GLP-1 Promotes Cortical and Medullary Perfusion in the Human Kidney and Maintains Renal Oxygenation During NaCl Loading

Background GLP-1 (glucagon-like peptide-1) receptor agonists exert beneficial long-term effects on cardiovascular and renal outcomes. In humans, the natriuretic effect of GLP-1 depends on GLP-1 receptor interaction, is accompanied by suppression of angiotensin II, and is independent of changes in renal plasma flow. In rodents, angiotensin II constricts vasa recta and lowers medullary perfusion. The current randomized, controlled, crossover study was designed to test the hypothesis that GLP-1 increases renal medullary perfusion in healthy humans. Methods and Results Healthy male participants (n=10, aged 27±4 years) ingested a fixed sodium intake for 4 days and were examined twice during a 1-hour infusion of either GLP-1 (1.5 pmol/kg per minute) or placebo together with infusion of 0.9% NaCl (750 mL/h). Interleaved measurements of renal arterial blood flow, oxygenation (R₂*), and perfusion were acquired in the renal cortex and medulla during infusions, using magnetic resonance imaging. GLP-1 infusion increased medullary perfusion (32±7%, P<0.001) and cortical perfusion (13±4%, P<0.001) compared with placebo. Here, NaCl infusion decreased medullary perfusion (-5±2%, P=0.007), whereas cortical perfusion remained unchanged. R₂* values increased by 3±2% (P=0.025) in the medulla and 4±1% (P=0.008) in the cortex during placebo, indicative of decreased oxygenation, but remained unchanged during GLP-1. Blood flow in the renal artery was not altered significantly by either intervention. Conclusions GLP-1 increases predominantly medullary but also cortical perfusion in the healthy human kidney and maintains renal oxygenation during NaCl loading. In perspective, suppression of angiotensin II by GLP-1 may account for the increase in regional perfusion. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT04337268.

Influence of blood haemoglobin concentration on renal haemodynamics and oxygenation during experimental cardiopulmonary bypass in sheep

AIM

Blood transfusion may improve renal oxygenation during cardiopulmonary bypass (CPB). In an ovine model of experimental CPB, we tested whether increasing blood haemoglobin concentration [Hb] from ~7 g dL-1 to ~9 g dL-1 improves renal tissue oxygenation.

METHODS

Ten sheep were studied while conscious, under stable isoflurane anaesthesia, and during 3 hours of CPB. In a randomized cross-over design, 5 sheep commenced bypass at a high target [Hb], achieved by adding 600 mL donor blood to the priming solution. After 90 minutes of CPB, PlasmaLyte® was added to the blood reservoir to achieve low target [Hb]. For the other 5 sheep, no blood was added to the prime, but after 90 minutes of CPB, 800-900 mL of donor blood was given to achieve a high target [Hb].

RESULTS

Overall, CPB was associated with marked reductions in renal oxygen delivery (-50 ± 12%, mean ± 95% confidence interval) and medullary tissue oxygen tension (PO2 , -54 ± 29%). Renal fractional oxygen extraction was 17 ± 10% less during CPB at high [Hb] than low [Hb] (P = .04). Nevertheless, no increase in tissue PO2 in either the renal medulla (0 ± 6 mmHg change, P > .99) or cortex (-19 ± 13 mmHg change, P = .08) was detected with high [Hb].

CONCLUSIONS

In experimental CPB blood transfusion to increase Hb concentration from ~7 g dL-1 to ~9 g dL-1 did not improve renal cortical or medullary tissue PO2 even though it decreased whole kidney oxygen extraction.

[Renomedullary interstitial cell tumor: review of a rare clinical case]

Autopsy study of a 49-year-old patient demonstrates a rare kidney tumor that had a structure of a mesenchymal renomedullary interstitial tumor (RIO), which had specific characteristics. They include a predominance of the cellular component in the tumor, represented by cells with processes elements (fibroblasts, myofibroblasts), which were surrounded by collagen fibers of interstitium. Apparently, these cells can be involved in the regulation of renin and bicarbonates, the exchange of components of connective tissue (collagen fibers, proteoglycans) and hormones (estrogen, progesterone). Since these tumor cells regulate the synthesis and secretion of the substances, numerous clinical manifestations of RIO can be explained. Most likely, these include increased blood pressure, water-electrolyte disorders, hormonal disorders due to the presence of estrogen receptors, progesterone in the tumor tissue and metabolic disorders (diabetes mellitus). However, these assumptions require further clinical, morphological and immunohistochemical studies.

Hydrogen peroxide stimulates chloride secretion in primary inner medullary collecting duct cells via mPGES-1-derived PGE2

We investigated the role and mechanism of H2O2 in regulation of NaCl transport in primary inner medullary collecting duct (IMCD) cells. IMCD cells were isolated from wild-type mice and grown onto semipermeable membranes, and short-circuit current (Isc) was determined by Ussing chamber. Exposure of IMCD cells to H2O2 at a range of 100-300 microM caused a rapid increase in Isc in a time- and dose-dependent manner. This increase was almost abolished by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel inhibitors diphenylamine-2-carboxylic acid (DPC) and CFTR inhibitor-172. In contrast, the magnitude of stimulation was unaffected by the epithelial Na+ channel (ENaC) inhibitor amiloride. The H2O2-induced Cl(-) secretion was significantly inhibited by indomethacin, as well as by microsomal PGE synthase-1 (mPGES-1) deficiency. Like H2O2, PGE2 treatment induced a twofold increase in Isc that was reduced by the protein kinase A (PKA) inhibitors H-89 and KT5720. These data suggest that H2O2 stimulates CFTR Cl(-) channel-mediated Cl(-) secretion through cyclooxygenase- and mPGES-1-dependent release of PGE2 and subsequent activation of PKA.

Downregulation of SGK1 by nucleotides in renal tubular epithelial cells

This study determined whether nucleotides that bind to purinergic receptors (P2R) regulate the expression or function of serum- and glucocorticoid-inducible kinase-1 (SGK1) in mouse renal inner medullar collecting duct cells (mIMCD-3). The SGK1 protein was detected by Western blotting. A significant reduction of cytosolic SGK1 expression was observed in the cells pretreated with P2R agonist adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), and the reduction could be reversed by P2R antagonists. This reduction was also observed in cells that were pretreated with agonists for P2R subtypes. Using ELISA, we observed a reduced SGK1 kinase activity in ATPgammaS-pretreated cells. This effect was reversed by P2R antagonists. Furthermore, an increase of SGK1 kinase activity in aldosterone-pretreated cells was suppressed by ATPgammaS. These studies demonstrate for the first time that SGK1 can be downregulated by nucleotides in renal collecting duct epithelial cells, likely via the activation of P2R, and suggest that activation of renal purinergic signaling regulates a SGK1-dependent pathway that is known to modulate ion transport in the renal collecting duct.

The role of Randall's plaques in the pathogenesis of calcium stones

PURPOSE

Knowledge of the inciting lesion in kidney stone formation has remained rudimentary until quite recently. Randall theorized that areas of apatite plaque on the renal papillae would be an ideal site for an overgrowth of calcium oxalate to develop into a calculus. We reviewed in vivo data that have further defined the role of Randall's plaques in stone disease.

