Weinstein A M
Department of Physiology and Biophysics, Cornell University Medical College, Rogosin Kidney Center, New York Hospital, New York 10021.
Am J Physiol. 1988 Mar;254(3 Pt 2):F297-305. doi: 10.1152/ajprenal.1988.254.3.F297.
When the proximal tubule epithelium is represented as cellular and lateral intercellular (LIS) compartments, the presence of a paracellular pathway can render the overall phenomenologic equations quite an indirect representation of intraepithelial transport processes. 1) Active sodium transport into the LIS may create a hypertonic region that drives water movement from lumen to peritubular blood, i.e., a term for active water transport may appear in the overall transport equations. The correlate of this uphill water flux is a solute polarization effect, such that the measured epithelial water permeability is less than that of the cell membranes. 2) Basolateral uptake of potassium by the cell may lower the LIS concentration and promote diffusive entry of K across the tight junction. Even without cellular uptake of K from the lumen, the epithelial transport equations may contain a term for active K reabsorption. The solute polarization correlate is a low epithelial reflection coefficient that does not represent a convective flux of K through a specific channel. 3) When there is convective flux of Na and Cl through the tight junction but none through the cell, then a fluid circuit around junction and cell may be present, even when net epithelial volume flux is absent. In this case, part of the net epithelial Cl flux must be represented in the overall transport equations as electroneutral Na-Cl cotransport.
当近端肾小管上皮细胞被表示为细胞内和细胞间外侧（LIS）区室时，细胞旁途径的存在会使整体现象学方程成为上皮内转运过程的相当间接的表示。1）主动钠转运进入LIS可能会产生一个高渗区域，驱动水从管腔向肾小管周血液移动，即主动水转运项可能会出现在整体转运方程中。这种向上的水通量的相关因素是溶质极化效应，使得测得的上皮水通透性低于细胞膜的水通透性。2）细胞对钾的基底外侧摄取可能会降低LIS浓度，并促进钾通过紧密连接的扩散性进入。即使细胞没有从管腔摄取钾，上皮转运方程中也可能包含主动钾重吸收项。溶质极化的相关因素是低上皮反射系数，它并不代表钾通过特定通道的对流通量。3）当钠和氯通过紧密连接有对流通量但通过细胞没有时，即使上皮净体积通量不存在，连接和细胞周围也可能存在流体回路。在这种情况下，上皮净氯通量的一部分必须在整体转运方程中表示为电中性的钠-氯共转运。
