Basic Functions of Erectile Dysfunction

Important Principles

• The study of male sexual function has a long history in Asia. With the current rapid clarification of erection and ejaculation mechanisms, the results of basic studies have led to remarkable progress in understanding the nature of male sexual dysfunction and the many factors that contribute to it.

• In the penis, the dominant structure is the corpus cavernosum which controls tumescence, erection, and detumescence after ejaculation. Contraction of this muscle maintains the non-erect state, while relaxation promotes tumescence and erection.

• Excitatory motor neurotransmission (adrenergic) elicits contraction, and inhibitory neurotransmission (nitrergic) produces relaxation. Dual (contractile and relaxant) affinities are present in corpus cavernosum muscle for adrenoceptors, cholino- ceptors, histamine receptors, and various other transmitters/modulators including prostaglandins.

• Physiological processes in the corpus cavernosum involved in controlling penile tumescence and detumescence include: (a) a-adrenoceptor-mediated motor res- ponses that maintain the non-erectile state; (b) inhibitory endogenous modulators that facilitate temporary withdrawal of a-adrenergic activity, e.g. prostaglandin E₁ (PGE₁); (c) unmasking of nitrergic (nitric oxide-mediated) inhibitory transmission which releases cyclic guanosine monophosphate (cGMP) for relaxation of caver- nous muscle; (d) endogenous autocoids such as histamine, acetylcholine and prostaglandins acting on contractile (for detumescence) and relaxant (for tume- scence) receptors; and (e) inter- and intracellular electrophysiological processes including ions, proteins, kinases and gap junctions that induce contraction or relaxation according to the input signal.

• Within the central nervous system, complex interactions of neurons within the medial preoptic area (MPOA), paraventricular nucleus (PVN) of the hypothalamus, and medial amygdala are also known to be involved in the regulation of penile erection.

• CNS neurotransmitters that are believed to facilitate penile erection include dopamine (acting via D₂ receptors), oxytocin, nitric oxide (NO), excitatory amino acids, adrenocorticotropic hormone and related peptides, and noradrenaline. Those believed to have an inhibitory role include g-aminobutyric acid (GABA), prolactin, and opioids (e.g. morphine, b-endorphin), while serotonin (5-hydroxytryptamine; 5-HT) has both a facilitatory and inhibitory role.

• Erectile dysfunction (ED) is a common sexual disorder that influences the social interactions, well-being and quality of life of sufferers. Its precise incidence is difficult to establish, but recent studies have indicated that its occurrence in Asian communities is considerably higher than was previously thought.

• Many factors contribute to the development of ED including aging, chronic diseases such as diabetes mellitus, cardiovascular disease, renal and hepatic failure, pelvic or transurethral surgery, pelvic trauma, spinal cord injury and, more contro- versially, lifestyle factors such as smoking, alcohol consumption and abnormal cholesterol levels.

• Certain drugs may also be associated with ED including some antihypertensive agents, digoxin, hormones (estrogens, progestins, GnRH agonists), and various psychotherapeutic drugs.

• An understanding of the prevalence and characteristics of ED are important in establishing therapeutic strategies for its treatment.