NBC3 expression in rabbit collecting duct: colocalization with vacuolar H+-ATPase

We have recently cloned and characterized a unique sodium bicarbonate cotransporter, NBC3, which unlike other members of the NBC family, is ethylisopropylamiloride (EIPA) inhibitable, DIDS insensitive, and electroneutral (A. Pushkin, N. Abuladze, I. Lee, D. Newman, J. Hwang, and I. Kurtz. J. Biol. Chem. 274: 16569-16575, 1999). In the present study, a specific polyclonal antipeptide COOH-terminal antibody, NBC3-C1, was generated and used to determine the pattern of NBC3 protein expression in rabbit kidney. A major band of approximately 200 kDa was detected on immunoblots of rabbit kidney. Immunocytochemistry of rabbit kidney frozen sections revealed specific staining of the apical membrane of intercalated cells in both the cortical and outer medullary collecting ducts. The pattern of NBC3 protein expression in the collecting duct was nearly identical to the same sections stained with an antibody against the vacuolar H+-ATPase 31-kDa subunit. In addition, the NBC3-C1 antibody coimmunoprecipitated the vacuolar H+-ATPase 31-kDa subunit. Functional studies in outer medullary collecting ducts (inner stripe) showed that type A intercalated cells have an apical Na+-dependent base transporter that is EIPA inhibitable and DIDS insensitive. The data suggest that NBC3 participates in H+/base transport in the collecting duct. The close association of NBC3 and the vacuolar H+-ATPase in type A intercalated cells suggests a potential structural/functional interaction between the two transporters.

Cell shrinkage triggers the activation of mitogen-activated protein kinases by hypertonicity in the rat kidney medullary thick ascending limb of the Henle's loop. Requirement of p38 kinase for the regulatory volume increase response

The kidney medulla is exposed to very high interstitial osmolarity leading to the activation of mitogen-activated protein kinases (MAPK). However, the respective roles of increased intracellular osmolality and of cell shrinkage in MAPK activation are not known. Similarly, the participation of MAPK in the regulatory volume increase (RVI) following cell shrinkage remains to be investigated. In the rat medullary thick ascending limb of Henle (MTAL), extracellular hypertonicity produced by addition of NaCl or sucrose increased the phosphorylation level of extracellular signal-regulated kinase (ERK) and p38 kinase and to a lesser extent c-Jun NH(2)-terminal kinase with sucrose only. Both hypertonic solutions decreased the MTAL cellular volume in a dose- and time-dependent manner. In contrast, hypertonic urea had no effect. The extent of MAPK activation was correlated with the extent of MTAL cellular volume decrease. Increasing intracellular osmolality without modifying cellular volume did not activate MAPK, whereas cell shrinkage without variation in osmolality activated both ERK and p38. In the presence of 600 mosmol/liter NaCl, the maximal cell shrinkage was observed after 10 min at 37 degrees C and the MTAL cellular volume was reduced to 70% of its initial value. Then, RVI occurred and the cellular volume progressively recovered to reach about 90% of its initial value after 30 min. SB203580, a specific inhibitor of p38, almost completely inhibited the cellular volume recovery, whereas inhibition of ERK did not alter RVI. In conclusion, in rat MTAL: 1) cell shrinkage, but not intracellular hyperosmolality, triggers the activation of both ERK and p38 kinase in response to extracellular hypertonicity; and 2) RVI is dependent on p38 kinase activation.

K(+)/Na(+) antagonism at cytoplasmic sites of Na(+)-K(+)-ATPase: a tissue-specific mechanism of sodium pump regulation

Tissue-distinct interactions of the Na(+)-K(+)-ATPase with Na(+) and K(+), independent of isoform-specific properties, were reported previously (A. G. Therien, N. B. Nestor, W. J. Ball, and R. Blostein. J. Biol. Chem. 271: 7104-7112, 1996). In this paper, we describe a detailed analysis of tissue-specific kinetics particularly relevant to regulation of pump activity by intracellular K(+), namely K(+) inhibition at cytoplasmic Na(+) sites. Our results show that the order of susceptibilities of alpha(1) pumps of various rat tissues to K(+)/Na(+) antagonism, represented by the ratio of the apparent affinity for Na(+) binding at cytoplasmic activation sites in the absence of K(+) to the affinity constant for K(+) as a competitive inhibitor of Na(+) binding at cytoplasmic sites, is red blood cell < axolemma approximately rat alpha(1)-transfected HeLa cells < small intestine < kidney < heart. In addition, we have carried out an extensive analysis of the kinetics of K(+) binding and occlusion to the cytoplasmic cation binding site and find that, for most tissues, there is a relationship between the rate of K(+) binding/occlusion and the apparent affinity for K(+) as a competitive inhibitor of Na(+) activation, the order for both parameters being heart >/= kidney > small intestine approximately rat alpha(1)-transfected HeLa cells. The notion that modulations in cytoplasmic K(+)/Na(+) antagonism are a potential mode of pump regulation is underscored by evidence of its reversibility. Thus the relatively high K(+)/Na(+) antagonism characteristic of kidney pumps was reduced when rat kidney microsomal membranes were fused into the dog red blood cell.

Cloning and preliminary characterization of a calcium-binding protein closely related to nucleolin on the apical surface of inner medullary collecting duct cells

Calcium stone crystal attachment to the urinary epithelium plays an essential role in the development of kidney stones by allowing small crystals to be retained in the kidney until they become macroscopic. We among others have described attachment of stone crystals to cultured renal epithelia (Wiessner, J. H., Kleinman, J. G., Blumenthal, S. S., Garancis, J. C., and Mandel, G. S. (1987) J. Urol. 138, 640-643). To isolate protein(s) that may participate in crystal attachment, apical membranes of cultured renal inner medullary collecting duct were biotinylated, the cells were lysed with detergent, the lysate was subjected to hydroxyapatite chromatography, and fractions were incubated with calcium oxalate monohydrate. Electrophoresis of material solubilized from the crystals showed several selectively adsorbed protein bands. A 110-kDa band stained positively for biotin and for glycosides and bound (45)Ca. The amino acid sequence of this band was determined to be that of a protein closely related to rat nucleolin (nucleolin-related protein; NRP). NRP was cloned and sequenced and was 83% homologous with the previously sequenced nucleolar protein nucleolin. Using temperature-induced phase partitioning with Triton X-114, NRP was associated with both the insoluble membrane skeleton pellet and the soluble aqueous phase but not the soluble detergent phase. This association with the membrane skeleton was increased in the presence of calcium. Thus, NRP is associated with the apical membranes of cultured renal tubular cells and is bound to membrane skeletal elements in a calcium-dependent fashion. The physiological role of NRP remains to be determined; however, a pathophysiological role may be that of mediating the attachment to the renal tubular epithelium of calcium stone crystals.

SNARE proteins regulate H(+)-ATPase redistribution to the apical membrane in rat renal inner medullary collecting duct cells

The interaction of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins provides the necessary steps for vesicle docking fusion. In inner medullary collecting duct (IMCD) cells, acid secretion is regulated in part by exocytotic insertion and endocytotic retrieval of an H(+)-ATPase to and from the apical membrane. We previously suggested a role for SNARE proteins in exocytotic insertion of proton pumps in IMCD cells. The purpose of the present study was to determine whether SNARE proteins are associated with the 31-kDa subunit of H(+)-ATPase in IMCD cells during exocytosis and to determine the effects of clostridial toxins on SNARE-mediated trafficking of H(+)-ATPase. Cell acidification induced a marked increment of H(+)-ATPase in the apical membrane. However, pretreating cells with clostridial toxins blocked the cellular translocation of the 31-kDa subunit. Immunoprecipitation of IMCD cell homogenate, using antibodies against either the 31-kDa subunit of H(+)-ATPase or vesicle-associated membrane protein-2, co-immunoprecipitated N-ethylmaleimide-sensitive factor, alpha-soluble NSF attachment protein (alpha-SNAP), synaptosome-associated protein-23, syntaxin, and vesicle-associated membrane protein-2. Pretreatment with clostridial toxin resulted in reduced co-immunoprecipitation of H(+)-ATPase and syntaxin. These experiments document, for the first time, a putative docking fusion complex in IMCD cells and a physical association of the H(+)-ATPase with the complex. The sensitivity to the action of clostridial toxin indicates the docking-fusion complex is a part of the exocytotic mechanism of the proton pump.

ERK activation by urea in the renal inner medullary mIMCD3 cell line

Urea- and NaCl-inducible extracellular signal-regulated kinase (ERK) phosphorylation exhibited dissimilar kinetics. Among cell lines examined, the effect of urea was unique to mIMCD3 inner medullary collecting duct cells and MDCK cells. Urea-inducible ERK activation was approximately 10-fold less sensitive to the MEK inhibitor, PD-98059, than was that of NaCl. This difference did not appear to be accounted for by differential activation of MEK isoforms. Interestingly, the inhibitor of p38 activation, SB-203580, abrogated the effect of both urea and NaCl upon both ERK and MEK activation; however, the former was much less sensitive to the inhibitor. Consistent with this observation, NaCl was much more effective than urea at inducing p38 phosphorylation. The effect of hypertonic stress (e.g., sorbitol 100 mM) could be blocked by appropriate medium dilution such that isotonicity was maintained. In marked contrast, the effect of hyperosmotic urea could not be blocked in this fashion, implying the absence of dependence upon cell volume. Together, these data suggest that cells of the renal inner medulla are potentially uniquely responsive to urea and that urea and hypertonic stressors induce ERK activation through distinct mechanisms.

