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

15-[F.sub.2t]-isoprostane (15-[F.sub.2t]-IsoP), also termed 8-isoprostaglandin [F.sub.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 vasoconstriction. We recently demonstrated that 15-[F.sub.2t]-IsoP is metabolized in humans to a major metabolite, 2,3-dinor-5,6-dihydro-15-[F.sub.2t]-IsoP (15-[F.sub.2t]-IsoP-M). 15-[F.sub.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.sub.2t]-IsoP-M is generated during oxidation of [Gamma]-linolenic acid and explored whether it may exhibit biological activity. 15-[F.sub.2t]-IsoP-M caused marked constriction of porcine surface retinal and intraparenchymal brain microvessels, comparable to that observed with 15-[F.sub.2t]-IsoP. These effects were associated with increased thromboxane [A.sub.2] ([TXA.sub.2]) formation and were virtually abolished by [TXA.sub.2]-synthase and -receptor inhibitors (CGS-12970 and L-670596). Vasoconstriction induced by either 15-[F.sub.2t]-IsoP or 15-[F.sub.2t]-IsoP-M on perfused ocular choroid was also abrogated by [TXA.sub.2]-synthase inhibition as well as by removal of endothelium. Similar to 15-[F.sub.2t]-IsoP, 15-[F.sub.2t]-IsoP-M evoked vasoconstriction and [TXA.sub.2] generation by activating [Ca.sub.2+] influx from nonvoltage-gated channels (SK&F96365 sensitive) in the retina and from both nonvoltage-and N-type voltage-gated [Ca.sub.2+] 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.sub.2t]-IsoP and 15-[F.sub.2t]-IsoP-M act on the same receptor mechanism. Findings reveal a novel concept by which a [Beta]-oxidation metabolite of 15-[F.sub.2t]-IsoP that can also be formed by nonenzymatic oxidation of [Gamma]-linolenic acid is equivalently bioactive to 15-[F.sub.2t]-IsoP and may prolong the vascular actions of [F.sub.2]-IsoPs. calcium; thromboxane; peroxidation