When intra-articular pressure is raised to pathological values (> 9 cmH2O) by saline, the hydraulic conductance of the synovial lining increases manyfold. The increase at 25 cmH2O is only partially accounted for by stretching of the tissue and has been ascribed to washout and/or dilution of interstitial matrix biopolymers. This suggestion was tested in this study by sampling synovium from control joints (rabbit knees) and from joints perfused with saline to 25 cmH2O, and analysing them quantitatively for collagen, chondroitin sulphate, heparan sulphate and hyaluronan. 2. Pressure and trans-synovial flow measurements showed that in samples taken at 25 cmH2O the conductance of the synovial lining had increased by a factor of 5.23 +/- 1.5 (mean +/- S.E.M.) over the conductance at low pressures (just above atmospheric pressure). 3. The tissue concentrations of collagen and the sulphated glycosaminoglycans (GAGs) were reduced by similar amounts after perfusion to 25 cmH2O, namely to 62.8-70.4% of control. The hyaluronan concentration by contrast was not significantly reduced (106% of control). 4. The reduction in collagen concentration (fixed material) indicated increased interstitial hydration. The closely similar reduction in sulphated GAGs indicated that dilution rather than washout of these components was occurring. The hyaluronan results could be explained by synthesis in vivo at a rate of > or = 91 micrograms h-1 (ml synovium)-1 (possibly a non-basal rate under the conditions of the experiment, i.e. raised pressure and a stretched hydrated membrane). 5. Because interstitial hydraulic drag is related to biopolymer concentration by a power function, the overall matrix dilution observed here was more than sufficient to explain the rise in synovial lining hydraulic conductance at 25 cmH2O when taken in conjunction with stretching of the synovial lining (increased area, reduced thickness).
