Nephron Physiology最新文献

Background/aims: Dopamine (DA) uptake inhibition in the renal cortex, elicited by angiotensin II (ANG II), is mediated by AT(1) receptors and signals through the phospholipase C pathway and activation of protein kinase C and CaM-kinase II. By this indirect way, ANG II stimulates renal Na(+),K(+)-ATPase activity through DA intracellular reduction. In the present work, we continued to study different aspects of renal DA metabolism in DA-ANG II interaction, such as DA synthesis, release, catabolism and turnover.

Methods: ANG II effects on DA synthesis, release, catabolism and turnover were measured in samples from the outer renal cortex of Sprague-Dawley rats.

Results: ANG II reduced renal aromatic acid decarboxylate activity without affecting basal secretion of DA or its KCl-induced release. Moreover, ANG II enhanced monoamine oxidase activity without altering catechol-o-methyl transferase activity and increased DA turnover.

Conclusion: Current results as well as previous findings show that ANG II modifies DA metabolism in rat renal cortex by reducing DA uptake, decreasing DA synthesis enzyme activity and increasing monoamine oxidase activity, and DA turnover. Together, all these effects may reduce DA accumulation into renal cells and decrease its endogenous content and availability. This would prevent D1 receptor recruitment and stimulation, while diminishing DA inhibition of Na(+),K(+)-ATPase activity and stimulating sodium reabsorption.

Background/aims: Nephrogenic diabetes insipidus (NDI) is a serious condition with large water losses in the urine and the risk of hypernatremic dehydration. Unrecognized, repeated episodes of hypernatremic dehydration can lead to permanent brain damage. Primary NDI is due to mutations in either AVPR2 or AQP2. NDI can also occur as a secondary complication, most commonly from obstructive uropathy or chronic lithium therapy. We observed NDI in patients with inherited tubulopathies and aimed to define the clinical and molecular phenotype.

Methods: We reviewed the medical notes of 4 patients with clinical NDI and an underlying molecularly confirmed diagnosis of nephropathic cystinosis, Bartter syndrome, nephronophthisis and apparent mineralocorticoid excess, respectively.

Results: The patients all failed to concentrate their urine after administration of 1-desamino[8-D-arginine] vasopressin. None had an identifiable mutation in AVPR2 or AQP2, consistent with secondary NDI. Patients experienced repeated episodes of hypernatremic dehydration, and in 2 cases, NDI was initially thought to be the primary diagnosis, delaying recognition of the underlying problem.

Conclusion: The recognition of this potential complication is important as it has direct implications for clinical management. The occurrence of NDI in association with these conditions provides clues for the etiology of aquaporin deficiency.

Introduction: X-linked hypophosphatemia (XLH) is characterized by renal phosphate wasting with hypophosphatemia, short stature, and rachitic manifestations.

Clinical picture: We describe a novel nonsense mutation in exon 3 of the PHEX gene (Glu(96)X (c.286G>T) causing XLH in a mother and daughter of Indian ancestry. The mother was noted to have concomitant vitamin D insufficiency.

Conclusion: Our report identifies a novel nonsense mutation in the PHEX gene causing XLH. It also highlights the fact that the presence of concomitant vitamin D insufficiency should not preclude the diagnosis of familial forms of hypophosphatemic rickets, especially if more than one family member is affected.

Background: Altered renal sodium handling has a major pathogenic role in salt-sensitive hypertension. Renal sodium transporters are present in urinary exosomes. We hypothesized that sodium transporters would be excreted into the urine in different amounts in response to sodium intake in salt-sensitive versus salt-resistant patients.

