A hormone that is indistinguishable from a plant extract used to make poison darts in South America and Africa appears to play a key role in the development of high blood pressure, a team of scientists said yesterday.
Researchers from the University of Maryland School of Medicine and the Upjohn Co. said the discovery could lead to a diagnostic test for hypertension -- a condition that affects an estimated 60 million Americans -- as well as drugs capable of blocking the hormone's activity.
During eight years of investigation, the scientists detected the hormone not only in human blood but also in every mammal they studied -- including dogs, cats, rats, sheep, pigs and cows.
"It's kind of surprising that something like a poison can be present normally in the circulation of every mammalian species we looked at," Dr. John M. Hamlyn, associate professor of physiology at the University of Maryland, said at a news briefing.
Yesterday's announcement occupied center stage during the first day of the American Heart Association's three-day conference on hypertension, which is being held at the Hyatt Regency Hotel in Baltimore. Pieces of the research were discussed by Dr. Hamlyn, Dr. Mordecai P. Blaustein, chairman of physiology at UM, and Upjohn scientists.
So far, the hormone has not been named, but the scientists said it appears to be the same compound as ouabaine -- pronounced WAH-bane -- which was traditionally extracted from tropical trees and shrubs to make poison darts.
Early in the century, physicians prescribed ouabaine to stimulate sluggish hearts. Later, they switched to digitalis -- a similar compound, extracted from the foxglove plant -- to treat heart failure because it was less toxic.
Apparently, the newly discovered hormone circulates in everyone's blood system. But the scientists said abnormally high levels of the ouabaine-like compound appear to impede the ability of cells lining the blood vessels to expel sodium. This, in turn, upsets the cells' sensitive chemical balance, causing them to retain abnormally high levels of calcium.
Elevated calcium causes the vessels to constrict more than they should, raising the force of the blood pushing against the vessel walls, said Dr. Blaustein. This effect is similar to the surge of water pressure that occurs when someone squeezes a flexible hose.
Researchers at UM and the Upjohn Co. processed hundreds of gallons of human blood plasma in their search for the hormone.
Last week, the Maryland scientists and the company applied for a patent on the process they used to isolate it. If granted, a patent would ensure that the parties would earn royalties if someone markets a diagnostic test that is based on the same process.
Hypertension is known as a "silent killer" because noticeable symptoms may not surface for many years. Hypertension causes the heart and the arteries to work harder and is a leading cause of heart attack, stroke, kidney failure and congestive heart failure.
There may be several reasons to measure levels of the ouabaine-like compound in people's blood, the scientists said. Children who have normal blood pressure but a family history of hypertension, for instance, might be tested to see if they have elevated hormone -- and thus are at risk for developing high blood pressure.
"Would you treat them? Perhaps the way to treat them is simply to find those individuals and ask them to lower their dietary salt intake," said Dr. Blaustein.
Additionally, a test might be used to see whether someone has bona fide hypertension or "white coat" hypertension -- a passing elevation in blood pressure that is caused by a patient's anxiety in the face of a doctor or nurse. A mistaken diagnosis "can condemn patients to years and years of drug therapy," said Dr. Hamlyn.
Development of a marketable test might be just a few years away; a drug capable of reducing hormone levels or blocking its effect on the cells is further off, the scientists said.
The scientists said they still do not fully understand how the hormone affects blood pressure. Also, they do not know whether the hormone accounts for a small or large portion of hypertension cases.
"We need to find out who has elevated levels of this substance and whether it's present in all forms of high blood pressure," Dr. Blaustein said.
Dr. Hamlyn said the compound may be responsible only for "salt-sensitive" hypertension, a condition in which the blood pressure surges in direct response to salt intake. About one-quarter of hypertensive Americans have the salt-sensitive condition.