Increased release of excitatory amino acids by the actions of ATP and peroxynitrite on volume-regulated anion channels (VRACs) in astrocytes.

Rapid swelling of astrocytes in primary culture by exposure to hyposmotic medium (or slower swelling by exposure to high K+ medium) leads to release of the excitatory amino acids (EAAs) glutamate and aspartate. One question that arises is whether these phenomena are only relevant to pathological states such as ischemia and trauma where marked astrocytic swelling occurs or whether much smaller astrocytic volume changes, that might be encountered under physiological states, will cause such release. We have recently found that extracellular ATP strongly potentiated volume-regulated anion channels (VRACs)-mediated-excitatory amino acid release in non-swollen and osmotically swollen primary astrocyte cultures. However, ATP does not seem to directly activate but instead positively modulates VRACs and we postulate that a minor fraction of these are active under isoosmotic conditions based on the finding that in hyperosmotic media the ATP-induced increase was inhibited. Agonist and inhibitor analysis suggests that the effect of ATP is mediated by several subtypes of metabotropic P2Y receptors. Thus, the concept of volume transmission may be extended to volume-mediated transmission, whereby moderate cell swelling causes release of neurotransmitter substances. The product of the superoxide oxygen radical and nitric oxide, peroxynitrite, formed under pathological conditions such as cerebral ischemia, also potentiated the release of D-[3H]aspartate from astrocyte cultures exposed to limited or marked swelling via intracellular signaling mechanisms involving tyrosine kinases (TKs). Thus, the enhancement of cell volume-dependent release of excitatory amino acids from astrocytes can be physiological or pathological and its magnitude depends on the degree of the cell volume increase.