MATERIALS AND METHODS

We examined a set of literature that tested 2 hypotheses, that is 1) Randall's plaques are a specialized disease that begins as apatite in a unique region of the kidney due to local driving forces and anatomy, and 2) stones that arise from causes different from common calcium oxalate stones do not necessarily arise on plaque.

RESULTS

Intraoperative papillary and cortical biopsy specimens obtained during percutaneous nephrolithotomy from the kidneys of 3 types of stone formers (idiopathic calcium stone formers, patients with stones due to bariatric procedures and brushite stone formers) showed unique histopathological findings.

CONCLUSIONS

The metabolic and surgical pathological findings in 3 distinct groups of stone formers demonstrate that the histology of the renal papillae from a stone former is particular to the clinical setting.

Endoscopic evidence of calculus attachment to Randall's plaque

PURPOSE

It has been proposed that calcium oxalate calculi begin as small stones attached to the renal papillae at sites of Randall's plaque. However, no study has investigated the prevalence of attached stones in calcium oxalate stone formers or the relationship between stone attachment site and Randall's plaque. In this study we used endoscopic examination of renal papillae in stone formers undergoing percutaneous nephrolithotomy to investigate both issues.

MATERIALS AND METHODS

Idiopathic calcium oxalate stone formers undergoing PNL for stone removal were enrolled in this study. Multiple papillae were examined and images were recorded by digital video. The presence or absence of papillary plaque and attached stones was noted, as was the site of stone attachment.

RESULTS

In 23 patients, 24 kidneys and 172 renal papillae were examined. All kidneys were found to have papillary plaque and 11 of the patients had attached stones. Most papillae (91%) contained plaque.

CONCLUSIONS

The prevalence of attached stones in calcium oxalate stone formers (48%) is greater than that previously reported for the general population. Attachment appears to be on Randall's plaque. The high prevalence of attached stones and the appearance of the attachment site are consistent with a mechanism of calcium oxalate stone formation in which stones begin as plaque overgrowth.

Intramedullary ectopic choroid plexus: report of a rare case

OBJECTIVE AND IMPORTANCE

Intracranial cysts containing an ectopic choroid plexus or choroid plexus-like tissue have seldom been described in the literature. However, there has been no report of a spinal intramedullary cyst containing an ectopic choroid plexus. This is the first case report in the available literature of an ectopic choroid plexus tissue in the spinal cord.

CLINICAL PRESENTATION

A 30-year-old man presented with complaints of progressive descending weakness of both upper limbs and increasing stiffness of the lower limbs along with numbness of all four limbs without a history of any bowel or bladder disturbances. Examination was suggestive of a C5-T2 intramedullary lesion. Magnetic resonance imaging revealed a C6-T2 intramedullary cystic lesion along with a small anterosuperiorly placed lesion enhancing with contrast.

INTERVENTION

A C6-T2 laminotomy and exploration of the intramedullary cyst and gross total microsurgical excision of the reddish vascular frond-like structure resembling the choroid plexus were performed along with a syringostomy. A laminoplasty with miniplates and screws was performed. Histopathological and immunohistochemical studies revealed a normal choroid plexus. The patient has been followed for 1 year and has demonstrated symptomatic improvement.

CONCLUSION

Although there have been rare case reports of drop metastasis of choroid plexus papillomas in the spine, this is the first case report in the available literature of normal but ectopic choroid plexus tissue in the spinal cord.

Vasopressin induces expression of the Cl−/HCO3− exchanger SLC26A7 in kidney medullary collecting ducts of Brattleboro rats

SLC26A7 is a newly identified basolateral Cl−/HCO3− exchanger specific to α-intercalated cells of the outer medullary collecting duct (OMCD). The purpose of the present experiments was to examine the expression of SLC26A7 in kidneys of vasopressin-deficient Brattleboro rats before and after treatment with desamino-Cys1,d-Arg8-vasopressin (dDAVP). Brattleboro rats were treated with dDAVP, a vasopressin analog, for 8 days, and their kidneys were examined for the expression of SLC26A7. The expression of SLC26A7 protein, as examined by immunofluorescence, was undetectable in kidneys of Brattleboro rats. However, treatment with dDAVP induced expression of SLC26A7 protein, restoring it to levels observed in normal rats. These results were verified by Western blot analysis. The mRNA expression of SLC26A7 remained unchanged in response to dDAVP. Immunofluorescent labeling demonstrated abundant levels of anion exchanger type 1 in the OMCD of Brattleboro rats and a mild reduction in response to dDAVP. The abundance of H+-ATPase was not affected by dDAVP. The increased SLC26A7 expression directly correlated with enhanced aquaporin-2 expression, which is proportional to increased interstitial osmolarity in the medulla. In conclusion, vasopressin increases the expression of SLC26A7 protein through posttranscriptional mechanisms in the OMCD. The induction of SLC26A7 by vasopressin in OMCD cells of Brattleboro rats is likely an attempt by cells to regulate their cell volume and maintain HCO3− absorption in a state associated with increased interstitial medullary tonicity.

Le carcinome médullaire du rein : une observation

Renal medullary carcinoma is an aggressive malignant tumour, recently reported in the literature. It is usually reported in the relatively young patients with drepanocytic trai. Histologically, the tumour is constituted by a tumoral proliferation with diffuse or glandular architecture and inflammatory stroma. The carcinomatous cells have plasmocytoid or rhabdoid aspect. We report a case of 40 years old man who presented macroscopic hematuria. Through this observation and the review of the literature we discuss the anatomoclinical and the prognostic aspects of this exceptional tumour.

Ureteroscopic laser papillotomy to treat papillary calcifications associated with chronic flank pain

OBJECTIVES

To evaluate retrospectively the efficacy and durability of a novel approach using ureteroscopic laser papillotomy for the treatment of painful papillary calcifications. Chronic pain due to renal papillary calcifications has not been addressed by current techniques.

METHODS

Ureteroscopic holmium laser lithotripsy and papillotomy were performed on patients with chronic pain and radiographically visible papillary calcifications without free collecting system calculi. The papillary urothelium overlying all cystic dilations and intraductal calcifications was vaporized. Treated patients answered a telephone survey to assess pain scores, duration of response, use of narcotics, and patient satisfaction. We reviewed the medical records to evaluate for procedure-related complications and serum creatinine measurements.

RESULTS

Of 20 patients who underwent laser papillotomy and responded to the telephone survey, 7 had bilateral procedures, yielding 27 renal units available for analysis. "Much less pain" was reported after 85% of the procedures, with a durable improvement reported after 59% of the procedures, at a median follow-up of 14.5 months. Significant improvements in the median pain scores were seen at 1 month (1.0, P <0.001), 6 months (2.0, P <0.001), and 1 year (1.5, P <0.001) compared with a median preoperative pain score of 9.0. The mean serum creatinine was unchanged after the procedure.

CONCLUSIONS

Ureteroscopic laser papillotomy appears to be an effective treatment option for the chronic pain associated with papillary calcifications. Laser papillotomy offers hope to patients who would otherwise have been denied an attempt at treatment because of a lack of free calculi within the collecting system.

Collecting duct carcinoma of the renal medulla presenting with paraplegia

We report an interesting case of a patient with collecting duct carcinoma arising from the left kidney who presented with paraplegia secondary to metastases. The diagnosis was based on CT and histology. To our knowledge this is the first case of collecting duct carcinoma to present with paraplegia. The literature review also highlights the rarity of this disease with less than a hundred cases reported to date and the aggressive nature and poor prognosis despite prompt interventions.

