The serum- and glucocorticoid-regulated kinase (sgk1) is induced by mineralocorticoids and, in turn, upregulates heterologously expressed renal epithelial Na+ channel (ENaC) activity in Xenopus oocytes. Accordingly, Sgk1 is considered to mediate the mineralocorticoid stimulation of renal ENaC activity and antinatriuresis. Here we show that at standard NaCl intake, renal water and electrolyte excretion is indistinguishable in sgk1-knockout (sgk1–/–) mice and wild-type (sgk1+/+) mice. In contrast, dietary NaCl restriction reveals an impaired ability of sgk1–/– mice to adequately decrease Na+ excretion despite increases in plasma aldosterone levels and proximal-tubular Na+ and fluid reabsorption, as well as decreases in blood pressure and glomerular filtration rate
The "classical" organic anion secretory pathway of the renal proximal tubule is critical for the renal excretion of the prototypic organic anion, para-aminohippurate, as well as of a large number of commonly prescribed drugs among other significant substrates. Organic anion transporter 1 (OAT1), originally identified as NKT (Lopez-Nieto, C. E., You, G., Bush, K. T., Barros, E. J. G., Beier, D. R., and Nigam, S. K. (1997) J. Biol. Chem. 272, 6471-6478), has physiological properties consistent with a role in this pathway. However, several other transporters (e.g. OAT2, OAT3, and MRP1) have also been proposed as important PAH transporters on the basis of in vitro studies; therefore, the relative contribution of OAT1 has remained unclear. We have now generated a colony of OAT1 knockout mice, permitting elucidation of the role of OAT1 in the context of these other potentially functionally redundant transporters. We find that the knock-out mice manifest a profound loss of organic anion transport (e.g. para-aminohippurate) both ex vivo (in isolated renal slices) as well as in vivo (as indicated by loss of renal secretion). In the case of the organic anion, furosemide, loss of renal secretion in the knock-out results in impaired diuretic responsiveness to this drug. These results indicate a critical role for OAT1 in the functioning of the classical pathway. In addition, we have determined the levels of ϳ60 endogenous organic anions in the plasma and urine of wild-type and knock-out mice. This has led to identification of several compounds with significantly higher plasma concentrations and/or lower urinary concentrations in knock-out mice, suggesting the involvement of OAT1 in their renal secretion. We have also demonstrated in xenopus oocytes that some of these compounds interact with OAT1 in vitro. Thus, these latter compounds might represent physiological substrates of OAT1.
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...
Bacterial sepsis is still a leading cause of neonatal morbidity and mortality. Early onset sepsis in particular, presents with a different clinical course and involves other pathogens than sepsis later in life. In this study, plasma concentrations and mRNA expression of granulocyte colony-stimulating factor (G-CSF), tumor necrosis factor-alpha (TNF-alpha), IL-1beta, IL-6, IL-8, and soluble intercellular adhesion molecule-1 (sICAM-1) of neonates with early onset sepsis were evaluated in cord blood and during the first days of life. Irrespective of prematurity, plasma levels of G-CSF, TNF-alpha, IL-1beta, IL-6, and IL-8, but not sICAM-1, were excessively elevated in septic neonates when compared with both healthy infants and infants with clinically suspected but not confirmed sepsis. Compared with the corresponding maternal levels, neonatal cytokine cord plasma levels were likewise highly elevated, indicating the endogenous cytokine production by the neonate. With the exception of TNF-alpha, mRNA expression in blood cells from septic infants was, however, not more frequently detectable than in those from nonseptic patients. Cytokine levels decreased significantly within the first days of life, whereas levels of sICAM-1 and C-reactive protein increased during the same time period. In summary, in contrast to C-reactive protein and sICAM-1, cord blood plasma levels, but not the presence of mRNA, of G-CSF, TNF-alpha, IL-1beta, IL-6, and IL-8 can predict neonatal early onset sepsis with a high sensitivity and specificity. Cell types other than blood cells are likely to contribute considerably to the high cytokine production in septic newborns.
Mutations in the gene encoding for the K ؉ channel ␣-subunit KCNQ1 have been associated with long QT syndrome and deafness. Besides heart and inner ear epithelial cells, KCNQ1 is expressed in a variety of epithelial cells including renal proximal tubule and gastrointestinal tract epithelial cells. At these sites, cellular K ؉ ions exit through KCNQ1 channel complexes, which may serve to recycle K ؉ or to maintain cell membrane potential and thus the driving force for electrogenic transepithelial transport, e.g., Na ؉ ͞glucose cotransport. Employing pharmacologic inhibition and gene knockout, the present study demonstrates the importance of KCNQ1 K ؉ channel complexes for the maintenance of the driving force for proximal tubular and intestinal Na ؉ absorption, gastric acid secretion, and cAMP-induced jejunal Cl ؊ secretion. In the kidney, KCNQ1 appears dispensable under basal conditions because of limited substrate delivery for electrogenic Na ؉ reabsorption to KCNQ1-expressing mid to late proximal tubule. During conditions of increased substrate load, however, luminal KCNQ1 serves to repolarize the proximal tubule and stabilize the driving force for Na ؉ reabsorption. In mice lacking functional KCNQ1, impaired intestinal absorption is associated with reduced serum vitamin B12 concentrations, mild macrocytic anemia, and fecal loss of Na ؉ and K ؉ , the latter affecting K ؉ homeostasis.
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