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Lithium is used in the treatment of bipolar disorder. We previously demonstrated that two types of transporters mediate the tubular reabsorption of lithium in rats, and suggested that sodium-dependent phosphate transporters play a role in lithium reabsorption with high affinity. In the present study, we examined sex differences in lithium reabsorption in rats. When lithium chloride was infused at 60 µg/min, creatinine clearance and the renal clearance of lithium were lower, and the plasma concentration of lithium was higher in female rats. These values reflected the higher fractional reabsorption of lithium in female rats. In rats infused with lithium chloride at 6 µg/min, the pharmacokinetic parameters of lithium examined were all similar in both sexes. The fractional reabsorption of lithium was decreased by foscarnet, a representative inhibitor of sodium-dependent phosphate transporters, in male and female rats when lithium chloride was infused at the low rate. Among the candidate transporters mediating lithium reabsorption examined herein, the mRNA expression of only PiT2, a sodium-dependent phosphate transporter, exhibited sexual dimorphism. The present results demonstrated sex differences in the tubular reabsorption of lithium with low affinity in rats.

Background: The transepithelial transport of electrolytes, solutes, and water in the kidney is a well-orchestrated process involving numerous membrane transport systems. Basolateral potassium channels in tubular cells not only mediate potassium recycling for proper Na+,K+-ATPase function but are also involved in potassium and pH sensing. Genetic defects in KCNJ10 cause EAST/SeSAME syndrome, characterized by renal salt wasting with hypokalemic alkalosis associated with epilepsy, ataxia, and sensorineural deafness. Methods: A candidate gene approach and whole-exome sequencing determined the underlying genetic defect in eight patients with a novel disease phenotype comprising a hypokalemic tubulopathy with renal salt wasting, disturbed acid-base homeostasis, and sensorineural deafness. Electrophysiologic studies and surface expression experiments investigated the functional consequences of newly identified gene variants. Results: We identified mutations in the KCNJ16 gene encoding KCNJ16, which along with KCNJ15 and KCNJ10, constitutes the major basolateral potassium channel of the proximal and distal tubules, respectively. Coexpression of mutant KCNJ16 together with KCNJ15 or KCNJ10 in Xenopus oocytes significantly reduced currents. Conclusions: Biallelic variants in KCNJ16 were identified in patients with a novel disease phenotype comprising a variable proximal and distal tubulopathy associated with deafness. Variants affect the function of heteromeric potassium channels, disturbing proximal tubular bicarbonate handling as well as distal tubular salt reabsorption. Keywords: KCNJ10; KCNJ15; KCNJ16; acid-base homeostasis; deafness; distal tubule; hypokalemia; potassium channels; proximal tubule; tubulopathy.

International audience; Aims: Interindividual variability in capacity to reabsorb glucose at the proximal renal tubule could contribute to risk of diabetic kidney disease. Our present study investigated, in patients with diabetes, the association between fractional reabsorption of glucose (FRGLU) and degree of renal disease as assessed by urinary albumin excretion (UAE) and estimated glomerular filtration rate (eGFR).Methods: FRGLU [1-(glucose clearance/creatinine clearance)] was assessed in 637 diabetes patients attending our tertiary referral centre, looking for correlations between FRGLU and UAE (normo-, micro-, macro-albuminuria) and Kidney Disease: Improving Global Outcomes (KDIGO) eGFR categories: >90 (G1); 90-60 (G2); 59-30 (G3); and11mmol/L).Results: Median (interquartile range, IQR) FRGLU levels were blood glucose-dependent: 99.90% (0.05) for low (n=106); 99.90% (0.41) for intermediate (n=288); and 96.36% (12.57) for high (n=243) blood glucose categories (P11mmol/L.

Inhibitors of sodium–glucose cotransporter 2 (SGLT2) have undoubtedly shifted the paradigm for diabetes medicine and research and, especially, diabetic kidney disease (DKD). The pharmacological action of SGLT2 inhibitors is simply the release of glucose into urine; however, precisely how SGLT2 inhibitors contribute to the health of those with diabetes has still not been completely elucidated. Towards this end, the present review provides a novel insight into the action of SGLT2 inhibitors by highlighting a neglected fuel-burning system found in proximal tubular cells—‘glycolysis’. In addition, exploring the details of the molecular mechanisms and clinical biomarkers of the organ protection conferred by SGLT2 inhibitors is now required to prepare for the next stage of clinical diabetes medicine—the ‘post-SGLT2 inhibitor era’.

Background: The discovery of Sodium-Glucose co-transporter-2 (SGLT2) inhibitors had rewritten the treatment of diabetes mellitus with an impressive fall in the incidence of death and associated complications. Introduction: The SGLT2 inhibitors by inhibiting the SGLT2 in the proximal nephron, helps in reducing the reabsorption of approximately 90% of the filtered glucose and increased urinary glucose excretion (UGE). Methods: The literature related to SGLT2 inhibitors has been thoroughly explored from various available public domains and reviewed extensively for this article. Detailed and updated information related to SGLT2 inhibitors with a major focus on the recently approved Ertuglifolzin is structured in this review. Result: The present review is an effort to understand the management of diabetes mellitus over the past few decades with a special focus on the role of SGLT2 receptor in the causes of therapeutic and preventive strategies for diabetes mellitus. Pragmatic placement of the currently available Canagliflozin, Dapagliflozin, and Empagliflozin as oral antidiabetic agents has been done. Well accommodated stereochemistry and a high docking score of Ertugliflozin in ligand-receptor simulation studies attribute to its high potency. Conclusion: This review highlights the unique mechanism of SGLT2 Inhibitors coupled with pleiotropic benefits on weight and blood pressure, which make it an attractive choice of therapy to diabetic patients, not controlled by other medications.

Angiotensin II (ANG II) stimulates proximal nephron transport via activation of classical protein kinase C (PKC) isoforms. Acute fructose treatment stimulates PKC and dietary fructose enhances ANG II’s ability to stimulate Na+transport, but the mechanisms are unclear. We hypothesized that dietary fructose enhances ANG II’s ability to stimulate renal proximal tubule Na+reabsorption by augmenting PKC-α activation and increases in intracellular Ca2+. We measured total and isoform-specific PKC activity, basal and ANG II-stimulated oxygen consumption, a surrogate of Na+reabsorption, and intracellular Ca2+in proximal tubules from rats given either 20% fructose in their drinking water (fructose group) or tap water (control group). Total PKC activity was measured by ELISA. PKC-α, PKC-β, and PKC-γ activities were assessed by measuring particulate-to-soluble ratios. Intracelluar Ca2+was measured using fura 2. ANG II stimulated total PKC activity by 53 ± 15% in the fructose group but not in the control group (−15 ± 11%, P < 0.002). ANG II stimulated PKC-α by 0.134 ± 0.026 but not in the control group (−0.002 ± 0.020, P < 0.002). ANG II increased PKC-γ activity by 0.008 ± 0.003 in the fructose group but not in the control group ( P < 0.046). ANG II did not stimulate PKC-β in either group. ANG II increased Na+transport by 454 ± 87 nmol·min−1·mg protein−1in fructose group, and the PKC-α/β inhibitor Gö6976 blocked this increase (−96 ± 205 nmol·min−1·mg protein−1, P < 0.045). ANG II increased intracellular Ca2+by 148 ± 53 nM in the fructose group but only by 43 ± 10 nM in the control group ( P < 0.035). The intracellular Ca2+chelator BAPTA blocked the ANG II-induced increase in Na+transport in the fructose group. We concluded that dietary fructose enhances ANG II’s ability to stimulate renal proximal tubule Na+reabsorption by augmenting PKC-α activation via elevated increases in intacellular Ca2+.

The diabetic kidney presents excess expression and activity of the SGLT2 transporter of the proximal tubule. This situation increases the renal reabsorption of Na and glucose and reduces their distal supply. In addition to the metabolic effects on the internal environment of this excess reabsorbed glucose, the renal tubule is subjected to glycosylated stress capable of locally activating both apoptosis and inflammasome. The result is a progressive loss of nephron units, activation of transition of mesangial epithelium and collagen deposition. Activation of insulin signalling by the MAP kinase pathway and resistance to the metabolic effects of insulin take place. This is simultaneously combined with afferent vasodilation due to hyperglycaemia, tubuloglomerular feedback inhibition due to reduced distal fluid supply, podocyte dedifferentiation and reduction in their number, the latter effects being due to insulin resistance. The result is self-feeding renal damage, with intraglomerular hyper-pressure, podocyte dedifferentiation, tubular apoptosis, and local and distant activation of inflammasome. All these effects are susceptible to be totally or partially corrected by inhibiting glucose transport via the SGLT transporters.