Collecting duct carcinoma of the kidney: CT and pathologic correlation

PURPOSE

We characterized CT findings of collecting duct carcinoma of the kidney and correlated these with the histopathologic findings.

MATERIALS AND METHODS

CT scans of 18 patients with pathologically proven collecting duct carcinoma of the kidney were retrospectively reviewed. We analyzed CT findings of collecting duct carcinoma and also correlated CT findings with the histopathologic findings.

RESULTS

The mean size of the tumors was 6.9 cm and all cases were solid. Seventeen (94%) tumors had a medullary location. Nine (69%) and 11 (85%) cases showed weak and heterogeneous enhancement, respectively. A cystic component (50%) was frequently seen within the tumors. Lymphadenopathy and metastasis were noted in 10 (56%) and 6 (33%) cases, respectively. Perinephric stranding and vascular invasion were present in 10 (56%) and 5 (28%) cases, respectively. In 17 (94%) of the 18 cases, involvement of the renal sinus was present. Infiltrative growth (67%) and preservation of the renal contour (61%) were more common than expansile growth (33%) and exophytic configuration (39%), respectively. These CT features were well correlated with the histopathologic findings.

CONCLUSION

Medullary location, weak and heterogeneous enhancement, involvement of the renal sinus, infiltrative growth, preserved renal contour, and a cystic component are CT findings frequently seen in patients with collecting duct carcinoma of the kidney. CT findings are nevertheless nonspecific and do not allow collecting duct carcinoma to be easily differentiated from the other subtypes of renal cell carcinoma. However, when CT demonstrates a renal tumor with these findings, collecting duct carcinoma can be considered in the differential diagnosis.

Cyclosporine decreases prostaglandin E2 production in mouse medullary thick ascending limb cultured cells

Intrarenal vasoconstriction is thought to be the major pathogenesis of cyclosporine (CsA) nephrotoxicity. Nitric oxide (NO) and prostaglandin E2 (PGE2) are two of the major intrarenal vasodilators, which protect kidney from ischemia. CsA inhibited NO production in renal epithelial cells. The interaction between CsA and intrarenal PGE2 and NO production is still unclear. The aim of the study is to evaluate the interaction of CsA with intrarenal PGE2 and NO production in renal epithelial cells. Models of cultured mouse thick ascending limb (TAL) cells are chosen to perform the experiments, as TAL cells are the major site of intrarenal PGE2 production and target of CsA nephrotoxicity. We investigated the PGE2 production by enzyme-linked immunosorbent assay, and cyclooxygenase (COX-1 and COX-2) mRNA expression by RT-PCR in cultured cells treated with or without CsA. TAL cells maintained the main characteristics of their parental cells. TAL cells produce PGE2 mainly by COX-1 in steady state and by COX-2 in stimulated state by lipopolysaccharide (LPS). CsA (100 ng/ml) significantly reduced the PGE2 production up to 43% in TAL cells in LPS stimulated status (control versus CsA: 375.1 +/- 15.5 vs. 187.2 +/- 12.2 nm/mg protein, n = 7, P < 0.001). The effects were dose-dependent. The mRNA expression of COX1 is not affected and COX-2 is decreased in CsA-treated TAL cells. NO donor could prevent the inhibitory effects of CsA. We concluded that CsA decreased intrarenal PGE2 production in stimulated status mainly by decreasing COX-2 expression. NO might play a role in the CsA effect. The results suggested the role possible of PGE2 in CsA nephrotoxicity.

Oxidative stress decreases klotho expression in a mouse kidney cell line

BACKGROUND/AIMS

Defects in klotho gene expression in the mouse result in a syndrome that resembles human aging. We recently identified expression of klotho in a mouse inner medullary collecting duct (mIMCD3) cell line for the first time, and in the present study we explored the physiological relevance of the regulation of klotho expression in the presence of oxidant stress injury.

METHODS

Klotho expression was analyzed by real-time PCR, Western blot, and immuocytochemical staining during exposure to hydrogen peroxide (H2O2). Overexpression of the klotho gene was induced by klotho adenoviruses, and the number of apoptotic cells was counted by flowcytometry.

RESULTS

Oxidant stress injury by H2O2 dose-dependently reduced klotho expression and diminished klotho staining. There were fewer apoptotic cells among the klotho-transfected cells than among the control cells.

CONCLUSION

Klotho is expressed in cell line mIMCD3, and the klotho gene may be involved in the process of oxidative stress injury and apoptosis in this cell line.

Significant responses to platinum-based chemotherapy in renal medullary carcinoma

Most patients with renal medullary carcinoma (RMC) have advanced disease at presentation and rarely respond to radiation or chemotherapy. We describe two adolescents with metastatic disease who had significant responses to cisplatin or carboplatin in combination with gemcitabine and paclitaxel.

Neoplasms of the Renal Medulla: Radiologic-Pathologic Correlation

Tumors of the renal medulla cover a wide spectrum, with characteristic histomorphology and variable biologic profiles. Renal medullary tumors can be categorized into benign and malignant neoplasms based on histologic features and clinico-biologic behavior. They can be further classified into pediatric and adult tumors based on the patient age group. When small, renal medullary tumors may be differentiated from the more common renal adenocarcinomas by their central location and certain demographic characteristics. Although most large malignant medullary tumors demonstrate imaging findings that are indistinguishable from those of other renal malignancies, some tumors demonstrate imaging findings that may suggest a specific diagnosis.

Medullary cystic kidney disease type 1 in a large Native-American kindred

BACKGROUND

Autosomal dominant medullary cystic kidney disease type 1 (MCKD1; Mendelian Inheritance in Man 174000) is a hereditary tubulointerstitial renal disease. For MCKD1, a locus on chromosome 1q21 is published. Although there are characteristic biopsy and imaging findings for MCKD, clinical diagnosis of this disorder is still very difficult because unique phenotypic features are not always present in individual cases.

METHODS

In a large Native-American kindred with apparent autosomal dominant nephropathy, clinical findings in more than 50 individuals were collected and evaluated. Haplotype analysis for 34 individuals was performed.

RESULTS

We report the difficulties establishing the diagnosis of MCKD in a large Native-American kindred solely by means of clinical criteria. This kindred shows a wider range of age of disease onset than previously reported. Gout and hypertension were common, but no patient reported symptoms of salt wasting. By means of haplotype analysis linkage was shown to the MCKD1 locus (logarithm of the odds score, 3.34).

CONCLUSION

Establishing a diagnosis of MCKD solely on clinical findings is difficult because signs and symptoms may be subtle, renal cysts may be absent in more than 50% of affected individuals, and renal histological abnormalities are nonspecific. In patients presenting with renal insufficiency from apparent interstitial disease, a thorough family history and genetic linkage studies are required to establish a diagnosis of MCKD. We suspect MCKD is underdiagnosed and the true incidence of MCKD1 in the general population may be underestimated. No further living-related transplantation should be performed until genetic testing can exclude potentially affected donors.