Methods: Urinary exosomes were isolated by ultracentrifugation, and their content of Na-K-2Cl cotransporter (NKCC2) and Na-Cl cotransporter (NCC) was analyzed by immunoblotting. Animal studies: NKCC2 and NCC excretion was measured in 2 rat models to test whether changes in sodium transporter excretion are indicative of regulated changes in the kidney tissue. Human studies: in hypertensive patients (n = 41), we investigated: (1) a possible correlation between sodium reabsorption and urinary exosomal excretion of sodium transporters, and (2) the profile of sodium transporter excretion related to blood pressure (BP) changes with salt intake. A 24-hour ambulatory BP monitoring and a 24-hour urine collection were performed after 1 week on a low- and 1 week on a high-salt diet.

Results: Animal studies: urinary NKCC2 and NCC excretion rates correlated well with their abundance in the kidney. Human studies: 6 patients (15%) were classified as salt sensitive. The NKCC2 and NCC abundance did not decrease after the high-salt period, when the urinary sodium reabsorption decreased from 99.7 to 99.0%. In addition, the changes in BP with salt intake were not associated with a specific profile of exosomal excretion.

Conclusions: Our results do not support the idea that excretion levels of NKCC2 and NCC via urinary exosomes are markers of tubular sodium reabsorption in hypertensive patients.

Background/aims: Patients treated with peroxisome proliferator-activated receptor analogs (PPAR) alpha or alpha/gamma may develop a transient and reversible increase in serum creatinine, the mechanism of which remains obscure. This study evaluates whether treatment with either PPAR-alpha or -alpha/gamma analogs, fenofibrate or tesaglitazar, may cause deterioration in renal hemodynamics or exert direct tubular or glomerular nephrotoxic effects in rats.

Methods: Male Sprague-Dawley rats (300-320 g) were treated per os with fenofibrate (300 mg/kg/day), tesaglitazar (1.2 mg/kg/day) or vehicle, for 14 days. Glomerular filtration rate (GFR) and renal blood flow (RBF) were measured by inulin clearance and ultrasonic flowmetry, and cumulative excretion of sodium and creatinine were assessed. Biomarkers of glomerular and tubular injury were measured, including urinary albumin excretion and renal mRNA levels of kidney injury molecule 1 (Kim-1), lipocalin 2 (Lcn2), and osteopontin (Spp1).

Results: Fenofibrate and tesaglitazar improved the lipid profile, but caused no detectable decrease in GFR or RBF compared with vehicle-treated rats. Furthermore, the cumulative excretions of sodium and creatinine were not altered by the drugs. Finally, there was no significant difference between drug- and vehicle-treated groups in urinary albumin excretion or in the expression of renal injury biomarkers.

Conclusions: In the rat, no direct nephrotoxic effect or deterioration in renal hemodynamics and function were observed following treatment with fenofibrate or tesaglitazar.

Background: The renal proximal tubule (PT) is clinically vulnerable to mitochondrial dysfunction; sub-lethal injury can lead to the Fanconi syndrome, with elevated urinary excretion of low-molecular-weight proteins. As the mechanism that couples mitochondrial dysfunction to impaired PT low-molecular weight protein uptake is unknown, we investigated the effect of respiratory chain (RC) inhibitors on endocytosis of FITC-albumin in PT-derived OK cells.

Methods: Uptake of FITC-albumin was quantified using confocal microscopy. Cytosolic ATP levels were measured in real time using both luciferin/luciferase assays and measurements of free [Mg(2+)]. Reactive oxygen species production was measured using mitosox.

Results: RC blockade produced only a small decrease in cytosolic ATP levels and had minimal effect on FITC-albumin uptake. Inhibition of glycolysis caused a much bigger decrease in both cytosolic ATP levels and FITC-albumin endocytosis. Rotenone led to higher rates of reactive oxygen species production than other RC inhibitors. Rotenone also caused widespread structural damage on electron microscopy, which was mimicked by colchicine and prevented by taxol; consistent with inhibition of microtubule polymerisation as the underlying mechanism.

Conclusions: Endocytosis of FITC-albumin is ATP-dependent in OK cells, but the cells are very glycolytic and therefore represent a poor metabolic model of the PT. Rotenone has toxic extra-mitochondrial structural effects.

