Medullary Interstitium Research Articles

Does Idiopathic Hypercalciuria Trigger Calcium-Sensing Receptor–Mediated Protection from Urinary Supersaturation?

Kidney stone formation is common, affecting 5 to 6% of the American adult population, and highly recurrent. Approximately 70% of stones are composed predominantly of calcium oxalate (CaOx) with small amounts of calcium phosphate (CaP); another 10% are largely CaP. The physicochemical requirement for stone formation is supersaturation of urine with respect to the stone minerals. In the case of calcium stones, supersaturation is driven by urine calcium concentration, which is a function of calcium excretion and urine volume. For CaP, supersaturation is also controlled by urine pH, because solubility of this salt decreases as urine pH rises; CaP stones are seen largely in patients with urine pH >6. Biopsies of renal medullary papillae of stone formers, taken during percutaneous nephrolithotomy, demonstrate that stone formers are also characterized by specific patterns of mineral deposition in tissue. Common idiopathic CaOx stone formers have interstitial deposits of CaP in the medullary interstitium, so-called Randall's plaques, which begin in the thin loops of Henle and extend downward toward the base of the papilla, where they can be seen beneath the urothelium.1 CaOx stones form as overgrowths on plaque, attached to the papillae.2 When the amount of plaque covering the papillary surface was quantified, it varied directly with urine calcium excretion and inversely with urine volume and pH.3 The factors that control urinary supersaturation also affect plaque formation, which seems to be critical to the initiation of most calcium stones. Medullary biopsy of patients with CaP stones have a different appearance. CaP deposits are found in the inner medullary collecting ducts and the terminal ducts of Bellini, associated with fibrosis and papillary deformity,4 and interstitial plaque abundance is low. The process of stone formation is less clear …

Dysnatriämien bei Intensivpatienten

Intensive care patients often suffer from hypo- or hypernatremia. These dysnatremias reflect an antidiuretic-hormone (ADH)-related water imbalance and are the result of the underlying disease. However, they are often triggered by drug side effects and exacerbated by an intentional or unintentional sodium imbalance. Dysnatremias are also caused by artificial ventilation; however, the mechanisms behind this are beyond the scope of this article. Considerations regarding etiology, water and sodium balance and, in particular, the variable in urine dilution or concentration, take priority over a brisk normalization of sodium concentration. Therefore, the 3 most important factors are: 1) delivery of water and sodium to the collecting duct; 2) generation and maintenance of an osmotic pressure gradient exerted by solutes present in the renal medullary interstitium; 3) the regulated water permeability of collecting duct cells under the control of antidiuretic hormone. With these, most disorders can already be identified from patient history and simple factors such as body weight and serum and urine osmolality.

Interstitial tonicity controls TonEBP expression in the renal medulla

Cells in the hyperosmotic kidney medulla, express a transcriptional activator termed tonicity responsive enhancer binding protein (TonEBP). Genes targeted by TonEBP protect kidney cells from the deleterious effects of hyperosmolality by inducing the expression of organic osmolytes and molecular chaperones, and other genes that mediate urine concentration such as aquaporin-2 and urea transporters. We tested here the effect of hypertonicity and hyperosmotic salt in the renal medullary interstitium on the expression TonEBP. When massive water diuresis was induced in rats the medullary sodium concentrations did not change, neither did TonEBP expression. In these animals the medullary tonicity was unchanged despite the production of dilute urine. On the other hand, treatment with the loop diurectic furosemide resulted in a dose-dependent decrease in the medullary sodium concentration causing a reduction in interstitial tonicity. Here, TonEBP expression was blunted in the outer and inner medulla which was due, in part, to decreased mRNA abundance. As expected, the expression of TonEBP target genes in the renal medulla also decreased in response to furosemide. Hence TonEBP expression in the renal medulla is stimulated by interstitial hypertonicity.

Why does the plasma urea concentration increase in acute dehydration?