Activation of the apical Na+/H+ exchanger NHE3 by formate: a basis of enhanced fluid and electrolyte reabsorption by formate in the kidney

Formate stimulates sodium chloride and fluid reabsorption in kidney proximal tubule; however, the exact cellular mechanism of this effect remains unknown. We hypothesized that the primary target of formate is the apical Na(+)/H(+) exchanger. Here, we demonstrate that formate directly enhances the apical Na(+)/H(+) exchanger (NHE3) activity in mouse kidney proximal tubule. In the absence of CO(2)/HCO(3)(-), addition of formate (500 microM) to the bath and lumen of microperfused mouse kidney proximal tubule caused significant intracellular alkalinization, with intracellular pH (pH(i)) increasing from baseline levels 7.17 +/- 0.01 to 7.55 +/- 0.01 (P < 0.001, n = 14), with a Delta pH of 0.38 +/- 0.02. Removal of luminal chloride did not block cell pH alkalinization by formate (baseline pH of 7.26 +/- 0.01 to 7.53 +/- 0.01 with formate, P < 0.001, n = 10), indicating that the apical Cl(-)/OH(-) exchanger was not the primary mediator of the effect of formate on cell pH. However, removal of sodium from the lumen or addition of EIPA completely prevented cell pH alkalinization. Addition of formate to the lumen and bath in the outer medullary collecting duct, which does not express any apical Na(+)/H(+) exchanger, did not cause any cell pH alkalinization. At lower concentrations (50 microM), formate caused significant pH(i) alkalinization in proximal tubule cells, with pH(i) increasing from baseline levels 7.15 +/- 0.02 to 7.36 +/- 0.02 (P < 0.02, n = 11). Acetate, at 50 microM, had no effect on pH(i). Formate's effect was observed both in the absence and presence of CO(2)/HCO(3)(-) in the media. We conclude that formate stimulates the apical Na(+)/H(+) exchanger NHE3 in the kidney proximal tubule. We propose that formate stimulation of chloride reabsorption in the proximal tubule is indirect and is secondary to the activation of apical Na(+)/H(+) exchanger NHE3, which then leads to the stimulation of the apical chloride/base exchanger.

Extracellular ATP-induced calcium signaling in mIMCD-3 cells requires both P2X and P2Y purinoceptors

Kidney tubules are targets for the activation of locally released nucleotides through multiple P2 receptor types. Activation of these P2 receptors modulates cellular Ca(2+) signaling and downstream cellular function. The purpose of this study was to determine whether P2 receptors were present in mIMCD-3 cells, a mouse inner medullary collecting duct cell line, and if so, to examine their link with intracellular Ca(2+) homeostasis. To monitor intracellular Ca(2+) concentration ([Ca(2+)](i)), experiments were conducted using the fluorescent dye fura 2. ATP (0.1-100 microM) produced a dose-dependent increase in [Ca(2+)](i) in a physiological Ca(2+)-containing solution, with an EC(50) of 2.5 microM. The P2-receptor antagonist PPADS reduced the effect of ATP on [Ca(2+)](i), and the P1-receptor agonist adenosine caused only a small increase in [Ca(2+)](i). Preincubation of cells with the phospholipase C antagonist U-73122 blocked the ATP-induced increase in [Ca(2+)](i), indicating P2Y receptors were involved in this process. In a Ca(2+)-free bath solution, thapsigargin and ATP induced intracellular Ca(2+) release from an identical pool. Nucleotides caused an increase in [Ca(2+)](i) in the potency order of UTP = ATP > ATP gamma S > ADP > UDP that is best fitted with the P2Y(2) subtype profile. Although the P2Y agonist UTP induced a similar large transient increase in [Ca(2+)](i) as did ATP, a small but sustained increase in [Ca(2+)](i) occurred only in ATP-stimulated cells, suggesting the role of P2X receptors in Ca(2+) influx. The sustained increase in [Ca(2+)](i) could be blocked by either nonselective cation channel blockers Gd(3+) or P2X antagonists PPADS and PPNDS. Furthermore, when either Gd(3+) or PPNDS was applied to the bath solution before ATP application, the ATP-induced increase in [Ca(2+)](i) was significantly reduced. Both RT-PCR and Western blotting corroborated the presence of P2X(1) and P2Y(2) receptors. These studies demonstrate that mIMCD-3 cells have both P2X and P2Y subtype receptors and that the activation of both P2X and P2Y receptors by extracellular ATP appears to be required to regulate intracellular Ca(2+) signaling.

ANG II reduces net acid secretion in rat outer medullary collecting duct

In rat outer medullary collecting duct (OMCD), the mechanism(s) and regulation of H+ secretion are not understood fully. The effect of changes in acid-base balance and the renin-angiotensin system on net H+ secretion was explored. Rats received NaCl, NaHCO3, NH4Cl, or nothing in their drinking water for 7 days. Total ammonia and total CO2 (JtCO2) fluxes were measured in OMCD tubules perfused in vitro from rats in each treatment group. JtCO2 was reduced in tubules from rats drinking NH4Cl relative to those drinking NaHCO3. Because NH4Cl intake increases plasma renin and aldosterone, we asked if upregulation of the renin-angiotensin system reduces net H+ secretion. Deoxycorticosterone pivalate administered in vivo did not affect JtCO2. However, ANG II given in vivo at 0.1 ng/min reduced JtCO2 by 35%. To determine if ANG II has a direct effect on acid secretion, JtCO2 was measured with ANG II applied in vitro. ANG II (10-8 M) present in the bath solution reduced JtCO2 by 35%. This ANG II effect was not observed in the presence of the AT1 receptor blocker candesartan. In conclusion, in rat OMCD, JtCO2 is paradoxically reduced with NH4Cl ingestion. Increased circulating ANG II, as occurs during metabolic acidosis, reduces JtCO2.

P2Y2 receptor-stimulated release of prostaglandin E2 by rat inner medullary collecting duct preparations

Extracellular nucleotides, acting through the P2Y2 receptor and the associated phosphoinositide-Ca2+ signaling pathway, inhibit AVP-stimulated osmotic water permeability in rat inner medullary collecting duct (IMCD). Because a rise in intracellular Ca2+ is frequently associated with enhanced arachidonic acid metabolism, we examined the effect of activation of the P2Y2 receptor on release of PGE2 in freshly prepared rat IMCD suspensions. Unstimulated IMCD released moderate, but significant, amounts of PGE2, which were more sensitive to cyclooxygenase (COX)-2 than COX-1 inhibition. Agonist activation of P2Y2 receptor by adenosine 5'-O-(3-thiotriphosphate) enhanced release of PGE2 from IMCD in a time- and concentration-dependent fashion. Purinergic-stimulated release of PGE2 was completely blocked by nonspecific COX inhibitors (flurbiprofen and 2-acetoxyphenylhept-2-ynyl sulfide). Differential COX inhibition studies revealed that purinergic-stimulated release of PGE2 was more sensitive to a COX-1-specific inhibitor (valeroyl salicylate) than a COX-2-specific inhibitor (NS-398). Thus purinergic stimulation resulted in significantly more release of PGE2 in the presence of COX-2 inhibitor than COX-1 inhibitor. If it is assumed that increased release of PGE2 is related to its increased production, our results suggest that purinergic stimulation of IMCD results in enhanced production and release of PGE2 in a COX-1-dependent fashion. Because PGE2 is known to affect transport of water, salt, and urea in IMCD, interaction of the purinergic system with the prostanoid system in IMCD can modulate handling of water, salt, and urea by IMCD and, thus, may constitute an AVP-independent regulatory mechanism.

