Moreover, platelet aggregation was largely suppressed in at least three other key pathways related to their function when platelets were stimulated with substances that normally trigger clot formation. Each of these tests involved mixing whole blood, or platelet-rich plasma, from aspirin-treated men and women with various concentrations of each of the main chemical compounds involved in the pathways - collagen, adenosine diphosphate, and epinephrine - to see how platelets responded.
For example, in aspirin-treated men, platelet clumping went down by 14.6 ohms when 1 microgram of collagen per milliliter was added to whole blood, and decreased by 2.4 ohms when exposed to a higher dose of 5 micrograms per milliliter. In treated women, reductions were the same, at 14.9 ohms and 2.42 ohms, respectively.
When 10 micromoles per liter of adenosine diphosphate were added to whole blood, platelet aggregation decreased the same amount, 0.19 ohms in men and 0.21 ohms in women. Addition of 2 micromoles per liter of epinephrine to platelet-rich plasma produced significantly greater reductions in platelet clumping in treated women, a drop of 36.9 percent, while it was less of a reduction for men, at 31.5 percent. Again, the researchers say, these changes would have been zero if aspirin had had no effect.
Further analysis of results highlighted mainly two factors, platelet reactivity levels before therapy starts and gender, as having played a significant role in predicting the effects of aspirin therapy on platelet clumping. Other factors, such as age, race or known risk factors for heart disease, including smoking, obesity and high blood pressure, were not found to be good predictors of aspirin's beneficial effects.
More than 500 men and 700 women participated in the study, called the Genetic Study of Aspirin Responsiveness (GeneSTAR). Conducted solely at Hopkins from June 2004 to November 2005, the study enrolled participants from across the country who ranged in age from 21 to 80; 31 percent were black and the rest were white. None had previous histories of heart problems, such as a heart attack, but all were considered to be at slightly increased risk of heart disease because of a family history. Fifty percent of women participants were postmenopausal.
Blood testing was conducted both before and after treatment. In total, more than 200 different tests of platelet reactivity were performed and analyzed in the study. Because whole blood contains other cells that affect platelet aggregation, testing was repeated using a purified version of test samples made up of strictly platelet-rich plasma.
At the start of the experiment, laboratory tests of blood platelets in women were found four times more likely than in men to aggregate when exposed to arachidonic acid, a clot-inducing chemical in the pathway that is most suppressed by aspirin.
While taking aspirin, participants maintained a strict and consistent dietary and exercise regimen, with no smoking or consumption of foods that by themselves affect platelet activity, such as caffeine, chocolate, wine or grapefruit juice. Physical examinations and pill counts were conducted to ensure that all participants adhered to the study protocol. Because aspirin reaches its maximal effect in the body at five days, the researchers say a longer study testing period was not required to determine the drug's effects on platelet function.
Funding for the study was provided by the National Heart, Lung and Blood Institute, a member of the National Institutes of Health, and the Johns Hopkins Clinical Research Center.
Besides Becker and Faraday, other investigators in this research were Jodi Segal, M.D.; Dhananjay Vaidya, M.D., Ph.D.; Lisa Yanek, M.P.H.; J. Enrique Herrera-Galeano, M.S.; Paul Bray, M.D.; Taryn Moy, M.S.; and Lewis Becker, M.D.
Contact: David March
Johns Hopkins Medical Institutions