Catecholamine responses to alpha-adrenergic blockade during exercise in women acutely exposed to altitude.

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 alpha-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 alpha-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 alpha-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 ( upward arrow 59%) vs. blocked ( upward arrow 35%) trial. A more dramatic effect of both altitude ( upward arrow 104% placebo vs. 95% blocked) and blockade ( upward arrow 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 alpha-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.