Resolution of Osmotic (Lpo) and Hydrostatic (Lph) Hydraulic Conductivities of Series Barriers in Bovine Retinal Pigment Epithelium (BRPE) and Bruch’s Membrane–Choroid Complex.

Abstract

Abstract: : 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 L_ps have been difficult to determine. We have applied a new method to resolve the L_ps 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) L_P^o was 0.016 µl×cm^–2×hr^–1×cmH₂O^–1 (n=13), ≈35–fold lower than L_P^h 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 L_p^h 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.

Keywords: aging • Bruch's membrane • choroid