PENIS
RESEARCH OFFERS ERECTION HOPE
A team of Hopkins scientists has greatly advanced the science
of penile erection, showing for the first time the mechanism for
continued production of nitric oxide that maintains an erection
over time. The findings are published in the March 19 issue of
the Proceedings of the National Academy of Sciences.
"The physiology of erection is like driving a car," says
Arthur Burnett, M.D., associate professor of urology at Hopkins
and senior author of the study. "You can't just turn the key
and expect to go anywhere. You also need to hit and hold the accelerator."
Researchers at Hopkins ten years ago discovered that release of
nitric oxide from nerve endings in the penis caused erection, but
the temporary release of nitric oxide - a neurotransmitter that
survives just a few seconds before breaking down - couldn't explain
how erections are naturally sustained over time.
Working with rats and genetically transformed mice, Hopkins scientists
found that after an initial burst of nitric oxide from nerve endings
triggers erection, blood vessels release more nitric oxide to harden
and maintain the erection.
"Once blood starts flowing into the penis, the source of
nitric oxide in the blood vessels is continuously activated so
that more nitric oxide is released, more tissue relaxes, more blood
comes in and a sustained erection is achieved," Burnett says.
This cascade of events, Burnett says, begins when erotic thoughts
or physical sensations produce nitric oxide release in nerve endings
in the penis. Nitric oxide is a relaxant that allows blood vessels
to open up or dilate, bringing increased blood flow and swelling
of tissues. The flow of blood also creates a minor stress on the
blood vessel wall which activates the release of more nitric oxide.
This time it is from cells in the wall of the blood vessel - the
endothelial cells - rather than from nerves. Endothelial nitric
oxide causes more tissue to relax and the process repeats until
the penis is fully erect.
A key element in the attainment of erection is the continuous
activation of the source of nitric oxide in blood vessel walls,
according to Burnett. Finding this source, a special form of the
enzyme called endothelial nitric oxide synthase (eNOS), plus the
discovery that the pressure of flowing blood against a vessel wall
could induce it to produce nitric oxide, were critical pieces of
the puzzle. These discoveries offer a fuller picture of the complex
physiology of erection.
In addition to its application towards new and improved therapies
for erectile dysfunction, Burnett says, this understanding of nitric
oxide's effect can be extended to other areas of the body that
are more difficult to observe, such as the heart and other internal
organs.
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