In vitro studies suggest that collecting duct-derived (CD-derived) endothelin-1 (ET-1) can regulate renal Na reabsorption; however, the physiologic role of CD-derived ET-1 is unknown. Consequently, the physiologic effect of selective disruption of the ET-1 gene in the CD of mice was determined. Mice heterozygous for aquaporin2 promoter Cre recombinase and homozygous for loxP-flanked exon 2 of the ET-1 gene (called CD-specific KO of ET-1 [CD ET-1 KO] mice) were generated. These animals had no CD ET-1 mRNA and had reduced urinary ET-1 excretion. CD ET-1 KO mice on a normal Na diet were hypertensive, while body weight, Na excretion, urinary aldosterone excretion, and plasma renin activity were unchanged. CD ET-1 KO mice on a high-Na diet had worsened hypertension, reduced urinary Na excretion, and excessive weight gain, but showed no differences between aldosterone excretion and plasma renin activity. Amiloride or furosemide reduced BP in CD ET-1 KO mice on a normal or high-Na diet and prevented excessive Na retention in salt-loaded CD ET-1 KO mice. These studies indicate that CD-derived ET-1 is an important physiologic regulator of renal Na excretion and systemic BP. IntroductionEndothelin-1 (ET-1) was initially described as a potent endothelial cell-derived vasoconstrictor (1); however, the peptide is now known to be produced by many cell types and to elicit multiple biologic effects (2). The kidney is likely an important target; ET-1 causes renal vasoconstriction, mesangial cell contraction, glomerular cell proliferation, ECM accumulation, and alterations in nephron fluid and electrolyte transport (2). While many renal cell types synthesize and bind ET-1, the collecting duct (CD) is of particular importance: The renal inner medulla contains the highest concentration of ET-1 in the body (3), and the inner medullary CD (IMCD) is the predominant renal site of ET-1 production (4-8) and receptor expression (9-11).In vitro studies suggest that ET-1 inhibits Na and water reabsorption in the cortical CD (CCD) and IMCD and that this occurs through activation of the ET B receptor (ETRB). ET-1 inhibits vasopressin-stimulated (AVP-stimulated) water flux in isolated CCD (9, 12) and reduces AVP-stimulated cyclic AMP accumulation (13-15) and osmotic water permeability (16, 17) in isolated IMCDs. ET-1 also inhibits mineralocorticoid and AVP-stimulated Na and Cl reabsorption in isolated CCDs (12,18,19) and decreases Na/K-ATPase activity in suspensions of IMCDs (20). Despite these data, demonstrating such an ET-1 effect in vivo and clarifying how CD-derived ET-1 physiologically regulates Na and water transport has been problematic. This difficulty stems, in part, from

. Collecting duct-specific knockout of the endothelin B receptor causes hypertension and sodium retention. Am J Physiol Renal Physiol 291: F1274 -F1280, 2006. First published July 25, 2006 doi:10.1152/ajprenal.00190.2006.-Collecting duct (CD)-derived endothelin-1 (ET-1) inhibits renal Na reabsorption and its deficiency increases blood pressure (BP). The role of CD endothelin B (ETB) receptors in mediating these effects is unknown. CD-specific knockout of the ETB receptor was achieved using an aquaporin-2 promoter-Cre recombinase transgene and the loxP-flanked ETB receptor gene (CD ETB KO). Systolic BP in mice with CD-specific knockout of the ETB receptor, ETA receptor (CD ETA KO) and ET-1 (CD ET-1 KO), and their respective controls were compared during normal-and high-salt diet. On a normal-sodium diet, CD ETB KO mice had elevated BP, which increased further during high salt feeding. However, the degree of hypertension in CD ETB KO mice and the further increase in BP during salt feeding were lower than that of CD ET-1 KO mice, whereas CD ETA KO mice were normotensive. CD ETB KO mice had impaired sodium excretion following acute sodium loading. Aldosterone and plasma renin activity were decreased in CD ETB KO mice on normal-and high-sodium diets, while plasma and urinary ET-1 levels did not differ from controls. In conclusion, the CD ETB receptor partially mediates the antihypertensive and natriuretic effects of ET-1. CD ETA and ETB receptors do not fully account for the antihypertensive and natriuretic effects of CD-derived ET-1, suggesting paracrine effects of this peptide. blood pressure; urinary sodium excretion; cell-specific gene targeting NUMEROUS LINES OF EVIDENCE indicate that collecting duct (CD)-derived endothelin-1 (ET-1) is an important regulator of renal Na reabsorption and systemic blood pressure (BP). First, the CD is the major renal site of ET-1 production (8,25,41,48,49), synthesizing more of the peptide than any other cell type (24). Second, the distribution of ET receptors in the kidney closely corresponds to the sites of ET production. Binding and RT-PCR studies using microdissected nephron segments indicate that endothelin receptors are primarily expressed by the inner medullary CD, moderately expressed by outer medullary CD and cortical CD, with much lower expression by other nephron segments (43,45). Third, cultured inner medullary CD cells secrete ET-1 from, and bind ET-1 to, the same (basolateral) side (27). Fourth, in vitro studies indicate that ET-1 inhibits CD Na transport. ET-1 decreases Na reabsorption by the isolated CD, an effect that may be mediated by inhibition of amiloride-sensitive sodium channel activity (14,31,32,46) and/or Na-K-ATPase activity (53). Perhaps the most compelling evidence implicating CD-derived ET-1 in the regulation of renal Na excretion and BP comes from gene targeting studies. Mice with CD-specific knockout of the ET-1 gene (CD ET-1 KO) are hypertensive on a normal-Na diet and this is exacerbated by high Na intake, with systolic BP increasing by almost 40...

