Critical role of NADPH oxidase-derived reactive oxygen species in generating Ca2+ oscillations in human aortic endothelial cells stimulated by histamine

There is increasing evidence that intracellular reactive oxygen species (ROS) play a role in cell signaling and that the NADPH oxidase is a major source of ROS in endothelial cells. At low concentrations, agonist stimulation of membrane receptors generates intracellular ROS and repetitive oscillations of intracellular Ca2+ concentration ([Ca2+](i)) in human endothelial cells. The present study was performed to examine whether ROS are important in the generation or maintenance of [Ca2+](i) oscillations in human aortic endothelial cells (HAEC) stimulated by histamine. Histamine (1 mum) increased the fluorescence of 2',7'-dihydrodichlorofluorescin diacetate in HAEC, an indicator of ROS production. This was partially inhibited by the NADPH oxidase inhibitor diphenyleneiodonium (DPI, 10 pm), by the farnesyltransferase inhibitor H-Ampamb-Phe-Met-OH (2 mum), and in HAEC transiently expressing Rac1(N17), a dominant negative allele of the protein Rac1, which is essential for NADPH oxidase activity. In indo 1-loaded HAEC, 1 mum histamine triggered [Ca2+](i) oscillations that were blocked by DPI or H-Ampamb-Phe-Met-OH. Histamine-stimulated [Ca2+](i) oscillations were not observed in HAEC lacking functional Rac1 protein but were observed when transfected cells were simultaneously exposed to a low concentration of hydrogen peroxide (10 muM), which by itself did not alter either [Ca2+](i) or levels of inositol 1,4,5-trisphosphate (Ins-1,4,5-P-3). Thus, histamine generates ROS in HAEC at least partially via NADPH oxidase activation. NADPH oxidase-derived ROS are critical to the generation of [Ca2+](i) oscillations in HAEC during histamine stimulation, perhaps by increasing the sensitivity of the endoplasmic reticulum to Ins-1,4,5-P-3.