Proximal Tubular Reabsorption Research Articles

Tumor‐induced osteomalacia: Lessons learned

Tumor-induced osteomalacia is a rare acquired metabolic disorder characterized by hypophosphatemia and inappropriately low serum levels of 1,25-dihydroxyvitamin D. Symptoms include chronic muscle and bone pain, weakness, and fatigue in association with a high risk of fragility fractures due to osteomalacia. The diagnosis is commonly delayed for years due to the nonspecific nature of the presenting symptoms, failure to include determination of serum phosphorus levels in blood chemistry testing, and difficulty in identifying the responsible tumor. The pathogenesis of tumor-induced osteomalacia involves tumor expression of fibroblast growth factor 23, a hormone that inhibits proximal renal tubular reabsorption of phosphate and down-regulates renal conversion of 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D. The metabolic abnormalities may be partially or completely corrected with phosphate supplementation and calcitriol. A definitive diagnosis and treatment require excision of the responsible tumor.

Physiological Regulation and Disorders of Phosphate Metabolism -Pivotal Role of Fibroblast Growth Factor 23-

Fibroblast growth factor (FGF) 23 has been identified as the last member of FGF family. FGF23 reduces serum phosphate level by suppressing proximal tubular phosphate reabsorption and intestinal phosphate absorption. FGF23 is produced by bone and acts on the kidney through a specific receptor system which is composed of Klotho and certain subtypes of FGF receptors. Excess actions of FGF23 cause several hypophosphatemic diseases characterized by impaired renal phosphate reabsorption and rickets/osteomalacia. In contrast, deficient actions of FGF23 result in hyperphosphatemic tumoral calcinosis with enhanced renal phosphate reabsorption. These results indicate that FGF23 works as a hormone to regulate the serum phosphate level.

Parathyroid hormone inhibits renal phosphate transport by phosphorylation of serine 77 of sodium-hydrogen exchanger regulatory factor–1

Parathyroid hormone (PTH), via activation of PKC and/or protein kinase A, inhibits renal proximal tubular phosphate reabsorption by facilitating the internalization of the major sodium-dependent phosphate transporter, Npt2a. Herein, we explore the hypothesis that the effect of PTH is mediated by phosphorylation of serine 77 (S77) of the first PDZ domain of the Npt2a-binding protein sodium-hydrogen exchanger regulatory factor-1 (NHERF-1). Using recombinant polypeptides representing PDZ I, S77 of NHERF-1 is phosphorylated by PKC but not PKA. When expressed in primate kidney epithelial cells (BSC-1 cells), however, activation of either protein kinase phosphorylates S77, suggesting that the phosphorylation of PDZ I by PKC and PKA proceeds by different biochemical pathways. PTH and other activators of PKC and PKA dissociate NHERF-1/Npt2a complexes, as assayed using quantitative coimmunoprecipitation, confocal microscopy, and sucrose density gradient ultracentrifugation in mice. Murine NHERF-1-/- renal proximal tubule cells infected with adenovirus-GFP-NHERF-1 containing an S77A mutation showed significantly increased phosphate transport compared with a phosphomimetic S77D mutation and were resistant to the inhibitory effect of PTH compared with cells infected with wild-type NHERF-1. These results indicate that PTH-mediated inhibition of renal phosphate transport involves phosphorylation of S77 of the NHERF-1 PDZ I domain and the dissociation of NHERF-1/Npt2a complexes.

Index of Suspicion

Index of Suspicion

Size and charge selectivity of the glomerular filter in early experimental diabetes in rats