Early transcriptional effects of aldosterone in a mouse inner medullary collecting duct cell line

The mineralocorticoid aldosterone is a major regulator of Na+ and acid-base balance and control of blood pressure. Although the long-term effects of aldosterone have been extensively studied, the early aldosterone-responsive genes remain largely unknown. Using DNA array technology, we have characterized changes in gene expression after 1 h of exposure to aldosterone in a mouse inner medullary collecting duct cell line, mIMCD-3. Results from three independent microarray experiments revealed that the expression of many transcripts was affected by aldosterone treatment. Northern blot analysis confirmed the upregulation of four distinct transcripts identified by the microarray analysis, namely, the serum and glucose-regulated kinase sgk, connective tissue growth factor, period homolog, and preproendothelin. Immunoblot analysis for preproendothelin demonstrated increased protein expression. Following the levels of the four transcripts over time showed that each had a unique pattern of expression, suggesting that the cellular response to aldosterone is complex. The results presented here represent a novel list of early aldosterone-responsive transcripts and provide new avenues for elucidating the mechanism of acute aldosterone action in the kidney.

Towards the identification of (a) gene(s) for autosomal dominant medullary cystic kidney disease

Medullary cystic kidney disease (MCKD) belongs with nephronophthisis (NPH) in a group of inherited tubulo-interstitial nephritis, which has been referred to as the NPH-MCKD complex. Although MCKD and NPH share morphological features, they differ in several respects. The most common variant is recessive juvenile NPH, with onset in childhood and leading to end-stage renal disease (ESRD) within the 2nd decade of life; the most frequent extrarenal involvement is tapeto-retinal degeneration. MCKD is a dominant condition recognized in later life and leading to ESRD at the age of 50 years; hyperuricemia and gout can be associated features. The first sign of MCKD is polyuria; later, the clinical findings relate to renal insufficiency. Originally, NPH and MCKD were considered separate entities. Subsequently, it has been suggested that the two diseases were a single disorder due to the clinico-pathological identity. This unifying conception was later refuted due to the identification of MCKD dominant families. Recently, considerable insight has been gained into the genetics of the NPH-MCKD complex. The majority of juvenile NPH cases are due to deletion of the NPHP1 gene on chromosome 2q13. Genes for infantile and adolescent NPH have been localized respectively to chromosome 9q22-q31 and 3q22. A new locus, NPHP4, has been recently identified on chromosome 1p36. Two genes predisposing to dominant MCKD, MCKD1 and MCKD2, have been localized to chromosome 1q21 and 16p12. Independent confirmation of the locations of MCKD1 and MCKD2 in other MCKD families, with or without hyperuricemia and gout, has been reported. The gene for familial juvenile hyperuricemic nephropathy (FJHN), a phenotype that is very similar to MCKD, was recently mapped to 16p12, in a region overlapping with the MCKD2 locus, raising the question as to whether MCKD2 and FJHN are allelic variants of the same disease entity. The ultimate proof of the allelism between MCKD2 and FJHN will be provided by the identification of the responsible gene(s). Identification and characterization of the MCKD and FJHN genes will help to clarify the pathogenesis and classification of hereditary tubulo-interstitial nephritides.

Calcium oxalate precipitates in a renomedullary interstitial cell tumor

We report a case of calcium oxalate deposition in a renomedullary interstitial cell tumor (RICT) in a patient dying of full-blown AIDS. The precipitates showed birefringence using a partially polaris ed light and were stained black in Yasue's silver nitrate-rubeanic acid method. The combination of calcium oxalosis and RICT has not been reported before and might possibly be due to systemic biochemical alterations of the glycosaminoglycans as a result of profound metabolic disturbances in AIDS patients.

C-peptide stimulates Na+,K+-ATPase activity via PKC alpha in rat medullary thick ascending limb

AIMS/HYPOTHESIS

C-peptide, the cleavage product of proinsulin processing exerts physiological effects including stimulation of Na(+),K(+)-ATPase in erythrocytes and renal proximal tubules. This study was undertaken to assess the physiological effects of connecting peptide on Na(+),K(+)-ATPase activity in the medullary thick ascending limb of Henle's loop.

METHODS

Na(+),K(+)-ATPase activity was measured as the ouabain-sensitive generation of (32)Pi from gamma[(32)P]-ATP and (86)Rb uptake on isolated rat medullary thick ascending limbs. The cell-surface expression of Na(+),K(+)-ATPase was evaluated by Western blotting of biotinylated proteins, and its phosphorylation amount was measured by autoradiography. The membrane-associated fraction of protein kinase C isoforms was evaluated by Western blotting.

RESULTS

Rat connecting peptide concentration-dependently stimulated Na(+),K(+)-ATPase activity with a threshold at 10(-9) mol/l and a maximal effect at 10(-7) mol/l. C-peptide (10(-7) mol/l) already stimulates Na(+),K(+)-ATPase activity after 5 min with a plateau from 15 to 60 min. C-peptide (10(-7) mol/l) stimulated Na(+),K(+)-ATPase activity and (86)Rb uptake to the same extent, but did not alter Na(+),K(+)-ATPase cell surface expression. The stimulation of Na(+),K(+)-ATPase activity was associated with an increase in Na(+),K(+)-ATPase alpha-subunit phosphorylation and both effects were abolished by a specific protein kinase C inhibitor. Furthermore, connecting peptide induced selective membrane translocation of PKC-alpha.

CONCLUSION/INTERPRETATION

This study provides evidence that in rat medullary thick ascending limb, C-peptide stimulates Na(+),K(+)-ATPase activity within a physiological concentration range. This effect is due to an increase in Na(+),K(+)-ATPase turnover rate that is most likely mediated by protein kinase C-alpha phosphorylation of the Na(+),K(+)-ATPase alpha-subunit, suggesting that C-peptide could control Na(+) reabsorption during non-fasting periods.

Modulation of peripheral-type benzodiazepine receptor levels in a reperfusion injury pig kidney-graft model

BACKGROUND

Ischemia-reperfusion injury is associated with an increased risk of acute rejection, delayed graft function, or chronic graft dysfunction. Mitochondria play a central role in this process.

METHODS

Using an autotransplantation pig kidney model, both early (40 min and 7 days) and late (2-16 weeks) changes in renal function and morphology were determined after different periods of cold ischemia in University of Wisconsin or Euro-Collins solutions. We have also investigated the expression of the peripheral-type benzodiazepine receptor (PBR), which is also critical in maintaining outer mitochondrial membrane stability.

RESULTS

Function of the kidneys was better preserved after 1 hr and 24 hr than after 48 hr and 72 hr in Euro-Collins and University of Wisconsin solutions. Medulla injury was reduced in 1 hr-preserved and 24 hr-preserved groups. PBR was found to be present in epithelial cells of the deep cortical and outer medulla in both normal human and well-preserved pig kidneys. PBR expression was modulated by ischemia-reperfusion injury and the concurrent tubular injury and repair processes.

CONCLUSION

This study indicates that PBR expression correlates with the quality of kidney preservation and might serve as an index of kidney and mitochondria viability. Moreover, these data suggest that PBR might be involved in membrane biogenesis during reperfusion. In addition, considering the identical localization of PBR in human and pig kidneys, these findings could have a direct application in human clinical settings of kidney pathology.