Imaging of renal medullary interstitial cells in situ by confocal fluorescence microscopy

Renal medullary interstitial cells are a prevalent and characteristic feature of the inner medulla of the kidney, but the physiological significance of this is unclear. We have developed a method for imaging renal medullary interstitial cells in situ by loading the cells with fluorescent dyes and monitoring their distribution using confocal microscopy. The pH-sensitive probe 2'7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester was used as a marker of cytoplasmic volume and therefore of cell morphology. Nile Red was used to demonstrate the presence of renal medullary interstitial cell lipid droplets. Papillae were excised from 100 g Sprague-Dawley rats and loaded with the appropriate dye. The papillae were then examined using a Leica TCS 4D confocal microscope and oil immersion lenses. Fluorescence was excited (488 nm) using an argon laser and emission wavelengths above 515 nm collected using a long pass filter. Images of papillae loaded with 2'7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester clearly demonstrate a ladder-like arrangement of renal medullary interstitial cells. More detailed examination revealed the presence of cytoplasmic extensions that appear to make close contact with adjacent loops of Henle. Three-dimensional reconstructions of serial sections revealed spiral arrangements in some ladders of renal medullary interstitial cells. Nile Red-labelled lipid droplets of 0.5-1.0 microm diameter were located throughout the cytoplasm of renal medullary interstitial cells and especially within the cytoplasmic extensions. These experiments highlight the ability of confocal microscopy to allow investigation of renal medullary interstitial cells in situ.

Regulation of cyclooxygenase-2 expression in renal medulla by tonicity in vivo and in vitro

Renal medullary prostaglandins are believed to exert an important functional role in antagonizing vasopressin effects in dehydration. Studies were undertaken to determine the effect of hyperosmolality on cyclooxygenase (COX) isoform expression in the renal medulla. COX-1 and COX-2 mRNA and protein levels were determined by RT-PCR or Western blotting in Sprague-Dawley rats on varying water intakes, in Brattleboro rats and in Long-Evans controls. Over a wide range of urinary tonicity, COX-2 expression correlated closely with urine osmolality levels (R = 0.872). COX-1 levels did not vary. Immunolocalization showed that the stimulation of COX-2 expression by dehydration occurred predominantly in the collecting duct. Hypertonicity caused by addition of NaCl produced a dose- and time-dependent stimulation of COX-2 expression in mIMCD-K2 cells as well as in MDCK cells. COX-1 was unaffected. In the same cell lines, mannitol, sucrose, and raffinose also had a stimulatory effect. The tonicity-stimulated COX-2 expression in mIMCD-K2 cells was almost completely blocked by a tyrosine kinase inhibitor, genistein at 100 microM. In MDCK cells transfected with a 2.7-kb COX-2 promoter and lacZ reporter construct, NaCl induced a twofold increase in beta-galactosidase activity. Using mIMCD-K2 cells, hypertonic NaCl (600 mosmol/kgH(2)O for 24 h) induced a 33-fold increase in PGE(2) release determined by enzyme immunoassay, an effect completely blocked by 3 microM indomethacin or the COX-2-specific blocker N-(2-cyclohexy-4-nitrophenyl)methanesulfonamide (NS-398). We conclude that in inner medulla, COX-2 but not COX-1 is upregulated by hyperosmolality.

Angiotensin II inhibits HCO-3 absorption via a cytochrome P-450-dependent pathway in MTAL

The role of ANG II in the regulation of ion reabsorption by the renal thick ascending limb is poorly understood. Here, we demonstrate that ANG II (10(-8) M in the bath) inhibits HCO-3 absorption by 40% in the isolated, perfused medullary thick ascending limb (MTAL) of the rat. The inhibition by ANG II was abolished by pretreatment with eicosatetraynoic acid (10 microM), a general inhibitor of arachidonic acid metabolism, or 17-octadecynoic acid (10 microM), a highly selective inhibitor of cytochrome P-450 pathways. Bath addition of 20-hydroxyeicosatetraenoic acid (20-HETE; 10(-8) M), the major P-450 metabolite in the MTAL, inhibited HCO-3 absorption, whereas pretreatment with 20-HETE prevented the inhibition by ANG II. The addition of 15-HETE (10(-8) M) to the bath had no effect on HCO-3 absorption. The inhibition of HCO-3 absorption by ANG II was reduced by >50% in the presence of the tyrosine kinase inhibitors genistein (7 microM) or herbimycin A (1 microM). We found no role for cAMP, protein kinase C, or NO in the inhibition by ANG II. However, addition of the exogenous NO donor S-nitroso-N-acetylpenicillamine (SNAP; 10 microM) or the NO synthase (NOS) substrate L-arginine (1 mM) to the bath stimulated HCO-3 absorption by 35%, suggesting that NO directly regulates MTAL HCO-3 absorption. Addition of 10(-11) to 10(-10) M ANG II to the bath did not affect HCO-3 absorption. We conclude that ANG II inhibits HCO-3 absorption in the MTAL via a cytochrome P-450-dependent signaling pathway, most likely involving the production of 20-HETE. Tyrosine kinase pathways also appear to play a role in the ANG II-induced transport inhibition. The inhibition of HCO-3 absorption by ANG II in the MTAL may play a key role in the ability of the kidney to regulate sodium balance and extracellular fluid volume independently of acid-base balance.

Urea-associated oxidative stress and Gadd153/CHOP induction

Urea treatment (100-300 mM) increased expression of the oxidative stress-responsive transcription factor, Gadd153/CHOP, at the mRNA and protein levels (at >/=4 h) in renal medullary mIMCD3 cells in culture, whereas other solutes did not. Expression of the related protein, CCAAT/enhancer-binding protein (C/EBP-beta), was not affected, nor was expression of the sensor of endoplasmic reticulum stress, grp78. Urea modestly increased Gadd153 transcription by reporter gene analysis but failed to influence Gadd153 mRNA stability. Importantly, upregulation of Gadd153 mRNA and protein expression by urea was antioxidant sensitive. Accordingly, urea treatment was associated with oxidative stress, as quantitated by intracellular reduced glutathione content in mIMCD3 cells. In addition, antioxidant treatment partially inhibited the ability of urea to activate transcription of an Egr-1 luciferase reporter gene. Therefore oxidative stress represents a novel solute-signaling pathway in the kidney medulla and, potentially, in other tissues.

Human renal medullary interstitial cells and analgesic nephropathy

The aim of this study was to investigate the effects of known papillotoxins using cultures of human renal interstital medullary cells (hRMIC). The culture of hMIC was based on the primary culture of human renal medullary explants, selective detachment of interstitial cells and selective overgrowth of these cells in a serum-rich medium after dilution cloning. The homogeneous population of cells obtained exhibited the characteristic morphological and functional characteristics of Type I interstitial cells, viz. stellate-shaped cells demonstrating numerous lipid droplets, abundant endoplasmic reticulum and mitochondria, fine filaments underlying the cell membrane and the production of extracellular matrix. Cytotoxicity studies using hMIC and known papillotoxins clearly demonstrated a reduction in cell viability that varied with bath exposure time and type of agent tested. While only phenylbutazone and mefenamic acid produced significant cytotoxicity after a 24 h incubation period, cell viability assessed using the MTT assay was only profoundly reduced by aspirin and paracetamol following sub-chronic exposure for 7 days. The rank order of cytotoxicity observed in hMIC was phenylbutazone > mefenamic acid > aspirin > paracetamol. The results demonstrate the potential of hMIC for investigating and defining the early cellular events in the pathogenesis of analgesic nephropathy.

Regulation of aquaporin mRNA expression in rat kidney by water intake

Three aquaporins (AQP) are present in the membrane of the principal collecting duct cells. On the apical side, the levels of AQP2 protein are increased in response to both arginine vasopressin and water deprivation. However, whether this change parallels changes in the abundance of AQP3 and AQP4 in the basolateral membrane is less well known. This study evaluates the effect of either dehydration or water loading on the rat kidney mRNA expression of AQP2, AQP3, and AQP4. Poly(A+)RNA was prepared from renal cortex and medulla of control, water-deprived, well hydrated, and water-deprived rats treated with OPC31260, a V2 receptor antagonist. Northern blots were done and mRNA levels were quantified using a PhosphorImager system. Relative to control, water deprivation increased the expression of cortical AQP2, -3, and -4, whereas water loading decreased the cortical and medullar expression of AQP2, -3, and -4. Therefore, in addition to AQP2 and -3, AQP4 expression is also regulated by water intake. Treatment with OPC31260 (40 mg/kg of weight per d) inhibited up to 20 to 30% the upregulation of AQP-mRNA induced by water deprivation. Blood values of arginine vasopressin and aldosterone were significantly increased by water deprivation, whereas they were unchanged by water overloading. Taken together, these results indicate that renal AQP2, -3, and -4 expression is regulated in a coordinated manner. Simultaneous up- or downregulation of the three transcripts occurred upon either water deprivation or water loading of animals, respectively. However, the signaling mechanism for the two long-term adaptive processes may be different, and, in addition to arginine vasopressin, other factors may be involved in the transcriptional regulatory processes.

