2,3-Dinor-5,6-dihydro-15-F2t-isoprostane:: a bioactive prostanoid metabolite

15-F-2t-isoprostane (15-F-2t-IsoP), also termed 8-isoprostaglandin F-2 alpha, is one of a series of prostanoids formed by free radical-mediated peroxidation of arachidonic acid and exerts potent biological actions such as vascoconstriction. We recently demonstrated that 15-F-2t-IsoP is metabolized in 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 free radical-induced oxidation of the low abundance fatty acid gamma -linolenic acid. We confirmed that 15-F-2t-IsoP-M is generated during oxidation of gamma -linolenic acid and explored whether it may exhibit biological activity. 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) (TXA(2)) formation and were virtually abolished by TXA(2)-synthase and -receptor 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. Similar to 15-F-2t-IsoP, 15-F-2t-IsoP-M evoked vascoconstriction and TXA(2) generation by activating Ca2+ influx from nonvoltage-gated channels (SK&F96365 sensitive) in the retina and from both non-voltage- and N-type voltage-gated Ca2+ channels (omega -conotoxin MVIIA sensitive), respectively, in brain endothelial and astroglial cells; smooth muscle cells were unresponsive to both 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 equivalently bioactive to 15-F-2t-IsoP and may prolong the vascular actions of F-2-IsoPs.