Microalbuminuria is an early sign of diabetic nephropathy. The aim of the present study was to investigate whether the changes of the glomerular filtration barrier in early experimental diabetes are due to size- or charge-selective alterations. Wistar rats, made diabetic by streptozotocin (STZ) and having their blood glucose maintained at approximately 20 mM for 3 or 9 wk, were compared with age-matched controls. Glomerular clearances of native albumin (Cl-HSA) and neutralized albumin (Cl-nHSA) were assessed using a renal uptake technique. Glomerular filtration rate and renal plasma flow were assessed using (51)Cr-EDTA and [125I]iodohippurate, respectively. In a separate set of animals, diabetic for 9 wk, and in controls, glomerular sieving coefficients (theta) for neutral FITC-Ficoll (molecular radius: 15-90 A) were assessed using size exclusion chromatography. At 3 wk of diabetes, Cl-HSA and Cl-nHSA remained unchanged, indicating no alteration in either size or charge selectivity. By contrast, at 9 wk of diabetes, there was a twofold increase of Cl-HSA, whereas Cl-nHSA remained largely unchanged, at first suggesting a glomerular charge defect. However, according to a two-pore model, the number of large pores, assessed from both Ficoll and Cl-HSA, increased twofold. In addition, a small reduction in proximal tubular reabsorption was observed at 3 wk, which was further reduced at 9 wk. In conclusion, no functional changes were observed in the glomerular filtration barrier at 3 wk of STZ-induced diabetes, whereas at 9 wk there was a decrease in size selectivity due to an increased number of large glomerular pores.

P1,P4-Diadenosine Tetraphosphate (Ap4A) Inhibits Proximal Tubular Reabsorption of Sodium in Rats

Background/Aims: P¹,P⁴-diadenosine tetraphosphate (Ap₄A) is a vasoactive dinucleotide possessing natriuretic activity. It is unclear, however, which part of the nephron is the target site of action for Ap₄A. Methods: We evaluated the tubular sites of Ap₄A action using the lithium clearance technique. Results: Ap₄A at a priming dose of 2 µmol/kg with subsequent infusion at 20 nmol/kg/min increased fractional water and sodium excretion 2.5- and 5.6-fold, respectively. Moreover, Ap₄A increased lithium clearance 1.9-fold and fractional lithium excretion 2.8-fold. Fractional water and sodium excretion from distal nephron segments was not significantly affected by Ap₄A. Conclusion: These results suggest that Ap₄A induces natriuresis mainly through inhibition of proximal tubular reabsorption of sodium.

Rapid Correction of Severe Hyponatremia After Computed Tomography With Intravenous Contrast

