A Ca2+-independent activation of a type IV cytosolic phospholipase A2 underlies the receptor stimulation of arachidonic acid-dependent noncapacitative calcium entry

The oscillatory [Ca2+](i) signals typically seen following physiologically relevant stimulation of phospholipase C-linked receptors are associated with a receptor-activated entry of Ca2+, which plays a critical role in driving the oscillations and influencing their frequency. We have recently shown that this receptor-activated entry of Ca2+ does not conform to the widely accepted "capacitative" model and, instead, reflects the activity of a distinct, novel Ca2+ entry pathway regulated by arachidonic acid (Shuttleworth, T. J., and Thompson, J. L. (1998) J. Biol. Chem. 273, 32636-32643). We now show that the generation of arachidonic acid under these conditions results from the activity of a type IV cytosolic phospholipase A(2) (cPLA(2)). Although cPLA(2) activation commonly involves a Ca2+-dependent translocation to the membrane, at these low agonist concentrations cPLA(2) activation was independent of increases in [Ca2+](i), and no detectable translocation to the membrane occurs. Nevertheless, stimulation of cPLA(2) activity was confined to the membrane fraction, where an increase in phosphorylation of the enzyme was observed. We suggest that, at the low agonist concentrations associated with oscillatory [Ca2+](i) signals, cPLA(2) activation involves an increased phosphorylation of a discrete pool of the total cellular cPLA(2) that is already localized within the membrane fraction at resting [Ca2+](i).