Characterisation of Inorganic Phosphate Transport in Bovine Articular Chondrocytes
In mineralising tissues such as growth plate cartilage extracellular organelles derived from the chondrocyte membrane are present. These matrix vesicles (MV) possess membrane transporters that accumulate Ca<sup>2+</sup> and inorganic phosphate (P<sub>i</sub>), 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. In the present study, P<sub>i</sub> transport pathways in isolated bovine articular chondrocytes have been characterised. P<sub>i</sub> uptake was temperature-sensitive and could be resolved into Na<sup>+</sup>-dependent and Na<sup>+</sup>-independent components. The Na<sup>+</sup>-dependent component saturated at high concentrations of extracellular P<sub>i</sub>, with a K<sub>m</sub> for P<sub>i</sub> of 0.17mM. In solutions lacking Na<sup>+</sup>, uptake did not fully saturate, implying that under these conditions carrier-mediated uptake is supplemented by a diffusive pathway. Both Na<sup>+</sup>-dependent and Na<sup>+</sup>-independent components were sensitive to the P<sub>i</sub> transport inhibitors phosphonoacetate and arsenate, although a fraction of Na<sup>+</sup>-independent P<sub>i</sub> uptake was resistant to these anions. Total P<sub>i</sub> uptake was optimal at pH 7.4, and reduced as pH was made more acidic or more alkaline, an effect that represented reduced Na<sup>+</sup>-dependent influx. RT-PCR analysis confirmed that two members of the NaPi III family, Pit-1 and Pit-2, are expressed, but that NaPi II transporters are not.