To the Editor: Overly rapid correction of hyponatremia is a major concern in severely hyponatremic patients because of the potential development of osmotic demyelination. Numerous factors can contribute to overly rapid correction of hyponatremia (Table 1).1Pham P-CT Pham P-MT Miller J et al.Treatment of chronic hyponatremia.Proc UCLA Healthcare. Fall 2002; 6: 32-36Google Scholar We report a case in which a patient developed osmotic diuresis after contrast dye injection that rapidly self-corrected the severe hyponatremia.TABLE 1Potential Causes of Overly Rapid Correction of HyponatremiaAdapted from Proceedings of UCLA HealthCare,1Pham P-CT Pham P-MT Miller J et al.Treatment of chronic hyponatremia.Proc UCLA Healthcare. Fall 2002; 6: 32-36Google Scholar with permission. Unawareness of the recommended rate of correction for hyponatremiaMiscalculationsUnrecognized sources of sodium Exogenous Sodium administration at another site before transfer to the current unitAddition of excess salt to patient's diet by patient's family or facility staff membersEndogenous Potassium supplementation with resultant extracellular sodium shiftExcess free water loss Water deprivation in primary polydipsiaWithdrawal of thiazide diuretics during treatment of hyponatremiaGlucocorticoid replacement in a cortisol-deficient patientRecovery from acute respiratory failureExcessive gastrointestinal, pulmonary, or skin hypotonic fluid lossSevere hyponatremia with subsequent cerebral edema and pituitary infarctionNonelectrolyte osmotic diuresis: urea, glucose, mannitol, contrast dye, total parenteral nutritionIntracellular free water shift SeizuresRhabdomyolysisCorrection of hyperglycemiaPotassium supplementation Open table in a new tab Report of a Case. A 59-year-old woman with diabetes mellitus and hypertension presented with a 1-week history of slow mentation, fatigue, nausea, vomiting, abdominal pain, and subjective left hemiparesthesia. A dietary history revealed that she routinely ate only fruits and cereals and drank only water. Her medications included hydrochlorothiazide, fosinopril, atenolol, metformin, and aspirin. Physical examination revealed dry oral mucosa, slow mentation, and generalized sluggish deep tendon reflexes. On initial evaluation, the serum sodium concentration was 115 mEq/L, and urinary studies revealed an osmolality of 526 mosm/kg, a sodium level of 130 mEq/L, and a chloride level of less than 10 mEq/L. The patient was given two 500-mL boluses of normal saline during the first 12 hours of hospitalization. In the interim, she underwentcomputed tomography of the head and abdomen with intravenous contrast (iohexol). Despite correction of the hypovolemia, the patient had no improvement in her symptoms or serum sodium concentration during the first 24 hours. However, approximately 24 to 26 hours after presentation, she developed progressive polyuria (up to 150-175 mL/h). During the polyuric phase, the urine osmolality ranged from 217 to 393 mosm/kg. During the subsequent few hours, the patient's mentation noticeably improved in association with a spontaneous rapid increase in serum sodium concentration (8 mEq/L over 6 hours). In fact, hypotonic saline had to be administered to halt the rapid self-correction of the hyponatremia. Given the patient's history of poor dietary nutrition, use of hydrochlorothiazide, and current pain and vomiting, we suspected that the hyponatremia resulted from ongoing water intake concurrent with the patient's inability to excrete free water maximally because of a reduced dietary solute load and syndrome of inappropriate secretion of antidiuretic hormone-induced enhanced free water reabsorption.2Fichman MP Vorherr H Kleeman CR Telfer N Diuretic-induced hyponatremia.Ann Intern Med. 1971; 75: 853-863Crossref PubMed Scopus (239) Google Scholar, 3Thaler SM Teitelbaum I Berl T “Beer potomania” in non-beer drinkers: effect of low dietary solute intake.Am J Kidney Dis. 1998; 31: 1028-1031Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar Despite saline infusion, the patient's serum sodium concentration remained at 115 mEq/L 12 to 18 hours after presentation, presumably a reflection of the sustained syndrome of inappropriate secretion of antidiuretic hormone caused by her nausea and pain. However, by 24 hours after presentation, the patient had a rapid increase in urinary output as well as serum sodium concentration despite the discontinuation of all intravenous fluids at least 6 hours previously. At that point, the patient possibly had adequate volume reexpansion, reduced proximal tubular reabsorption of salt and water, and resultant increased distal delivery for excretion. In addition, the control of nausea and pain and the volume reexpansion may have inhibited antidiuretic hormone secretion sufficiently to result in the aquaretic phase. However, subsequent urinary electrolyte studies revealed high osmolality (217-393 mosm/kg) relative to serum osmolality (240-264 mosm/kg), which suggested that she had osmotic diuresis, not aquaresis. Further analysis of her urine by ultraviolet spectrophotometry revealed that the predominant source of the urine osmolality during the polyuric phase was the contrast dye iohexol. The findings in this case are important because computed tomography with intravenous contrast is performed routinely for the evaluation of nonspecific complaints in severely hyponatremic patients.

Renal function during normal pregnancy and preeclampsia

Glomerular filtration rate and renal plasma flow increase by 40 to 65 and 50 to 85%, respectively, during normal pregnancy in women. Studies using the gravid rat as a model have greatly enhanced our understanding of mechanisms underlying these remarkable changes in the renal circulation during gestation. Hyperfiltration is largely due to increased renal plasma flow, the latter attributable to profound reductions in both the renal afferent and efferent arteriolar resistances. The ovarian hormone, relaxin, mediates renal vasodilation during pregnancy. Relaxin increases vascular gelatinase activity, thereby converting big ET to ET(1-32), which leads to renal vasodilation, hyperfiltration and reduced myogenic reactivity of small renal arteries via the endothelial ET(B) receptor and nitric oxide. Serum concentration of uric acid falls during normal pregnancy as a consequence of increased GFR and/or reduced proximal tubular reabsorption. The elevated urinary excretion of protein during pregnancy is secondary to increased GFR, reduced proximal tubular reabsorption, and perhaps alteration in the electrostatic charge of the glomerular filter. Whether the tubular secretion of Tamm-Horsfall protein increases during normal pregnancy is uncertain. In most women with preeclampsia, renal plasma flow and glomerular filtration rate are at most only modestly decreased as a consequence of increased afferent arteriolar resistance and/or reduced ultrafiltration coefficient. Serum uric acid concentrations are increased mainly as a consequence of reduced renal clearance. Reduced GFR leads to decreased filtered load of uric acid, and plasma volume contraction contributes to increased proximal tubular reabsorption coupled to sodium. The increase in urinary protein excretion in preeclampsia occurs secondary to alterations in the size and/or charge selectivity of the glomerular filter, possible increases in glomerular capillary pressure, and compromise of proximal tubular reabsorption. The renal histologic lesion characteristic of preeclampsia is termed "glomerular endotheliosis". Recent evidence suggests that anti-angiogenic factors emanating from the placenta in preeclampsia contribute to glomerular endotheliosis, proteinuria, and hypertension during disease.

