Site-specific proteoglycan characteristics of third carpal articular cartilage in exercised and nonexercised horses

The relevance of site and the influence of exercise on third carpal articular cartilage proteoglycan (PG) were assessed in 16 horses. Six horses were exercised (exercised group) for 30 minutes, 3 times/wk, for 6 weeks. The other 10 horses (nonexercised group) were housed in box stalls for the same 6-week period. At week 6, articular cartilage from the proximal surface of the right third carpal bone was harvested and cultured with radioactive sulfate to label newly synthesized PG. Endogenous PG was measured by use of a uronic acid assay. Newly synthesized and endogenous PG were characterized by use of Sepharose CL-2B chromatography, composite gel electrophoresis, and/or immunoblot analysis with monoclonal antibody 1C6 directed against the hyaluronic acid-binding region on PG. There was a significant (P = 0.0002) effect of exercise, but not site, on newly synthesized PG, which was increased in the exercised horses, compared with the nonexercised horses at the end of the 6-week study period. The increase in newly synthesized PG was not reflected in the existing cartilage matrix as there was no significant difference between groups in endogenous PG. However, there was a significant (P = 0.01) effect of site on endogenous PG, with the nest of sites located in the palmar aspect of the radial facet containing a greater concentration of endogenous PG than the nests of sites located on the dorsal aspect of the radial facet or all sites on the intermediate facet. Most newly synthesized PG in both groups consisted of hydrodynamically small PG monomers. However, there was a change in the profile of newly synthesized PG at some sites in the exercised horses to include an early elution peak on Sepharose CL-2B, which may contain aggregating PG. All sites in both groups contained a diverse population of endogenous large and small PG on toluidine blue-stained composite gels that reacted with monoclonal antibody 1C6, indicating the potential to bind to hyaluronic acid.