We used patch-clamp electrophysiology to investigate regulation of the epithelial Na ϩ channel (ENaC) by endothelin-1 (ET-1) in isolated, split-open rat collecting ducts. ET-1 significantly decreases ENaC open probability by about threefold within 5 min. ET-1 decreases ENaC activity through basolateral membrane ETB but not ETA receptors. In rat collecting duct, we find no role for phospholipase C or protein kinase C in the rapid response of ENaC to ET-1. ET-1, although, does activate src family tyrosine kinases and their downstream MAPK1/2 effector cascade in renal principal cells. Both src kinases and MAPK1/2 signaling are necessary for ET-1-dependent decreases in ENaC open probability in the split-open collecting duct. We conclude that ET-1 in a physiologically relevant manner rapidly suppresses ENaC activity in native, mammalian principal cells. These findings may provide a potential mechanism for the natriuresis observed in vivo in response to ET-1, as well as a potential cause for the salt-sensitive hypertension found in animals with impaired endothelin signaling.salt-sensitive hypertension; systemic blood pressure ENDOTHELIN-1 (ET-1) is a powerful vasoconstricting peptide hormone that is an important regulator of systemic blood pressure (53). Independent of its vascular effects, ET-1 also affects renal Na ϩ and water handling favoring natriuresis and diuresis. While circulating ET-1 arises from endothelial cells, local ET-1 systems also exist. For instance in the kidney, the collecting duct produces significant amounts of 25,38,51). ET-1 targets cells through two distinct receptor subtypes, ET A and ET B (32, 41). Renal collecting duct cells have both types of receptors and are able to bind 49,50). Thus, collecting duct-derived ET-1, acting in a paracrine/ autocrine manner, is an important regulator of renal Na ϩ handling (2,20,26,42).Regulated Na ϩ reabsorption in the renal collecting duct, in part, controls blood pressure. Here, activity of the aldosteronesensitive epithelial Na ϩ channel (ENaC) is limiting for Na ϩ transport (reviewed in Refs. 19,30,31). Dysfunction and inappropriate regulation of ENaC consequently result in improper renal Na ϩ handling and thus, blood pressure disorders. For instance, gain of ENaC function in rodents and humans is causative for hypertension associated with the hallmarks of low plasma renin activity and aldosterone levels (1,22,23,45,46). Amiloride, an ENaC blocker, ameliorates this hypertension.Spotting lethal (sl) rats have a naturally occurring null mutation of ET B (17). These rats, when rescued from lethal intestinal aganglionosis by directed ET B transgene expression in the enteric nervous system, are particularly sensitive to DOCA and salt-induced hypertension (18,33,34). Similarly, mice with collecting duct-specific knockout of the ET B receptor have elevated blood pressure that further increases with high salt feeding (20). Collecting duct-specific ET-1 knockout, moreover, leads to hypertension exacerbated by high salt (2, 42). Plasma renin activity and aldoste...