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Complex pattern of inhibition by Mg2+ of exocytosis from permeabilised eosinophils
1Department of Physiology, University College London, UK.
Abstract:
Inhibition by Mg2+ ions of exocytotic secretion from permeabilised eosinophils, stimulated by Ca2+ and GTP gamma S, and in the presence and absence of ATP, has been examined. While Mg2+ inhibits release of aryl sulphatase, hexosaminidase and peroxidase, we found no evidence that this occurs by competition at a Ca(2+)-binding site. On the other hand, the IC50 for Mg2+ approximates a simple inverse relationship to EC50 for GTP gamma S over a wide range of concentrations, indicative of a possible competition with events directly controlled by a GTP-binding protein. However, for secretion stimulated by GTP gamma S in the absence of Ca2+ (which necessitates provision of ATP), the effect of Mg2+ becomes biphasic. Initially, secretion is dependent on the presence of Mg2+ as a component of the complex ligand Mg.ATP. At high concentrations, Mg2+ inhibits secretion and the IC50 was found to be fixed at a concentration of about 8 mM regardless of the strength of the stimulus. The presence of ATP appears to divert the site of inhibition due to Mg2+.
Insights
Magnesium ions (Mg2+) inhibit exocytotic secretion in eosinophils. This inhibition may involve GTP-binding proteins, with Mg2+ playing a complex, biphasic role dependent on ATP presence.
Area of Science:
- Cell Biology
- Immunology
- Biochemistry
Background:
- Exocytotic secretion is crucial for eosinophil function.
- Calcium (Ca2+) and GTP-binding proteins (G proteins) are key regulators of secretion.
- The role of magnesium ions (Mg2+) in eosinophil secretion is not fully understood.
Purpose of the Study:
- To investigate the inhibitory effects of Mg2+ on exocytotic secretion in permeabilized eosinophils.
- To elucidate the mechanisms by which Mg2+ influences secretion stimulated by Ca2+ and GTPγS, with and without ATP.
Main Methods:
- Utilized permeabilized eosinophils stimulated with Ca2+ and GTPγS.
- Assessed the release of specific enzymes (aryl sulphatase, hexosaminidase, peroxidase) as indicators of secretion.
- Varied concentrations of Mg2+ and ATP to determine dose-dependent effects and identify IC50 values.
Main Results:
- Mg2+ inhibited the release of aryl sulphatase, hexosaminidase, and peroxidase.
- Mg2+ inhibition did not appear to compete with Ca2+-binding sites.
- Mg2+ showed an inverse relationship with GTPγS efficacy, suggesting interaction with G protein-dependent events.
- Mg2+ exhibited biphasic effects when ATP was present and Ca2+ was absent, initially promoting and then inhibiting secretion.
Conclusions:
- Mg2+ plays a complex regulatory role in eosinophil exocytosis.
- Inhibition by Mg2+ likely involves G protein pathways rather than direct competition with Ca2+.
- The presence of ATP modulates Mg2+ inhibition, highlighting the intricate interplay of ions and cofactors in secretion.