Reduction of urinary 8-epi-prostaglandin F-2 alpha during cyclooxygenase inhibition in rats but not in man

1 8-epi-prostaglandin (PG) F-2 alpha, a major F-2 isoprostane, is produced in viva by free radical-dependent peroxidation of lipid-esterified arachidonic acid, Both cyclo-oxygenase isoforms (COX-1 and COX-2) may also form free 8-epi-PGF(2 alpha) as a minor product. It has been recently 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 stress in vivo. 2 To test the validity of this marker in the rat, we evaluated in vivo the effect of COX inhibition on the formation of 8-epi-PGF(2 alpha) vs prostanoids, Two structurally unrelated COX inhibitors (naproxen: 30 mg kg(-1) day(-1); indomethacin: 4 mg kg(-1) day(-1)) were given i.p. to rats kept in metabolic cages. In vivo formation of 8-epi-PGF(2 alpha) was assessed by measuring its urinary excretion, Prostanoid biosynthesis was assessed by measuring urinary excretion of major metabolites of thromboxane (TX) and prostacyclin (2,3-dinor-TXB1 and 2,3-dinor-6-keto-PGF(1 alpha)). All compounds were selectively measured by immunopurification/gas chromatography-mass spectrometry. 3 Naproxen reduced urinary excretion of 2,3-dinor-TXB1 and 2,3-dinor-6-keto-PGF(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 and plasma remained unchanged after indomethacin. 4 These findings prompted 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-epi-PGF(2 alpha) remained unchanged in the two groups (9.63+/-0.99 before 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 metabolites of thromboxane and prostacyclin (about 90% for both 11-dehydro-TXB2 and 2,3-dinor-TXB2; >50% for 2,3-dinor-6-keto-PGF(1 alpha)). 5 To investigate whether rat COX-1 produces 8-epi-PGF(2 alpha) more efficiently 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 conclude 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 implies that urinary 8-epi-PGF(2 alpha) cannot be used as an index of near-basal oxidant stress in rats. On the other hand, our data further confirm the validity of this marker in man.