ENDOTHELIN-1 AND ENDOTHELIN-3 BINDING TO RAT NEPHRONS

The existence of endothelin (ET) receptor subtypes has recently been implicated in the different biological effects of ET in various tissues. Indeed, the cDNAs for two types of ET receptors, ET(A) and ET(B), have been cloned. To further gain insights into ET function in the kidney we examined I-125-labeled ET-1 and ET-3 binding to microdissected rat nephron segments. Specific ET-1 binding was highest in the inner medullary collecting duct, whereas the cortical and outer medullary collecting ducts as well as glomeruli showed moderate binding. There was low, although not significant, ET-1 binding to the early portion of the proximal tubule. Other nephron segments displayed little ET-1 binding. The binding profile of ET-3 along the nephron markedly resembled that of ET-1. Scatchard analyses of binding of ET-1 and ET-3 using cortical collecting ducts revealed a single class of receptor for both ET-1 and ET-3; apparent dissociation constants were 2.05 +/- 0.72 and 2.58 +/- 0.32 nM, and maximal binding capacity values were 0.408 +/- 0.058 and 0.511 +/- 0.047 fmol/mm, respectively. Displacement of I-125-ET-1 binding by unlabeled ET-3 was similar to that produced by unlabeled ET-1. Furthermore, a specific ET(B) agonist, BQ 3020, almost completely inhibited I-125-ET-1 binding in cortical collecting ducts, whereas a specific ETA antagonist, BQ 123, had little effect. These data indicate that cortical collecting ducts express ET(B) receptors, to which both ET-1 and ET-3 bind equally.