MOLECULAR CHANGES IN PROXIMAL TUBULE FUNCTION IN DIABETES MELLITUS (original) (raw)
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Na + -glucose cotransporter 1 (SGLT1)-mediated glucose uptake leads to activation of Na + -H + exchanger 3 (NHE3) in the intestine by a process that is not dependent on glucose metabolism. This coactivation may be important for postprandial nutrient uptake. However, it remains to be determined whether SGLTmediated glucose uptake regulates NHE3-mediated NaHCO 3 reabsorption in the renal proximal tubule. Considering that this nephron segment also expresses SGLT2 and that the kidneys and intestine show significant variations in daily glucose availability, the goal of this study was to determine the effect of SGLTmediated glucose uptake on NHE3 activity in the renal proximal tubule. Stationary in vivo microperfusion experiments showed that luminal perfusion with 5 mM glucose stimulates NHE3-mediated bicarbonate reabsorption. This stimulatory effect was mediated by glycolytic metabolism but not through ATP production. Conversely, luminal perfusion with 40 mM glucose inhibited NHE3 because of cell swelling. Notably, pharmacologic inhibition of SGLT activity by Phlorizin produced a marked inhibition of NHE3, even in the absence of glucose. Furthermore, immunofluorescence experiments showed that NHE3 colocalizes with SGLT2 but not SGLT1 in the rat renal proximal tubule. Collectively, these findings show that glucose exerts a bimodal effect on NHE3. The physiologic metabolism of glucose stimulates NHE3 transport activity, whereas, supraphysiologic glucose concentrations inhibit this exchanger. Additionally, Phlorizin-sensitive SGLT transporters and NHE3 interact functionally in the proximal tubule.
Intracellular sodium in proximal tubules of diabetic rats. Role of glucose
Kidney International, 1988
Intracellular sodium in proximal tubules of diabetic rats. Role of glucose. Renal hypertrophy is a common consequence of diabetes mellitus that precedes and possibly accounts for the increased glomerular filtration rate. We have postulated that the glucose-mediated increase in the intracellular concentration of sodium [Na]1 initiates the chain of events leading to the increase in cell size and eventually cell number. Experiments were conducted on Sprague-Dawley rats made diabetic by the intravenous injection of 45 mg/kg body wt of streptozotocin dissolved in a 5 m citrate buffer solution. Control animals were injected with the vehicle alone. Ninety-six hours and 11 weeks later, measurements of [Na]1 were done by NMR spectroscopy on suspensions of proximal tubules, using dysprosium tripolyphosphate as an extracellular shift reagent. At 96 hours after the induction of the diabetes, there was a 60% increase in [Na]1 compared to control (P < 0.01). No further increase in [Na]1 was observed during the subsequent II weeks of observation. Addition of ouabain (1.0 mM) resulted in a fourfold increase in [Na]1 in tubules from control animals, and a 2.5-fold increase in tubules from 96-hour diabetic rats. Ouabain-inhibitable Na-K-ATPase activity was substantially higher in the renal tubules of diabetic rats, the increase being proportional to that of [Na]1. In order to ascertain the effect of hyperglycemia on [Na]1, proximal tubules prepared from kidneys of normal and diabetic rats were exposed to low (5 mM) and high (25 mM) concentration of glucose in the media. The rise in glucose concentration resulted in a 52% increase (P < 0.001) in [Na]1 in proximal tubules of non-diabetic animals, whereas the increase was only 11% (P > 0.2) in the 96-hour diabetics, and 5% (P > 0.2) in Il-week diabetic rats. Based on these findings, it is reasonable to propose that the rise in [Na]1 observed in the proximal tubules of diabetic rats is due to increased filtered load of glucose that stimulates the entry of glucose and Na into renal cells. The relationship between the rise in [Na]1 and cell growth remains speculative, although an association between these events has been documented consistently in the literature.
Dipeptidyl peptidase IV inhibition downregulates Na -H exchanger NHE3 in rat renal proximal tubule
In the microvillar microdomain of the kidney brush border, sodium hydrogen exchanger type 3 (NHE3) exists in physical complexes with the serine protease dipeptidyl peptidase IV (DPPIV). The purpose of this study was to explore the functional relationship between NHE3 and DPPIV in the intact proximal tubule in vivo. To this end, male Wistar rats were treated with an injection of the reversible DPPIV inhibitor Lys [Z(NO2)]-pyrrolidide (I40; 60 mg.kg(-1).day(-1) ip) for 7 days. Rats injected with equal amounts of the noninhibitory compound Lys[Z(NO2)]-OH served as controls. Na(+) - H(+) exchange activity in isolated microvillar membrane vesicles was 45 +/- 5% decreased in rats treated with I40. Membrane fractionation studies using isopycnic centrifugation revealed that I40 provoked redistribution of NHE3 along with a small fraction of DPPIV from the apical enriched microvillar membranes to the intermicrovillar microdomain of the brush border. I40 significantly increased urine output (67 +/- 9%; P < 0.01), fractional sodium excretion (63 +/- 7%; P < 0.01), as well as lithium clearance (81 +/- 9%; P < 0.01), an index of end-proximal tubule delivery. Although not significant, a tendency toward decreased blood pressure and plasma pH/HCO(3)(-) was noted in I40-treated rats. These findings indicate that inhibition of DPPIV catalytic activity is associated with inhibition of NHE3-mediated NaHCO3 reabsorption in rat renal proximal tubule. Inhibition of apical Na(+) - H(+) exchange is due to reduced abundance of NHE3 protein in the microvillar microdomain of the kidney brush border. Moreover, this study demonstrates a physiologically significant interaction between NHE3 and DPPIV in the intact proximal tubule in vivo.
