Altitude and [beta]-blockade augment glucose utilization during
submaximal exercise.
Roberts, A. C., J. T. Reeves, G. E. Butterfield, R. S. Mazzeo, J. R.
Sutton, E. E. Wolfel, and G. A. Brooks.
University of California at Berkeley, Berkeley, CA, 94720,
University of Colorado Health Sciences Center, Denver, CO, 80262 and
GRECC, Palo Alto VA Medical Center, Palo Alto, CA, 95304; Faculty of
Health Sciences, University of Sydney, and Department of Human
Biodynamics, University of California
APStracts 2:0430A, 1995.
We tested the hypothesis that altitude exposure increases glucose
utilization and that this increment is mediated by a [beta]
-adrenergic mechanism. Therefore, the effects of hypobaric hypoxia and
[beta]-blockade on glucose rates of appearance (Ra), disappearance
(Rd), oxidation (Rox), and leg uptake [G^ = 2 (arterio-venous glucose
difference) (1-leg blood flow)] were measured during rest and a given
submaximal exercise task. We studied six healthy [beta]-blocked men
([beta]) (mean + SEM = 26.7+1.2 yr., 74.0+6.6 kg) and five matched
controls (C) (26+1.2 yr., 69.3+2.6 kg) in energy and nitrogen balance
during rest and leg cycle ergometry exercise at sea level, on acute
altitude exposure to 4,300 m (Pb = 463 mm Hg), and after 3 weeks
habituation. Subjects received a primed continuous infusion of [6,6
-2H]- and [1-13C]glucose and rested for a minimum of 90 min, followed
immediately by 45 min of exercise at 89 W which elicited 49% of the
sea level peak O2 consumption (VO2peak; 65 % of altitude VO2peak). At
sea level, resting glucose appearance (Ra) was (C = 1.47+0.19, [beta]
= 1.66 +0.16 mg.kg-1.min-1); and increased to (C=3.04 +0.25, [beta]=
3.56+0.27 mg.kg-1.min-1) during exercise. Thus, glucose Ra was
significantly increased by [beta]-blockade during rest and exercise
at sea level. At sea level, [beta]-blockade increased leg G^ which
accounted for 49 and 69% of glucose disposal during exercise in
control and [beta]-blocked subjects respectively. On acute altitude
exposure, glucose Ra rose significantly during rest (C=1.99+0.11,
[beta]=2.25+0.17 mg.kg-1.min-1) and exercise (C=4.42+0.5,
[beta]=5.76+0.47 mg.kg-1.min-1) relative to sea level while blockade
continued to augment this increment. During exercise upon acute
exposure, G^ (C= 3.34+0.53, [beta]= 5.56+1.1 mg.kg-1.min-1) increased
more than at sea level and accounted for a greater percentage (80 and
97%) of glucose disappearance (Rd) in control and [beta]-blocked
subjects during exercise. Similarly, rates of glucose oxidation,
particularly during exercise, were increased significantly at
altitude (C=1.39+0.13, [beta]=2.17+0.40 mg.kg-1.min-1) relative to
sea level values (C=0.82+0.03, [beta]=0.98+0.11 mg.kg-1.min-1), and
[beta]-blockade potentiated this effect. During a given submaximal
exercise task after acclimatization, glucose Ra, Rox and net leg
uptake were increased relative to sea level, but these increments
were less than those in response to exercise measured upon acute
exposure. We conclude that altitude exposure increases glucose use
during rest and a given submaximal exercise bout and [beta]-blockade
exaggerates the response.
Received 12 September 1994; accepted in final form 21 September
1995.
APS Manuscript Number A958-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95