Novel ligands BQ123 and BQ3020 characterize endothelin receptor subtypes ETA and ETB in human kidney

Abstract

Endothelins (ET), a group of vasoconstrictor peptides, are expressed in a wide range of tissues and species and bind to specific receptors of which there are at least two subtypes, ETA and ETB. The kidney has been found in animal studies to be highly sensitive to the effects of ET. We have used two new peptide ligands, which are selective for particular ET receptor subtypes (the antagonist BQ123 for ETA and the agonist BQ3020 for ETB) in binding studies to characterize human renal ET receptors. Saturation studies gave equilibrium dissociation constants (Kd) for [I125]ET-1 of 0.17 ± 0.04 nM and for [I125]BQ3020 of 0.36 ± 0.06 nM. Hill coefficients were 0.86 ± 0.03 and 0.77 ± 0.04, respectively. Macro- and microautoradiography using [I125]-labeled ligands with BQ123 and BQ3020 as competing blockers showed the majority of ET binding to be in the medulla with concentration in the vasa recta bundles, and ETA binding localizing to vascular smooth muscle. The ETB subtype predominated over ETA receptors by about 2:1, and was more generally distributed, with concentration in the collecting system. These findings were confirmed by competition binding assays giving Bmax values (ETB/ETA, fmol/mg protein), for medulla and cortex, respectively, of 18.7 ± 2.2/11.3 ± 2.7 and 12.7 ± 3.9/7.6 ± 3.5 for BQ3020; and 36.2 ± 5.6/11.1 ± 4.1 and 14.9 ± 1.6/5.3 ± 0.2 for BQ123. This study establishes ETA and ETB receptor distribution in human kidney and demonstrates that the novel ligands BQ123 and BQ3020 have at least thousand-fold selectivities for the ETA- and ETB-receptor subtypes, respectively. The results show that, in contrast with other human tissues such as heart, kidney contains predominantly the ETB subtype.