Heterogeneity of thromboxane A₂ (TP‐) receptors: evidence from antagonist but not agonist potency measurements

Abstract

1 Thromboxane A2 (TP-) receptors in human, rat and rabbit platelets and in smooth muscle of guineapig trachea, rat aorta and rabbit aorta have been characterized by measurement of the potencies of agonists and antagonists having considerable variations in chemical structure. 2 On each washed platelet system, eight prostanoids induced maximal irreversible aggregation (full agonists) and the potency ranking was EP 171 > STA2 > 9,11-azo PGH2 > 9,11-endoxy-lOa-homo PGH2 > U-46619 (standard) > PGH2 = 16-p-fluorophenoxy-co-tetranor PGF2. > 16,16-dimethyl PGF2,. Correlations between the three platelet preparations for both absolute and relative potencies were good. On human platelets, STA2, at concentrations above that required for maximum aggregation, exerted an inhibitory effect which was independent of its interaction with the TP-receptor. 3 Five prostanoids, EP 109, EP 167, EP 204, PTA2 and 16,20-methano PTA2, exhibited partial agonist activity on the platelet and smooth muscle preparations. There was good agreement between absolute potencies on the six preparations; on platelets potency was assessed from shape change measurements, since aggregation, when present, always showed a very shallow concentration-response relationship. The magnitude of the maximum response induced by each compound decreased in the order listed above, to the extent that 16,20-methano PTA2 could be treated as a pure antagonist. 4 With U-46619 as agonist, the pA2 values of seven antagonists were found to be very similar on human and rat platelets. The potency ranking was EP 169 > AH 23848 > EP 092 > ONO 11120 > EP 115 = 16,20-methano PTA2 > BM 13177. There was a similar trend on rabbit platelets but pA2 values were 1.0-1.5 log units smaller; the exception was BM 13177 which had similar affinities. The antagonism produced by EP 169 and AH 23848 was surmountable on rabbit platelets but not on human and rat platelets. 5 None of the antagonists was highly potent on the rabbit aorta (pA2 values < 7.5 by Schild analysis). Affinities on the guinea-pig trachea and the rat aorta were higher and in the same range as those obtained for human and rat platelets. However the correlations of pA2 values between any pair of smooth muscle preparations and between any pair of platelet/smooth muscle preparations were either weak or not significant (P > 0.05). 6 The excellent agreement for both full and partial agonist potencies between the six preparations provides no evidence for TP-receptor subtypes and further suggests that the agonist recognition sites of the TP-receptors could be very similar, if not identical, in nature. In contrast, the different antagonist affinities found in this and other published studies indicate heterogeneity of TP-receptors. However, classification into TP,-, TP2-receptors, etc. on the basis of the limited antagonist data available does not appear appropriate at this time.

Investigation of the prostaglandin E (EP‐) receptor subtype mediating relaxation of the rabbit jugular vein

“…We have recently reported low affinities for several TP-receptor antagonists (e.g. EP 092, EP 169 and ONO 11120) on two other rabbit isolated preparations, the thoracic aorta ring and blood platelets (Tymkewycz et al, 1991). In early experiments on the jugular vein it was found that none of the three antagonists showed greater blocking potency than GR 32191 (unpublished observations).…”

International Union of Basic and Clinical Pharmacology. LXXXIII: Classification of Prostanoid Receptors, Updating 15 Years of Progress

“…With higher affinity TP antagonists in human and rat platelet assays, surmountability is seen for the shapechange response, whereas insurmountability occurs for aggregation. This profile is still compatible with a reversible-competitive mechanism, because slow dissociation of the high-affinity antagonist from the TP receptor retards agonist (U-46619) occupancy in the early stage of the aggregation response, thereby favoring the disaggregation process and insurmountability; in contrast, the nonfading shape-change response affords a truer measure of dose ratios at steady state (Armstrong et al, 1985;Jones et al, 1989;Lumley et al, 1989;Tymkewycz et al, 1991;Ogletree et al, 1993).…”

“…There has been much debate about the existence of different TP receptor subtypes in platelet and vascular smooth systems (Mais et al, 1985(Mais et al, , 1988Swayne et al, 1988;Morinelli et al, 1989;Masuda et al, 1991;Tymkewycz et al, 1991;Folger et al, 1992). It is certainly clear that species heterology exists; for example, higher affinities are often found for human and rat platelet TP receptors compared with rabbit platelet TP receptors (Tymkewycz et al, 1991).…”

“…10). U-46619 (the most commonly used standard agonist), 9,11-azo PGH 2 and stable TxA 2 analogs behave as full agonists on isolated vascular and respiratory smooth muscle and human platelet preparations (Tymkewycz et al, 1991), as does the cyclic carbonate AGN-191976 (Fig. 10) (Krauss et al, 1996).…”

Ca2+ Influx Induced by the Agonist U46619 Is Inhibited by Hyperpolarization Induced by the K+ Channel Opener Cromakalim in Canine Coronary Artery.

“…U46619 is a stable and full ago nist analogue of thromboxane A2 (17,18). Recently, the human thromboxane A2 receptor was cloned, and its amino-acid sequence shows that it is one of the G protein coupled receptors (19).…”