Modulation of platelet-activating factor (PAF) synthesis and release from human polymorphonuclear leukocytes (PMN): Role of extracellular Ca²⁺

Extracellular Ca²⁺ regulated the synthesis and release of platelet-activating factor (PAF) from human polymorphonuclear leukocytes (PMN) stimulated with N′-formyl-methionyl-leucyl-phenylalanine (FMLP) in the presence of cytochalasin B. Maximum PAF synthesis and release required the presence of 0.14 mm Ca²⁺ whereas 1.4 mm Ca²⁺ was necessary for maximum lysosomal enzyme secretion. The synthesis of PAF occurred within 2.5 min after PMN stimulation in the presence of 1.4 mm Ca²⁺; however, PAF release did not occur until 5 min after stimulation. Peak PAF release occurred by 7.5 min but accounted for only 30-40% of the total amount of PAF synthesized, the remainder being retained on or within the PMN. Stimulation of PMN in the presence of 0.01 m EDTA or EGTA decreased PAF synthesis and release by greater than 95%. In the absence of extracellular Ca²⁺, stimulated PMN synthesized PAF in amounts that were 10-30% of maximum, but there was no release of the newly synthesized PAF. At Ca²⁺ concentrations greater than 0.01 mm, there was a dose-dependent (up to 0.14 mm) increase in PAF synthesis that was associated with the initiation and concomitant increase in the amount of PAF released. These data suggest the presence of a PAF synthesis-release coupling mechanism in which the extracellular Ca²⁺-dependent release of PAF stimulates additional PAF synthesis.