Kidney size and glomerular filtration rate (GFR) often increase with the onset of diabetes, and elevated GFR is a risk factor for the development of diabetic kidney disease. Hyperfiltration mainly occurs in response to signals passed from the tubule to the glomerulus: high levels of glucose in the glomerular filtrate drive increased reabsorption of glucose and sodium by the sodium–glucose cotransporters SGLT2 and SGLT1 in the proximal tubule. Passive reabsorption of chloride and water also increases. The overall capacity for proximal reabsorption is augmented by growth of the proximal tubule, which (alongside sodium–glucose cotransport) further limits urinary glucose loss. Hyperreabsorption of sodium and chloride induces tubuloglomerular feedback from the macula densa to increase GFR. In addition, sodium–glucose cotransport by SGLT1 on macula densa cells triggers the production of nitric oxide, which also contributes to glomerular hyperfiltration. Although hyperfiltration restores sodium and chloride excretion it imposes added physical stress on the filtration barrier and increases the oxygen demand to drive reabsorption. Tubular growth is associated with the development of a senescence-like molecular signature that sets the stage for inflammation and fibrosis. SGLT2 inhibitors attenuate the proximal reabsorption of sodium and glucose, normalize tubuloglomerular feedback signals and mitigate hyperfiltration. This tubule-centred model of diabetic kidney physiology predicts the salutary effect of SGLT2 inhibitors on hard renal outcomes, as shown in large-scale clinical trials.

The kidneys filter large amounts of glucose. To prevent the loss of this valuable fuel, the tubular system of the kidney, particularly the proximal tubule, has been programmed to reabsorb all filtered glucose. The machinery involves the sodium-glucose cotransporters SGLT2 and SGLT1 on the apical membrane and the facilitative glucose transporter GLUT2 on the basolateral membrane. The proximal tubule also generates new glucose, particularly in the post-absorptive phase but also to enhance bicarbonate formation and maintain acid-base balance. The glucose reabsorbed or formed by the proximal tubule is primarily taken up into peritubular capillaries and returned to the systemic circulation or provided as an energy source to further distal tubular segments that take up glucose by basolateral GLUT1. Recent studies provided insights on the coordination of renal glucose reabsorption, formation, and usage. Moreover, a better understanding of renal glucose transport in disease states is emerging. This includes the kidney in diabetes mellitus, when renal glucose retention becomes maladaptive and contributes to hyperglycemia. Furthermore, enhanced glucose reabsorption is coupled to sodium retention through the sodium-glucose cotransporter SGLT2, which induces secondary deleterious effects. As a consequence, SGLT2 inhibitors are new anti-hyperglycemic drugs that can protect the kidneys and heart from failing. Recent studies discovered unique roles for SGLT1 with implications in acute kidney injury and glucose sensing at the macula densa. This review discusses established and emerging concepts of renal glucose transport, and outlines the need for a better understanding of renal glucose handling in health and disease.

Aims/Introduction The renal threshold for glucose (RTg) corresponds to a blood glucose level of ~180 mg/dL; however, in hospitals, patients are often encountered who are hyperglycemic, but urine glucose test strip‐negative, who remain negative for urine glucose even at blood glucose concentrations >180 mg/dL, implying a high RTg value. In this study, we aimed to identify factors determining high RTg in Japanese patients with type 2 diabetes mellitus. Materials and Methods We estimated RTg (eRTg) using urinalysis data from 67 type 2 diabetes mellitus patients for whom the glucose infusion rate (GIR) was determined by hyperinsulinemic‐euglycemic clamp. After allocating patients to two groups according to their baseline eRTg (, The renal threshold for glucose (RTg) corresponds to a blood glucose level of ~180 mg/dL; however, in hospitals, patients are often encountered who are hyperglycemic, but urine glucose test strip‐negative, implying a high RTg value. In this study, we aimed to identify factors determining high RTg in Japanese patients with type 2 diabetes mellitus. We estimated RTg (eRTg) using urinalysis data from 67 type 2 diabetes mellitus patients for whom the glucose infusion rate was determined by hyperinsulinemic‐euglycemic clamp, and found that a high eRTg is associated with a low glucose infusion rate and high glycated hemoglobin, with the glucose infusion rate making a substantial contribution.

Background: Anaemia is a common finding in diabetes, particularly in those patients with albuminuria or renal dysfunction and is associated with impaired erythropoietin (EPO) secretion. This review focuses on mechanisms involved in the regulation of erythropoiesis in diabetic patients in an effort to elucidate the competing effects of the renin angiotensin system (RAS) blockade and sodium-glucose cotransporter-2 (SGLT2) inhibitors on haemoglobin concentration and hematocrit values. Summary: The RAS shows significant activation in diabetic subjects. Angiotensin II, its active octapeptide, causes renal tubulointerstitial hypoxia, which stimulates hypoxia-inducible factors (HIF) and increases EPO secretion and erythropoiesis. As expected, drugs that inactivate RAS, such as angiotensin converting enzyme inhibitors or angiotensin receptor blockers (ACEi/ARB) are associated with a significant hematocrit-lowering effect and/or anaemia in various clinical conditions, including diabetes. Dual blockade by a combination of ACEi and ARB in diabetic patients achieves a better RAS inhibition, but at the same time a worse drop of haemoglobin concentration. Increased glucose reabsorption by SGLTs in diabetic subjects generates a high-glucose environment in renal tubulointerstitium, which may impair HIF-1, damage renal erythropoietin-producing cells (REPs) and decrease EPO secretion and erythropoiesis. SGLT2 inhibitors, which inhibit glucose reabsorption, may attenuate glucotoxicity in renal tubulointerstitium, allowing REPs to resume their function and increase EPO secretion. Indeed, EPO levels increase within a few weeks after initiation of therapy with all known SGLT2 inhibitors, followed by increased reticulocyte count and a gradual elevation of haemoglobin concentration and hematocrit level, which reach zenith values after 2–3 months. Key Messages: The competing effects of RAS blockade and SGLT2 inhibitors on erythropoiesis may have important clinical implications. The rise of hematocrit values by SGLT2 inhibitors given on top of RAS blockade in recent outcome trials may significantly contribute to the cardiorenal protection attained. The relative contribution of each system to erythropoiesis and outcome remains to be revealed in future studies.

Background Zinc deficiency can change the concentrations of minerals and trace elements in the body. However, previous studies still had many limitations. Objective To reveal the effects of zinc deficiency on homeostasis of 16 minerals and trace elements. Methods Forty-five rats were divided randomly into three groups: normal zinc diet (30 mg/kg), low zinc diet (10 mg/kg), and pair-fed diet(30 mg/kg). The concentrations of 16 minerals and trace elements in serum, feces, urine, and liver were measured by inductively coupled plasma mass spectrometry. The excretion of 16 elements in urine and feces were calculated and compared. Results Zinc-deficient rats exhibited significant changes in up to 12 minerals and trace elements. The low zinc diet induced decreased excretion of zinc and concentrations of zinc in serum, feces, urine, and liver. Zinc deficiency increased feces concentrations of Mg, Cu, Se, K, Ag, Fe and Mn; decreased the concentrations of Mg, Cu, Se, K in liver and urine, and a diminished amount of Ag was observed in serum. Decreased urinary concentrations of Zn Ca, Mg, Cu, Se, K, Na, As and Cr, suggested that zinc-deficient rats increased the 9 elements’ renal reabsorption. Decreased concentrations of Ca in liver, urine, and feces, decreased excretion in urine and feces and increased serum total Ca suggested that zinc deficiency increased the redistribution of Ca in serum or other tissues. Zinc deficiency increased excretion of Cu, Se, Fe; and decreased the excretion of other 8 elements except for Ag. Conclusions Zinc deficiency changed the excretion, reabsorption and redistribution of 12 minerals and trace elements in rats. Our findings are the first to show that zinc deficiency alters the concentrations of Ag, Cr, and As. Graphical abstract

GLUT9 is generally considered to be associated with the uric acid transport, which plays an important role in the regulation of serum uric acid level. In this study, the expression level of miR-143-3p was significantly decreased in hyperuricemia mice model group compared with the normal control by miRNA microarray, the same results were confirmed in the hyperuricemia patients and the healthy control group. It is predicted that GLUT9 may be the target gene of miR-143-3p by target scan and other net-software. GLUT9 as the downstream target gene of miR-143-3p was determinated by fluorescence enzyme activity assay. Western blotting and qRT-PCR indicated that the expression of GLUT9 in human renal tubular epithelial cells transfected with miR-143-3p mimics was significantly reduced. Meanwhile inflammatory factors IL-1β and MCP-1 significantly decreased. In conclusion, miR-143-3p can reduce uric acid reabsorption by inhibiting its downstream target gene GLUT9.

