Accumulation of [³H]glutamate in cultured rat calvarial osteoblasts

In the present study, we have attempted to demonstrate constitutive and functional expression in bone of particular glutamate transporters (GluTs) required for signal termination in glutamatergic signaling process. Reverse transcription polymerase chain reaction revealed constitutive expression of mRNA for the neuronal GluT subtype excitatory amino acid carrier-1, in addition to glial subtypes such as glutamate aspartate transporter and glutamate transporter-1, in rat calvarial osteoblasts cultured for 7-21 days in vitro (DIV). The accumulation of [³H]glutamate (Glu) occurred in a temperature- and sodium-dependent manner with pharmacological profiles similar to those for brain GluTs in osteoblasts cultured for 7 DIV, while three different agonists at ionotropic Glu receptors significantly inhibited the accumulation of [³H]Glu in osteoblasts. Although [³H]Glu accumulation consisted of a single component with a K_m value of 26.0±5.8 μM and a V_max value of 960±122 nmol/(min mg protein), respectively, in osteoblasts cultured for 7 DIV, in vitro maturation led to a significant decrease in V_max value to 290±33 nmol/(min mg protein) without significantly affecting K_m values on 21 DIV. These results suggest that Glu could be incorporated into intracellular locations through glial and/or neuronal GluT subtypes expressed in cultured rat calvarial osteoblasts.