Facilitation of glutamate release by ionotropic glutamate receptors in osteoblasts

Constitutive expression of mRNA was seen for the vesicular glutamate transporter brain-specific Na⁺-dependent inorganic phosphate cotransporter (BNPI), but not differentiation-associated Na⁺-dependent inorganic phosphate cotransporter, in rat calvarial osteoblasts cultured for 7 and 21 days in vitro (DIV). Three different agonists for ionotropic glutamate receptors (iGluR) at 1 mM, as well as 50 mM KCl, significantly increased the release of endogenous L-glutamate from osteoblasts cultured for 7 DIV when determined 5 min after the addition by using a high performance liquid chromatograph. The inhibitor of desensitization of DL-α-amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA) receptors cyclothiazide significantly potentiated and prolonged the release of endogenous L-glutamate evoked by AMPA in a dose-dependent manner. The release evoked by AMPA was significantly prevented by the addition of an AMPA receptor antagonist as well as by the removal of Ca²⁺ ions. These results suggest that endogenous L-glutamate could be released from intracellular vesicular constituents associated with BNPI through activation of particular iGluR subtypes expressed in cultured rat calvarial osteoblasts.