~ Nitric oxide is produced in the cells surrounding blood vessels to relax the blood vessels and lower blood pressure.
~ Decreased endothelial nitric oxide synthase (eNOS) can be a cause of heart disease, high blood pressure, and even affect brain health.
~ Genetic variants in the NOS3 gene are linked to lower eNOS and higher blood pressure.
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The Endothelium and Nitric Oxide Synthase:
Let’s dig into the creation of nitric oxide, what it does in the body, and why it is so important for heart health and healthy aging.
Nitric oxide (NO) is a molecule that acts as a signaling molecule in the body. It is also a free radical, so the body tightly regulates it. The body creates nitric oxide in specific reactions, but it only hangs around for less than 5 seconds.[ref]
Quick aside: Nitric oxide is not the same thing as nitrous oxide (laughing gas).
What does nitric oxide do in the body?
Nitric oxide’s composition consists of a nitrogen atom bound to an oxygen atom. This pairing leaves one electron free, thus making it a free radical.
Nitric oxide in the environment reacts to form acid rain, participates in ozone layer depletion, and it can kill you if you breathe it at high concentrations. Same molecule, but different roles when created inside the body.
Nitric oxide is a tiny molecule; it can diffuse across cell membranes and move without needing to be transported. This tiny gaseous NO molecule acts as a signaling molecule in pretty much every living organism (bacteria, plants, fungi, animals).
One important role that nitric oxide plays in the body is to act as a signaling molecule from the endothelium to the surrounding layers of the blood vessels, causing the blood vessel to relax. This relaxation (vasodilation) increases blood flow. Vasodilation usually causes a decrease in blood pressure.
Producing nitric oxide in the endothelium:
We need nitric oxide at the right amount in the endothelial cells that make up blood vessels.
The enzyme responsible for producing nitric oxide in the endothelium is known as eNOS (endothelium nitric oxide synthase), and it is coded for by the NOS3 gene.
Acetylcholine signals the creation of eNOS, and then the nitric oxide synthase causes the creation of nitric oxide from the amino acid arginine.
The NO produced in the endothelium regulates the constriction of the blood vessels, the stickiness of platelets, and leukocyte adhesion. It can diffuse from the endothelial cells into neighboring smooth muscle cells, causing the muscle to relax.
Let’s pause for a moment here for a quick explanation of the endothelium and then get into the chemistry of creating NO.
Background: What is the endothelium?
The single layer of endothelial cells forming the lining of blood vessels and lymph vessels is called the endothelium. This thin cell layer is essential to the health of your blood vessels. Endothelial cells can divide, and if there is a tear in a blood vessel, neighboring endothelial cells can proliferate and repair the tear. The endothelium also assists in controlling the rate of blood flow.[ref]
The endothelium acts as a barrier between everything in the bloodstream and the rest of the tissues of the body. But it is way more than just a passive lining.
Endothelial cells are selectively permeable – allowing in certain chemicals and white blood cells, and giving off signaling molecules.
Creation of nitric oxide: arginine, oxygen, citrulline
Arginine, an amino acid, converts into nitric oxide with the addition of oxygen. The product of the reaction is l-citrulline and NO.
Two essential proteins needed in the reaction are tetrahydrobiopterin (BH4) and nitric oxide synthase (NOS3, eNOS). Nitric oxide synthase (eNOS) is the enzyme that catalyzes the reaction.
Nitric oxide synthase turns up in other areas of the body, so in the endothelium, nitric oxide synthase (NOS) is referred to as eNOS to differentiate it from NOS elsewhere.
eNOS (NOS3) consists of two identical proteins joined together with BH4, a cofactor essential to this process. Without BH4 creating the bond, the reaction using just one nitric oxide synthase molecule will actually create a superoxide anion, which is a deleterious free radical. Bad stuff.[ref][ref]
When NOS joins together (with BH4 to help), it is called ‘coupled’. When not joined together, it is called uncoupled eNOS.
Excess oxidative stress is a driver of cardiovascular disease. One problem with oxidative stress is that it diminishes the levels of BH4. Without BH4 coupling the NOS3 molecules together, the reaction to form nitric oxide from l-arginine cannot happen. Instead, this results in the production of superoxide, a source of oxidative stress in the cell. Thus, excess oxidative stress decreases NOS3, driving the production of more oxidative stress.[ref]
What causes nitric oxide to be released?
The endothelium controls nitric oxide formation in multiple ways.
