Ca2+-dependence of arachidonic acid redistribution among phospholipids of cultured mouse keratinocytes

Mouse keratinocytes cultured in a medium containing less than 0.1 mM Ca²⁺ (low Ca²⁺) incorporated [1-^l4C]arachidonic acid (AA) into phospholipids by kinetics including; (i) a rapid labelling of phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer) and both acid-stable and alkenylacyl forms of phosphatidylcholine (PtdCho); and (ii) a slow but long-lasting radiolabel incorporation into both acid-stable and alkenylacyl forms of phosphatidylethanolamine (PtdEtn), partly associated with a net radioactivity loss from acid stable-PtdCho. Under low Ca²⁺ conditions no radioactivity transfer apparently occurred between PtdIns and other phospholipid classes. When cells were prelabelled for 24 h with [1-¹⁴C]AA and reincubated in label-free medium containing 1.2 mM Ca²⁺ (normal Ca²⁺), an early and extensive loss of radioactivity from PtdIns was observed, reasonably in connection with Ca²⁺ stimulation of phosphoinositide turnover [1]. Cell shift to normal Ca²⁺ did not result in an increased synthesis of labelled eicosanoids, but was consistent with an increase of radioactivity incorporation into diacylglycerol (DAG) and with a complex pattern of [1-¹⁴C]AA redistribution, eventually leading to a marked radioactivity incorporation into acid stable-PtdEtn (but not into alkenylacyl-PtdEtn) and to a labelling decrease of acid stable-PtdCho. The possible mechanisms driving AA recycling after cell shift to normal Ca²⁺ are discussed.