Evidence for the distinct nature of F 2-isoprostane receptors from those of thromboxane a 2

-In rat glomeruli and mesangial cells, the thromboxane A ₂ (TxA ₂) mimetic, U-46,619, but not 8-iso-prostaglandin F _2α (8-isoPGF _2α), reduced glomerular inulin space and increased inositol 1,4,5-trisphosphate production, effects abolished by SQ29,548. In competitive binding studies using 8-iso-[ ³H]PGF _2α or [ ³H]SQ-29,548, mesangial cells displayed TxA ₂ binding sites but not ones for 8-iso-PGF _2α. In contrast, rat aortic smooth muscle cells possessed specific binding sites for both TxA ₂ and S-iso-PGF _2α and displayed functional responses to both agonists, such as time- and dose-dependent activation of mitogen-activated protein kinases. In these cells, the mean dissociation constant value for the isoprostane receptor was 31.8 ± 5.7 nM. When human TxA ₂ receptor cDNA was expressed in Xenopus oocytes injected with the Ca ²⁺-specific photoprotein, aequorin, 8-iso-PGF _2α gave much weaker responses than U-46,619. These studies provide the first radio-ligand binding characteristics of the F ₂-isoprostane receptor and demonstrate its specific and heterologous cellular localization. These studies support the distinct nature and biological significance of isoprostane receptors and provide a tool for their further molecular characterization.