Hernández J A, Fischbarg J
Sección Biofísica, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay.
Biophys J. 1994 Oct;67(4):1464-72. doi: 10.1016/S0006-3495(94)80620-4.
The processes of membrane transport exhibiting permeability coefficients depending on the species activities do not obey the "independence principle" and are assumed to take place by a mechanism of discrete nature, analyzable by a kinetic formalism. In this article, we study the dependence of the osmotic permeability coefficient on the water activities, from the steady-state analysis of a kinetic model of single-file water transport that simultaneously incorporates the vacancy-mediated and "knock-on" mechanisms into the state diagram. In particular, we study the relation between the near-equilibrium osmotic permeability (Pe) and the equilibrium water activity of the compartments (w). The analysis and numerical calculations performed for a simple case of the model show that, for values of the parameters consistent with experimental data, Pe exhibits only a small variation with w within the physiological range in the majority of the situations considered here. It is not possible to predict, from the study of these simple models, whether more complicated kinetic diagrams of water transport may be characterized by permeability coefficients with a more evident dependence on the water activities. Nevertheless, the results obtained here suggest that, for the case of physiological water pores, the analysis of the kinetic dependence of the permeability coefficients on the water activities may not yield evidence pointing to a discrete nature for the transport process.
表现出取决于物种活性的渗透系数的膜运输过程并不遵循“独立原理”，并被认为是通过一种离散性质的机制发生的，这种机制可用动力学形式进行分析。在本文中，我们从单排水分子运输动力学模型的稳态分析出发，研究渗透系数对水活性的依赖性，该模型同时将空位介导机制和“撞击”机制纳入状态图。特别地，我们研究了近平衡渗透系数（Pe）与隔室平衡水活性（w）之间的关系。对该模型的一个简单情况进行的分析和数值计算表明，对于与实验数据一致的参数值，在此处考虑的大多数情况下，Pe在生理范围内随w仅表现出很小的变化。从这些简单模型的研究中无法预测，更复杂的水分子运输动力学图是否可能具有对水活性更明显依赖的渗透系数。然而，此处获得的结果表明，对于生理水孔的情况，分析渗透系数对水活性的动力学依赖性可能无法得出指向运输过程具有离散性质的证据。
