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Competition for Viagra?

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WebMD Health News
Reviewed by Gary D. Vogin, MD

Jan. 2, 2001 -- There's no doubt about it: Viagra is one of the most popular and best-selling drugs ever to hit the pharmacy. Since it first appeared on the market in 1998, more than 20 million prescriptions have been written for it. But while countless men have bade farewell to impotence or erectile dysfunction, others have found that Viagra does not work well for them. For these men, good news may be on the way, as researchers have discovered yet another promising treatment for sexual dysfunction.

"Many different types of treatments exist for men with erectile difficulties," says Craig Niederberger, MD, FACS, who explains that much of Viagra's popularity is credited to the fact that it can be taken as a pill.

Other treatments, he adds, are less convenient and may involve injection of medication directly into the penis. Niederberger, who was not involved in the recent research, is chief of the division of andrology at the University of Illinois, in Chicago.

"Traditionally, treatments for erectile dysfunction have focused on the processes that turn on smooth muscle relaxation and produce erection, rather than by blocking the contraction of smooth muscle," says study author Christopher J. Wingard, MS, PhD. In their study, which appears in the January issue of Nature Medicine, Wingard and his colleagues looked at the latter process.

When men are sexually aroused, blood flow into the penis increases, filling up spongy cylinders called corpora cavernosa. When the cylinders fill with blood, the penis hardens and becomes erect. Acting like flood gates, muscular blood vessels called arterioles control the flow of blood into the penis. A chemical called nitric oxide is a signal for the gates to open, and Viagra works by increasing the amount of nitric oxide, signaling the gates to open and increasing the blood flow.

Researchers from the Medical College of Georgia, in Augusta decided to explore a different approach. They found that an enzyme called Rho-kinase is present in the spongy cylinders. Elsewhere, this enzyme enhances the activity of muscle, like the muscle in those flood gate muscular vessels. The researchers reasoned that if they inhibited the activity of this enzyme, the muscle would relax, opening the flood gates.

Sure enough, it worked. They injected a drug called Y-27632, a known inhibitor of Rho-kinase, into the spongy cylinders of rats, causing penile erections. In further testing, they showed that the Rho-kinase inhibition worked completely independently of the way Viagra works.

It's estimated that about half of all American men between the ages of 40 and 70 are affected with impotence to some degree, and the underlying causes vary. Sometimes, psychological reasons or lifestyle factors, such as excessive alcohol consumption, can lead to impotence. However, a persistent problem is usually due to a chronic illness or a side effect of certain drugs.

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