PAF binding sites. Characterization by [3H]52770 RP, a pyrrolo[1,2-c]thiazole derivative, in rabbit platelets.
52770 RP, the N-(3-chlorophenyl)-3-(3-pyridinyl)-1H,3H-pyrrolo[1,2-c]thiazole -7-carboxamide, displaces in a potent, specific and competitive manner [3H]PAF from its binding sites on rabbit platelets. Since 52770 RP is not structurally related to PAF and has low liposolubility with respect to PAF, it was selected as a potential radioligand for PAF receptor sites. [3H]52770 RP displayed high-affinity, specificity, as well as saturable and displaceable binding to a single class of recognition sites in intact platelets and crude platelet membranes. In these preparations, the values of binding parameters were, respectively, 8.5 and 7.6 nM for Kd, 0.2 pmol/5 X 10(7) platelets and 3.66 pmol/mg protein for Bmax and 0.96 and 0.91 for nH. Inasmuch as the (+)-52770 RP was 300-fold more potent than the (-)-isomer at displacing [3H]52770 RP in intact platelets, the studied binding site manifested stereospecific discrimination. A variety of pharmacological agents including pro- and anti-aggregant compounds did not exhibit affinity for [3H]52770 RP binding sites. In contrast, PAF, some of its active analogues and several recognized PAF antagonists (BN 52021, brotizolam, L-652,731, triazolam), displaced the [3H]52770 RP binding. Studies carried out using [3H]PAF demonstrated that 52770 RP was approximately 4- and 200-fold more potent than L-652,731 and BN 52021 respectively, as a PAF-receptor antagonist. In washed rabbit platelets, the rank order of potency (Ki) for several analogues of 52770 RP, to displace [3H]PAF from its binding sites, was highly correlated (r = 0.96) to their ability to antagonize [3H]52770 RP binding. In functional studies, 52770 RP antagonized not only the PAF-induced aggregation in washed rabbit platelets but also the hypotension evoked by PAF in the anesthetized rat. In this respect, it was 26 and 2 times more potent than L-652,731, respectively. In conclusion, [3H]52770 RP might represent a novel interesting tool for furthering our understanding of the role of PAF binding sites in pathophysiological processes.
Robaut
C
C
Rhône-Poulenc Santé, Centre de Recherches de Vitry, Department of Biology, Vitry-sur-Seine, France.
Durand
G
G
James
C
C
Lave
D
D
Sedivy
P
P
Floch
A
A
Mondot
S
S
Pacot
D
D
Cavero
I
I
Le Fur
G
G
D000818:Animals; D001792:Blood Platelets / Q000032:analysis*; D004224:Diterpenes; D005663:Furans / Q000494:pharmacology; D046934:Ginkgolides; D066298:In Vitro Techniques; D007700:Kinetics; D007783:Lactones; D008297:Male; D010936:Plant Extracts / Q000494:pharmacology; D010972:Platelet Activating Factor / Q000378:metabolism; D010974:Platelet Aggregation / Q000187:drug effects; D010980:Platelet Membrane Glycoproteins; D011725:Pyridines / Q000378:metabolism* / Q000494:pharmacology; D011817:Rabbits; D011869:Radioligand Assay; D011956:Receptors, Cell Surface / Q000032:analysis*; D043562:Receptors, G-Protein-Coupled; D013237:Stereoisomerism; D013844:Thiazoles / Q000378:metabolism* / Q000494:pharmacology; D014229:Triazolam / Q000494:pharmacology; D014316:Tritium
D004224:Diterpenes; D005663:Furans; D046934:Ginkgolides; D007783:Lactones; D010936:Plant Extracts; D010972:Platelet Activating Factor; D010980:Platelet Membrane Glycoproteins; D011725:Pyridines; D011956:Receptors, Cell Surface; D043562:Receptors, G-Protein-Coupled; D013844:Thiazoles; C039351:platelet activating factor receptor; D014316:Tritium; D014229:Triazolam; C053706:N-(3-chlorophenyl)-3-(3-pyridinyl)-1H,3H-pyrrolo(1,2-c)thiazole-7-carboxamide; C047504:2,5-bis(3,4,5-trimethoxyphenyl)tetrahydrofuran; C045856:ginkgolide B