Characterization of platelet thromboxane A2/prostaglandin H2 receptor by a novel thromboxane receptor antagonist, [3H]S-145.
The specific binding sites for S-145, a novel thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor antagonist with weak partial agonistic activity, were studied in human platelet membranes. [3H]S-145 displayed high affinity and specificity, as well as saturable and displaceable binding, to a single class of recognition sites with the same maximum number of sites (2100 fmol/mg protein) as the other two TXA2/PGH2 receptor antagonists, [3H]SQ29,548 and [3H]ONO3708. Binding of S-145 to the platelet membranes was enhanced by divalent cations (Mg2+ and Ca2+), and the binding affinity in the presence of 20 mM MgCl2 was 0.75 nM, a value which was smaller than those of SQ29,548 (8.7 nM) and ONO3708 (3.7 nM). The rank order of potency (Ki) for a series of TXA2/PGH2 receptor antagonists to displace [3H]S-145 binding to the membranes was correlated with those determined from [3H]SQ29,548 or [3H]ONO3708 binding to the same preparations. Kinetic analysis for the binding of the above radiolabeled antagonist to the crude platelet membranes revealed that the dissociation rate constant (K-1) for S-145 was much smaller than that for other ligands in human, rat and rabbit platelets. The extremely slow dissociation of S-145 from the receptors may explain the long-lasting characteristic of this compound in vivo as well as the abolishment of partial agonistic activity.
Hanasaki
K
K
Shionogi Research Laboratories, Shionogi & Co., Ltd., Osaka, Japan.
Nagasaki
T
T
Arita
H
H
D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; D000818:Animals; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism; D001643:Bridged Bicyclo Compounds / Q000494:pharmacology*; D001952:Bridged-Ring Compounds / Q000494:pharmacology*; D002412:Cations; D002462:Cell Membrane / Q000378:metabolism; D005229:Fatty Acids, Monounsaturated / Q000494:pharmacology*; D006801:Humans; D011450:Prostaglandin Endoperoxides, Synthetic / Q000494:pharmacology; D011463:Prostaglandins H / Q000378:metabolism; D011817:Rabbits; D051381:Rats; D011982:Receptors, Prostaglandin / Q000378:metabolism*; D017482:Receptors, Thromboxane; D044244:Receptors, Thromboxane A2, Prostaglandin H2; D013928:Thromboxane A2 / Q000378:metabolism
D001643:Bridged Bicyclo Compounds; D001952:Bridged-Ring Compounds; D002412:Cations; D005229:Fatty Acids, Monounsaturated; D011450:Prostaglandin Endoperoxides, Synthetic; D011463:Prostaglandins H; D011982:Receptors, Prostaglandin; D017482:Receptors, Thromboxane; D044244:Receptors, Thromboxane A2, Prostaglandin H2; C056566:S 145; D013928:Thromboxane A2; D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid