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* To whom correspondence should be addressed. E-mail: hepple{at}ucalgary.ca.
The anatomical size of the capillary-to-fiber interface plays an important role in O2 flux from blood to tissue by determining the anatomical surface area available for diffusion, and is maintained in relative proportion to fiber mitochondrial volume across a wide range of muscle aerobic capacity. In the current study we examined an estimate of the anatomical size of the capillary-to-fiber interface (the quotient of the individual capillary-to-fiber ratio and fiber perimeter, CFPE Index), relative to fiber oxidative capacity in different skeletal muscles, or muscle regions, to test the hypothesis that capillarization would be maintained in relative excess of reduced fiber oxidative capacity in aged muscles. The right gastrocnemius, plantaris and soleus muscles from young adult (8 mo old) and late middle aged (28-30 mo old) Fischer 344 x Brown Norway F1-hybrid rats were excised for evaluation of flux through electron transport chain complexes I-III and/or morphometric estimation of capillarization. Muscle mass was lower in the gastrocnemius muscles of the older animals (2076 ± 32 versus 1825 ± 47 mg in young adult versus late middle aged, respectively; mean ± SE), but not the plantaris or soleus muscles. Fibers were smaller in the white region of gastrocnemius muscles, but larger in the red region of gastrocnemius muscles of the older animals. There was no difference in the number of capillaries around a fiber, the individual capillary-to-fiber ratio or the CFPE index between groups for any muscle/region, whereas flux through complexes I-III was reduced by 29-43% in late middle aged animals. Thus, the quotient of individual capillary-to-fiber ratio and fiber oxidative capacity was greater in soleus and the white region of gastrocnemius muscles but unchanged in the red region of gastrocnemius muscles of the older animals, showing that anatomical capillarity is maintained in relative excess of oxidative capacity in some muscle regions in late middle aged rats.
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