2,3-Dinor-5,6-dihydro-15-F-2t-isoprostane: a bioactive prostanoid metabolite

15-F-2t-isoprostane (15-F-2t-IsoP), also termed 8-isoprostaglandin F-2 alph a, is one of a series of prostanoids formed by free radical-mediated peroxi dation of arachidonic acid and exerts potent biological actions such as vas coconstriction. We recently demonstrated that 15-F-2t-IsoP is metabolized i n humans to a major metabolite, 2,3-dinor-5,6-dihydro-15-F-2t-IsoP (15-F-2t -IsoP-M). 15-F-2t-IsoP-M can also potentially be formed as a product of fre e radical-induced oxidation of the low abundance fatty acid gamma -linoleni c acid. We confirmed that 15-F-2t-IsoP-M is generated during oxidation of g amma -linolenic acid and explored whether it may exhibit biological activit y. 15-F-2t-IsoP-M caused marked constriction of porcine surface retinal and intraparenchymal brain microvessels, comparable to that observed with 15-F -2t-IsoP. These effects were associated with increased thromboxane A(2) (TX A(2)) formation and were virtually abolished by TXA(2)-synthase and -recept or inhibitors (CGS-12970 and L-670596). Vasoconstriction induced by either 15-F-2t-IsoP or 15-F2t-IsoP-M on perfused ocular choroid was also abrogated by TXA(2)-synthase inhibition as well as by removal of endothelium. Simila r to 15-F-2t-IsoP, 15-F-2t-IsoP-M evoked vascoconstriction and TXA(2) gener ation by activating Ca2+ influx from nonvoltage-gated channels (SK&F96365 s ensitive) in the retina and from both non-voltage- and N-type voltage-gated Ca2+ channels (omega -conotoxin MVIIA sensitive), respectively, in brain e ndothelial and astroglial cells; smooth muscle cells were unresponsive to b oth agents. Cross-desensitization experiments further suggest that 15-F-2t- IsoP and 15-F-2t-IsoP-M act on the same receptor mechanism. Findings reveal a novel concept by which a beta -oxidation metabolite of 15-F-2t-IsoP that can also be formed by nonenzymatic oxidation of gamma -linolenic acid is e quivalently bioactive to 15-F-2t-IsoP and may prolong the vascular actions of F-2-IsoPs.