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1 Department of Biology, University of California, Riverside, Riverside, California, United States
* To whom correspondence should be addressed. E-mail: enrico.rezende{at}ebd.csic.es.
We studied relations between maximal oxygen consumption during forced exercise (VO2max) and subordinate traits associated with blood O2 transport and cellular respiration in four lines of mice selectively bred for high voluntary wheel running (S lines) and four non-selected control (C) lines. Previously, we reported VO2max of 59 females at three PO2 (hypoxia = 14% O2, normoxia = 21%, hyperoxia = 30%). Here, we test the hypothesis that variation in VO2max can be explained, in part, by hemoglobin concentration [Hb] and P50 (an estimate of Hb affinity for O2) of the blood as well as citrate synthase (CS) activity and [myoglobin] of ventricles and gastrocnemius muscle. Statistical analyses controlled for body mass, compared S and C lines, and also considered effects of the mini-muscle phenotype (present only in S lines and resulting from a Mendelian recessive allele), which reduces hindlimb muscle mass while increasing mass-specific aerobic capacity. Although S lines had higher VO2max than C, subordinate traits showed no statistical differences when the presence of the mini-muscle phenotype was controlled. However, subordinate traits did account for some of the individual variation in VO2max. Ventricle size was a positive predictor of VO2max at all three PO2. Blood [Hb] was a positive predictor of VO2max in S lines but a negative predictor in C lines, indicating that the physiological underpinnings of VO2max have been altered by selective breeding. Mice with the mini-muscle phenotype had enlarged ventricles, with higher mass-specific CS activity and [myoglobin], which may account for their higher VO2max in hypoxia.
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