ED in women: Drugs for men might not help

Watch television, especially stations with a large male audience, and you’ll have a hard time avoiding commercials for pills that promise “male enhancement” — a euphemism for tackling ED (itself a coy abbreviation for erectile dysfunction). Well, it turns out that women suffer from ED as well, and for much the same reason as men, according to Kyan Allahdadi, a vascular physiologist at the Medical College of Georgia. Unfortunately, his new animal data indicate, women aren’t likely to gain the same benefits from the little blue pill and its kin.

Male impotence is a common side effect of aging. Actually, it’s a side effect of aging arteries and blood vessels. With the development of arteriosclerosis, blood vessels tend to stiffen and even contract. But drugs that inhibit enzymes in the phosphodiesterase-5 — or PDE-5 — family help the body produce nitric oxide.

A vasodilator, nitric oxide allows arterial and vessel walls to relax. And that allows more blood to flow into and through vessels, especially the small ones. And male enhancement, as those commercials note, is all about getting plenty of blood to you know where, whenever he and his lady are “ready.”

It turns out that the anatomies of men and women have a lot in common. Which isn’t surprising, because in early fetal development there’s no difference between the genders. We females are the default gender. Unless the Y-chromosome turns on testosterone flood gates early in development, we all will develop into women, regardless of whether we inherited  two X chromosomes or an X-Y pair.

But once the sex hormones for male development turn on, genes direct a certain little button-headed organ in females to morph into a long, tubular structure: the penis. Allahdadi points out that the female default organ also has the potential to engorge with blood during sexual arousal. He says it can gain an extra 10 milliliters or so from the internal pudendal artery — the same “highway of blood that delivers blood for male erectile responses.”

Female erections weren’t really appreciated up until four or five years ago, he says. That’s when MRI studies of women before and after becoming sexually aroused by a video demonstrated and effectively quantified their erection. In fact, Allahdadi notes, most of that erection occurs internally — out of sight. But getting proper blood flow to a woman’s reproductive organs is just as important for her arousal and sexual “function” as it is in men, he reported this week at the Experimental Biology meeting in New Orleans.

Owing to the similarity in vasculature relating to arousal in both genders, Allahdadi’s team decided to evaluate the ability of the most common PDE-5 inhibiting drugs for ED — those sold under the trade names Viagra, Cialis and Levitra — in both male and female rats. The animals had been pretreated with a drug to contract the internal pudendal artery. This would mimic arteriosclerotic vessels.

In males, blood flow in the artery and to the gonads increased, and the vessel relaxation responsible for these changes depended on the dose of PDE-5 inhibitor administered. In females the story was similar — but with a twist.

Instead of the relaxation persisting as it did in males, the females’ arteries would relax and constrict and relax and constrict in an oscillatory fashion.

I asked Allahdadi if this meant the females’ vessels were attempting to compensate for the drug-driven relaxation and were periodically, repeatedly succeeding — only to be temporarily countered again by the drug. “That’s exactly what we think is happening,” he said. Whatever mechanism is behind the vessels’ oscillatory re-constriction “is really interesting. We don’t know what it is yet, but we have a couple ideas.”

Such as? “Well, we’re thinking that there may be potentially different subtypes of PDE-5s” in the females and that the current line of drugs doesn’t inhibit them all — or at least as well as they do these enzymes in males.

Still, he says, his team’s initial findings in the females suggest they’re on the right track since the arteries responsible for the females’ ED were at least trying to respond. The researchers also discovered gender-related differences in sensitivity to the drugs. Of the three tried, Viagra worked most effectively in female rats, and male rats derived the most vessel-relaxation from Levitra.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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