Extrusion of pyrophosphate into extracellular media by osteoarthritic cartilage incubates

The distribution of calcium pyrophosphate mineral phase, almost exclusively confined to articular cartilage in chondrocalcinosis, and the high level of pyrophosphate (PPi) ion relative to serum in synovial fluid in patients with either chondrocalcinosis or advanced osteoarthritis led to an investigation of whether cartilage cells elaborate PPi ions. Incubates of articular cartilage from young rabbits but not from mature rabbits, as well as growth plates cartilage, released PPi into incubation media during a 4h period. Control rabbit ear cartilage and synovial membrane elaborated negligible amounts of PPi. The PPi was shown to be undialyzable but could be dissociated from the alkaline phosphatase by ultracentrifugation. In 16 patients with osteoarthritis, a substantial output of PPi by samples of articular cartilage from the knee was demonstrated. It is postulated that either rapid cell division and matrix synthesis found in the base of ulcerating osteoarthritic cartilage or remodeling calcified sites are the source of the PPi in such osteoarthritic cartilage. It is further hypothesized that this PPi output accounts at least in part for the elevated PPi levels found in synovial fluid of patients with osteoarthritis.