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For example, it can interact with transition metals such as those in the haem group of guanylyl cyclase enzymes, which produce cyclic GMP — a messenger molecule involved in many biological processes.

This enzyme prevents the release of certain neurotransmitters--most notably one called cyclic GMP--that cause the smooth muscles surrounding arteries to relax, allowing the arteries to expand.

While PDE5 is always present in the penis, cyclic GMP is produced only during arousal.

The catch in impotent men is that they may not produce enough cyclic GMP to temporarily "win out" over the PDE5.

Thus the efficacy of Viagra: by strong-arming PDE5, it allows a little bit of one's cyclic GMP to go a long way.