Elevated Serum Adiponectin Level is Associated with a Decrease in Rate of Proximal Tubular Sodium Reabsorption

Elevated Serum Adiponectin Level is Associated with a Decrease in Rate of Proximal Tubular Sodium Reabsorption

Role of brain angiotensin AT1 receptor in the carbachol-induced natriuresis and expression of nNOS in the locus coeruleus and proximal convoluted tubule.

Central administration of losartan effectively blocked the increase of blood pressure and drinking response induced by angiotensin II (Ang II) or carbachol. However, the relationship between angiotensin AT(1) receptors and the natriuresis induced by brain cholinergic stimuli is still not clear. The purpose of the study is to reveal the role of brain angiotensin AT(1) receptor in the carbachol-induced natriuresis and expression of neuronal nitric oxide synthase (nNOS) in the locus coeruleus (LC) and proximal convoluted tubule (PCT). Our results indicated that 40 min after intracerebroventricular (ICV) injection of carbachol (0.5 microg), urinary sodium excretion was significantly increased to 0.548+/-0.049 micromol x min(-1) x 100 g(-1). Immunohistochemistry showed that carbachol induced an increase of neuronal nitric oxide synthase immunoreactivity (nNOS-IR) in the LC and renal proximal tubular cells. After pretreatment with losartan (20 microg), carbachol-induced urinary sodium excretion was reduced to 0.249+/-0.067 micromol x min(-1) x 100 g(-1). The same was true for carbachol-induced increase of nNOS-IR in the LC and PCT. The present data suggest that ICV cholinergic stimulation could induce a natriuresis and upregulate the activity of nNOS in the LC and PCT. The blockade of AT(1) receptors might downregulate the effects induced by carbachol in the LC and PCT. Consequently, we provide a new evidence that brain angiotensinergic pathway and NO-dependent neural pathway contribute to the natriuresis following brain cholinergic stimulation and thus play an important role in the regulation of fluid homeostasis. Furthermore, the final effect of nitric oxide on proximal tubular sodium reabsorption participated in the natriuresis induced by brain cholinergic stimulation.