- The blood flowing through your veins causes a shear force on the endothelium, and this mechanical force acts to regulate NO release. It goes on all day long, every day. Plus, this blood flow regulation keeps platelets from activating and sticking together.[ref]
- You can have a decrease in eNOS due to a lack of oxygen (hypoxia) or due to insufficient BH4. A lack of eNOS causes blood pressure to rise. Alternatively, too much eNOS is also detrimental, so balance is needed.[ref]
- Hyperglycemia, or high blood sugar, can also inhibit eNOS. It may play a significant role in cardiovascular problems in people with diabetes.[ref]
Histamine (H1) receptors on the endothelial cell cause nitric oxide to be released in response to histamine. It is how histamine increases vascular permeability.[ref][ref]
Nitric oxide, endothelial function, and aging:
Cardiovascular disease (CVD) is the #1 cause of death in most countries, and about 70% of people over age 60 meet the criteria for CVD.
Researchers discovered decades ago that nitric oxide production in the endothelial cells lining the arteries is key to the health of the cardiovascular system.
As we age, there is an increase in oxidative stress in the body and a decrease in nitric oxide bioavailability. This reduction in nitric oxide directly drives the stiffening of the arteries seen in CVD.[ref]
Cardiovascular disease can arguably be considered a problem of imbalanced reactive oxygen species – e.g., oxidative stress. A simplified explanation is that this increase in oxidative stress decreases eNOS, thus increasing blood pressure.[ref]
What causes the increase in ROS over what the cell needs as we age? Things like chronic inflammation, mold toxins, gum disease, and heavy metals.
Exercise, heart health, and NOS3
Good blood flow and oxygen transport are essential for athletic performance, and nitric oxide is one variable athletes target to increase their endothelial function.
Exercise causes increased blood flow and increased shear stress in the blood vessels. This increase causes eNOS to be released, which is why exercise helps lower blood pressure for some people.[ref]
NOS3 Genotype Report
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NOS3 gene: encodes nitric oxide synthase
Quite a few studies on the NOS3 gene variants point to interactions with cardiovascular disease risk and blood pressure. In many studies, though, the increase in risk is statistically significant but not really very large. I encourage you to read through the research studies referenced.
Genetics may increase your susceptibility to coronary artery disease, but CAD is going to end up being due to a combination of lifestyle and environmental factors along with the genes.
One more caveat: there may be gender or population group differences as well with the NOS3 variants. One large, prospective study found that the NOS3 variants did not impact hypertension risk for Caucasian women.[ref]
Check your genetic data for rs891512 G24943A or IVS25+15 (23andMe v5; AncestryDNA):
- G/G: typical (more common allele)
- A/G: higher blood pressure, increased risk of coronary artery disease
- A/A: higher blood pressure[ref][ref], increased risk of coronary artery disease[ref]
Members: Your genotype for rs891512 is —.
Check your genetic data for rs1800779 (23andMe v4, v5; AncestryDNA):
- A/A: typical
- A/G: decreased NOS3 expression (compared to AA); increased risk of high blood pressure and cardiovascular disease
- G/G: decreased NOS3 expression (compared to AA); increased risk of high blood pressure and cardiovascular disease[ref], increased risk of primary open-angle glaucoma (women)[ref]
Members: Your genotype for rs1800779 is —.
Check your genetic data for rs4496877 (23andMe v4; AncestryDNA):
- G/G: typical
- G/T: increased risk of hypertension in older males (Chinese population)[ref], increased bilirubin in people who are older or who drink[ref]
- T/T: increased risk of hypertension in older males (Chinese population), increased bilirubin in people who are older or who drink[ref]
Members: Your genotype for rs4496877 is —.
Check your genetic data for rs2070744 (23andMe v5):
- C/C: increased risk of coronary artery disease (Asian, Caucasian)[ref], increased risk of osteoporosis[ref], decreased risk of hemorrhagic stroke, atrial fibrillation[ref][ref]
- C/T: increased risk of hypertension
- T/T: typical
Members: Your genotype for rs2070744 is —.
Nitric oxide is needed at just the right levels, specific to the tissue in question.
Increasing endothelial nitric oxide synthase may help with hypertension for some people. It may especially be true in conjunction with the above NOS3 genetic variants.[ref]
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Debbie Moon is the founder of Genetic Lifehacks. Fascinated by the connections between genes, diet, and health, her goal is to help you understand how to apply genetics to your diet and lifestyle decisions. Debbie has a BS in engineering and also an MSc in biological sciences from Clemson University. Debbie combines an engineering mindset with a biological systems approach to help you understand how genetic differences impact your optimal health.