Arteriolar and Venular Capillary Distribution in Skeletal Muscles of Old Rats

Decreased skeletal muscle mass and proportion of fast-twitch glycolytic fibers are well-documented correlates of aging; however, data on concomitant changes on capillary-to-fiber ratio (C:F) are inconsistent. We simultaneously examined fiber-type composition and arteriolar and venular portions of capillaries in the distal hind-limb muscles of 12-, 24-, and 35-month old F1 hybrid F344 Brown Norway rats. Aging significantly increased C:F of venular capillaries in muscles, which also presented significant age-related increase in slow-(type I) and fast-(type IIa) oxidative fibers (plantaris, tibialis anterior, medial gastrocnemius; p,.05). In contrast, arteriolar and venular capillary proportions did not change in the soleus, extensor digitorum longus, or lateral gastrocnemius. These data suggest that age-associated increases in skeletal muscle capillarity may be due to the venular portion of capillaries and that the increase occurs primarily in muscles that demonstrate increased oxidative potential with age. Key Words: Fiber type—Capillary density—Capillary-to-fiber ratio—Skeletal muscle— Morphometry. AGING effects on the structure and function of skeletalmuscles have been intensely studied for decades; however, age-related changes in skeletal muscle capillarit