REDUCTION OF URINARY 8-EPI-PROSTAGLANDIN F2-ALPHA DURING CYCLOOXYGENASE INHIBITION IN RATS BUT NOT IN MAN

1 8-epi-prostaglandin (PG) F-2 alpha, a major F-2 isoprostane, is prod uced in viva by free radical-dependent peroxidation of lipid-esterifie d arachidonic acid, Both cyclo-oxygenase isoforms (COX-1 and COX-2) ma y also form free 8-epi-PGF(2 alpha) as a minor product. It has been re cently seen in human volunteers that the overall basal formation of 8- epi-PGF(2 alpha), in vivo is mostly COX-independent and urinary 8-epi- PGF(2 alpha) is therefore an accurate marker of 'basal' oxidative stre ss in vivo. 2 To test the validity of this marker in the rat, we evalu ated in vivo the effect of COX inhibition on the formation of 8-epi-PG F(2 alpha) vs prostanoids, Two structurally unrelated COX inhibitors ( naproxen: 30 mg kg(-1) day(-1); indomethacin: 4 mg kg(-1) day(-1)) wer e given i.p. to rats kept in metabolic cages. In vivo formation of 8-e pi-PGF(2 alpha) was assessed by measuring its urinary excretion, Prost anoid biosynthesis was assessed by measuring urinary excretion of majo r metabolites of thromboxane (TX) and prostacyclin (2,3-dinor-TXB1 and 2,3-dinor-6-keto-PGF(1 alpha)). All compounds were selectively measur ed by immunopurification/gas chromatography-mass spectrometry. 3 Napro xen reduced urinary excretion of 2,3-dinor-TXB1 and 2,3-dinor-6-keto-P GF(1 alpha) but, unexpectedly, also that of 8-epi-PGF(2 alpha) (82, 49 and 52% inhibition, respectively). Indomethacin had a similar effect (77, 69 and 55% inhibition). Esterified 8-epi-PGF(2 alpha), in liver a nd plasma remained unchanged after indomethacin. 4 These findings prom pted us to re-assess the contribution of COX activity to the systemic production of 8-epi-PGF(2 alpha) in man. We gave naproxen (1 g day(-1) ) to healthy subjects (four nonsmokers and four smokers). Urinary 8-ep i-PGF(2 alpha) remained unchanged in the two groups (9.63+/-0.99 befor e vs 10.24+/-1.01 after and 20.14+/-3.00 vs 19.03+/-2.45 ng h(-1) 1.73 m(-2)), whereas there was a marked reduction of major urinary metabol ites of thromboxane and prostacyclin (about 90% for both 11-dehydro-TX B2 and 2,3-dinor-TXB2; >50% for 2,3-dinor-6-keto-PGF(1 alpha)). 5 To i nvestigate whether rat COX-1 produces 8-epi-PGF(2 alpha) more efficien tly than human COX-I, we measured the ex vivo formation of 8-epi-PGF(2 alpha) and TXB2 simultaneously in whole clotting blood. Serum levels of 8-epi-PGF(2 alpha) and TXB2 were similar in rats and man. 6 We conc lude that a significant amount of COX-dependent 8-epi-PGF(2 alpha) is present in rat but not in human urine under normal conditions. This im plies that urinary 8-epi-PGF(2 alpha) cannot be used as an index of ne ar-basal oxidant stress in rats. On the other hand, our data further c onfirm the validity of this marker in man.