Viagra. Levitra. Hydrogen sulfide?

The compound responsible for the smell of rotten eggs could be a new way to treat erectile dysfunction, based on an early study in rats by a team featuring the UCLA Nobel laureate pharmacologist, Louis Ignarro.

Ignarro's team injected the penile smooth-muscle of rats with hydrogen sulfide, which relaxed them, allowing more blood to flow in — just like Viagra.

"In the future, that could help humans have an erection," said Jim Cummings, a urologist at St. Louis University and expert in erectile dysfunction, who was not involved in the new research.

Though it smells extremely bad, hydrogen sulfide is a bizarre chemical that has some strange and unexpected effects on living things. In mice, it's been shown to induce a state like "suspended animation" while other scientists say that 2 H 2 S was responsible for a mass extinction on Earth several hundred million years ago. It could eventually help soldiers survive blood loss resulting from war injuries.

And now, just maybe, it could help impotent men for whom Viagra and similar drugs haven't helped. Pfizer has reported that half the men taking Viagra don't refill their prescriptions, suggesting that it doesn't work for everyone. A meta-review of the drug's clinical trials found that even when it works generally, it doesn't work during every sexual attempt.

Viagra works by stimulating the production of nitric oxide, which usually relaxes the penile tissue. The physical result of the hydrogen sulfide treatment appears to be the same — relaxing the smooth-muscle in the penile tissue known as the corpus cavernosum, but it exploits a different chain of molecular tools.

"This is a completely different pathway," Cummings said. "If it were to work out in humans, it would be a way to help out people that aren't responding to Viagra and drugs like it."

To turn this animal research into a human therapy will likely take years, if not decades. The goal would probably be to develop a formulation that could be taken as a pill, just like Viagra, Cummings said.

"What I would foresee is not that we'd inject the gas into our own penises, we'd look for a drug that would make you generate more of this compound in your own tissues," he said.

The early results published in the journal Proceedings of the National Academy of Sciences suggest that this could be possible, although no one knows what the side effects might be. Their data "strongly support the case" for a role for hydrogen sulfide as a key component in the human erection process.

The team secured human penile tissue from men having male-to-female sex-change operations. They found that the human penis tissue expressed measurable amounts of hydrogen sulfide, which raises the hope that some drug could be found to cause more H 2 S production.

"These observations may help to unravel the complex mechanisms underlying the pathophysiology of human penile erection and may lead to the development of therapeutic approaches in the treatment of ED and sexual arousal disorders," the authors wrote in PNAS.

Author's Note, 9:16 pm: This post has been updated to correct the description of the corpus cavernosum in the 7th paragraph.

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