Rat renomedullary interstitial cells possess bradykinin B2 receptors in vivo and in vitro

1. Renomedullary interstitial cells (RMIC), abundant throughout the medulla of the kidney, have been demonstrated to have binding sites for many vasoactive peptides, including atrial natriuretic peptide, endothelin, angiotensin II and bradykinin (BK). These observations would support the hypothesis that interactions between RMIC and vasoactive peptides are important in the regulation of renal function. 2. We aimed to localize the BK B2 receptor binding site to RMIC in vivo and to also demonstrate that these receptors are biologically active in vitro. 3. The present study demonstrates BK B2 binding sites on RMIC of the inner stripe of the outer medulla and the inner medulla of the rat kidney in vivo. 4. We further demonstrate that the BK B2 radioligand [125I]-HPP-Hoe140 specifically bound to rat RMIC in vitro. In addition, reverse transcription-polymerase chain reaction detected the mRNA for the BK B2 receptor subtype in cell extracts. 5. For RMIC in vitro, cAMP levels were increased at 1 min and cGMP levels were increased at 2 min after treatment with 10(-10) and 10(-7) mol/L BK, respectively. Inositol 1,4,5-trisphosphate was increased at 10 s treatment with both 10(-6) and 10(-7) mol/L BK. 6. For RMIC in vitro, BK induced an increase in cell proliferation ([3H]-thymidine incorporation) and an increase in extracellular matrix synthesis (ECM; trans-[35S] incorporation), both effects mediated by BK B2 receptors. 7. We conclude that BK B2 receptors are present on RMIC both in vivo and in vitro. These receptors are coupled to intracellular second messenger systems and, in vitro, their stimulation results in cellular proliferation and synthesis of ECM.

K-Cl cotransporter expression in the human kidney

The K-Cl cotransporter protein KCC1 is a membrane transport protein that mediates the coupled, electroneutral transport of K and Cl across plasma membranes. The precise cell type(s) in the kidney that express the K-Cl cotransporter have remained unknown. The aim of the present investigation was to define the distribution of KCC1 mRNA in the human kidney. We used in situ hybridization with a nonradioactive digoxigenin-labeled riboprobe. We identified abundant KCC1 mRNA expression in the epithelial cells throughout the distal and proximal renal tubular epithelium. The transporter was also expressed in glomerular mesangial cells and endothelial cells of the renal vessels. These findings suggest that the K-Cl cotransporter may have an important role in transepithelial K and Cl reabsorption.

Cellular response to osmotic stress in the renal medulla

Cells of the renal medulla, which are exposed under normal physiological conditions to widely fluctuating extracellular solute concentrations, respond to hypertonic stress by accumulating the organic osmolytes glycerophosphorylcholine (GPC), betaine, myo-inositol, sorbitol and free amino acids. Increased intracellular contents of these osmolytes are achieved by a combination of increased uptake (myo-inositol and betaine) and synthesis (sorbitol, possibly GPC), decreased degradation (GPC) and reduced osmolyte release. In the medulla of the concentrating kidney, accumulation of organic osmolytes, which do not perturb cell function even at high concentrations, allows the maintenance of "normal" intracellular concentrations of inorganic electrolytes. Adaptation to decreasing extracellular solute concentrations, e.g. diuresis, is achieved primarily by activation of pathways allowing the efflux of organic osmolytes, and secondarily by inactivation of production (sorbitol) and uptake (betaine, myo-inositol) and stimulation of degradation (GPC). Apart from modulation of the osmolyte content, osmolality-dependent reorganization of the cytoskeleton and expression of specific stress proteins (heat shock proteins) may be further, as yet poorly characterized, components of the regulatory systems involved in the adaptation of medullary cells to osmotic stress.

Cycles and separations in a model of the renal medulla

This study gives the first quantitative analysis of net steady-state transmural fluxes of water, urea, and NaCl in a numerical model of the rat renal medulla in antidiuresis, revealing the model's predictions of water, urea, and NaCl cycling patterns. These predictions are compared both to in vivo micropuncture data from the literature and to earlier qualitative proposals (e.g., K. V. Lemley and W. Kriz. Kidney Int. 31: 538-548, 1987) of cycling and exchange patterns based on medullary anatomy and available permeability and transport parameter measurements. The analysis is based on our most recent three-dimensional model [X. Wang, S. R. Thomas, and A. S. Wexler. Am. J. Physiol. 274 (Renal Physiol. 43): F413-F424, 1998]. In general agreement with earlier proposed patterns, this analysis predicts the following: 1) important water short-circuiting from descending structures to ascending vasa recta in most medullary regions, 2) massive urea recycling in the upper inner medulla, 3) a progressive increase of the ratio of urea to total osmoles along the corticopapillary axis, 4) urea dumped from the collecting ducts (CD) into the deep papilla is returned to the cortex essentially via outer medullary short vasa recta, bearing witness to a shift from the long descending limbs and vasa recta of the inner medulla (IM) to short structures in the outer medulla (OM). The analysis also shows that the known radial heterogeneity of the inner stripe (IS) implies unequal osmolalities in long descending limbs, vasa recta, and CDs entering the IM across the OM/IM border and explains the model's unorthodox osmolality profile along the CD. In conflict with micropuncture evidence of a doubling of urea flow in superficial Henle's loops (SHL) between the end proximal and early distal tubule (T. Armsen and H. W. Reinhardt. Pflügers Arch. 326: 270-280, 1971), the model predicts net urea loss from SHL due to the model's inclusion of nonneglible measured urea permeability of medullary thick ascending limbs [M. A. Knepper, Am. J. Physiol. 245 (Renal Fluid Electrolyte Physiol. 14): F634-F639, 1983]. We present a suite of adjusted model permeabilities that improves agreement with the micropuncture data on this point. In conclusion, this modeling analysis of solute and water recycling serves as a quantitative check on qualitative propositions in the literature and allows closer critical comparison of model behavior with published experimental results than was heretofore possible.

Activation of H+-ATPase by hypotonicity: a novel regulatory mechanism for H+ secretion in IMCD cells

The effect of hypotonicity on H+-ATPase activity was examined in cultured inner medullary collecting duct (mIMCD-3) cells. mIMCD-3 cells were grown to confluence, loaded with 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and assayed for H+-ATPase activity measured as the Na+- and K+-independent intracellular pH (pHi) recovery following an acid load. Exposure of mIMCD-3 cells to a hypotonic solution (150 mosmol/kgH2O) increased pHi recovery by approximately 350% (P < 0.0001). This effect was inhibited by diethylstilbestrol (an inhibitor of H+-ATPase) and was not dependent on external K+, indicating lack of involvement of H+-K+-ATPase. H+-ATPase activation was acute, independent of cell calcium, and was not secondary to Cl- channel activation. The magnitude of H+-ATPase upregulation was dependent on the osmolarity of the media, with maximum stimulation at 150 mosmol/kgH2O. H+-ATPase upregulation in hypotonicity was significantly blocked in the presence of staurosporine or calphostin C or in cells pretreated with phorbol 12-myristate 13-acetate (PMA), indicating involvement of protein kinase C. Hypotonicity inhibited the Na+/H+ exchanger activity in mIMCD-3 cells, indicating that its stimulatory effect is specific to H+-ATPase. In conclusion, a novel regulatory mechanism of H+-ATPase by hypotonicity is described. The increased H+-ATPase activity in hypotonicity may be responsible for increased HCO-3 reabsorption and maintained acid-base homeostasis in hyposmolar states.

Localization and interactions of vasoactive peptide receptors in renomedullary interstitial cells of the kidney

Vasoactive peptides regulate renal medullary microcirculation and tubular function, but the localization of their receptors and mechanisms of actions are currently unknown. Using electron microscopic autoradiography, we have mapped the receptors for angiotensin II (Ang II [AT1 and AT2]), endothelin (ET(A) and ET(B)), and bradykinin (B2) in the rat renal medulla. Although these peptide receptors show distinct vascular and tubular distributions, they overlap strikingly in renomedullary interstitial cells (RMICs) of the inner stripe and the papilla. Using reverse transcription-polymerase chain reaction (RT-PCR) and Southern analysis, mRNAs for AT1A, ET(A), and B2 receptors were detected in cultured adult RMICs. Ang II increases intracellular inositol 1,4,5-triphosphate (IP3) and [Ca2+]i and stimulates [3H]thymidine incorporation and extracellular matrix (ECM) synthesis via AT1A receptors. Endothelin and bradykinin also stimulate cell proliferation and ECM synthesis in RMICs through ET(A) and B2 receptors, respectively, but the actions of endothelin are modulated by concurrent nitric oxide production. By contrast, AT2 receptor mRNA was detected only in embryonic RMICs, in which Ang II inhibits cell proliferation through this receptor. These results suggest that multiple vasoactive peptides may interact with RMICs to exert endocrine and/or paracrine influences on renal medullary microcirculation and tubular function.

