I'll put this in complete layman's terms...or try to at least.
We're all familiar with PDE5 inhibitors. They are most commonly used for erectile disfunction. The big names are Viagra (sildenafil), Cialis (tadalafil), Levitra (vardenafil), and Stendra (avanafil).
The way they work is by blocking the effects of PDE5 (Phosphodiesterase-5). PDE5 is an enzyme that prevents NO (Nitric Oxide) from causing a necessary chemical reaction needed to relax the muscles surrounding the blood vessels in the penis. When PDE5 is blocked, NO is able to complete the chemical reaction resulting in a chemical called cGMP (cyclic guanosine monophosphate). cGMP is responsible for relaxing the muscles surrounding the blood vessels allowing the vessels to dilate and allow more blood flow.
Now...why is Cialis the best choice when it comes to muscle pumps and blood pressure regulation? Well, this is due to selectivity. PDE5 is just one member of the Phosphodiesterase family. There are 11 in total and they are names PDE1, PDE2, PDE3.....up to PDE11. PDE5 inhibitors like Cialis and such are designed to be selective in that they "mostly" only target PDE5 and tend to leave the other Phosphodiesterases alone or have minimal interaction. PDE5 is associated with the smooth muscles in your body, such as the corpus cavernosum muscle in the penis which is what inflates with blood during an erection...not to be confused with skeletal muscle. However, Cialis (tadalafil) is the LEAST selective when it comes to PDE11 interaction. This means Cialis also inhibits PDE11 to a degree whereas the other 3 PDE5 inhibitors are highly selective AGAINST PDE11 and basically leave it alone. See chart below showing each PDE5 inhibitor and it's selectivity:
Why is this important? Well, PDE11 is a specific Phosphodiesterase that is highly present in skeletal muscle. Much more so than any other Phosphodiesterase. Because Cialis is NOT highly selective against PDE11, it will interact with it and block it's effects. This allows the blood vessels in your skeletal muscles to dilate and allow more blood flow, thus enhancing your pump and slightly lowering overall blood pressure. None of the other PDE5 inhibitors interact with PDE11 like Cialis does.
Therefore, anyone who tells you they took a Viagra before training and it works just as good as Cialis for muscle pumps....they are mistaken. On paper and in theory, Viagra should have virtually no effect on skeletal muscle blood flow.
References:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3776492/
