Evidence for sequential signals in the induction of the arachidonic acid cascade in macrophages

We have examined the requirement for Na+, Ca2+, and protein synthesis in the induction of the arachidonic acid (20:4) cascade in cultured murine peritoneal macrophages. Replacement of extracellular Na+ with choline or with K+ inhibited receptor-mediated 20:4 release by 60-90%, but did not inhibit release stimulated by the soluble triggers PMA and A23187. Cells that had preingested zymosan particles in a K+ medium could be induced to secrete 20:4 metabolites merely by changing the medium to one containing Na+. The Ca2+ ionophore A23187 caused cells in Na+-free medium to release and metabolize 20:4 to prostacyclin, PGE2, leukotriene C, and hydroxyeicosatetraenoic acids, suggesting that the phospholipase(s), cyclooxygenase, and lipoxygenase enzymes do not have a requirement for extracellular Na+. These data suggest that receptor-mediated 20:4 secretion has a requirement for extracellular Na+, while 20:4 release triggered by soluble stimuli do not. Immune complex- and A23187-induced 20:4 release was absolutely dependent on extracellular Ca2+. PMA-triggered 20:4 secretion was inhibited 50% in Ca2+-free medium, but could be inhibited completely by preloading the cells with the Ca2+ antagonist quinine. Protein and RNA synthesis was required for 20:4 release induced by zymosan, immune complex, and PMA, but not by A23187. Cycloheximide and emetine were effective within 15 min of addition, while actinomycin D was an effective inhibitor within 45 min. We suggest that receptor-mediated signal response coupling in the 20:4 cascade in macrophages comprises a sequential series of signals that includes an Na+ influx, synthesis of a rapid turnover-protein, and finally an increase in intracellular Ca2+.