Vivek Bhalla

Renal sympathetic denervation in patients with treatment-resistant hypertension (The Symplicity HTN-2 Trial): a randomised controlled trial.

BACKGROUND Activation of renal sympathetic nerves is key to pathogenesis of essential hypertension. We aimed to assess effectiveness and safety of catheter-based renal denervation for reduction of blood pressure in patients with treatment-resistant hypertension. METHODS In this multicentre, prospective, randomised trial, patients who had a baseline systolic blood pressure of 160 mm Hg or more (≥150 mm Hg for patients with type 2 diabetes), despite taking three or more antihypertensive drugs, were randomly allocated in a one-to-one ratio to undergo renal denervation with previous treatment or to maintain previous treatment alone (control group) at 24 participating centres. Randomisation was done with sealed envelopes. Data analysers were not masked to treatment assignment. The primary effectiveness endpoint was change in seated office-based measurement of systolic blood pressure at 6 months. Primary analysis included all patients remaining in follow-up at 6 months. This trial is registered with ClinicalTrials.gov, number NCT00888433. FINDINGS 106 (56%) of 190 patients screened for eligibility were randomly allocated to renal denervation (n=52) or control (n=54) groups between June 9, 2009, and Jan 15, 2010. 49 (94%) of 52 patients who underwent renal denervation and 51 (94%) of 54 controls were assessed for the primary endpoint at 6 months. Office-based blood pressure measurements in the renal denervation group reduced by 32/12 mm Hg (SD 23/11, baseline of 178/96 mm Hg, p<0·0001), whereas they did not differ from baseline in the control group (change of 1/0 mm Hg [21/10], baseline of 178/97 mm Hg, p=0·77 systolic and p=0·83 diastolic). Between-group differences in blood pressure at 6 months were 33/11 mm Hg (p<0·0001). At 6 months, 41 (84%) of 49 patients who underwent renal denervation had a reduction in systolic blood pressure of 10 mm Hg or more, compared with 18 (35%) of 51 controls (p<0·0001). We noted no serious procedure-related or device-related complications and occurrence of adverse events did not differ between groups; one patient who had renal denervation had possible progression of an underlying atherosclerotic lesion, but required no treatment. INTERPRETATION Catheter-based renal denervation can safely be used to substantially reduce blood pressure in treatment-resistant hypertensive patients. FUNDING Ardian.

Mechanisms of ENaC Regulation and Clinical Implications

The epithelial Na+ channel (ENaC) transports Na+ across tight epithelia, including the distal nephron. Different paradigms of ENaC regulation include extrinsic and intrinsic factors that affect the expression, single-channel properties, and intracellular trafficking of the channel. In particular, recent discoveries highlight new findings regarding proteolytic processing, ubiquitination, and recycling of the channel. Understanding the regulation of this channel is critical to the understanding of various clinical phenomena, including normal physiology and several diseases of kidney and lung epithelia, such as blood pressure (BP) control, edema, and airway fluid clearance. Significant progress has been achieved in this active field of research. Although ENaC is classically thought to be a mediator of BP and volume status through Na+ reabsorption in the distal nephron, several studies in animal models highlight important roles for ENaC in lung pathophysiology, including in cystic fibrosis. The purpose of this review is to highlight the various modes and mechanisms of ENaC regulation, with a focus on more recent studies and their clinical implications.

AMP-activated kinase inhibits the epithelial Na- channel through functional regulation of the ubiquitin ligase Nedd4-2