Acute dehydration biochemically manifests itself with mild uremia and a normal creatinine concentration. The reason for this, as quoted in popular medical textbooks, is that there is increased reabsorption of urea by the kidneys ([1][1]). This explanation is rarely taken further by clinical tutors

Opposed effects of prostaglandin E2 on perfusion of rat renal cortex and medulla: interactions with the renin–angiotensin system

While prostaglandin E(2) (PGE(2)) is an established renal vasodilator, studies of prostaglandin EP receptors suggest that it also has vasoconstrictor potential. Prostaglandin E(2) is much more abundant in the medulla than in the cortex, yet likely differences in effects between zones have not been defined. This study is focused on different vascular effects in the cortex and medulla and interaction with the renin-angiotensin system (RAS). In anaesthetized rats, the effects of cyclo-oxygenase blockade and of PGE(2) infused into the renal artery or renal interstitium were examined. Total renal blood flow was measured by ultrasonic renal artery probe, and local perfusion, separately, of the superficial cortex, outer- and inner medulla, as laser-Doppler fluxes. Indomethacin (5 mg kg(-1) i.v.) increased cortical perfusion (by approximately 10%) and decreased medullary perfusion (by approximately 20%). Renal artery infusion of PGE(2) (15-30 microg kg(-1) h(-1)) increased cortical and medullary perfusion only transiently. Previous inactivation of the RAS using losartan or captopril, and background infusion of exogenous angiotensin II, prevented the transient increase and enhanced the subsequent stable decrease in perfusion. Prostaglandin E(2) infused into the medullary interstitium (7-22 microg kg(-1) h(-1)) increased medullary perfusion by 13%, while cortical perfusion decreased by 6%. Misoprostol, an agonist of constrictor EP(3) receptors, decreased perfusion of the cortex and medulla, with both renal artery and medullary interstitial infusion. In conclusion, in rat renal cortex the dominating stable PGE(2) effect is vasoconstriction, most probably mediated by EP(3) receptors and unrelated to activation of the RAS. Prostaglandin E(2) applied to the cortical or medullary interstitium, a natural route for paracrine agents, induces medullary vasodilatation.

Obstructive Protein Cast Nephropathy in Cynomolgus Monkeys Treated with Small Organic Molecules

We have observed a renal toxicity consistent with an obstructive protein cast nephropathy in cynomolgus macaques but not in other species treated with different therapeutic candidates having a common carboxylic acid moiety, suggesting a species-specific sensitivity. Here, we present renal toxicity findings consistent with a protein cast nephropathy in a 2-week safety study in cynomolgus monkeys. Light microscopic changes consisted of intratubular cast formation, tubular dilatation, interstitial inflammation, and expansion of the medullary interstitium. Tubular cast material was identified as Tamm-Horsfall protein (THP) and, on ultrastructure, crystalloid material was present in vacuoles of tubular epithelium. It is hypothesized that microcrystal formation in the urinary tubular spaces induces aggregation of THP protein and cast formation in monkeys. Drug-induced obstructive nephropathy is not identified as a major problem in humans; thus, the clinical relevance of the above findings in monkeys is not clear.

Cell-type-specific expression of murine multifunctional galectin-3 and its association with follicular atresia/luteolysis in contrast to pro-apoptotic galectins-1 and -7

Galectin-3 is a multifunctional protein with modular design. A distinct expression profile was determined in various murine organs when set into relation to homodimeric galectins-1 and -7. Fittingly, the signature of putative transcription-factor-binding sites in the promoter region of the galectin-3 gene affords a toolbox for a complex combinatorial regulation, distinct from the respective sequence stretches in galectins-1 and -7. A striking example for cell-type specificity was the ovary, where these two lectins were confined to the surface epithelium. Immunohistochemically, galectin-3 was found in macrophages of the cortical interstitium between developing follicles and medullary interstitium, matching the distribution of the F4/80 antigen. With respect to atresia and luteolysis strong signals in granulosa cells of atretic preantral but not antral follicles and increasing positivity in corpora lutea upon regression coincided with DNA fragmentation. Labeled galectin-3 revealed lactose-inhibitable binding to granulosa cells. Also, slender processes of vital granulosa cells which extended into the zona pellucida were positive. This study demonstrates cell-type specificity and cycle-associated regulation for galectin-3 with increased presence in atretic preantral follicles and in late stages of luteolysis.

