Viagra
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Viagra is used for the treatment of male impotency. The active ingredient in Viagra is a specific inhibitor of phosphodiesterase 5. Phosphodiesterase 5 (PDE5) cleaves the ring form of cyclic GMP (cGMP) a cellular second messenger similar to cAMP. The inhibition of the phosphodiesterase allows for the persistence of cGMP which promotes the effects of cGMP production. In this case, it allows for increased blood flow into the penis which may result in an erection.
Normally, sexual stimulation in the male is followed by release of nitric oxide (NO) in the corpus cavernosum of the penis. NO activates the enzyme guanylate cyclase in the smooth muscle cells which allows for the relaxation of the penile arteries and an increase flow of blood into the penile cavity. The effect of Viagra is to allow the accumulation of cGMP by inhibiting the phosphodiesterase action which tends to magnify the effect of NO release.