Lithium use in special populations

Byline: E. Mohandas, V. Rajmohan Lithium, a monovalent cation, was first used for the treatment of mania in the 1940s. Half a century into its use, the drug continues to be the preeminent choice for bipolar disorder with antimanic, antidepressant (modest) and antisuicidal property. Lithium is the "gold standard" mood stabilizer against which potential mood stabilizer agents are judged. The therapeutic uses of lithium also include use as an augmenting agent in depression, schizoaffective disorder, aggression, impulse control disorder, eating disorders, attention deficit disorder and in certain subsets of alcoholism. Lithium has been used in many medical disorders, especially cluster headache and dermatological disorders (seborrheic dermatitis, eczematoid dermatitis, genital herpes).[sup] [1] The drug is however associated with neurologic, endocrine, cardiovascular, renal, dermatologic and gastrointestinal adverse effects and possible teratogenicity. History Lithium was first discovered and defined by Johan August Arfvedson in 1817 when he did an analysis of the mineral petalite [LiAl(Si2O5)2]. Petalite was first found by Brazilian scientist Jose Bonifacio Andrade e Silva in 1800. Lepidolite, spodumene, petalite and amblygonite are the more important minerals containing lithium. It was Arfwedson's laboratory chief John Jacob Berzelius who named this alkali metal "lithion." Arfvedson was never able to fully isolate lithium, and it wasn't until 1855 that it was isolated by William Thomas Brande. Brande and Sir Humphrey Davy earlier had done electrolysis on lithium oxide in 1818. Lithium was first produced commercially in 1923 by Metallgesellschaft AG.[sup] [2] The use of lithium for medicinal purposes can be traced back 1,800 years to the Greek physician Galen, who treated patients with mania by having them bathe in alkaline springs and drink the water, which probably contained lithium. In 1843 Alexander Ure introduced lithium into modern medicine, and he showed the in vitro reduction of weight of a uric acid bladder stone in a lithium carbonate solution. Sir Alfred Garrod later discovered that gouty uric acid deposits also were soluble in lithium solution. The view in that time was that uric acid imbalances caused a wide range of diseases, and Armand Trousseau and Alexander Haig proposed that mania and depression also may result from this imbalance and lithium may be effective in these conditions. In the 1840s, lithium was mixed with carbonate or citrate to form a salt and was used to treat gout, epilepsy, diabetes, cancer and insomnia. In the 1870s, the then American Surgeon General William Hammond had provided anecdotal evidence for the use of lithium bromide in the treatment of acute mania. In the 1880s and 1890s the Lange brothers Carl and Fritz used lithium in depression, and Carl Lange was the first to systematically use lithium in the acute and prophylactic treatment of depression.[sup] [2] The introduction of lithium preparations and tablets in the 1900s brought to the fore the toxic effects of the drug; and there were reports of weakness, tremor, diarrhea, vomiting and deaths. The drug disappeared from the British Pharmacopoeia by 1932, but later in the 1940s it was used as a sodium substitute in low sodium diets; but the reports of severe intoxication led to its removal from American markets in 1949. [sup] [1] The appearance in 1949 in the Medical Journal of Australia of a paper entitled "Lithium salts in the treatment of psychotic excitement" by John F. J. Cade was an unspectacular entry into a new era of psychiatry. Manic patients showed improvement, with the patient becoming calmer after four to five days. There was no improvement in the excited schizophrenic patients, though there was a calming effect. There was no improvement or deepening of depression. The paper also gave details of initial dosage, maintenance doses, appearance of toxic symptoms and warning about lithium over-dosage. …

OCTN2-Mediated Carnitine Uptake in a Newly Discovered Human Proximal Tubule Cell Line (Caki-1)

The proximal tubular reabsorption of carnitine in the human kidney is significant because more than 95% of the carnitine filtered in the kidney is reabsorbed by the proximal tubules therefore maintaining the homeostatic balance of carnitine in the body. Objectives of this study include the characterization of OCTN2 function in the Caki-1 cell line and the potential interactions of carnitine uptake with renally secreted drugs, including drugs of quaternary ammonium structure. Caki-1 cells were additionally characterized to be of proximal tubule nature, and an apical membrane expression pattern of OCTN2 in Caki-1 cells was discovered. Uptake studies with radiolabeled L-carnitine in Caki-1 cells revealed a Na+- and temperature-dependent carrier-mediated process (K(m) = 15.90 microM) which was unaffected by pH in a range from 6.5 to 8.5. All drugs tested were able to inhibit the carnitine uptake process to different degrees. The quaternary ammonium compounds ciclotropium bromide and ipratropium bromide were strong inhibitors with IC(50) values of 30 microM and 95 microM, respectively. The observed kinetics, immunohistolocalization, and inhibition studies indicate that the high-affinity uptake of carnitine in the Caki-1 cell line is most likely mediated by OCTN2. The interaction of drugs at the renal level with OCTN2 indicates a possible pathway for the final step of cationic secretion into the urine.

