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.

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SummaryThe acquisition of monoclonal antibodies specific for human fibrin has been impaired by the similarity in chemical composition between fibrinogen and fibrin and the conformational difference between immobilised and soluble fibrinogen. Five monoclonal antibodies (mabs) with a known affinity for fibrin have been subjected to screening procedures which involved the presentation of different forms of both fibrinogen and fibrin to the test mabs. It was observed by scanning electron microscopy that dried fibrin (denoted fibrin D), immobilised on the wells of PVC plates was morphologically similar to the fibrin found in human clots whereas PVC-immobilised fibrin monolayers (fibrin M) and a homogenised form of fibrin (fibrin FF) presented two very different morphological appearances. It was shown that lack of cross reactivity of a mab with an antigen (e.g. fibrinogen) was validly demonstrated only when both mab and antigen were present in the soluble state. These findings have allowed the generation of a screening procedure which involves the use of fibrin D on PVC plates in conjunction with whole human plasma incubated with the test antibody. This screening procedure has been validated using two mabs, one of which has an exclusive fibrin affinity while the other has a broad spectrum crossreactivity with both fibrinogen and fibrin. This procedure would ensure the acquisition of all the five fibrin-specific mabs used in this study while other less reliable screening procedures might not.

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