Increased capillarity in leg muscle of finches living at altitude.
An increased ratio of muscle capillary to fiber number (capillary/fiber number) at altitude has been found in only a few investigations. The highly aerobic pectoralis muscle of finches living at 4,000-m altitude (Leucosticte arctoa; A) was recently shown to have a larger capillary/fiber number and greater contribution of tortuosity and branching to total capillary length than sea-level finches (Carpodacus mexicanus; SL) of the same subfamily (O. Mathieu-Costello, P. J. Agey, L. Wu, J. M. Szewczak, and R. E. MacMillen. Respir. Physiol. 111: 189-199, 1998). To evaluate the role of muscle aerobic capacity on this trait, we examined the less-aerobic leg muscle (deep portion of anterior thigh) in the same birds. We found that, similar to pectoralis, the leg muscle in A finches had a greater capillary/fiber number (1.42 +/- 0.06) than that in SL finches (0.77 +/- 0.05; P < 0.01), but capillary tortuosity and branching were not different. As also found in pectoralis, the resulting larger capillary/fiber surface in A finches was proportional to a greater mitochondrial volume per micrometer of fiber length compared with that in SL finches. These observations, in conjunction with a trend to a greater (rather than smaller) fiber cross-sectional area in A than in SL finches (A: 484 +/- 42, SL: 390 +/- 26 micrometer2, both values at 2.5-micrometer sarcomere length; P = 0.093), support the notion that chronic hypoxia is also a condition in which capillary-to-fiber structure is organized to match the size of the muscle capillary-to-fiber interface to fiber mitochondrial volume rather than to minimize intercapillary O2 diffusion distances.
Hepple
R T
RT
Department of Medicine, University of California, San Diego, La Jolla 92093-0623, California, USA.
Agey
P J
PJ
Hazelwood
L
L
Szewczak
J M
JM
MacMillen
R E
RE
Mathieu-Costello
O
O
D000531:Altitude; D000818:Animals; D001835:Body Weight; D002196:Capillaries / Q000502:physiology / Q000648:ultrastructure; D005260:Female; D007866:Leg / Q000098:blood supply*; D008931:Mitochondria, Muscle / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure; D018485:Muscle Fibers, Skeletal / Q000502:physiology / Q000648:ultrastructure; D018482:Muscle, Skeletal / Q000098:blood supply* / Q000166:cytology / Q000648:ultrastructure; D010101:Oxygen Consumption / Q000502:physiology; D012039:Regional Blood Flow / Q000502:physiology; D020308:Songbirds / Q000502:physiology*