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Protein kinase C is not involved in the desensitization of platelet activating factor receptor in rabbit platelets
1Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan, Republic of China.
Abstract:
Rabbit platelets pretreated with platelet activating factor (PAF) became refractory to further stimulation by PAF. The effect was specific for PAF. In this study, the alteration in the specific agonist binding to PAF receptor in platelets following desensitization was investigated. As revealed by the Scatchard analysis of radioligand binding data, the affinity for specific PAF binding to desensitized platelet membranes was substantially lowered as compared with that to control platelet membranes. Guanine nucleotide triphosphate, which was shown to decrease the affinity of specific PAF binding to platelet membranes, had less effect on the PAF binding affinity to the desensitized preparation. In platelets pretreated with phorbol 12-myristate-13-acetate, the binding affinity of PAF receptor remained unaltered. Pretreatment of platelets with 1-(5-isoquinolinesulphonyl)-2-methylpiperazine, a protein kinase C inhibitor, or neomycin, an inhibitor of the polyphosphoinositide breakdown, failed to prevent the reduction of specific PAF binding affinity following subsequent exposure to PAF. These results suggest that the agonist-induced desensitization of PAF receptor in rabbit platelets is independent of activation of protein kinase C.
Insights
Platelet activating factor (PAF) desensitization in rabbit platelets lowers PAF receptor affinity. This desensitization is independent of protein kinase C activation, suggesting a novel mechanism for PAF receptor regulation.
Area of Science:
- Biochemistry
- Pharmacology
- Cell Biology
Background:
- Platelet activating factor (PAF) is a potent lipid mediator involved in inflammation and thrombosis.
- Platelets desensitized to PAF exhibit reduced responsiveness to subsequent PAF stimulation.
- The specific mechanisms underlying PAF receptor desensitization remain incompletely understood.
Purpose of the Study:
- To investigate alterations in specific agonist binding to the PAF receptor in desensitized rabbit platelets.
- To determine the role of protein kinase C (PKC) and polyphosphoinositide (PPI) breakdown in PAF receptor desensitization.
Main Methods:
- Rabbit platelets were pretreated with PAF to induce desensitization.
- Radioligand binding assays and Scatchard analysis were employed to assess PAF receptor affinity.
- Experiments involved guanine nucleotide triphosphate, phorbol 12-myristate-13-acetate, a PKC inhibitor, and a PPI breakdown inhibitor.
Main Results:
- Desensitized platelet membranes showed substantially lowered affinity for specific PAF binding compared to control membranes.
- Guanine nucleotide triphosphate had a diminished effect on PAF binding affinity in desensitized platelets.
- PKC activation or inhibition, and PPI breakdown inhibition, did not prevent the reduction in PAF receptor binding affinity.
Conclusions:
- Agonist-induced desensitization of the PAF receptor in rabbit platelets involves a significant reduction in receptor affinity.
- This desensitization process appears to be independent of protein kinase C activation.
- The findings suggest novel pathways regulating PAF receptor function during desensitization.