Kay Alan R
Department of Biology, University of IowaIowa City, IA, USA.
Front Cell Dev Biol. 2017 May 8;5:41. doi: 10.3389/fcell.2017.00041. eCollection 2017.
The ability of all cells to set and regulate their size is a fundamental aspect of cellular physiology. It has been known for sometime but not widely so, that size stability in animal cells is dependent upon the operation of the sodium pump, through the so-called pump-leak mechanism (Tosteson and Hoffman, 1960). Impermeant molecules in cells establish an unstable osmotic condition, the Donnan effect, which is counteracted by the operation of the sodium pump, creating an asymmetry in the distribution of Na and K staving off water inundation. In this paper, which is in part a tutorial, I show how to model quantitatively the ion and water fluxes in a cell that determine the cell volume and membrane potential. The movement of water and ions is constrained by both osmotic and charge balance, and is driven by ion and voltage gradients and active ion transport. Transforming these constraints and forces into a set of coupled differential equations allows us to model how the ion distributions, volume and voltage change with time. I introduce an analytical solution to these equations that clarifies the influence of ion conductances, pump rates and water permeability in this multidimensional system. I show that the number of impermeant ions () and their average charge have a powerful influence on the distribution of ions and voltage in a cell. Moreover, I demonstrate that in a cell where the operation of active ion transport eliminates an osmotic gradient, the size of the cell is directly proportional to . In addition, I use graphics to reveal how the physico-chemical constraints and chemical forces interact with one another in apportioning ions inside the cell. The form of model used here is applicable to all membrane systems, including mitochondria and bacteria, and I show how pumps other than the sodium pump can be used to stabilize cells. Cell biologists may think of electrophysiology as the exclusive domain of neuroscience, however the electrical effects of ion fluxes need to become an intimate part of cell biology if we are to understand a fundamental process like cell size regulation.
所有细胞设定和调节自身大小的能力是细胞生理学的一个基本方面。虽然人们已经知晓这一点有一段时间了，但了解得并不广泛，动物细胞的大小稳定性依赖于钠泵的运作，通过所谓的泵 - 漏机制（托斯滕森和霍夫曼，1960年）。细胞内的非渗透性分子建立了一种不稳定的渗透状态，即唐南效应，而钠泵的运作会抵消这种效应，在钠和钾的分布上产生不对称性，从而避免细胞被水淹没。在本文中，部分内容是教程性质的，我展示了如何定量模拟细胞中决定细胞体积和膜电位的离子和水通量。水和离子的移动受到渗透平衡和电荷平衡的限制，并由离子梯度、电压梯度以及主动离子运输驱动。将这些限制和驱动力转化为一组耦合的微分方程，使我们能够模拟离子分布、体积和电压如何随时间变化。我引入了这些方程的一个解析解，它阐明了在这个多维系统中离子电导、泵速率和水渗透性的影响。我表明非渗透性离子的数量（）及其平均电荷对细胞内离子分布和电压有强大影响。此外，我证明在一个主动离子运输的运作消除了渗透梯度的细胞中，细胞大小与直接成正比。另外，我用图表揭示了物理化学限制和化学力在细胞内分配离子时是如何相互作用的。这里使用的模型形式适用于所有膜系统，包括线粒体和细菌，并且我展示了除钠泵之外的其他泵如何用于稳定细胞。细胞生物学家可能认为电生理学是神经科学的专属领域，然而，如果我们要理解像细胞大小调节这样的基本过程，离子通量的电效应就需要成为细胞生物学的一个重要组成部分。
