Figure 58

Figure 58 Mechanism of action of sildenafil (Viagra): nitric oxide (NO) is produced by the enzyme nitric oxide synthase (NOS), found in
both nerve endings and endothelial cells. NO acts as a neurotransmitter to stimulate guanylate cyclase to produce cyclic guanosine
monophosphate (cGMP) from guanosine triphosphate (GTP). cGMP in the corpora is broken down by phosphodiesterase type 5 (PDE5).
Sildenafil inhibits PDE5, thus potentiating the effect of NO