Erectile Dysfunction (ED) and pharmacological treatments

We believe that up to 10% of men experience erectile dysfunction although different writers quote different figures. The most comprehensive figures still come from the  Kinsey Reports (see Bancroft 1989). Kinsey and his fellow researchers found that 1.6% of men in their sample of American men reported more or less permanent impotence, with this figure getting proportionately higher in older age groups, meaning that by the age of 70 27% of men reported being impotent. Additionally, 35% of men stated that they experienced erectile difficulties occasionally and 7.1% stated that they had problems more frequently. It is clear from these figures that erection problems are normal for most men at some point in their lives and that they impact a sizable proportion of men quite heavily.

Before the advent of Viagra only a handful of other pharmaceutical treatment options had been found. In the 1990's, a widely used treatment for all levels of erectile difficulty was to inject papaverine directly into the corpora cavernosa before intercourse was due to happen. Papaverine relaxes the smooth muscles we referred to above: but this treatment can be painful and sometimes leads to scarring and damage to the tunica albuginea - which may lead to Peyronie's disease.

The unsatisfactory nature of these side effects lead to the development of Muse, a trans-urethral drug which was introduced through the end of the penis into the urethra. Side effects included a burning sensation, so this was not universally accepted by men as a solution for erection problems. And then came Viagra....of which more in a moment.

The smooth muscles of the penis relax when they receive a small amount of nitric oxide from either the endothelial cells around the smooth muscles or the nerves connected to the smooth muscle. Nitric oxide is a chemical molecule the body uses as a messenger to modulate muscle function.

Nitric oxide is produced from L-arginine, an amino acid, by an enzyme called nitric oxide synthase; the resultant nitric oxide enters the smooth muscle cells and then activates the conversion of guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP), a chemical pathway mediated by guanylate cyclase. cGMP then causes other intracellular enzymes; these in turn produce substances which relax the smooth muscle. Various enzymes, known as phosphodiesterases (PDEs), will inactivate cGMP, or more exactly change it into an inactive compound known as 5-GMP.

Obviously, since PDE enzymes render cGMP ineffective which is essential for relaxation of the penile smooth muscles, any chemical compound which stops PDE enzymes will effectively increase the level of cGMP and therefore provide the smooth muscles with more stimulation to relax, which should in turn enhance the capacity of the penis to become erect.

Viagra, unsurprisingly, is a powerful inhibitor of PDEs. It effectively blocks the capacity of the phosphodiesterase enzyme (PDE) to turn cGMP into 5-GMP. As a consequence, cGMP levels remain high, and so promote erection in the penis. Thus relaxation of the smooth muscle of the corpus cavernosum tissue and the resultant erection is a product of higher cGMP levels, all due to the role of PDE inhibitor that is fulfilled by Viagra. Viagra is responsible for increased cGMP levels in the corpus cavernosum since it stops a certain proportion of cGMP from being destroyed. It is also the agent which allows relaxation of the smooth muscle around the corpora, thereby promoting erection.

However, despite the fact that Viagra has revolutionized the treatment of erectile dysfunction, it is not a miracle cure for all men. Viagra works on a very complicated series of biochemical reactions, and it doesn't affect all men in the same way. Also, Viagra helps to create an erection, but it does not heighten libido. So, if you don't feel like having sex, Viagra will not help to increase your sexual desire, and you may find it hard to cope with the side effects (flushing of the face, headaches, nasal congestion, and so on.). Additionally, most men want to use Viagra to have sex with a partner, but if that partner does not accept or support his use of Viagra it will probably not result in a pleasant sexual experience for him even though his erection may be reliable. The impact of Viagra on a man's partner is something that needs to be taken into account when it is prescribed.

To reiterate an important fact about Viagra, let me say again that it doesn't produce desire or arousal. Therefore, if a man takes Viagra but doesn't feel sexually stimulated, using Viagra will not result in an erection. On the other hand, if a man using Viagra has an erection during sex with his partner, it is his partner that turns him on, the Viagra only supplies the chemical messenger to let that arousal turn into a physiological process, an erection. Viagra is best taken on an empty stomach and takes about 45-60 minutes to achieve its best results. It has been shown that its usefulness increases for a man between the first to the eighth dose, suggesting that it takes time and practice to really learn how to use it well (Althof 2002).

References:

Althof, S.E.(2002) When an erection alone is not enough: biopsychosocial obstacles to lovemaking. International Journal of Impotence Research, Suppl 1, 99-104