Mechanisms of macromolecular transport through articular cartilage: Relevance to loading

Cartilage matrix integrity is critical to the normal load bearing function of articulating joints. Controlled solute transport in a tissue without vasculature is essential not only to maintain appropriate nutrient supply and waste removal but also to provide regulatory enzymes, inhibitors, cytokines and growth factors essential to maintain chondrocyte metabolic activity. Previous studies have demonstrated that mechanical compression can regulate chondrocyte metabolic response in vitro and in vivo. Physiologic compression of cartilage produces matrix compaction, fluid flow, streaming potentials and currents, as well as changes in the physicochemical composition of cartilage. The effects of these individual components on solute transport through the cartilage matrix are the focus of this study.

We have studied the contributions of diffusion, fluid flow and electrical migration to molecular transport through adult articular cartilage explants using neutral and charged solutes that were either radiolabeled or fluorescently tagged. In order to induce fluid flow within the cartilage matrix without mechanical deformation, electric current densities were applied across cartilage disks. These currents produced electroosmotic fluid velocities of 1-2 $\mu$m/sec, magnitudes that have been reported to exist during joint loading in vivo. This fluid convection was observed to enhance neutral solute flux relative to passive diffusion alone by a factor that increased with the size of the solute. The enhancement factor seen for low molecular weight molecules such as $\sp3$H-water was 2.3-fold, that for $\sp3$H-raffinose (594 Da) and similar sized neutral solutes was 10-fold, suggesting that the effect of fluid flow may be important even for small solutes. Enhancement of much higher magnitudes were observed for larger molecules such as Lissamine-dextran, and proteins such as TIMP, the endogenous inhibitor of matrix metalloproteinases (enzymes known to degrade cartilage), and IGF-1, a growth factor known to influence chondrocyte metabolic activity. Our results confirmed that fluid convection is even more important to the transport of large solutes. We also studied the electrophoretic contribution to the flux of charged solutes which is relevant in the presence of intratissue streaming potentials induced during loading in vivo. Using the negatively charged $\sp{35}$S-sulfate ion with a range of current densities, as much as a 10-fold enhancement in flux was observed. To separate the contributions of fluid flow and electrical migration to transport of charged proteins, we titrated the charge of TIMP and IGF-1 and found that the contribution of fluid flow dominates over the electrical migration. From these data, values for the intrinsic transport properties of the solutes (e.g., diffusivity, electrical mobility, hydrodynamic hindrance factor) were obtained. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)