Background/aims: Mutations in the type 2 vasopressin receptor gene (AVPR2) underlie X-linked recessive nephrogenic diabetes insipidus (NDI). Here, we report on a family with a mutation in AVPR2, c.262G>A (p.V88M). This recurrently identified mutation was previously shown to abolish AVPR2 function, yet in some affected members, urine osmolalities of up to 570 mosm/kg were observed. We detail the variable clinical phenotype and investigate its molecular basis.

Methods: Retrospective analysis of clinical data and in vitro assessment of wild-type and V88M-mutant receptors.

Results: Clinical data were available on 6 patients. Four of these demonstrated a substantial increase in urinary concentration after 1-desamino[8-D-arginine] vasopressin, consistent with partial NDI, while 2 did not respond. In vitro analysis revealed a reduced cell surface expression and decreased binding affinity for arginine-vasopressin of the mutant receptor, leading to blunted signaling activity. Treatment with the pharmacological chaperone SR121463 enhanced cell surface expression.

Conclusion: The V88M mutation is associated with phenotypical diversity, which may be explained by the fact that both the expression level and the hormone-binding affinity are affected by the mutation. Our results provide a rational basis for treatment trials with vasopressin analogues in combination with pharmacologic chaperones in patients with this recurrently identified mutation.

Recent evidence obtained in rodents indicates that gap junctions in the juxtaglomerular apparatus play an important role in the control of renin-producing cells and in tubuloglomerular signaling. These gap junctions are formed by cell-specific expression patterns of the vascular connexins Cx37, Cx40, Cx43 and Cx45. In order to obtain a first indication if gap junctions might play a similar important functional role in the juxtaglomerular apparatus of human kidneys, this study aimed to characterize the juxtaglomerular localization of Cx40, Cx37, Cx43 and Cx45 in human kidney specimens. We found Cx37, Cx40 and Cx43, but not Cx45 expression in high density in the extraglomerular mesangium. Renin-producing cells displayed strong immunoreactivity for Cx40 and Cx37. Cx37, Cx40 and Cx43 were also seen in the endothelium of arteries/arterioles outside of the glomeruli, whereas Cx45 was located in vascular smooth muscle cells. All four connexins were also expressed within the glomeruli. These findings indicate that the expression pattern of vascular connexins in the human kidney cortex is very similar to that previously found for mouse and rat kidneys, suggesting that the intrarenal expression pattern of vascular connexins is conserved among the mammalian species. Because of this similarity, and in particular in view of the strong expression of Cx37 and Cx40 in the juxtaglomerular area, we infer that those functions of connexins that have already been demonstrated for rodent kidneys, such as a central role of Cx40 for the development and function of renin-producing cells and for tubuloglomerular signal transmission, might hold for human kidneys as well.

Background/aim: It has been recently demonstrated that acute obstructive jaundice is associated with modifications in the renal expression and function of organic anion transporters such as Oat1, Oat3, Oatp1 and Mrp2. This study examined the expression and function of bilitranslocase in liver and kidney from rats with bile duct ligation (BDL).

Methods: Bilitranslocase expression was evaluated in renal homogenates (H), renal basolateral plasma membranes (KBLM) and liver plasma membranes (LPM) by immunoblotting. Bilitranslocase function was studied by measuring the kinetic parameters of electrogenic bromosulfophthalein (BSP) uptake in KBLM and LPM by a spectrophotometric technique.

Results: An increased abundance of bilitranslocase in KBLM without modifications in renal H and in LPM from BDL rats was observed compared with Sham rats. BDL rats showed a higher V(max) for BSP uptake in KBLM. No differences between groups were observed for Michaelis-Menten parameters in LPM.

Conclusion: The higher renal expression and function of bilitranslocase in renal basolateral membranes from rats with obstructive cholestasis might also contribute to the dramatic increase in BSP renal excretion observed in this experimental model. This would be another compensation mechanism to overcome the hepatic dysfunction in the elimination of organic anions.