Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 84990 Midreshet Ben-Gurion, Israel.
Am J Physiol Regul Integr Comp Physiol. 2012 Sep 1;303(5):R551-61. doi: 10.1152/ajpregu.00076.2012. Epub 2012 Jul 11.
Many wild birds fast during reproduction, molting, migration, or because of limited food availability. Species that are adapted to fasting sequentially oxidize endogenous fuels in three discrete phases. We hypothesized that species not adapted to long fasts have truncated, but otherwise similar, phases of fasting, sequential changes in fuel oxidization, and similar changes in blood metabolites to fasting-adapted species. We tested salient predictions in house sparrows (Passer domesticus biblicus), a subspecies that is unable to tolerate more than ~32 h of fasting. Our main hypothesis was that fasting sparrows sequentially oxidize substrates in the order carbohydrates, lipids, and protein. We dosed 24 house sparrows with [(13)C]glucose, palmitic acid, or glycine and measured (13)CO(2) in their breath while they fasted for 24 h. To ascertain whether blood metabolite levels reflect fasting-induced changes in metabolic fuels, we also measured glucose, triacylglycerides, and β-hydroxybutyrate in the birds' blood. The results of both breath (13)CO(2) and plasma metabolite analyses did not support our hypothesis; i.e., that sparrows have the same metabolic responses characteristic of fasting-adapted species, but on a shorter time scale. Contrary to our main prediction, we found that recently assimilated (13)C-tracers were oxidized continuously in different patterns with no definite peaks corresponding to the three phases of fasting and also that changes in plasma metabolite levels accurately tracked the changes found by breath analysis. Notably, the rate of recently assimilated [(13)C]glycine oxidization was significantly higher (P < 0.001) than that of the other metabolic tracers at all postdosing intervals. We conclude that the inability of house sparrows to fast for longer than 32 h is likely related to their inability to accrue large lipid stores, separately oxidize different fuels, and/or spare protein during fasting.
许多野生鸟类在繁殖、换羽、迁徙或由于食物供应有限时会禁食。适应禁食的物种会按顺序经历三个离散阶段氧化内源性燃料。我们假设那些不适应长时间禁食的物种尽管阶段较短，但仍具有类似的禁食阶段、燃料氧化的顺序变化以及与禁食适应物种相似的血液代谢物变化。我们在家麻雀（Passer domesticus biblicus）中检验了一些重要的预测，家麻雀是一个亚种，不能耐受超过约 32 小时的禁食。我们的主要假设是禁食的麻雀会按碳水化合物、脂肪和蛋白质的顺序依次氧化底物。我们给 24 只家麻雀注射了 [(13)C]葡萄糖、棕榈酸或甘氨酸，并在它们禁食 24 小时时测量它们呼出的 (13)CO2。为了确定血液代谢物水平是否反映了禁食引起的代谢燃料变化，我们还测量了鸟血液中的葡萄糖、三酰甘油和 β-羟丁酸。呼吸 [(13)CO2]和血浆代谢物分析的结果都不支持我们的假设，即麻雀具有与禁食适应物种相同的代谢反应，但时间较短。与我们的主要预测相反，我们发现最近同化的 [(13)C]示踪剂以不同的模式连续氧化，没有与禁食的三个阶段相对应的明确峰值，而且血浆代谢物水平的变化准确地跟踪了呼吸分析中发现的变化。值得注意的是，最近同化的 [(13)C]甘氨酸氧化速率明显高于其他代谢示踪剂（P < 0.001），在所有给药后间隔时间内都是如此。我们得出结论，家麻雀不能禁食超过 32 小时的原因可能与它们不能积累大量脂肪储备、分别氧化不同的燃料以及/或在禁食期间节省蛋白质有关。
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