SYMPATHETIC RESPONSE DURING 21 DAYS AT HIGH-ALTITUDE (4,300-M) AS DETERMINED BY URINARY AND ARTERIAL CATECHOLAMINES

The sympathoadrenal system plays a major role in adjustments to both short- and long-term high-altitude exposure. Thus, this study investigated catecholamine responses in blood, urine, and muscle during 3 weeks' exposure to 4,300 m in control and beta-blocked subjects. Eleven healthy, sea level (SL)-resident men (aged 26 +/- 1 years) were studied under resting conditions at SL and on arrival and during 21 days at 4,300 m (Pikes Peak). Six subjects received 240 mg/d propranolol, and five were administered a placebo. Compared with SL values (38.7 +/- 4.3 v 32.4 +/- 2.8 mu g/d for control end B-blocked, respectively). urinary norepinephrine (NE) excretion increased significantly during altitude exposure, reaching peak values on days 6 to 7 (105.5 +/- 16.1 v 88.4 +/- 12.3 mu g/d, respectively). Furthermore, resting arterial NE levels (up arrow 87%), as well as net NE release (up arrow 219%) across the leg, both increased during acclimatization, indicating elevated sympathetic activity. Systemic vascular resistance and arterial pressures increased with time at altitude and correlated with NE measurements. Resting heart rates increased initially and then declined steadily after day 4 at altitude in both groups of subjects. Urinary epinephrine (EPI) excretion increased with initial exposure as compared with SL values (5.1 +/- 0.8 to 6.6 +/- 0.7 mu g/d for control, 4.5 +/- 0.5 to 5.2 +/- 1.3 mu g/d for beta-blocked); however, no consistent pattern was observed for the following 20 days at altitude. Arterial EPI increased upon acute exposure, but declined after 21 days' acclimatization. No changes in dopamine excretion were observed with beta blockade or altitude exposure. It was concluded that although sympathetic and adrenal activity differ, alterations in sympathoadrenal activity correlate with several cardiovascular adjustments associated with acclimatization to high altitude. Copyright (C) 1994 by W.B. Saunders Company