Identification of conventional and novel endothelin receptors in sheep choroid plexus cells

Previous studies have demonstrated the presence of super-high affinity endothelin receptors with apparent Kd's on the order of pM in different brain tissues. This study was designed to characterize, in detail, the receptors present in SCP cells, a non-transformed sheep choroid plexus cell line. Competitive binding assays with receptor-selective ligands indicated the presence of at least three classes of binding sites: a conventional receptor of the ETA subtype with a Kd = 0.4 nM that mediates an increase in intracellular levels of inositol 1,4,5-trisphosphate (IP3)in response to ET-1 and two additional sites with much higher binding affinities. The latter two sites are not coupled to the common signal transduction pathways of IP3, cAMP and cGMP. Northern blot analysis confirmed the presence of only the ETA subtype mRNA in SCP cells. It remains to determined if the multiple binding sites are distinct gene products, multiple affinity states of a single receptor molecule or a result of cooperative association of one site with either the ligand or with other proteins.