Amyloidosis in Black-footed Cats (Felis nigripes)

A high prevalence of systemic amyloidosis was documented in the black-footed cat (Felis nigripes) based on a retrospective review of necropsy tissues (n = 38) submitted as part of ongoing disease surveillance. Some degree of amyloid deposition was present in 33 of 38 (87%) of the examined cats, and amyloidosis was the most common cause of death (26/38, 68%). Amyloid deposition was most severe in the renal medullary interstitium (30/33, 91%) and glomeruli (21/33, 63%). Other common sites included the splenic follicular germinal centers (26/31, 84%), gastric lamina propria (9/23, 39%), and intestinal lamina propria (3/23, 13%). Amyloid in all sites stained with Congo red, and in 13 of 15 (87%) cats, deposits had strong immunoreactivity for canine AA protein by immunohistochemistry. There was no association with concurrent chronic inflammatory conditions (P = .51), suggesting that amyloidosis was not secondary to inflammation. Adrenal cortical hyperplasia, a morphologic indicator of stress that can predispose to amyloid deposition, was similarly not associated (P = .09) with amyloidosis. However, adrenals were not available from the majority of cats without amyloidosis; therefore, further analysis of this risk factor is warranted. Heritability estimation suggested that amyloidosis might be familial in this species. Additionally, tissues from a single free-ranging black-footed cat had small amounts of amyloid deposition, suggesting that there could be a predilection for amyloidosis in this species. Research to identify the protein sequence of serum amyloid A (SAA) in the black-footed cat is needed to further investigate the possibility of an amyloidogenic SAA in this species.

Natriuretic activity of prehypertensive Dahl salt‐sensitive (DS) and salt‐resistant (SS13BN) rats

We have recently demonstrated that activation of the ETB receptor in the renal medulla induces a natriuretic response in anesthetized rats and that sodium excretion of an acute salt load in the conscious rat requires the ETB receptor. We hypothesized that animals genetically susceptible to salt‐induced hypertension (DS) would have reduced ETB‐mediated natriuresis compared to salt‐resistant controls (SS13BN). Therefore, the aims of the current studies were to determine the natriuretic response to 1) intramedullary ETB activation and 2) an acute oral salt load in DS and SS13BN rats. Sarafotoxin 6c (S6c; 1–10 pmol/kg/min; n = 6), a selective ETB agonist, infused into the renal medullary interstitium dose‐dependently increased the sodium excretion without affecting systemic blood pressure, although no difference was observed in the response between DS and SS13BN (10 pmol/kg/min; 2.3 ± 0.2 μmol/min and 2.0 ± 0.3 μmol/min, respectively). In a separate series of experiments, conscious rats received an acute salt challenge of 900 μEq Na p.o. Both strains exhibited significant sodium excretion within the first 12 hours post challenge (DS: 1.48 ± 0.13 mEq; SS13BN: 1.68 ± 0.11 mEq). Again, there was no difference between the strains. In conclusion, these studies suggest that acute ETB‐dependent natriuretic mechanisms are intact in both salt‐resistant and salt‐sensitive strains of rats.

Elevated Arterial Perfusion Pressure Increases H2O2 in the Renal Medullary Interstitium

