Evidence for the distinct nature of F2-isoprostane receptors from those of thromboxane A2

In rat glomeruli and mesangial cells, the thromboxane A₂ (TxA₂) mimetic, U-46,619, but not 8-iso-prostaglandin F(2α) (8-iso-PGF(2α)), reduced glomerular inulin space and increased inositol 1,4,5-trisphosphate production, effects abolished by SQ-29,548. In competitive binding studies using 8-iso-[³H]PGF(2α) or [³H]SQ-29,548, mesangial cells displayed TxA₂ binding sites but not ones for 8-iso-PGF(2α). In contrast, rat aortic smooth muscle cells possessed specific binding sites for both TxA₂ and 8-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 TxA₂ receptor cDNA was expressed in Xenopus oocytes injected with the Ca²⁺-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 F₂-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.