Expression of synaptotagmin VIII in rat kidney

The synaptotagmins are a family of integral membrane proteins proposed to function as regulators of both exocytosis and endocytosis. Here, we have used immunochemical techniques and RT-PCR to assess sites of renal expression of synaptotagmin VIII. A polyclonal antibody was raised to a synthetic peptide corresponding to the carboxy-terminal 21 amino acids of mouse synaptotagmin VIII. On immunoblots of membrane fractions from renal cortex and medulla (and in several other tissues), the antibody labeled a 52-kDa band (absent with preimmune IgG). Immunofluorescence localization was carried out in tissue sections from rat kidney. The synaptotagmin VIII antibody labeled early proximal tubules, thin ascending limbs, thick ascending limbs, connecting tubules, and collecting ducts. In collecting ducts, both type A and B intercalated cells exhibited basolateral labeling, whereas principal cells were labeled chiefly in the apical and subapical portion of the cells. Thick ascending limbs were labeled in both the basolateral and apical regions. RT-PCR experiments using total RNA extracted from cortex and medulla or microdissected inner medullary collecting ducts gave a single band of appropriate size. Sequencing of the PCR product confirmed that the amplified target is synaptotagmin VIII. We conclude that synaptotagmin VIII is broadly expressed among renal tubule epithelia, raising the possibility that it is involved in regulation of transport and/or cell remodeling at several sites in the nephron and collecting duct.

Regulation of cAMP production in initial and terminal inner medullary collecting ducts

BACKGROUND

The inner medullary collecting duct (IMCD) is composed of at least two functionally and morphologically distinct segments, the initial (IMCDi) and the terminal (IMCDt) portions. However, most studies of receptor signaling have been performed on cells obtained from the entire inner medulla. The purpose of this study was to determine whether the patterns of receptor-activated cAMP accumulation were different between these segments.

METHODS

We measured cAMP accumulation stimulated by vasopressin and isoproterenol, and the effect of epinephrine in freshly dissected IMCDi and IMCDt segments cultured and IMCDi and IMCDt cells in primary culture.

RESULTS

The maximum response to vasopressin was twofold higher in fresh IMCDt verus IMCDi (P < 0.05), however, it increased in cultured IMCDi by 40% verus fresh cells with no change in the response in fresh verus cultured IMCDt. The maximum response to isoproterenol was small in fresh cells but increased by five- and sixfold, respectively, in cultured IMCDi and IMCDt cells. alpha 2-Adrenoceptor stimulation almost completely inhibited both vasopressin and isoproterenol-stimulated cAMP accumulations in fresh IMCDi and IMCDt cells, but only partially inhibited either accumulation by 34 to 49% in cultured cells.

CONCLUSIONS

(1) IMCDi and IMCDt cells are both subject to vasopressin and alpha 2- and beta-adrenergic regulation of adenylyl cyclase activity; (2) the relative influence of beta-adrenergic, alpha 2-adrenergic and V2 receptors to affect cAMP accumulation is altered in primary culture versus freshly dissected IMCD segments, suggesting that caution must be exercised in the extrapolation of data from cultured IMCD cells to in vivo models.

Characteristics of urea transport of cells derived from rabbit thick ascending limb of Henle's loop

BACKGROUND

The thick ascending limb of Henle's loop (TALH) is thought to be involved in the regulation of the renal urea gradient.

METHODS

We have characterized the uptake of urea (oil density centrifugation and 2-compartment-culture) and volume regulation (impedance measurement) in highly differentiated cells derived from rabbit outer medulla.

RESULTS

TALH cells exposed to 600 mOsm/liter (300 mM urea) shrunk to 72 +/- 5% of the isoosmotic volume. Due to a regulatory volume increase (RVI), the cell volume was almost completely regained at 92 +/- 6% after five minutes. The uptake of 14C-urea in the presence of urea concentrations up to 600 mM did not show any saturation. In the presence of phloretin the urea uptake decreased to 69 +/- 14%. The transport was sodium and chloride independent. Changing the membrane potential caused an increase of regulatory volume increase and urea uptake. Hyperosmolarity induced by sucrose (300 mM) and NaCl (150 mM) caused a decrease of urea uptake to 70 +/- 14% and 53 +/- 11%, respectively. The permeability coefficient (P) in a two compartment culture was P = 1.7 . 10(-6) +/- 0.39.10(-6) cm/second, suggesting a relatively low permeability.

CONCLUSION

Due to the low permeability, it seems impossible to achieve a physiologically significant participation of the TALH in the urea circulation within the nephron. However, the results of this study provides significant hints about the existence of a specific urea transport mechanism that enables the cell to adapt rapidly to different osmolarities.

A combination of NaCl and urea enhances survival of IMCD cells to hyperosmolality

Physiological adaptation to the hyperosmolar milieu of the renal medulla involves a complex series of signaling and gene expression events in which NaCl and urea activate different cellular processes. In the present study, we evaluated the effects of NaCl and urea, individually and in combination, on the viability of murine inner medullary collecting duct (mIMCD3) cells. Exposure to hyperosmolar NaCl or urea caused comparable dose- and time-dependent decreases in cell viability, such that 700 mosmol/kgH2O killed >90% of the cells within 24 h. In both cases, cell death was an apoptotic event. For NaCl, loss of viability at 24 h paralleled decreases in RNA and protein synthesis at 4h, whereas lethal doses of urea had little or no effect on these biosynthetic processes. Cell cycle analysis showed both solutes caused a slowing of the G2/M phase. Surprisingly, cells exposed to a combination of NaCl + urea were significantly more osmotolerant such that 40% survived 900 mosmol/kgH2O. Madin-Darby canine kidney cells but not human umbilical vein endothelial cells also exhibited a similar osmotolerance response. Enhanced survival was not associated with a restoration of normal biosynthetic rates or cell cycle progression. However, the combination of NaCl + urea resulted in a shift in Hsp70 expression that appeared to correlate with survival. In conclusion, hyperosmolar NaCl and urea activate independent and complementary cellular programs that confer enhanced osmotolerance to renal medullary epithelial cells.

Hepatocyte growth factor protects renal epithelial cells from apoptotic cell death

Hepatocyte growth factor (HGF) is a pleiotropic factor that plays an essential role in renal tubular repair and regeneration following injury. Studies indicate that administration of exogenous HGF to animals stimulates renal epithelial cell DNA synthesis and accelerates recovery from acute renal failure (ARF). However, whether increased cell proliferation accounts for all of the beneficial effects of HGF in ARF is unknown. In this study, we demonstrate that HGF protects renal epithelial cells from undergoing apoptotic cell death. Treatment of renal epithelial mIMCD-3 cells with 25 microM cisplatin in the serum-free medium induced significant apoptosis, as assessed by fluorescent Dye H-33342 staining, TUNEL staining, light and electron microscopy, and DNA laddering analysis. However, constitutive expression of HGF by transfection in mIMCD-3 cells resulted in resistance to cisplatin-induced apoptotic death. The survival rate of HGF-producing C1 cells was more than 2-fold greater as compared to control, mIMCD-3 cells following treatment with 25 microM cisplatin for 2 days. These results suggest that HGF may not only activate tubular repair processes but also ameliorate the initial injury by protecting renal epithelial cells from undergoing apoptosis.

Hyperosmolality causes growth arrest of murine kidney cells. Induction of GADD45 and GADD153 by osmosensing via stress-activated protein kinase 2

Murine kidney cells of the inner medullary collecting duct (mIMCD) were exposed to either isosmotic (300 mosmol/kg) or hyperosmotic medium (isosmotic medium + 150 mM NaCl) after seeding. We determined cell numbers, total nucleic acid, DNA, and RNA contents in both groups every day for a total period of 7 days. Based on all 4 parameters it was evident that growth of mIMCD3 cells is arrested for approximately 18 h following onset of hyperosmolality. However, none of the parameters measured indicated cell death because of hyperosmolality. Growth curves of hyperosmotic samples were shifted compared with isosmotic samples showing a gap of 18 h but had the same shape otherwise. We demonstrated that at 24 and 48 h after onset of hyperosmolality, but not in isosmotic controls, growth arrest and DNA damage-inducible (GADD) proteins GADD45 and GADD153 are strongly induced. This result is consistent with growth arrest observed in hyperosmotic medium. We tested if mitogen- and stress-activated protein kinase (SAPK) cascades are involved in osmosignaling that leads to GADD45 and GADD153 induction. Using phosphospecific antibodies we showed that extracellular signal-regulated kinases 1 and 2 (ERK), SAPK1 (JNK), and SAPK2 (p38) are hyperosmotically activated in mIMCD cells. Hyperosmotic GADD45 induction was significantly decreased by 37.5% following inhibition of the SAPK2 pathway, whereas it was significantly increased (65.2%) after inhibition of the ERK pathway. We observed similar, although less pronounced effects of SAPK2 and ERK inhibition on hyperosmotic GADD153 induction. In conclusion, we demonstrate that mIMCD cells arrest growth following hyperosmotic shock, that this causes strong induction of GADD45 and GADD153, that GADD induction is partially dependent on osmosignaling via SAPK2 and ERK, and that SAPK2 and ERK pathways have opposite effects on GADD expression.

