PDE5A and Priapism: Therefore, when an erectile stimulus occurs, NO activates GCs that convert GTP to cGMP, but cGMP is not efficiently degraded by reduced PDE5 activity, so high levels of cGMP accumulate in the smooth muscle cell of the corpus cavernosum, promoting smooth muscle relaxation and finally, prolonged penile erection (priapism) [6, 7, 9, 10].