Na+-glucose cotransporter (SGLT)1 mediates glucose reabsorption in late proximal tubules. SGLT1 also mediates macula densa (MD) sensing of an increase in luminal glucose, which increases nitric oxide (NO) synthase 1 (MD-NOS1)-mediated NO formation and potentially glomerular filtratrion rate (GFR). Here, the contribution of SGLT1 was tested by gene knockout (−/−) in type 1 diabetic Akita mice. A low-glucose diet was used to prevent intestinal malabsorption in Sglt1−/−mice and minimize the contribution of intestinal SGLT1. Hyperglycemia was modestly reduced in Sglt1−/−versus littermate wild-type Akita mice (480 vs. 550 mg/dl), associated with reduced diabetes-induced increases in GFR, kidney weight, glomerular size, and albuminuria. Blunted hyperfiltration was confirmed in streptozotocin-induced diabetic Sglt1−/−mice, associated with similar hyperglycemia versus wild-type mice (350 vs. 385 mg/dl). Absence of SGLT1 attenuated upregulation of MD-NOS1 protein expression in diabetic Akita mice and in response to SGLT2 inhibition in nondiabetic mice. During SGLT2 inhibition in Akita mice, Sglt1−/−mice had likewise reduced blood glucose (200 vs. 300 mg/dl), associated with lesser MD-NOS1 expression, GFR, kidney weight, glomerular size, and albuminuria. Absence of Sglt1 in Akita mice increased systolic blood pressure, associated with suppressed renal renin mRNA expression. This may reflect fluid retention due to blunted hyperfiltration. SGLT2 inhibition prevented the blood pressure increase in Sglt1−/−Akita mice, possibly due to additive glucosuric/diuretic effects. The data indicate that SGLT1 contributes to diabetic hyperfiltration and limits diabetic hypertension. Potential mechanisms include its role in glucose-driven upregulation of MD-NOS1 expression. This pathway may increase GFR to maintain volume balance when enhanced MD glucose delivery indicates upstream saturation of SGLTs and thus hyperreabsorption.

BACKGROUND Renal sodium handling could be a potential mediator linking adipokines to hypertension. The aim of the study was to assess the relationship of leptin with urinary sodium excretion and proximal sodium reabsorption in humans. METHODS This cross-sectional study was conducted on participants of hypertensive families from the Seychelles Island. A split urine (daytime and nighttime) collection and plasma leptin were measured. Endogenous lithium clearance was used to assess proximal sodium reabsorption. Mixed multiple linear regression tests adjusted for confounding factors were used. RESULTS Three hundred and sixty-five participants (57% women) were included in this analysis. Leptin and adiponectin were higher in women (P < 0.001). Leptin was associated positively with daytime (coefficient [c]: 0.16, standard deviation (SD): 0.03, P < 0.001), nighttime urinary sodium excretion (c: 0.17, SD: 0.04), P < 0.01), daytime lithium clearance (c: 0.40, SD: 0.08, P < 0.001), and nighttime lithium clearance (c: 0.39, SD: 0.10, P < 0.001) after adjusting for sex. The association was lost or mitigated only when BMI was introduced in the model. When BMI was categorized in normal vs. overweight participant, leptin was associated with daytime and nighttime sodium excretion rates (c: 0.14, SD: 0.05, P = 0.011 and c: 0.22, SD: 0.07, P = 0.002, respectively) only in overweight participants. CONCLUSION Leptin is associated positively with daytime and nighttime sodium excretion and lithium clearance suggesting a natriuretic rather than a sodium retaining effect of leptin. Sex and body mass index (BMI) are major confounders in this association. These results highlight the importance of sex and obesity in our understanding of the relationships between leptin, blood pressure, and renal sodium handling.

The article explores the functional state of kidneys in animals with acetaminophen-induced intoxication, which were maintained under the conditions of different protein supply. The research model involved the division of animals into next equal groups: group I – rats, which were maintained on a complete semi-synthetic diet (C); group II – rats, which were kept on a low-protein diet (LPD); ІІІ – rats with acetaminophen-induced injury, maintained on a complete semi-synthetic diet (TI); IV – rats with acetaminophen-induced injury, which were previously kept on the low-protein diet (LPD + TI). Determination of creatinine and protein levels and activity of γ-glutamyltransferase in urine was performed using a photocolorimetric method and standard sets of reagents (Filisit-Diagnostics). Urine Na+ level was determined spectrophotometrically using the standard set of reagents (Human, Germany). It was established, that kidney function of animals, which were maintained in conditions of the alimentary deprivation of protein, was characterized by the disturbances of the filtration capacity on the background of a slight decrease in reabsorption capacity. It is evidenced by a minor increase in GGT activity and urine Na+ level, along with significant proteinuria against the background of GFR reduction and preservation of plasma creatinine level. In animals with acetaminophen-induced injury, an increase in GGT activity, urine Na+ level and proteinuria in the absence of GFR and plasma creatinine changes, indicates the primary damage to renal tubular cells, while maintaining the filtration capacity of the kidney. The most significant changes in the filtration capacity of kidney were recorded in animals receiving toxic doses of acetaminophen on the background of alimentary deficiency of protein: a significant increase in plasma creatinine on the background of a 4-fold decrease in glomerular filtration. Proteinuria, increased γ-glutamyltransferase activity, and an increase in urine Na+ level indicates the damage to tubular cells and impaired renal reabsorption capacity. The conclusion was made, that lack of protein in the diet is a factor leading to a worsening of kidney dysfunction in animals with acetaminophen-induced intoxication since under those conditions disturbances of both filtration and reabsorption capacity of the kidney are observed. The obtained results can be used for a biochemical substantiation of the approaches to correction and elimination of the renal dysfunction caused by acetaminophen in people with protein deficiency.

The proximal tubule (PT) is responsible for the majority of calcium reabsorption by the kidney. Most PT calcium transport appears to be passive, although the molecular facilitators have not been well established. Emerging evidence supports a major role for PT calcium transport in idiopathic hypercalciuria and the development of kidney stones. This review will cover recent developments in our understanding of PT calcium transport and the role of the PT in kidney stone formation.

Epithelial glucose transport is accomplished by Na+ -glucose co-transporters, SGLT1 and SGLT2. In the intestine, uptake of dietary glucose is for its majority mediated by SGLT1, and humans with mutations in the SGLT1 gene show glucose/galactose malabsorption. In the kidney, both transporters, SGLT1 and SGLT2, are expressed and recent studies identified that SGLT2 mediates up to 97% of glucose reabsorption. Humans with mutations in the SGLT2 gene show familial renal glucosuria. In the last three decades, significant progress was made in understanding the physiology of these transporters and their potential as therapeutic targets. Based on the structure of phlorizin, a natural compound acting as a SGLT1/2 inhibitor, initially several SGLT2, and later SGLT1 and dual SGLT1/2 inhibitors have been developed. Interestingly, SGLT2 knockout or treatment with SGLT2 selective inhibitors only causes a fractional glucose excretion in the magnitude of ∼60%, an effect mediated by up-regulation of renal SGLT1. Based on these findings the hypothesis was brought forward that dual SGLT1/2 inhibition might further improve glycaemic control via targeting two distinct organs that express SGLT1: the intestine and the kidney. Of note, SGLT1/2 double knockout mice completely lack renal glucose reabsorption. This review will address the rationale for the development of SGLT1 and dual SGLT1/2 inhibitors and potential benefits compared to sole SGLT2 inhibition.

Tubuloglomerular feedback (TGF) responses become anomalous in rats fed high-NaCl diet after subtotal nephrectomy (STN), such that stimulating TGF causes single nephron GFR (SNGFR) to increase rather than decrease. Micropuncture experiments were performed to determine whether this anomaly results from heightened nitric oxide response to distal delivery, which is a known mechanism for resetting TGF, or from connecting tubule TGF (cTGF), which is a novel amiloride-inhibitable system for offsetting TGF responses. Micropuncture was done in Wistar Froemter rats fed high-NaCl diet (HS) for 8–10 days after STN or sham nephrectomy. TGF was manipulated by orthograde microperfusion of Henle’s loop with artificial tubular fluid with or without NOS inhibitor, LNMMA, or the cell-impermeant amiloride analog, benzamil. SNGFR was measured by inulin clearance in tubular fluid collections from the late proximal tubule. TGF responses were quantified as the increase in SNGFR that occurred when the perfusion rate was reduced from 50 to 8 nl/min in STN or 40 to 8 nl/min in sham animals. The baseline TGF response was anomalous in STN HS (−4 ± 3 vs 14 ± 3 nl/min, P < 0.001). TGF response was normalized by perfusing STN nephron with LNMMA (14 ± 3 nl/min, P < 0.005 for ANOVA cross term) but not with benzamil (−3 ± 4 nl/min, P = 0.4 for ANOVA cross term). Anomalous TGF occurs in STN HS due to heightened effect of tubular flow on nitric oxide signaling, which increases to the point of overriding the normal TGF response. There is no role for cTGF in this phenomenon.

