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.

红玉喉北蜂鸟几乎完全依赖花蜜糖存活，其血糖调节面临极端挑战。跨膜糖转运能力由易化葡萄糖转运蛋白（facilitative glucose transporters, GLUTs）的活性及其在质膜（plasma membrane, PM）上的定位所介导。本研究通过定制抗体开展免疫印迹实验，测定了红玉喉北蜂鸟（Archilochus colubris）飞行肌、心脏与肝脏的全组织及质膜组分中GLUT1、GLUT2、GLUT3和GLUT5的相对蛋白丰度。所检测的GLUTs在几乎所有受试组织中均有检出。肝脏全组织内GLUT1仅微量存在，而在质膜组分中未检出。GLUT5在飞行肌中的表达水平与肝脏相当，这与蜂鸟飞行肌被推测的极高果糖摄取与氧化能力相符。为评估GLUT的调控机制，我们让红玉喉北蜂鸟自由采食1M蔗糖溶液24小时，随后将其分为两组：一组禁食1小时，另一组持续喂食直至采样。我们测定了各组的GLUT相对丰度及循环血糖浓度。结果显示，禁食蜂鸟的血液果糖浓度出现下降（约5 mM降至约0.18 mM），但质膜组分中负责转运果糖的GLUT2与GLUT5丰度未发生变化。喂食组与禁食组蜂鸟的血糖浓度均维持在较高水平（约30 mM），而分别在飞行肌和肝脏的质膜组分中，负责转运葡萄糖的GLUT1与GLUT3在禁食蜂鸟体内的丰度出现下调。本研究结果表明，禁食状态下葡萄糖摄取能力会发生动态降低，从而维持较高的血糖水平；而果糖摄取能力则始终维持在组成型高表达状态，促使禁食最初1小时内血液总果糖被快速消耗。