We established a direct role of arterial renal perfusion pressure (RPP) in renal injury using servo‐controlled techniques to maintain the left kidney of Sprague Dawley (SD) rats at normal RPP while exposing the right kidney to a high RPP during the development of hypertension (Hypertension 43: 752, 2004). These results showed that pressure‐induced renal injury was associated with activation of NF‐kβ and TGF–β1 in the renal outer medulla indicating oxidative stress was involved in these mechanically‐induced responses. In the present study, we determined whether medullary interstitial [H2O2], a reactive oxygen species, could be induced by a rapid increase in RPP. Left kidneys of SD rats were denervated, a linear microdialysis fiber implanted in the outer medulla, a ureteral catheter inserted, and an adjustable occluder placed around the aorta proximal and distal to the renal arteries to raise and lower RPP. Following 60 min. equilibration, dialysate was collected (2 μl/min) for 30 min and H2O2 measured at three levels of RPP (Amplex Red). As RPP was increased from 73±1 to 93±1 mmHg, medullary H2O2 was relatively constant (650±62 to 733±89 nM) but when RPP increased to 138±3, H2O2 levels increased to 1237±78 nM (P<0.05) with parallel increases in urinary H2O2 excretion. We conclude that elevations of RPP >100 mmHg rapidly and markedly increase the levels of H2O2 within the renal medulla. (Support: HL‐29587)

CCN3 is a novel endogenous PDGF-regulated inhibitor of glomerular cell proliferation

CCN proteins affect cell proliferation, migration, attachment, and differentiation. We identified CCN3 as a suppressed gene following platelet-derived growth factor (PDGF)-BB or -DD stimulation in a cDNA-array analysis of mesangial cells. In vitro growth-arrested mesangial cells overexpressed and secreted CCN3, whereas the addition of the recombinant protein inhibited cell growth. Induction of mesangial cell proliferation by PDGF-BB or the specific PDGF beta-receptor ligand PDGF-DD led to downregulation of CCN3 mRNA, confirming the array study. Specific PDGF alpha-receptor ligands had no effect. CCN3 protein was found in arterial smooth muscle cells, the medullary interstitium, and occasional podocytes in the healthy rat kidney. Glomerular CCN3 was low prior to mesangial proliferation but increased as glomerular cell proliferation subsided during mesangioproliferative glomerulonephritis (GN). Inhibition of PDGF-B in mesangioproliferative disease led to overexpression of glomerular CCN3 mRNA. CCN3 localized mostly to podocytes in human glomeruli, but this expression varied widely in different human glomerulonephritides. Glomerular cell proliferation negatively correlated with CCN3 expression in necrotizing GN. Our study identifies CCN3 as an endogenous inhibitor of mesangial cell growth and a modulator of PDGF-induced mitogenesis.

Fructose intake as a risk factor for kidney stone disease

Taylor and Curhan report that consumption of fructose is independently associated with an increased risk of incident kidney stones. What could be the mechanisms underlying the relation between fructose intake and stone risk? And how should we incorporate this finding into the dietary advice that we give to our patients to prevent kidney stone formation?

Vasopressin receptor subtype 2 activation increases cell proliferation in the renal medulla of AQP1 null mice

Aquaporin (AQP) 1 null mice have a defect in the renal concentrating gradient because of their inability to generate a hyperosmotic medullary interstitium. To determine the effect of vasopressin on renal medullary gene expression, in the absence of high local osmolarity, we infused 1-deamino-8-d-arginine vasopressin (dDAVP), a V(2) receptor (V(2)R)-specific agonist, in AQP1 null mice for 7 days. cDNA microarray analysis was performed on the renal medullary tissue, and 5,140 genes of the possible 12,000 genes on the array were included in the analysis. In the renal medulla of AQP1 null mice, 245 transcripts were identified as increased by dDAVP infusion and 200 transcripts as decreased (1.5-fold or more). Quantitative real-time PCR measurements confirmed the increases seen for cyclin D1, early growth response gene 1, and activating transcription factor 3, genes associated with changes in cell cycle/growth. Changes in mRNA expression were correlated with changes in protein expression by semiquantitative immunoblotting; cyclin D1 and ATF3 were increased significantly in abundance following dDAVP infusion in the renal medulla of AQP1 null mice (161 and 461%, respectively). A significant increase in proliferation of medullary collecting ducts cells, following V(2)R activation, was identified by proliferating cell nuclear antigen immunohistochemistry; colocalization studies with AQP2 indicated that the increase in proliferation was primarily observed in principal cells of the inner medullary collecting duct (IMCD). V(2)R activation, via dDAVP, increased AQP2 and AQP3 protein abundance in the cortical collecting ducts of AQP1 null mice. However, V(2)R activation did not increase AQP2 protein abundance in the IMCD of AQP1 null mice.