Objective: Hyperuricemia is an excess of urate in blood. The kidneys play important parts in urate excretion, which involves handling reabsorption and secretion. A series of urate transporters is responsible for this process: urate transporter (URAT)1, glucose transporter (GLUT)9, organic anion transporter (OAT)1 and OAT3. Excessive fructose intake may result in increased serum urate levels. Chicory (Cichorium intybus L.) has been used as an edible vegetable and traditional Chinese medicine. Studies have shown that chicory is a promising anti-hyperuricemia agent and we explored the mechanism of its uricosuric effect via a renal pathway. Methods: Hyperuricemia was induced in rats by administration of 10% fructose. The uricosuric effect was evaluated by determining the serum urate level. Renal excretory function was detected by the clearance rate of creatinine, clearance rate of uric acid and histology. The location and expression of URAT1, GLUT9, OAT1 and OAT3 their mRNA expression in kidneys were analyzed. Results: Chicory decreased serum levels of urate and creatinine significantly, and promoted the clearance of creatinine and urate, as well as improving renal pathologic changes due to hyperuricemia. Chicory inhibited expression of URAT1 and GLUT9 markedly in a dose-dependent manner, but showed no influence on expression of OAT1 or OAT3. Conclusion: Chicory might be a promising anti-hyperuricemia agent. It can promote renal excretion of urate by inhibiting urate reabsorption, which may be related to down-regulation of mRNA and protein expression of URAT1 and GLUT9. Keywords: Chicory, Fructose, Hyperuricemia, Renal resorption, Urate transporters

Inhibitors of the Na+-glucose cotransporter SGLT2 enhance urinary glucose and urate excretion and lower plasma urate levels. The mechanisms remain unclear, but a role for enhanced glucose in the tubular fluid, which may interact with tubular urate transporters, such as the glucose transporter GLUT9 or the urate transporter URAT1, has been proposed. Studies were performed in nondiabetic mice treated with the SGLT2 inhibitor canagliflozin and in gene-targeted mice lacking the urate transporter Glut9 in the tubule or in mice with whole body knockout of Sglt2, Sglt1, or Urat1. Renal urate handling was assessed by analysis of urate in spontaneous plasma and urine samples and normalization to creatinine concentrations or by renal clearance studies with assessment of glomerular filtration rate by FITC-sinistrin. The experiments confirmed the contribution of URAT1 and GLUT9 to renal urate reabsorption, showing a greater contribution of the latter and additive effects. Genetic and pharmacological inhibition of SGLT2 enhanced fractional renal urate excretion (FE-urate), indicating that a direct effect of the SGLT2 inhibitor on urate transporters is not absolutely necessary. Consistent with a proposed role of increased luminal glucose delivery, the absence of Sglt1, which by itself had no effect on FE-urate, enhanced the glycosuric and uricosuric effects of the SGLT2 inhibitor. The SGLT2 inhibitor enhanced renal mRNA expression of Glut9 in wild-type mice, but tubular GLUT9 seemed dispensable for the increase in FE-urate in response to canagliflozin. First evidence is presented that URAT1 is required for the acute uricosuric effect of the SGLT2 inhibitor in mice.

It has been theorized that Na-glucose cotransporter (SGLT2) blockade slows progression of diabetic kidney disease by reducing physical strain on the glomerulus. This is the first direct measurement of intraglomerular pressure during SGLT2 blockade. Findings confirmed that SGLT2 blockade does reduce glomerular capillary pressure, that this is mediated through tubuloglomerular feedback, and that the tubuloglomerular feedback response to SGLT2 blockade involves preglomerular vasoconstriction and postglomerular vasorelaxation.

A heterozygous mutation (c.643C.A; p.Q215X) in the creatine transporter SLC16A12 has been proposed to cause a syndrome with juvenile cataracts, microcornea, and glucosuria in humans. To further explore the role of SLC16A12 in renal physiology and decipher the mechanism underlying the phenotype of humans with the SLC16A12 mutation, we studied Slc16a12 knockout (KO) rats. Slc16a12 KO rats had lower plasma levels and increased absolute and fractional urinary excretion of creatine and its precursor guanidinoacetate (GAA). Slc16a12 KO rats displayed lower plasma and urinary creatinine levels, but the glomerular filtration rate was normal. The phenotype of heterozygous rats was indistinguishable from wild-type (WT) rats. Renal artery to vein (RAV) concentration differences in WT rats were negative for GAA and positive for creatinine. However, RAV differences for GAA were similar in Slc16a12 KO rats, indicating incomplete compensation of urinary GAA losses by renal GAA synthesis. Together, our results reveal that Slc16a12 in the basolateral membrane of the proximal tubule is critical for the reabsorption of creatine and GAA. Our data suggest a dominant-negative mechanism underlying the phenotype of humans affected by the heterozygous SLC16A12 mutation. Furthermore, in the absence of Slc16a12, urinary losses of GAA are not adequately compensated by increased tubular synthesis, likely caused by feedback inhibition of the rate-limiting enzyme l-arginine:glycine amidinotransferase by creatine in proximal tubular cells.NEW & NOTEWORTHY SLC16A12 is a recently identified creatine transporter of unknown physiological function. A heterozygous mutation in the human SLC16A12 gene causes juvenile cataracts and reduced plasma guanidinoacetate (GAA) levels with an increased fractional urinary excretion of GAA. Our study with transgenic SLC16A12-deficient rats reveals that SLC16A12 is critical for tubular reabsorption of creatine and GAA in the kidney. Our data furthermore indicate a dominant-negative mechanism underlying the phenotype of humans affected by the heterozygous SLC16A12 mutation.

Calcium (Ca+2) is a divalent cation that plays a critical role in numerous body functions such as skeletal mineralization, signal transduction, nerve conduction, muscle contraction, and blood coagulation. Ca+2 metabolism is linked to magnesium (Mg+2) and phosphate metabolism. Ca+2 homeostasis is dependent on intestinal absorption, bone turnover, and renal reabsorption. The hormonal regulators of these processes are the parathyroid hormone (PTH), calcitriol {1,25-dihydroxyvitamin D [1,25(OH)2D]}, and serum ionized Ca+2. Cloning of the Ca+2-sensing receptor (CaSR) has greatly advanced the understanding of Ca+2 metabolism. Disorders of Ca+2 metabolism are easily recognized because Ca+2 is included in routine chemistry panels. Measurement of ionized Ca+2 is the preferred way to ascertain the diagnosis of hypocalcemia and hypercalcemia.

Sexual dimorphic variations are present in many aspects of biology and involve the structure and/or function of nearly every organ system. Acid-base homeostasis is critical for optimal health, and renal ammonia metabolism has a major role in the maintenance of acid-base homeostasis. Recent studies have shown sex-dependent differences in renal ammonia metabolism with regard to both basal ammonia excretion and the response to an exogenous acid load. These sexual dimorphisms are associated with structural changes in the proximal tubule and the collecting duct and variations in the expression of multiple proteins involved in ammonia metabolism and transport. Studies using orchiectomy-induced testosterone deficiency and physiological testosterone replacement have shown that testosterone underlies much of the sex-dependent differences in the proximal tubule. This parallels the finding that the canonical testosterone target receptor, androgen receptor (AR), is present exclusively in the proximal tubule. Thus testosterone, possibly acting through AR activation, regulates multiple components of renal structure and ammonia metabolism. The lack of detectable AR in the remainder of the nephron and collecting duct suggests that some dimorphisms in renal structure and ammonia transporter expression are mediated through mechanisms other than direct testosterone-dependent AR activation. A better understanding of the mechanism and biological implications of sex’s effect on renal structure and ammonia metabolism is critical for optimizing our ability to care for both men and women with acid-base disturbances.