Lipid Changes and Statins in Chronic Renal Insufficiency

It has been known for a long time that chronic kidney disease (CKD) is associated with dyslipidemia, but the full extent of abnormalities has been appreciated only recently, because routine laboratory tests fail to disclose the entire spectrum of lipid abnormalities. Lipids, particularly HDL cholesterol, are predictive of cardiovascular events, but a paradoxic inverse relation between cholesterol concentration and cardiovascular death has been noted in uremic patients. This currently is thought to be explained by the confounding effect of microinflammation and possibly calcification, but this is not definitely proved. Several retrospective analyses that included patients with mild or moderate CKD documented benefit from lowering of cholesterol by statins. In contrast, the Die Deutsche Diabetes Dialyse (4D) study and a small Scandinavian study failed to show a benefit from lowering of cholesterol by statins in ESRD. Pathomechanistically, it is possible that nonclassical pathomechanisms override statin-sensitive mechanisms as also suggested by the observation that statins fail to reduce carotid intima-media thickening. Although, experimentally, exposure to lipids (particularly oxidized lipids) aggravates progression, data on the effect of statins on progression in patients with CKD are not definite. The most likely explanation is that the impact of numerous confounders obscures their effect on progression. The increase in urinary protein excretion of patients who are treated with statins had been a cause of concern, but the underlying mechanism (i.e. interference with proximal tubular reabsorption of protein) meanwhile has been well documented.

Mechanisms of renal blood flow autoregulation: dynamics and contributions

Autoregulation of renal blood flow (RBF) is caused by the myogenic response (MR), tubuloglomerular feedback (TGF), and a third regulatory mechanism that is independent of TGF but slower than MR. The underlying cause of the third regulatory mechanism remains unclear; possibilities include ATP, ANG II, or a slow component of MR. Other mechanisms, which, however, exert their action through modulation of MR and TGF are pressure-dependent change of proximal tubular reabsorption, resetting of RBF and TGF, as well as modulating influences of ANG II and nitric oxide (NO). MR requires < 10 s for completion in the kidney and normally follows first-order kinetics without rate-sensitive components. TGF takes 30-60 s and shows spontaneous oscillations at 0.025-0.033 Hz. The third regulatory component requires 30-60 s; changes in proximal tubular reabsorption develop over 5 min and more slowly for up to 30 min, while RBF and TGF resetting stretch out over 20-60 min. Due to these kinetic differences, the relative contribution of the autoregulatory mechanisms determines the amount and spectrum of pressure fluctuations reaching glomerular and postglomerular capillaries and thereby potentially impinge on filtration, reabsorption, medullary perfusion, and hypertensive renal damage. Under resting conditions, MR contributes approximately 50% to overall RBF autoregulation, TGF 35-50%, and the third mechanism < 15%. NO attenuates the strength, speed, and contribution of MR, whereas ANG II does not modify the balance of the autoregulatory mechanisms.

Abnormal Circadian Rhythm of Diuresis or Nocturnal Polyuria in a Subgroup of Children With Enuresis and Hypercalciuria is Related to Increased Sodium Retention During Daytime

In a subgroup of children with enuresis an increase in nighttime water and solute excretion has been documented. To investigate if modifications in renal function are involved in nocturnal enuresis, we assessed circadian variation in natriuresis and tubular sodium handling in polyuric hypercalciuric children. A total of 10 children with proved hypercalciuria and nocturnal polyuria and 10 age matched controls were included in the study. A 24-hour urine collection was performed in 8 sampling periods for measurement of urinary sodium excretion. Segmental tubular sodium transport was investigated during a daytime oral water load test and calculated according to standardized clearance methodology. The children with enuresis showed a marked increase in the fractional excretion of sodium during the night (0.93% +/- 0.36%), while daytime sodium excretion was decreased (0.84% +/- 0.23%). Analysis of segmental tubular sodium transport revealed decreased delivery of sodium to distal tubule (C(H2O) + C(Na) = 10.7 ml/100 ml glomerular filtration rate), indicating increased proximal tubular sodium reabsorption but also stimulation of distal sodium reabsorption as demonstrated by increased fractional distal sodium reabsorption (92.9% +/- 2.2%, controls 90.5% +/- 2.9%). Increased distal reabsorption was associated with increased fractional potassium excretion (17.5% +/- 2.7%, controls 13.6% +/- 6.4%), indicating increased distal tubular sodium/potassium exchange. No intrinsic defect in renal tubular sodium transport was found, but during the day increased sodium reabsorption in proximal and distal tubules was observed, suggesting extrarenal factors to be involved in altered circadian variation in solute and water excretion by the kidney.

