Hydrogen Peroxide Activates Agonist-sensitive Ca2+-flux Pathways In Canine Venous Endothelial Cells
The effect of the biological oxidant H2O2 on purinergic-receptor-stimulated Ca2+ signalling was determined in canine venous endothelial cells. H2O2 increased cytosolic free [Ca2+] ([Ca2+]i), the rate of rise of which was dose-dependently related to H2O2 concentration. The response of [Ca2+]i to H2O2 resulted in part from release of Ca2+ from internal stores. The H2O2-sensitive intracellular Ca2+ pool was characterized in cells suspended in Ca(2+)-free/EGTA buffer and stimulated in sequence with H2O2 and ionomycin or ATP. Under this condition, the rank order of apparent compartment size sensitive to each compound was ionomycin > H2O2 > ATP. Stimulation of cells with H2O2 eliminated any response of [Ca2+]i to subsequent addition of ATP. To test more directly whether H2O2 accesses the inositol trisphosphate-sensitive Ca2+ store, cells were pretreated with thapsigargin, a selective inhibitor of that store's Ca2+ pump. Release of Ca2+ from internal Ca2+ stores by H2O2 declined as the interval after thapsigargin addition increased, a finding that supports the contention that H2O2 accesses the inositol trisphosphate-sensitive Ca2+ store. H2O2-stimulated Ca2+ influx across the cell membrane was sensitive to Ni2+, La3+, and 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole HCl (SKF-96365), a selective inhibitor of the agonist-stimulated Ca(2+)-influx pathway. Ca2+ entry triggered by H2O2 appears to occur via the agonist-sensitive Ca2+ influx pathway. Together, these results suggest that H2O2, which is normally secreted by activated neutrophils and monocytes, may act as an intercellular messenger and stimulate Ca2+ signalling in target endothelial cells.