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

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‐PGF1α > thromboxane B2 (TXB2) > PGE2 > 6‐oxo‐PGE1 (measured biologically) > PGF2α. 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.