Effect of proteoglycan removal on solute mobility in articular cartilage

Transport of nutrients, cytokines, pharmacologic agents, and matrix components through articular cartilage is critical for the viability and structural integrity of the tissue. To understand the role of the extracellular matrix in regulating this process, we measured the diffusivity of three uncharged solutes of different molecular size (glucose, MW 180; inulin, MW 5000; dextran, MW 70,000) into intact cartilage and cartilage that had its proteoglycan (PG) component removed. Solute diffusivity was measured by performing transient (nonsteady state) one-dimensional diffusion tests using radiolabelled solutes. Compared to intact cartilage, the diffusivity of glucose was unchanged after PG removal, inulin was unchanged but dextran increased by 1.7 times after 71% PG removal, and both inulin and dextran increased by 1.6 and 2.0 times, respectively, after 93% PG removal. The diffusivities of inulin and dextran were inversely proportional to the PG content. While no change was found in the tissue's bulk fluid content, PG depletion resulted in an increase in fluid content in the upper regions of the tissue and a decrease in the lower regions. These results indicate that in intact tissue small uncharged solutes have free mobility through the inter-molecular and intra-molecular PG volumes, larger molecules have limited intra-molecular mobility, and very large molecules are excluded from the intra-molecular space.