N-acetylneuraminic acids (nana): a potential key in renal calculogenesis

N-Acetylneuraminic acids (NANA) promote binding of calcium ions to macromolecules and cells, increase the intrinsic viscosity of glycoproteins and facilitate gel formation in water. Since these properties are crucial in urinary calculogenesis, we evaluated NANA levels in urine and serum as well as their expression in kidney tissues. Using a modified thiobarbituric acid assay, the evaluation of free and bound NANA in 24-h urine samples revealed a ratio of 1.87 in 33 non-stone-formers but a reversed ratio of 0.84 in 41 recurrent calcium oxalate stone-formers. Time kinetics revealed a gradual rise in NANA expression until 48 h of culture and a significantly higher release into supernatants of papillary renal epithelial cells (REC) when compared with cortical REC. To examine NANA distribution in kidney tissues, paraffin-embedded biopsies from five normal and six stone-forming kidneys were labeled with the biotinylated NANA-specific lectins Maackia amurensis (MAA) and Sambucus nigra (SNA). Immunohistochemistry revealed intense luminal MAA reactivity of distal tubular REC and collecting ducts in 96.7% and 91.5% of normal and stone-forming kidneys respectively. By contrast, there was a marked difference between normal and stone-forming kidneys for SNA reactivity (17.7% vs 95%) at the same locations. Finally, the glycocalyx of recurrent stone-formers showed altered sialylglycoside linkages [alpha(2,6) instead of alpha(2,3)] that may indicate an altered REC function. Given the calcium-binding potential of NANA, their increased local concentration within the glycocalyx layer in the distal nephron may either initiate stone formation or facilitate attachment of microcrystals to REC.

Establishment of a mouse clonal early proximal tubule cell line and outer medullary collecting duct cells expressing P2 purinoceptors

The purpose of this study was to establish tubule cells expressing P2 purinoceptors. Use of the polymerase chain reaction coupled with reverse transcription showed that mouse clonal early proximal tubule (S1) cell line (NF-5) and outer medullary collecting tubule (OMCT) cells expressed mRNA for P2X4, P2Y1 and P2Y2 purinoceptors. ATP and its analogues induced a dose-dependent increase in cytosolic free calcium concentration ([Ca2+]i) in both NF-5 and OMCT cells. ATP enhanced [3H]-thymidine uptake by both NF-5 and OMCT cells in a dose-dependent manner. In conclusion, NF-5 and OMCT cells can be used in the further analysis of the function and regulation of activity and expression of P2 purinoceptors in the renal tubule.

Cortical and medullary hemodynamics in deoxycorticosterone acetate-salt hypertensive mice

The effect of acutely increasing renal perfusion pressure or extracellular fluid volume on renal medullary and cortical blood flow was examined in the low-renin deoxycorticosterone acetate (DOCA)-salt hypertension model in mice. A 50-mg DOCA tablet was implanted, and 1% saline was given as drinking water for 3 wk. Medullary and cortical blood flow were determined with laser-Doppler flowmetry, and whole-kidney blood flow was measured with a transit-time ultrasound flowprobe around the renal artery. In control mice, total renal blood flow ranged from 6.3 and 7.6 ml/min per g kidney weight and in DOCA-salt mice from 4.3 and 4.7 ml/min per g kidney weight, respectively, and was minimally affected as renal perfusion pressure was increased. Renal vascular resistance increased correspondingly. During stepwise increases in renal artery pressure from 90 to 140 mmHg, medullary blood flow progressively increased in control mice to 125% of baseline values, whereas cortical blood flow did not change. In DOCA-salt mice, increasing BP from 100 to 154 mmHg had no effect on either cortical or medullary blood flow. Urine flow and sodium excretion were lower in DOCA-salt mice than in controls and increased nearly to the same extent in both groups after volume expansion with isotonic saline. Total renal blood flow increased after saline loading, more in controls than in DOCA-salt mice. Increases in medullary blood flow after saline loading were up to 122% of baseline values in controls and demonstrated a significantly steeper slope than the 110% of baseline increases in DOCA-salt mice. Cortical blood flow, however, was not different between the groups. Thus, medullary blood flow is not as tightly autoregulated as cortical blood flow in normal mice. Natriuresis with acute volume loading is facilitated by increased medullary blood flow. In DOCA-salt mice, the medullary blood flow reaction to renal perfusion pressure increases is abolished, whereas flow increases with extracellular volume expansion are diminished. These results suggest that diminished pressure-natriuresis responses in DOCA-salt mice are related to perturbed medullary blood flow.

Evidence for atrial natriuretic factor induced natriuretic peptide receptor subtype switching in rat proximal tubular cells during culture

Culture and natriuretic peptide dependent changes in the expression of the natriuretic peptides atrial natriuretic factor (ANF), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) and the natriuretic peptide receptors A, B, and C in primary cultures of rat proximal tubular cells were demonstrated using polymerase chain reaction analysis and cyclic guanosine monophosphate response to ANF and CNP. Freshly isolated cells expressed mRNA coding for the natriuretic peptide receptor C only, with no expression of the natriuretic peptides or the natriuretic peptide receptors A or B. At confluence natriuretic peptide receptor C expression was lost, while mRNA transcripts for both ANF and BNP and the A and B receptors became apparent. The appearance of mRNA transcripts for the natriuretic peptide receptors A and B during cell growth correspond with a significant increase in the cyclic guanosine monophosphate response to both ANF and CNP, confirming the presence of functionally active guanylate cyclase linked A and B natriuretic peptide receptors. The observed changes in peptide receptor expression during culture were preceded by changes in natriuretic peptide mRNA expression, suggesting the possibility that natriuretic peptide receptor subtype switching may be under the control of endogenous peptide release. Incubation of freshly isolated proximal tubular cells with ANF, BNP, or CNP for 3 h induced similar changes in receptor expression. Incubation with ANF induced expression of the natriuretic peptide receptor B and CNP while inhibiting natriuretic peptide receptor C. Incubation with BNP induced expression of the natriuretic peptide receptor B and CNP. Incubation with CNP induced expression of the natriuretic peptide receptors A and B and CNP. These results suggest that primary cultures of rat proximal tubular cells may experience natriuretic peptide and natriuretic peptide receptor subtype switching as they approach confluence under the control of endogenously expressed natriuretic peptides.

Properties of a polarized primary culture from rat renal inner medullary collecting duct (IMCD) cells

A primary culture from rat renal IMCD cells was established to investigate the permeability characteristics of the luminal and contraluminal plasma membranes of the papillary collecting duct in vitro. Freshly isolated IMCD cells were grown on filters in a special "epithelial cell" medium. Confluency was proved with an epithelial volt/ohm meter. After 7 d of culture the transepithelial resistance reached more than 1000 omega x cm2. A polarization of the cells with regard to a basolateral localization of a lactate efflux system, and an L-alanine transport system was achieved. The hypotonicity-activated release systems for the organic osmolytes sorbitol and betaine were also located basolaterally, whereas taurine, glycerophosphorylcholine, and myo-inositol left the cells at both cell poles but with different capacity. Morphological observations revealed also that the monolayer was well differentiated. Thus, a model of a renal collecting duct epithelium was established which can be used to analyze polarized and differentiated transport processes across the epithelial cells and their plasma membranes.

Downregulation of endothelin B receptors in cardiomyopathic hamsters

The mechanisms responsible for abnormal fluid retention in congestive heart failure (CHF) are unclear. Studies were conducted to elucidate how endothelin (ET) may contribute to salt and water retention. Cardiomyopathic (CM) hamsters with moderate heart failure were employed for in vivo and in vitro trials. Clearance methods were used to compare the level of renal function in CM hamsters and control animals. Radioligand binding studies were also performed to determine ET receptor distribution in the inner medullary collecting ducts. CM hamsters exhibited an attenuated response to ANF infusion (FENa: 2.7 +/- 0.5 vs. 5.9 +/- 0.8%, p < 0.01; FEH2O: 1.7 +/- 0.3 vs. 3.2 +/- 0.4%, p < 0.01; UcGMP: 11.2 +/- 2.3 vs. 16.6 +/- 2.0 pmol/min, p < 0.05) and a decrease in total ET receptor density (532 +/- 77 vs. 959 +/- 154 fmol/mg protein, p < 0.005). Particularly ETB receptors were significantly reduced (214 +/- 26 vs. 483 +/- 88 fmol/mg protein, p < 0.003). Enalapril therapy simultaneously restored the natriuretic and diuretic effects of ANF and ET receptor density in the diseased animals. These studies suggest that the renin-angiotensin-aldosterone system and ET hormonal system act together, via ETB receptor downregulation, to promote the abnormal fluid retention observed in CHF.

Ureteral obstruction enhances eicosanoid production in cortical and medullary tubules of rat kidneys

We examined prostaglandin (PG) E2, 6-keto PGF1alpha, and thromboxane B2 (TxB2) production in cortical and medullary tubules from sham-operated control (SOC) rats and rats with bilateral ureteral obstruction (BUO) of 24 h duration. In SOC rats medullary tubules produced significantly greater amounts of the three eicosanoids than cortical tubules. Again, the production of PGE2, 6-keto PGF1alpha, and TxB2 by cortical and medullary tubules was significantly greater in BUO rats than in SOC rats. To elucidate the mechanisms involved, we examined the activity of phospholipase A2 (PLA2) reactive against phosphatidylcholine or phosphatidylethanolamine (PE), the activity of phospholipase C (PLC), and the levels of cyclooxygenase (COX) in cortical and medullary tubules from SOC and BUO rats. In SOC rats the activity of phosphatidylcholine-PLA2 and PE-PLA2, the activity of PLC, and the mass of COX were significantly greater in medullary tubules than in cortical tubules. On the other hand, the activity of PLC in membranes of cortical tubules and the activity of PE-PLA2 and PLC in membranes of medullary tubules, which were in active location, were significantly greater in BUO rats than in SOC rats. COX levels were also significantly greater in cortical and medullary tubules of BUO rats than in those of SOC rats. Thus, we indicate that medullary tubules from SOC rats have greater production of eicosanoids through increased activity of the PLA2 and PLC-COX pathway than cortical tubules from the same group of rats. Again, in rats with BUO, the tubular eicosanoid production may be enhanced via activation of the PLC-COX pathway in cortical tubules or through activation of the PE-PLA2 and PLC-COX pathway in medullary tubules. The enhanced production of tubular eicosanoids observed in rats with BUO may affect tubular function, particularly sodium and water reabsorption.

