Sugar daddies of the world, rejoice. If Viagra doesn't work for you, scientists have discovered what may be the next big cure for impotence: hydrogen sulfide, a gas better known as the foul smell of rotten eggs.
High concentrations of hydrogen sulfide are toxic, but recent studies have shown that smaller quantities play an important role in the body. Our cells produce hydrogen sulfide from the amino acid L-cysteine, thanks to help from two enzymes, cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE). The gas seems to serve as a neurotransmitter, promoting blood vessel relaxation and hormone secretion. Researchers have even begun to test whether hydrogen sulfide can protect us against the damage associated with heart attacks and strokes (Science, 30 May 2008, p. 1155).
So what's the link to erections? It turns out that creating and sustaining them depends on a delicate balance between relaxation and contraction of blood vessels in the penis's corpus cavernosum, a spongelike tissue. Hydrogen sulfide is known to be involved in erection in mice, and pharmacologist Giuseppe Cirino of the University of Naples Federico II in Italy decided to investigate the issue in humans. Cirino and his colleagues obtained penis tissue from eight sex-change operations. They found that not only does the tissue harbor CBS and CSE, but CSE is also present in the smooth muscle cells and peripheral nerves that control erection.
To test the effect of hydrogen sulfide, the team cut off thin strips of the penis tissue and added a chemical that turns into the gas. The blood vessels in the tissue relaxed, the researchers report online this week in the Proceedings of the National Academy of Sciences, which in real life would lead to increased blood supply and an erection. When the team injected varying amounts of the hydrogen sulfide precursor into the penises of anesthetized rats, the rats experienced stronger erections with increasing dosage.
Taken together, the results imply that hydrogen sulfide relaxes the penile nerves, expands the blood vessels, and causes erections in humans, says Cirino. Viagra works in a similar way but through another gas--nitric oxide. That difference suggests that drugs based on hydrogen sulfide may work in men who do not respond to Viagra and similar classes of drugs.
Physiologist Rui Wang of Lakehead University in Ontario, Canada, welcomes the findings, but he says talk of hydrogen sulfide–based drugs is premature. The hydrogen sulfide concentrations used to cause erections in this study are toxic, he notes, and therefore unlikely to be useful for clinical application.
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