Continued divergence in V̇o2 maxof rats artificially selected for running endurance is mediated by greater convective blood O2delivery

Abstract

We previously showed that after seven generations of artificial selection of rats for running capacity, maximal O2uptake (V̇o2 max) was 12% greater in high-capacity (HCR) than in low-capacity runners (LCR). This difference was due exclusively to a greater O2uptake and utilization by skeletal muscle of HCR, without differences between lines in convective O2delivery to muscle by the cardiopulmonary system (Q̇o2 max). The present study in generation 15 (G15) female rats tested the hypothesis that continuing improvement in skeletal muscle O2transfer must be accompanied by augmentation in Q̇o2 maxto support V̇o2 maxof HCR. Systemic O2transport was studied during maximal normoxic and hypoxic exercise (inspired Po2∼70 Torr). V̇o2 maxdivergence between lines increased because of both improvement in HCR and deterioration in LCR: normoxic V̇o2 maxwas 50% higher in HCR than LCR. The greater V̇o2 maxin HCR was accompanied by a 41% increase in Q̇o2 max: 96.1 ± 4.0 in HCR vs. 68.1 ± 2.5 ml stpd O2·min−1·kg−1in LCR ( P < 0.01) during normoxia. The greater G15 Q̇o2 maxof HCR was due to a 48% greater stroke volume than LCR. Although tissue O2diffusive conductance continued to increase in HCR, tissue O2extraction was not significantly different from LCR at G15, because of the offsetting effect of greater HCR blood flow on tissue O2extraction. These results indicate that continuing divergence in V̇o2 maxbetween lines occurs largely as a consequence of changes in the capacity to deliver O2to the exercising muscle.