Sodium-glucose cotransporter (SGLT) 2 inhibitors, which inhibit renal reabsorption of glucose and stimulate urinary glucose excretion, have been proposed as useful drugs for treating diabetes. This study examined the effects of the SGLT2 inhibitor ipragliflozin on blood glucose-dependent urinary glucose excretion in mice. To investigate the antihyperglycemic effects of ipragliflozin, several oral glucose tolerance tests (glucose load: 1-4 g/kg) were performed. Ipragliflozin significantly suppressed the increases in blood glucose levels concomitant with urinary glucose excretion. To investigate the risk of hypoglycemia, the effects of ipragliflozin were examined under fasting conditions in mice that prefasted for various time periods (0-24 h). Ipragliflozin significantly decreased blood glucose levels in mice that had prefasted for 0 or 6 h (non-hypoglycemic conditions), but not in mice that had prefasted for 12 h or more (hypoglycemic conditions). These ipragliflozin-induced hypoglycemic and urinary glucose excretion effects were well correlated with blood glucose levels. These results suggest that ipragliflozin-induced increases in urinary glucose excretion and accompanying hypoglycemic effects were caused by blood glucose-dependent mechanisms. Thus, ipragliflozin may be a useful antidiabetic agent that achieves ideal blood glucose control in diabetic patients with little risk of hypoglycemia.

Hyperuricemia is more prevalent among people with obesity and metabolic syndrome, and is associated with adverse clinical outcomes. We hypothesized that increased renal reabsorption of uric acid (UA) in obesity and metabolic syndrome may be an adaptive response of the kidney when faced with fatty acid-induced oxidative stress. To test this hypothesis, we examined lipid accumulation, markers of oxidative stress, and renal UA handling in Zucker diabetic fatty (ZDF) rats, and in matched lean control animals. Rats were randomized to either normal rodent chow or a diet supplemented with antioxidants (α-tocopheryl acetate, sodium selenite, zinc sulfate, and ascorbic acid), and were followed up for either 4 or 20 weeks after randomization. Dietary antioxidant supplementation had no significant effects in lean control rats but led to partial improvement in markers of elevated oxidative stress in the kidney of ZDF rats. Renal UA handling was not affected by antioxidant supplementation. We observed robust correlations between renal lipid content and oxidative stress markers in the pooled experimental groups, particularly in older animals after 20 weeks on the study diets. Dietary antioxidant supplementation did not prevent the gradual decline in renal function observed in older ZDF rats. These findings suggest that hyperuricemia in the ZDF rat model of obesity and the metabolic syndrome is not caused by renal oxidative stress, that there may be a pathophysiological link between lipid accumulation and oxidative stress in the kidney, and that antioxidant supplementation does not prevent age-related decline in renal function in ZDF rats.

Studies have shown that epoxyeicosatrienoic acids (EETs) can regulate glucose homeostasis, but the specific mechanisms need further exploration. The sodium-glucose co-transporter 2 (SGLT2) is highly expressed in diabetic kidneys, which further promotes renal reabsorption of glucose to respond to the hyperglycemic state of diabetes. Herein, whether EETs can be a latent inhibitor of SGLT2 to regulate glucose homeostasis in diabetic state needs to be elucidated. Our study demonstrated that EETs attenuated the glucose reabsorption via renal tubular epithelial cells in diabetic mice, which partly accounted for the beneficial effects of EETs on glucose homeostasis. Moreover, 14,15-EET suppressed SGLT2 expression in both diabetic kidney and renal tubular epithelial cells. Further, inhibition of NF-κB with BAY 11-7082 decreased insulin-induced SGLT2 expression while NF-κB overexpression reversed the above effects. In addition, 14,15-EET attenuated SGLT2 expression via inactivating NF-κB. Mechanistically, 14,15-EET attenuated NF-κB mediated SGLT2 transcription at the -1821/-1812 P65-binding site. These results showed that EETs ameliorated glucose homeostasis via preventing NF-κB-mediated transcription of SGLT2 in renal tubular epithelial cells, providing a unique therapeutic strategy for insulin resistance and diabetes.

Hyperuricemia is mainly the result of relative underexcretion of urate. Urate is mainly eliminated by kidney and several important transporters expressed on the membrane of renal tubular cells involved in urate excretion. Olsalazine sodium was screened from 3167 authorized small compounds/drugs, targeting xanthine oxidoreductase. In previous study, we reported that olsalazine sodium significantly reduced the serum urate levels, and the anti-hyperuricemic activity linked with inhibiting urate formation by reducing the activity of xanthine oxidoreductase. The current research aimed to assess olsalazine sodium renal urate excretion and likely molecular mechanism. The results showed that administration of olsalazine sodium 5.0 mg/kg decreased the levels of serum urate in hyperuricemic rats, and noticeably improved the fractional excretion of urate and urate clearance, exhibiting an uricosuric action. Moreover, olsalazine sodium (2.5, 5.0, 10.0 mg/kg) reduced the level of blood urea nitrogen in rats. Further study showed that olsalazine sodium reduced the mRNA expression of urate reabsorptive transporter glucose transporter 9 (GLUT9), increased the mRNA expression of urate secretory transporters, organic anion transporter 1 (OAT1), OAT3 and type 1 sodium-dependent phosphate transporter (NPT1) as well as the protein expression of OAT3 in the kidney in hyperuricemic mice. In conclusion, olsalazine sodium exhibited a promotion of urate excretion in kidney by increasing the expression of OAT3.

Vitamin D is well known for its important roles in maintaining calcium homeostasis and bone mineralization via the regulation of calcium mobilization and renal reabsorption, and the intestinal absorption of both calcium and phosphorus [...]

Introduction: The availability of L-arginine is the rate-limiting factor in Nitric Oxide (NO) synthesis, a potent vasodilatory factor. L-arginine plasma levels are blunted in diabetes, thus L-arginine supplementation decreases BP in animal diabetic models, and type 2 diabetic patients. Renal reabsorption of filtered L-arginine in the proximal tubule is mainly sodium dependent. Higher filtered glucose load in diabetes may alter sodium gradient via higher glucose reabsorption by sodium-glucose cotransporters (SGLTs). Purpose: The mechanism by which SGLT2 inhibitors, such as Empagliflozin, decrease BP in type 2 diabetic patients, is still unknown. We hypothesize that SGLT2 inhibitors could decrease the urinary excretion of L-arginine, by restoring sodium gradient necessary for L-arginine reabsorption in the proximal tubule. Methods: Male obese Zucker diabetic fatty/Spontaneously hypertensive heart failure hybrid (ZSF1) rats and their lean littermates (CTR) received either Empagliflozin or vehicle for 6 weeks. Levels of L-arginine in urine and plasma were quantified via Liquid Chromatography-Mass Spectrometry. Plasmatic cGMP level, a marker of NO production and urinary 8-hydroxydeoxyguanosine (8-OHdG) level, a marker of oxidative stress, were measured. Results: The urinary level of L-arginine was higher in ZSF-1 rats compared to CTR ones, and plasmatic concentrations of L-arginine were lower in ZSF-1 rats compared to CTR ones. Both parameters were significantly reversed by Empagliflozin. Empagliflozin treatment increased plasmatic cGMP and decreased urinary 8-OHdG in diabetic rats. Conclusion: Empagliflozin decreased urinary excretion of L-arginine, associated with an increased plasmatic concentration of L-arginine and cGMP, and decreased oxidative stress in diabetic rats. This suggests that Empagliflozin increased NO production and bioavailability. Such effects might contribute to the BP lowering effect of Empagliflozin in diabetics. Disclosure S. Khemais-Benkhiat: Employee; Self; Boehringer Ingelheim International GmbH. D. Veyel: Employee; Self; Boehringer Ingelheim Pharmaceuticals, Inc. V. Schini-Kerth: Research Support; Self; Boehringer Ingelheim International GmbH. B. Krawczyk: Employee; Self; Boehringer Ingelheim Pharmaceuticals, Inc. M. Pieper: Employee; Self; Boehringer Ingelheim Pharmaceuticals, Inc. M. Mark: Employee; Self; Boehringer Ingelheim International GmbH. E. Mayoux: Employee; Self; Boehringer Ingelheim Pharmaceuticals, Inc.

