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S. Tripathi, T. Banzon, S. Jalickee, S.S. Miller; Resolution of Osmotic (Lpo) and Hydrostatic (Lph) Hydraulic Conductivities of Series Barriers in Bovine Retinal Pigment Epithelium (BRPE) and Bruch’s Membrane–Choroid Complex. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1092.
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Purpose:The BRPE drives solute–coupled water transport from the retinal side of the RPE towards the choroidal vasculature through Bruch’s membrane. Because this complex is in series with the BRPE, not easily isolated without damage, individual Lps have been difficult to determine. We have applied a new method to resolve the Lps of the series components. Methods: The capacitance probe technique was refined (Adelman and Miller, 1994; Jalickee et al., 2003) for measuring transepithelial water flow. This was accomplished by abolition of evaporation, improved temperature control, and immobilization of the tissue. Results: Retina–to–choroid hydrostatic steps of 1–5 cmH2O failed to drive transepithelial water flow (our resolution is 0.5 µl×cm–2×hr–1). Simultaneous application of osmotic steps, hypo–osmotic on the retinal side by 35 mOsm and hyper–osmotic on the choroidal side by 35 mOsm produced a sustained increment of 16.9+3.3 µl×cm–2×hr–1 (n=7) over spontaneous volume flow. Application of the 70 mOsm gradient in the reverse direction attenuated spontaneous flows by 15.2+2.6 µl×cm–2×hr–1 (n=6). This non–rectifying (p>0.67) LPo was 0.016 µl×cm–2×hr–1×cmH2O–1 (n=13), ≈35–fold lower than LPh reported for mammalian(dog) RPE (Tsuboi, 1987). Choroid–to–retina hydrostatic steps of 1–6 cmH2O reversibly opened the tight junctions, eliminating the BRPE as a significant barrier. The minimum residual Lph of the Bruch’s membrane–choroid complex permitting these high flows is 17.1+3.1 µl×cm–2×hr–1×cmH2O–1 (n=5). Conclusions: Spontaneous fluid absorption by the BRPE of the order of 5 µl×cm–2×hr–1 is driven by transepithelial osmotic gradients of the order of 10 mOsm. Once this fluid crosses the RPE, it requires a pressure of <1cmH2O to drive it across the highly permeable Bruch’s membrane–choroid. This new method could serve as a sensitive tool to detect small changes in water permeability of Bruch’s membrane–choroid that occur in disease or senescence.
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