ATP can be released from neurons and act as a neuromodulator in the nervous system. Besides neurons, cortical astrocytes also are capable of releasing ATP from acidic vesicles in a Ca2+-dependent way. In the present work, we investigated the release of ATP from Müller glia cells of the chick embryo retina by examining quinacrine staining and by measuring the extracellular levels of ATP in purified Müller glia cultures. Our data revealed that glial cells could be labeled with quinacrine, a reaction that was prevented by incubation of the cells with 1 μM bafilomycin A1 or 2 μM Evans blue, potent inhibitors of vacuolar ATPases and of the vesicular nucleotide transporter, respectively. Either 50 mM KCl or 1 mM glutamate was able to decrease quinacrine staining of the cells, as well as to increase the levels of ATP in the extracellular medium by 77% and 89.5%, respectively, after a 5 min incubation of the cells. Glutamate-induced rise in extracellular ATP could be mimicked by 100 μM kainate (81.5%) but not by 100 μM NMDA in medium without MgCl2but with 2 mM glycine. However, both glutamate- and kainate-induced increase in extracellular ATP levels were blocked by 50 μM of the glutamatergic antagonists DNQX and MK-801, suggesting the involvement of both NMDA and non-NMDA receptors. Extracellular ATP accumulation induced by glutamate was also blocked by incubation of the cells with 30 μM BAPTA-AM or 1 μM bafilomycin A1. These results suggest that glutamate, through activation of both NMDA and non-NMDA receptors, induces the release of ATP from retinal Müller cells through a calcium-dependent exocytotic mechanism. © 2010 Elsevier Ltd. All rights reserved.
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