Arachidonic acid and docosahexaenoic acid suppress thrombin-evoked Ca2+ response in rat astrocytes by endogenous arachidonic acid liberation

Arachidonic (AA) and docosahexaenoic acid (DHA) are the major polyunsaturated fatty acids (PUFAs) in the brain. However, their influence on intracellular Ca2+ signalling is still widely unknown. In astrocytes, the amplitude of thrombin- induced Ca2+ response was time-dependently diminished by AA and DHA, or by the AA tetraynoic analogue ETYA, but not by eicosapentaenoic acid (EPA). Thrombin-elicited Ca2+ response was reduced (20-30%) by 1-min exposure to AA or DHA. Additionally, 1-min application of AA or DHA together with thrombin in Ca2+-free medium blocked Ca2+ influx, which followed after readdition of extracellular Ca2+. EPA and ETYA, however, were ineffective. Long-term treatment of astrocytes with AA and DHA, but not EPA reduced the amplitude of the thrombin-induced Ca2+ response by up to 80%. AA and DHA caused a comparable decrease in intracellular Ca2+ store content. Only DHA and AA, but not EPA or ETYA, caused liberation of endogenous AA by cytosolic phospholipase A2 (cPLA2). Therefore, we reasoned that the suppression of Ca2+ response to thrombin by AA and DHA could be due to release of endogenous AA. Possible participation of AA metabolites, however, was excluded by the finding that specific inhibitors of the different oxidative metabolic pathways of AA were not able to abrogate the inhibitory AA effect. In addition, thrombin evoked AA release via activation of cPLA2. From our data we propose a novel model of positive/negative-feed-back in which agonist-induced release of AA from membrane phospholipids promotes further AA release and then suppresses agonist-induced Ca2+ responses.