Release of inorganic pyrophosphate (PPI) by rabbit articular chondrocytes in vitro was measured by a newly developed assay which utilizes radioactive orthophosphate (32Pi) labeling and anion exchange high performance liquid chromatography. Chondrocytes in monolayer and high density culture failed to release PPi. Explants (cartilage fragments), however, released newly formed PPi into the culture medium. Trypsin treatment of cartilage fragments almost completely blocked the PPi extrusion. Collagenase treatment had no effect on PPi extrusion. There was no clear correlation between proteoglycan synthesis, measured by 35S04 incorporation, and PPi release. Suppression of proteoglycan synthesis with tunicamycin did not influence the PPi release of the explants.Calcification of articular cartilage above the tidemark (chondrocalcinosis), either by calcium pyrophosphate dihydrate (CPPD) crystals or by hydroxyapatite (HA) crystals, is a relatively frequent pathologic finding. Both CPPD crystal deposition and HA crystal deposition are associated with osteoarthritis (OA) (1,2). These calcifications may be locally elaborated. CPPD crystal deposition has been found in humans following meniscectomy (3) and as a postarthritis phenomenon (4), and in animals after trauma From the Jan van Breemen Institute,

Medial sesamoid bones from the metacarpophalangeal joints of calves were used for prolonged culture of anatomically intact articular cartilage on its natural bone support. The cartilage remained viable during culture, without signs of degeneration. After 1 wk of culture the cartilage showed an increased proteoglycan synthesis, and some minor changes in the composition of newly synthesized proteoglycans were observed. In the next 7 wk all studied parameters remained constant, except for the rate of proteoglycan synthesis, which declined between 4 and 8 wk to values just below those measured at the start of culture. Despite the fact that newly synthesized proteoglycans showed some altered biochemical properties, the composition of the total pool of proteoglycans did not change during 8 wk of culture. The significance of this phenomenon is discussed. This new in vitro model of intact articular cartilage offers a promising alternative to in vivo studies because in contrast to other in vitro models no surgical injury of the cartilage is introduced.

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