In 1987 Louis Ignarro published a paper in the Proceedings of the National Academy of Science that was the culmination of years of work trying to identify a molecule known before then only by its function – Endothelium-derived relaxing factor, or EDRF. As the name implies this was a substance that came from the lining of blood vessels, the endothelium. One of EDRF’s primary effects was to relax the smooth muscle embedded in blood vessels. It caused the pipes to open up, a process called vasodilation. When I was a UCLA medical student in 1995, Dr. Ignarro was our professor of pharmacology. There were already whispers that he was a shoo-in for the Nobel Prize for discovering that the true identity of EDRF was nitric oxide, a signaling molecule made up of two atoms, nitrogen and oxygen. For this discovery Dr. Ignarro was indeed awarded the Nobel Prize in Physiology or Medicine in 1998. So it was with no little amount of trepidation that I was ushered into his office to discuss why exactly my pharmacology examination score was…ahem…shall we say… decidedly not Nobel prize-worthy. I risked failing a class taught by a future Nobel-prize winner, but at least I got some one-on-one time with the great scientist himself. (I ended up doing fine on the final exam, by the way.)

So what was so special about this mysterious molecule nitric oxide, or NO? Nitric oxide relaxes blood vessels, and having sufficent NO activity helps prevent hypertension and helps provide sufficient blood flow to vital organs, including the heart. Dr. Ignarro went on to pen several books touting his favorite molecule and its importance in preventing strokes and heart attacks. Indeed, NO is why we give nitroglycerin to patients with heart attacks and those with heart pain, or angina. The drug works by increasing the amount of NO in cardiac vessels. This opens up blocked pipes, bringing blood flow and oxygen to dying heart muscle cells and relieving pain. NO is an important molecule to be sure. Our bodies control blood vessel dilation and regulate blood pressure with an enzyme called eNOS, endothelial nitric oxide synthase, which has a critical role in regulating healthy blood pressure. Lou Ignarro became a huge fan of NO and it’s effects.

NO has its detractors too. It seemed to have a dark side too. The inducible form of NOS (iNOS) produces both NO and superoxide that combine to make peroxynitrite during acute inflammation. This interaction damages the lining of blood vessels. During sepsis, iNOS is activated along with inflammation, causing blood vessels to dilate and making blood pressure in some patients become dangerously low.

Low blood pressure (hypotension) is a hallmark of severe sepsis. Septic patients are routinely subjected to interventions – IV fluids, catecholamine, vasopressin, phenylephrine – aimed at increasing the blood pressure.

Should we then block iNOS in sepsis to raise blood pressure and prevent cardiovascular collapse? As you might expect, it has been tried. Lopez et al showed that inhibiting nitric oxide production had an effect on mortality. It wasn’t the effect on mortality that the investigators expected. The nitric oxide inhibitor 546C88 caused more deaths in the treated group.

Could this outcome have been predicted? One possible reason for the failure of nitric oxide inhibition in sepsis is the finding that nitric oxide has antimicrobial activity. Burggraaf et al. (2011) wrote: “the inducible form of NOS (iNOS) generates NO, which serves as a potent anti-viral molecule to combat infection in combination with acute phase proteins and cytokines.” For their part, many pathogens block the action of NOS as part of their virulence program. For example, influenza A inhibits nitric oxide production during infection (Lyon and Hinshaw 1993).

Andreakis et al. evaluated 181 NOS proteins in 33 invertebrate and 63 vertebrate species and showed that its function is remarkably conserved. Nitric oxide is an important molecule used by invertebrates to combat infection. Ana Rivero showed that NO is used by insects and other invertebrates to kill parasites.

Given that iNOS function is conserved, it makes little sense that impairing its function would improve survival during severe infection and sepsis. Not so fast, a detractor might say. Just because NO plays a host defense role in a wide variety of vertebrates and invertebrates across the tree of life does not mean necessarily that NO has the same function in people. After all, many people, physicians included, ascribe to a human exceptionalism point of view. However, the One Health movement provides ample evidence to the contrary. And the Lopez et al study was done in humans; the inhibitor made patients worse. This result and ones like it suggest that NOS has a defense function during infection that we should endeavor to understand better in order to improve outcomes in critically ill patients.

Update: nitric oxide therapy was just shown to improve viral clearance in patients with COVID. Read this Lancet article.

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Joe Alcock

Emergency Physician, Educator, Researcher, interested in the microbiome, evolution, and medicine