Determinants of maximal O 2 uptake in rats selectively bred for endurance running capacity

O 2 transport during maximal exercise was studied in rats bred for extremes of exercise endurance, to determine whether maximal O 2 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 max was 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 2 difference, or lung O 2 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 O 2 convection (cardiac output times arterial blood O 2 content) was also similar in both groups. The major difference observed was in capillary-to-tissue O 2 transfer: both the O 2 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 −1 in 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 O 2 at the tissue level.