Physiological significance of nitrergic transmission in human penile erection

The corpora cavernosa (CC) muscles of the human penis and their structural arrangements are essential for the physiology of erection. Contraction of this muscle causes detumescence, and relaxation, tumescence. The motor excitatory neurotransmission is adrenergic, acting through the alpha adrenoceptors. Continuous adrenergic transmitter (noradrenaline) release is necessary for the maintenance of non-erectile (contractile) state of the penis. The inhibitory neurotransmitter that relaxes CC muscle to produce erection is nitrergic i.e., the chemical messenger being nitric oxide (NO). The latter can also be released from cavernous endothelium. Presence of NO increases intracellular cGMP through activation of the enzyme guanylate cyclase. This causes relaxation of CC muscle. Phosphodiesterase type 5 (PDE5) is responsible for the degradation of cGMP and regulation of CC muscle tone. Specific PDE inhibitors such as sildenafil enhance the intracellular cGMP to improve erection. Increase in intracellular cAMP can also bring about pharmacological erection in man (e.g. PGE1, papaverine and histamine). Inhibition of excessive adrenergic tone with appropriate alpha-adrenergic blocking agents (e.g. phentolamine) can also contribute to the onset of pharmacological erection.