Penile erection begins with relaxation of smooth muscle in the penis which increases blood flow in and decreases flow out. Nitric oxide plays a role in prolonged erections known as priapism. Medications that inhibit an enzyme called PDE can prevent breakdown of cGMP and block the effects of nitric oxide, thereby inhibiting smooth muscle relaxation in the penis.

4. Penile erection begins with
relaxation of smooth muscle of
cavernosal arteries and tissue,
leading to increased inflow and
decreased venous outflow
Nitric oxide is an important mediator
in development of priapism.
Use of PDE inhibitors inhibit
breakdown of cGMP and inhibit NO
mediated penile smooth muscle
relaxation