Electrolyte and fluid secretion by cultured human inner medullary collecting duct cells

Inner medullary collecting ducts (IMCD) are the final nephron segments through which urine flows. To investigate epithelial ion transport in human IMCD, we established primary cell cultures from initial (hIMCD(i)) and terminal (hIMCD(t)) inner medullary regions of human kidneys. AVP, PGE(2), and forskolin increased cAMP in both hIMCD(i) and hIMCD(t) cells. The effects of AVP and PGE2 were greatest in hIMCD(i); however, forskolin increased cAMP to the same extent in hIMCD(i) and hIMCD(t). Basal short-circuit current (I(SC)) of hIMCD(i) monolayers was 1.4 +/- 0.5 microA/cm2 and was inhibited by benzamil, a Na+ channel blocker. 8-Bromo-cAMP, AVP, PGE(2), and forskolin increased I(SC); the current was reduced by blocking PKA, apical Cl- channels, basolateral NKCC1 (a Na+ - K+ - 2Cl- cotransporter), and basolateral Cl-/HCO(3)(-) exchangers. In fluid transport studies, hIMCD(i) monolayers absorbed fluid in the basal state and forskolin reversed net fluid transport to secretion. In hIMCD(t) monolayers, basal current was not different from zero and cAMP had no effect on I(SC). We conclude that AVP and PGE2 stimulate cAMP-dependent Cl- secretion by hIMCD(i) cells, but not hIMCD(t) cells, in vitro. We suggest that salt secretion at specialized sites along human collecting ducts may be important in the formation of the final urine.

Contribution of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) to transepithelial transport of H(+), NH(4)(+), K(+), and Na(+) in rat outer medullary collecting duct

In rat kidney, the "secretory" isoform of the Na-K-Cl cotransporter, NKCC1 (BSC-2), localizes to the basolateral membrane of the alpha intercalated cell, the acid secreting cell of the outer medullary collecting duct (OMCD). This laboratory has reported that NKCC1 mediates Cl(-) uptake across the basolateral membrane in series with Cl(-) secretion across the apical membrane in rat OMCD. NKCC1 transports NH(4)(+), K(+), and Na(+) as well as Cl(-); therefore, a role for the cotransporter in the process of HCl, NH(4)Cl, KCl, and NaCl secretion has been suggested. Thus, it was determined if bumetanide, an inhibitor of NKCC1, alters transepithelial cation transport in rat OMCD. OMCD tubules from deoxycorticosterone pivalate (DOCP)-treated rats were perfused in vitro. Hydration of CO(2), rather than NH(4)(+), provides the principle source of H(+) for net acid secretion. In HCO(3)(-)/CO(2)-buffered solutions, no effect of bumetanide on net K(+) flux was detected. Under some conditions, bumetanide addition resulted in a small reduction in secretion of net H(+) equivalents. Transepithelial Na(+) flux, J(Na), was -1.5 +/- 1.7 pmol/mm per min, values not different from zero. However, with the application of bumetanide to the bath, J(Na) was +5.2 +/- 1.3 pmol/mm per min (P < 0.05), which indicates net Na(+) absorption. In conclusion, inhibition of NKCC1 in rat OMCD changes transepithelial movement of Na(+) and Cl(-). The role of NKCC1 in the secretion of net H(+) equivalents is small.

Cl- channels in basolateral TAL membranes. XVI. MTAL and CTAL cells each contain the mRNAs encoding mmClC-Ka and mcClC-Ka

BACKGROUND

Our prior data indicate that two separate but homologous basolateral chloride (Cl-) channels, mmClC-Ka and mcClC-Ka, are the principal mediators of net Cl- absorption in mouse medullary thick ascending limb (MTAL) and cortical thick ascending limb (CTAL) cells, respectively. In the present studies, we evaluated the possibility that there might be translational or post-translational suppression of mmClC-Ka and mcClC-Ka activity in CTAL and MTAL cells, respectively.

METHODS

Polymerase chain reaction (PCR) fragments were prepared that were highly specific for either mmClC-Ka or mcClC-Ka, the cDNAs encoding mmClC-Ka and mcClC-Ka, respectively.

RESULTS

Using reverse transcription (RT)-PCR with these highly specific products, mRNAs specific for non-homologous channel sequences in either mmClC-Ka or mcClC-Ka were present in both MTAL and CTAL cells.

CONCLUSIONS

Both mouse MTAL and CTAL cells contain the mRNAs encoding mmClC-Ka and mcClC-Ka. There may be translational or post-translational suppression of mmClC-Ka activity in CTAL cells, and of mcClC-Ka activity in MTAL cells.

Renal handling of NH4+ in relation to the control of acid-base balance by the kidney

The major component of urinary acid excretion is NH4+. To be appropriately excreted in urine, NH4+ must be synthesized by proximal tubular cells, secreted into the proximal tubular fluid, reabsorbed by the medullary thick ascending limb (MTAL) to be accumulated in the medullary interstitium, and finally secreted in medullary collecting ducts. Each step of this renal pathway is highly regulated and, in addition to acute events mediated by peptide hormones such as parathyroid hormone, the control of gene expression explains how the renal handling of NH4+ fully adapts to chronic changes in the acid-base status. Several targets have been identified at the gene expression level to account for the adaptation of renal NH4+ synthesis and transport in response to an acid load. These are the key enzymes of ammoniagenesis (mitochondrial glutaminase and glutamate dehydrogenase) and gluconeogenesis (phosphoenolpyruvate carboxykinase) in the proximal tubule, the apical Na(+)-K+(NH4+)-2Cl- cotransporter of the MTAL, and the basolateral Na(+)-K+(NH4+)-2Cl- cotransporter of medullary collecting ducts. At least two factors control the expression of these genes during metabolic acidosis: an acid pH and glucocorticoids, which appear to act in concert to coordinate the adaptation of various tubular cell types. The present review focuses on some aspects of these regulations that have been recently elucidated.

Coupling of vasopressin-induced intracellular Ca2+ mobilization and apical exocytosis in perfused rat kidney collecting duct

Arginine vasopressin (AVP) regulates the osmotic water permeability of the kidney collecting duct by inducing exocytotic insertion of aquaporin-2 into apical membrane. The coupling between AVP-induced intracellular Ca2+ mobilization and apical exocytosis was investigated in isolated perfused rat inner medullary collecting duct (IMCD) segments using confocal fluorescence microscopy. Changes of [Ca2+]i in IMCD cells were measured with fluo-4. A novel confocal imaging technique using a styryl dye, FM1-43, was developed to monitor real-time exocytosis induced by arginine vasopressin. AVP (0.1 nM) triggered a rapid increase of [Ca2+]i in IMCD cells, followed by sustained oscillations. Ratiometric measurement of [Ca2+]i confirmed that the observed [Ca2+]i oscillation was a primary event and was not secondary to changes in cell volume. The frequencies of [Ca2+]i oscillations in each IMCD cell were independent and time variant. 1-Deamino-8-D-arginine vasopressin (a V2 receptor agonist, 0.1 nM) simulated the effects of AVP by triggering [Ca2+]i oscillations. In the absence of extracellular Ca2+, ryanodine (0.1 mM) inhibited AVP-induced Ca2+ mobilization. AVP (0.1 nM) triggered accumulative apical exocytosis in IMCD cells within 20 s after application. Pre-incubating the IMCD with an intracellular Ca2+ chelator, BAPTA, prevented AVP-induced intracellular Ca2+ mobilization, apical exocytosis, and increase of osmotic water permeability. These results indicate that AVP, via the V2 receptor, triggers a calcium signalling cascade observed as [Ca2+]i oscillations in the IMCD and that intracellular Ca2+ mobilization is required for exocytotic insertion of aquaporin-2.

