LOS ANGELES 2/3 — LOS ANGELES -- Researchers say they have found the physiological explanation for the vast majority of cases of male impotence: a failure to produce a simple chemical that controls a wide variety of biological functions.
The chemical, nitric oxide, initiates a series of events that cause the penis to become engorged with blood and remain erect, said the man who made the discovery, Dr. Jacob Rajfer, a professor of urology at the University of California at Los Angeles.
Defects in the nitric oxide system of the penis cause blood to flow out and account for 7 million to 8 million of the 10 million cases of impotence among American men, he said.
The finding will help urologists develop better methods for diagnosing impotence and could lead to new kinds of treatments, experts said, but it is not expected to lead to a pill that would cure the problem.
Dr. Rajfer's research is described in today's issue of the New England Journal of Medicine.
Dr. Robert Kessler, an associate professor of urology at Stanford University, called the research "an important piece in the overall process of how an erection is mediated," but added, "I don't know that it will be all that useful clinically, compared to what we have now."
Medicines injected directly into the penis are popular for treating impotence, Dr. Kessler said, "but what you'd really like is a pill."
The new research will not help make such a pill feasible, said Dr. Tom Lue, an associate professor of urology at the University of California at San Francisco, but could lead to other kinds of treatment, such as a removable patch that could be placed on a penis to bring on an erection.
Dr. Rajfer said nitric oxide is emerging as one of the most powerful known biological substances.
Nitric oxide, a primary component of smog and acid rain, is produced in tiny amounts for fractions of a second throughout the body. The brain uses it to send messages and store memories. The immune system uses it to kill invading bacteria. And when nitric oxide touches the muscles that encircle blood vessels, the muscles relax instantly.
It was the latter observation that led Dr. Rajfer, an expert on impotence, to wonder whether nitric oxide played a role in erections.
An erection begins with a brain signal that travels down the spinal cord and into the penis along a special set of nerves, Dr. Rajfer said. These nerves make connections with the blood vessels and smooth muscle of the penis, triggering the production of nitric oxide and the muscle relaxation.
The smooth muscles are usually constricted, Dr. Rajfer said, keeping the penis flaccid. But when the muscles relax, the blood vessels open, and blood pours into spaces inside the penis. The rise in pressure blocks the veins that normally drain the penis, keeping it engorged.
The notion that muscle relaxation brings on an erection seems contradictory, Dr. Rajfer said, but it is what starts the process. After ejaculation or loss of nerve stimulation to the penis, the smooth muscles reconstrict, the veins open, and blood flows out.
"We used to think that impotence was caused by poor inflow of blood to the penis," he said. "We now know that most impotent men have a problem with holding on to blood in the penis."
Dr. Louis J. Ignarro, a professor of pharmacology at UCLA who collaborated on the research, said the findings shed light on why certain drugs injected into the penis help achieve erections.
Tens of thousands of men in the United States routinely give themselves such injections with one of four drugs, he said. The drugs, papavarine, phentolamine, prostaglandin E1 and nitroprusside, work by raising nitric oxide levels or relaxing smooth muscles in other ways.