Both Na(+)-dependent and Na(+)-independent components were sensitive to the P(i) transport inhibitors phosphonoacetate and arsenate, although a fraction of Na(+)-independent P(i) uptake was resistant to these anions. In solutions lacking Na(+), uptake did not fully saturate, implying that under these conditions carrier-mediated uptake is supplemented by a diffusive pathway. The Na(+)-dependent component saturated at high concentrations of extracellular P(i), with a K(m) for P(i) of 0.17 mM. P(i) uptake was temperature-sensitive and could be resolved into Na(+)-dependent and Na(+)-independent components. In the present study, P(i) transport pathways in isolated bovine articular chondrocytes have been characterised. MV are also present in articular cartilage, and hydroxyapatite crystals are believed to promote cartilage degradation in osteoarthritic joints. These matrix vesicles (MV) possess membrane transporters that accumulate Ca(2+) and inorganic phosphate (P(i)), and initiate the formation of hydroxyapatite crystals. In mineralising tissues such as growth plate cartilage extracellular organelles derived from the chondrocyte membrane are present.
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