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Reversible or irreversible modification of [3H]PAF binding on rabbit platelet membranes differentiates various PAF
M T Domingo1, F Piro, C Broquet
1Institut Henri Beaufour, Les Ulis, France.
Abstract:
[3H]Platelet-activating factor (PAF) binding to rabbit platelet membranes was examined before and after 20 min preincubation at 25 degrees C in the presence of PAF, lysoPAF, or of five different PAF receptor antagonists (L 652731, BN 52021, WEB 2086, BN 52111 and BN 52115). When platelet membranes were not washed after preincubation with PAF or PAF antagonists, no significant specific binding of [3H]PAF was observed, which suggests full occupancy of the binding sites. When membranes were extensively washed, full recovery of specific [3H]PAF binding was attained with L 652731 and partial recoveries (60%, 55% and 30%) were reached with BN 52021, WEB 2086 and PAF, respectively; no recovery was seen with the dioxolanes BN 52111 and BN 52115. Scatchard analysis of the binding data indicated that no significant change in the dissociation constant (Kd) and maximum number of binding sites (Bmax) occurred after preincubation of platelet membrane with L 652731, whereas a reduction of Bmax was observed when PAF and BN 52021 were measured. When platelet membranes were preincubated with WEB 2086, Bmax and Kd significantly increased. The data suggest differing binding properties for PAF and the PAF antagonists. Some of the PAF antagonists may tightly bind to the PAF receptor site(s) and/or irreversibly modify or downregulate PAF recognition sites. Our results also suggest that the interaction of PAF receptor antagonists with PAF receptor can be divided into at least two components, namely a reversible component and an irreversible one.
Insights
Platelet-activating factor (PAF) receptor antagonists exhibit varied binding behaviors. Some antagonists may irreversibly bind or downregulate receptor sites, suggesting distinct interaction mechanisms beyond simple reversibility.
Area of Science:
- Pharmacology
- Biochemistry
- Molecular Biology
Background:
- Platelet-activating factor (PAF) is a potent lipid mediator involved in inflammation and thrombosis.
- Understanding the interaction between PAF and its receptor is crucial for developing targeted therapies.
Purpose of the Study:
- To investigate the binding characteristics of various PAF receptor antagonists to rabbit platelet membranes.
- To elucidate the nature of PAF and antagonist interactions with the PAF receptor, differentiating between reversible and irreversible binding components.
Main Methods:
- Radioligand binding assays using [3H]PAF to quantify binding to rabbit platelet membranes.
- Preincubation of membranes with PAF or different antagonists (L 652731, BN 52021, WEB 2086, BN 52111, BN 52115) under specific conditions.
- Scatchard analysis to determine binding parameters (Kd and Bmax) after washing steps.
Main Results:
- Washing after preincubation with L 652731 fully restored [3H]PAF binding, with no changes in Kd or Bmax.
- Preincubation with PAF, BN 52021, or WEB 2086 resulted in partial or no recovery of binding, with alterations in Bmax and/or Kd.
- Dioxolane antagonists (BN 52111, BN 52115) showed no recovery of binding, suggesting irreversible interaction.
Conclusions:
- PAF receptor antagonists display heterogeneous binding properties, with some exhibiting irreversible binding or receptor downregulation.
- The interaction between PAF receptor antagonists and the receptor can involve both reversible and irreversible components.
- These findings provide insights into the distinct mechanisms of action for different PAF antagonists.