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Action of polysaccharides of similar average mass but differing molecular volume and charge on fluid drainage through synovial interstitium in rabbit knees

D. Scott, P. J. Coleman, R. M. Mason* and J. R. Levick

1. Hyaluronan (HA), an anionic polysaccharide of synovial fluid, attenuates fluid loss from joints as joint pressure is raised ('outflow buffering'). The buffering is thought to depend on the expanded molecular domain of the polymer, which causes reflection by synovial extracellular matrix, leading to flow-dependent concentration polarization. We therefore assessed the effects of polysaccharides of differing average molecular volume and charge.

2. Trans-synovial fluid drainage( s) was measured at controlled joint fluid pressure (P_j) in knees of anaesthetized rabbits. The joints were infused with polydisperse HA of weight-average mass 2100 kDa (4 mg ml^-1, n = 17), with polydisperse neutral dextran of similar average mass (2000 kDa; n = 7) or with Ringer solution vehicle (n = 2). The role of polymer charge was assessed by infusions of neutral or sulphated dextran of average molecular mass 500 kDa (n = 6).

3. When HA was present, s increased little with pressure, forming a virtual plateau of ~4 µl min^-1 from 10 to 25 cmH₂O. Neutral dextran 2000 failed to replicate this effect. Instead, s increased steeply with P_j, reaching eight times the HA value by 20 cmH₂O (P = 0·0001, ANOVA). Dextran 2000 reduced flows in comparison with Ringer solution.

4. Analysis of the aspirated joint fluid showed that 31 ± 0·07 % (s.e.m.) of dextran 2000 in the filtrand was reflected by synovium, compared with 79 % for HA. The viscometric molecular radius of the dextran, ~31 nm, was smaller than that of HA (101-181 nm), as was its osmotic pressure.

5. Anionic dextran 500 failed to buffer fluid drainage, but it reduced fluid escape and synovial conductance d s/dP_j more than neutral dextran 500 (P < 0·0001, ANOVA). The anionic charge increased the molecular volume and viscosity of dextran 500.

6. The results support the hypothesis that polymer molecular volume influences its reflection by interstitial matrix and outflow buffering. Polymer charge influences flow through an effect on viscosity and possibly electrostatic interactions with negatively charged interstitial matrix.

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