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-adrenergic blockade during
exercise in women acutely exposed to altitude
1 Department of Kinesiology and Applied Physiology, University of Colorado, Boulder, Colorado 80309; 2 Palo Alto Veterans Affairs Health Care System, Palo Alto, California 94304 - 1290; 3 US Army Research Institute of Environmental Medicine, Natick, Massachusetts 01760 - 5007; and 4 University of Colorado, Health Sciences Center, Denver, Colorado 80220
We have previously
documented the importance of the sympathetic nervous system in
acclimatizing to high altitude in men. The purpose of
this investigation was to determine the extent to which 
-adrenergic
blockade affects the sympathoadrenal responses to exercise during acute
high-altitude exposure in women. Twelve eumenorrheic women (24.7 ± 1.3 yr, 70.6 ± 2.6 kg) were studied at sea level and on
day 2 of high-altitude exposure (4,300-m hypobaric chamber)
in either their follicular or luteal phase. Subjects performed two
graded-exercise tests at sea level (on separate days) on a bicycle
ergometer after 3 days of taking either a placebo or an -blocker (3 mg/day prazosin). Subjects also performed two similar exercise tests
while at altitude. Effectiveness of blockade was determined by
phenylephrine challenge. At sea level, plasma norepinephrine levels
during exercise were 48% greater when subjects were -blocked
compared with their placebo trial. This difference was only 25% when
subjects were studied at altitude. Plasma norepinephrine values were
significantly elevated at altitude compared with sea level but to a
greater extent for the placebo (
59%) vs. blocked (35%) trial. A
more dramatic effect of both altitude (104% placebo vs. 95%
blocked) and blockade (50% sea level vs. 44% altitude) was
observed for plasma epinephrine levels during exercise. No phase
differences were observed across any condition studied. It was
concluded that -adrenergic blockade 1) resulted in a
compensatory sympathoadrenal response during exercise at sea level and
altitude, and 2) this effect was more pronounced for plasma epinephrine.
epinephrine; norepinephrine; sympathetics; hypoxia
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