Inorganic phosphate transport in matrix vesicles from bovine articular cartilage

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 (P_i), 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 P_i transport in MV derived from articular cartilage. Methods: Matrix vesicles were harvested from collagenase digests of bovine articular cartilage by serial centrifugation. P_i uptake by MV was measured using radioactive phosphate (³³[P]HPO). The Na⁺ dependence, pH sensitivity and effects of P_i analogues that inhibit P_i transport were determined. Results: P_i uptake was temperature‐sensitive and comprised Na⁺‐dependent and Na⁺‐independent components. The Na⁺‐dependent component saturated at high extracellular P_i 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 P_i uptake persisted. Total P_i 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 P_i uptake by chondrocytes, suggesting that MV inherit P_i transporters of the chondrocyte membrane from which they are derived. Na⁺‐independent P_i 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.