When a man is sexually stimulated, the body releases nitric oxide in the network of blood vessels forming the corpus cavernosum — a sponge-like area of tissue inside the penis that absorbs an inflow of blood to create the erection. Nitrous oxide stimulates the production of an enzyme called guanylate cyclase (GMP) which, in turn, generates cyclic guanosine monophosphate (cGMP) in the muscle cells in the walls of the arteries carrying blood through the penis. This should relax the arteries and allow more blood to flow into the corpus cavernosum. But if the body also produces phosphodiesterase type 5 (PDE5), this process may be slowed or stopped.
Levitra inhibits the production of PDE5 inside the penis. On its own, this has no effect on the penis because sexual stimulation is required for an erection to occur. Thus, Levitra allows the body to go through its natural cycle of increasing the flow of blood through the penis to produce an erection. In this, Levitra is similar to Viagra. Both lose their effectiveness about six hours after being taken. Both are fully eliminated from the body in between eight and ten hours.
In the condition Pulmonary Arterial Hypertension (PAH), the flow of blood through the lungs is also affected by the production of PDE5. Viagra already has approval for the treatment of PAH, and both Levitra and Cialis are being tested for the same purpose. Given that Viagra was first developed as a treatment for angina, researchers hypothesise that all three PDE5 inhibitors should have a beneficial effect in the arterial system within the lungs in general, and on PAH in particular. There is also interest in whether these drugs will support the regeneration of neurons after a stroke.
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