Structural basis of muscle O₂ diffusing capacity: evidence from muscle function in situ

Although evidence for muscle O₂ diffusion limitation of maximal O₂ uptake has been found in the intact organism and isolated muscle, its relationship to diffusion distance has not been examined. Thus we studied six sets of three purpose-bred littermate dogs (aged 10–12 mo), with 1 dog per litter allocated to each of three groups: control (C), exercise trained for 8 wk (T), or left leg immobilized for 3 wk (I). The left gastrocnemius muscle from each animal was surgically isolated, pump-perfused, and electrically stimulated to peak O₂ uptake at three randomly applied levels of arterial oxygenation [normoxia, arterial$P{O}_2$ (${\text{Pa}}_{O_2}$ ) 77 ± 2 (SE) Torr; moderate hypoxia, ${\text{Pa}}_{O_2}$ : 33 ± 1 Torr; and severe hypoxia, ${\text{Pa}}_{O_2}$ : 22 ± 1 Torr]. O₂ delivery (ml ⋅ min⁻¹ ⋅ 100 g⁻¹) was kept constant among groups for each level of oxygenation, with O₂ delivery decreasing with decreasing ${\text{Pa}}_{O_2}$ . O₂extraction (%) was lower in I than T or C for each condition, but calculated muscle O₂ diffusing capacity (Dm${\text{us}}_{O_2}$ ) per 100 grams of muscle was not different among groups. After the experiment, the muscle was perfusion fixed in situ, and a sample from the midbelly was processed for microscopy. Immobilized muscle showed a 45% reduction of muscle fiber cross-sectional area (P < 0.05), and a resulting 59% increase in capillary density (P < 0.05) but minimal reduction in capillary-to-fiber ratio (not significant). In contrast, capillarity was not significantly different in T vs. C muscle. The results show that a dramatically increased capillary density (and reduced diffusion distance) after short-term immobilization does not improve Dm${\text{us}}_{O_2}$ in heavily working skeletal muscle.