Impaired sodium excretion after stress contributes to hypertension in blacks

May 16, 2002

Some young, healthy blacks continue to retain sodium even after the stress that drove their blood pressure up is gone, a situation that keeps blood pressure and volume elevated for prolonged periods and puts them at risk for hypertension, researchers said.

One third of the 118 black males and females age 15 to 18 in the Medical College of Georgia study had an impaired ability to eliminate sodium in their urine hours after the stress subsided, according to the study presented May 16 at the 17th Annual Scientific Session of the American Society of Hypertension in New York.

The body naturally increases blood pressure during stress, immediately by constricting blood vessels and longer term by directing the kidneys to retain more sodium and so increase blood volume, said Dr. Gregory A. Harshfield, first author on the presentation, Impaired Stress-Induced Pressure Natriuresis Increases Cardiovascular Load in Black Youths.

Many previous studies looking at why blacks are at increased risk for hypertension have supported the hypothesis that they have an exaggerated response to stress, Dr. Harshfield said. "However the physiologic response to stress is dynamic and includes both the response to the stress per se and the changes that restore blood pressure to pre-stress levels," he said in his presentation.

His study showed one mechanism key to restoring blood pressure - natriuresis or secreting sodium in the urine - is impaired in some black youths with normal blood pressures. "As such, we hypothesize that the interaction between salt and stress is more important than either of the factors considered separately," he wrote.

"This says to me that the stresses of the day are affecting these individuals differently in that they are affecting their long-term blood pressure control system," Dr. Harshfield said, noting his previous work at Charles R. Drew University in California which showed that the blood pressure of blacks doesn't drop, even when sleeping, as much as that of whites.

"These individuals are under stress, they are holding onto salt, their blood pressure is high, it takes longer to bring their blood pressure down. That increases their cardiovascular load and can lead to premature development of essential hypertension," said the physiological psychologist at MCG's Georgia Prevention Institute.

Researchers designed a protocol that put the youths on a diet for three days to bring their sodium balance to similar levels. On test day, the youths relaxed for two hours at the Georgia Prevention Institute, were stressed for an hour by playing competitive video games, then relaxed for two more hours. Blood pressure and sodium excretion were checked throughout the five-hour protocol.

Two-thirds of participants showed a normal response, with blood pressure and sodium conservation increasing during stress and decreasing afterward. But one-third had decreased sodium excretion during stress, which increased their blood volume and pressure and kept it up during the recovery period.

"... (T)he results of this study provide support for our hypothesis that stress-induced impaired pressure natriuresis leads to an extended period of elevated blood pressure," Dr. Harshfield said in his presentation. "As such, this study provides evidence for an alternative hypothesis to the traditional reactivity hypothesis to explain the causative role of stress in the development of essential hypertension. This hypothesis is particularly appealing for the black population whose blood pressure is more sensitive to the effects of sodium."

Sodium is involved in all nervous system regulation, physiologic functions and fluid volumes, he said. The body doesn't produce sodium but tightly regulates its levels through a number of mechanisms. These include angiotensin 2, a vasoconstrictor that also directs the kidneys to reabsorb salt. Additionally, angiotensin 2 stimulates aldosterone, a hormone produced by the adrenal gland that also gets the kidneys to absorb more sodium. If the blood pressure drops because of bleeding or a stressful situation triggers the basic survival instinct to fight or flee, these are some primary ways the body increases blood pressure.

Dr. Harshfield has long wondered whether reactivity alone was how behavioral factors contribute to hypertension. As a postdoctoral fellow at Cornell University Medical Center in New York in the 1980s, he worked on a program project grant from the National Institutes of Health examining behavioral factors contributing to blood pressure regulation variation.

These studies in animal models found that even though blood pressures increased significantly in response to stress, that alone did not produce cardiovascular damage, such as thickening of the muscle wall in the pumping chamber of the heart. But his more recent work, published in the current issue of the journal Hypertension, documents structural changes in the left ventricles of blacks who retain sodium during stress.

"Hypertension is not a disease in and of itself; it's a symptom of some other underlying disorder," said Dr. Harshfield, an expert in the kidneys' role in blood pressure regulation. "Hypertension can result from multiple impaired systems but they all lead to impaired renal function. So, no matter what the etiology, the underlying characteristic of hypertension is that it requires a higher level of blood pressure to maintain a sodium balance."

He hopes that by better defining the causes of elevated blood pressure, patients can get more targeted, effective treatments so that resulting damage, such as kidney failure and heart disease, can be avoided.
The work was funded by the NIH.

Medical College of Georgia at Augusta University

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