Two conventional protein kinase C isoforms, α and βI, are involved in the ATP-induced activation of volume-regulated anion channel and glutamate release in cultured astrocytes

Volume-regulated anion channels (VRACs) are activated by cell swelling and are permeable to inorganic and small organic anions, including the excitatory amino acids glutamate and aspartate. In astrocytes, ATP potently enhances VRAC activity and glutamate release via a P2Y receptor-dependent mechanism. Our previous pharmacological study identified protein kinase C (PKC) as a major signaling enzyme in VRAC regulation by ATP. However, conflicting results obtained with potent PKC blockers prompted us to re-evaluate the involvement of PKC in regulation of astrocytic VRACs by using small interfering RNA (siRNA) and pharmacological inhibitors that selectively target individual PKC isoforms. In primary rat astrocyte cultures, application of hypoosmotic medium (30% reduction in osmolarity) and 20 mu M ATP synergistically increased the release of excitatory amino acids, measured with a non-metabolized analog of L-glutamate, D-[(3)H]aspartate. Both Go6976, the selective inhibitor of Ca(2+)-sensitive PKC alpha, beta I/II, and gamma, and MP-20-28, a cell permeable pseudosubstrate inhibitory peptide of PKC alpha and beta I/II, reduced the effects of ATP on D-[(3)H]aspartate release by similar to 45-55%. Similar results were obtained with a mixture of siRNAs targeting rat PKC alpha and beta I. Surprisingly, down-regulation of individual alpha and beta I PKC isozymes by siRNA was completely ineffective. These data suggest that ATP regulates VRAC activity and volume-sensitive excitatory amino acid release via cooperative activation of PKC alpha and beta I.