Enzymatic flux capacities in hummingbird flight muscles: a “one size fits all” hypothesis
Abstract:Hummingbirds (family Trochilidae) are among the smallest endothermic vertebrates representing an extreme, among birds, in their physiological design. They are unique in their ability to sustain hovering flight, one of the most energetically demanding forms of locomotion. Given that hovering metabolic rate (HMR) in hummingbirds scales allometrically as M 0.78 (M is mass), we tested the hypothesis that variation in HMR may be correlated with variation in maximal enzyme activities (V max values) of key enzymes in glucose and fatty acid oxidation pathways in the flight muscles of four species of hummingbirds ranging in body mass from 4 to 20 g. We also estimated metabolic flux rates from respirometric data obtained during hovering flight. The data are striking in the lack of correlation between V max values and flux rates at most steps in energy metabolism, particularly at the hexokinase and carnitine palmitoyltransferase reactions. In the context of hierarchical regulation analysis, this finding suggests that metabolic regulation (resulting from variation in substrate, product, or allosteric regulator concentrations) dominates as the proximate explanation for the interspecific variation in flux. On the other hand, we found no evidence of hierarchical regulation of flux, which results from variation in V max and is based on variation in enzyme concentration [E]. The evolutionary conservation of pathways of energy metabolism suggests that “one size fits all” among hummingbirds.
Document Type: Research Article
Affiliations: 1: Department of Integrative Biology, University of California, Berkeley, and Museum of Vertebrate Zoology, University of California, Berkeley, CA 94720, USA. 2: Center for Advanced Studies in Ecology and Biodiversity, Departamento de Ecología, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago 651-3677, Chile. 3: Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106-9610, USA.
Publication date: October 16, 2011
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