Effect of protein S-glutathionylation on Ca2+ homeostasis in cultured aortic endothelial cells

Diamide is a membrane-permeable, thiol-oxidizing agent that rapidly and reversibly oxidizes glutathione to GSSG and promotes formation of protein-glutathione mixed disulfides. In the present study, the acute effect of diamide on free cytosolic Ca2+ concentration ([Ca2+]i) was examined in fura-2-loaded bovine aortic endothelial cells. At low concentrations (50, 100 μM), diamide reversibly increased spontaneous, asynchronous Ca2+ oscillations, whereas, at higher concentrations (250, 500 μM), diamide caused an immediate synchronized Ca2+ oscillation in essentially all cells of the monolayer, followed by a time-dependent rise in basal [Ca2+]i. The effects of diamide on [Ca2+]i dynamics were independent of extracellular Ca2+. Inhibition of phospholipase C by U-73122 prevented the observed changes in [Ca2+]i. Additionally, the diamide-induced oscillations, but not the rise in basal [Ca2+]i, were blocked by inhibition of the inositol-1,4,5-trisphosphate (IP3) receptor (IP3R) by 2-aminoethyl diphenyl borate. However, diamide failed to alter the plasmalemmal distribution of a green fluorescent protein-tagged phosphatidylinositol-4,5-bisphosphate binding protein, demonstrating that diamide does not activate phospholipase C. Inhibition of glutathione reductase by N,N'-bis(2-chloroethyl)-N-nitrosourea or depletion of glutathione by l-buthionine-sulfoximine enhanced the effects of diamide, which, under these conditions, could only be reversed by addition of dithiothreitol to the wash buffer. Biochemical assays showed that both the IP3R and the plasmalemmal Ca2+-ATPase pump could be reversibly glutathionylated in response to diamide. These results demonstrate that diamide promotes Ca2+ release from IP3-sensitive internal Ca2+ stores and elevates basal [Ca2+]i in the absence of extracellular Ca2+, effects that may be related to a diamide-induced glutathionylation of the IP3R and the plasmalemmal Ca2+-ATPase Ca2+ pump, respectively.