Fellowship in Sexology and Psychosexual Counseling. For More Info WhatsApp- 9604715783. No activates enzyme Guanylate cyclase, allowing production of cyclic Guanosine monophosphate (cGMP) which causes relaxation of smooth muscles. As arterioles and the muscles relax, lacunar spaces are filled with blood, increasing the pressure on the veins and occluding the venous return and causing the penis to become erect. If there is no further sexual stimulation, cGMP is metabolized by Phosphodiesterase 5 (PDE5) resulting in loss of erection.