The term Bartter syndrome encompasses a heterogeneous group of autosomal recessive salt-losing nephropathies that are caused by disturbed transepithelial sodium chloride reabsorption in the distal nephron. Mutations have been identified in the NKCC2 (Na(+)-K(+)-2Cl(-)) cotransporter and ROMK potassium channel, which cooperate in the process of apical sodium chloride uptake, and ClC-Kb chloride channels, which mediate basolateral chloride release. Recently, mutations in barttin, a protein not related to any known ion transporter or channel, were described in BSND, a variant of Bartter syndrome associated with sensorineural deafness. Here we show that barttin functions as an activator of ClC-K chloride channels. Expression of barttin together with ClC-K in Xenopus oocytes increased ClC-K current amplitude, changed ClC-K biophysical properties, and enhanced ClC-K abundance in the cell membrane. Co-immunoprecipitation revealed a direct interaction of barttin with ClC-K. We performed in situ hybridization on rat kidney slices and RT-PCR analysis on microdissected nephron segments to prove co-expression of barttin, ClC-K1 and ClC-K2 along the distal nephron. Functional analysis of BSND-associated point mutations revealed impaired ClC-K activation by barttin. The results demonstrate regulation of a CLC chloride channel by an accessory protein and indicate that ClC-K activation by barttin is required for adequate tubular salt reabsorption.

Lipopolysaccharide (LPS) as a major component of the outer membrane of gram‐negative bacteria stimulates various cells to initiate a signalling cascade which ultimately leads to cell activation and expression of immunoregulatory or inflammatory cytokines. The human respiratory epithelium is an important environmental interface, but differences in LPS‐induced cell activation between bronchial and alveolar epithelial cells have not yet been investigated in detail. First, the expression of Toll‐like receptors (TLRs), as pattern‐recognition receptors, was investigated for the bronchial epithelial cells and type II‐like pneumocytes, demonstrating that they fulfil the prerequisites for LPS signalling. Thereafter, the effects of LPS, soluble CD14 (sCD14) and LPS‐binding protein (LBP) on the release of interleukin‐6 (IL‐6) and IL‐8 were studied. In the presence of LPS, sCD14 induced a significant and concentration‐dependent cytokine release in type II‐like pneumocytes, whereas the response of bronchial epithelial cells to sCD14 stimulation was low, implicating sCD14‐independent activation mechanisms. Furthermore, LBP revealed inhibitory effects on the activation of alveolar epithelial cells, which may represent a novel local defence mechanism during gram‐negative infection. We conclude that distinct pathways exist for LPS‐induced activation of bronchial and alveolar epithelial cells.

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Abstract-The expression of cyclooxygenase 2 (COX-2) in the late thick ascending limb, including the macula densa, is found to be upregulated in an activated renin-angiotensin system. How this upregulation is managed is not yet known. We therefore considered the possibility that the stimulation of COX-2 expression is triggered by the activation of the renin-angiotensin system. For this purpose, we treated male Sprague-Dawley rats with the angiotensin I-converting enzyme inhibitor ramipril (10 mg/kg per day), the angiotensin II type 1 (AT 1 ) receptor blocker losartan (30 mg/kg per day), and the angiotensin II type 2 (AT 2 ) receptor blocker PD123319 (6 mg/kg per day) for 4 days. We determined the expression of COX-2 mRNA and protein in the renal cortex. We found that ramipril and the AT 1 receptor blocker losartan increased COX-2 mRNA and COX-2 immunoreactivity in the macula densa Ϸ4-fold, whereas the AT 2 blocker PD123319 showed no effect. A low-salt diet (0.02% wt/wt) stimulated COX-2 expression in the kidney cortex Ͻ2-fold.The combination of a low-salt diet with ramipril led to a further increase of COX-2 mRNA and COX-2 immunoreactivity compared with low salt or ramipril alone. These data indicate that endogenous angiotensin II apparently inhibits COX-2 expression in the macula densa via AT 1 receptors and can therefore not account for the stimulation of COX-2 expression associated with an activated renin-angiotensin system. Because macula densa-derived prostaglandins are considered stimulators of renin secretion and renin synthesis, inhibition of macula densa COX-2 by angiotensin II could form a novel indirect negative feedback control of the renin system. (Hypertension. 1999;34:503-507.)Key Words: renin Ⅲ prostaglandins Ⅲ angiotensin II W ithin the renal cortex, cyclooxygenase 2 (COX-2) is almost exclusively expressed in cells of the late thick ascending limb of Henle, including the macula densa cells. [1][2][3] The macula densa cells are directly adjoined to the reninproducing cells of the afferent arterioles. Because prostaglandins in general 4 -6 and macula densa-derived prostaglandins in particular 7,8 are well-known stimulators of renin synthesis and of renin secretion, it is possible to assume a major function of COX-2 in the control of the renin system. This assumption is supported by the observation that the stimulation of the renin system during low salt intake, 9 renal artery stenosis, 10 or furosemide treatment 11 is attenuated by cyclooxygenase inhibitors. Moreover, it was found that the expression of COX-2 in the macula densa is markedly upregulated in the aforementioned conditions associated with an activated renin-angiotensin system (References 1 and 3 and K. Wolf, unpublished data, 1999). The signal pathways that lead to upregulation of COX-2 in the macula densa are not yet known. Because macula densa cells express angiotensin II (Ang II) receptors, 12 it would be an obvious explanation that it is the activation of the renin-angiotensin system itself that induces and maintains COX-2 expressio...