Functional characterization of N-methyl-d-aspartic acid-gated channels in bone cells

Our recent identification of glutamate receptors in bone cells suggested a novel means of paracrine communication in the skeleton. To determine whether these receptors are functional, we investigated the effects of the excitatory amino acid, glutamate, and the pharmacological ligand, N-methyl-d-aspartic acid (NMDA), on glutamate-like receptors in the human osteoblastic cell lines MG63 and SaOS-2. Glutamate binds to osteoblasts, with a Kd of approximately 10−4 mol/L and the NMDA receptor antagonist, dl-2-amino-5-phosphonovaleric acid (D-APV), inhibits binding. Using the patch-clamp technique, we measured whole-cell currents before and after addition of l-glutamate or NMDA and investigated the effects of the NMDA channel blockers, dizolcipine maleate (MK801), and Mg2+, and the competitive NMDA receptor antagonist, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphoric acid (R-CPP), on agonist-induced currents. Both glutamate and NMDA induced significant increases in membrane currents. Application of Mg2+ (200 μmol/L) and MK801 (100 μmol/L) caused a significant decrease in inward currents elicited in response to agonist stimulation. The competitive NMDA receptor antagonist, R-CPP (100 μmol/L), also partially blocked the NMDA-induced currents in MG63 cells. This effect was reversed by addition of further NMDA (100 μmol/L). In Fura-2-loaded osteoblasts, glutamate induced elevation of intracellular free calcium, which was blocked by MK801. These results support the hypothesis that glutamate plays a role in bone cell signaling and suggest a possible role for glutamate agonists/antagonists in the treatment of bone diseases.