We recently found that the metabolic sensor AMP-activated kinase (AMPK) inhibits the epithelial Na+ channel (ENaC) through decreased plasma membrane ENaC expression, an effect requiring the presence of a binding motif in the cytoplasmic tail of the β-ENaC subunit for the ubiquitin ligase Nedd4-2. To further examine the role of Nedd4-2 in the regulation of ENaC by AMPK, we studied the effects of AMPK activation on ENaC currents in Xenopus oocytes co-expressing ENaC and wild-type (WT) or mutant forms of Nedd4-2. ENaC inhibition by AMPK was preserved in oocytes expressing WT Nedd4-2 but blocked in oocytes expressing either a dominant-negative (DN) or constitutively active (CA) Nedd4-2 mutant, suggesting that AMPK-dependent modulation of Nedd4-2 function is involved. Similar experiments utilizing WT or mutant forms of the serum- and glucocorticoid-regulated kinase (SGK1), modulators of protein kinase A (PKA), or extracellular-regulated kinase (ERK) did not affect ENaC inhibition by AMPK, suggesting that these pathways known to modulate the Nedd4-2-ENaC interaction are not responsible. AMPK-dependent phosphorylation of Nedd4-2 expressed in HEK-293 cells occurred both in vitro and in vivo, suggesting a potential mechanism for modulation of Nedd4-2 and thus cellular ENaC activity. Moreover, cellular AMPK activation significantly enhanced the interaction of the β-ENaC subunit with Nedd4-2, as measured by co-immunoprecipitation assays in HEK-293 cells. In summary, these results suggest a novel mechanism for ENaC regulation in which AMPK promotes ENaC-Nedd4-2 interaction, thereby inhibiting ENaC by increasing Nedd4-2-dependent ENaC retrieval from the plasma membrane. AMPK-dependent ENaC inhibition may limit cellular Na+ loading under conditions of metabolic stress when AMPK becomes activated.

Melamine nephrotoxicity: an emerging epidemic in an era of globalization

Recent outbreaks of nephrolithiasis and acute kidney injury among children in China have been linked to ingestion of milk-based infant formula contaminated with melamine. These cases provide evidence in humans for the nephrotoxicity of melamine, which previously had been described only in animals. The consequences of this outbreak are already severe and will likely continue to worsen. Herein we summarize the global impact of the melamine milk contamination, the reemergence of melamine-tainted animal feed, and potential mechanisms of melamine nephrotoxicity. Large-scale epidemiologic studies are necessary to further characterize this disease and to assess its potential long-term sequelae. This epidemic of environmental kidney disease highlights the morbidity associated with adulterated food products available in todays global marketplace and reminds us of the unique vulnerability of the kidney to environmental insults. Melamine is the latest in a growing list of diverse potentially toxic compounds about which nephrologists and other health-care providers responsible for the diagnosis and management of kidney disease must now be aware.

Recurrent and de novo diabetic nephropathy in renal allografts.

Background. Histologic findings of diabetic nephropathy (DN) are observed in allografts of patients with pretransplant (PreTx) diabetes mellitus (DM) and in patients who develop DM posttransplant (PostTx). Patients with allograft biopsies (Bx) were retrospectively studied to determine the incidence of recurrent and de novo DN and to ascertain what, if any, risk factors predispose to histologic DN in either patient population. Methods. From the renal transplant services at four hospitals from 1992 to 2000, the authors identified all patients with PreTxDM and PostTxDM (n=81). Those with renal biopsies performed ≥18 months PostTx were classified according to the presence or absence of histologic DN (Bx-positive, n=23; Bx-negative, n=35). Patients were then subdivided into four categories—recurrent DN (n=16), de novo DN (n=7), no recurrent DN (n=27), and no de novo DN (n=8)—for analyses. Results. Among these 58 patients, 74.1% had PreTx and 25.9% had PostTx diabetes. Of those with histologic DN, 69.6% were recurrent DN and 30.4% were de novo DN, making de novo DN at least as likely to develop as recurrent DN. After the onset of diabetes in the de novo population, the time to development of histologic DN was similar in the recurrent and the de novo patients (6.68±3.86 years vs. 5.90±3.13 years, P =0.66) and more rapid than previously reported. Apart from a more frequent family history of hypertension in patients with allograft DN compared with those without allograft DN, known risk factors for the development of native DN did not significantly differ among patients in the four cohorts. Proposed risk factors related to transplantation did not correlate with the development of recurrent or de novo DN. Conclusion. Among patients with histologic DN, de novo DN occurred at least as frequently as recurrent DN, and the time to onset of histologically apparent DN was more rapid than previously reported. Neither the usual clinical predictors of DN nor clinical variables related to transplantation clearly distinguished the group with DN from the group without it, potentially implicating novel mechanisms in its pathogenesis.

