Novel driving forces for water transport & osmoregulation: carbonate precipitation and osmotic coefficients

包括人类在内的脊椎动物由大约70%的水组成。水的摄入和输出的平衡显然对健康至关重要，但大多数人没有意识到体内水分的巨大内部运动一直在进行。例如，肾脏、肠道和胰腺每天总共运输8倍于我们身体总水量的水分进出这些组织。因此，了解这些器官使用的机制，以及排列在这些器官上的细胞(称为上皮细胞)如何运行这种水运输是重要的。尽管这很重要，但水上运输机制仍然是许多争论的主题。话虽如此，50多年来，人们一直认为，水的净传输需要盐(特别是氯化钠或氯化钠)首先向一个特定方向传输，然后通过一种称为渗透的过程(第二次)驱动液体向同一方向传输。尽管达成了这一共识，但水通过上皮细胞运输的确切路线仍存在激烈争议。例如，它是否通过一种名为水通道蛋白的特殊蛋白质穿过细胞膜？或者它会挤在细胞之间？本提案的新颖性在于发现了两种新的影响水上运输的机制，它们在概念上与目前辩论的其他领域截然不同。这些想法挑战了既定的教条，不依赖于盐与水的运输方向相同，这代表了我们对水的理解的根本变化，并为水在动物上皮中的运输机制提供了一个新的模型。这一发现是通过研究海水鱼类如何饮用海水并通过肠道处理这些液体以避免脱水而得出的。就像人类喝普通液体一样，这些动物首先将氯化钠从肠道输送到血液中，然后是水，然后是渗透作用。然而，海鱼还有另一个诀窍，可以最大限度地提高它们的取水能力。它们分泌一种不同的化合物，称为小苏打(与小苏打中发现的相同)进入肠道，方向与水吸收相反。这会在被吞噬的海水中引起化学反应，导致海水中所含的高水平钙以固体的白色碳酸钙(如石灰石)的形式沉淀。这些“肠道结石”最终会被排泄出来，但对鱼类的好处是减少了肠液中的总溶解化合物，这反过来又更容易将水分提取到血液中。我们建议通过使用三种不同的海鱼(比目鱼、罗非鱼和鲑鱼)来进一步研究这一新的过程，这三种海鱼产生的碳酸氢盐数量非常不同，因此被预测有不同的吸水效率。寒冷和高压也会抑制降水，所以我们将比较鱼在低温和高压下(在大气压下)的吸水率。碳酸盐的沉淀存在于人类疾病中，如肾结石和胰腺结石，因此在鱼类身上研究这一过程可能有助于我们了解这种病理情况。鱼类使用的第二种新工艺涉及到它们饮用的海水中镁和硫酸盐的含量很高。这些物质不会被吸收，因此，当水被提取时，吞咽的液体沿着肠道移动时，这些物质预计会越来越集中。这最终会阻碍水渗透到血液中。然而，镁和硫酸盐的不同寻常之处在于，它们引起渗透的可能性只有其他化合物的一半。只是因为海水中的硫酸镁含量如此之高，鱼才能如此高效地提取水。这将使用从动物身上取出的肠道组织样本进行探索，并在试管中(体外)研究其水分传输特性。许多人类泻药使用硫酸镁(Epsom盐)，所以这项研究可以揭示这些治疗实际上是如何起作用的。

Vertebrates, including humans, are made up of about 70% water. Balancing water intake and output is obviously vital for health, but most people don't realise the vast internal movements of water going on all the time within their bodies. For example, the kidney, gut and pancreas collectively transport 8 times our total body water volume in and out of these tissues each day. Understanding the mechanisms these organs use, and how the cells that line them (called epithelia) operates this water transport is therefore important. Despite this importance, the mechanism of water transport is still the subject of much debate. Having said that, for more than 50 years it has been understood that the net transport of water requires salts (especially sodium chloride, or NaCl) to be transported in one particular direction first, to then drive fluid transport (secondarily) in the same direction by a process known as osmosis. Despite this consensus the precise route that water transport takes across epithelia is hotly disputed. For example, does it pass through the cell membranes, via special protein called aquaporins? or does it squeeze between the cells? The novelty of the present proposal lies in the discovery of two new mechanisms for influencing water transport that are conceptually very different to the other current areas of debate. These ideas challenge the established dogma by not relying on salt being transported in the same direction as water, representing a fundamental change in our understanding and providing a novel model for the mechanism of water transport in animal epithelia. The discovery has been made by studying how marine fish drink seawater and process this fluid through the intestine to avoid dehydration. Like humans drinking ordinary fluids, these animals first transport NaCl from the gut into the blood, and water then follows by osmosis. However, marine fish have another trick up their sleeve that maximises their water extraction capability. They secrete a different compound called bicarbonate (same as found in baking soda) into the intestine, in the opposite direction to water absorption. This causes a chemical reaction within the swallowed seawater that causes the high levels of calcium it contains to precipitate as solid, white clumps of calcium carbonate (like limestone). These 'gut rocks' are eventually excreted but the advantage to the fish is to reduce the total dissolved compounds in the gut fluid, which in turn makes it easier to extract water into the blood. We propose to study this novel process further by using 3 different species of marine fish (flounder, tilapia and trout) that produce very different quantities of bicarbonate, and are therefore predicted to have different efficiencies of water absorption. Cold and high pressure also inhibit precipitation, so we will compare water absorption in fish at cold temperature and high pressure (in a barometric chamber). Precipitation of carbonate occurs in human diseases such as kidney and pancreatic stones, so studying this process in fish may help us understand this pathological condition. A second novel process that fish use involves the high levels of magnesium and sulphate in the sea water that they drink. These are not absorbed, and would therefore be expected to get more and more concentrated as swallowed fluid moves down the intestine as water is extracted. This would eventually retard the osmosis of water into the blood. However, magnesium and sulphate are unusual in only having half the potential of other compounds to causes osmosis. It is only because seawater happens to have such high levels of magnesium sulphate that fish extract water so efficiently. This will be explored using samples of gut tissue taken out of the animal and studying its water transport properties in a test tube (in vitro). Many human laxatives use magnesium sulphate (Epsom salts) so this research could reveal insights into how these treatments actually work.