PHYSIOLOGICAL-PROPERTIES OF THE NORMAL LENS

The lens is an avascular organ suspended between the aqueous and vitre ous humors of the eye. The cellular structure is symmetric about an ax is passing through its anterior and posterior poles but asymmetric abo ut a plane passing through its equator. Because of its asymmetric stru cture, the lens has historically been assumed to perform transport bet ween the aqueous and vitreous humors. Indeed, when anterior and poster ior surfaces were isolated in an Ussing chamber, a translens current w as measured. However, in the eye, the two surfaces are not isolated. T he vibrating probe technique showed the current densities at the surfa ce of a free-standing lens were surprisingly large, about an order of magnitude greater than measured in an Ussing chamber, and were not dir ected across the lens. Rather, they were inward in the region of eithe r anterior or posterior pole and outward at the equator. This circulat ing current is the most dramatic physiological property of a normal le ns. We believe it is essential to maintain clarity; hence, this review focuses on factors Likely to drive and direct it. We review propertie s and spatial distribution of lens Na+/K+ pumps, ion channels, and gap junctions. Based on these data, we propose a model in which the diffe rence in electromotive potential of surface versus interior cell membr anes drives the current, whereas the distribution of gap junctions dir ects the current in the observed pattern. Although this model is clear ly 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 o f the current is to create an internal circulatory system for the avas cular lens.