Axial Flow Effects in Proximal Tubule

描述(申请人提供)：肾小管平衡(GTB)是近端小管运输的一个重要方面，随着肾小球滤过率(GFR)的变化，其对Na+、HCO3-、C1-和水的重吸收几乎保持成比例的变化。GTB可防止肾小球滤过率增加后肾脏溶质流失，还可在低肾小球滤过率时保存足够的远端钠盐输送，从而限制远端肾单位酸和钾排泄的损害。GFR调节近端重吸收的机制尚不清楚，尽管已经确定肾小管液轴向流动的变化本身可以引起Na+转运的比例变化。我们推测刷状边缘微绒毛是传递流速增加信号的传感器，而根尖膜NHE3下游的离子交换器是增加Na+、HCO3-和液体重吸收的关键效应器。最近一个关于近端小管刷子边界内流体流动的数学模型支持这样的观点，即微绒毛结构非常适合作为近端小管流体流动的机械传感器。在拟议的工作中，将进行体内和体外微灌流实验，目的如下：1)测试刷状边缘微绒毛是否作为机械换能器在大鼠和小鼠近端小管轴向流中起作用；2)检查近端小管转运的增加是否归因于通过NHE3的跨细胞转运增加，而不是通过改变改变上皮通透性和增加细胞旁运输的物理因素；以及3)利用基因敲除动物和抑制剂来确定血流依赖运输的关键胞浆介质。 我们提议的合作的独到之处包括：(1)比较在小鼠和敲除动物体内和体外的近端小管血流依赖的微灌流；(2)重吸收通量随水动力和扭矩对微绒毛的影响的表示；(3)开发一个模型，解释流动和重吸收之间的非线性关系，其中考虑到小管和微绒毛几何形状的变化；以及(4)在近端小管运输的数学模型中对传输和渗透性数据的评估。这些研究将为GTB的发病机制以及生理和病理条件下肾液和HCO3-转运方面提供新的信息。

DESCRIPTION (provided by applicant): Glomerulotubular balance (GTB) is a critical aspect of proximal tubule transport, that maintains nearly proportional change in reabsorption of Na+, HCO3-, C1-, and water with variation in glomerular filtration (GFR). GTB acts to prevent renal solute loss following a GFR increase, and also allows preservation of adequate distal isodium delivery in times of low GFR, thus limiting compromise of distal nephron acid and potassium excretion. The mechanisms by which GFR can modulate proximal reabsorption are uncertain, although it is well established that variation in axial flow of tubular fluid can, by itself, induce proportional changes in Na+ transport. We hypothesize that brush border microvilli are the sensors for transducing the signal of increased flow rate, and that the downstream ion exchanger on the apical membrane NHE3 is the key effector for increasing the reabsorption of Na+, HCO3- and fluid. A recent mathematical model of fluid flow within the proximal tubule brush border supports the idea that the microvilli configuration is ideally suited for them to serve as the mechanosensors of proximal tubule fluid flow. In the work proposed, both in vivo and in vitro microperfusion experiments will be conducted with the following three aims: 1) To test whether brush border microvilli act as mechanotransducers that sense axial flow in rat and mouse proximal tubules; 2) To examine whether the increment of proximal tubule transport attributed to enhanced flow rate is due to increasing transcellular transport via NHE3, rather than by changing the physical factors that alter the epithelial permeability and increase paracellular transport; and 3) utilize both knockout animals and inhibitors to define key cytosolic mediators of flow-dependent transport. The unique features of our proposed collaboration are: (1) the comparison of flow-dependent proximal tubule transport, both in vivo and in vitro microperfusion in mice and in knockout animals; (2) the representation of reabsorptive fluxes as a function of hydrodynamic forces and torques on microvilli; (3) the development of a model to explain the non-linear relationship between flow and reabsorption that takes account of changing tubule and microvilli geometry and (4) the assessment of transport and permeability data within a mathematical model of proximal tubule transport. These studies will provide new information on mechanisms of GTB and aspects of renal fluid and HCO3- transport in physiological and pathophysiological conditions.