Local osmosis and isotonic transport
Local osmosis and isotonic transport
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DOI：
10.1007/s00232-005-0817-9
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发表时间：
2005-11-01
影响因子：
2.4
通讯作者：
Wang, H
中科院分区：
文献类型：
--
作者：
Mathias, RT;Wang, H
在两个具有相同渗透压的溶液c(o)（哺乳动物为300 mM）之间，由渗透驱动的水流u (cm/s)，其理论等渗最大值为u = j/c(o)，其中j（摩尔/cm(2)/s）为盐输送速率。在许多实验研究中，发现输运与等渗是无法区分的。本工作的目的是研究它接近等渗的条件。一个必要条件是膜盐/水渗透比epsilon必须小：典型的生理值为epsilon = 10（-3）至10(-5)，因此epsilon一般较小，但这不足以保证近等压输运。如果我们考虑两个系列膜的最简单模型，它们分泌一滴眼泪或一滴汗水（即分泌物没有外部施加的边界条件），扩散是可以忽略不计的，预测的渗透压是：基底= c(o)，细胞内近似于（1 + epsilon）c(o)，分泌近似于（1 + 2 epsilon）c(o)， u近似于（1 - 2 epsilon）j/c(o)。注意，该模型也适用于实验收集输送溶液的情况。因此，在没有外部边界条件的情况下，输运在实验上与等压输运难以区分。然而，如果外部边界条件使上皮两侧的盐浓度为c(o)，则流体运输依赖于侧向空间的分布渗透梯度。如果横向空间太短太宽，则扩散主导对流，降低渗透梯度，流体流动明显小于等压。此外，由于顶端和基底侧膜的水通量是由细胞内渗透压联系在一起的。当基底外侧膜的总透水性等于根尖膜的透水性时，水流最大。在肾近端小管的情况下，数据表明它在接近最佳条件下运输。然而，典型的生理值表明，新过滤的液体以大约0.86 j/c(o)的速率被重吸收，因此高渗溶液正在被重吸收。因此，滤液的渗透压c(F) (M)将随着离滤过部位（肾小球）的距离而减小，直到被输送的溶液与滤液等渗，u = J/c(F)。在此稳态条件下，分布模型近似等效于串联的两个膜。现在的渗透压是：c(F)近似于（1 - 2）j/c(o)，细胞内近似于（1 -）c(o)，侧边空间近似于c(o)， u近似于（w + 2）j/c(o)。c(F) -的变化预计在长度常数约为0.3 cm时发生。因此，膜运输倾向于向epsilon c(o)方向调整跨膜渗透梯度，这导致水流等渗到epsilon数量级内。这些发现为近端小管或其他上皮如何运输等渗溶液提供了一个合理的假设。
Osmotically driven water flow, u (cm/s), between two solutions of identical osmolarity, c(o) (300 mM in mammals), has a theoretical isotonic maximum given by u = j/c(o), where j (moles/cm(2)/s) is the rate of salt transport. In many experimental studies, transport was found to be indistinguishable from isotonic. The purpose of this work is to investigate the conditions for it to approach isotonic. A necessary condition is that the membrane salt/water permeability ratio, epsilon, must be small: typical physiological values are epsilon = 10(-3) to 10(-5), so epsilon is generally small but this is not sufficient to guarantee near-isotonic transport. If we consider the simplest model of two series membranes, which secrete a tear or drop of sweat (i.e., there are no externally-imposed boundary conditions on the secretion), diffusion is negligible and the predicted osmolarities are: basal = c(o), intracellular approximate to (1 + epsilon)c(o), secretion approximate to (1 + 2 epsilon)c(o), and u approximate to (1 - 2 epsilon)j/c(o). Note that this model is also appropriate when the transported solution is experimentally collected. Thus, in the absence of external boundary conditions, transport is experimentally indistinguishable from isotonic. However, if external boundarv conditions set salt concentrations to c(o) on both sides of the epithelium, then fluid transport depends on distributed osmotic gradients in lateral spaces. If lateral spaces are too short and wide, diffusion dominates convection, reduces osmotic gradients and fluid flow is significantly less than isotonic. Moreover, because apical and basolateral membrane water fluxes are linked by the intracellular osmolarity. water flow is maximum when the total water permeability of basolateral membranes equals that of apical membranes. In the context of the renal proximal tubule, data suggest it is transporting at near optimal conditions. Nevertheless, typical physiological values suggest the newly filtered fluid is reabsorbed at a rate u approximate to 0.86 j/c(o), so a hypertonic solution is being reabsorbed. The osmolarity of the filtrate c(F) (M) will therefore diminish with distance from the site of filtration (the glomerulus) until the solution being transported is isotonic with the filtrate, u = J/c(F). With this steady-state condition, the distributed model becomes approximately equivalent to two membranes in series. The osmolarities are now: c(F) approximate to (1 - 2 epsilon)j/c(o), intracellular approximate to (1 - epsilon)c(o), lateral spaces approximate to c(o), and u approximate to (w + 2 epsilon)j/c(o). The change in c(F) -is predicted to occur with a length constant of about 0.3 cm. Thus, membrane transport tends to adjust transmembrane osmotic gradients toward epsilon c(o), which induces water flow that is isotonic to within order epsilon. These findings provide a plausible hypothesis on how the proximal tubule or other epithelia appear to transport an isotonic solution.