Unique endothelin receptor binding in kidneys of ETB receptor deficient rats

Departments of 1 Pharmacology and Toxicology and 4 Surgery, 3 Vascular Biology Center, Medical College of Georgia, Augusta, Georgia 30912; and 2 Department of Pediatrics, University of Michigan, Ann Arbor, Michigan 48109 Gariepy and colleagues (Gariepy CE, Williams SC, Richardson JA, Hammer RE, and Yanagisawa M. J Clin Invest 102: 1092-1101, 1998.) developed rescued spotting-lethal rats that carry a naturally occurring deletion of the endothelin (ET) type B receptor gene resulting in a lack of functional renal ET B receptor expression. It has been shown that rats homozygous ( sl/sl ) for the deletion have elevated plasma ET-1 levels; thus, the purpose of this study was to determine whether this deletion would result in a downregulation of ET A receptors in renal tissue. ET-1 and ET-3 binding experiments were performed with cortex, outer medullary, and inner medullary membranes of heterozygous ( sl/+ ) and sl/sl ET B receptor-deficient rats. 125 I-labeled ET-1 binding in sl/sl cortex and outer medulla was significantly lower than cortex and outer medulla from sl/+ rats. In contrast to sl/+ rats, [ 125 I]ET-3 binding was not detected in the cortex and outer medulla of sl/sl rats, indicating a lack of ET B receptor expression. The inner medulla of sl/+ rats also demonstrated an abundance of ET B receptors. Surprisingly, however, we also observed significant [ 125 I]ET-3 binding in the sl/sl inner medulla. Furthermore, ET-3 binding in the inner medulla could be blocked with an ET A receptor antagonist in sl/sl rats but not in tissue from sl/+ rats. These studies indicate that rats deficient in ET B receptors have decreased renal cortical and outer medullary ET A receptor number, most likely in response to elevated plasma ET-1 levels. In addition, homozygous ET B -deficient rats express a novel inner medullary ET-3 binding site. endothelin B receptor; kidney; receptor binding; spotting-lethal rats