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

Department of Medicine, University of California at San Diego, La Jolla, California 92093-0623 Although evidence for muscle O 2 diffusion limitation of maximal O 2 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 O 2 uptake at three randomly applied levels of arterial oxygenation [normoxia, arterial P O 2 (Pa O 2 ) 77 ± 2 (SE) Torr; moderate hypoxia, Pa O 2 : 33 ± 1 Torr; and severe hypoxia, Pa O 2 : 22 ± 1 Torr]. O 2 delivery (ml · min 1 · 100 g 1 ) was kept constant among groups for each level of oxygenation, with O 2 delivery decreasing with decreasing Pa O 2 . O 2 extraction (%) was lower in I than T or C for each condition, but calculated muscle O 2 diffusing capacity (Dmus 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