Chronic hypoxia affects capillary density and geometry in pigeon pectoralis muscle

We examined fiber capillarization and ultrastructure in pectoralis muscle of 11 pigeons (Columbia livia; body mass 603 +/- 12 (SE) g) i.e. nine birds kept at 3800 m for 5 months (three in a small aviary (A1) and five in smaller cages, A2) compared to three sea-level (SL) controls. There was no difference between groups in either the relative area or number of aerobic and glycolytic fibers per total fibers, fiber size or mitochondrial volume density. Hematocrit was significantly greater in A1 and A2 (59 +/- 1%) than SL (50 +/- 2%). In A1, capillary density relative to the sectional area of aerobic/total fibers, capillary diameter and the contribution of tortuosity and branching to capillary length were significantly greater than SL, yielding greater capillary length and surface area per volume of fiber. Capillary length and surface densities very close to those in A1 and significantly greater than SL for the relative sectional area of aerobic/total fibers were also found in four out of five A2 birds, without alteration in capillary geometry or diameter. The size of the capillary-fiber interface (i.e. capillary-to-fiber surface ratio) in aerobic fibers was also greater in A1 and A2 than SL, indicating a greater capacity for oxygen supply of the muscle fibers in the altitude groups.