Molecular sieving of hyaluronan by synovial interstitial matrix and lymphatic capillary endothelium evaluated by lymph analysis in rabbits

Abstract

Synovial fluid hyaluronan (HA) profoundly buffers fluid loss from joints. This is attributed to the osmotic pressure of HA reflected by the joint lining. The aims were to quantify HA sieving during fluid drainage from joint to lymphatics and to compare the contributions of the synovial lining and lymphatic endothelium to sieving. HA (2100 kDa) and fluorescein-dextran (FD, 20 kDa) were infused under constant pressure into the knee cavity in anaesthetised rabbits. Samples were taken of femoral lymph and, after ∼3-h transsynovial filtration, subsynovial fluid and mixed intra-articular fluid. [HA] and [FD] were analysed by HPLC and reflection was calculated as one minus transmitted fraction. Subsynovial and lymph [HA] were 16 and 12% of intra-articular [HA], which increased to 2.6 times infusate [HA] ( P < 0.001, ANOVA, n = 19). [FD] was not significantly changed in infusate, aspirate, and subsynovial fluid but fell to 62% in femoral lymph due to dilution by skin/muscle lymph. The HA reflected fraction for the cavity-to-lymph barrier, R lymph, was 0.54 ± 0.03 ( n = 82, mean ± SEM), compared with 0.51 ± 0.07 for cavity-to-subsynovium ( R syn, P > 0.05, Bonferroni) and 0.07 ± 0.18 for subynovium-to-lymph ( R endo, P < 0.0001, Bonferroni). Lymphatic capillary endothelial reflection R endo was not significantly different from zero (one-sample t test). It is concluded that HA is partially sieved out of fluid leaving the joint cavity, and the sieve is the synovial lining interstitial matrix, not lymphatic capillary endothelium.