Studies of articular cartilage macromolecules in the equine middle carpal joint, in joint pathology and training

Lameness caused by osteoarthritis (OA) is the most common reason for failure to race among horses. Degenerative changes to the articular cartilage are commonly seen often with subchondral bone sclerosis. Cartilage is a connective tissue with tensile strength and resilience, consisting of cells and abundant extracellular matrix. The cells are adapting the matrix to environmental conditions by synthesis and degradation of the matrix structural elements. A change in the concentration of macromolecules in synovial fluid and serum can indicate early biochemical changes in the joint. This provides a tool for research into and monitoring pathogenic mechanisms of OA. The content of COMP, aggrecan and collagen type II was measured in serum and synovial fluid (sf) from trotters and riding horses with either normal joints or joints with different stages of cartilage pathology. The trotters with a training background and cartilage degeneration had low concentrations of COMP and aggrecan in synovial fluid and serum as well as sf-collagen (paper I). The effect of long-term training on the concentration of sf- COMP, -aggrecan and -collagen type II from young trotters was measured (paper II). The amount of training and age influenced sf-COMP, where lower concentrations were related to more training and higher age. The concentration of collagen type II degradation products increased with total days of training. In vitro dynamic compression of cartilage explants from trained horses showed a down-regulation of COMP synthesis compared to untrained horses (paper II). COMP in cartilage matrix was presented in an ultrastructural study (paper III), where COMP concentration, was much less abundant in loaded and unloaded matrix of articular cartilage from a strenuously trained horse compared to untrained horses. The untrained horses often displayed a higher immunolabeling in loaded areas compared to unloaded areas, indicating that dynamic load can promote COMP synthesis and /or retention, while excessive load have opposite effect. Galloping horses with osteochondral fractures had higher concentration of sf-COMP compared to normal or osteoarthtritic joints from trotters or galloping horses (paper IV). In conclusion the present work shows that strenuously training of racehorses alter the expression of macromolecules in the articular cartilage of the third carpal bone.