ACCLIMATIZATION TO HIGH-ALTITUDE INCREASES MUSCLE SYMPATHETIC ACTIVITY BOTH AT REST AND DURING EXERCISE

This investigation examined the relationship between alterations in pl asma norepinephrine associated with 21 days of high-altitude exposure and muscle sympathetic activity both at rest and during exercise. Heal thy sea level residents, divided into a control group (n = 5) receivin g a placebo or a drug group (n = 6) receiving 240 mg/day of propranolo l, were studied while at sea level, upon arrival (acute), and after 21 days of residence (chronic) at 4,300 m. Arterial norepinephrine level s and net leg uptake and release of norepinephrine were determine both at rest and during 45 min of submaximal exercise via samples collecte d from femoral arterial and venous catheters. Arterial norepinephrine levels increased significantly after chronic altitude exposure both at rest (84%) and during exercise (174%) compared with sea level and acu te values. A net uptake of norepinephrine was found in resting legs at sea level (0.28 +/- 0.05 nmol/min) and with acute exposure (0.07 +/- 0.06 nmol/min); however, a significant switch to net leg norepinephrin e release was observed with chronic altitude exposure (0.51 +/- 0.11 n mol/min). With exercise, a net release of norepinephrine by the leg oc curred across all conditions with chronic exposure, again eliciting th e greatest values (5.3 +/- 0.6, 8.0 +/- 1.7, and 14.4 +/- 3.1 nmol/min for sea level, acute, and chronic exposure, respectively). It was con cluded that muscle sympathetic activity is significantly elevated both at rest and during submaximal exercise as a result of chronic high-al titude exposure, and muscle is a major contributor to the increase in plasma norepinephrine levels associated with prolonged altitude exposu re. The presence of dense beta-blockade did not alter this adaptation to altitude.