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The Mechanism of Increasing Outflow Facility during Washout in the Bovine Eye

¹ From the Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts; the ² Department of Ophthalmology, Boston University School of Medicine, Boston, Massachusetts; and the ³ Department of Biomedical Engineering, Northwestern University, Evanston, Illinois.

PURPOSE. To investigate the relationship between outflow facility and separation between the inner wall of the aqueous plexus and the juxtacanalicular connective tissue (JCT) during washout in the bovine eye.

METHODS. Facility was recorded during 3 hours of anterior chamber perfusion at 15 mm Hg in eight pairs of bovine eyes. One eye of each pair was then lowered to 0 mm Hg for 1 hour, whereas the fellow eye was kept at 15 mm Hg. After a brief perfusion at 15 mm Hg, both eyes were perfusion fixed and processed for electron microscopy. Micrographs of the inner wall were analyzed for separation from the JCT. To study the role of cellular adhesion between the inner wall and JCT, 12 additional pairs were perfused with integrin-binding peptide (RGD: Arg-Gly-Asp) or sham control peptide (RGE: Arg-Gly-Glu) at 2 µM to 2 mM, before IOP was reduced.

RESULTS. During the first 3 hours, facility increased in both eyes because of "washout." However, after 1 hour of 0 mm Hg, facility decreased by 13% (P < 0.006), whereas facility increased by 20% (P < 0.001) in the fellow eyes maintained at 15 mm Hg. Two types of separation were observed between the inner wall and JCT: cell–matrix separation between the endothelial cell and basal lamina and matrix–matrix separation between the basal lamina and JCT. A significant positive correlation (P = 0.042) was found between the degree of matrix–matrix separation and the change in outflow facility after 1 hour of 0 mm Hg. Compared with RGE control, RGD had no apparent effect on outflow facility (P > 0.35) or on the change in outflow facility after 1 hour at 0 mm Hg (P > 0.15).

CONCLUSIONS. The increase in outflow facility that occurs during washout in the bovine eye is reversible and correlates with the degree of separation between the basal lamina of the inner wall endothelium and the JCT. Therefore, adhesions tethering the inner wall to the JCT may be important ultrastructural features involved in the regulation of aqueous humor outflow resistance.

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