Long-distance running causes site-dependent decrease of cartilage glycosaminoglycan content in the knee joints of beagle dogs.

OBJECTIVE: To study the effects of a long-term (1-year) program of running exercise (up to 40 km/day) on the thickness and glycosaminoglycan (GAG) content of articular cartilage in the knee and humeral head cartilage of young dogs.
METHODS: Samples for histologic analysis were obtained from 12 different locations of the joints. We conducted a detailed, area-specific analysis, measuring the thickness of articular cartilage and analyzing the distribution of Safranin O stain that binds stoichiometrically to GAG as determined by quantitative microspectrophotometry.
RESULTS: Running exercise decreased the GAG content of the uncalcified articular cartilage in the weight-bearing summits of the femoral condyles by 5-13% (P < 0.05), while at margins of these areas the GAG content was equivalent to control levels. In the lateral condyle of the femur, the reduction was most prominent in the superficial zone (up to 28% decrease; P < 0.05), and extended into the intermediate zone (11% decrease; P < 0.05). GAG content was also significantly reduced in the superficial zone at the lateral condyle of the tibia and the head of the humerus, by 35% (P < 0.01) and 15% (P < 0.05), respectively. Running did not alter GAG concentration in the patellofemoral region.
CONCLUSION: The GAG depletion caused by 40-km/day running exercise is restricted to prominent weight-bearing areas of the joint and begins from the superficial cartilage without signs of degeneration. The different degree and type of joint loading can explain the site-dependent cartilage response to long-distance running. The loss of GAGs was possibly due to breakdown of proteoglycans, which could not be compensated for by neosynthesis of molecules. With time, this may affect the condition of articular cartilage, especially if the joint is exposed to loading for lengthy periods.