Identification of Two Corticotropin‐Releasing Factor Receptors from Xenopus laevis with High Ligand Selectivity: Unusual Pharmacology of the Type 1 Receptor

: Two cDNA clones encoding distinct members of the corticotropin‐releasing factor (CRF) receptor family have been isolated from Xenopus laevis with PCR‐based approaches. The first full‐length cDNA amplified from Xenopus brain encoded a 415‐amino acid protein with ∼80% identity to mammalian CRF receptor type 1 (CRF‐R1). The second full‐length cDNA isolated from Xenopus brain and heart encoded a 413‐amino acid protein with ∼81% identity to the α‐variant of mammalian CRF receptor, type 2 (CRF‐R2). No evidence could be obtained that the β‐variant of CRF‐R2 existed in Xenopus laevis. Binding studies using human embryonic kidney 293 (HEK 293) cells stably transfected with xenopus CRF‐R2 showed that the CRF analogues urotensin I, urocortin, and sauvagine were bound with higher affinities than human/rat CRF, xenopus CRF, and ovine CRF. In contrast to human CRF‐R1, xenopus CRF‐R1 (xCRF‐R1) was very selective for different CRF ligands. Urotensin I, urocortin, human/rat CRF, and xenopus CRF were bound with significantly (10–22‐fold) higher affinities than ovine CRF (KD = 31.7 nM) and sauvagine (KD = 51.4 nM). In agreement with these binding data, EC50 values of 39.7 and 1.1 nM were found for sauvagine and for human/rat CRF or xenopus CRF, respectively, when the cyclic AMP production in HEK 293 cells stably transfected with xCRF‐R1 was determined.