ARACHIDONIC-ACID INHIBITS CAPACITATIVE CALCIUM-ENTRY IN RAT THYMOCYTES AND HUMAN NEUTROPHILS

Emptying the intracellular Ca2+ stores by treatment with the endomembr ane Ca2+-ATPase inhibitor thapsigargin activates capacitative Ca2+ ent ry (CCE). This can be evidenced in fura-2-loaded cells by an increase of [Ca2+](i) or by an acceleration of Mn2+ entry. Micromolar concentra tions of arachidonic acid inhibited CCE induced by treatment with thap sigargin in rat thymocytes and in human neutrophils. This inhibitory a ction was shared by other unsaturated fatty acids, but not by the satu rated arachidic acid nor by arachidonic acid methyl ester. The effect was not due to metabolites derived from arachidonic acid since several non-metabolizable analogs were able to reproduce it. Phorbol dibutyra te (PDB) acted similarly, suggesting that the inhibitory effect could be mediated by activation of protein kinase C (PKC). However, whereas the inhibition of CCE by PDB was reversed by treatment with the PKC in hibitor staurosporin, the inhibition by arachidonic acid was not. We f ind that unsaturated fatty acids antagonized microsomal dealkylation o f benzyl-resorufin, a cytochrome P450-mediated activity, with the same specificity profile as for inhibition of CCE. These results are consi stent with previous proposals suggesting that a microsomal cytochrome P450 may be involved in the regulation of CCE. (C) 1997 Elsevier Scien ce B.V.