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Part
of the physiological process of erection involves the parasympathetic
nervous system causing the release of nitric oxide (NO) in the
corpus cavernosum of the penis. NO binds to the receptors of
the enzyme guanylate cyclase which results in increased levels
of cyclic guanosine monophosphate (cGMP), leading to smooth
muscle relaxation (vasodilation) in the corpus cavernosum, resulting
in increased inflow of blood and an erection.
Sildenafil is a potent and selective inhibitor of cGMP specific
phosphodiesterase type 5 (PDE5) which is responsible for degradation
of cGMP in the corpus cavernosum. The molecular structure of
sildenafil is similar to that of cGMP and acts as a competitive
binding agent of PDE5 in the corpus cavernosum, resulting in
more cGMP and better erections. Without sexual stimulation,
and therefore lack of activation of the NO/cGMP system, sildenafil
should not cause an erection. Other drugs that operate by the
same mechanism include tadalafil (Cialis®) and vardenafil
(Levitra®).
Sildenafil is metabolised by hepatic enzymes and excreted by
both the liver and kidneys. If taken with a high-fat meal, there
may be a delay in absorption of sildenafil and the peak effect
might be reduced slightly as the plasma concentration will be
lowered.
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