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Effects of exercise training on acclimatization to hypoxia: systemic O₂ transport during maximal exercise

¹Laboratoire "Reponses Cellulaires et Fonctionelles à l'Hypoxie," Association pour la Recherche en Physiologie de l'Environnement, Université Paris XIII, 93017 Bobigny, France; and ²Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, Kansas 66160

Submitted 12 December 2001 ; accepted in final form 24 June 2003

Acclimatization to hypoxia has minimal effect on maximal O₂ uptake (O_{2 max}). Prolonged hypoxia shows reductions in cardiac output (), maximal heart rate (HR-_max), myocardial -adrenoceptor (-AR) density, and chronotropic response to isoproterenol. This study tested the hypothesis that exercise training (ET), which attenuates -AR downregulation, would increase HR_max and of acclimatization and result in higher O_{2 max}. After 3 wk of ET, rats lived at an inspired PO₂ of 70 Torr for 10 days (acclimatized trained rats) or remained in normoxia, while both groups continued to train in normoxia. Controls were sedentary acclimatized and nonacclimatized rats. All rats exercised maximally in normoxia and hypoxia (inspired PO₂ of 70 Torr). Myocardial -AR density and the chronotropic response to isoproterenol were reduced, and myocardial cholinergic receptor density was increased after acclimatization; all of these receptor changes were reversed by ET. Normoxic O_{2 max} (in ml·min^-1·kg^-1) was 95.8 ± 1.0 in acclimatized trained (n = 6), 87.7 ± 1.7 in nonacclimatized trained (P < 0.05, n = 6), 74.2 ± 1.4 in acclimatized sedentary (n = 6, P < 0.05), and 72.5 ± 1.2 in nonacclimatized sedentary (n = 8; P > 0.05 acclimatized sedentary vs. nonacclimatized sedentary). A similar distribution of O_{2 max} values occurred in hypoxic exercise. was highest in trained acclimatized and nonacclimatized, intermediate in nonacclimatized sedentary, and lowest in acclimatized sedentary groups. ET preserved in acclimatized rats thanks to maintenance of HR_max as well as of maximal stroke volume. preservation, coupled with a higher arterial O₂ content, resulted in the acclimatized trained rats having the highest convective O₂ transport and O_{2 max}. These results show that ET attenuates -AR downregulation and preserves and O_{2 max} after acclimatization, and support the idea that -AR downregulation partially contributes to the limitation of O_{2 max} after acclimatization in rats.

maximal oxygen consumption; heart rate; stroke volume; tissue oxygen extraction myocardial -adrenoceptors; myocardial cholinergic receptors

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