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Department of Visual Science, Institute of Ophthalmology, London.

1. When the volume of water per unit time which flows through natural elastic basement membrane is divided by the applied pressure, the value-the hydraulic conductivity-is not constant but decreases as pressure increases. In contrast when the same membrane is tanned with glutaraldehyde and rendered inelastic, the hydraulic conductivity is constant at all pressures. 2. Over a pressure range of 0-6.7 kPa equivalent to a membrane stress of 0-195 kPa in natural elastic membrane the hydraulic conductivity (Lp) can be related by the linear equation Lp = Lp.0 + apP where P is the hydraulic pressure, Lp.0 is the initial hydraulic conductivity and ap is a constant which is the decreased hydraulic conductivity per unit pressure (correlation coefficient 0.764. P less than 0.001). 3. The initial conductivity of the basement membrane of the crystalline lens of the adult rat (lens capsule) was 47.6 +/- 7.3 x 10(-12) m s-1 Pa-1 while the decrease in hydraulic conductivity per unit increase in pressure was -3.38 x 10(-15) m s-1 Pa-2. 4. Following tanning with glutaraldehyde the hydraulic conductivity was constant at 27.4 +/- 4.0 x 10(-12) m s-1 Pa-1. 5. A change in the configuration of the superhelices of the filaments of type IV collagen which form the framework of basement membrane is termed. 'The deformation matrix theory' and can satisfactorily account for the changes in hydraulic conductivity of both natural and tanned membrane. 6. In natural membrane the filaments deform easily and the pitch of the filament superhelices is increased by axial stress induced by pressure. The filaments straighten and become compacted together and the hydraulic permeability is thereby decreased. 7. In tanned membrane the filaments become more rigid and axial stress barely deforms them: moreover the pitch of the filament superhelices is decreased so that the filaments become more closely coiled and compacted together. Because of these changes the hydraulic conductivity is reduced as compared with unstressed natural membrane and remains unaltered by increasing pressure.

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