Transient concentrations and agonist potency of PGH2 in platelet activation by endogenous arachidonate

Human platelets were prelabelled with [14C]arachidonate and stimulated with thrombin or methyl mercury. [14C]PGH2 and the more stable of the other [14C]eicosanoids formed were rapidly extracted with organic solvent cooled to -30 degrees C and analyzed by radio-TLC. TXB2 and PGH2 were also quantified by radioimmunoassay, the latter as its index metabolite PGE2. PGH2 reached its peak concentration of 12 nmol/l after 20-30 s when it amounted to approximately 2/3 of the TXB2 concentration. In the presence of the thromboxane synthase inhibitor dazoxiben, PGH2 peaked after 60 s and afterwards declined in favour of PGE2, PGD2 and PGF2 alpha. Thirty seconds after stimulation with thrombin 1 IU/ml or methyl mercury 20 mumol/l, PGH2 amounted to 35 or 28% of the cyclooxygenase products in the absence and to 66 or 63% in the presence of dazoxiben, respectively. The platelet-activating potency of PGH2 was evaluated with purified PGH2 in platelets pretreated with acetylsalicylic acid. The EC50 values of PGH2 were 0.69 and 19 nmol/l for shape change and aggregation, respectively. U 46619 produced the same effects at 4.1 and 23 nmol/l. PGH2-induced [3H]serotonin release did not exceed 25%, whereas U 46619 was able to induce approximately 50% [3H]serotonin release. Dazoxiben enhanced the aggregation induced by PGH2. Human serum albumin inhibited the aggregating effect of PGH2, suppressed the enhancing effect of dazoxiben and shifted the metabolism of PGH2 to the inhibitory PGD2. The TXA2/PGH2 receptor antagonist daltroban suppressed the agonistic effects of endogenous or added PGH2, demonstrating that the TXA2/PGH2 receptor was its site of action.(ABSTRACT TRUNCATED AT 250 WORDS)