Ionic modulation of flow resistance in an immobilized proteoglycan model of the trabecular meshwork

Resistance to aqueous humor outflow, which is essential for the maintenance of normal intraocular pressures, is thought to reside within the final few layers of the trabecular meshwork, adjacent to Schlemm's canal. Trabecular proteoglycans, with their extensive, negatively charged glycosaminoglycan (GAG) side-chains, are thought to contribute most of this outflow resistance. We hypothesize that one mechanism by which trabecular cells can regulate aqueous outflow is through modulation of the ionic extracellular microenvironment at or near their surfaces within the outflow pathway. To examine this possibility, we have developed an experimental model of the trabecular extracellular matrix. In this model, a large proteoglycan, aggrecan, is attached covalently by its core protein to the matrix of a tortuous-pore membrane. When viewed in scanning electron micrographs, an interesting similarity between the flow model and the human trabecular meshwork is observed. The fluid flow rate through the model membranes, measured by constant-pressure perfusion, decreases in a saturable, dose-dependent manner in response to increased proteoglycan binding. The proteoglycan is tightly bound, as judged by retention of radiolabeled proteoglycan, under the perfusion conditions used. Degradation of the GAGs without removal of the proteoglycan core protein, achieved by treatment with the enzyme chondroitinase AC, return flow rates to values near those measured through control membranes. Increasing the sodium chloride concentration in the perfusate produces a dose-dependent increase in flow rate, which plateaus by 100 mM. Increasing the calcium concentration produces a similar response, although the plateau is reached by 10 mM. Maximum flow is observed near physiological pH, declining at values above or below neutrality. We conclude that this membrane model with the immobilized proteoglycans is a useful system for evaluating potential roles of trabecular proteoglycans in modulating aqueous humor outflow, free from contributions of trabecular cellular dynamics. These studies support our hypothesis that trabecular cells can modulate flow by changing the ionic microenvironment of the extracellular matrices near their surfaces within the flow channels.