Group III metabotropic glutamate receptor activation inhibits Ca2+ influx and nitric oxide synthase activity in bone marrow stromal cells

Nitric oxide (NO) is pivotal to bone physiology. In the central nervous system constitutive, Ca2+-calmodulin regulated NO synthase activity and glutamate signalling are intimately linked. Since L-glutamate signalling occurs in bone and is implicated in bone regulation, we have investigated the effect Of L-glutamate on NO synthase in bone-derived cells. Treatment of marrow stromal cells with L-glutamate reduced basal NO synthase activity by 40%. Imaging showed that L-glutamate caused a rapid, usually localised and slowly-reversible fall in [Ca2+]. This effect was resistant to disruption of intracellular Ca2+ stores but sensitive to extracellular La3+ or omission of extracellular Ca2+, demonstrating that glutamate acts by inhibition of membrane Ca2+ influx. The only previous description Of Such an effect Of L-glutamate is via activation of the group 111 receptor, mGluR6, in the retina. Using Western blotting and RT-PCR we detected mGIuR6 protein and transcripts in marrow stromal cells. The effects Of L-glutamate on NOS activity and [Ca2+] in marrow stromal cells were abolished by a group III mGIuR inhibitor, (S)-2-amino-2-methyl-4-phosphonobutyric acid. Recording of membrane potential showed that, similarly to the effects of retinal mGIuR6 activation, L-glutamate induced membrane hyperpolarisation (-16 +/- 2 mV), which was also sensitive to group III mGIuR inhibition. L-glutamate had no effect on cAMP levels. We conclude that activation of a group III mGIuR in bone marrow stromal cells inhibits a Ca2+ permeable plasma membrane channel, reducing [Ca2+](i) and suppressing generation of NO. These observations directly link bone L-glutamate signalling to processes central to bone growth and regulation. (c) 2005 Wiley-Liss, Inc.