title: Glucose transporter expression in an avian nectarivore: the ruby-throated hummingbird (Archilochus colubris).
abstract: Glucose transporter (GLUT) proteins play a key role in the transport of monosaccharides across cellular membranes, and thus, blood sugar regulation and tissue metabolism. Patterns of GLUT expression, including the insulin-responsive GLUT4, have been well characterized in mammals. However, relatively little is known about patterns of GLUT expression in birds with existing data limited to the granivorous or herbivorous chicken, duck and sparrow. The smallest avian taxa, hummingbirds, exhibit some of the highest fasted and fed blood glucose levels and display an unusual ability to switch rapidly and completely between endogenous fat and exogenous sugar to fuel energetically expensive hovering flight. Despite this, nothing is known about the GLUT transporters that enable observed rapid rates of carbohydrate flux. We examined GLUT (GLUT1, 2, 3, & 4) expression in pectoralis, leg muscle, heart, liver, kidney, intestine and brain from both zebra finches (Taeniopygia guttata) and ruby-throated hummingbirds (Archilochus colubris). mRNA expression of all four transporters was probed using reverse-transcription PCR (RT-PCR). In addition, GLUT1 and 4 protein expression were assayed by western blot and immunostaining. Patterns of RNA and protein expression of GLUT1-3 in both species agree closely with published reports from other birds and mammals. As in other birds, and unlike in mammals, we did not detect GLUT4. A lack of GLUT4 correlates with hyperglycemia and an uncoupling of exercise intensity and relative oxidation of carbohydrates in hummingbirds. The function of GLUTs present in hummingbird muscle tissue (e.g. GLUT1 and 3) remain undescribed. Thus, further work is necessary to determine if high capillary density, and thus surface area across which cellular-mediated transport of sugars into active tissues (e.g. muscle) occurs, rather than taxon-specific differences in GLUT density or kinetics, can account for observed rapid rates of sugar flux into these tissues.
authors.affiliation: Department of Biological Sciences, University of Toronto Scarborough, Toronto, Ontario, Canada.
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Welch
authors.forename: Amina
authors.initials: A
authors.lastname: Allalou
authors.forename: Prateek
authors.initials: P
authors.lastname: Sehgal
authors.forename: Jason
authors.initials: J
authors.lastname: Cheng
authors.forename: Aarthi
authors.initials: A
authors.lastname: Ashok
mesh_terms: D000818:Animals; D001483:Base Sequence; D001717:Birds / Q000235:genetics*; D018076:DNA, Complementary / Q000235:genetics; D005247:Feeding Behavior; D020869:Gene Expression Profiling; D005786:Gene Expression Regulation; D051246:Glucose Transport Proteins, Facilitative / Q000235:genetics* / Q000378:metabolism; D007150:Immunohistochemistry; D009928:Organ Specificity / Q000235:genetics; D057048:Plant Nectar / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid; D013194:Staining and Labeling
chemical_list: D018076:DNA, Complementary; D051246:Glucose Transport Proteins, Facilitative; D057048:Plant Nectar; D012333:RNA, Messenger