Evidence of high transport and phosphorylation capacity for both glucose and fructose in the ruby-throated hummingbird (Archilochus colubris)

Hummingbirds are able to fuel hovering flight entirely with recently ingested glucose or fructose. Among vertebrates, several steps of sugar flux from circulation to skeletal muscle are potentially rate-limiting, including transport into muscle and subsequent phosphorylation. While capacities for glucose flux are substantial, capacities for fructose flux are comparatively low. The mechanisms underlying apparent high rates of glucose and fructose oxidation in hummingbird flight muscle remain unclear. We examined relative expression of facilitative sugar transporters (GLUTs) and enzymes of fructolysis in ruby-throated hummingbird (Archilochus colubris) tissues involved in energy homeostasis and flight, via qPCR and measured hexokinase activity in pectoralis in vitro. We hypothesized that expression of these genes was upregulated in hummingbird flight muscle compared to other vertebrates. We found that hummingbird pectoralis had high relative transcript abundance of GLUT1 and GLUT5 compared to expression profiles of other vertebrates. In particular, GLUT5 expression in pectoralis was similar to that of intestine. We demonstrated minimal relative densities of fructolytic enzymes in pectoralis, suggesting that the ketohexokinase pathway does not rapidly metabolize fructose in these muscles. Instead, we found that the capacity for phosphorylation of either glucose or fructose by hexokinase is very high in pectoralis in vitro. The contributions of individual hexokinase isoforms remain to be determined. Our results further characterize the strategies by which hummingbirds, and perhaps other nectarivores, accomplish rapid sugar flux. High transport and sugar phosphorylation capacities appear to exist in flight muscle, though the enzymatic pathways that catalyze the phosphorylation of sugar in muscle remain uncertain.