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Binding of a radioiodinated 13‐azapinane thromboxane antagonist to platelets: correlation with antiaggregatory activity in different species
Author(s) -
Narumiya Shuh,
Okuma Minoru,
Ushikubi Fumitaka
Publication year - 1986
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1986.tb10208.x
Subject(s) - platelet , thromboxane , chemistry , prostaglandin h2 , thromboxane a2 , agonist , dissociation constant , antagonist , thromboxane b2 , prostaglandin , thromboxane receptor , receptor , medicine , endocrinology , thromboxane a synthase , stereochemistry , biochemistry , biology
1 Binding of a 125 I‐labelled derivative of the 13‐azapinane thromboxane antagonist (ONO‐11120), [ 125 I]‐9,11‐dimethylmethano‐11,12‐methano‐16‐(3‐iodo‐4‐hydroxyphenyl)‐13,14‐dihydro‐13‐aza‐15‐β‐ω‐tetranor‐thromboxane A 2 ([ 125 I]‐PTA‐OH), to washed platelets of human, dog and rabbit was studied. Results were compared with the in vitro inhibitory potency of ONO‐11120 on platelet aggregation induced by arachidonate and a thromboxane agonist, 9,11‐epithio‐11,12‐methanothromboxane A 2 (STA 2 ). 2 [ 125 I]‐PTA‐OH bound to washed human platelets in a reversible, saturable and temperature‐dependent manner, and specific binding displaced by 20 μM ONO‐11120 constituted about 40% of the total binding. Scatchard analyses revealed a single class of specific binding and the equilibrium dissociation constant ( K D ) and maximal concentration of binding sites ( B max ) were 22 nM and 390 fmol per 10 8 platelets (about 2,300 sites per platelet), respectively. In addition to ONO‐11120, STA 2 and another thromboxane receptor agonist, (15S)‐hydroxy‐11,9‐epoxymethano‐prosta‐5Z,13E‐dienoic acid (U‐46619), effectively displaced the binding with IC 50 values of 44 and 125 nM respectively. Prostaglandin D 2 (PGD 2 ) partially displaced the binding only at a concentration above 1 μM. PGE 1 and thromboxane B 2 (TXB 2 ) were without effect up to 100 μM. 3 Similar binding of [ 125 I]‐PTA‐OH was observed on dog platelets. The K D and B max were 12 nM and 110 fmol per 10 8 platelets (about 680 sites per platelet), respectively, and these values did not change significantly after adrenaline treatment which potentiated arachidonate‐induced aggregation of platelets in this species. On the other hand, no specific binding of [ 125 I]‐PTA‐OH was found on rabbit platelets. 4 Consistent with the results from binding studies, ONO‐11120, 0.5 μM, completely suppressed arachidonate‐induced aggregation of human platelets, whereas, at concentrations up to 5 μM, this agent did not significantly inhibit aggregation of rabbit platelets induced by the same stimulus. STA 2 ‐induced aggregation of rabbit platelets also showed less sensitivity to ONO‐11120. When a similar extent of irreversible aggregation was induced by STA 2 and the inhibitory potency of ONO‐11120 was compared in human and rabbit platelets, about one hundred times greater concentration of ONO‐11120 was required to suppress aggregation of rabbit platelets than that of human platelets. 5 These results suggest that [ 125 I]‐PTA‐OH binds to a platelet thromboxane receptor, and that the structure of the binding site(s) on the receptor may vary between species.