Poor relationship between arterial [lactate] and leg net release during exercise at 4,300 m altitude.
We evaluated the hypotheses that on acute exposure to hypobaric hypoxia, sympathetic stimulation leads to augmented muscle lactate production and circulating [lactate] through a beta-adrenergic mechanism and that beta-adrenergic adaptation to chronic hypoxia is responsible for the blunted exercise lactate response after acclimatization to altitude. Five control and 6 beta-blocked men were studied during rest and exercise at sea level (SL), on acute exposure to 4,300 m (A1), and after a 3-wk sojourn at altitude (A2). Exercise was by leg cycling at 49% of SL peak O2 consumption (VO2 peak) (65% of altitude VO2 peak or 87 +/- 2.6 W); beta-blockade was by propranolol (80 mg 3x daily), femoral arterial and venous blood was sampled; leg blood flow (Q) was measured by thermodilution, leg lactate net release [ = (2) (1-leg Q) venous-arterial concentrationL] was calculated, and vastus lateralis needle biopsies were obtained. Muscle [lactate] increased with exercise and acute altitude exposure but regressed to SL values with acclimatization; beta-blockade had no effect on muscle [lactate]. Arterial [lactate] rose during exercise at SL (0.9 +/- 0.1 to 1.5 +/- 0.3 mM); exercise at A1 produced the greatest arterial [lactate] (4.4 +/- 0.8 mM), and exercise at A2 an intermediate response (2.1 +/- 0.6 mM). beta-Blockade reduced circulating [lactate] approximately 45% during exercise under all altitude conditions. increased transiently at exercise onset but then declined over time under all conditions. Blood and muscle "lactate paradoxes" occurred independent of beta-adrenergic influences, and the hypotheses relating the blood lactate response at altitude to beta-adrenergic mechanisms are rejected. During exercise at altitude, arterial [lactate] is determined by factors in addition to hypoxemia, circulating epinephrine, and net lactate release from active muscle beds.
Brooks
G A
GA
Department of Integrative Biology, University of California, Berkeley, California 94720, USA.
Wolfel
E E
EE
Butterfield
G E
GE
Cymerman
A
A
Roberts
A C
AC
Mazzeo
R S
RS
Reeves
J T
JT
D000064:Acclimatization; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000531:Altitude; D005080:Exercise Test; D005263:Femoral Artery / Q000502:physiology; D005268:Femoral Vein / Q000502:physiology; D006801:Humans; D000860:Hypoxia; D007773:Lactates / Q000097:blood / Q000378:metabolism*; D007866:Leg; D008297:Male; D018482:Muscle, Skeletal / Q000098:blood supply / Q000502:physiology*; D010101:Oxygen Consumption; D005082:Physical Exertion / Q000502:physiology*; D011433:Propranolol / Q000494:pharmacology; D012039:Regional Blood Flow; D012146:Rest
D000319:Adrenergic beta-Antagonists; D007773:Lactates; D011433:Propranolol