Erectile Function and Viagra

In 1990, we discovered that the long sought-after neurotransmitter released from the pelvic nerves that brings about penile erection is nitric oxide. When these nerves are stimulated either centrally (brain) by sexual thoughts or visual images or peripherally by touch or other tactile stimuli, the nerves immediately produce and discharge NO into the corpus cavernosum, which consists of both vascular and nonvascular smooth muscle. The result is smooth muscle relaxation and vasodilation, which dramatically increases blood flow into the penis, causing engorgement or an erection. Like all other vasodilator responses, NO works by increasing tissue levels of cyclic GMP, which in turn causes vasodilation. Viagra works by preventing the tissue destruction of cyclic GMP, allowing it to accumulate and cause profound vasodilation. Viagra inhibits the enzyme phosphodiesterase type 5, thereby promoting the accumulation of cyclic GMP.