Background and Aims The thyroid hormone (TH) plays an important role in glucose metabolism. Recently, we showed that the TH improves glycemia control by decreasing cytokines expression in the adipose tissue and skeletal muscle of alloxan-induced diabetic rats, which were also shown to present primary hypothyroidism. In this context, this study aims to investigate whether the chronic treatment of diabetic rats with T3 could affect other tissues that are involved in the control of glucose homeostasis, as the liver and kidney. Method Adult male Wistar rats were divided into nondiabetic, diabetic, and diabetic treated with T3 (1.5 ?g/100 g BW for 4 weeks). Diabetes was induced by alloxan monohydrate (150 mg/kg, BW, i.p.). Animals showing fasting blood glucose levels greater than 250 mg/dL were selected for the study. Results After treatment, we measured the blood glucose, serum T3, T4, TSH, and insulin concentration, hepatic glucose production by liver perfusion, liver PEPCK, GAPDH, and pAKT expression, as well as urine glucose concentration and renal expression of SGLT2 and GLUT2. T3 reduced blood glucose, hepatic glucose production, liver PEPCK, GAPDH, and pAKT content and the renal expression of SGLT2 and increased glycosuria. Conclusion Results suggest that the decreased hepatic glucose output and increased glucose excretion induced by T3 treatment are important mechanisms that contribute to reduce serum concentration of glucose, accounting for the improvement of glucose homeostasis control in diabetic rats.

Familial hyperkalemic hypertension (FHHt; also called pseudohypoaldosteronism type II) is a hereditary hypertensive disease which can be caused by mutations in four genes: WNK1 [with no lysine (K) 1], WNK4, Kelch-like3 (KLHL3), and cullin3 (CUL3). Decreased KLHL3 expression was identified as being involved in the pathogenesis of FHHt caused by cullin 3 disease mutations. Recent studies have revealed an increased WNK4 and hence Na-Cl cotransporter (NCC) activity in the db/db mice, resulting from PKC-mediated KLHL3 phosphorylation, which impairs the degradation of its substrate, WNK4. However, whether WNK4 and NCC were activated in type 1 diabetes still remains unclear. We created streptozotocin-induced type 1 diabetic mice and revealed that renal WNK-oxidative stress response kinase-1/STE20/SPS1-related proline alanine-rich kinase (OSR1/SPAK)-NCC cascade was activated, whereas KLHL3 expression was markedly decreased and CUL3 was heavily neddylated. Moreover, decreased KLHL3 was reversed and WNK1 and WNK4 abundance increased by MLN4924, a neddylation inhibitor. In vitro, our study also showed decreased KLHL3 abundance without any significant change in phosphorylated KLHL3 under high glucose exposure. These results indicate that decreased KLHL3 likely plays a role in the pathogenesis of renal sodium reabsorption in hyperglycemic conditions.

For years, physicians and scientists were enthralled by the enigmatic phenomenon of fasting-associated diuresis and natriuresis and their reversal by feeding. This abrupt response is most prominent in obese and hypertensive individuals, and if repeated once and again may lead to the attenuation of blood pressure and improve insulin sensitivity. The mechanisms involved in early natriuresis and diuresis remain speculative as the renin-angiotensin-aldosterone axis and natriuretic peptides are initially suppressed. Based on gained insight using sodium-glucose transporter 2 (SGLT-2) inhibitors, herein, we propose a role for enhanced post-prandial proximal tubular sodium uptake, mediated by increased glucose-sodium co-transport, as daily filtered glucose increases, and reduced sodium uptake when glucose reabsorption diminishes. This phenomenon might be more pronounced in diabetics due to prolonged post-prandial hyperglycemia and intense SGLT-driven transport. Our hypothesis may also provide a physiologic basis for fasting-related reduced blood pressure in hypertension. This theory deserves challenging by experimental and clinical studies.

The beneficial effects of fatty acids (FAs) on human health have attracted widespread interest. However, little is known about the impact of FAs on the handling of urate, the end-product of human purine metabolism, in the body. Increased serum urate levels occur in hyperuricemia, a disease that can lead to gout. In humans, urate filtered by the glomerulus of the kidney is majorly re-absorbed from primary urine into the blood via the urate transporter 1 (URAT1)-mediated pathway. URAT1 inhibition, thus, contributes to decreasing serum urate concentration by increasing net renal urate excretion. Here, we investigated the URAT1-inhibitory effects of 25 FAs that are commonly contained in foods or produced in the body. For this purpose, we conducted an in vitro transport assay using cells transiently expressing URAT1. Our results showed that unsaturated FAs, especially long-chain unsaturated FAs, inhibited URAT1 more strongly than saturated FAs. Among the tested unsaturated FAs, eicosapentaenoic acid, α-linolenic acid, and docosahexaenoic acid exhibited substantial URAT1-inhibitory activities, with half maximal inhibitory concentration values of 6.0, 14.2, and 15.2 μM, respectively. Although further studies are required to investigate whether the ω-3 polyunsaturated FAs can be employed as uricosuric agents, our findings further confirm FAs as nutritionally important substances influencing human health.

The renal circadian clock has a major influence on the function of the kidney. Aryl hydrocarbon receptor nuclear translocator-like protein 1 [ARNTL; also known as brain and muscle ARNT-like 1 (BMAL1)] is a core clock protein and transcription factor that regulates the expression of nearly half of all genes. Using male and female kidney-specific cadherin BMAL1 knockout (KS-BMAL1 KO) mice, we examined the role of renal distal segment BMAL1 in blood pressure control and solute handling. We confirmed that this mouse model does not express BMAL1 in thick ascending limb, distal convoluted tubule, and collecting duct cells, which are the final locations for solute and fluid regulation. Male KS-BMAL1 KO mice displayed a substantially lower basal systolic blood pressure compared with littermate control mice, yet their circadian rhythm in pressure remained unchanged [male control mice: 127 ± 0.7 mmHg ( n = 4) vs. male KS-BMAL KO mice: 119 ± 2.3 mmHg ( n = 5), P < 0.05]. Female mice, however, did not display a genotype difference in basal systolic blood pressure [female control mice: 120 ± 1.6 mmHg ( n = 5) vs. female KS-BMAL1 KO mice: 119 ± 1.5 mmHg ( n = 7), P = 0.4]. In addition, male KS-BMAL1 KO mice had less Na+ retention compared with control mice in response to a K+-restricted diet (15% less following 5 days of treatment). However, there was no genotype difference in Na+ handling after a K+-restricted diet in female mice. Furthermore, there was evidence indicating a sex-specific response to K+ restriction where female mice reabsorbed less Na+ in response to this dietary challenge compared with male mice. We propose that BMAL1 in the distal nephron and collecting duct contributes to blood pressure regulation and Na+ handling in a sex-specific manner.

SLC34A3/NPT2c/NaPi‐2c/Npt2c is a growth‐related NaPi cotransporter that mediates the uptake of renal sodium‐dependent phosphate (Pi). Mutation of human NPT2c causes hereditary hypophosphatemic rickets with hypercalciuria. Mice with Npt2c knockout, however, exhibit normal Pi metabolism. To investigate the role of Npt2c in Pi homeostasis, we generated α‐klotho−/−/Npt2c−/− (KL2cDKO) mice and analyzed Pi homeostasis. α‐Klotho−/− (KLKO) mice exhibit hyperphosphatemia and markedly increased kidney Npt2c protein levels. Genetic disruption of Npt2c extended the lifespan of KLKO mice similar to that of α‐Klotho−/−/Npt2a−/− mice. Adult KL2cDKO mice had hyperphosphatemia, but analysis of Pi metabolism revealed significantly decreased intestinal and renal Pi (re)absorption compared with KLKO mice. The 1,25‐dihydroxy vitamin D3 concentration was not reduced in KL2cDKO mice compared with that in KLKO mice. The KL2cDKO mice had less severe soft tissue and vascular calcification compared with KLKO mice. Juvenile KL2cDKO mice had significantly reduced plasma Pi levels, but Pi metabolism was not changed. In Npt2cKO mice, plasma Pi levels began to decrease around the age of 15 days and significant hypophosphatemia developed within 21 days. The findings of the present study suggest that Npt2c contributes to regulating plasma Pi levels in the juvenile stage and affects Pi retention in the soft and vascular tissues in KLKO mice., SLC34A3/NPT2c/NaPi‐2c/Npt2c is a growth‐related NaPi cotransporter that mediates the uptake of renal sodium‐dependent phosphate (Pi). Mutation of human NPT2c causes hereditary hypophosphatemic rickets with hypercalciuria. Npt2c contributes to regulating plasma Pi levels in the juvenile stage and affects Pi retention in the soft and vascular tissues in KLKO mice.

