Mathias R T, Rae J L, Baldo G J
Department of Physiology and Biophysics, State University of New York, Stony Brook, USA.
Physiol Rev. 1997 Jan;77(1):21-50. doi: 10.1152/physrev.1997.77.1.21.
The lens is an avascular organ suspended between the aqueous and vitreous humors of the eye. The cellular structure is symmetric about an axis passing through its anterior and posterior poles but asymmetric about a plane passing through its equator. Because of its asymmetric structure, the lens has historically been assumed to perform transport between the aqueous and vitreous humors. Indeed, when anterior and posterior surfaces were isolated in an Ussing chamber, a translens current was measured. However, in the eye, the two surfaces are not isolated. The vibrating probe technique showed the current densities at the surface of a free-standing lens were surprisingly large, about an order of magnitude greater than measured in an Ussing chamber, and were not directed across the lens. Rather, they were inward in the region of either anterior or posterior pole and outward at the equator. This circulating current is the most dramatic physiological property of a normal lens. We believe it is essential to maintain clarity; hence, this review focuses on factors likely to drive and direct it. We review properties and spatial distribution of lens Na+/K+ pumps, ion channels, and gap junctions. Based on these data, we propose a model in which the difference in electromotive potential of surface versus interior cell membranes drives the current, whereas the distribution of gap junctions directs the current in the observed pattern. Although this model is clearly too simple, it appears to quantitatively predict observed currents. However, the model also predicts fluid will move in the same pattern as ionic current. We therefore speculate that the physiological role of the current is to create an internal circulatory system for the avascular lens.
晶状体是一个无血管的器官，悬浮于眼的房水和玻璃体之间。其细胞结构关于一条穿过前后极的轴呈对称，但关于一条穿过赤道的平面呈不对称。由于其不对称结构，晶状体一直以来被认为在房水和玻璃体之间进行物质运输。实际上，当将晶状体的前后表面分离并置于尤斯灌流小室中时，可检测到跨晶状体电流。然而，在眼中，这两个表面并非分离的。振动探针技术显示，独立晶状体表面的电流密度出奇地大，比在尤斯灌流小室中测得的约大一个数量级，且并非穿过晶状体。相反，它们在前极或后极区域向内，在赤道处向外。这种循环电流是正常晶状体最显著的生理特性。我们认为维持晶状体的透明度至关重要；因此，本综述聚焦于可能驱动和引导该电流的因素。我们回顾了晶状体钠钾泵、离子通道和缝隙连接的特性及空间分布。基于这些数据，我们提出一个模型，其中表面细胞膜与内部细胞膜之间的电动势差异驱动电流，而缝隙连接的分布则将电流引导成所观察到的模式。尽管这个模型显然过于简单，但它似乎能定量预测所观察到的电流。然而，该模型还预测液体将以与离子电流相同的模式流动。因此，我们推测该电流的生理作用是为无血管的晶状体创建一个内部循环系统。
