Reduction of urinary 8‐epi‐prostaglandin F2α during cyclo‐oxygenase inhibition in rats but not in man

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Abstract
1. 8-epi-prostaglandin (PG) F2 alpha, a major F2 isoprostane, is produced in vivo 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-PGF2 alpha as a minor product. It has been recently seen in human volunteers that the overall basal formation of 8-epi-PGF2 alpha in vivo is mostly COX-independent and urinary 8-epi-PGF2 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-PGF2 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-PGF2 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-PGF1 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-PGF1 alpha but, unexpectedly, also that of 8-epi-PGF2 alpha (82, 49 and 52% inhibition, respectively). Indomethacin had a similar effect (77, 69 and 55% inhibition). Esterified 8-epi-PGF2 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-PGF2 alpha in man. We gave naproxen (1 g day-1) to healthy subjects (four nonsmokers and four smokers). Urinary 8-epi-PGF2 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-PGF1 alpha). 5. To investigate whether rat COX-1 produces 8-epi-PGF2 alpha more efficiently than human COX-1, we measured the ex vivo formation of 8-epi-PGF2 alpha and TXB2 simultaneously in whole clotting blood. Serum levels of 8-epi-PGF2 alpha and TXB2 were similar in rats and man. 6. We conclude that a significant amount of COX-dependent 8-epi-PGF2 alpha is present in rat but not in human urine under normal conditions. This implies that urinary 8-epi-PGF2 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.