Identification of endothelin receptor subtypes in sheep choroid plexus

Endothelin (ET) and its G-protein-coupled receptors are distributed in a wide variety of tissues, including the brain. In this study, we have identified and characterized the endothelin receptor subtypes in sheep choroid plexus. Competitive binding experiments using [125I]ET-1 and the receptor subtype-selective ligands, ET-1, ET-3, BQ-123, Sarafotoxin 6c, and [Ala1,3,11,15] ET-1 demonstrated the presence of both ETA and ETB receptor subtypes in the ratio of 30:70. In addition, a small fraction of the total binding sites exhibited affinities for ET-1 in the subpicomolar range. Chemical crosslinking of [125I]ET-1 with bis(sulfosuccinimidyl)-suberate (BS3) to choroid plexus membranes revealed the presence of two bands, with apparent molecular masses of 89 and 45 kDa, corresponding to the ETA receptor, and three bands, with apparent molecular masses of 75, 58, and 33 kDa, corresponding to the ETB receptor. Of considerable interest was the finding that dimers of the [125I]ET-1-occupied ETA receptor could be identified by crosslinking, as could apparent dimers and tetramers of [125I]ET-1, but only when bound to receptor. In addition to mapping the distribution of ET receptors in sheep choroid plexus, our results strongly suggest that ET-1 binding to the ETA receptor leads to dimer formation.