Weinstein A M
Biophys J. 1983 Nov;44(2):153-70. doi: 10.1016/S0006-3495(83)84287-8.
The rat proximal tubule epithelium is represented as well-stirred, compliant cellular and paracellular compartments bounded by mucosal and serosal bathing solutions. With a uniform pCO2 throughout the epithelium, the model variables include the concentrations of Na, K, Cl, HCO3, H2PO4, HPO4, and H, as well as hydrostatic pressure and electrical potential. Except for a metabolically driven Na-K exchanger at the basolateral cell membrane, all membrane transport within the epithelium is passive and is represented by the linear equations of nonequilibrium thermodynamics. In particular, this includes the cotransport of Na-Cl and Na-H2PO4 and countertransport of Na-H at the apical cell membrane. Experimental constraints on the choice of ionic conductivities are satisfied by allowing K-Cl cotransport at the basolateral membrane. The model equations include those for mass balance of the nonreacting species, as well as chemical equilibrium for the acidification reactions. Time-dependent terms are retained to permit the study of transient phenomena. In the steady state the energy dissipation is computed and verified equal to the sum of input from the Na-K exchanger plus the Gibbs free energy of mass addition to the system. The parameter dependence of coupled water transport is studied and shown to be consistent with the predictions of previous analytical models of the lateral intercellular space. Water transport in the presence of an end-proximal (HCO3-depleted) luminal solution is investigated. Here the lower permeability and higher reflection coefficient of HCO3 enhance net sodium and water transport. Due to enhanced flux across the tight junction, this process may permit proximal tubule Na transport to proceed with diminished energy dissipation.
大鼠近端肾小管上皮被表示为搅拌良好、顺应性的细胞内和细胞旁隔室，由黏膜和浆膜浴液界定。在整个上皮中pCO2均匀的情况下，模型变量包括Na、K、Cl、HCO3、H2PO4、HPO4和H的浓度，以及静水压力和电势。除了基底外侧细胞膜上由代谢驱动的Na-K交换器外，上皮内的所有膜转运都是被动的，并用非平衡热力学的线性方程表示。特别地，这包括顶端细胞膜上Na-Cl和Na-H2PO4的协同转运以及Na-H的逆向转运。通过允许基底外侧膜上的K-Cl协同转运，满足了对离子电导率选择的实验限制。模型方程包括非反应物种的质量平衡方程以及酸化反应的化学平衡方程。保留了时间相关项以允许研究瞬态现象。在稳态下，计算并验证能量耗散等于来自Na-K交换器的输入之和加上系统质量增加的吉布斯自由能。研究了耦合水转运的参数依赖性，并表明其与先前细胞间外侧间隙分析模型的预测一致。研究了在近端末端（HCO3耗尽）管腔溶液存在下的水转运。在这里，HCO3的较低通透性和较高反射系数增强了钠和水的净转运。由于跨紧密连接的通量增加，这个过程可能允许近端肾小管的钠转运以减少的能量耗散进行。
