Maximal oxygen consumption in relation to subordinate traits in lines of house mice selectively bred for high voluntary wheel running

We studied relations between maximal O2 consumption (V̇o2 max) during forced exercise 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 their four nonselected control (C) lines. Previously, we reported V̇o2 max of 59 females at three Po2 (hypoxia = 14% O2, normoxia = 21%, hyperoxia = 30%). Here, we test the hypothesis that variation in V̇o2 max can be explained, in part, by hemoglobin concentration and Po2 necessary to obtain 50% O2 saturation of Hb (an estimate of Hb affinity for O2) of the blood as well as citrate synthase activity and myoglobin concentration 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 muscle mass-specific aerobic capacity. Although S lines had higher V̇o2 max 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 V̇o2 max. Ventricle size was a positive predictor of V̇o2 max at all three Po2. Blood Hb concentration was a positive predictor of V̇o2 max in S lines but a negative predictor in C lines, indicating that the physiological underpinnings of V̇o2 max have been altered by selective breeding. Mice with the mini-muscle phenotype had enlarged ventricles, with higher mass-specific citrate synthase activity and myoglobin concentration, which may account for their higher V̇o2 max in hypoxia.