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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.

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In addition, viagra® is approximately 700 times more active against PDE5 than against PDE6, an enzyme found in the retina of the eye and responsible for phototransmission. viagra® also exhibits an effect 9,000 times more potent against PDE5 compared to its effect on PDE8, PDE9 and PDE10, and 14 times more potent against PDE5 than PDE11. Tissue distribution and physiological effects of inhibition of PDE8 - PDE11 have not yet been clarified.

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viagra® in healthy individuals does not cause significant changes in systolic and diastolic pressure in comparison with placebo in the supine position (the average maximum decrease is 1.6 / 0.8 mm Hg, respectively) and in the standing position (the average maximum decrease is 0.2 / 4.6 mm Hg. Art., respectively). viagra® does not cause significant changes in heart rate.

viagra® does not cause changes in color recognition (blue / green), which is explained by its low affinity for PDE6 compared to PDE5. In addition, the effect of viagra® on visual acuity, electroretinogram, intraocular pressure and pupil size is not observed.