Earlier, we talked about the enzyme PDE. To better understand its function, consider that an enzyme is a specially folded protein that can speed up a chemical reaction. (For example, the article How Cells Work describes the maltase enzyme.)
In the accompanying diagram, you see that maltose is made of two glucose molecules bonded together (1). The maltase enzyme is a protein perfectly shaped to accept a maltose molecule and break the bond (2). The two glucose molecules are then released (3). A single maltase enzyme can break more than 1,000 maltose bonds per second, and will only accept maltose molecules.
PDE5 is an enzyme that accepts cGMP and breaks it down. Pfizer needed a chemical that would gum up PDE5 and keep it from doing its job. The chemical that Pfizer discovered is called sildenafil citrate. It fits right into the PDE5 enzyme and disables it.
Viagra contains sildenafil citrate packaged as a pill. When a man takes a Viagra pill, the sildenafil citrate flows throughout his body, but it really only affects the PDE5 enzyme in the penis. The drug stays in the bloodstream for about four hours, and then it's washed out of the blood by the liver and kidneys.
And that's the end of the "how it works" part of the Viagra story:
- A man takes a Viagra pill.
- The sildenafil citrate enters his bloodstream and flows throughout his body.
- The sildenafil citrate attaches to the PDE5 enzyme in his penis and disables most of it.
- When the man becomes sexually aroused, the brain sends the normal message to nerve cells in his penis, which produce nitric oxide as usual.
- The nitric oxide creates cGMP, which starts relaxing the arteries in his penis.
- Since the PDE5 has been disabled, the cGMP in the penis doesn't break down. Instead, it builds up and lets the arteries in the penis fully dilate.
- His penis inflates with blood, and the man gets a full erection.
This works perfectly for the majority of men -- except for a few minor problems.