates to be considered as substitutes for prescribed antihypertensive therapy.
The relationship with endothelial function
The effect of nitrates does not concern only blood pressure.
Flow-mediated dilation (FMD) is one of the main parameters used to study endothelial function.
A recent systematic review and meta-analysis of 13 randomized studies found an improvement in FMD after taking inorganic nitrate, with a greater effect in acute studies than in chronic ones. However, the authors highlighted marked heterogeneity among the studies, urging caution in interpreting the results.
A broader dose-response meta-analysis published in 2025, including 75 randomized studies and 1,823 participants, highlighted dose-dependent increases in plasma nitrate and nitrite concentrations and associations with reductions in blood pressure and improvements in some vascular biomarkers. In this case too, more homogeneous studies and better-characterized populations are needed to define the actual clinical relevance of the observed effects.
From the endothelium to atherosclerosis
Endothelial dysfunction is considered an important step in the development of atherosclerosis.
When NO bioavailability decreases, leukocyte adhesion to the endothelium, platelet activity, and vascular smooth muscle cell proliferation may increase.
At the same time, LDL lipoprotein oxidation and the increase in adhesion molecules can promote the recruitment of inflammatory cells into the arterial wall.
NO physiologically produced by the endothelium, on the other hand, helps maintain a vascular environment less favorable to platelet aggregation and vascular proliferation.
Maintaining proper NO signaling therefore represents one of the elements of endothelial protection.
However, the relationship is extremely complex: NO does not act in isolation, but within a network involving prostacyclin, endothelin-1, angiotensin II, reactive oxygen species, the immune system, and lipid metabolism.
A delicate balance between NO and oxidative stress
Perhaps the most important concept emerging from NO physiology is that of redox balance.
It is not enough to produce NO: the molecule must remain available to perform its signaling function.
When superoxide increases, NO can be rapidly sequestered through the formation of peroxynitrite. This phenomenon reduces NO availability and simultaneously increases oxidative pressure.
Furthermore, excessive oxidative stress can impair eNOS function by altering the BH4-eNOS system.
Thus, a circular relationship is established:
oxidative stress → eNOS uncoupling → increased production of reactive species → decreased NO bioavailability → endothelial dysfunction.
This mechanism helps explain why the endothelium is one of the first targets of metabolic and vascular alterations associated with aging and the main cardiovascular risk factors.
One molecule, many systems
Nitric oxide demonstrates how reductive it is to rigidly separate the cardiovascular system, metabolism, and immunity.
The same molecule can participate in the regulation of vascular tone, platelet activity, cell communication, and immune response.
In the cardiovascular system, the eNOS-NO-cGMP pathway represents a fundamental mechanism of vasodilation.
In inflammation, NO production can increase through iNOS and interact with reactive oxygen species.
In hypertension, reduced NO bioavailability can be associated with increased vasoconstrictor activity and oxidative stress.
Finally, in nutrition, the nitrate-nitrite-NO pathway represents another system through which dietary components can influence vascular biology.
NO can therefore be considered a central hub of vascular physiology, where mechanical, metabolic, nutritional, oxidative, and inflammatory signals converge.
Perspectives
Contemporary research on nitric oxide is increasingly shifting from the simple observation of its vasodilatory function toward an integrated view of endothelial redox signaling.
The possibility of modulating NO availability through diet, physical activity, cardiovascular risk factor control, and pharmacological interventions represents a field of great interest.
The available evidence on dietary nitrates is particularly interesting, but it must be interpreted in the context of study quality, the doses used, the duration of the interventions, and the characteristics of the populations examined.
The most solid biological data remains physiological: an endothelium capable of correctly producing and using NO is essential for maintaining vascular homeostasis.
Preserving this function therefore means intervening not on a single molecule, but on the entire balance that connects metabolism, oxidative stress, inflammation, and cardiovascular regulation.
NOTE WELL
The information contained in this article is for informational and educational purposes and does not constitute medical advice. It does not replace a medical evaluation, diagnosis, or the prescription of therapy. Hypertension and cardiovascular diseases require appropriate clinical assessment and, when indicated, medical treatment. Evidence related to dietary nitrates and NO modulation should not be interpreted as an indication to replace prescribed medications or therapies.
Bibliographic references
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