DALLAS, June 19 -- Response to a stress chemical may help explain racial differences in the prevalence of high blood pressure, according to a report in this month's Hypertension: Journal of the American Heart Association.
In a study of blood flow in people during mentally stressful conditions, Julio A. Panza, M.D., and researchers at the National Heart, Lung, and Blood Institute, report that the blood vessels of healthy blacks don't relax as much as healthy whites. Blood vessels relax and widen during stress to redirect blood flow to muscles involved in the "flight or fight" response.
"The opening up of blood vessels in response to mental stress is a typical response and, normally, is partly mediated by nitric oxide," adds Panza, head of echocardiography at NHLBI. "But the activity of nitric oxide during mental stress is reduced in blacks."
To sort out the racial differences in the role of nitric oxide, the study compared the rate of blood flow in the forearms of healthy whites and blacks as they performed an increasingly difficult set of math problems.
The rate of forearm blood flow in whites increased more than twice that of blacks as they performed the math problems.
To determine the role of nitric oxide, Panza repeated the experiment after giving participants a drug that inhibits its production in the body. In whites, the rate of blood flow dropped significantly, while in blacks, it had no significant effect.
This test suggests that the activity of nitric oxide is reduced in blacks and is responsible for their lower response to stress-induced dilation of blood vessels. Therefore, a third test was given to determine differences in responsiveness to nitric oxide. Researchers administered a drug that produces nitric oxide and, again, they saw a significantly reduced response in blacks.
"Blood flow increases, but significantly less than in whites, indicating their response to nitric oxide is reduced," Panza says.
Nitric oxide, a small molecule released by a lining of cells on the inner walls of blood vessels, "signals" the vessels to dilate and increase blood flow to skeletal muscles, such as those in the arms or legs, when they may be needed to respond to a stressful situation.
The finding may help pinpoint why blacks are far more susceptible than whites to high blood pressure and associated heart disease and illustrates the role of stress in heart disease, he says.
Blacks develop high blood pressure at an earlier age than whites and high blood pressure tends to be more severe in blacks than in whites. According to the American Heart Association, the death rate from high blood pressure for black men is about 356 percent higher than for white men. For black women, the death rate was 344 percent higher than for white women.
The finding is important, says Panza, because it identifies a potentially significant contributor to a condition that is a major national health problem. Co-authors include Carmine Cardillo, M.D.; Crescence M. Kilcoyne R.N., M.S. and Richard O. Cannon III M.D.
Media advisory: Dr. Panza can be reached by phone at 301-496-2634.