Songbirds meet the extreme metabolic demands of migration by burning both stored fat and protein. However, catabolizing these endogenous tissues for energy leads to organ atrophy, and reductions in gastrointestinal tissue can be as great as 50% of the pre-flight mass. Remarkably, during stopover refuelling birds quickly regain digestive mass and performance. Aminopeptidase-N (APN) is a brush-border enzyme responsible for late-stage protein digestion and may critically assist tissue reconstruction during the stopover, thus compensating for reduced gut size. We hypothesized that birds recovering from a fast would differentially upregulate APN activity relative to disaccharidases to rapidly process and assimilate dietary protein into lean mass. We fasted 23 wild-caught migratory white-throated sparrows (Zonotrichia albicollis) for 48 h to mimic mass reductions experienced during migratory flight and measured intestinal APN activity before the fast, immediately after the fast, and during recovery at 24 h and 48 h post-fast. Total fat mass, lean mass and basal metabolic rate were measured daily. We show that fasted birds maintain APN activity through the fast, despite a 30% reduction in intestine mass, but during refuelling, APN activity increases nearly twofold over pre-fasted individuals. This suggests that dynamically regulating APN may be necessary for rapid protein reconstruction during the stopover.

中文翻译：

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氨基肽酶-N 调节有助于白喉麻雀加油期间的瘦体重合成代谢

鸣禽通过燃烧储存的脂肪和蛋白质来满足迁徙的极端代谢需求。然而，将这些内源性组织分解代谢以获取能量会导致器官萎缩，胃肠道组织的减少可能高达飞行前质量的 50%。值得注意的是，在中途停留加油期间，鸟类迅速恢复消化质量和性能。氨基肽酶-N (APN) 是一种负责后期蛋白质消化的刷状缘酶，可在中途停留期间关键地协助组织重建，从而补偿肠道大小的减少。我们假设从禁食中恢复的鸟类会相对于双糖酶差异上调 APN 活性，以快速将日粮蛋白质加工和同化为瘦肉。我们禁食 23 只野生捕捞的迁徙白喉麻雀 (Zonotrichia albicollis) 48 小时，以模拟迁徙飞行过程中的体重减少，并在禁食前、禁食后立即以及在 24 小时和 48 小时后恢复期间测量肠道 APN 活性-快速地。每天测量总脂肪量、瘦体重和基础代谢率。我们表明，禁食的鸟类在禁食期间保持 APN 活性，尽管肠道质量减少了 30%，但在加油期间，APN 活性比禁食前的个体增加了近两倍。这表明动态调节 APN 对于中途停留期间的快速蛋白质重建可能是必要的。每天测量总脂肪量、瘦体重和基础代谢率。我们表明，禁食的鸟类在禁食期间保持 APN 活性，尽管肠道质量减少了 30%，但在加油期间，APN 活性比禁食前的个体增加了近两倍。这表明动态调节 APN 对于中途停留期间的快速蛋白质重建可能是必要的。每天测量总脂肪量、瘦体重和基础代谢率。我们表明，禁食的鸟类在禁食期间保持 APN 活性，尽管肠道质量减少了 30%，但在加油期间，APN 活性比禁食前的个体增加了近两倍。这表明动态调节 APN 对于中途停留期间的快速蛋白质重建可能是必要的。