Ca2+‐Independent, Ca2+‐Dependent, and Carbachol‐ Mediated Arachidonic Acid Release from Rat Brain Cortex Membrane

Publisher Website
Google Scholar
Abstract
Synaptoneurosomes obtained from the cortex of rat brain prelabeled with [14C] arachidonic acid ([14C]AA) were used as a source of substrate and enzyme in studies on the regulation of AA release. A significant amount of AA is liberated in the presence of 2 mM EGTA, independently of Ca2+, primarily from phosphatidic acid and polyphosphoinositides (poly‐PI). Quinacrine, an inhibitor of phospholipase A2 (PLA2), suppressed AA release by about 60% and neomycin, a putative inhibitor of phospholipase C(PLC), reduced AA release by about 30%. An additive effect was exhibited when both inhibitors were given together. Ca2+ activated AA release. The level of Ca2+ present in the synaptoneurosomal preparation (endogenous level) and 5 μM CaC12 enhance AA liberation by approximately 25%, whereas 2 mM CaC12 resulted in a 50% increase in AA release relative to EGTA. The source for Ca2+‐dependent AA release is predominantly phosphatidylinositol (PI); however, a small pool may also be liberated from neutral lipids. Carbachol, an agonist of the cholinergic receptor, stimulated Ca2+‐dependent AA release by about 17%. Bradykinin enhanced the effect of carbachol by about 10–15%. This agonist‐mediated AA release occurs specifically from phosphoinositides (PI + poly‐PI). Quinacrine almost completely suppresses calcium‐ and carbachol‐mediated AA release. Neomycin inhibits this process by about 30% and totally suppresses the effect of bradykinin. Our results indicate that both phospholipases PLA2 and PLC with subsequent action of DAG lipase are responsible for Ca2+‐ independent AA release. Ca2+‐dependent and carbachol‐mediated AA liberation occurs mainly as the result of PLA2action. A small pool of AA is probably also released by PLC, which seems to be exclusively responsible for the effect of bradykinin.