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Ali RS, Dick MF, Muhammad S, Sarver D, Hou L, Wong GW, Welch KC. Glucose transporter expression and regulation following a fast in the ruby-throated hummingbird, Archilochus colubris. J Exp Biol 2020; 223:jeb229989. [PMID: 32895327 PMCID: PMC10668337 DOI: 10.1242/jeb.229989] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2020] [Accepted: 09/01/2020] [Indexed: 12/18/2022]
Abstract
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-designed antibodies in whole-tissue homogenates and PM fractions of flight muscle, heart and liver of ruby-throated hummingbirds (Archilochus colubris). The GLUTs examined were detected in nearly all tissues tested. Hepatic GLUT1 was minimally present in whole-tissue homogenates and absent win PM fractions. GLUT5 was expressed in flight muscles at levels comparable to those of the liver, consistent with the hypothesised uniquely high fructose uptake and oxidation capacity of hummingbird flight muscles. To assess GLUT regulation, we fed ruby-throated hummingbirds 1 mol l-1 sucrose ad libitum for 24 h followed by either 1 h of fasting or continued feeding until sampling. We measured relative GLUT abundance and concentration of circulating sugars. Blood fructose concentration in fasted hummingbirds declined (∼5 mmol l-1 to ∼0.18 mmol l-1), while fructose-transporting GLUT2 and GLUT5 abundance did not change in PM fractions. Blood glucose concentrations remained elevated in fed and fasted hummingbirds (∼30 mmol l-1), 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.
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Affiliation(s)
- Raafay S Ali
- Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, ON, Canada M5S 3G5
- Department of Biological Sciences, University of Toronto Scarborough Campus, 1265 Military Trail, Toronto, ON, Canada M1C 1A4
| | - Morag F Dick
- Department of Biological Sciences, University of Toronto Scarborough Campus, 1265 Military Trail, Toronto, ON, Canada M1C 1A4
| | - Saad Muhammad
- Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, ON, Canada M5S 3G5
- Department of Biological Sciences, University of Toronto Scarborough Campus, 1265 Military Trail, Toronto, ON, Canada M1C 1A4
| | - Dylan Sarver
- Department of Physiology and Center for Metabolism and Obesity Research, School of Medicine, The Johns Hopkins University, Baltimore, MD 21205, USA
| | - Lily Hou
- Department of Biological Sciences, University of Toronto Scarborough Campus, 1265 Military Trail, Toronto, ON, Canada M1C 1A4
| | - G William Wong
- Department of Physiology and Center for Metabolism and Obesity Research, School of Medicine, The Johns Hopkins University, Baltimore, MD 21205, USA
| | - Kenneth C Welch
- Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, ON, Canada M5S 3G5
- Department of Biological Sciences, University of Toronto Scarborough Campus, 1265 Military Trail, Toronto, ON, Canada M1C 1A4
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Ban Y, Rizzolo LJ. Regulation of glucose transporters during development of the retinal pigment epithelium. BRAIN RESEARCH. DEVELOPMENTAL BRAIN RESEARCH 2000; 121:89-95. [PMID: 10837896 DOI: 10.1016/s0165-3806(00)00028-6] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
The retinal pigment epithelium (RPE) separates the outer retina from its blood supply. To satisfy the retina's large requirement for glucose, the RPE expresses high levels of glucose transporters. In most rat cells, the transporter GLUT3 provides a basal level of transport, but the expression of GLUT1 can be regulated. The opposite is true in chicken (P. Wagstaff, H.Y. Kang, D. Mylott, P.J. Robbins, M.K. White, Characterization of the avian GLUT1 glucose transporter: differential regulation of GLUT1 and GLUT3 in chicken embryo fibroblasts, Mol. Biol. Cell 6 (1995) 1575-1589). We examined chick RPE to determine which isoform is regulated during development, and if the neural retina regulates GLUT expression. By RT-PCR, RPE expressed GLUT1 and GLUT3, but not GLUT2. Only the level of GLUT1 increased between E5 and E18. A corresponding increase in GLUT1 protein was observed by immunoblotting. Most of the increase occurred between E14 and E18, which corresponds to the late stage of tight junction development. A culture model of development was used to examine the intermediate phase, which extends from E7 to E14. While medium conditioned by the neural retina decreased paracellular diffusion across the tight junctions, it increased diffusion through the glucose transporters. Unlike mammals, chick upregulates different isoforms in quiescent RPE and proliferating fibroblasts. Further, the upregulation of glucose transport is coordinated with the development of tight junctions in the blood-retinal barrier.
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Affiliation(s)
- Y Ban
- Departments of Surgery and Ophthalmology and Visual Science, Yale University School of Medicine, 333 Cedar Street, Box 208062, New Haven, CT 06520-8062 USA
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