Reduction of urinary 8‐epi‐prostaglandin F 2 α during cyclo‐oxygenase inhibition in rats but not in man

1 8‐epi‐prostaglandin (PG) F 2α , a major F 2 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‐PGF 2α as a minor product. It has been recently seen in human volunteers that the overall basal formation of 8‐epi‐PGF 2α in vivo is mostly COX‐independent and urinary 8‐epi‐PGF 2α 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 2α 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 2α 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‐TXB 1 and 2,3‐dinor‐6‐keto‐PGF 1α ). All compounds were selectively measured by immunopurification/gas chromatography‐mass spectrometry. 3 Naproxen reduced urinary excretion of 2,3‐dinor‐TXB 1 and 2,3‐dinor‐6‐keto‐PGF 1α but, unexpectedly, also that of 8‐epi‐PGF 2α (82, 49 and 52% inhibition, respectively). Indomethacin had a similar effect (77, 69 and 55% inhibition). Esterified 8‐epi‐PGF 2α 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 2α in man. We gave naproxen (1 g day −1 ) to healthy subjects (four nonsmokers and four smokers). Urinary 8‐epi‐PGF 2α 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‐TXB 2 and 2,3‐dinor‐TXB 2 ; >50% for 2,3‐dinor‐6‐keto‐PGF 1α ). 5 To investigate whether rat COX‐1 produces 8‐epi‐PGF 2α more efficiently than human COX‐1, we measured the ex vivo formation of 8‐epi‐PGF 2α and TXB 2 simultaneously in whole clotting blood. Serum levels of 8‐epi‐PGF 2α and TXB 2 were similar in rats and man. 6 We conclude that a significant amount of COX‐dependent 8‐epi‐PGF 2α is present in rat but not in human urine under normal conditions. This implies that urinary 8‐epi‐PGF 2α 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.British Journal of Pharmacology (1997) 121 , 1770–1774; doi: 10.1038/sj.bjp.0701321