Our recent identification of glutamate receptors in bone cells suggested a
novel means of paracrine communication in the skeleton. To determine whethe
r these receptors are functional, we investigated the effects of the excita
tory amino acid, glutamate, and the pharmacological ligand, N-methyl-D-aspa
rtic acid (NMDA), on glutamate like receptors in the human osteoblastic cel
l lines MG63 and SaOS-2, Glutamate binds to osteoblasts, with a K-d of appr
oximately 10(-4) mol/L and the NMDA receptor antagonist, DL-2-amino-5-phosp
honovaleric 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, dizolcip
ine maleate (MK801), and Mg2+, and the competitive NMDA receptor antagonist
, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1 phosphoric acid (R-CPP), on agon
ist-induced currents. Both glutamate and NMDA induced significant increases
in membrane currents. Application of Mg2+ (200 mu mol/L) and MK801 (100 mu
mol/L) caused a significant decrease in inward currents elicited in respon
se to agonist stimulation. The competitive NMDA receptor antagonist, R-CPP
(100 mu mol/L), also partially blocked the NMDA-induced currents in MG63 ce
lls. This effect was reversed by addition of further NMDA (100 mu mol/L). I
n Pura-2-loaded osteoblasts, glutamate induced elevation of intracellular f
ree calcium, which was blocked by MK801, These results support the hypothes
is that glutamate plays a role in bone cell signaling and suggest a possibl
e role for glutamate agonists/antagonists in the treatment of bone diseases
. (C) 1999 by Elsevier Science Inc. All rights reserved.