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(2alpha), 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(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-F(2t)-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 vasoconstriction and TXA(2) generation by activating Ca(2+) influx from nonvoltage-gated channels (SK&F96365 sensitive) in the retina and from both nonvoltage- and N-type voltage-gated Ca(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(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.