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Modulation of platelet-activating factor (PAF) synthesis and release from human polymorphonuclear leukocytes (PMN):
Abstract:
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.
Insights
Extracellular calcium (Ca2+) is crucial for platelet-activating factor (PAF) synthesis and release from human polymorphonuclear leukocytes (PMN). Calcium levels directly influence PAF production and secretion, indicating a synthesis-release coupling mechanism.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Platelet-activating factor (PAF) is a potent lipid mediator involved in inflammatory responses.
- Polymorphonuclear leukocytes (PMN) are key immune cells that synthesize and release PAF.
- Extracellular calcium (Ca2+) is known to play a role in various cellular processes, including immune cell activation.
Purpose of the Study:
- To investigate the role of extracellular Ca2+ in the regulation of PAF synthesis and release from human PMN.
- To elucidate the relationship between Ca2+ concentration and PAF production and secretion.
- To explore the potential coupling mechanism between PAF synthesis and release.
Main Methods:
- Human PMN were stimulated with N'-formyl-methionyl-leucyl-phenylalanine (FMLP) in the presence of cytochalasin B.
- PAF synthesis and release were measured at various extracellular Ca2+ concentrations.
- The effects of Ca2+ chelators (EDTA, EGTA) on PAF production were assessed.
Main Results:
- Maximum PAF synthesis and release required 0.14 mM Ca2+.
- Ca2+ was essential for PAF release, with significantly reduced synthesis and no release in its absence.
- A dose-dependent increase in PAF synthesis and release was observed with increasing Ca2+ concentrations up to 0.14 mM.
- Approximately 60-70% of synthesized PAF remained within the PMN.
Conclusions:
- Extracellular Ca2+ is a critical regulator of both PAF synthesis and release from human PMN.
- A Ca2+-dependent coupling mechanism exists where PAF release stimulates further PAF synthesis.
- These findings provide insights into the intricate regulation of inflammatory mediator production by calcium ions.