Fiber capillarization and ultrastructure of pigeon pectoralis muscle aftercold acclimation

We investigated the effect of 2 months of exposure to cold conditions (0-5 degrees C) on capillarization and on fiber size, distribution and ultrastru cture in the pectoralis muscle of nine pigeons (Columbia livia; mean body m ass 700+/-31 g) and compared the results with measurements from four contro l birds (mean mass 715+/-42 g) kept at normal ambient temperature (22-23 de grees C) for the same period. Superficial and deep portions of the muscles, taken from the central area of the right or left pectoralis major muscle, were perfusion-fixed irt situ, processed for electron microscopy and analyz ed by morphometry. Aerobic fibers represented the vast majority of fibers ( 93+/-1 % , mean +/- S.E.M.) in all samples, After cold-acclimation, fiber s ectional area was reduced and capillary density increased proportionally. T here was no change in the degree of orientation (anisotropy) of capillaries , capillary-to-fiber ratio or fiber type distribution compared with control s, The volume density of mitochondria and lipid droplets in aerobic fibers and capillary diameter increased in response to cold, while the linear rela tionship between capillary length per fiber volume and fiber mitochondrial volume density remained unchanged, Capillary surface area, intrafiber lipid deposition and fiber mitochondrial volume density were all correlated in c old-acclimated pigeons. The results indicate a close match between the aero bic capacity of the highly aerobic fibers of the pectoralis muscle and thei r vascularization to meet the increased energetic demand of shivering.