Inhibition of storen-dependent capacitative Ca 2+ influx by unsaturated fatty acids

Abstract

The effects of the unsaturated fatty acids, arachidonic and oleic acid, on the influx of Ca 2+ activated by depletion of intracellular stores with thapsigargin were investigated in various cell types. By using a Ca 2+free/Ca 2+ reintroduction protocol, we observed that arachidonic acid (2 to 5 μM) inhibited thapsigargin-induced rises in cytosolic free Ca 2+ ([Ca 2+] i) in Ehrlich tumor cells, Jurkat T lymphocytes, rat thymocytes, and Friend erythroleukemia and PC12 rat pheochromocytoma cells. This effect was attributed to the inhibition of Ca 2+ entry, since arachidonate also inhibited thapsigargin-stimulated unidirectional entry of the Ca 2+ surrogates Ba 2+ and Mn 2+. In Ehrlich cells, the IC, for arachidonic and oleic acid was 1.2 and 1.8 μM, respectively. The inhibition appeared to depend on the ratio [fatty acid]/[cells] rather than on the absolute fatty acid concentration. Experiments with [ 3H]-oleic acid revealed that the inhibitory activity was not correlated with cell internalisation and metabolism of the fatty acid. The inhibition was reverted by removal of the fatty acid bound to cell membrane by fatty acid-free albumin treatment. The unsaturated fatty acids had no effect on ATP/ADP cell levels and plasma membrane potential. Pharmacological evidence indicated that cell phosphorylation/dephosphorylation events, and pertussis toxin-sensitive G proteins were not involved. Other amphipathic lipophilic compounds, i.e. 2-bromopalmitic acid, retinoic acid, sphingosine, and dihydrosphingosine, mimicked arachidonic/oleic acid as they inhibited thapsigargin-stimulated Ca 2+ influx in an albumin-reversible fashion. These results suggest that physiologically relevant (unsaturated) fatty acids can inhibit capacitative Ca 2+ influx possibly because they intercalate into the plasma membrane and directly affect the activity of the channels involved.