Coping with a Watery Diet: Integration of Metabolic, Digestive, and Osmoregulatory Processes

维持水和电解质平衡对陆生动物来说是一个严峻的挑战。因此，陆生动物节约用水的策略已经得到了很好的研究。这一建议强调了陆地动物渗透调节的另一个研究较少的方面。与许多生活在干燥环境中的动物不同，以花蜜为食的动物必须应对过量的水分摄入。以花蜜为食的鸟类会根据花蜜中含糖量的变化来调节它们的食量。它们随着糖浓度的增加而减少摄取量。食蜜鸟类对糖浓度的摄入反应表明，在低至中等糖浓度下，鸟类会摄入大量的水。以花蜜为食的鸟类所表现出的饮水速度会导致其他陆生脊椎动物的病理后果。这个项目调查了食蜜的鸟类如何应对这种明显的多饮（水摄入过多）。蜂鸟可以通过两种非排他机制来应对自然的自发性多饮：1)它们可以避免在肠道中吸收水分，2)它们可以通过产生大量的尿液来排出摄入和吸收的水分。该项目的一个主要目标是确定这两种机制的相对重要性。因为鸟类在泄殖腔中混合了尿液和粪便，实现这一目标并不容易。设计了一种基于药代动力学数学模型的微创方法来测量不受约束的鸟类肠道吸收的摄入水的比例。初步数据表明，蜂鸟摄入的大部分水分在肠道中被吸收。因此，在这些鸟类中，处理摄入的水的任务似乎分配给了肾脏。肾脏过滤血浆的速度是一个很好的例子，它的大小是由许多潜在的竞争需求决定的。节约用水的需要和消除代谢副产物的需要可能是不协调的。在这个建议中，另一个潜在的冲突被确定：处理吸收的水的需要由高肾小球滤过率更好地满足，但这可能伴随着有价值的，尽管容易过滤的代谢物，如葡萄糖的损失。蜂鸟表现出极高的血浆葡萄糖浓度（比糖尿病人高几倍），并且可能表现出较高的肾小球滤过率，以排出吸收的饮食水。然而，这些鸟排泄物中的葡萄糖浓度极低。该项目的一个目标是阐明蜂鸟如何在面对高血浆葡萄糖水平和肾脏高过滤水平时设法避免排出葡萄糖。该项目意义重大，因为它将允许对鸟类在水负荷和水合作用条件下的肾脏功能进行综合定量描述。此外，了解水处理的限制将提供对动物如何设计的一般见解，设计方面如何约束动物的生态性能，以及一个生理系统的设计方面如何对其他系统施加限制。花蜜摄食鸟类的渗透调节过程强调了理解肠道中发生的事件对摄食行为、肾脏和代谢功能的影响的相关性。比较生理学家强调，低水可用性限制了能源和微生境的利用。我们的研究将强调水能相互作用的一个新方面，并填补我们对陆地动物渗透调节的理解空白。

The maintenance of water and electrolyte balance can pose a serious challenge for terrestrial animals. Consequently, the strategies used by terrestrial animals to conserve water have been well studied. This proposal emphasizes another, much less studied, aspect of osmoregulation in land animals. Unlike many animals living in dry environments, nectar feeding animals must cope with excessive water ingestion. Nectar-feeding birds respond to variation in the sugar concentration of floral nectar by modulating how much they eat. They decrease ingestion as sugar concentration increases. The intake response of nectar-feeding birds to sugar concentration has the consequence that at low to moderate sugar concentrations the birds ingest phenomenal amounts of water. The drinking rates exhibited by nectar-feeding birds would lead to pathological consequences in other terrestrial vertebrates. This project investigates how nectar-feeding birds cope with this apparent polydipsia (water over-ingestion). Hummingbirds can cope with natural voluntary polydipsia through two non-exclusive mechanisms: 1) They can avoid absorbing water in the intestine, and 2) they can get rid of ingested and absorbed water by producing copious urine. A primary objective of this project is to determine the relative importance of these two mechanisms. Because birds mix urine and feces in the cloaca, achieving this objective is not easy. A minimally invasive method that relies on a pharmacokinetic mathematical model was designed to measure the fraction of ingested water that is absorbed in the intestines of unrestrained birds. Preliminary data suggests that in hummingbirds most of the water ingested is absorbed in the intestine. Consequently, in these birds the task of disposing of ingested water seems to be assigned to the kidneys. The rate at which the kidney filters plasma is a good example of a trait whose magnitude is determined by many potentially competing demands. The need to conserve water and the need to eliminate metabolic by-products can be in discord. In this proposal another potential conflict is identified: The need to dispose of absorbed water is better served by high glomerular filtration rates, but these can be accompanied by the loss of valuable, albeit easily filterable, metabolites like glucose. Hummingbirds exhibit extremely high plasma glucose concentrations (several times higher than those of a diabetic human), and presumably show high glomerular filtration rates to get rid of absorbed dietary water. However, the concentration of glucose in the excreta of these birds is extremely low. A goal of this project is to elucidate how hummingbirds manage to avoid excreting glucose in the face of high plasma glucose levels and high levels of filtration by the kidney.The project is significant because it will allow development of an integrated quantitative description of kidney function under the range of water loads and hydration conditions that birds experience. Also, understanding the limits to water processing will provide general insights into how animals are designed, on how aspects of design constrain ecological performance of animals, and into how aspects of design in one physiological system can impose limits on other systems. The osmoregulatory processes of nectar feeding birds highlight the relevance of understanding the impact that events taking place in the gut can have for feeding behavior, and renal and metabolic function. Comparative physiologists have emphasized low water availability as a constraint on energy and microhabitat use. Our research will emphasize a novel aspect of the water-energy interaction and fill a gap in our understanding of osmoregulation in land animals.