Niles W D, Cohen F S, Finkelstein A
Rush Medical College, Department of Physiology, Chicago, Illinois 60612.
J Gen Physiol. 1989 Feb;93(2):211-44. doi: 10.1085/jgp.93.2.211.
When phospholipid vesicles bound to a planar membrane are osmotically swollen, they develop a hydrostatic pressure (delta P) and fuse with the membrane. We have calculated the steady-state delta P, from the equations of irreversible thermodynamics governing water and solute flows, for two general methods of osmotic swelling. In the first method, vesicles are swollen by adding a solute to the vesicle-containing compartment to make it hyperosmotic. delta P is determined by the vesicle membrane's permeabilities to solute and water. If the vesicle membrane is devoid of open channels, then delta P is zero. When the vesicle membrane contains open channels, then delta P peaks at a channel density unique to the solute permeability properties of both the channel and the membrane. The solute enters the vesicle through the channels but leaks out through the region of vesicle-planar membrane contact. delta P is largest for channels having high permeabilities to the solute and for solutes with low membrane permeabilities in the contact region. The model predicts the following order of solutes producing pressures of decreasing magnitude: KCl greater than urea greater than formamide greater than or equal to ethylene glycol. Differences between osmoticants quantitatively depend on the solute permeability of the channel and the density of channels in the vesicle membrane. The order of effectiveness is the same as that experimentally observed for solutes promoting fusion. Therefore, delta P drives fusion. When channels with small permeabilities are used, coupling between solute and water flows within the channel has a significant effect on delta P. In the second method, an impermeant solute bathing the vesicles is isosmotically replaced by a solute which permeates the channels in the vesicle membrane. delta P resulting from this method is much less sensitive to the permeabilities of the channel and membrane to the solute. delta P approaches the theoretical limit set by the concentration of the impermeant solute.
当与平面膜结合的磷脂囊泡发生渗透膨胀时，它们会产生静水压力（ΔP）并与膜融合。我们根据控制水和溶质流动的不可逆热力学方程，计算了两种一般渗透膨胀方法的稳态ΔP。在第一种方法中，通过向含有囊泡的隔室中添加溶质使其高渗来使囊泡膨胀。ΔP由囊泡膜对溶质和水的渗透率决定。如果囊泡膜没有开放通道，那么ΔP为零。当囊泡膜含有开放通道时，ΔP在通道密度处达到峰值，该通道密度对于通道和膜的溶质渗透特性而言是独特的。溶质通过通道进入囊泡，但通过囊泡 - 平面膜接触区域泄漏出去。对于对溶质具有高渗透率的通道以及在接触区域中膜渗透率低的溶质，ΔP最大。该模型预测产生压力大小递减的溶质顺序如下：氯化钾大于尿素大于甲酰胺大于或等于乙二醇。渗透剂之间的差异在数量上取决于通道的溶质渗透率和囊泡膜中通道的密度。有效性顺序与实验观察到的促进融合的溶质顺序相同。因此，ΔP驱动融合。当使用渗透率小的通道时，通道内溶质和水流之间的耦合对ΔP有显著影响。在第二种方法中，用一种渗透囊泡膜中通道的溶质等渗替代浸泡囊泡的非渗透性溶质。这种方法产生的ΔP对通道和膜对溶质的渗透率不太敏感。ΔP接近由非渗透性溶质浓度设定的理论极限。
