Human placental membranes contain predominantly ETB receptors.
The endothelin (ET) receptors on human placental membranes (HPMs), coupled to fluomicrospheres, have been characterized by examining the binding of 125I-ET-1 in a scintillation proximity assay (SPA). Specific binding of 125I-ET-1 was potently inhibited by ET-1 (IC50: 80 pM), ET-3 (IC50: 170 pM), and the ETB receptor-selective agonists sarafotoxin S6c (S6c; IC50: 210 pM) and alanine1,3,11,15-ET-1 (4-Ala-ET-1; IC50: 3.56 nM). In contrast, the ETA receptor-selective antagonist BQ123 (D-Val-Leu-D-Trp-D-Asp-Pro) only weakly (28% at 10 microM) inhibited 125I-ET-1 binding. In addition, the inhibition curves for ET-3 and 4-Ala-ET-1 were shallow, with slopes less than unity, indicating binding-site heterogeneity. In keeping with this finding, the presence of a small population of ETA receptors was confirmed by the ability of BQ123 (1 microM) to reduce the maximum binding capacity of the HPMs for 125I-ET-1 by approximately 17%, without affecting the affinity for the radioligand. In conclusion, these results suggest that the HPM-SPA system contains predominantly (approximately 80%) ETB receptors, with a small ETA receptor population. These findings should be taken into account when this assay system is used to identify novel endothelin receptor ligands.
Buchan
K W
KW
Pharmacology Division, Glaxo Group Research, Ware, Hertfordshire, England.
Sumner
M J
MJ
Watts
I S
IS
D000595:Amino Acid Sequence; D065128:Endothelin Receptor Antagonists; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007457:Iodine Radioisotopes; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D008969:Molecular Sequence Data; D010456:Peptides, Cyclic / Q000494:pharmacology; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D011869:Radioligand Assay; D017466:Receptors, Endothelin / Q000187:drug effects / Q000378:metabolism*
D065128:Endothelin Receptor Antagonists; D007457:Iodine Radioisotopes; D010456:Peptides, Cyclic; D017466:Receptors, Endothelin; C072247:cyclo(Trp-Asp-Pro-Val-Leu)