So how does Viagra block PDE5?
Start
PDE5 is what is known as an enzyme. 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. Maltase is shaped so that a maltose molecule can fit right in, and when it does, the maltase enzyme breaks the maltose molecule into two glucose 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 is 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 the NANC 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 does not 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.