Most of the filtered glucose is reabsorbed in the early proximal tubule by the sodium‐glucose cotransporter SGLT2. The glycosuric effect of the SGLT2 inhibitor ipragliflozin is linked to a diuretic and natriuretic effect that activates compensatory increases in fluid and food intake to stabilize body fluid volume (BFV). However, the compensatory mechanisms that are activated on the level of renal tubules remain unclear. Type 2 diabetic Goto‐Kakizaki (GK) rats were treated with vehicle or 0.01% (in diet) ipragliflozin with free access to fluid and food. After 8 weeks, GK rats were placed in metabolic cages for 24‐hr. Ipragliflozin decreased body weight, serum glucose and systolic blood pressure, and increased fluid and food intake, urinary glucose and Na+ excretion, urine volume, and renal osmolar clearance, as well as urine vasopressin and solute‐free water reabsorption (TcH2O). BFV, measured by bioimpedance spectroscopy, and fluid balance were similar among the two groups. Urine vasopressin in ipragliflozin‐treated rats was negatively and positively associated with fluid balance and TcH2O, respectively. Ipragliflozin increased the renal membrane protein expression of SGLT2, aquaporin (AQP) 2 phosphorylated at Ser269 and vasopressin V2 receptor. The expression of SGLT1, GLUT2, AQP1, and AQP2 was similar between the groups. In conclusion, the SGLT2 inhibitor ipragliflozin induced a sustained glucosuria, diuresis, and natriuresis, with compensatory increases in fluid intake and vasopressin‐induced TcH2O in proportion to the reduced fluid balance to maintain BFV. These results indicate that the osmotic diuresis induced by SGLT2 inhibition stimulates compensatory fluid intake and renal water reabsorption to maintain BFV., We show in this study that SGLT2 inhibitor ipragliflozin induced a sustained glucosuria, diuresis, and natriuresis, with compensatory increases in fluid intake and vasopressin‐induced aquaporin‐2 phosphorylation and solute‐free water reabsorption in proportion to the reduced fluid balance to maintain body fluid volume These results indicate that the osmotic diuresis induced by SGLT2 inhibition stimulates compensatory fluid intake and renal water reabsorption to maintain body fluid volume.

Calcium-sensing receptor (CaSR) is a plasma-membrane G protein-coupled receptor activated by extracellular calcium and expressed in kidney tubular cells. It inhibits calcium reabsorption in the ascending limb and distal convoluted tubule when stimulated by the increase of serum calcium levels; therefore, these tubular segments are enabled by CaSR to play a substantial role in the regulation of serum calcium levels. In addition, CaSR increases water and proton excretion in the collecting duct and promotes phosphate reabsorption and citrate excretion in the proximal tubule. These CaSR activities form a network in which they are integrated to protect the kidney against the negative effects of high calcium concentrations and calcium precipitates in urine. Therefore, the CaSR gene has been considered as a candidate to explain calcium nephrolithiasis. Epidemiological studies observed that calcium nephrolithiasis was associated with polymorphisms of the CaSR gene regulatory region, rs6776158, located within the promoter-1, rs1501899 located in the intron 1, and rs7652589 in the 5'-untranslated region. These polymorphisms were found to reduce the transcriptional activity of promoter-1. Activating rs1042636 polymorphism located in exon 7 was associated with calcium nephrolithiasis and hypercalciuria. Genetic polymorphisms decreasing CaSR expression could predispose individuals to stones because they may impair CaSR protective effects against precipitation of calcium phosphate and oxalate. Activating polymorphisms rs1042636 could predispose to calcium stones by increasing calcium excretion. These findings suggest that CaSR may play a complex role in lithogenesis through different pathways having different relevance under different clinical conditions.

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4− and HPO42−). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.

Circulating fatty acid binding protein 4 (FABP4), secreted from adipocytes, is a potential biomarker for metabolic and cardiovascular diseases. Circulating FABP4 levels are positively associated with adiposity and adrenergic stimulation, but negatively with renal function. In this study, we addressed the issue of how the kidney regulates clearance of circulating FABP4. Tracing study revealed remarkable accumulation of 125I-labeled FABP4 in the kidney. Exogenous FABP4 was exclusively detected in the apical membrane of proximal tubule epithelial cells (PTECs). Bilateral nephrectomy resulted in marked elevation of circulating FABP4 levels. Accelerated lipolysis by β-3 adrenergic stimulation led to a marked elevation in circulating FABP4 in mice with severe renal dysfunction. Megalin, an endocytic receptor expressed in PTECs, plays a major role in reabsorption of proteins filtered through glomeruli. Quartz-crystal microbalance study revealed that FABP4 binds to megalin. In kidney-specific megalin knockout mice, a large amount of FABP4 was excreted in urine while circulating FABP4 levels were significantly reduced. Our data suggest that circulating FABP4 is processed by the kidney via the glomerular filtration followed by megalin-mediated reabsorption. Thus, it is likely that circulating FABP4 levels are determined mainly by balance between secretion rate of FABP4 from adipocytes and clearance rate of the kidney.

Phosphate homeostasis is primarily maintained in the renal proximal tubules, where the expression of sodium/phosphate cotransporters (Npt2a and Npt2c) is modified by the endocrine actions of both fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH). However, the specific contribution of each regulatory pathway in the proximal tubules has not been fully elucidated in vivo. We have previously demonstrated that proximal tubule-specific deletion of the FGF23 coreceptor Klotho results in mild hyperphosphatemia with little to no change in serum levels of FGF23, 1,25(OH)2D3, and PTH. In the present study, we characterized mice in which the PTH receptor PTH1R was specifically deleted from the proximal tubules, either alone or in combination with Klotho ( PT-PTH1R−/−and PT-PTH1R/KL−/−, respectively). PT-PTH1R−/−mice showed significant increases in serum FGF23 and PTH levels, whereas serum phosphate levels were maintained in the normal range, and Npt2a and Npt2c expression in brush border membrane (BBM) did not change compared with control mice. In contrast, PT-PTH1R/KL−/−mice displayed hyperphosphatemia and an increased abundance of Npt2a and Npt2c in the renal BBM, along with increased circulating FGF23 levels. While serum calcium was normal, 1,25(OH)2D3levels were significantly decreased, leading to extremely high levels of PTH. Collectively, mice with a deletion of PTH1R alone in proximal tubules results in only minor changes in phosphate regulation, whereas deletion of both PTH1R and Klotho leads to a severe disturbance, including hyperphosphatemia with increased sodium/phosphate cotransporter expression in BBM. These results suggest an important interplay between the PTH/PTH1R and FGF23/Klotho pathways to affect renal phosphate handling in the proximal tubules.

Hypertensive individuals are at greater risk for developing chronic kidney disease (CKD). Reducing proteinuria has been suggested as a possible therapeutic approach to treat CKD. However, the mechanisms underlying the development of proteinuria in hypertensive conditions are incompletely understood. Cardiac and vascular dysfunction is associated with changes in the O-GlcNAcylation pathway in hypertensive models. We hypothesized that O-GlcNAcylation is also involved in renal damage, especially development of proteinuria, associated with hypertension. Using the spontaneously hypertensive rat (SHR) model, we observed higher renal cortex O-GlcNAcylation, glutamine–fructose aminotransferase (GFAT), and O-GlcNAc transferase (OGT) protein expression, which positively correlated with proteinuria. Interestingly, this was observed in hypertensive, but not pre-hypertensive, rats. Pharmacological inhibition of GFAT decreased renal cortex O-GlcNAcylation, proteinuria, and albuminuria in SHR. Using a proximal tubule cell line, we observed that increased O-GlcNAcylation reduced megalin surface expression and albumin endocytosis in vitro, and the effects were correlated in vivo. Moreover, megalin is O-GlcNAcylated both in vitro and in vivo. In conclusion, our results demonstrate a new mechanism involved in hypertension-associated proteinuria.

Gestational potassium retention, most of which occurs during late pregnancy, is essential for fetal development. The purpose of this study was to examine mechanisms underlying changes in potassium handling by the kidney and colon in pregnancy. We found that potassium intake and renal excretion increased in late pregnancy while fecal potassium excretion remained unchanged and that pregnant rats exhibited net potassium retention. By quantitative PCR we found markedly increased H+-K+-ATPase type 2 (HKA2) mRNA expression in the cortex and outer medullary of late pregnant vs. virgin. Renal outer medullary potassium channel (ROMK) mRNA was unchanged in the cortex, but apical ROMK abundance (by immunofluorescence) was decreased in pregnant vs. virgin in the distal convoluted tubule (DCT) and connecting tubule (CNT). Big potassium-α (BKα) channel-α protein abundance in intercalated cells in the cortex and outer medullary collecting ducts (by immunohistochemistry) fell in late pregnancy. In the distal colon we found increased HKA2 mRNA and protein abundance (Western blot) and decreased BKα protein with no observed changes in mRNA. Therefore, the potassium retention of pregnancy is likely to be due to increased collecting duct potassium reabsorption (via increased HKA2), decreased potassium secretion (via decreased ROMK and BK), as well as increased colonic reabsorption via HKA2.

