Ho N F, Burton P S, Conradi R A, Barsuhn C L
Upjohn Laboratories, Upjohn Company, Kalamazoo, MI 49007.
J Pharm Sci. 1995 Jan;84(1):21-7. doi: 10.1002/jps.2600840107.
This report is aimed at the biophysical modeling of transmembrane events involving a passive diffusion and directional pumplike mechanism at the apical (AP) and basolateral (BL) membranes of cultured cell monolayers. The essence of the model is based on experimental evidences for the existence of a saturable, apically polarized transport system in Caco-2 cells for peptides which hindered apical to basolateral flux, enhanced basolateral to apical flux, and showed substrate specificity. This system was further inhibited by verapamil, suggesting some homology with P-glycoprotein, the principal mediator of drug resistance in multidrug resistant cancer cells. Preliminary evidence was also obtained suggesting an additional polarized uptake system for the same peptides in the basolateral membrane. Upon saturation and/or inhibition of the active transport mechanisms with verapamil, the peptide fluxes in apical-to-basolateral direction and the basolateral-to-apical direction converged and became controlled by the passive mechanism. Since the intent of the modeling was to provide useful templates for the design of probing experiments and to delineate and quantify mass transfer mechanisms at the AP and BL membranes and their interrelationships, theoretical equations were developed for a host of kinetic boundary conditions: (a) AP-->BL and BL-->AP transfluxes, (b) bidirectional effluxes from substrate-preloaded cells, (c) undirectional efflux across the AP or BL membrane from preloaded cells, and (d) uptake kinetics via the AP or BL membrane leading to equilibrium. Furthermore, flux expressions were reduced to membrane permeability coefficients to accommodate passive diffusion, saturation, inhibition, and directionality. The diffusional mass transport resistances of the aqueous boundary layers and microporous filter support of the cell monolayer were necessarily included.
本报告旨在对跨膜事件进行生物物理建模，这些事件涉及培养的细胞单层顶端（AP）和基底外侧（BL）膜上的被动扩散和定向泵样机制。该模型的本质基于以下实验证据：在Caco-2细胞中存在一种可饱和的、顶端极化的肽转运系统，该系统阻碍顶端到基底外侧的通量，增强基底外侧到顶端的通量，并表现出底物特异性。维拉帕米可进一步抑制该系统，这表明其与P-糖蛋白存在一定同源性，P-糖蛋白是多药耐药癌细胞中耐药性的主要介导因子。还获得了初步证据，表明在基底外侧膜中存在针对相同肽的另一种极化摄取系统。在用维拉帕米使主动转运机制饱和和/或抑制后，肽在顶端到基底外侧方向和基底外侧到顶端方向的通量趋于一致，并由被动机制控制。由于建模的目的是为探测实验的设计提供有用的模板，并描述和量化AP和BL膜处的传质机制及其相互关系，因此针对一系列动力学边界条件推导了理论方程：（a）AP→BL和BL→AP的跨通量，（b）来自预先加载底物的细胞的双向流出，（c）预先加载的细胞通过AP或BL膜的单向流出，以及（d）通过AP或BL膜的摄取动力学直至达到平衡。此外，通量表达式简化为膜渗透系数，以适应被动扩散、饱和、抑制和方向性。细胞单层水相边界层和微孔滤膜支撑体的扩散传质阻力也必然被纳入考虑。
