FMLP-induced arachidonic acid release, phospholipid metabolism, and calcium mobilization in human monocytes

Radiolabeled human peripheral blood monocytes released [³H]arachidonic acid upon challenge with the calcium ionophore A23187 (10?M), or f-Met-LeuPhe (FMLP, 1?M). Chromatographic analysis of [³H]arachidonic acid labeled phospholipids showed that stimulation by FMLP reduced the amount of labeled phosphatidylcholine exclusively. Treatment of the monocytes with 10^-3 M dibutyryl cyclic AMP (d-cAMP) or 5×10^-4 M isobutylmethylxanthine (IBMX) substantially inhibited [³H]arachidonic acid release (30%) and depletion from labeled phosphatidylcholine (PC) in FMLP—but not calcium ionophore—stimulated cells. Using the fluorescent probe Indo-1, the FMLP-induced cytosolic calcium increase was unaffected by 10^-3 M dibutyryl cyclic AMP. The results suggest that FMLP-stimulated phospholipase activity is regulated by cyclic AMP, but not by depressing receptor-medicated increases in cytoplasmic free calcium.