Brzek Paweł, Caviedes-Vidal Enrique, Hoefer Keeshia, Karasov William H
Department of Forest and Wildlife Ecology, University of Wisconsin, 1630 Linden Drive, Madison, Wisconsin 53706, USA.
Physiol Biochem Zool. 2010 May-Jun;83(3):501-11. doi: 10.1086/651098.
Size and hydrolytic activity of the gastrointestinal tracts of altricial birds undergo large and rapid changes during ontogeny. However, nothing is known about the development of the capacity of absorption of products of digestion, a factor that can limit total digestive performance. Using pharmacokinetic methods applied to wild-collected and laboratory-raised altricial nestlings of house sparrows (Passer domesticus), we addressed several questions of general significance about absorption in young birds. We found that both rate and efficiency of absorption of radiolabeled 3-O-methyl-D-glucose (3-OMD-glucose; absorbed by both transporter-mediated and nonmediated mechanisms) increased significantly between days 3 and 12 posthatch. We hypothesize that these changes can explain improvements in whole-diet digestion rate and efficiency observed in the young of house sparrows and of many other avian species, even after intestinal growth has ceased. We also tested the hypothesis that a high level of nonmediated, paracellular glucose absorption, as is typical in adult house sparrows, would already be observed in nestlings, and that their glucose absorption efficiency would not depend on glucose load because absorption rate is nonsaturable and is matched to substrate concentration. Using l-glucose (which is absorbed by nonmediated mechanism[s]), we found that, as predicted, paracellular absorption accounted for the majority of total absorption in nestlings of all ages, and starch content (0% vs. 25%) in the diet of laboratory-raised nestlings had no effect on efficiency of absorption of 3-OMD-glucose. Presumably, reliance on nonmediated absorption in young sparrows can save energy for growth. Also, during the transition from an almost starch-free, insect-based diet during the first days posthatch to the starch-rich, seed-based diet that is typical of adults, reliance on passive absorption is advantageous because the rate of absorption can easily match the current carbohydrate level in the intestines and the activity of hydrolytic enzymes.
晚成雏鸟类胃肠道的大小和水解活性在个体发育过程中会经历巨大且快速的变化。然而，对于消化产物吸收能力的发育情况却一无所知，而这一因素可能会限制整体消化性能。我们运用药代动力学方法，对野外采集和实验室饲养的家麻雀（Passer domesticus）晚成雏幼鸟进行研究，探讨了有关幼鸟吸收的几个具有普遍意义的问题。我们发现，放射性标记的3 - O - 甲基 - D - 葡萄糖（3 - OMD - 葡萄糖；通过转运体介导和非介导机制吸收）的吸收速率和效率在孵化后第3天到第12天之间显著增加。我们推测，这些变化可以解释在家麻雀以及许多其他鸟类幼鸟中观察到的全日粮消化速率和效率的提高，即使肠道生长已经停止。我们还检验了这样一个假设，即像成年家麻雀那样高水平的非介导性、细胞旁葡萄糖吸收在雏鸟中已经存在，并且它们的葡萄糖吸收效率不依赖于葡萄糖负荷，因为吸收速率是非饱和的，且与底物浓度相匹配。使用L - 葡萄糖（通过非介导机制吸收），我们发现，正如预期的那样，细胞旁吸收在所有年龄段的雏鸟中占总吸收的大部分，并且实验室饲养雏鸟日粮中的淀粉含量（0%对25%）对3 - OMD - 葡萄糖的吸收效率没有影响。据推测，幼麻雀依赖非介导吸收可以节省生长所需的能量。此外，在从孵化后最初几天几乎不含淀粉的昆虫性日粮向成年鸟类典型的富含淀粉的种子性日粮转变过程中，依赖被动吸收是有利的，因为吸收速率可以轻松地与肠道中当前的碳水化合物水平以及水解酶的活性相匹配。
