EFFECT OF FLYING ACTIVITY ON CAPILLARY-FIBER GEOMETRY IN PIGEON FLIGHT-MUSCLE

The effect of flying activity on capillary density and geometry was in vestigated in pectoralis muscle of 4 wild-caught (W) pigeons (BW 233-3 48 g) perfusion-fixed in situ and processed for electron microscopy. M orphometric analysis revealed both differences and similarities with s imilar sampling sites (superficial and deep in central area of right o r left pectoralis major muscle, approximately midway along cranio-caud al and lateral axis) in sedentary (S) pigeons. Differences were the gr eater fractional cross-sectional area of aerobic fibers (W, 82 +/- 2%; S, 63 +/- 6%; p = 0.006) and the greater volume density of mitochondr ia per volume of fiber (W, 22.0 +/- 1.3%; S, 15.7 +/- 1.7%; p = 0.011) in wild-caught pigeons. While glycolytic fibers were significantly na rrower in W, the size of the majority of fibers comprising the muscles , i.e. aerobic fibers, was similar in the two groups. Other similariti es were found in capillary-to-fiber ratio (W, 2.0 +/- 0.2; S, 2.1 +/- 0.2) and in the degree of orientation of capillaries in the two groups . In addition, both capillary density at a given fractional cross-sect ional area of aerobic fibers and capillary length per fiber volume at a given mitochondrial volume density were similar in the two groups, i ndicating a proportional increase in capillarity and muscle aerobic ca pacity in W compared with S. Comparison of capillary numbers around ae robic fibers at a given mitochondrial volume per mu m length of fiber showed no difference between W and S groups nor with previous data in muscles with wide differences in fiber size and mitochondrial density such as rat soleus, bat muscles and hummingbird flight muscles. This s upported the notion of a tight correlation between capillary numbers a round individual fibers and mitochondrial volume per unit length of fi ber in aerobic muscles. It also supported the idea that it is the numb er of capillaries around the fibers rather than diffusion distance whi ch determines O-2 flux rates in highly aerobic muscles.