At the macroscopic level, the ocular lens appears as a relatively simple structure with the sole role of focusing light upon the retina. However, numerous studies have underscored the dynamic nature of this organ with a host of compartmentalized physiological processes necessary for transparency. Yet, to date, this contemporary appreciation has not resulted in a universally accepted model for normal lens physiology and controversies remain. Based on original findings of this laboratory, this application proposes to test 2 hypotheses related to electrolyte and fluid transport mechanisms by the lens - 1) that a fluid circulation exists inside the avascular lens. This fluid enters and leaves the lens at different regions completing a loop around the lens surface. Further, this circulation is driven by ionic transport mechanisms that are also non-uniformly distributed around the lens surface;and 2) that a volume change due to fluid traversing the surface of the lens occurs during accommodation. Although both processes involve fluid movement, they operate in a different time scale. The fluid circulation is thought to be a continuous and slow flow, whereas the fluid that moves in and out of the lens occurs in milliseconds and only during accommodation. The work proposed to address the first hypothesis is designed to empirically test an earlier theoretical concept initially proposed by Mathias, Rae and Baldo (1997) that evolved into the lens fluid circulation model (FCM), as it is presently known. However, this model has now become controversial, and legitimate questions have arisen. Although we acquired in recent years data consistent with the FCM, we now plan additional experiments to rigorously test the validity of the model. The second hypothesis is related to the general theme of this project, which focuses on the distribution of fluid flows across the lens surface, and is based upon data that were acquired during the previous funding period, which indicate that the normal lens changes its volume during the accommodation process. To examine the above hypotheses, electrophysiological, volumetric and biophysical techniques will be used in two specific aims on bovine and rabbit lens models: 1) to characterize the putative coupling between circulating electrolyte currents around the surface of the lens and internal movement of fluid to test the validity of the lens FCM;and 2) to further establish that the lens volume changes during accommodation, and to determine the relationship between mechanical and osmotic forces on lens shape and volume.
This aim will expand upon our recent observations that mechanical stretching forces and hypo-osmotic forces have oppositely directed influences on lens volume. From the accomplishment of these aims, data relevant to the understanding of the mechanisms underlying lens homeostasis will be obtained. Advancing our knowledge of normal lens function is essential for the future development of potential anti-cataract and presbyopia therapies.
It is generally accepted that a depletion of anti-oxidants in the interior lens nucleus leads to age- related nuclear cataracts (ANC). If an internal circulation of fluid indeed exists within the lens, future studies directed towards pharmacological approaches to augment the rate of fluid flow would be warranted in order to increase the convection of anti-oxidants to the lens interior. In accommodation, if our ideas are correct, it may turn out that another factor contributing toward presbyopia might be that the lens loses its fluid permeability with ageing.
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