Inorganic pyrophosphate release by rabbit articular chondrocytes in vitro

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 35SO4 incorporation, and PPi release. Suppression of proteoglycan synthesis with tunicamycin did not influence the PPi release of the explants.