Osteoclast response to low extracellular sodium

RAW 264.7 murine osteoclastic cells were cultured with recombinant cytokines (RANKL and M-CSF) in culture medium with normal sodium (NS; Na=140 mmol/l) and low sodium and reduced sodium (Na=120 mmol.l) while maintaining normal ostmolality with the addition of mannitol. After 24 hours, total RNA was extracted using PureLink kit (Invitrogen, Carlsbad, CA) according to manufacturer’s instructions, and purified using the Rneasy Mini Kit (Invitrogen). 5 µg RNA from each sample was converted into biotin-labeled cDNA and hybridized to GeneChip Mouse Genome 430 2.0 arrays containing 45,101 probe sets corresponding to known genes and expressed sequence tags. Total RNA was extracted using PureLink kit (Invitrogen, Carlsbad, CA) according to manufacturer’s instructions, and purified using the Rneasy Mini Kit (Invitrogen). 5 µg RNA from each sample was converted into biotin-labeled cDNA and hybridized to GeneChip Mouse Genome 430 2.0 arrays containing 45,101 probe sets corresponding to known genes and expressed sequence tags. Growing evidence indicated that chronic hyponatremia causes osteoporosis by increasing osteoclastic bone resorption, thereby liberating stored sodium from bone. Cell culture studies suggested that low extracellular sodium directly increases osteoclast function, but the underlying mechanisms remained to be defined.