Involvement of macromolecule synthesis, endonuclease activation and c-fos expression in cisplatin-induced apoptosis of mouse proximal tubule cells

We have previously demonstrated that cisplatin-induced nephrotoxicity is associated with the induction of apoptosis using mouse renal cells derived from the terminal proximal tubule (S3) which is the major target site of cisplatin-induced injury. The purpose of this study was to elucidate the intracellular mechanisms leading to the cisplatin-induced apoptosis of S3 cells. Actinomycin D (an inhibitor of RNA synthesis), cycloheximide (an inhibitor of protein synthesis) and aurintricarboxylic acid (an endonuclease inhibitor) reduced the extent of DNA fragmentation, a biochemical parameter of apoptosis, in cisplatin-treated S3 cells. Furthermore, cisplatin-induced apoptosis of S3 cells was accompanied by an increase in the level of c-fos mRNA expression, which is inhibited by pretreatment of the cells with actinomycin D, but not with cycloheximide or aurintricarboxylic acid. In contrast, outer medullary collecting duct cells treated with cisplatin exhibited morphological changes characteristic of apoptosis and an increase in the level of c-fos mRNA expression, but no increase in the extent of DNA fragmentation. In conclusion, the synthesis of macromolecules such as RNA and protein, endonuclease activation and c-fos expression appear to be involved in the intracellular pathways leading to the induction of apoptosis in cisplatin-treated S3 cells. In addition, the response to cisplatin may be different in different cells.

Dominant-negative c-Jun NH2-terminal kinase 2 sensitizes renal inner medullary collecting duct cells to hypertonicity-induced lethality independent of organic osmolyte transport

The c-Jun NH2-terminal protein kinases (JNKs), as well as the extracellular signal-regulated protein kinases (ERKs) and p38 mitogen-activated protein kinase, are activated in renal cells in response to extracellular hypertonicity. To determine whether activation of JNKs by hypertonicity is isoform-specific, renal inner medullary collecting duct cells were stably transfected with cDNA's encoding hemagglutinin (HA)-tagged JNK1 and JNK2 isoforms, and the expressed kinases were immunoprecipitated with an anti-HA antibody. Whereas both recombinant kinases were equivalently expressed, only immunoprecipitates from the HA-JNK2 cells displayed hypertonicity-inducible JNK activity. Furthermore, expression of dominant-negative JNK2 (HA-JNK2-APF) in stable clones inhibited hypertonicity-induced JNK activation by 40-70%, whereas expression of dominant-negative JNK1 (HA-JNK1-APF) had no significant inhibitory effect. Independent HA-JNK2-APF (but not HA-JNK1-APF) clones displayed greatly reduced viability relative to neomycin controls after 16 h of exposure to 600 mosM/kg hypertonic medium with percent survival of 20.5 +/- 2.7 and 31.5 +/- 7.3 for two independent HA-JNK2-APF clones compared with 80.1 +/- 1.0 for neomycin controls (p < 0.001, n = 5, mean +/- S.E.). However, neither JNK mutant blocked either regulatory volume increase or hypertonicity-induced enhancement of uptake of inositol, an organic osmolyte putatively involved in long term adaptation to hypertonicity. These results define JNK2 as the primary hypertonicity-activated JNK isoform in IMCD-3 cells and demonstrate its central importance in cellular survival in a hypertonic environment by a mechanism independent of acute regulatory volume increase as well as regulation of organic osmolyte uptake.

Detection of dopamine receptor D1A subtype-specific mRNA in rat kidney by in situ amplification

In recent years, both molecular biological and immunohistochemical techniques, utilizing receptor subtype-specific probes and antibodies to cloned central nervous system dopamine receptors, have revealed their presence in a number of peripheral organs and tissues. Molecular techniques have been hindered by the low abundance of receptor mRNA in these sites, and reverse transcription-polymerase chain reaction (RT-PCR) has been utilized to address this problem. However, RT-PCR is most often employed on either isolated mRNA or microdissected tissue samples, thereby limiting interpretation of whole tissue distribution. The present paper describes the use of a novel self-sustained sequence replication system (3SR) to amplify a target mRNA sequence in situ within the tissue or cell of interest that is then detected with the use of an internal labeled probe, using standard nonisotopic in situ hybridization. Specifically, D1A receptor mRNA was amplified and detected in kidney sections of Wistar-Kyoto rats (WKY). The amplified D1A receptor mRNA was localized to renal arterioles, juxtaglomerular apparatus, and both proximal and distal tubules. mRNA was colocalized to regions shown also to contain D1A receptor protein. D1A receptor mRNA was predominantly localized in the cortex. Specificity of D1A receptor mRNA detection was confirmed by appropriate localization in rat brain sections known to express D1A receptor mRNA. In addition, we confirmed the presence of renal D1A receptor mRNA by RT-PCR. We conclude that D1A receptor mRNA is expressed in a site-specific manner in the WKY kidney. The use of 3SR in situ permits elucidation of site specific mRNA localization in a manner not reported previously.

BMP-2 and OP-1 exert direct and opposite effects on renal branching morphogenesis

The bone morphogenetic proteins, BMP-2 and OP-1, are candidates for growth factors that control renal branching morphogenesis. We examined their effects in embryonic kidney explants and in the mIMCD-3 cell model of collecting duct morphogenesis (mIMCD-3 cells are derived from the terminal inner medullary collecting duct of the SV40 mouse). Osteogenic protein-1 (OP-1), at a dose of 0.25 nM, increased explant growth by 30% (P = 0.001). In contrast, 100-fold greater concentrations of OP-1 (28 nM) decreased explant growth by 10% (P < 0.001). BMP-2 was entirely inhibitory (maximum inhibition of 7% at 5 nM, P < 0.0004). In an in vitro model for branching morphogenesis utilizing the kidney epithelial cell line, mIMCD-3, low doses of OP-1 (< 0.5 nM) increased the number of tubular structures formed by 28 +/- 5% (P = 0.01), whereas concentrations > 0.5 nM decreased that number by 22 +/- 8% (P = 0.02). All concentrations of BMP-2 (0.05-10 nM) were inhibitory (maximum inhibition at 10 nM of 88 +/- 3%, P < 0.0001). Stimulatory doses of OP-1 increased tubular length (P = 0.003) and the number of branch points/structure (3.2-fold increase, P = 0.0005) compared with BMP-2. To determine the molecular basis for these effects, we demonstrated that BMP-2 is bound to mIMCD-3 cells by the type I serine/threonine kinase receptor, ALK-3, and that OP-1 bound to an approximately 80-kDa protein using ligand-receptor affinity assays. To demonstrate that OP-1 can exert both stimulatory and inhibitory effects within a developing kidney, embryonic explants were treated with agarose beads saturated with 2 microM OP-1. OP-1 decreased the number of ureteric bud/collecting duct branches adjacent to the beads by 58 +/- 1% (P < 0.0001). In contrast, the number of branches in tissue distal to the OP-1 beads was enhanced, suggesting a stimulatory effect at lower doses of OP-1. We conclude that OP-1 and BMP-2 directly control branching morphogenesis and that the effects of OP-1 are dependent on its local concentration within developing kidney tissue.

Localization and functional properties of angiotensin II AT1 receptors in the kidney: focus on renomedullary interstitial cells

The renal medulla plays an important role in maintaining body fluid and electrolyte balance and long-term blood pressure homeostasis through its unique structural and functional properties. Among several humoral, paracrine factors or autocoids, angiotensin II (Ang II) has been implicated in the regulation of renal medullary function, including the medullary/papillary microcirculation, urine concentration, and blood pressure, but the mechanisms by which Ang II exerts influences in the renal medulla are largely unknown. The purpose of this review is to summarize the cellular localization, regulation, and functional properties of Ang II AT1 receptors in the kidney, with special emphasis on type I renomedullary interstitial cells (RMICs) in the renal medulla and cultured RMICs. High densities of AT1 receptors have been localized in type I RMICs in the inner stripe of the outer medulla by high resolution light and electron microscopic autoradiography following in vitro or in vivo labelling, or in cultured RMICs. Furthermore, reverse transcription polymerase chain reaction and Southern blot analysis now confirm that AT1 receptors in cultured RMICs are exclusively of the AT1A subtype. In cultured RMICs, Ang II markedly increases intracellular inositol 1,4,5-triphosphate (IP3) concentration, and stimulates cell proliferation and extracellular matrix synthesis, and these cellular responses are exclusively mediated by AT1 receptors. Considering the co-occurrence of high levels of renin, renin substrate angiotensinogen, and Ang II in the interstitial fluid compartment, and AT1 receptors in type I RMICs of the renal medulla, the AT1 receptor-bearing RMICs may be more responsive to the locally formed interstitial Ang II than to the circulating peptide. Since RMICs also contain the receptors for other vasoactive peptides, such as endothelin (ET[A] and ET[B]), natriuretic peptides (NPR[A] and NPR[B]), and bradykinin (B2), and synthesize prostaglandins and medullipins, they may serve as an important site for functional interactions between Ang II and other vasoactive peptides in modulating renal medullary function. More studies using different experimental approaches are therefore required to explore and elucidate the functional role of renal interstitial Ang II and AT1 receptors in RMICs in the physiological control of renal medullary function and in the pathophysiology of hypertension and progressive renal diseases.