Analgesic nephropathy

Analgesic nephropathy

Downregulation of renal TonEBP in hypokalemic rats

Hypokalemia causes a significant decrease in the tonicity of the renal medullary interstitium in association with reduced expression of sodium transporters in the distal tubule. We asked whether hypokalemia caused downregulation of the tonicity-responsive enhancer binding protein (TonEBP) transcriptional activator in the renal medulla due to the reduced tonicity. We found that the abundance of TonEBP decreased significantly in the outer and inner medullas of hypokalemic rats. Underlying mechanisms appeared different in the two regions because the abundance of TonEBP mRNA was lower in the outer medulla but unchanged in the inner medulla. Immunohistochemical examination of TonEBP revealed cell type-specific differences. TonEBP expression decreased dramatically in the outer and inner medullary collecting ducts, thick ascending limbs, and interstitial cells. In the descending and ascending thin limbs, TonEBP abundance decreased modestly. In the outer medulla, TonEBP shifted to the cytoplasm in the descending thin limbs. As expected, transcription of aldose reductase, a target of TonEBP, was decreased since the abundance of mRNA and protein was reduced. Downregulation of TonEBP appeared to have also contributed to reduced expression of aquaporin-2 and UT-A urea transporters in the renal medulla. In cultured cells, expression and activity of TonEBP were not affected by reduced potassium concentrations in the medium. These data support the view that medullary tonicity regulates expression and nuclear distribution of TonEBP in the renal medulla in cell type-specific manners.

Downregulation of Renal Sodium Transporters and Tonicity-Responsive Enhancer Binding Protein by Long-Term Treatment with Cyclosporin A

Tonicity-responsive enhancer binding protein (TonEBP) is a transcriptional activator that is regulated by ambient tonicity. TonEBP protects the renal medulla from the deleterious effects of hyperosmolality and regulates the urinary concentration by stimulating aquaporin-2 and urea transporters. The therapeutic use of cyclosporin A (CsA) is limited by nephrotoxicity that is manifested by reduced GFR, fibrosis, and tubular defects, including reduced urinary concentration. It was reported recently that long-term CsA treatment was associated with decreased renal expression of TonEBP target genes, including aquaporin-2, urea transporter, and aldose reductase. This study tested the hypothesis that long-term CsA treatment reduces the salinity/tonicity of the renal medullary interstitium as a result of inhibition of active sodium transporters, leading to downregulation of TonEBP. CsA treatment for 7 d did not affect TonEBP or renal function. Whereas expression of sodium transporters was altered, the medullary tonicity seemed unchanged. Conversely, 28 d of CsA treatment led to downregulation of TonEBP and overt nephrotoxicity. The downregulation of TonEBP involved reduced expression, cytoplasmic shift, and reduced transcription of its target genes. This was associated with reduced expression of active sodium transporters-sodium/potassium/chloride transporter type 2 (NKCC2), sodium/chloride transporter, and Na(+),K(+)-ATPase-along with increased sodium excretion and reduced urinary concentration. Infusion of vasopressin restored the expression of NKCC2 in the outer medulla as well as the expression and the activity of TonEBP. It is concluded that the downregulation of TonEBP in the setting of long-term CsA administration is secondary to the reduced tonicity of the renal medullary interstitium.

Acute increases of renal medullary osmolality stimulate endothelin release from the kidney

