A clear, colorless gas that performs a number of important functions in the body. It seems to play a role in several medical conditions, including septic shock, dementia, and impotence, and it may partially account for the high rate of heart disease seen in people with diabetes. Nitric oxide is considered such an important chemical in the body that the journal Science named it the “Molecule of the Year” in 1992, and in 1998 three scientists won the Nobel Prize in Medicine for researching its role in the body.
Scientists have long been familiar with nitric oxide as a component of car exhaust, cigarette smoke, and air pollution, but it wasn’t until the late 1980’s that researchers discovered that mammal cells also produce the gas and use it to communicate with other cells. Abnormalities in the body’s production of nitric oxide have been implicated in high blood pressure, atherosclerosis (narrowing of the arteries), diabetes, erectile dysfunction, and stroke.
Scientists are furthest along in their understanding of nitric oxide in the area of heart disease. Normally, cells lining the inner wall of blood vessels secrete nitric oxide, which travels to the muscle cells around the blood vessels and causes them to widen, allowing easier blood flow. Impaired nitric oxide production may contribute to constricted blood vessels, high blood pressure (especially in the 90% of cases with no identifiable cause), and atherosclerosis. Scientists now know that nitroglycerin and similar drugs dilate blood vessels and help alleviate angina — temporary chest pain resulting from insufficient oxygen to the heart — because the drugs release nitric oxide. Nitric oxide also keeps blood platelets from clumping together and sticking to the inner walls of arteries, protecting the body from blood clots and atherosclerosis. Researchers are currently looking into drugs called nitric oxide donors that will increase nitric oxide levels in people with heart disease.
Nitric oxide also helps to initiate erections. During sexual stimulation, nerves in the penis release nitric oxide. Nitric oxide converts the chemical GTP to cyclic GMP, which causes the corpora cavernosa — the rods of spongy tissue within the penis — to fill with blood and become erect. Researchers believe that men who smoke and men with diabetes, atherosclerosis, or high blood pressure may have defective nitric oxide production, which can cause impotence. In fact, the discovery of nitric oxide’s role in the body led to the development of the popular impotence drugs sildenafil (brand name Viagra), vardenafil (Levitra), and tadalafil (Cialis). Like nitric oxide, these drugs work by increasing levels of cyclic GMP in the penis.
However, nitric oxide is not purely a good thing, since too much of it can contribute to severe medical problems. When there is a bloodborne infection, cells of the immune system produce vast amounts of nitric oxide to destroy the pathogen. This dilates blood vessels and can cause blood pressure to plummet — a condition called septic shock, which kills about 100,000 Americans each year. Also, when brain cells are deprived of oxygen during a stroke, they release lots of nitric oxide, which is believed to cause much of the destruction of brain tissue seen in strokes. To treat these attacks, researchers are trying to find ways to selectively inhibit the production of nitric oxide where it is harming the body — without decreasing the overall levels of beneficial nitric oxide. Research into the potential clinical applications of nitric oxide is still in its infancy.