Cl- channels in basolateral renal medullary membranes. XII. Anti-rbClC-Ka antibody blocks MTAL Cl- channels

Cl- channels in the medullary thick ascending limb (MTAL) studied by either patch-clamp technique or reconstitution into lipid bilayers are activated by increases in intracellular Cl- concentrations. rbClC-Ka, a ClC Cl- channel, may represent this channel. We therefore evaluated the role of rbClC-Ka in transcellular MTAL Cl- transport in two separate ways. First, an antibody was raised against a fusion protein containing a 153-amino acid fragment of rbClC-Ka. Immunostaining of rabbit kidney sections with the antibody was localized to basolateral regions of MTAL and cortical thick ascending limb (CTAL) segments and also to the cytoplasm of intercalated cells in the cortical collecting duct. Second, Cl- uptake and efflux were measured in suspensions of mouse MTAL segments. Cl- uptake was bumetanide sensitive and was stimulated by treatment with a combination of vasopressin + forskolin + dibutyryl adenosine 3',5-cyclic monophosphate (DBcAMP). Cl- efflux was also increased significantly by vasopressin + forskolin + DBcAMP from 114 +/- 20 to 196 +/- 36 nmol.mg protein-1.45 s-1 (P = 0.003). Cl- efflux was inhibited by the Cl- channel blocker diphenylamine-2-carboxylate (154 +/- 26 vs. 70 +/- 21 nmol.mg protein-1.45 s-1, P = 0.003). An anti-rbClC-Ka antibody, which inhibits the activity of MTAL Cl- channels in lipid bilayers, reduced Cl- efflux from intact MTAL segments (154 +/- 28 vs. 53 +/- 14 nmol.mg protein-1.45 s-1, P = 0.02). These results support the view that rbClC-Ka is the basolateral membrane Cl- channel that mediates vasopressin-stimulated net Cl- transport in the MTAL segment.

Effect of morphine on renomedullary interstitial cell proliferation and matrix accumulation

Renal interstitial scarring is an important feature of heroin-associated nephropathy. We studied the effect of morphine, an active metabolite of heroin, on cultured rat renal medullary interstitial cell (RMIC) proliferation and matrix accumulation. Morphine (10(-12) M) enhanced (p < 0.001) the proliferation of RMIC (control, 15.0+/-0.5 vs. morphine, 20.4+/-1.1 x 10(4) cells/ml). This effect of morphine was dose and time dependent. [3H]thymidine and bromodeoxyuridine incorporation studies confirmed the mitogenic effect of morphine on RMIC. Morphine also enhanced mRNA expression for c-jun and c-myc on RMIC. However, nalbuphine, a non-addicting alkaloid did not modulate the proliferation of RMIC. Morphine enhanced the accumulation of collagen type I in a dose-dependent manner and also increased (p < 0.001) the accumulation of collagen type III at a high concentration (control, 1,291+/-55.8 vs. morphine, 10(-4) M, 2,697.6+/-257.8 ng/microg protein). Morphine did not modulate the accumulation of laminin or fibronectin. Neutralizing antibody to IL-6 inhibited the effect of morphine on RMIC. H7, a protein kinase C inhibitor, also attenuated the morphine-induced RMIC proliferation. The present study provides a basis for a hypothesis that morphine may be playing a role in the development of renal interstitial pathology in patients with heroin addiction.

Transformation of rat inner medullary fibroblasts to myofibroblasts in vitro

Renal fibroblasts play a major role in the pathogenesis of renal interstitial fibrosis. This process is associated at least in some forms of interstitial fibrosis with a differentiation of fibroblasts into myofibroblasts, characterized by the de novo expression of alpha-smooth muscle (alpha-sm) actin and/or desmin. Both the mechanisms underlying this differentiation and their effects on cellular function are poorly understood. In vitro studies are difficult since the phenotypes of fibroblasts in culture have as yet not been well defined. We have, therefore, examined the phenotype of inner medullary fibroblasts (IMF) during the transition from in vivo to in vitro in various cell fractions derived from the inner medulla of healthy rats. IMF were positive for the lectin BSL-1 and negative for markers of endothelial cells. IMF first lost their prominent lipid droplets in vitro. Subsequently they developed cytoplasmic processes accompanied by a decrease in their reactivity for the lectin BSL-1 from strong to weak. From day 3 in primary culture, exclusively these weakly positive BSL-1 cells showed a de novo expression of alpha-sm actin (day 4 of primary culture, 75 +/- 4%; day 20, 94 +/- 2%) and desmin (day 4, 43 +/- 8%; day 20, 66 +/- 6%), classifying them as myofibroblasts. This transformation depended on culture conditions. In a mixed coculture with inner medullary collecting duct (IMCD) cells the transformation of IMF was largely absent: a significantly greater number of strong BSL-1 positive cells contained prominent lipid droplets (39 +/- 4 vs. 19 +/- 4%, P < 0.05) on day 4 of primary culture, and the transition of strongly to weakly positive BSL-1 IMF was almost completely blocked. By reducing the seeding density of IMCD cells the effect of this condition on IMF transformation could be largely abolished. This first detailed phenotypic characterization of rat fibroblasts during the transition from in vivo to in vitro demonstrates that these cells-depending on culture conditions-differentiate to myofibroblasts within a few days of primary culture and that subcultured IMF exhibit predominantly this phenotype. The presented model may serve as a useful tool for the in vitro study of myofibroblast formation and the consequences of such a differentiation for the physiological functions of IMF.

Hypotonicity increases transcription, expression, and action of Egr-1 in murine renal medullary mIMCD3 cells

In cells of the murine renal inner medullary collecting duct (mIMCD3) cell line, acute hypotonic shock (50% dilution of medium with sterile water but not with sterile 150 mM NaCl) increased Egr-1 mRNA abundance 2.5-fold at 6 h, as determined by Northern analysis. This increase was accompanied by increased Egr-1 transcription, as quantitated by luciferase reporter gene assay. Increased transcription was dose dependent, additive with other Egr-1 transcriptional activators, and occurred in the absence of overt cytotoxicity, as quantitated via a fluorometric viability assay. In addition, hypotonic stress increased Egr-1 protein abundance, which was accompanied by augmented Egr-1-specific DNA binding ability, as measured via electrophoretic mobility shift assay. Increased DNA binding was further associated with increased transactivation by Egr-1, demonstrated through transient transfection of mIMCD3 cells with a luciferase reporter gene driven by tandem repeats of the Egr-1 DNA consensus sequence. Taken together, these data indicate that hypotonic stress activates Egr-1 transcription, translation, DNA binding, and transactivation in renal medullary cells. This phenomenon might play a role in the acquisition of the adaptive phenotype in response to hypotonic stress in cells of the renal medulla in vivo.

Effect and distribution of intravenously injected 125I-guanylin in rat kidney examined by high-resolution light microscopic radioautography

125I-guanylin was injected intravenously into rats, and their kidney and intestinal tract were processed for light microscopic radioautography using semithin sections to examine the binding sites. Various doses of unlabeled guanylin were also injected to examine the morphological effects of guanylin on the kidney. Dense labeling of silver grains due to 125I-guanylin were observed only in the kidney. In the cortex, silver grains were localized on the luminal side of the proximal tubules at 5-30 min. In the medulla, silver grains appeared at the basal side of the collecting ducts, capillaries and loops of Henle after 5 min. Silver grains then accumulated in the cytoplasm of the collecting ducts after 10 min, and disappeared after 30 min. The cell height of the inner medullary collecting ducts (IMCD) decreased and their luminal spaces increased dose-dependently 5 min after the injection of both labeled and unlabeled guanylin. These structural changes returned to control levels within 30 min. These results indicate a high density localization of guanylin receptors on the luminal surface of proximal tubules in the renal cortex and also rapid excretion of guanylin through the IMCD. The morphological changes of the IMCD suggest a diuretic effect of guanylin.

Cloning, expression, and regulation of rabbit cyclooxygenase-2 in renal medullary interstitial cells

Prostaglandin synthesis requires cyclooxygenase-1 (COX1) or -2 (COX2), which mediate the conversion of arachidonate to prostaglandin H2. COX1 is the predominant constitutive isoform, whereas COX2 expression is typically low. In the present studies we cloned rabbit COX2 and determined its distribution in unstimulated tissues. Screening rabbit eye and uterine libraries yielded two cDNAs containing identical inserts with a 1,812-nucleotide open-reading frame. This encoded a 604-amino acid polypeptide, 90% identical to human, rat, and mouse COX2. Expression of the rabbit COX2 in HEK-293 cells enhanced prostanoid synthesis. Constitutive COX2 mRNA expression was highest in kidney and urinary bladder. COX2 expression was primarily in renal outer medullary interstitial cells with cortical expression in macula densa. In cultured medullary interstitial cells, COX2 mRNA predominated, with little COX1 expression. Interstitial cell COX2 mRNA but not COX1 was induced by phorbol ester and epidermal growth factor but suppressed by dexamethasone. Phorbol ester also upregulated immunoreactive COX2. Constitutive COX2 in these tissues has important implications for side effects of COX2-selective inhibitors.

