Identification of 6‐oxo‐prostaglandin E1 as a naturally occurring prostanoid generated by rat lung

Abstract

1 The spontaneous release of prostanoids from rat isolated perfused lungs was studied after acid/organic extraction of perfusates by bioassay, radioimmunoassay, thin layer and high performance liquid chromatographic methods and by gas chromatography-negative ion mass spectroscopy (g.c.-n.i.m.s.). 2 An acid/organic extractable anti-aggregatory vasodilator prostaglandin which inhibited the twitch response of the field-stimulated guinea-pig vas deferens was released from the Krebs-perfused rat lung in nanogram amounts similar to those of other detected prostanoids. Parallel biological assay suggested that this prostaglandin had very closely similar pharmacological activity to authentic 6-oxo-prostaglandin E₁ (6-oxo-PGE₁), a metabolite of prostacyclin (PGI₂) generated by the action of the enzyme 9-hydroxyprostaglandin dehydrogenase (9-PGDH). 3 6-oxo-PGE₁ was identified conclusively in extracts of rat lung perfusate by thin layer chromatography, high performance liquid chromatography and g.c./m.s. combined with bioassay (inhibition of platelet aggregation), and its covalent structure was defined by g.c. negative ion chemical ionization mass spectroscopy. 4 The rank order of spontaneous release of prostanoids (measured by radioimmunoassay) from the perfused rat lung was 6-oxo-PGF_1α > thromboxane B₂ (TXB₂) > PGE₂ > 6-oxo-PGE₁ (measured biologically) > PGF_2α. Release of all five prostanoids was inhibited by indomethacin, but only that of 6-oxo-PGE₁ was inhibited by naringenin. 5 Rat lung 100,000 g cytosolic supernatants contained 9-PGDH activity capable of removing 9β-tritium from labelled prostacyclin and forming an acid/organic extractable 6-oxo-PGE₁-like anti-aggregatory substance. This 9-PGDH activity was inhibited by naringenin (IC₅₀ 10.3 μM). 6 The relevance of these findings to the possible physiological role of 6-OXO-PGE₁ in the lung is discussed, and we propose that 6-oxo-PGE₁ should be accorded the status of a physiologically relevant, naturally occurring metabolite of arachidonic acid.