The nephron segments in the inner medulla are part of the urine concentrating mechanism. Depending on the diuretic state, they are facing a large range of extracellular osmolality. We investigated whether water homeostasis affects tubular transport and permeability properties in inner medullary descending thin limb (IMdTL) and ascending thin limb (IMaTL). Three experimental groups of rats under different diuretic states were investigated on metabolic cages: waterload, furosemide-induced diuresis, and control (antidiuresis). Urine production and osmolalities reflected the 3-day treatment. To functionally investigate tubular epithelial properties, we performed experiments in freshly isolated inner medullary thin limbs from these animals. Tubular segments were acutely dissected and investigated for trans- and paracellular properties by in vitro perfusion and electrophysiological analysis. IMdTL and IMaTL were distinguished by morphological criteria. We confirmed absence of transepithelial electrogenic transport in thin limbs. Although diffusion potential measurements showed no differences between treatments in IMdTLs, we observed increased paracellular cation selectivity under waterload in IMaTLs. NaCl diffusion potential was −5.64 ± 1.93 mV under waterload, −1.99 ± 1.72 mV under furosemide-induced diuresis, and 0.27 ± 0.40 mV under control. The corresponding permeability ratio PNa/Clwas 1.53 ± 0.21 (waterload), 1.22 ± 0.18 (furosemide-induced diuresis), and 0.99 ± 0.02 (control), respectively. Claudins are main constituents of the tight junction responsible for paracellular selectivity; however, immunofluorescence did not show qualitative differences in claudin 4, 10, and 16 localization. Our results show that IMaTLs change tight junction properties in response to diuretic state to allow adaptation of NaCl reabsorption.

The donor glomerular filtration rate (GFR) measured before kidney donation is a strong determinant of recipient graft outcome. No tubular function markers have been identified that can similarly be used in donors to predict recipient outcomes. In the present study we investigated whether the pre-donation tubular maximum reabsorption capacity of phosphate (TmP-GFR), which may be considered a functional tubular marker in healthy kidney donors, is associated with recipient GFR at 1 yr after transplantation, a key determinant of long-term outcome. We calculated the pre-donation TmP-GFR from serum and 24-h urine phosphate and creatinine levels in 165 kidney donors, and recipient125I-iothalamate GFR and eGFR (CKD-EPI) at 12 mo after transplantation. Kidney donors were 51 ± 10 yr old, 47% were men, and mean GFR was 118 ± 26 ml/min. The donor TmP-GFR was associated with recipient GFR 12 mo after transplantation (GFR 6.0 ml/min lower per 1 mg/dl decrement of TmP-GFR), which persisted after multivariable adjustment for donor age, sex, pre-donation GFR, and blood pressure and other potential confounders. Results were highly similar when eGFR at 12 mo was taken as the outcome. Tubular damage markers kidney injury molecule-1 and neutrophil gelatinase-associated lipocalin were low and not associated with recipient GFR. A lower donor TmP-GFR before donation, which may be considered to represent a functional measure of tubular phosphate reabsorption capacity, is independently associated with a lower recipient GFR 1 yr after transplantation. These data are the first to link donor tubular phosphate reabsorption with recipient GFR post-transplantation.

We previously reported the contribution of sodium-phosphate cotransporter to the tubular reabsorption of lithium in rats. In the present study, the dose dependency of the renal handling of lithium was examined in rats. When lithium chloride at 1.25 mg/kg, 2.5 mg/kg and 25 mg/kg was intravenously injected as a bolus, the areas under the plasma concentration-time curve of lithium until 60 minutes were calculated to be 6.23 mEq·min/l, 8.77 mEq·min/l and 64.6 mEq·min/l, respectively. The renal clearance of lithium and its fractional excretion increased with increments in the dose administered. The renal clearance of lithium strongly correlated with the urinary excretion rate of phosphate in the 1.25 mg/kg group (r = 0.840) and 2.5 mg/kg group (r = 0.773), whereas this correlation was weak in the 25 mg/kg group (r = 0.306). The infusion of foscarnet, a typical inhibitor of sodium-phosphate cotransporter, decreased the fractional reabsorption of lithium in rats administered lithium chloride at 2.5 mg/kg, but did not affect it in rats administered 25 mg/kg. These results demonstrate the nonlinearity of the renal excretion of lithium in rats, with the saturation of lithium reabsorption by the sodium-phosphate cotransporter potentially being involved.

We summarize the current understanding of the physiology of the renal handling of potassium (K+), and present an integrative view of the renal response to K+ depletion caused by dietary K+ restriction. This renal response involves contributions from different nephron segments, and aims to diminish the rate of excretion of K+ as a result of: decreasing the rate of electrogenic (and increasing the rate of electroneutral) reabsorption of sodium in the aldosterone-sensitive distal nephron (ASDN), decreasing the abundance of renal outer medullary K+ channels in the luminal membrane of principal cells in the ASDN, decreasing the flow rate in the ASDN, and increasing the reabsorption of K+ in the cortical and medullary collecting ducts. The implications of this physiology for the association between K+ depletion and hypertension, and K+ depletion and formation of calcium kidney stones are discussed.

Background: The serum phosphorus concentration ([P]s) is the sum of EP/Ccr and TRP/Ccr, where Ccr is creatinine clearance and EP and TRP are rates of excretion and reabsorption of phosphate. In chronic kidney disease (CKD), parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) mediate reduction of TRP/Ccr, and [PTH] and [FGF23] are linear functions of EP/Ccr. If controls and patients with CKD are considered together, TRP/Ccr is a hyperbolic function of EP/Ccr. Given these observations, we hypothesized that hyperbolas would describe relationships of phosphate reabsorption to [PTH] and [FGF23]. Methods: We studied 30 patients and 28 controls with mean eGFR of 29.5 and 86.0 mL/min/1.73 m2, respectively. All analyses combined both subsets. We measured fasting [PTH] 1–84 and intact [FGF23], and determined contemporaneous EP/Ccr, TRP/Ccr, fractional excretion of phosphorus (FEP), and phosphate tubular maximum per volume of filtrate (TmP/GFR). We examined linear regressions of TRP/Ccr and TmP/GFR on 100/[PTH] and 100/[FGF23]; from linear equations we derived hyperbolic equations relating reabsorptive parameters to hormone concentrations. Results: TRP/Ccr and TmP/GFR were linear functions of 100/[PTH] and 100/[FGF23] and hyperbolic functions of [PTH] and [FGF23]. TRP/Ccr and TmP/GFR fell minimally over the ranges of EP/Ccr, [PTH], and [FGF23] seen in CKD. FEP rose with EP/Ccr despite stable phosphate reabsorption. Conclusions: Hyperbolas describe relationships of TRP/Ccr and TmP/GFR to [PTH] and [FGF23] if subjects with normal and reduced GFR are analyzed together. Although FEP rises with [PTH] and [FGF23] as GFR falls, the simultaneous increments do not signify hormonally mediated reductions in phosphate reabsorption.

Renal phosphate handling critically determines plasma phosphate and whole body phosphate levels. Filtered phosphate is mostly reabsorbed by Na+-dependent phosphate transporters located in the brush border membrane of the proximal tubule: NaPi-IIa (SLC34A1), NaPi-IIc (SLC34A3), and Pit-2 (SLC20A2). Here we review new evidence for the role and relevance of these transporters in inherited disorders of renal phosphate handling. The importance of NaPi-IIa and NaPi-IIc for renal phosphate reabsorption and mineral homeostasis has been highlighted by the identification of mutations in these transporters in a subset of patients with infantile idiopathic hypercalcemia and patients with hereditary hypophosphatemic rickets with hypercalciuria. Both diseases are characterized by disturbed calcium homeostasis secondary to elevated 1,25-(OH)2 vitamin D3 as a consequence of hypophosphatemia. In vitro analysis of mutated NaPi-IIa or NaPi-IIc transporters suggests defective trafficking underlying disease in most cases. Monoallelic pathogenic mutations in both SLC34A1 and SLC34A3 appear to be very frequent in the general population and have been associated with kidney stones. Consistent with these findings, results from genome-wide association studies indicate that variants in SLC34A1 are associated with a higher risk to develop kidney stones and chronic kidney disease, but underlying mechanisms have not been addressed to date.