Effect of linoleic acid hydroperoxide on endothelial cell calcium homeostasis and phospholipid hydrolysis

The relationship between intracellular free calcium ion concentrations ([Ca2+]i) and release of arachidonic acid from membrane phospholipids following peroxidation was examined in rabbit aortic endothelial cells treated with linoleic acid hydroperoxide (LOOH). LOOH (0.1-0.4) mumol/10(6) cells) caused a rapid and dose-dependent transient increase in [Ca2+]i in the presence of extracellular Ca2+ that remained elevated over baseline for 15 to 30 s. In the absence of extracellular Ca2+, LOOH also evoked a transient increase in [Ca2+]i of lesser magnitude which immediately returned to basal (or below basal) levels. In this regard, the rise in intracellular Ca2+ after LOOH or vasopressin (AVP) treatments involved, at least in part, related intracellular pools that in each case was followed by influx of extracellular Ca2+. The intracellular membrane sources known to be affected by vasopressin were not directly involved. Most notably, the LOOH evoked rise in [Ca2+]i was not associated with release of IP3, suggesting that the source of intracellular Ca2+ is not IP3-sensitive pools. However, pretreatment with LOOH strongly inhibited the rise in [Ca2+]i upon subsequent addition of AVP or LOOH and the extent of such inhibition was dependent on the availability of free intracellular Ca2+ and presence of extracellular Ca2+. These findings suggest that reuptake of Ca2+ into intracellular membrane pools is reduced in the presence of LOOH and/or the availability of Ca2+ from agonist-sensitive sites is inhibited by LOOH. An increase in free 20:4 levels was found after LOOH treatment that was only partly prevented using intracellular Ca2+ chelators which maintained [Ca2+]i at basal levels after LOOH treatment. These findings suggest that LOOH induction of phospholipid hydrolysis proceeds following small transients in [Ca2+]i that are considerably less than that evoked by agents such as AVP, approximating basal Ca2+ concentrations. Inhibition of LOOH-induced lipid peroxidation by vitamin E also prevented the rise in [Ca2+]i and 20:4 release indicating that phospholipid hydrolysis is dependent, at least in part, on membrane lipid peroxidation. Inhibition of protein kinase C (PKC) completely blocked LOOH-induced release of 20:4 but had little effect on the LOOH-induced rise in [Ca2+]i, suggesting an indirect relationship between LOOH-induced membrane Ca2+ signalling events, with intervention via PKC-mediated induction of phospholipid hydrolysis. A rapid and progressive translocation of PKC to the membrane fraction was evident after LOOH addition over the time course corresponding to the maximal release of 20:4 which was also inhibited by vitamin E.(ABSTRACT TRUNCATED AT 400 WORDS)