Fiber capillarization in flight muscle of pigeons native and flying at altitude

We examined structural characteristics for fiber O 2 supply in the highly aerobic flight muscle of 5 pigeons ( Columbia livia; body mass 223–317 g) native and actively flying at altitude (La Paz, Bolivia; 3750 m). Whereas deep sites were significantly more aerobic and highly vascularized than superficial in altitude (A) but not sea-level (SL) group, both sites showed number of similarities between the two groups. The cross-sectional area of aerobic fibers (≥ 90% of fiber number) linearly increased with body mass ( r = 0.84; P < 0.0005) but was not smaller in A for their body mass, and there was no reduction in the size of glycolytic fibers compared to SL. The relationships between fiber capillarization and the sectional area of aerobic fibers in transverse sections or mitochondrial volume density were not altered in A compared to SL. The results indicate that the factors which determine the relationships between fiber capillarization and ultrastructure in the muscles were not altered by the adaptation to altitude. Differences between samples were related to the relative sectional area of aerobic fibers and mitochondrial volume density, not the chronic exposure to altitude.