Glucose transporter expression and regulation following a fast in the ruby-throated hummingbird, Archilochus colubris

Hummingbirds, subsisting almost exclusively on nectar sugar, face extreme
challenges to blood sugar regulation. The capacity for transmembrane sugar
transport is mediated by the activity of facilitative glucose transporters
(GLUTs) and their localisation to the plasma membrane (PM). In this study,
we determined the relative protein abundance of GLUT1, GLUT2, GLUT3, and
GLUT5 via immunoblot using custom antibodies in whole-tissue and PM
fractions of flight-muscle, heart, and liver of ruby-throated hummingbirds
(Archilochus colubris). GLUTs examined were detected in nearly all tissues
tested. Hepatic GLUT1 was minimally present in whole-tissue and absent in
PM fractions. GLUT5 was expressed in flight-muscles at levels comparable
to that of their liver, consistent with hummingbird flight-muscles’
hypothesised uniquely high fructose-uptake and oxidation capacity. To
assess GLUT regulation, we fed ruby-throated hummingbirds 1M sucrose ad
libitum for 24 hours followed by either 1 hour of fasting or continued
feeding until sampling. We measured relative GLUT abundance and
concentrations of circulating sugars. Blood fructose concentration in
fasted hummingbirds declined (∼5 mM to ∼0.18 mM), while
fructose-transporting GLUT2 and GLUT5 abundance did not change in PM
fractions. Blood glucose concentrations remained elevated in fed and
fasted hummingbirds (∼30 mM), while glucose-transporting GLUT1 and GLUT3
in flight muscle and liver PM fractions, respectively, declined in fasted
birds. Our results suggest that glucose uptake capacity is dynamically
reduced in response to fasting, allowing for maintenance of elevated blood
glucose levels, while fructose uptake capacity remains constitutively
elevated promoting depletion of blood total fructose within the first hour
of a fast.