Normally in the smooth muscles lining the blood vessels acts a degradative enzyme known as cyclic guanosine monophoshate-specific phosphodiesterase type 5 (cGMP – PDE5). This is responsible for breaking down cGMP which is a chemical responsible for smooth muscle relaxation among other effects. When this is broken down, the blood vessels tend to contract faster and the amount of blood flow is reduced. Pde5 inhibitor drugs work to reverse this effect by preventing the action of the pde5. As a result, cGMP is not broken down and it accumulates in the smooth muscles together with nitric oxide which is a chemical released in response to stimulation of the penile tissue in case of an erection. It is this nitric oxide which activates guanylate cyclase which is an enzyme responsible for accumulation of cGMP. This causes the smooth muscles to relax increasing the diameter of the blood vessels and subsequently increased blood flow is experienced in the area. When used to treat erectile dysfunction, blood flow to the penis is increased causing an erection and in case of pulmonary hypertension, the blood flowing to the lungs is increased reducing stress on the heart’s ventricles. In both conditions the pde5 inhibitor drugs work to enhance the vasodilatory effects of nitric oxide.
There are several drugs that fall under this category including sildenafil commonly known as Viagra, Tadalafil commonly known as Cialis, Vardenafil commonly known as Levitra and Avanafil commonly known as Stendra. The use of all these in treating erectile dysfunction has been proven as well as the use of sildenafil for treating pulmonary hypertension. Their working mechanism is similar and they are only pharmacologically active when the synthesis of cyclic guanosine monophosphate is activated.