Racial/ethnic differences in the prevalence of proteinuric and nonproteinuric diabetic kidney disease.

OBJECTIVE To examine racial/ethnic differences in the prevalence of diabetic kidney disease (DKD), with and without proteinuria, in an outpatient health care organization. RESEARCH DESIGN AND METHODS We examined electronic health records for 15,683 persons of non-Hispanic white (NHW), Asian (Asian Indian, Chinese, and Filipino), Hispanic, and non-Hispanic black (NHB) race/ethnicity with type 2 diabetes and no prior history of kidney disease from 2008 to 2010. We directly standardized age- and sex-adjusted prevalence rates of proteinuric DKD (proteinuria with or without low estimated glomerular filtration rate [eGFR]) or nonproteinuric DKD (low eGFR alone). We calculated sex-specific odds ratios of DKD in racial/ethnic minorities (relative to NHWs) after adjustment for traditional DKD risk factors. RESULTS Racial/ethnic minorities had higher rates of proteinuric DKD than NHWs (24.8–37.9 vs. 24.8%) and lower rates of nonproteinuric DKD (6.3–9.8 vs. 11.7%). On adjusted analyses, Chinese (odds ratio 1.39 for women and 1.56 for men), Filipinos (1.57 for women and 1.85 for men), Hispanics (1.46 for women and 1.34 for men), and NHBs (1.50 for women) exhibited significantly (P < 0.01) higher odds of proteinuric DKD than NHWs. Conversely, Chinese, Hispanic, and NHB women and Hispanic men had significantly lower odds of nonproteinuric DKD than NHWs. CONCLUSIONS We found novel racial/ethnic differences in DKD among patients with type 2 diabetes. Racial/ethnic minorities were more likely to have proteinuric DKD and less likely to have nonproteinuric DKD. Future research should examine diverse DKD-related outcomes by race/ethnicity to inform targeted prevention and treatment efforts and to explore the etiology of these differences.

Phosphopeptide screen uncovers novel phosphorylation sites of Nedd4-2 that potentiate its inhibition of the epithelial Na+ channel.

The E3 ubiquitin ligase Nedd4-2 regulates several ion transport proteins, including the epithelial Na+ channel (ENaC). Nedd4-2 decreases apical membrane expression and activity of ENaC. Although it is subject to tight hormonal control, the mechanistic basis of Nedd4-2 regulation remains poorly understood. To characterize regulatory inputs to Nedd4-2 function, we screened for novel sites of Nedd4-2 phosphorylation using tandem mass spectrometry. Three of seven identified Xenopus Nedd4-2 Ser/Thr phosphorylation sites corresponded to previously identified target sites for SGK1, whereas four were novel, including Ser-293, which matched the consensus for a MAPK target sequence. Further in vitro and in vivo phosphorylation experiments revealed that Nedd4-2 serves as a target of JNK1, but not of p38 MAPK or ERK1/2. Additional rounds of tandem mass spectrometry identified two other phosphorylated residues within Nedd4-2, including Thr-899, which is present within the catalytic domain. Nedd4-2 with mutations at these sites had markedly inhibited JNK1-dependent phosphorylation, virtually no ENaC inhibitory activity, and significantly reduced ubiquitin ligase activity. These data identify phosphorylatable residues that activate Nedd4-2 and may work together with residues targeted by inhibitory kinases (e.g. SGK1 and protein kinase A) to govern Nedd4-2 regulation of epithelial ion transport.

Low-level lead exposure and the prevalence of gout: an observational study.

