Previous electrophysiological experiments on renal cortical collecting ducts indicated that dietary sodium intake and variations in aldosterone plasma levels regulate the abundance of functional epithelial Na channels (ENaC) in the apical plasma membrane. In mouse kidney we investigated by immunohistochemistry whether feeding for 3 wk a diet with high (3% Na) and low (0.05% Na) Na content influences the distribution pattern of ENaC. In mice of all experimental groups, ENaC was apparent in cells from the late portion of the distal convoluted tubule (DCT2) down to the medullary collecting duct (CD). In mice on a high-Na diet (plasma aldosterone: 40.8 +/- 2.0 ng/dl), the alpha-subunit was undetectable, and the beta- and gamma-ENaC were detected in the cytoplasm, but not in the apical plasma membrane of the cells. In contrast, in mice on a low-Na diet (plasma aldosterone: 93.6 +/- 9.3 ng/dl) all three ENaC subunits were displayed in the subapical cytoplasm and in the apical membrane of DCT2, connecting tubule (CNT), and, although less prominent, in cortical CD cells. Apical plasma membrane immunostaining progressively decreased along the cortical CD, simultaneously with increasing cytoplasmic staining for beta- and gamma-ENaC. Thus our data on mice adapted to moderately low and high Na intake suggest that regulation of ENaC function in vivo involves shifts of beta- and gamma-subunits from the cytoplasm to the apical plasma membrane and vice versa, respectively. The insertion of these subunits into the apical plasma membrane coincides with upregulation of the alpha-subunit and its insertion into the apical plasma membrane.

Abstract. Gitelman's syndrome, an autosomal recessive renal tubulopathy caused by loss-of-function mutations in the thiazide-sensitive NaCl co-transporter (NCC) of the distal convoluted tubule (DCT), is characterized by mild renal Na ϩ wasting, hypocalciuria, hypomagnesemia, and hypokalemic alkalosis. For gaining further insights into the pathophysiology of Gitelman's syndrome, the impact of NCC ablation on the morphology of the distal tubule, on the distribution and abundance of ion transport proteins along its length, and on renal tubular Na ϩ and Ca 2ϩ handling in a gene-targeted mouse model was studied. NCC-deficient mice had significantly elevated plasma aldosterone levels and exhibited hypocalciuria, hypomagnesemia, and compensated alkalosis. Immunofluorescent detection of distal tubule marker proteins and ultrastructural analysis revealed that the early DCT, which physiologically lacks epithelial Na ϩ (ENaC) and Ca 2ϩ (TRPV5) channels, was virtually absent in NCC-deficient mice. In contrast, the late DCT seemed intact and retained expression of the apical ENaC and TRPV5 as well as basolateral Naexchanger. The connecting tubule exhibited a marked epithelial hypertrophy accompanied by an increased apical abundance of ENaC. Ca 2ϩ reabsorption seemed unaltered in the distal convolution (i.e., the DCT and connecting tubule) as indicated by real-time reverse transcription-PCR, Western blotting, and immunohistochemistry for TRPV5 and Na ϩ -Ca 2ϩ exchanger and micropuncture experiments. The last experiments further indicated that reduced glomerular filtration and enhanced fractional reabsorption of Na ϩ and Ca 2ϩ upstream and of Na ϩ downstream of the DCT provide some compensation for the Na ϩ transport defect in the DCT and contribute to the hypocalciuria. Thus, loss of NCC leads to major structural remodeling of the renal distal tubule that goes along with marked changes in glomerular and tubular function, which may explain some of the clinical features of Gitelman's syndrome.The renal distal convolution (DC), comprising the distal convoluted tubule (DCT) and the connecting tubule (CNT), plays an important role in the fine tuning of renal Na ϩ and K ϩ excretion. Moreover, it is the site of regulated transcellular Ca 2ϩ and Mg 2ϩ transport in the kidney [reviewed in (1,2)]. The thiazide-sensitive NaCl co-transporter (NCC) and the amiloride-sensitive epithelial sodium channel (ENaC) are the major apical Na ϩ transport pathways in the DCT and in the CNT, respectively (1). In rodents (3,4) and humans (5), both are co-expressed in the late DCT. High amounts of Ca 2ϩ transporting proteins such as the apical calcium channel (TRPV5/ECaC1) and the basolateral Na ϩ -Ca 2ϩ -exchanger (NCX) have been revealed in the DCT and CNT [reviewed in (6,7)]. Likewise, proteins implicated in renal Mg 2ϩ handling, such as the apical TRPM6 cation channel (8,9) and the basolateral ␥ subunit of the Na-K-ATPase (10), are highly expressed in the DC.NCC loss-of-function mutations cause human Gitelman's syndrome, an autosomal recessive tubu...