Cyclic guanosine monophosphate
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Activated by the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation. Most notably in the corpus cavernosum, resulting in increased inflow of blood to the penis and an erection.

The original article can be found at: http://en.wikipedia.org/wiki/Cyclic_guanosine_monophosphate.
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