title: Binding kinetics of PAF-acether (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to intact human platelets.
abstract: The binding of [3H]PAF-acether (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to intact human gel-filtered platelets was measured at 22 degrees C. Specific binding reached saturation within 15 min at high doses of [3H]PAF-acether (0.5-0.9 nM), whereas about 90 min were required when low doses (0.02-0.5 nM) were used. Above 1 nM, [3H]PAF-acether non-specific binding increased progressively, which together with the demonstration of a 3H-labelled metabolite suggested uptake and metabolism of [3H]PAF-acether. Equilibrium analysis revealed one class of specific receptors with a Ka of 18.86 +/- 4.82 X 10(9) M-1 and 242 +/- 64 binding sites per platelet. Non-equilibrium binding revealed a similar Ka (16.87 X 10(9) M-1). Specific binding became irreversible after prolonged incubation, a process that was enhanced at increasing concentrations of [3H]PAF-acether. Platelets made desensitized to PAF-acether by prior incubation with unlabelled PAF-acether failed to bind a second dose of PAF-acether (3H-labelled), suggesting that desensitization resulted from loss of available binding sites. Under the conditions of the binding studies, PAF-acether induced exposure of the fibrinogen receptor, aggregation (in a stirred suspension) and alterations in (poly)-phosphatidylinositides. These results suggest that PAF-acether initiates platelet responses via receptor-mediated processes.
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D001665:Binding Sites; D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000097:blood; D002850:Chromatography, Gel; D005340:Fibrinogen / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D010716:Phosphatidylinositols / Q000097:blood; D010972:Platelet Activating Factor / Q000378:metabolism*; D010974:Platelet Aggregation / Q000187:drug effects; D011485:Protein Binding
chemical_list: D000317:Adrenergic alpha-Antagonists; D010716:Phosphatidylinositols; D010972:Platelet Activating Factor; D005340:Fibrinogen; D002118:Calcium