WASHINGTON -- Researchers studying why American blacks have such alarming rates of high blood pressure are finding new evidence that hypertension may be a symptom of different underlying conditions and not one disease.
There have long been differing views on whether genetics or environmental influences play the most important role in the increased risk of high blood pressure in blacks.
"We don't yet know the real causes of hypertension, and we should look at physiological mechanisms as well as genetics," said Dr. Elijah Saunders, the head of the hypertension division at the University of Maryland medical school in Baltimore.
"It is doubtful that any one of these physiological mechanisms people are studying will account for all the excessive hypertension seen in blacks," Dr. Saunders said. "It's not going to be like a sickle cell problem, which is caused by one genetic defect, but a multiplicity of things, genetic and environmental."
Overall, blacks in the United States have twice the rate of high blood pressure of whites, and five to seven times the rate of severe hypertension.
But they also have much higher rates of elevated blood pressure than blacks in Africa. This leads most scientists to believe that environmental factors like diet, stress and smoking play a large role in triggering the condition in blacks who may have a genetic vulnerability to it.
Whatever the precipitating events and the steps that can be taken to ameliorate them, researchers said, it is important to know what happens to cause and sustain damaging fluid pressures in blood vessels, the heart and other organs.
And there is increasing evidence that mechanisms that result in hypertension may vary between groups -- defined by race, sex and national origin -- as well as within those groups. Data gained about any group could be helpful in explaining hypertension in others.
Dr. Robert F. Murray Jr., professor of pediatric medicine and genetics at Howard University's College of Medicine, said hypertension was really a symptom, a very general physical response to something else being amiss in the body. Like high blood sugar, it can occur in different people for varying reasons, he said.
"The value of these studies that find different mechanisms or influences involving hypertension in blacks is that they help to distinguish subgroups who may respond to different external factors that trigger their disease," Dr. Murray said. "Whatever the genetic component of hypertension in blacks, it is not a dominant one where one gene itself is the only cause of disease. There has to be an environmental factor for the disease to express itself."
Hypertension may turn out to be a number of diseases under the same name, Dr. Murray said, much like cancer or diabetes. Researchers know, for instance, that the net effect of cancers -- uncontrolled cell growth -- can be influenced in individuals by many different things, including carcinogens, diet and genetics.
Scientists looking for the causes of cancer or diabetes quickly recognized the different forms of the diseases, Dr. Murray noted. The same may prove true for hypertension in blacks, he said.
Dr. Roger J. Allen and his colleagues at the University of Maryland have linked a chemical produced by the body during stress with the high prevalence of hypertension in blacks.
The chemical, a blood vessel-constricting hormone called norepinephrine, is formed as a byproduct of production of the skin pigment melanin, which is much more abundant in blacks than whites.
Dr. Allen and Daniel L. Luxenberg, a graduate student, paired 40 black and 40 white students who were matched for age, sex, diet, medical history and fitness levels.
tTC The researchers used a stress trial called a cold pressor test, in which each subject had a hand placed in ice for 30 seconds to mimic the physiological response of a stressful situation like a job interview.
Skin surface sensors showed that all of the students, who were not normally hypertensive, quickly experienced a rise in norepinephrine and blood pressure, but blacks maintained the elevated pressure at least 10 times longer than whites.
Other researchers have previously theorized that stress and the frustration of encountering racial bias are linked as factors in black hypertension. Dr. Allen said his findings suggest a physiological mechanism for how the bodies of some blacks respond to stress.
"We were observing the seeds of future health problems by the way the subjects were reacting to stress in their youth," he said.
Dr. Allen said he eventually wants to conduct similar tests on people undergoing skin tanning, when melanin increases, to see if blood pressure is affected.
If the melanin-norepinephrine link to hypertension proves valid, he said, researchers would have to explain why American Indians and some dark-skinned Hispanic people do not suffer from excessive high blood pressure.