HIGHLIGHTED TOPICS
Commentary

Journal of Applied Physiology

	ARTICLE

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For years, a large part of our evolution has involved the development of metabolic pathways to deal with oxygen as the final electron acceptor for energy transfer. From this viewpoint, it is not surprising that aerobic capacity appears to be central in the divide between health and disease. Aerobic capacity can be operationally divided into two kinds: 1) intrinsic and 2) that accrued as a result of adaptation. Aerobic capacity is a complex trait in the sense of being determined by both genetic and environmental factors. The inherent complexity of traits such as aerobic capacity enhances the importance of adopting a unified conceptual approach for which genetic models are a critical component. In the first Highlighted Topics article featured in this issue of the Journal of Applied Physiology, "Variation and heritability for the adaptational response to exercise in genetically heterogeneous rats," Troxell et al. define substrates that are suitable for the initiation of artificial selection for the adaptational response to exercise training in rats. Models developed by artificial selection are ideal for three reasons. First, the low- and high-line traits can be made to differ substantially, which increases the signal measurements. Second, if the coefficient of inbreeding is kept low, the contrasting alleles causative of trait difference will be concentrated in the divergent lines. Third, selection across many generations interprets into selection for lack of sensitivity to subtle differences in environment. This can be of large benefit because inbred strains that differ markedly for a trait, which did not originate from selection, often demonstrate wide trait variation in response to similar environments. Development of contrasting genetic models requires long-term effort but delivers substrate for resolving function at all levels of organization.

The second article featured in this issue, "Skeletal muscle capillarity and enzyme activity in rats selectively bred for running endurance," by Howlett et al., utilizes the technique of artificial selection to explore differences in running endurance capacity created by divergent artificial selection. It is important to stress that this approach does not involve exercise training but allows selective breeding of those rats found to naturally exhibit high or low phenotypes for exercise capacity. These investigators then examined the structural and functional elements that explain the differences in exercise capacity, an approach that can lead to identification of the responsible genes. The present study complements an earlier report (1) in which the oxygen transport chain was functionally dissected into contributions from the lungs, heart, blood, and muscle. What appeared to separate the high exercise capacity rats from those exhibiting low capacity was enhanced oxygen transport from the blood to the mitochondria within muscle. Thus there was little or no difference in pulmonary, cardiac, or hematological function between groups. The present study extends these previous observations by examining muscle structure and metabolic capacity in the same rats. There was a strong (r = 0.92) correlation between muscle oxygen transport conductance and muscle capillary density across subjects, with the high-capacity runners showing 27% smaller total muscle mass (gastrocnemius) and 20% lower individual muscle fiber area (but the same proportion of type I fibers and the same total number of fibers). The total number of capillaries was also not different between groups. Oxidative but not glycolytic enzyme activities per unit muscle mass were greater in the high-capacity runners relative to those of the low. These results suggest that the structural and functional differences between animals selected for running endurance capacity occur in the skeletal muscles and involve complex differences in contractile proteins, oxidative enzymes, and vascular supply.

	FOOTNOTES

10.1152/japplphysiol.00055.2003

	REFERENCES

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1.   Henderson, KW, Wagner H, Favret F, Britton SL, Koch LG, Wagner PD, and Gonzalez NC. Determinants of maximal O₂ uptake in rats selectively bred for endurance running capacity. J Appl Physiol 93: 1265-1274, 2002[Abstract/Free Full Text].