Hummingbirds may fuel hovering with both glucose and fructose [with video]

Unlike most vertebrates, hummingbird flight muscle expresses transcripts that encode proteins that enable high transport and oxidation capacity for not just glucose, but fructose as well. This suggests the cellular physiology of hummingbird muscles is specialized to efficiently extract energy from their unusual fructose-rich diet.

HFSP Program Grant holder Kenneth Welch and colleagues
authored on Fri, 28 September 2018

Almost all vertebrates typically oxidize a mixture of fat and carbohydrate to power muscles and thus activity. Most of these muscles are particularly reliant on intramuscular glycogen, and the glucose produced from its breakdown, especially as exercise intensity increases. Despite its increasing abundance in the western diet, most vertebrate muscles are not good at using fructose, and most ingested fructose is instead metabolized in tissues like the liver. Unlike most vertebrates, hummingbirds feed on sugar rich floral nectar which contains roughly equal proportions of the simple sugars glucose and fructose. By tracking the oxidation of isotopically labeled sugars in freely hover-feeding hummingbirds, our research program has previously shown that these birds can oxidize fructose just as fast as they can glucose, raising the question of whether their muscles are able to uptake and metabolize fructose directly, as effectively as they do glucose.

We designed custom primers and used quantitative polymerase chain reaction to quantify RNA transcripts of key sugar transporters that controlled the movement of sugars among hummingbird tissues. We found that glucose transporter 5 (GLUT5: a fructose-specific transporter) transcript was unusually abundant in hummingbird flight muscle and heart compared to other vertebrates. Glucose transporter 1 (GLUT1: a glucose-specific transporter) transcript was also abundant in flight muscle and heart. Further, GLUT5 was not abundant in the much less metabolically active hummingbird leg muscles. These findings suggest that hummingbird muscle and heart may have the capacity to rapidly take up both circulating glucose and, uniquely, fructose if transcript abundance reflects protein abundance in these tissues.

Using the above technique and western blotting, we found that transcript and protein for aldolase B, the rate-limiting enzyme of fructolysis, the pathway by which most dietary fructose is phosphorylated in the liver in other vertebrates, was only found in low quantities in flight muscle and heart. This suggested that, if fructose is rapidly taken up by these tissues, it is phosphorylated by a pathway distinct from liver fructolysis.

Lastly, we performed enzyme assays to measure the capacity for glucose and fructose phosphorylation in hummingbird flight muscle. We confirmed that the capacity for glucose phosphorylation was higher than in any other vertebrate muscles thus far examined. The capacity for fructose phosphorylation, while only roughly 50% that for glucose, was nonetheless much higher than that found in any other vertebrate muscles.  Together our results suggest that hummingbirds’ flight muscle has the capacity to rapidly uptake and phosphorylate both dietary glucose and fructose, possibly enabling them to more efficiently extract energy from their unusually fructose-rich diet than animals that do not specialize on such diets.

Reference

Evidence of high transport and phosphorylation capacity for both glucose and fructose in the ruby-throated hummingbird (Archilochus colubris). Myrka, A. M., Welch Jr., K. C. 2018. Comparative Biochemistry and Physiology, Part B 224: 253-261.

PubMed link