Osmotic adaptation of renal medullary cells during transition from chronic diuresis to antidiuresis

The renal antioxidative effect of losartan involves heat shock protein 70 in proximal tubule cells

Angiotensin II exerts a cardinal role in the pathogenesis of hypertension and renal injury via action of angiotensin II type 1 (AT1) receptors. Local renin-angiotensin system (RAS) activity is essential for the mechanisms mediating pathophysiological functions. Proximal tubular angiotensinogen and tubular AT1 receptors are augmented by intrarenal angiotensin II. Caveolin 1 plays an important role as a regulatory molecule for the compartmentalization of redox signaling events through angiotensin II-induced NADPH oxidase activation in the kidney. A role for the renin-angiotensin system in the development and/or maintenance of hypertension has been demonstrated in spontaneously hypertensive rats (SHRs). Many effects of angiotensin II are dependent on the AT1 stimulation of reactive oxygen species (ROS) production by NADPH oxidase. Angiotensin II upregulation stimulates oxidative stress in proximal tubules from SHR. The NADPH oxidase 4 (Nox4) is abundantly expressed in kidney proximal tubule cells. Induction of the stress response includes synthesis of heat shock protein 70, a molecular chaperone that has a critical role in the recovery of cells from stress and in cytoprotection, guarding cells from subsequent insults. HSP70 chaperones function in part by driving the molecular triage decision, which determines whether proteins enter the productive folding pathway or result in client substrate ubiquitination and proteasomal degradation. This review examines regulation of losartan-mediated antioxidative stress responses by the chaperone HSP70 in proximal tubule cells of spontaneously hypertensive rats.

Clinical implications with tolvaptan on monitored bioimpedance-defined fluid status in patients with cirrhotic ascites: an observational study

Background

Prognostic value or clinical implications of fluid status monitoring in liver cirrhosis are not fully elucidated. Tolvaptan, an orally available, selective vasopressin V2-receptor antagonist approved for hyponatremia in the United States and European Union. It is also used for cirrhotic ascites at a relatively low dose (3.75 mg to 7.5 mg) in Japan, exerts its diuretic function by excreting electrolyte-free water. We hypothesized that bioimpedance-defined dynamic changes in fluid status allow prediction of response of V2 antagonism and survival in cirrhotic patients.

Methods

In this prospective observational study, 30 patients with decompensated liver cirrhosis who were unresponsive to conventional diuretics were enrolled. Detailed serial changes of body composition that were assessed by using non-invasive bioimpedance analysis (BIA) devices, along with biochemical studies, were monitored at 5 time points.

Results

Sixteen patients were classified as short-term responders (53%). Rapid and early decrease of BIA-defined intracellular water, as soon as 6 h after the first dose (ΔICW_BIA%-6 h), significantly discriminated responders from non-responders (AUC = 0.97, P < 0.0001). ΔICW_BIA%-6 h was highly correlated with the change of BIA-derived phase angle of trunk, e.g. reduced body reactance operated at 50 kHz after 24 h of the first dose of tolvaptan. Lower baseline blood urea nitrogen and lower serum aldosterone were predictive of a rapid and early decrease of ICW_BIA. A rapid and early decrease of ICW_BIA in response to tolvaptan was also predictive of a better transplant-free survival.

Conclusions

BIA-defined water compartment monitoring may help predict short-term efficacy and survival in decompensated cirrhotic patients treated with tolvaptan.

Immune cells control skin lymphatic electrolyte homeostasis and blood pressure

The skin interstitium sequesters excess Na+ and Cl- in salt-sensitive hypertension. Mononuclear phagocyte system (MPS) cells are recruited to the skin, sense the hypertonic electrolyte accumulation in skin, and activate the tonicity-responsive enhancer-binding protein (TONEBP, also known as NFAT5) to initiate expression and secretion of VEGFC, which enhances electrolyte clearance via cutaneous lymph vessels and increases eNOS expression in blood vessels. It is unclear whether this local MPS response to osmotic stress is important to systemic blood pressure control. Herein, we show that deletion of TonEBP in mouse MPS cells prevents the VEGFC response to a high-salt diet (HSD) and increases blood pressure. Additionally, an antibody that blocks the lymph-endothelial VEGFC receptor, VEGFR3, selectively inhibited MPS-driven increases in cutaneous lymphatic capillary density, led to skin Cl- accumulation, and induced salt-sensitive hypertension. Mice overexpressing soluble VEGFR3 in epidermal keratinocytes exhibited hypoplastic cutaneous lymph capillaries and increased Na+, Cl-, and water retention in skin and salt-sensitive hypertension. Further, we found that HSD elevated skin osmolality above plasma levels. These results suggest that the skin contains a hypertonic interstitial fluid compartment in which MPS cells exert homeostatic and blood pressure-regulatory control by local organization of interstitial electrolyte clearance via TONEBP and VEGFC/VEGFR3-mediated modification of cutaneous lymphatic capillary function.

