Greater capillary-fiber interface per fiber mitochondrial volume in skeletal muscles of old rats

The objective was to examine whether muscle structural capacity for O2 flux (i.e., capillary-to-fiber surface ratio) relative to fiber mitochondrial volume deteriorates with the muscle atrophy of aging in predominantly slow- (soleus, S) and fast-twitch (extensor digitorum longus, EDL) muscles of old (24 mo) and very old (35 mo) F344BN rats compared with adult (12 mo old). Wet muscle mass decreased 29% (196 ± 4 to 139 ± 5 mg) in S and 22% (192 ± 3 to 150 ± 3 mg) in EDL between 12 and 35 mo of age, without decline in body mass. Capillary density increased 65% (1,387 ± 54 to 2,291 ± 238 mm−2) in S and 130% (964 ± 95 to 2,216 ± 311 mm−2) in EDL, because of the muscle fiber atrophy, whereas capillary per fiber number remained unchanged. Altered capillary geometry, i.e., lesser contribution of tortuosity and branching to capillary length, was found in S at 35 compared with 12 and 24 mo, and not in EDL. Accounting for capillary geometry revealed 55% (1,776 ± 78 to 2,750 ± 271 mm−2) and 113% (1,194 ± 112 to 2,540...