Confirmation of a gene locus for medullary cystic kidney disease (MCKD2) on chromosome 16p12

BACKGROUND

Autosomal-dominant medullary cystic kidney disease (MCKD) is an interstitial nephropathy characterized by structural renal tubular defects that result in salt wasting and a reduction in urinary concentration. The condition has clinical and morphological similarities to autosomal-recessive juvenile nephronophthisis. Two genes predisposing to MCKD have been localized. MCKD1 on chromosome 1q21 was localized in two Cypriot families, and MCKD2 on chromosome 16p12 was localized in a single Italian family. We have evaluated a large Welsh MCKD family for linkage at these two loci.

METHODS

Clinical data and DNA samples were collected from affected family members. Polymorphic microsatellite markers spanning the critical regions on chromosome 1 and chromosome 16 that encompass MCKD1 and MCKD2 were analyzed. Two-point and multipoint LOD scores were calculated.

RESULTS

The family fulfilled previously published criteria for the diagnosis of MCKD, but hyperuricemia and gout were not prominent features. Twenty-one affected individuals were identified. Mean age at death or end-stage renal disease was 47 years (37 to 60). Linkage and haplotype analysis generated strongly negative results at MCKD1 on chromosome 1q21 (two-point LOD score = -5.32). Strong evidence of linkage to MCKD2 was generated with a maximum multi-point LOD score of 3.75.

CONCLUSION

These results provide the first independent confirmation of a gene predisposing to MCKD on chromosome 16p12 and indicate that mutation of this gene is not restricted to a single family or population. The absence of hyperuricemia and gout in our family indicates that these are not obligatory features of MCKD2 mutations.

Further evidence for linkage of autosomal-dominant medullary cystic kidney disease on chromosome 1q21

BACKGROUND

Autosomal-dominant medullary cystic kidney disease (ADMCKD) is characterized by the development of cysts at the corticomedullary border of the kidneys. It resembles nephronophthisis (NPH) with an autosomal-recessive mode of inheritance. Genetic linkage has been shown either on chromosome 1q21 (ADMCKD1) or 16p12 (ADMCKD2), and families exist who are not linked to the aforementioned loci. No disease-causing gene underlying this disorder has been reported.

METHODS

The Finnish Transplantation Register and hospital records were searched to identify all of the ADMCKD families in the Finnish population. Detailed clinical information of the patients was collected. Linkage analysis was used to study whether the Finnish families originating from a homogeneous population showed genetic linkage to the ADMCKD1 or ADMCKD2 loci. Also, the coding region of a strong candidate gene, natriuretic peptide receptor A (NPRA), located on the chromosome 1q21 critical region, was sequenced using polymerase chain reaction sequencing with an ABI 377XL Automated DNA sequencer (Applera Corp., Norwalk, CT, USA).

RESULTS

Five of the six families showed linkage to the previously identified region of chromosome 1q21. Family 6 with hyperuricemia as a prominent clinical feature was linked to neither of the ADMCKD loci. Wide interfamiliar and intrafamiliar variability in the clinical picture of the patients was detected. The NPRA gene mutation was excluded as a causative gene by sequencing.

CONCLUSION

This study locates the gene for ADMCKD1 close to a marker D1S1595 in a region <5 cM, and further confirms the existence of at least three loci for the medullary cystic kidney disease. Heterogeneity of the symptoms complicates the clinical diagnosis and classification of the patients. Further studies are needed to identify the disease-causing gene.

Alpha-2 and beta-adrenergic receptors mediate NE's biphasic effects on rat thick ascending limb chloride flux

The sympathetic neurotransmitter norepinephrine (NE) influences renal sodium excretion via activation of adrenergic receptors. The thick ascending limb (THAL) possesses both alpha-2 and beta-adrenergic receptors. However, the role(s) different adrenergic receptors play in how isolated THALs respond to NE are unclear. We tested the hypothesis that both alpha-2 and beta-adrenergic receptors are responsive to NE in the isolated THAL, with alpha-2 receptors inhibiting and beta-receptors stimulating chloride flux (J(Cl)). THALs from male Sprague-Dawley rats were perfused in vitro, and the effects of 1) incremental NE, 2) the alpha-2 agonist clonidine, and 3) the beta-agonist isoproterenol on J(Cl) were measured. Low concentrations (0.1 nM) of NE decreased J(Cl) from a rate of 114.2 +/- 8.1 to 93.5 +/- 14.6 pmol. mm(-1). min(-1) (P < 0.05), with the nadir occurring at 1 nM (67.7 +/- 8.8 pmol. mm(-1). min(-1); P < 0.05). In contrast, greater concentrations of NE significantly increased J(Cl) from the nadir to a maximal rate of 131.0 +/- 28.5 pmol. mm(-1). min(-1) at 10 microM (P < 0.05). To evaluate the adrenergic receptors mediating these responses, the THAL J(Cl) response to NE was measured in the presence of selective antagonists of beta- and alpha-2 receptors. A concentration of NE (1 microM), which alone tended to increase J(Cl), decreased THAL J(Cl) (from 148.9 +/- 16.4 to 76.2 +/- 13.6 pmol. mm(-1). min(-1); P < 0.01) in the presence of the beta-antagonist propranolol. In contrast, a concentration of NE (0.1 microM), which alone tended to decrease J(Cl), increased THAL J(Cl) (from 85.5 +/- 20.1 to 111.8 +/- 20.1 pmol. mm(-1). min(-1); P < 0.05) in the presence of the alpha-2 antagonist rauwolscine. To further clarify the role of different adrenergic receptors, selective adrenergic agonists were used. The alpha-2 agonist clonidine decreased J(Cl) from 102.4 +/- 9.9 to 54.0 +/- 15.7 pmol. mm(-1). min(-1), a reduction of 49.1 +/- 11.0% (P < 0.02). In contrast, the beta-agonist isoproterenol stimulated J(Cl) from 95.3 +/- 11.6 to 144.1 +/- 15.0 pmol. mm(-1). min(-1), an increase of 56 +/- 14% (P < 0.01). We conclude that 1) the sympathetic neurotransmitter NE exerts concentration-dependent effects on J(Cl) in the isolated rat THAL, 2) selective alpha-2 receptor activation inhibits THAL J(Cl), and 3) selective beta-receptor activation stimulates THAL J(Cl). These data indicate the response elicited by the isolated rat THAL to NE is dependent on the neurotransmitter concentration, such that application of NE in vitro biphasically modulates J(Cl) via differential activation of alpha-2 and beta-adrenergic receptors in a concentration-dependent manner.

