Composition and glycosaminoglycan metabolism of articular cartilage from habitually loaded and habitually unloaded sites.

The uronic acid (proteoglycan, PG) content of cartilage from habitually unloaded sites of normal canine femoral condyles has been shown to be lower than that from habitually loaded regions, even though the glycosaminoglycan (GAG) synthesis is similar. We investigated whether the GAG degradation in unloaded cartilage would be greater than that in loaded cartilage, and we obtained comparative biochemical data concerning the PGs and organization of the extracellular matrix of normal loaded and unloaded cartilage. PG extractability (determined by sequential guanidinium chloride extracts of cartilage), percentage of PGs forming large aggregates, and hydrodynamic size of the PG monomers (determined by Sepharose 2B chromatography) were essentially the same in loaded and unloaded cartilage. As expected, the uronic acid content of unloaded cartilage was 20% lower than that of loaded cartilage (P less than 0.02), while the water and DNA contents of the 2 tissues were not statistically different. There was no difference in the rate of net 35SO4-GAG synthesis in organ cultures of loaded and unloaded cartilage. Moreover, there was no appreciable difference in the rates of 35SO4-GAG degradation of loaded and unloaded cartilage, as determined by 35SO4 pulse-chase studies. We have previously shown that selective cyclic compressive stresses applied in vitro to cartilage from loaded areas of canine femoral condyles may increase 35SO4-GAG synthesis. The present results suggest that the rates of GAG metabolism in loaded and unloaded cartilage under atmospheric pressure in vitro may not reflect the rates which exist in articular joints under compressive loads in vivo.

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