A selective defect in arachidonic acid release from macrophage membranes in high potassium media.

ABSTRACT

Murine peritoneal macrophages cultured in minimal essential medium (alpha-MEM; 118 mM Na+, 5 mM K+) released arachidonic acid (204) from phospholipids on encountering a phagocytic stimulus of unopsonized zymosan. In high concentrations of extracellular K+ (118 mM), 3H release from cells prelabeled with [3H]204 was inhibited 80% with minimal reduction (18%) in phagocytosis. The inhibitory effect of K+ on 204 release was fully reversed on returning cells to medium containing Na+ (118 mM). Preingestion of zymosan particles by macrophages maintained in high K+ medium resulted in cells being "primed" for 204 release, which was only effected (without the further addition of particles) by changing the medium to one containing Na+. In contrast, 204 release from cells stimulated with the calcium ionophore A23187 was unimpaired by the elevated K+ medium, suggesting no direct effect of high K+ on the phospholipase. Macrophages stimulated with zymosan in alpha-MEM metabolized the released 204 to prostacyclin, prostaglandin E2 (PGE2), and leukotriene C (LTC). The smaller quantity of released 204 in high K+ medium was recovered as 6-Keto-PGF1 alpha, the breakdown product of prostacyclin, and PGE2. No LTC was synthesized. In high K+, resting (no zymosan) macrophages synthesized hydroxyeicosatetraenoic acids from exogeneously supplied 204 in proportions similar to cells maintained in alpha-MEM. These findings and the similarity of products (including LTC) produced by A23187 stimulated cells in alpha-MEM and high K+ medium indicated that the cyclooxygenase and lipoxygenase pathway enzymes were not directly inhibited by high extracellular K+. We conclude that high concentrations of extracellular K+ uncouple phagocytosis of unopsonized zymosan from the induction of the phospholipase responsible for the 204 cascade and suggest that the lesion is at the level of signal transduction between the receptor-ligand complex and the phospholipase.