Erection

Cellular perspective of the erection pathway and mechanism of action of PDE5 inhibitors

• During sexual stimulation signals in the brain cause nitric oxide (NO) to be released in the penis.
• NO is transported into the smooth-muscle cells where it activates the enzyme guanilate cyclase, which produces cGMP (cyclic guanosine monophosphate).
• Increased concentration of cGMP causes the outflow of calcium ions (Ca⁺⁺) from the cell, resulting in smooth-muscle relaxation.
• Consequently the arteries dilate, inflow of blood into the corpora cavernosa increases and blood accumulates, which results in an erection.

• After ejaculation the enzyme phosphodiesterase type 5 (PDE5) reduces cGMP levels and the penis becomes flaccid.

PDE5 inhibitors (e.g. sildenafil) prevent the reduction of cGMP levels and thus help achieve and maintain an erection.