Immunohistochemical localization of ANG II AT1 receptor in adult rat kidney using a monoclonal antibody

Molecular and functional studies have suggested that AT1 receptors are present in most nephron segments, yet direct demonstration of AT1 at these sites is lacking. The present study was performed to determine the intrarenal localization of the AT1 receptor utilizing a monoclonal anti-peptide (amino acid residues 8-17) antibody (6313/G2) in adult male Sprague-Dawley rats. Western blot analysis of kidney protein extracts showed a predominant 41-kDa immunoreactive band corresponding to the molecular weight of the deduced cDNA sequence. To determine optimal fixation conditions, kidney tissues were immersion fixed in Bouin's solution, 10% buffered Formalin, or 4% paraformaldehyde. Specificity of immunostaining was documented by preadsorption of the antibody with the immunogenic peptide sequence. Prominent AT1 immunostaining was visualized in the proximal tubule brush-border and basolateral membranes. In addition, distal tubules, cortical and medullary collecting ducts, and the renal arterial vasculature exhibited specific immunoreactivity. Glomerular staining for AT1 was observed in mesangial cells and podocytes. Macula densa cells stained positively. Similar localization of the AT1 receptor was obtained using the three tissue fixation methods, although the intensity of vascular and glomerular staining was highest in Bouin-fixed tissues. The present study demonstrates that the AT1 receptor is more widely distributed along the nephron than previously described and includes renal vascular smooth muscle and proximal and distal epithelial sites.

Kidney cell survival in high tonicity

The kidney medulla of mammals undergoes large changes in tonicity in parallel with the tonicity of the final urine that emerges from the kidney at the tip of the medulla. When the medulla is hypertonic, its cells accumulate the compatible osmolytes myo-inositol, betaine, taurine, sorbitol and glycerophosphorylcholine. The mechanisms by which the compatible osmolytes are accumulated have been explored extensively in kidney-derived cells in culture. Myo-inositol, betaine and taurine are accumulated by increased activity of specific sodium-coupled transporters, sorbitol by increased synthesis of aldose reductase that catalyses the synthesis of sorbitol from glucose. Glycerophosphorylcholine accumulates primarily because its degradation is reduced in cells in hypertonic medium. cDNAs for the cotransporters and for aldose reductase have been cloned and used to establish that hypertonicity increases the transcription of the genes for the cotransporters for myo-inositol, betaine and for aldose reductase. The region 5' to the promoter of the gene for the betaine cotransporter and for aldose reductase confer osmotic responsiveness to a heterologous promoter. The 12-bp sequence responsible for the transcriptional response to hypertonicity has been identified in the 5' region of the gene for the betaine cotransporter.

Induction of molecular chaperones by hyperosmotic stress in mouse inner medullary collecting duct cells

The extreme hyperosmotic conditions that exist in the renal inner medulla enable the urinary concentrating mechanism to function. In this study, we evaluated whether stress-related molecular chaperones are induced in response to hyperosmotic stress in mouse inner medullary collecting duct (mIMCD3) cells. Exposure of cells to medium supplemented with 100 mM NaCl for 4 or 24 h resulted in an increase in heat shock protein-72 (HSP-72) (inducible form) by Western blot. Immunocytochemistry confirmed the increase of HSP-72 and showed that hyperosmotic stress resulted in a localization of HSP-72 predominantly to the nucleoplasm that surrounds the nucleoli and to the cytoplasm, a subcellular distribution pattern different from that seen with heat shock. Using a denatured protein (casein)-affinity column with ATP elution, we identified a number of putative molecular chaperones (46, 60, 78, and 200 kDa) that are upregulated in response to 4 h of hyperosmotic NaCl treatment. Microsequencing identified one of these proteins to be the mitochondrial chaperone mtHSP-70, a member of HSP-70 family, and another to be similar to beta-actin. We also found high levels of HSP-72 in cells chronically adapted to hypertonicity, indicating that chaperones are still required to maintain certain cellular functions even after nonperturbing organic osmolytes are known to accumulate. These results suggest an important role for molecular chaperones in the adaptation of renal medullary epithelial cells to the hyperosmotic conditions that exist in the inner medulla in vivo.

Immunolocalization of the ubiquitin-protein ligase Nedd4 in tissues expressing the epithelial Na+ channel (ENaC)

The epithelial Na+ channel (ENaC) was previously shown to be expressed in several Na(+)- and fluid-absorbing epithelia, particularly those of the kidney, colon, and lung. We have recently identified the ubiquitin-protein ligase Nedd4 as an interacting protein with ENaC and demonstrated that Nedd4 binds by its WW domains to the proline-rich PY motifs of ENaC. These PY motifs were recently shown to be deleted/mutated in patients afflicted with Liddle's syndrome, a hereditary form of systemic renal hypertension. Such mutations cause elevated channel activity by an increase in channel number/stability at the plasma membrane and by increased channel opening. We then proposed that Nedd4, by regulating channel stability/ degradation, may be a suppressor of ENaC. To test whether Nedd4 is localized to those tissues/regions that express ENaC, we performed immunocytochemical analysis of rat Nedd4 (rNedd4) distribution in rat kidney, colon, and lung tissues. Our results show that, in the kidney, rNedd4 is primarily localized to the cortical collecting tubules and outer and inner medullary collecting ducts. These tubular segments were previously shown to express ENaC. The epithelium lining medullary calyxes was also intensely stained, and microvillar borders of proximal convoluted tubules expressed variable amounts of rNedd4. In the lung, rNedd4 was mainly expressed in the epithelia lining the airways, in the submucosal glands and ducts, and in the distal respiratory epithelium. These sites resemble the pattern of ENaC expression. In contrast, in the distal colon, rNedd4 was strongly expressed in the epithelia lining the crypts but not in the ENaC-expressing surface epithelium. Low-salt diet (to elevate serum aldosterone levels) had no effect on rNedd4 distribution in the kidney or colon. Thus Nedd4 is coexpressed and likely colocalizes with ENaC in specific regions within the kidney and lung but not in the colon. We speculate this difference in colocalization may reflect differences in the regulation of channel stability in those tissues.

K+/NH4+ antiporter: a unique ammonium carrying transporter in the kidney inner medulla

The mechanism of NH4+ transport in inner medulla is not known. The purpose of these experiments was to study the process that is involved in ammonium (NH4+) transport in cultured inner medullary collecting duct (mIMCD-3) cells. Cells grown on coverslips were exposed to NH4+ and monitored for pHi changes by the use of the pH-sensitive dye BCECF. The rate of cell acidification following the initial cell alkalinization was measured as an index of NH4+ transport. The rate of NH4+ transport was the same in the presence or absence of sodium in the media (0.052 +/- 0.003 vs 0.048 +/- 0.004 pH/min. P > 0.05), indicating that NH4+ entry into the cells was independent of sodium. The presence of ouabain, bumetanide, amiloride, barium, or 4,4'-di-isothiocyanostilbene-2-2'-disulfonic acid (DIDS) did not block the NH4(+)-induced cell acidification, indicating lack of involvement of Na+:K(+)-ATPase, Na+:K+:2Cl- transport, Na+:H+ exchange, K+ channel, or Cl-/base exchange, respectively, in NH4+ transport. The NH4(+)-induced cell acidification was significantly inhibited in the presence of high external [K+] as compared to low external [K+] (0.018 +/- 0.001 vs. 0.049 +/- 0.003 pH/min for 140 mM K+ vs. 1.8 mM K+ in the media, respectively, P < 0.001). Inducing K+ efflux by imposing an outward K+ gradient caused intracellular acidification by approximately 0.3 pH unit in the presence but not the absence of NH4+. This K+ efflux-induced NH4+ entry increased by extracellular NH4+ in a saturable manner with a Km of approximately 5 mM, blocked by increasing extracellular K+ and was not inhibited by barium. The K+ efflux-coupled NH4+ entry was electroneutral as monitored by the use of cell membrane potential probe 3,3'-dipropylthiadicarbocyanine. These results are consistent with the exchange of internal K+ with external NH4+ in a 1:1 ratio. The K(+)-NH4+ antiporter was inhibited by verapamil and Schering 28080 in a dose-dependent manner, was able to work in reverse mode, and did not show any affinity for H+ as a substrate, indicating that it is distinct from other NH4(+)-carrying transporters. We conclude that a unique transporter, a potassium-ammonium (K+/NH4+) antiport, is responsible for NH4+ transport in renal inner medullary collecting duct cells. This antiporter is sensitive to verapamil and Schering 28080, is electroneutral, and is selective for NH4+ and K+ as substrates. The K+/NH4+ antiporter may play a significant role in acid-base regulation by excretion of ammonium and elimination of acid.