Overactivation of two receptors for histamine, normally associated with common allergies and acid reflux, may explain why some people, including highly trained athletes, pass out soon after heightened physical activities, according to researchers at the University of Oregon.
A series of studies led researchers in incremental steps to the discovery that the use of two commonly used antihistamines (fexofenadine and ranitidine) prior to exercise dramatically lower or completely eliminate low blood pressure following exertion. The drugs worked by preventing post-exercise hyperemia, an increased flow of blood, in the skeletal muscle during the critical 90-minute recovery period after exercise. In all, the pre-exercise consumption of the two antihistamines reduced the blood flow that occurs during recovery by 80 percent.
The study, funded by the American Heart Association, was posted online ahead of regular publication in the Journal of Applied Physiology. While fainting after exercise, a condition called syncope, can indicate a serious heart disorder, most cases are linked to low blood pressure and low blood flow to the brain.
"There is reason to believe that histamine is the primary vasodilator contributing to post-exercise hypotension, but we cannot say for certain," cautioned principal investigator John R. Halliwill, a professor of human physiology. "Some people have problems regulating blood pressure during and after exercise. Trained athletes have had fainting bouts at the end of exercise. It may be that these result from a natural overactivation of these two receptors for histamine."
The histamine receptors involved are known as H1 and H2. Fexofenadine, which is the generic name for Allegra, works against H1, reducing the occurrence of such allergy symptoms as sneezing and runny nose. Ranitidine, or Zantac, acts against H2 in the treatment of acid reflux.
For the study, 28 sedentary and endurance-trained men and women were monitored closely throughout a session that covered a pre-exercise period, a 60-minute ride on a cycling machine and a 90-minute recovery period. The participants were all non-smokers without blood pressure problems and between the ages of 19 and 34. The group given the histamine blockers consumed them with water 60 minutes before beginning the exercise regimen.
The studies in Halliwill's Exercise and Environmental Physiology Laboratories were designed to pursue the mechanisms involved in the exercise recovery period. The findings that the two antihistamine products worked as they did do not mean that athletes or sedentary-turned-active people should head to their medicine cabinets before exercising.
The amount of fexofenadine used in the study was almost three times the strongest dose used for respiratory allergies, while the dosage for ranitidine matched the common starting dose for battling heartburn. Also, there may be a benefit to the normal activation of these receptors during physical activity, because routine exercise helps to reduce or prevent the development of hypertension, or high blood pressure. "Activating these receptors might be an important part of the health benefits of daily exercise," Halliwill said.
The two drugs, however, did not appear to affect the central nervous system or cause sedation during the exercise experiments, Halliwill and co-author Jennifer L. McCord, a doctoral student, noted in the study.
The big question now, Halliwill said, is what triggers the histamine responses during exercise.
"The body tends to be very good at recycling mechanisms," he said. "The body may be using these same receptors for other things. A bout of exercise appears to turn on a program for remodeling blood vessels in the body, and these receptors may be an important part of that program."
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