Unchanged expression of the sodium-dependent phosphate cotransporter NaPi-IIa despite diurnal changes in renal phosphate excretion

Renal phosphate excretion is subjected to circadian rhythmicity. The bulk of filtered inorganic phosphate (P(i)) is reabsorbed by the sodium-dependent phosphate cotransporter NaPi-IIa. The regulation of proximal tubular phosphate reabsorptive capacity is largely attributed to the altered abundance of NaPi-IIa residing in the brush border membrane (BBM) of proximal tubular cells. Therefore, we examined if the diurnal rise in renal phosphate excretion is accompanied by a corresponding change in NaPi-IIa expression. Renal phosphate excretion, creatinine clearance, and serum phosphate were determined at consecutive time points in rats, starting from 8 a.m. until 5 p.m. During this period, renal phosphate excretion (fractional P(i) excretion) increased more than eightfold until 5 p.m. compared to the morning values at 8 a.m. In addition, serum phosphate and creatinine clearance as well as the calculated tubular phosphate threshold increased. Neither immunoblot analysis of BBMs nor immunohistochemical staining for NaPi-IIa yielded evidence for a lower abundance of NaPi-IIa in kidneys collected in the afternoon compared to those in the morning. However, kidneys sampled in the afternoon showed a small decrease (14%) in (32)P uptakes into BBM vesicles (BBMVs). Thus, the diurnal rise in renal phosphate excretion was associated with a mild reduction in the sodium-dependent phosphate transport rate in proximal tubular BBMs. There was no apparent downregulation of NaPi-IIa abundance and only a small reduction in Na(+)-dependent Pi-transport activity. Thus, the diurnal changes in urinary phosphate excretion appear to be mainly related to changes in serum phosphate and tubular threshold but not to NaPi-IIa expression.

Aquaporins in endothelia

Aquaporin-1 (AQP1) water channels are expressed widely in microvascular endothelia outside of the central nervous system, including renal vasa recta and tumor microvessels, as well as in non-vascular endothelia in pleura, peritoneum, cornea, and lymphatics. In kidney, AQP1-facilitated water transport in outer medullary descending vasa recta is likely an important component of the urinary concentrating mechanism. However, in most vascular endothelia outside of kidney, it remains uncertain whether AQP1 expression and high water permeability are physiologically important. AQP1 in non-vascular endothelia at the inner corneal surface is involved in the maintenance of corneal transparency. Recently, a new role of AQP1 in endothelial cell migration was discovered in analyzing the cause of defective tumor angiogenesis in AQP1-deficient mice. AQP1 facilitates endothelial cell migration by a mechanism that may involve facilitated water transport across cell protrusions (lamellipodia). AQP1 inhibitors may thus have aquaretic and antiangiogenic activity.

The effect of single oral low-dose losartan on posture-related sodium handling in post-TIPS ascites-free cirrhosis

Post-TIPS ascites-free patients with cirrhosis and previous refractory ascites demonstrate subtle sodium retention when challenged with a high sodium load. This is also observed in pre-ascitic patients with cirrhosis. This phenomenon is dependent on an intrarenal angiotensin II (ANG II) mechanism related to the assumption of erect posture. We investigated whether similar mechanisms were involved in post-TIPS ascites-free patients, by studying 10 patients with functioning TIPS and no ascites. We measured the effect of changing from supine to erect posture on sodium excretion at baseline and after single oral low dose losartan (7.5 mg) which has been shown to blunt proximal and distal tubular sodium reabsorption in pre-ascites. At baseline, the assumption of erect posture produced a reduction in sodium excretion (from 0.30+/-0.06 to 0.13+/-0.02 mmol/min, P=.05), which was mainly due to an increase in proximal tubular reabsorption of sodium (PTRNa) (69.7+/-3.1% to 81.1+/-1.8%, P=.003). The administration of losartan resulted in a blunting of PTRNa (supine 69.7+/-3.1% to 63.9+/-3.9%, P=.01 and erect 81.1+/-1.8% to 73.8+/-2.4%, P=.01), accompanied by an increased distal tubular reabsorption of sodium in both postures, with no overall improvement in sodium excretion on standing. In conclusion, post-TIPS ascites-free patients with cirrhosis exhibit erect posture-induced sodium retention. We speculate that (1) this effect is partly mediated by the effect of ANG II on PTRNa and (2) that the inability of low dose losartan to block the erect posture-induced sodium retention may be related to the erect posture-induced rise in aldosterone which is unmodified by losartan.

