Determinants of maximal O₂ uptake in rats selectively bred for endurance running capacity

O2 transport during maximal exercise was studied in rats bred for extremes of exercise endurance, to determine whether maximal O2 uptake (V˙o 2 max) was different in high- (HCR) and low-capacity runners (LCR) and, if so, which were the phenotypes responsible for the difference. V˙o 2 maxwas determined in five HCR and six LCR female rats by use of a progressive treadmill exercise protocol at inspired Po 2 of ∼145 (normoxia) and ∼70 Torr (hypoxia). Normoxic V˙o 2 max (in ml · min−1 · kg−1) was 64.4 ± 0.4 and 57.6 ± 1.5 ( P < 0.05), whereas V˙o 2 max in hypoxia was 42.7 ± 0.8 and 35.3 ± 1.5 ( P < 0.05) in HCR and LCR, respectively. Lack of significant differences between HCR and LCR in alveolar ventilation, alveolar-to-arterial Po 2difference, or lung O2 diffusing capacity indicated that neither ventilation nor efficacy of gas exchange contributed to the difference in V˙o 2 max between groups. Maximal rate of blood O2 convection (cardiac output times arterial blood O2 content) was also similar in both groups. The major difference observed was in capillary-to-tissue O2transfer: both the O2 extraction ratio (0.81 ± 0.002 in HCR, 0.74 ± 0.009 in LCR, P < 0.001) and the tissue diffusion capacity (1.18 ± 0.09 in HCR and 0.92 ± 0.05 ml · min−1 · kg−1 · Torr−1in LCR, P < 0.01) were significantly higher in HCR. The data indicate that selective breeding for exercise endurance resulted in higher V˙o 2 max mostly associated with a higher transfer of O2 at the tissue level.