Influence of dehydration on glycerophosphorylcholine and choline distribution along the rat nephron

Glycerophosphorylcholine is one of the four major organic osmolytes in renal medullary cells, changing their intracellular osmolyte concentration in parallel with extracellular tonicity during cellular osmoadaptation. In this study, the tubular content of glycerophosphorylcholine was quantified in untreated and 48-h-dehydrated male rats. A chemiluminescence ultra-micromethod was developed to measure choline at the picomolar level in single tubules microdissected from collagenase-treated kidneys. The glycerophosphorylcholine level was calculated as the difference between total choline after acid hydrolysis and the free tubular choline content. In accordance with the glycerophosphorylcholine distribution pattern in different renal zones of untreated rats, low amounts of glycerophosphorylcholine were found in all cortical and outer medullary structures (< 35 pmol/mm), whereas increasing amounts were detected towards the papillary tip (163 pmol/mm). As a percentage of total choline, the level of free tubular choline varied from 4.2% in outer medullary proximal tubules to 30.3% in the inner medullary collecting ducts adjacent to the outer medulla (IMCD1). Antidiuresis led to a nearly twofold increase in glycerophosphorylcholine content in papillary collecting ducts. The osmolality-dependent regulation of organic osmolytes in single microdissected tubules has been demonstrated for the first time. Furthermore, the high tubular glycerophosphorylcholine concentration compared to sorbitol and myo-inositol emphasizes the predominance of glycerophosphorylcholine in the inner medulla and papilla of the rat kidney.

Collecting duct carcinoma of the kidney: are imaging findings suggestive of the diagnosis?

OBJECTIVE

Collecting duct carcinoma derives from the renal medulla and has an infiltrative growth pattern at pathologic examination. The purpose of our study was to characterize the imaging features of this aggressive malignancy and determine whether the diagnosis can be reliably suggested from imaging findings.

MATERIALS AND METHODS

Radiologic studies from 17 patients with pathologically proven collecting duct carcinoma were analyzed by two reviewers.

RESULTS

The tumors varied in size from 1.5 to 19 cm (mean, 7.7 cm). Medullary involvement was present on CT in 16 (94%) of 17 cases, but cortical involvement or an exophytic component was also present in 15 cases (88%) and 10 cases (59%), respectively. The reniform contour of the kidney was preserved in seven cases (41%) and correlated with a smaller tumor size (p<0.01). Tumors showed an infiltrative appearance on CT in 11 cases (65%), but an expansile component was also present in eight of these cases. A cystic component was present on CT in six (35%) of 17 cases. On sonography, the solid tumor component was hyperechoic to normal renal parenchyma in six of seven cases and isoechoic in the other. On MR imaging, all tumors (4/4) were hypointense on T2-weighted imaging. On urography, all lesions (5/5) distorted the intrarenal collecting system. On angiography, all tumors (3/3) were hypovascular.

CONCLUSION

Medullary involvement and an infiltrative appearance are common findings on cross-sectional imaging and may suggest the diagnosis of collecting duct carcinoma. In large tumors, however, these features are frequently overshadowed by an exophytic or expansile component that cannot be distinguished from the more common cortical renal cell carcinoma.

Urea sensitizes mIMCD3 cells to heat shock-induced apoptosis: protection by NaCl

Urea, with NaCl, constitutes the osmotic gradient that allows water reabsorption in mammalian kidneys. Because NaCl induces heat shock proteins, we tested the responses to heat shock of mIMCD3 cells adapted to permissive urea and/or NaCl concentrations. We found that heat-induced cell death was stronger after adaptation to 250 mM urea. This effect was reversible, dose dependent, and, interestingly, blunted by 125 mM NaCl. Moreover, we have shown that urea-adapted cells engaged in an apoptotic pathway upon heat shock, as shown by DNA laddering. This sensitization is not linked to a defect in the heat shock response, because the induction of HSP70 was similar in isotonic and urea-adapted cells. Moreover, it is not linked to the presence of urea inside cells, because washing urea away did not restore heat resistance and because applying urea and heat shock at the same time did not lead to heat sensitivity. Together, these results suggest that urea modifies the heat shock response, leading to facilitated apoptosis.

Molecular characterization of a novel urea transporter from kidney inner medullary collecting ducts

In the terminal part of the kidney collecting duct, rapid urea reabsorption is essential to maintaining medullary hypertonicity, allowing maximal urinary concentration to occur. This process is mediated by facilitated urea transporters on both apical and basolateral membranes. Our previous studies have identified three rat urea transporters involved in the urinary concentrating mechanism, UT1, UT2 and UT3, herein renamed UrT1-A, UrT1-B, and UrT2, which exhibit distinct spatial distribution in the kidney. Here we report the molecular characterization of an additional urea transporter isoform, UrT1-C, from rat kidney that encodes a 460-amino acid residue protein. UrT1-C has 70 and 62% amino acid identity to rat UrT1-B and UrT2 (UT3), respectively, and 99% identity to a recently reported rat isoform (UT-A3; Karakashian A, Timmer RT, Klein JD, Gunn RB, Sands JM, and Bagnasco SM. J Am Soc Nephrol 10: 230-237, 1999). We report the anatomic distribution of UrT1-C in the rat kidney tubule system as well as a detailed functional characterization. UrT1-C m RNA is primarily expressed in the deep part of the inner medulla. When expressed in Xenopus laevis oocytes, UrT1-C induced a 15-fold stimulation of urea uptake, which was inhibited almost completely by phloretin (0.7 mM) and 60-95% by thiourea analogs (150 mM). The characteristics are consistent with those described in perfusion studies with inner medullary collecting duct (IMCD) segments, but, contrary to UrT1-A, UrT1-C-mediated urea uptake was not stimulated by activation of protein kinase A. Our data show that UrT1-C is a phloretin-inhibitable urea transporter expressed in the terminal collecting duct that likely serves as an exit mechanism for urea at the basolateral membrane of IMCD cells.

Vasopressin-stimulated chloride transport in transimmortalized mouse cell lines derived from the distal convoluted tubule and cortical and inner medullary collecting ducts

BACKGROUND

The fine control of NaCl absorption takes place in the distal parts of the renal tubule, but the regulation of Cl(-) transport in this region has not been fully elucidated. We have analysed the effects of dD-arginine vasopressin (dDAVP) on Cl(-) fluxes in cultured mouse distal convoluted tubule (mpkDCT), cortical collecting duct (mpkCCD) and inner medullary collecting duct (mpkIMCD) cell lines.

METHODS

RT-PCR and Western blotting were used to detect the amiloride-sensitive sodium channel (ENaC) and cystic fibrosis transmembrane conductance regulator (CFTR) mRNAs and protein in cultured mpkDCT, mpkCCD and mpkIMCD cells. Cl(-) fluxes were analysed by measuring the short-circuit current (I(sc)) and bidirectional (36)Cl(-) fluxes on confluent cells grown on filters.

RESULTS

All three cell lines expressed ENaC and CFTR and had I(sc) stimulated by dDAVP. The rise in I(sc) caused by dDAVP (10(-8) M) was inhibited by amiloride, and to a lesser extent by 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) in all three cell lines. The dDAVP-dependent I(sc) measured under apical Na(+)-free condition was reduced by Cl(-) channel blockers with a profile (NPPB>glibenclamide>DIDS), similar to that for rat CFTR. dDAVP stimulated the apical-to-basal (36)Cl(-) flux and to a lesser extent the basal-to-apical (36)Cl(-) flux under open-circuit condition in all three cultured cell lines. Adding NPPB to the apical side reduced the basal-to-apical (36)Cl(-) flux but not the opposite (36)Cl(-) flux from dDAVP-treated cells.

CONCLUSION

These results indicate that dDAVP stimulates the bi-directional flux of Cl(-), resulting in net Cl(-)absorption, in these cultured mouse distal and collecting duct cells. I(sc) experiments also suggest the presence of a minor component of electrogenic Cl(-) secretion, possibly mediated by CFTR.