Nephron Function in Transgenic Mice with Selective Vascular or Tubular Expression of Angiotensin-Converting Enzyme

Angiotensin-converting enzyme (ACE) null mice display aberrant renal pathology. Inadequate formation of angiotensin II (Ang II) results in hypotension, loss of fluid homeostasis, lack of urine concentration, and failure to regulate GFR through the tubuloglomerular feedback (TGF) mechanism. For examining the tissue-specific role of ACE in renal structure and regulation of renal filtrate formation, single-nephron GFR, proximal tubular fluid reabsorption, and TGF responsiveness were determined in mice that expressed ACE in only one tissue. Maximum TGF responses in mice that expressed somatic ACE (sACE) in proximal tubule cells (Gs strain) or germinal ACE in the serum (Pg strain) were reduced significantly compared with wild-type (WT) mice. In contrast, TGF responses in mice that expressed sACE in vascular endothelial cells (Ts strain) were not different from control. Single-nephron GFR was reduced in Ts compared with WT mice, but fractional reabsorption and therefore glomerulotubular balance were not distinguishable. BP responses to exogenous Ang I were diminished in Ts, Gs, and Pg mice, whereas those to Ang II were the same in the different strains. Plasma and renal tissue Ang I of all transgenic mouse strains was significantly higher than WT, whereas Ang II levels were generally lower; aldosterone levels were significantly lower than WT in Ts mice but not in the two other transgenic strains. Our results demonstrate that vascular expression of sACE can largely but not completely restore TGF regulation of GFR. Proximal fluid reabsorption in the chronic absence of proximal tubule ACE is normal.

Is tenofovir involved in hypophosphatemia and decrease of tubular phosphate reabsorption in HIV-positive adults?

Tubulopathy with hypophosphatemia have been observed in HIV-positive patients receiving a tenofovir-containing regimen. However, the real incidence and prevalence of hypophosphatemia and their relation to tubular reabsorption disorders in tenofovir-treated patients remain uncertain. The aim of our study was to explore the effect of tenofovir on phosphatemia and on tubular phosphate reabsorption. In a first transversal study, 145 HIV-positive adults (44+/-9 years) receiving tenofovir 300 mg daily with a mean exposure of 11+/-9 months were included. In a second prospective study, 29 HIV-positive antiretroviral experienced adults (44+/-10 years) were evaluated before introduction of tenofovir 300 mg daily (M0) and at 3 months (M3) and 6 months (M6), thereafter. Phosphate, creatinine, glucose and protein levels were determined in plasma and urine. The ratio of maximal reabsorption capacity (TmPO4)/glomerular filtration rate (GFR) was determined by using the normogramm of Walton and Bijvoet. In the transversal study, 26% of patients had hypophosphatemia (<0.84 mmol/l) while 47% of patients had a decreased TmPO4/GFR (<0.8 mmol/l). In the prospective study, baseline prevalence of hypophosphatemia (<0.84 mmol/l) and decreased TmPO4/GFR (<0.8mmol/l) was 31 and 41%, respectively. Three and 6 months after starting tenofovir, there is no significant change in mean phosphate levels (M0:0.91 mmol/l, M3:0.97 mmol/l, M6:0.98 mmol/l) and mean TmPO4/GFR (M0:0.80 mmol/l, M3:0.88 mmol/l, M6:0.84 mmol/l). Moreover, prevalence of hypophosphatemia (M3:28%, M6:28%) and decreased TmPO4/GFR (M3:41%, M6:45%) remained stable. Hypophosphatemia linked to a decreased proximal tubular reabsorption was frequently observed in HIV-positive adults independently of the use of tenofovir. In this preliminary study, no worsening effect on phosphatemia and tubular phosphate reabsorption was observed 6 months after introduction of tenofovir in treatment experienced patients.