PMID- 25031038 OWN - NLM STAT- MEDLINE DCOM- 20150713 LR - 20211021 IS - 1432-136X (Electronic) IS - 0174-1578 (Linking) VI - 184 IP - 8 DP - 2014 Dec TI - Sugar flux through the flight muscles of hovering vertebrate nectarivores: a review. PG - 945-59 LID - 10.1007/s00360-014-0843-y [doi] AB - 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. FAU - Welch, Kenneth C Jr AU - Welch KC Jr AD - Department of Biological Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada, kwelch@utsc.utoronto.ca. FAU - Chen, Chris C W AU - Chen CC LA - eng PT - Journal Article PT - Research Support, Non-U.S. Gov't PT - Review DEP - 20140717 PL - Germany TA - J Comp Physiol B JT - Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology JID - 8413200 RN - 0 (Glucose Transport Proteins, Facilitative) RN - 30237-26-4 (Fructose) RN - IY9XDZ35W2 (Glucose) SB - IM MH - Animals MH - Birds/metabolism/*physiology MH - Chiroptera/metabolism/*physiology MH - Energy Metabolism/*physiology MH - Flight, Animal/*physiology MH - Fructose/metabolism MH - Glucose/*metabolism MH - Glucose Transport Proteins, Facilitative/metabolism MH - *Models, Biological MH - Muscle, Skeletal/metabolism/*physiology MH - Oxidation-Reduction MH - Oxygen Consumption/physiology MH - Phosphorylation MH - Physical Exertion/physiology MH - Species Specificity EDAT- 2014/07/18 06:00 MHDA- 2015/07/15 06:00 CRDT- 2014/07/18 06:00 PHST- 2014/04/22 00:00 [received] PHST- 2014/06/20 00:00 [accepted] PHST- 2014/06/15 00:00 [revised] PHST- 2014/07/18 06:00 [entrez] PHST- 2014/07/18 06:00 [pubmed] PHST- 2015/07/15 06:00 [medline] AID - 10.1007/s00360-014-0843-y [doi] PST - ppublish SO - J Comp Physiol B. 2014 Dec;184(8):945-59. doi: 10.1007/s00360-014-0843-y. Epub 2014 Jul 17.