A malodorous gas behind the smell of rotting eggs has been found to play a key role in giving men erections. Scientists believe the discovery could lead to the development of a male impotence drug to rival Viagra.
The whiff of hydrogen sulphide – a gas not traditionally associated with lovemaking – accompanies the biological degradation of sulphur-containing substances. It also belches from the exhausts of cars fitted with catalytic converters.
A study has shown that, prior to sexual intercourse, minute quantities of hydrogen sulphide are released within the key nerve cells of the penis which control the engorgement of the male organ with blood to stiffen it.
Scientists believe that the discovery could lead to the development of a new class of drugs to combat erectile dysfunction by affecting a different biochemical pathway to the one targeted by Viagra.
The research, published in the journal Proceedings of the National Academy of Sciences, was based on experiments performed on the intact erectile tissue of eight men who had undergone sex-change surgery in a hospital in Italy.
"We found that hydrogen sulphide is involved in human penile erection. That was proved in this study," said Professor Giuseppe Cirino, of the University of Naples Federico II.
"Of course, the hydrogen sulphide pathway represents a new therapeutic target for erectile dysfunction and it should be possible in future to develop drugs that either deliver hydrogen sulphide or that control the hydrogen sulphide production," Professor Cirino said.
The discovery that hydrogen sulphide gas helps penile erection mirrors the earlier discovery that another gas, nitric oxide, is involved in a similar biochemical process. That led to Viagra being used as an anti-impotence drug.
The release of nitric oxide at the nerve endings close to the blood vessels of the penis controls the human erection. By relaxing those blood vessels, blood flows into the penis and collects in special cavities, the corpus cavernosum, causing the organ to swell into an erection.
Viagra works on that biochemical pathway by stimulating the flow of blood into the penis while decreasing the flow out of the organ – maintaining an erection that might otherwise dissipate quickly or not happen at all.
Both hydrogen sulphide and nitric oxide work as "cell mediators", communicating signals between the cells that control the flow of blood within the arteries and veins. When released, the gases cause certain muscle cells to relax, allowing blood to flow more freely into the penis, leading to an erection. "Hydrogen sulphide, like nitric oxide, was best known as a toxic pollutant until recent years, when it has been proposed to be a gaseous neurotransmitter," Professor Cirino and his colleagues said.
"These observations may help to unravel the complex mechanisms underlying the patho-physiology of human penile erection and may lead to the development of therapeutic approaches in the treatment of erectile dysfunction and sexual arousal disorders."
It is estimated that about half of all men who are aged between 40 and 70 have suffered at some stage from erectile dysfunction, which is often a symptom of a wider disorder of the circulatory system, such as diabetes or heart disease.
About one third of men with erectile dysfunction do not respond to Viagra, and it is those men that Professor Cirino said may benefit from the development of a drug targeted at hydrogen sulphide rather than nitric oxide.