Does A New Hypothesis Help Explain Higher Levels Of Hypertension Among African-Americans?

(Augusta, GA)  – Some 50 million Americans have high blood pressure, according to the American Heart Association, yet the prevalence of essential hypertension -- high blood pressure with no identifiable cause -- is much higher in African-Americans than in Caucasians. While this disparity is well documented, the mechanisms by which stress might contribute to these differences are far less clear.

Most studies have used a “reactivity hypothesis” to help identify the reasons behind the differences. This hypothesis assumes that blacks exhibit exaggerated blood pressure (BP) responses to stress which produce vascular damage that underlies the premature development of essential hypertension.  An alternative to this widely accepted approach is the “pressure natriuresis” hypothesis. It supposes that stress-induced impaired sodium (salt) regulation leads to an extended period of elevated BP in blacks, and that the resulting increase in BP load leads to the early development of hypertension and its consequences. 

A New Study

The latter approach has been put to the test by a team of researchers. The authors of a new study entitled “Race Differences in Stress-Induced Salt Sensitivity and Resulting Blood Pressure Load,” are Gregory A. Harshfield, Martha E. Wilson, Kathryn McLeod, Coral Hanevold, Gaston Kapuka, Lynne Mackey, Lesley Edmunds and Delores Gillis, all of the Medical College of Georgia, Augusta, GA. They will present their findings during the upcoming scientific conference, Understanding Renal and Cardiovascular Function Through Physiological Genomics, a meeting of the American Physiological Society (APS) (www.the-aps.org), being held October 1-4, 2003 at the Radisson Riverfront Hotel and Convention Center, Augusta, GA.

Methodology

To test the hypothesis that stress is a contributing factor through its effects on pressure natriuresis (the excretion of sodium in urine, usually in excessive amounts), the research team developed a protocol to examine race differences in “stress-induced salt sensitivity” by measuring changes in urinary sodium excretion (U_NaV) in response to a stress-induced increase in blood pressure (BP).

A total of 189 African-American) and 32 Caucasian Americans (CA) aged 15-18 years were brought into similar levels of sodium balance prior to testing. The stress protocol consisted of a 2-hour baseline period, followed by a 1-hour period during which the subjects played a competitive video game task and a 2-hour recovery period.

Hemodynamic measures were obtained at 15-minute intervals and blood and urine samples were obtained hourly.

Results

The researchers noted the following:

• The AA subjects had lower U_NaV stress (16±7 versus 19±6 mEq/hr) despite similar levels of systolic BP (118± 9 versus 119± 11 mmHg).

• The BP of the AA subjects was significantly higher than the CA subjects 2 hours following the cessation of the stressor (114± 9 versus 110± 8 mmHg; P<0.05).

• Within the AA subjects, stress BP was associated with stroke volume (r=0.15; P<0.03) and the U_NaV was inversely associated with angiotensin II (r=0.29; P<0.002).

Conclusions

The major finding of this study is that stress-induced salt sensitivity and the associated increased blood pressure load contribute to racial differences in the prevalence of hypertension.  These results may help to further explain how the interactions between salt, stress and blood pressure contribute to the increased incidence and prevalence of hypertension of African-Americans.

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The American Physiological Society (APS) is one of the world’s most prestigious organizations for physiological scientists. These researchers specialize in understanding the processes and functions by which animals live, and thus ultimately underlie human health and disease.  Founded in 1887 the Bethesda, MD-based Society has more than 11,000 members and publishes 3,800 articles in its 14 peer-reviewed journals each year.

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EDITOR’S NOTE: Members of the press are invited to attend the conference and interview the researchers in person or by phone. Please contact Donna Krupa at (703) 527-7357 (office); (703) 967-2751 (cell) or djkrupa1@aol.com (email) for more information.