Reduced albumin reabsorption in the proximal tubule of early-stage diabetic rats
Histochemistry and Cell Biology, 2001
The aim of this study is to investigate the role of the proximal tubule in microalbuminuria in the early stage of diabetic nephropathy. Diabetes was induced in male Sprague-Dawley rats by an injection of streptozotocin (50 mg/kg, i.v.). After 2 weeks, albumin delivery in the proximal tubule was measured using micropuncture and the endocytosis process of FITC-labeled albumin was evaluated with immunoelectron microscopy. Albumin was significantly reabsorbed in the proximal convoluted tubule (PCT) of controls (0.39±0.05 ng/min at early PCT to 0.17±0.08 at late PCT, P<0.05), whereas albumin reabsorption was inhibited in diabetic rats (0.27±0.05 to 0.21±0.08). Immunogold study revealed that FITC-albumin was significantly less reabsorbed in endosomes and lysosomes of S1 segments in diabetic rats than in controls (endosome: 1.20±0.10 vs 2.16±0.15 µm–1, P<0.0001; lysosome: 0.26±0.03 vs 0.83±0.07, P<0.0001). The expression of megalin, an endocytosis receptor, was decreased at the apical membrane of PCT in diabetic rats. The lipid peroxidation production in the proximal tubule was significantly increased in diabetic rats. In conclusion, albuminuria in early-stage diabetic rats can be partly explained by a decreased albumin endocytosis with reduced megalin expression and with increased lipid peroxidation in the proximal tubule.
Fructose Acutely Stimulates NHE3 Activity in Kidney Proximal Tubule
2012
Background/Aims: Fructose causes a sodium-sensitive hypertension and acutely reduces the urinary Na + excretion, suggesting that it may regulate the activity of renal tubular sodium transporters. NHE3 is highly expressed in proximal tubule (PT), along with proteins that mediate fructose transport and metabolism. The present work was outlined to investigate whether fructose modulates proximal NHE3 activity and to elucidate the molecular mechanisms underlying this modulation. Methods/Results: Using in vivo stationary microperfusion, we observed that fructose stimulates NHE3 mediated JHCO 3 reabsorption. The MAPK pathway is not involved in this activation, as demonstrated by using of MEK/MAPK inhibitors, whereas experiments using a PKA inhibitor suggest that PKA inhibition plays a role in this response. These results were confirmed in vitro by measuring the cell pH recovery rate after NH 4 Cl pulse in LLC-PK1, a pig PT cell line, which showed reduced cAMP levels and NHE3 phosphorylation at serine-552 (PKA consensus site) after fructose treatment. Conclusions: NHE3 activity is stimulated by fructose, which increases proximal tubule Na + reabsorption. The molecular mechanisms involved in this process are mediated, at least in part, by downregulation of the PKA signaling pathway. Future studies are needed to address whether fructose-stimulated NHE3 activity may contribute to renal injury and hypertension.
Increased NHE3 abundance and transport activity in renal proximal tubule of rats with heart failure
AJP: Regulatory, Integrative and Comparative Physiology, 2012
Heart failure (HF) is associated with a reduced effective circulating volume that drives sodium and water retention and extracellular volume expansion. We therefore hypothesized that Na+/H+ exchanger isoform 3 (NHE3), the major apical transcellular pathway for sodium reabsorption in the proximal tubule, is upregulated in an experimental model of HF. HF was induced in male rats by left ventricle radiofrequency ablation. Sham-operated rats (sham) were used as controls. At 6 wk after surgery, HF rats exhibited cardiac dysfunction with a dramatic increase in left ventricular end-diastolic pressure. By means of stationary in vivo microperfusion and pH-dependent sodium uptake, we demonstrated that NHE3 transport activity was significantly higher in the proximal tubule of HF compared with sham rats. Increased NHE3 activity was paralleled by increased renal cortical NHE3 expression at both protein and mRNA levels. In addition, the baseline PKA-dependent NHE3 phosphorylation at serine 552 wa...