Male impotence drugs may
deserve a second look in women
Friday, April 17,
2009
New studies indicate the three drugs used to treat male impotence
also appear to work in females, albeit a little differently, and
should give the scientific community pause to take a second look at
their potential in the 40 percent of women who report sexual
dysfunction, researchers say.
In one of the first studies of the effect of phosphodiesterase Type
5 inhibitors - Viagra®, Levitra® and Cialis® - on the pudendal
arteries that supply the penis, vagina and clitoris the blood
needed to produce a satisfying sexual experience, Medical College
of Georgia researchers showed the drugs relax the artery in male
and female rats.
"It shows the drugs need to be investigated more for women and
small alterations could make these compounds more effective for
women living with these disorders," says Dr. Kyan J. Allahdadi,
postdoctoral fellow in physiology at MCG. He's presenting the
findings during the 122nd Annual Meeting of the American
Physiological Society held in New Orleans April 18-22 as part of
the Experimental Biology 2009 scientific conference.
Although there was talk years ago of a pink pill for women to
parallel the blue Viagra for men, early clinical trials found
essentially no response in women.
MCG researchers decided to look again, first giving a drug to
constrict the internal pudendal arteries in male and female rats –
as they would be in a non-erect state – then giving doses of each
impotency drug to see the impact. The arteries from male rats
displayed a relatively standard concentration-dependent relaxation
– the more drug they got, the more they relaxed - while in females
arteries, there was an initial relaxation then an odd oscillation
between relaxation and contraction with subsequent dosing.
While they don't fully understand the swing, the unique female
response likely provides more evidence that sexual function is more
complex in females, says Dr. R. Clinton Webb, chair of the MCG
Department of Physiology and a study author. Scientists define
female sexual dysfunction as a multifaceted disorder that includes
anatomical, psychological, physiological and social-interpersonal
aspects.
MCG researchers have shown part of that complexity may be the
smooth muscle cells in the internal pudendal arteries of females
communicate, agreeing to contract and relax, while male smooth
muscle cells make independent decisions to just relax.
They found one other distinction: females were more sensitive to
Viagra®, or sildenafil, while males were most sensitive to
Levitra®, or vardenafil.
Previous studies on the effectiveness of these drugs focused on the
cavernosal tissue, or penis. The internal pudendal artery actually
feeds the penile artery which is buried deep in the penis where
numerous caverns enable it to be flaccid when not engorged with
blood. Physical stimulation of the area causes the tissue,
endothelial cells and nerves to release nitric oxide, a powerful
dilator of blood vessels. The system works pretty much the same way
in the vagina and clitoris.
"If you have too much constriction or not enough relaxation to
allow blood to go through the internal pudendal artery, you are not
going to get the net effect of an erection," Dr. Allahdadi says.
"That is why we wanted to begin to characterize what was going on
in this blood vessel."
Perhaps as importantly, the MCG scientists and others are beginning
to believe sexual dysfunction provides an early, or at least
visible, clue of vascular disease. Vascular problems, that can
result from diabetes, hypertension, high cholesterol and the like,
are a major cause of sexual dysfunction in men and women. "You
don't feel atherosclerosis but you know darn well if you are not
getting an erection," Dr. Webb says. In fact, the MCG scientists
are beginning to look at animal models of disease states, such as
diabetes, to see what it does to these internal pudendal
arteries.
"What we have seen preliminarily is there is big difference in
responsiveness in these arteries. The diabetic pudendal arteries
are much more sensitive to contraction," Dr. Allahdadi says. They
will look at how drugs like Viagra impact that contraction in the
days ahead.
In fact MCG scientists suspect one reason that many of the women
participants in previous studies of Viagra did not seem to respond
is because they did not have vascular problems that could have been
circumvented by a drug that relaxes arteries so blood can enter. In
men with a healthy vasculature, the drugs likely would still
produce a longer erection.
Dr. Rita C. Tostes, associate professor in the MCG Department of
Physiology, is a co-author who contributed to the design and
analysis of the study.
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Medical College of Georgia
Thanks to Medical College of
Georgia for this article.