Continued divergence in V̇O 2 max of rats artificially selected for running endurance is mediated by greater convective blood O 2 delivery

Gonzalez, Norberto C., Scott D. Kirkton, Richard A. Howlett, Steven L. Britton, Lauren G. Koch, Harrieth E. Wagner, and Peter D. Wagner. Continued divergence in V̇O2 max of rats artificially selected for running endurance is mediated by greater convective blood O2 delivery. J Appl Physiol 101: 1288–1296, 2006. First published June 15, 2006; doi:10.1152/japplphysiol.01527.2005.—We previously showed that after seven generations of artificial selection of rats for running capacity, maximal O2 uptake (V̇O2 max) was 12% greater in high-capacity (HCR) than in low-capacity runners (LCR). This difference was due exclusively to a greater O2 uptake and utilization by skeletal muscle of HCR, without differences between lines in convective O2 delivery 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 O2 transfer must be accompanied by augmentation in Q̇O2 max to support V̇O2 max of HCR. Systemic O2 transport was studied during maximal normoxic and hypoxic exercise (inspired PO2 70 Torr). V̇O2 max divergence between lines increased because of both improvement in HCR and deterioration in LCR: normoxic V̇O2 max was 50% higher in HCR than LCR. The greater V̇O2 max in 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 1 in LCR (P 0.01) during normoxia. The greater G15 Q̇O2 max of HCR was due to a 48% greater stroke volume than LCR. Although tissue O2 diffusive conductance continued to increase in HCR, tissue O2 extraction was not significantly different from LCR at G15, because of the offsetting effect of greater HCR blood flow on tissue O2 extraction. These results indicate that continuing divergence in V̇O2 max between lines occurs largely as a consequence of changes in the capacity to deliver O2 to the exercising muscle.