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-2 alpha (8-iso-PGF(2 alpha)), 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-[H-3]PGF(2 alpha) or [H-3]SQ-29,548, mesangial cells displayed TxA(2) binding sites but not ones for 8-iso-PGF(2 alpha). In contrast, rat aortic smooth muscle cells possessed specific binding sites for both TxA(2) and 8-iso-PGF(2 alpha) 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 Ca2+-specific photoprotein, aequorin, 8-iso-PGF(2 alpha) gave much weaker responses than U-46,619. These studies provide the first radioligand 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.