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Department of Medicine, University of California at San Diego, La Jolla, California 92093-0623
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
PO2
(PaO2) 77 ± 2 (SE) Torr; moderate
hypoxia, PaO2: 33 ± 1 Torr; and severe hypoxia, PaO2: 22 ± 1 Torr]. O2 delivery
(ml · min
1 · 100 g1) was kept constant among groups
for each level of oxygenation, with O2 delivery decreasing
with decreasing PaO2. O2
extraction (%) was lower in I than T or C for each condition, but
calculated muscle O2 diffusing capacity
(DmusO2) 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 DmusO2 in heavily
working skeletal muscle.
capillarization; diffusion distance; exercise; maximal oxygen uptake
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