In vitro studies have shown that viagra® is a selective PDE5 inhibitor. PDE5 is an enzyme found in the smooth muscles of the corpus cavernosum, in the smooth muscles of the vessels of the internal organs, in skeletal muscles, platelets, kidneys, lungs and cerebellum. The action of viagra® on PDE5 is more active than on other phosphodiesterases. viagra® is 10,000 times more potent for PDE5 than for PDE1, PDE2, PDE4 and PDE7, which are localized in the heart, brain, blood vessels, liver, leukocytes, skeletal muscles and other organs. viagra® is 10,000 times more active in blocking PDE5 than PDE3, an enzyme found in the heart and blood vessels. This selectivity for PDE5 over PDE3 is important because PDE3 is an enzyme involved in cardiac muscle contraction.