Activation of voltage-gated Na+ and Ca2+ channels is required for glutamate release from retinal glial cells implicated in cell volume regulation

Gliotransmitters such as glutamate and ATP play an essential role in the prevention of the osmotic swelling of retinal glial (Muller) cells. It has been shown that vascular endothelial growth factor (VEGF) induces a Ca²⁺-dependent release of glutamate from the cells [Wurm et al. (2008), J Neurochem 104:386-399]. In the present study, we investigated with cell swelling experiments on freshly isolated retinal glial cells of the rat whether activation of voltage-gated Na⁺ (Na(v)) and Ca²⁺ channels (VGCCs) is implicated in mediating the VEGF-induced release of glutamate. We found that the inhibitory effect of VEGF on the osmotic swelling of retinal glial cells, used as an indicator of glutamate release, is prevented in the presence of selective blockers of T-type VGCCs (kurtoxin, mibefradil, Ni²⁺) and Na(v) channels (TTX, saxitoxin, phenytoin). In contrast, the swelling-inhibitory effect of glutamate, that is mediated by a downstream release of ATP, remained unaffected in the presence of the blockers. The cells displayed immunolabeling for VGLUT3, Ca(v)1.2, Ca(v)3.1, and Na(v)1.6. In addition to VEGF, various other receptor agonists including neuropeptide Y, progesterone, erythropoietin, and endothelin-1 evoked a VGCC- and Na(v) channel-dependent release of glutamate. It is concluded that activation of T-type VGCCs and Na(v) channels is implicated in mediating the ligand-induced release of glutamate from retinal glial cells of the rat. The involvement of VLGUTs might suggest that glutamate is released by vesicular exocytosis.