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Peripheral oxygen transport and utilization in rats following continued selective breeding for endurance running capacity

¹University of California-San Diego, La Jolla, California; ²Union College, Schenectady, New York; ³University of Kansas Medical Center, Kansas City, Kansas; and ⁴University of Michigan, Ann Arbor, Michigan

Submitted 24 February 2007 ; accepted in final form 2 April 2008

Untrained rats selectively bred for either high (HCR) or low (LCR) treadmill running capacity previously demonstrated divergent physiological traits as early as the seventh generation (G7). We asked whether continued selective breeding to generation 15 (G15) would further increase the divergence in skeletal muscle capillarity, morphometry, and oxidative capacity seen previously at G7. At G15, mean body weight was significantly lower (P < 0.001) in the HCR rats (n = 11; 194 ± 3 g) than in LCR (n = 12; 259 ± 9 g) while relative medial gastrocnemius muscle mass was not different (0.23 ± 0.01 vs. 0.22 ± 0.01% total body weight). Normoxic (FI_O2 = 0.21) O_2max was 50% greater (P < 0.001) in HCR despite the lower absolute muscle mass, and skeletal muscle O₂ conductance (measured in hypoxia; FI_O2 = 0.10) was 49% higher in HCR (P < 0.001). Muscle oxidative enzyme activities were significantly higher in HCR (citrate synthase: 16.4 ± 0.4 vs. 14.0 ± 0.6; β-hydroxyacyl-CoA dehydrogenase: 5.2 ± 0.2 vs. 4.2 ± 0.2 mmol·kg^–1·min^–1). HCR rats had 36% more total muscle fibers and also 36% more capillaries in the medial gastrocnemius. Because average muscle fiber area was 35% smaller, capillary density was 36% higher in HCR, but capillary-to-fiber ratio was the same. Compared with G7, G15 HCR animals showed 38% greater total fiber number with an additional 25% decrease in mean fiber area. These data suggest that many of the skeletal muscle structural and functional adaptations enabling greater O₂ utilization in HCR at G7 continue to progress following additional selective breeding for endurance capacity. However, the largest changes at G15 relate to O₂ delivery to skeletal muscle and not to the capacity of skeletal muscle to use O₂.

oxidative capacity; diffusive conductance; capillarity

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				S. D. Kirkton, R. A. Howlett, N. C. Gonzalez, P. G. Giuliano, S. L. Britton, L. G. Koch, H. E. Wagner, and P. D. Wagner Continued artificial selection for running endurance in rats is associated with improved lung function J Appl Physiol, June 1, 2009; 106(6): 1810 - 1818. [Abstract] [Full Text] [PDF]