Acclimatization to high altitude increases muscle sympathetic
activity both at rest and during exercise .
Mazzeo, Robert S., George A. Brooks, Gail. E. Butterfield, Deborah A.
Podolin, Eugene E. Wolfel, and John T. Reeves.
Department of Kinesiology, University of Colorado, Boulder, CO,
University of California, Berkeley, CA, University of Colorado,
Health Sciences Center, Denver, CO, GRECC VA Medical Center, Palo
Alto, CA.
APStracts 2:0059R, 1995.
This investigation examined the relationship between alterations in
plasma norepinephrine associated with 21 days of high-altitude
exposure and muscle sympathetic activity both at rest and during
exercise. Healthy sea level residents, divided into a control group
(n=5) receiving a placebo or a drug group (n=6) receiving 240 mg/day
of propranolol, were studied while at sea level, upon arrival (acute)
and after 21 days residence at (chronic) 4,300 m. Arterial
norepinephrine levels and net leg uptake/release of norepinephrine
were determine both at rest and during 45 min of submaximal exercise
via samples collected from femoral arterial and venous catheters.
Arterial norepinephrine levels increased significantly after chronic
altitude exposure both at rest (84%) and during exercise (174%)
compared to sea level and acute values. A net uptake of
norepinephrine was found in resting legs at sea level (0.28+/-0.05
nmole/min) and with acute exposure (0.07+/-0.06 nmole/min), however,
a significant switch to net leg norepinephrine release was observed
with chronic altitude exposure (0.51+/-0.11 nmole/min). With
exercise, a net release of norepinephrine by the legs occurred across
all conditions with chronic exposure again eliciting the greatest
values (5.3+/-0.6, 8.0+/-1.7, and 14.4+/-3.1 nmole/min for sea level,
acute and chronic exposure, respectively). It was concluded that
muscle sympathetic activity is significantly elevated both at rest
and during submaximal exercise as a result of chronic high-altitude
exposure and muscle is a major contributor to the increase in plasma
norepinephrine levels associated with prolonged altitude exposure.
The presence of dense [beta]-blockade did not alter this adaptation
to altitude.
Received 25 October 1994; accepted in final form 15 February
1995.
APS Manuscript Number R618-4.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 7 March 1995.