Experiments conducted in vitro suggest that high osmolality stimulates endothelin production and release by renal tubular epithelial cells. Whether hyperosmotic solutions exert similar effects in vivo is unknown. Therefore, we tested the hypothesis that increasing renal medullary osmolality enhances urinary excretion of endothelin in anesthetized rats. Isosmotic NaCl (284 mosmol/kgH(2)O) was infused either intravenously (1.5 ml/h) or into the renal medullary interstitium (0.5 ml/h) during a 1-h equilibration period and 30-min baseline urine collection period, followed by either isosmotic or hyperosmotic NaCl (921 or 1,664 mosmol/kgH(2)O iv; 1,714 mosmol/kgH(2)O into renal medulla) for two further 30-min periods. Compared with isosmotic NaCl, infusion of hyperosmotic NaCl into the renal medulla significantly increased the endothelin excretion rate (P < 0.05; from 0.30 +/- 0.02 to 0.49 +/- 0.03 fmol/min). Intravenous infusion of hyperosmotic NaCl also significantly increased endothelin excretion rate in a concentration-dependent manner (from 0.79 +/- 0.07 to 1.77 +/- 0.16 fmol/min and 0.59 +/- 0.04 to 1.11 +/- 0.08 fmol/min for 1,664 and 921 mosmol/kgH(2)O, respectively). To differentiate between effects of osmolality and NaCl, similar experiments were performed using mannitol solutions. Compared with isosmotic mannitol, medullary interstitial infusion of hyperosmotic mannitol (1,820 mosmol/kgH(2)O) significantly increased endothelin excretion rate (P < 0.05; from 0.54 +/- 0.03 to 0.94 +/- 0.12 fmol/min). Thus exposing the renal medulla to hyperosmotic concentrations of either NaCl or mannitol stimulates endothelin release in vivo, consistent with medullary osmolality being an important regulator of renal endothelin synthesis.

V2 receptor activation induces renal medullary cell proliferation in mice with a reduced urinary concentrating ability

Increases in circulating vasopressin trigger receptor activated pathways in the kidney and increase the renal concentrating capacity of the medulla. The renal medulla encounters an increase in osmotic stress following increases in vasopressin. Mice which lack the AQP1 gene fail to generate a hypertonic medullary interstitium thus have a urinary concentrating defect. This study used AQP1 null mice as a tool to determine the specific effects of vasopressin on medullary gene expression, in the absence of high osmotic stress. We infused dDAVP, a V2 receptor agonist, into AQP1 null mice for 7 days. Growth regulated genes cyclin D1, early growth response gene 1 and activated transcription factor 3 (ATF3) mRNA were significantly increased in the medulla of AQP1 null mice following dDAVP infusion. Western blot analysis confirmed the increase in cyclin D1 protein (dDAVP infused 161±9% v 100±6% AQP1 null controls, p<0.05) and ATF3 (dDAVP infused 461±49% v 100±19% AQP1 null controls, p<0.05). Using a proliferating cell nuclear antigen (PCNA) antibody we determined that dDAVP increased renal medullary cell proliferation in AQP1 null mice. AQP2 and PCNA dual labeling was performed; proliferating cells were positive for AQP2. dDAVP did not increase proliferation in normal mice. We conclude that dDAVP infusion induces renal medullary collecting duct cell proliferation in mice with a decreased urinary concentrating ability.

Type 2 Pseudohypoaldosteronism: New Insights Into Renal Potassium, Sodium, and Chloride Handling

Type 2 Pseudohypoaldosteronism: New Insights Into Renal Potassium, Sodium, and Chloride Handling

Crystal Retention in Renal Stone Disease

The mechanisms that are involved in renal stone disease are not entirely clear. In this article, the various concepts that have been proposed during the past century are reviewed briefly and integrated into current insights. Much attention is dedicated to hyaluronan (HA), an extremely large glycosaminoglycan that may play a central role in renal stone disease. The precipitation of poorly soluble calcium salts (crystal formation) in the kidney is the inevitable consequence of producing concentrated urine. HA is a major constituent of the extracellular matrix in the renal medullary interstitium and the pericellular matrix of mitogen/stress-activated renal tubular cells. HA is an excellent crystal-binding molecule because of its size, negative ionic charge, and ability to form hydrated gel-like matrices. Crystal binding to HA leads to crystal retention in the renal tubules (nephrocalcinosis) and to the formation of calcified plaques in the renal interstitium (Randall's plaques). It remains to be determined whether one or both forms of renal crystal retention are involved in the development of kidney stones (nephrolithiasis).