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

Gonzalez, Norberto C., Richard L. Clancy, Yoshihiro Moue,and Jean-Paul Richalet. Increasing maximal heart rate increases maximal O₂ uptake in ratsacclimatized to simulated altitude. J. Appl.Physiol. 84(1): 164-168, 1998.Maximal exerciseheart rate (HR_max) is reducedafter acclimatization to hypobaric hypoxia. The lowHR_max contributes to reducemaximal cardiac output(_max) andmay limit maximal O₂ uptake(O_{2 max}). Theobjective of these experiments was to test the hypothesisthat the reduction in_max afteracclimatization to hypoxia, due, in part, to the lowHR_max, limitsO_{2 max}. Ifthis hypothesis is correct, an increase in _max wouldresult in a proportionate increase inO_{2 max}. Rats acclimatized to hypobaric hypoxia [inspiredPO₂(PI_O2) = 69.8 ± 3 Torr for 3 wk] exercised on a treadmill in hypoxic (PI_O2 = 71.7 ± 1.1 Torr) or normoxic conditions(PI_O2 = 142.1 ± 1.1 Torr). Each rat ran twice: in one bout the rat was allowed to reach itsspontaneous HR_max, which was 505 ± 7 and 501 ± 5 beats/min in hypoxic and normoxic exercise,respectively; in the other exercise bout,HR_max was increased by 20% to the preacclimatization value of 600 beats/min by atrial pacing. This resulted in an ~10% increase in_max, since theincrease in HR_max was offset by a10% decrease in stroke volume, probably due to shortening of diastolicfilling time. The increase in_max was accompanied by a proportionate increase in maximal rate of convective O₂ delivery(_max × arterial O₂ content), maximal workrate, and O_{2 max} inhypoxic and normoxic exercise. The data show that increasingHR_max topreacclimatization levels increasesO_{2 max}, supportingthe hypothesis that the lowHR_max tends to limitO_{2 max} after acclimatization to hypoxia.