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

We have previously documented the importance of the sympathetic nervous sys tem in acclimatizing to high altitude in men. The purpose of this investiga tion was to determine the extent to which alpha -adrenergic blockade affect s the sympathoadrenal responses to exercise during acute high-altitude expo sure 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 per formed two graded-exercise tests at sea level (on separate days) on a bicyc le ergometer after 3 days of taking either a placebo or an alpha -blocker ( 3 mg/day prazosin). Subjects also performed two similar exercise tests whil e at altitude. Effectiveness of blockade was determined by phenylephrine ch allenge. At sea level, plasma norepinephrine levels during exercise were 48 % greater when subjects were alpha -blocked compared with their placebo tri al. This difference was only 25% when subjects were studied at altitude. Pl asma norepinephrine values were significantly elevated at altitude compared with sea level but to a greater extent for the placebo (up arrow 59%) vs. blocked (up arrow 35%) trial. A more dramatic effect of both altitude (up a rrow 104% placebo vs. 95% blocked) and blockade (up 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 con cluded that alpha -adrenergic blockade 1) resulted in a compensatory sympat hoadrenal response during exercise at sea level and altitude, and 2) this e ffect was more pronounced for plasma epinephrine.