Inorganic phosphate transport in matrix vesicles from bovine articular cartilage
Solomon DH., Browning JA., Wilkins RJ.
Aims: In mineralizing tissues such as growth plate cartilage extracellular organelles derived from the chondrocyte membrane are present. These matrix vesicles (MV), possess membrane transporters that accumulate Ca2+ and inorganic phosphate (Pi), and initiate the formation of hydroxyapatite crystals. MV are also present in articular cartilage, and hydroxyapatite crystals are believed to promote cartilage degradation in osteoarthritic joints. This study characterizes Pi transport in MV derived from articular cartilage. Methods: Matrix vesicles were harvested from collagenase digests of bovine articular cartilage by serial centrifugation. Pi uptake by MV was measured using radioactive phosphate ( 33[P]HPO42-). The Na+ dependence, pH sensitivity and effects of Pi analogues that inhibit Pi transport were determined. Results: Pi uptake was temperature-sensitive and comprised Na+-dependent and Na +-independent components. The Na+-dependent component saturated at high extracellular Pi concentrations, with a K m of 0.16 mm. In Na+-free solutions, uptake did not fully saturate implying that carrier-mediated uptake is supplemented by a diffusive pathway. Uptake was inhibited by phosphonoacetate and arsenate, although a fraction of Na+-independent Pi uptake persisted. Total Pi uptake was maximal at pH 6.5, and reduced at more acidic or alkaline values, representing inhibition of both components. Conclusion: These properties are highly similar to those of Pi uptake by chondrocytes, suggesting that MV inherit Pi transporters of the chondrocyte membrane from which they are derived. Na+-independent Pi uptake has not previously been described in MV from growth plate cartilage and is relatively uncharacterized, but warrants further attention in articular cartilage, given its likely role in initiating inappropriate mineral formation. © 2007 The Authors.