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J Appl Physiol 84: 164-168, 1998;
8750-7587/98 $5.00
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Vol. 84, Issue 1, 164-168, January 1998

Increasing maximal heart rate increases maximal O2 uptake in rats acclimatized to simulated altitude

Norberto C. Gonzalez, Richard L. Clancy, Yoshihiro Moue, and Jean-Paul Richalet

Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, Kansas 66160; and Association Pour la Recherche en Physiologie de l'Environment, Unité de Formation et de Recherche de Médecine, 93012 Bobigny Cedex, France

Gonzalez, Norberto C., Richard L. Clancy, Yoshihiro Moue, and Jean-Paul Richalet. Increasing maximal heart rate increases maximal O2 uptake in rats acclimatized to simulated altitude. J. Appl. Physiol. 84(1): 164-168, 1998.---Maximal exercise heart rate (HRmax) is reduced after acclimatization to hypobaric hypoxia. The low HRmax contributes to reduce maximal cardiac output (Qmax) and may limit maximal O2 uptake (VO2 max). The objective of these experiments was to test the hypothesis that the reduction in Qmax after acclimatization to hypoxia, due, in part, to the low HRmax, limits VO2 max. If this hypothesis is correct, an increase in Qmax would result in a proportionate increase in VO2 max. Rats acclimatized to hypobaric hypoxia [inspired PO2 (PIO2) = 69.8 ± 3 Torr for 3 wk] exercised on a treadmill in hypoxic (PIO2 = 71.7 ± 1.1 Torr) or normoxic conditions (PIO2 = 142.1 ± 1.1 Torr). Each rat ran twice: in one bout the rat was allowed to reach its spontaneous HRmax, which was 505 ± 7 and 501 ± 5 beats/min in hypoxic and normoxic exercise, respectively; in the other exercise bout, HRmax was increased by 20% to the preacclimatization value of 600 beats/min by atrial pacing. This resulted in an ~10% increase in Qmax, since the increase in HRmax was offset by a 10% decrease in stroke volume, probably due to shortening of diastolic filling time. The increase in Qmax was accompanied by a proportionate increase in maximal rate of convective O2 delivery (Qmax × arterial O2 content), maximal work rate, and VO2 max in hypoxic and normoxic exercise. The data show that increasing HRmax to preacclimatization levels increases VO2 max, supporting the hypothesis that the low HRmax tends to limit VO2 max after acclimatization to hypoxia.

aerobic capacity; hypoxic exercise; chronic hypoxia; acclimatization to hypoxia; convective oxygen delivery; cardiac output; stroke volume

The Journal of Applied Physiology 84(1):164-168
0161-7567/98 $5.00 Copyright © 1998 the American Physiological Society


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