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 F2alpha (8-iso-PGF2alpha), 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]PGF2alpha or [3H]SQ-29,548, mesangial cells displayed TxA2 binding sites but not ones for 8-iso-PGF2alpha. In contrast, rat aortic smooth muscle cells possessed specific binding sites for both TxA2 and 8-iso-PGF2alpha 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-PGF2alpha gave much weaker responses than U-46,619. These studies provide the first radioligand 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.