HOW VIAGRA WORKS?
Sexual stimulation leads to the local release of nitric oxide (NO) from nerve endings and endothelial cells in the spongy erectile tissue - the corpus cavernosum - of the penis. NO switches on the enzyme guanylate cyclase, which converts guanosine triphosphate into cyclic guanosine monophosphate (cGMP). This key second messenger is a vasodilator - it relaxes the vascular smooth muscle of the blood vessels of the corpus cavernosum, so that the blood flows more strongly leading to an erection. However, at the same time cGMP is hydrolysed by phosphodiesterase type 5 enzyme (PDE5), to inactive GMP.Men who have erectile dysfunction (ED) are often producing insufficient amounts of NO. So although they are producing a small amount of cGMP, it is being broken down at the same rate. Viagra works by blocking part of the cycle. It selectively inhibits the PDE5, by binding with PDE5's active site. This prevents the hydrolysis of cGMP to inactive GMP, allowing cGMP to accumulate and prolong the vasodilation effect.
Because of this mechanism of action, Viagra works regardless of the underlying cause of ED. It is effective in men with ED associated with a variety of medical conditions, including vascular and neural disease, diabetes, prostate surgery, depression and spinal injury (providing sufficient nerve has been left intact). Whatever the cause of reduced NO formation, Viagra still works. However, Viagra is not an aphrodisiac. Because the drug is potentiating the natural effects of cGMP rather than stimulating its production, it only works in response to sexual stimulation.