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Journal of Comparative Physiology B

, Volume 184, Issue 8, pp 945–959 | Cite as

Sugar flux through the flight muscles of hovering vertebrate nectarivores: a review

  • Kenneth C. WelchJr.Email author
  • Chris C. W. Chen
Review

Abstract

In most vertebrates, uptake and oxidation of circulating sugars by locomotor muscles rises with increasing exercise intensity. However, uptake rate by muscle plateaus at moderate aerobic exercise intensities and intracellular fuels dominate at oxygen consumption rates of 50 % of maximum or more. Further, uptake and oxidation of circulating fructose by muscle is negligible. In contrast, hummingbirds and nectar bats are capable of fueling expensive hovering flight exclusively, or nearly completely, with dietary sugar. In addition, hummingbirds and nectar bats appear capable of fueling hovering flight completely with fructose. Three crucial steps are believed to be rate limiting to muscle uptake of circulating glucose or fructose in vertebrates: (1) delivery to muscle; (2) transport into muscle through glucose transporter proteins (GLUTs); and (3) phosphorylation of glucose by hexokinase (HK) within the muscle. In this review, we summarize what is known about the functional upregulation of exogenous sugar flux at each of these steps in hummingbirds and nectar bats. High cardiac output, capillary density, and blood sugar levels in hummingbirds and bats enhance sugar delivery to muscles (step 1). Hummingbird and nectar bat flight muscle fibers have relatively small cross-sectional areas and thus relatively high surface areas across which transport can occur (step 2). Maximum HK activities in each species are enough for carbohydrate flux through glycolysis to satisfy 100 % of hovering oxidative demand (step 3). However, qualitative patterns of GLUT expression in the muscle (step 2) raise more questions than they answer regarding sugar transport in hummingbirds and suggest major differences in the regulation of sugar flux compared to nectar bats. Behavioral and physiological similarities among hummingbirds, nectar bats, and other vertebrates suggest enhanced capacities for exogenous fuel use during exercise may be more wide spread than previously appreciated. Further, how the capacity for uptake and phosphorylation of circulating fructose is enhanced remains a tantalizing unknown.

Keywords

Glucose Fructose Hummingbird Bat Fuel use Sugar homeostasis 

Notes

Acknowledgments

Much of the work summarized in this review was supported by grants from the National Sciences Foundation, UC MEXUS-CONACYT, and the Natural Sciences and Engineering Research Council of Canada. The authors thank D. Groom, A. Myrka, R. K. Suarez, and B. Velten for comments on a draft of this work.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Department of Biological SciencesUniversity of Toronto ScarboroughTorontoCanada
  2. 2.Department of Cell and Systems BiologyUniversity of TorontoTorontoCanada
  3. 3.School of Kinesiology and Health ScienceYork UniversityTorontoCanada

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