A hyperpolarization-activated, cyclic nucleotide-gated, (Ih-like) cationic current and HCN gene expression in renal inner medullary collecting duct cells

The cation conductancein primary cultures of rat renal inner medullary collecting duct was studied using perforated-patch and conventional whole cell clamp techniques. Hyperpolarizations beyond -60 mV induced a time-dependent inward nonselective cationic current (I(vti)) that resembles the well-known hyperpolarization-activated, cyclic nucleotide-gated I(h) and I(f) currents. I(vti) showed a half-maximal activation around -102 mV with a slope factor of 25 mV. It had a higher conductance (but, at its reversal potential, not a higher permeability) for K(+) than for Na(+) (gK(+)/gNa(+) = 1.5), was modulated by cAMP and blocked by external Cd(2+) (but not Cs(+) or ZD-7288), and potentiated by a high extracellular K(+) concentration. We explored the expression of the I(h) channel genes (HCN1 to -4) by RT-PCR. The presence of transcripts corresponding to the HCN1, -2, and -4 genes was observed in both the cultured cells and kidney inner medulla. Western blot analysis with HCN2 antibody showed labeling of approximately 90- and approximately 120-kDa proteins in samples from inner medulla and cultured cells. Immunocytochemical analysis of cell cultures and inner medulla showed the presence of HCN immunoreactivity partially colocalized with the Na(+)-K(+)-ATPase at the basolateral membrane of collecting duct cells. This is the first evidence of an I(h)-like cationic current and HCN immunoreactivity in either kidney or any other nonexcitable mammalian cells.

CELL SURVIVAL IN THE HOSTILE ENVIRONMENT OF THE RENAL MEDULLA

The countercurrent system in the medulla of the mammalian kidney provides the basis for the production of urine of widely varying osmolalities, but necessarily entails extreme conditions for medullary cells, i.e., high concentrations of solutes (mainly NaCl and urea) in antidiuresis, massive changes in extracellular solute concentrations during the transitions from antidiuresis to diuresis and vice versa, and low oxygen tension. The strategies used by medullary cells to survive in this hostile milieu include accumulation of organic osmolytes and heat shock proteins, the extensive use of the glycolysis for energy production, and a well-orchestrated network of signaling pathways coordinating medullary circulation and tubular work.

Effect of ammonium on the expression of osmosensitive genes in Madin-Darby canine kidney cells

The cells of the kidney medulla are exposed routinely to high extracellular concentrations of various solutes including NaCl, urea and ammonium (NH4+). Although it is well established that the expression of a variety of osmosensitive genes and proteins, which confer cytoprotection on renal medullary cells, is induced by high NaCl concentrations, the role of NH4+ in these cellular responses is unclear. This study thus addressed the effect of NH4+ on the expression of the betaine/GABA transporter (BGT-1), the sodium/myo-inositol cotransporter (SMIT), aldose reductase (AR), and heat shock protein 70 (HSP70) in Madin-Darby canine kidney (MDCK) cells, using Northern and Western blot analyses and enzyme-linked immunosorbent assay (ELISA). The incidence of apoptosis was monitored by determining caspase-3 activity and annexin V binding. Addition of NH4Cl (50 mM; total osmolality 400 mosmol (kg H2O)(-1) to the medium was more effective than equiosmolar NaCl in increasing BGT-1 and HSP70 mRNA abundance, but less effective in enhancing BGT-1 and HSP70 expression at the protein level. Qualitatively similar results were obtained for SMIT and AR mRNAs. Exposure to both isotonic and hypertonic, NH4Cl-containing medium enhanced apoptosis compared with equiosmolar, NaCl-containing media. These results suggest that, in addition to NaCl, NH4Cl may play a role in regulating the intracellular accumulation of organic osmolytes, the abundance of HSP70 and cell turnover in the renal medulla in vivo.

