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

We have previouslydocumented the importance of the sympathetic nervous system inacclimatizing to high altitude in men. The purpose ofthis investigation was to determine the extent to which -adrenergicblockade affects the sympathoadrenal responses to exercise during acutehigh-altitude exposure in women. Twelve eumenorrheic women (24.7 ± 1.3 yr, 70.6 ± 2.6 kg) were studied at sea level and onday 2 of high-altitude exposure (4,300-m hypobaric chamber)in either their follicular or luteal phase. Subjects performed twograded-exercise tests at sea level (on separate days) on a bicycleergometer after 3 days of taking either a placebo or an -blocker (3 mg/day prazosin). Subjects also performed two similar exercise testswhile at altitude. Effectiveness of blockade was determined byphenylephrine challenge. At sea level, plasma norepinephrine levelsduring exercise were 48% greater when subjects were -blockedcompared with their placebo trial. This difference was only 25% whensubjects were studied at altitude. Plasma norepinephrine values weresignificantly elevated at altitude compared with sea level but to agreater extent for the placebo (59%) vs. blocked (35%) trial. Amore dramatic effect of both altitude (104% placebo vs. 95%blocked) and blockade (50% sea level vs. 44% altitude) wasobserved for plasma epinephrine levels during exercise. No phasedifferences were observed across any condition studied. It wasconcluded that -adrenergic blockade 1) resulted in acompensatory sympathoadrenal response during exercise at sea level andaltitude, and 2) this effect was more pronounced for plasma epinephrine.