title: Inorganic phosphate transport in matrix vesicles from bovine articular cartilage.
abstract: OBJECTIVE
  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 Ca(2+) 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(4)(2-)). 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, UK.
authors.forename: D H
authors.initials: DH
authors.lastname: Solomon
authors.forename: J A
authors.initials: JA
authors.lastname: Browning
authors.forename: R J
authors.initials: RJ
authors.lastname: Wilkins
mesh_terms: D000818:Animals; D001149:Arsenates / Q000494:pharmacology; D002358:Cartilage, Articular / Q000187:drug effects / Q000378:metabolism*; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D022162:Cytoplasmic Vesicles / Q000187:drug effects / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000187:drug effects / Q000502:physiology; D009994:Osmolar Concentration; D010710:Phosphates / Q000378:metabolism*; D010746:Phosphonoacetic Acid / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D013997:Time Factors
chemical_list: D001149:Arsenates; D004791:Enzyme Inhibitors; D010710:Phosphates; D012964:Sodium; C025657:arsenic acid; D010746:Phosphonoacetic Acid