BACKGROUND Blood lead levels (BLLs) less than 1.21 µmol/L (<25 µg/dL) among adults are considered acceptable by current national standards. Lead toxicity can lead to gouty arthritis (gout), but whether the low lead exposure in the contemporary general population confers risk for gout is not known. OBJECTIVE To determine whether BLLs within the range currently considered acceptable are associated with gout. DESIGN Population-based cross-sectional study. SETTING The National Health and Nutrition Examination Survey for 2005 through 2008. PATIENTS 6153 civilians aged 40 years or older with an estimated glomerular filtration rate greater than 10 mL/min per 1.73 m2. MEASUREMENTS Outcome variables were self-reported physician diagnosis of gout and serum urate level. Blood lead level was the principal exposure variable. Additional data collected were anthropometric measures, blood pressure, dietary purine intake, medication use, medical history, and serum creatinine concentration. RESULTS The prevalence of gout was 6.05% (95% CI, 4.49% to 7.62%) among patients in the highest BLL quartile (mean, 0.19 µmol/L [3.95 µg/dL]) compared with 1.76% (CI, 1.10% to 2.42%) among those in the lowest quartile (mean, 0.04 µmol/L [0.89 µg/dL]). Each doubling of BLL was associated with an unadjusted odds ratio of 1.74 (CI, 1.47 to 2.05) for gout and 1.25 (CI, 1.12 to 1.40) for hyperuricemia. After adjustment for renal function, diabetes, diuretic use, hypertension, race, body mass index, income, and education level, the highest BLL quartile was associated with a 3.6-fold higher risk for gout and a 1.9-fold higher risk for hyperuricemia compared with the lowest quartile. LIMITATION Blood lead level does not necessarily reflect the total body lead burden. CONCLUSION Blood lead levels in the range currently considered acceptable are associated with increased prevalence of gout and hyperuricemia.

Na+-sensitive elevation in blood pressure is ENaC independent in diet-induced obesity and insulin resistance

The majority of patients with obesity, insulin resistance, and metabolic syndrome have hypertension, but the mechanisms of hypertension are poorly understood. In these patients, impaired sodium excretion is critical for the genesis of Na(+)-sensitive hypertension, and prior studies have proposed a role for the epithelial Na(+) channel (ENaC) in this syndrome. We characterized high fat-fed mice as a model in which to study the contribution of ENaC-mediated Na(+) reabsorption in obesity and insulin resistance. High fat-fed mice demonstrated impaired Na(+) excretion and elevated blood pressure, which was significantly higher on a high-Na(+) diet compared with low fat-fed control mice. However, high fat-fed mice had no increase in ENaC activity as measured by Na(+) transport across microperfused cortical collecting ducts, electrolyte excretion, or blood pressure. In addition, we found no difference in endogenous urinary aldosterone excretion between groups on a normal or high-Na(+) diet. High fat-fed mice provide a model of metabolic syndrome, recapitulating obesity, insulin resistance, impaired natriuresis, and a Na(+)-sensitive elevation in blood pressure. Surprisingly, in contrast to previous studies, our data demonstrate that high fat feeding of mice impairs natriuresis and produces elevated blood pressure that is independent of ENaC activity and likely caused by increased Na(+) reabsorption upstream of the aldosterone-sensitive distal nephron.

New perspective of ClC-Kb/2 Cl− channel physiology in the distal renal tubule

Since its identification as the underlying molecular cause of Bartters syndrome type 3, ClC-Kb (ClC-K2 in rodents, henceforth it will be referred as ClC-Kb/2) is proposed to play an important role in systemic electrolyte balance and blood pressure regulation by controlling basolateral Cl(-) exit in the distal renal tubular segments from the cortical thick ascending limb to the outer medullary collecting duct. Considerable experimental and clinical effort has been devoted to the identification and characterization of disease-causing mutations as well as control of the channel by its cofactor, barttin. However, we have only begun to unravel the role of ClC-Kb/2 in different tubular segments and to reveal the regulators of its expression and function, e.g., insulin and IGF-1. In this review we discuss recent experimental evidence in this regard and highlight unexplored questions critical to understanding ClC-Kb/2 physiology in the kidney.