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1 Department of Medicine, University of California, San Diego, La Jolla, California 92093-0623, USA
* To whom correspondence should be addressed. E-mail: odile{at}ucsd.edu.
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-month) and very old (35-month) F344BN rats compared to adult (12-month 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-month of age, without decline in body mass. Capillary density increased 65% (1387±54 to 2291±238 mm-2) in S and 130 % (964±95 to 2216±311 mm-2) in EDL, due to the muscle fiber atrophy while 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 to 12- and 24-month, and not in EDL. Accounting for capillary geometry revealed 55% (1776±78 to 2750±271 mm-2) and 113% (1194±112 to 2540±343 mm-2) increases in capillary length / fiber volume between 12- and 35- month of age in S and EDL, respectively. Fiber mitochondrial volume density was unchanged over the same period, causing mitochondrial volume / µm fiber length to decrease in proportion to the fiber atrophy in both muscles. As a result of the smaller fiber mitochondrial volume in the face of the unchanged capillary / fiber number, capillary-to-fiber surface ratio relative to fiber mitochondrial volume not only did not deteriorate, but in fact increased 2-fold in both muscles between 12- and 35-month of age, independent of their different fiber type.
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