Abstract
Several physiological agonists that induce elevation of cytosolic free calcium (Ca 2+)-levels act via receptor coupled G-proteins, involving activation of phospholipase C (PLC) and hydrolysis of phosphatidylinositol 4,5-bisphosphate. Activation of the inositol signal transduction pathway that precedes Ca 2+ ion mobilization is a well accepted signaling pathway in endothelial cell eicosanoid synthesis. This study was designed to examine possible involvement of phosphoinositides in the effects of oxygen free radicals on Ca 2+ liberation and eicosanoid synthesis in human umbilical venous endothelial cells (HUVEC). Hydrogen peroxide (H 2O 2) was chosen as oxygen radicals generating agent. Stimulation of HUVEC with H 2O 2 (0.1 mmol/l) led to significant rises in inositol phosphate and diacylglycerol (DAG) levels within 300 seconds and an inhibition of Ca 2+ release from internal stores. Eicosanoid formation was detectable despite unchanged levels of cytosolic free Ca 2+ and no detectable activation of membrane associated phospholipase A 2 (PLA 2). This suggests that eicosanoid formation may be mediated through the activation of a Ca 2+ independent, cytosolic 40 kDa PLA 2 isoenzyme and that DAG could serve as an alternative source for arachidonic acid and seems to sensitize a cytosolic PLA 2. © 1997 Elsevier Science Ltd
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