Mechanism of arachidonic acid-induced Ca²⁺ mobilization from rat liver microsomes

Recent evidence indicates that unesterified arachidonic acid functions as a mediator of intracellular Ca²⁺ mobilization by inducing Ca²⁺ release from the endoplasmic reticulum of pancreatic islet beta cells in a manner closely similar to that of inositol 1,4,5-trisphosphate. To test the generality and explore the mechanism of this phenomenon we have examined the effects of arachidonic acid on calcium accumulation and release by hepatocyte subcellular fractions enriched in endoplasmic reticulum (microsomes). At concentrations above 0.017 μmol/mg microsomal protein, arachidonate induced rapid (under 2 min) ⁴⁵Ca²⁺ release from microsomes that had been preloaded with ⁴⁵Ca²⁺. Arachidonate also suppressed microsomal ⁴⁵Ca²⁺ accumulation when present during the loading period, as reflected by reduction both of ⁴⁵Ca²⁺ accumulation at steady state and of the rate of uptake. Neither the cyclooxygenase inhibitor indomethacin nor the lipoxygenase/cyclooxygenase inhibitor BW755C suppressed arachidonate-induced ⁴⁵Ca²⁺ release, indicating that this effect was not dependent upon oxygenation of the fatty acid to metabolites. The long-chain unsaturated fatty acids oleate and linoleate were less potent than arachidonate in inducing ⁴⁵Ca²⁺ release, and the saturated fatty acid stearate did not exert this effect. Albumin prevented ⁴⁵Ca²⁺ release by arachidonate, presumably by binding the fatty acid. As is the case for inositol 1,4,5-trisphosphate, the ability of arachidonate to induce ⁴⁵Ca²⁺ release was dependent on the ambient free Ca²⁺ concentration. Arachidonate did not influence microsomal membrane permeability or Ca²⁺-ATPase activity and may exert its effects on microsomal Ca²⁺ handling by activation of a Ca²⁺ extrusion mechanism or by dissociating Ca²⁺ uptake from Ca²⁺-ATPase activity.