In vitro studies suggest that collecting duct-derived (CD-derived) endothelin-1 (ET-1) can regulate renal Na reabsorption; however, the physiologic role of CD-derived ET-1 is unknown. Consequently, the physiologic effect of selective disruption of the ET-1 gene in the CD of mice was determined. Mice heterozygous for aquaporin2 promoter Cre recombinase and homozygous for loxP-flanked exon 2 of the ET-1 gene (called CD-specific KO of ET-1 [CD ET-1 KO] mice) were generated. These animals had no CD ET-1 mRNA and had reduced urinary ET-1 excretion. CD ET-1 KO mice on a normal Na diet were hypertensive, while body weight, Na excretion, urinary aldosterone excretion, and plasma renin activity were unchanged. CD ET-1 KO mice on a high-Na diet had worsened hypertension, reduced urinary Na excretion, and excessive weight gain, but showed no differences between aldosterone excretion and plasma renin activity. Amiloride or furosemide reduced BP in CD ET-1 KO mice on a normal or high-Na diet and prevented excessive Na retention in salt-loaded CD ET-1 KO mice. These studies indicate that CD-derived ET-1 is an important physiologic regulator of renal Na excretion and systemic BP. IntroductionEndothelin-1 (ET-1) was initially described as a potent endothelial cell-derived vasoconstrictor (1); however, the peptide is now known to be produced by many cell types and to elicit multiple biologic effects (2). The kidney is likely an important target; ET-1 causes renal vasoconstriction, mesangial cell contraction, glomerular cell proliferation, ECM accumulation, and alterations in nephron fluid and electrolyte transport (2). While many renal cell types synthesize and bind ET-1, the collecting duct (CD) is of particular importance: The renal inner medulla contains the highest concentration of ET-1 in the body (3), and the inner medullary CD (IMCD) is the predominant renal site of ET-1 production (4-8) and receptor expression (9-11).In vitro studies suggest that ET-1 inhibits Na and water reabsorption in the cortical CD (CCD) and IMCD and that this occurs through activation of the ET B receptor (ETRB). ET-1 inhibits vasopressin-stimulated (AVP-stimulated) water flux in isolated CCD (9, 12) and reduces AVP-stimulated cyclic AMP accumulation (13-15) and osmotic water permeability (16, 17) in isolated IMCDs. ET-1 also inhibits mineralocorticoid and AVP-stimulated Na and Cl reabsorption in isolated CCDs (12,18,19) and decreases Na/K-ATPase activity in suspensions of IMCDs (20). Despite these data, demonstrating such an ET-1 effect in vivo and clarifying how CD-derived ET-1 physiologically regulates Na and water transport has been problematic. This difficulty stems, in part, from

In vitro studies suggest that endothelin-1 (ET-1) inhibits vasopressin (AVP)-stimulated water permeability in the collecting duct (CD). To evaluate the role of CD-derived ET-1 in regulating renal water metabolism, the ET-1 gene was selectively disrupted in the CD (CD ET-1 KO). During normal water intake, urinary osmolality (Uosm), plasma Na concentration, urine volume, and renal aquaporin-2 (AQP2) levels were unchanged, but plasma AVP concentration was reduced in CD ET-1 KO animals. CD ET-1 KO mice had impaired ability to excrete an acute, but not a chronic, water load, and this was associated with increased CD ET-1 mRNA in control, but not CD ET-1 KO, mice. In response to continuous infusion of 1-desamino-8-D-arginine vasopressin, CD ET-1 KO mice had greater increases in Uosm, V2 and AQP2 mRNA, and phosphorylation of AQP2. CD suspensions from CD ET-1 KO mice had enhanced AVP- and forskolin-stimulated cAMP accumulation. These data indicate that CD ET-1 KO increases renal sensitivity to the urinary concentrating effects of AVP and suggest that ET-1 functions as a physiological autocrine regulator of AVP action in the CD.