Relationship between intracellular ionic strength and expression of tonicity-responsive genes in rat papillary collecting duct cells

Intracellular ionic strength may play an important role in regulating the expression of genes encoding osmolyte-accumulating molecules. To establish whether a strict relation exists between these variables, intracellular ionic strength (sum of Na+, Cl- and K+ concentrations) and the relative abundance of mRNA derived from various tonicity-sensitive genes was examined using electron microprobe analysis and Northern blots on primary cultures of rat papillary collecting duct (PCD) cells following acute or long-term alterations in medium tonicity. Hypertonic medium (450 mosmol kg(-1)) evoked an initial rise in intracellular ionic strength (269 +/- 5 vs. 194 +/- 7 mmol (kg wet weight (wt))(-1) in isotonic controls; means +/- S.E.M.), which subsequently declined gradually, and a significantly higher abundance of bgt1 (Na+- and Cl- -dependent betaine transporter), smit (Na+/myo-inositol cotransporter), ar (aldose reductase) and osp94 (osmotic stress protein 94) mRNAs. Conversely, exposure to hypotonic medium (200 mosmol kg(-1)) for 12 h was associated with significantly reduced intracellular ionic strength (153 +/- 4 mmol (kg wet wt)(-1)) and significantly reduced the abundance of smit and ar mRNAs. PCD cells preconditioned in hypotonic medium and re-exposed to isotonic medium showed significantly higher abundance of these mRNAs than isotonic controls, although the intracellular ionic strength did not differ. Two further tonicity-sensitive genes responded differently to medium tonicity: while the abundance of hsp70 (heat shock protein 70) mRNA increased significantly following both hypo- and hypertonic stress, inos (inducible nitric oxide synthase) mRNA abundance correlated inversely with medium tonicity. These findings support the view that the effect of intracellular ionic strength on the expression of bgt1, smit, ar and osp94 is modulated by additional factors such as cell volume, and that its effect on the pathways regulating hsp70 and inos is even more complex.

Molecular chaperones in the kidney: distribution, putative roles, and regulation

Molecular chaperones are intracellular proteins that prevent inappropriate intra- and intermolecular interactions of polypetide chains. A specific group of highly conserved molecular chaperones are the heat shock proteins (HSPs), many of which are constitutively expressed but most of which are inducible by diverse (in some cases specific) stress factors. HSPs, either alone or in cooperation with "partner" chaperones, are involved in cellular processes as disparate as correct folding and assembly of proteins, transport of proteins to specific intracellular locations, protein degradation, and preservation and restructuring of the cytoskeleton. The characteristic distribution of individual HSPs in the kidney, and their response to different challenges, suggests that a number of HSPs may fulfill specific, kidney-related functions. HSP72 and the osmotic stress protein 94 (Osp94) appear to participate in the adaptation of medullary cells to high extracellular salt and urea concentrations; the small HSPs (HSP25/27 and crystallins) may be involved in the function of mesangial cells and podocytes and contribute to the volume-regulatory remodeling of the cytoskeleton in medullary cells during changes in extracellular tonicity. HSP90 contributes critically to the maturation of steroid hormone receptors and may thus be a critical determinant of the aldosterone sensitivity of specific renal epithelial cells. Certain HSPs are also induced in various pathological states of the kidney. The observation that the expression of individual HSPs in specific kidney diseases often displays characteristic time courses and intrarenal distribution patterns supports the idea that HSPs are involved in the recovery but possibly also in the initiation and/or maintenance phases of these disturbances.

Bidirectional regulation of tonicity-responsive enhancer binding protein in response to changes in tonicity

Tonicity-responsive enhancer binding protein (TonEBP) regulates transcription of tonicity responsive genes such as the sodium-myo-inositol cotransporter (SMIT), the sodium-chloride-betaine cotransporter (BGT1), and aldose reductase (AR). To characterize signals that activate TonEBP in Madin-Darby canine kidney (MDCK) cells, the abundance and nuclear distribution of TonEBP were studied after the osmolality of the culture medium was changed. Hypertonicity but not hyperosmolality is effective in activation of TonEBP as expected. Surprisingly, exposure to hypotonic medium leads to a dramatic downregulation of TonEBP both in abundance and nuclear distribution, indicating that under isotonic conditions, TonEBP is at a low-level activated state and can respond to both increase and decrease in tonicity. Additional experiments suggest that cellular ionic strength is the signal that initiates regulation of TonEBP. The increase in abundance of TonEBP is mediated by an increase in mRNA abundance and a parallel increase in synthesis of TonEBP. The stability of TonEBP mRNA is not affected by hypertonicity indicating that transcription plays a major role in the induction of TonEBP by hypertonicity.