Intrinsic gluconeogenesis is enhanced in renal proximal tubules of Zucker diabetic fatty rats

Recent studies indicate that renal gluconeogenesis is substantially stimulated in patients with type 2 diabetes, but the mechanism that is responsible for such stimulation remains unknown. Therefore, this study tested the hypothesis that renal gluconeogenesis is intrinsically elevated in the Zucker diabetic fatty rat, which is considered to be an excellent model of type 2 diabetes. For this, isolated renal proximal tubules from diabetic rats and from their lean nondiabetic littermates were incubated in the presence of physiologic gluconeogenic precursors. Although there was no increase in substrate removal and despite a reduced cellular ATP level, a marked stimulation of gluconeogenesis was observed in diabetic relative to nondiabetic rats, with near-physiologic concentrations of lactate (38%), glutamine (51%) and glycerol (66%). This stimulation was caused by a change in the fate of the substrate carbon skeletons resulting from an increase in the activities and mRNA levels of the key gluconeogenic enzymes that are common to lactate, glutamine, and glycerol metabolism, i.e., mainly of phosphoenolpyruvate carboxykinase and, to a lesser extent, of glucose-6-phosphatase and fructose-1,6-bisphosphatase. Experimental evidence suggests that glucocorticoids and cAMP were two factors that were responsible for the long-term stimulation of renal gluconeogenesis observed in the diabetic rats. These data provide the first demonstration in an animal model that renal gluconeogenesis is upregulated by a long-term mechanism during type 2 diabetes. Together with the increased renal mass (38%) observed, they lend support to the view so far based only on in vivo studies performed in humans that renal gluconeogenesis may be stimulated by and crucially contribute to the hyperglycemia of type 2 diabetes.     

Intracellular distribution and depletion of glutathione in rabbit renal proximal tubules

The intracellular compartmentation of glutathione (GSH) in rabbit renal proximal tubules under various conditions was examined using the digitonin fractionation technique. Tubules with GSH contents similar to those found in vivo (13.4 +/- 0.8 nmol . mg protein-1) and with decreasing amounts of GSH had an apparently constant mitochondrial GSH pool of 1.9 +/- 0.1 nmol . mg protein-1. This renal mitochondrial GSH pool is similar in size to that of hepatic mitochondria and represents 10 to 15 percent of the total cellular GSH. Using phorone and diethyl maleate to decrease tubular GSH concentrations, the cytosolic GSH pool could be depleted without affecting the mitochondrial GSH pool. Depletion of the cytosolic GSH pool and decreases in the mitochondrial pool of up to 42 percent were not associated with mitochondrial dysfunction nor loss of tubular viability.     

Role of caspases in hypoxia-induced necrosis of rat renal proximal tubules.

The role of the caspases, a newly discovered group of cysteine proteases, was investigated in a model of hypoxia-induced necrotic injury of rat renal proximal tubules. An assay for caspases in freshly isolated rat proximal tubules was developed. There was a 40% increase in tubular caspase activity after 15 min of hypoxia in association with increased cell membrane damage as indicated by a threefold increase in lactate dehydrogenase release. The specific caspase inhibitor Z-Asp-2,6-dichlorobenzoyloxymethylketone (Z-D-DCB) attenuated the increase in caspase activity during 15 min of hypoxia and markedly decreased lactate dehydrogenase release in a dose-dependent manner. In the proximal tubules, Z-D-DCB also inhibited the hypoxia-induced increase in calpain activity, another cysteine protease. In contrast, when Z-D-DCB was added to purified calpain in vitro, there was no inhibition of calpain activity. The calpain inhibitor (2)-3-(4-iodophenyl)-2-mercapto-2-propenoic acid (PD150606) also inhibited the hypoxia-induced increase in caspase activity in proximal tubules, but did not inhibit the activity of purified caspase 1 in vitro. In these experiments, caspase activity was detected with the fluorescence substrate Ac-Tyr-Val-Ala-Asp-7-amido-4-methyl coumarin (Ac-YVAD-AMC), which is preferentially cleaved by caspase 1. However, minimal caspase activity was detected with the fluorescence substrate Ac-Asp-Glu-Val-Asp-7-amido-4-methyl coumarin (Ac-DEVD-AMC), which is cleaved by caspases 2, 3, and 7. The present study in proximal tubules demonstrates that (1) caspase inhibition protects against necrotic injury by inhibition of hypoxia-induced caspase activity; and (2) caspase 1 may be the caspase involved. Thus, although the role of caspases in apoptotic cell death is well established, this study provides new evidence that caspases contribute to necrotic cell death as well.     

Focal induction of IGF binding proteins in proximal tubules of diabetic rat kidney.

Diabetes-associated kidney enlargement is associated with increased kidney insulinlike growth factor I (IGF-I) binding. IGF-I binds to the type I IGF receptor, which mediates most of its actions, and to specific binding proteins (IGFBPs), which modulate its actions. To explore the nature and extent of IGF-I binding in the kidney, in vitro autoradiography was used to map the distribution of IGF binding in control and diabetic rat kidney. Specificity studies were performed with increasing concentrations of unlabeled IGF-I, IGF-II, des(1-3)IGF-I (an IGF-I derivative that binds to receptors normally but with decreased affinity to binding proteins), and insulin. In control rats, diffuse binding was found throughout the kidney with increased density in the papilla. Binding specificity in the cortex and outer medulla was typical of the type I IGF receptor (IGF-I = des[1-3]IGF-I greater than IGF-II much greater than insulin). Binding in the outer medulla of diabetic kidney was typical of the type I IGF receptor. A marked focal increase in proximal tubular binding occurred in 13 of 22 postpubertal diabetic rats. Binding specificity of the proximal tubular binding was consistent with the predominance of an IGF binding protein (IGF-I = IGF-II greater than des[1-3]IGF-I with minimal displacement by insulin). Northern-blot analysis revealed increased IGFBP-1 and IGFBP-3 mRNA in cortical tissue from diabetic rats displaying increased proximal tubular binding but not from diabetic rats not displaying this phenomenon. As cell surface association of IGFBPs is linked to potentiation of IGF activity, a possible mechanism for potentiation of local IGF-I action may be provided.     

Angiotensin stimulates glucose and fluid absorption by rat proximal straight tubules

I investigated the effect of angiotensin on glucose and fluid absorption by isolated, perfused rat proximal straight tubules. During the control period, tubules absorbed fluid at 0.68 +/- 0.05 nL/mm.min and glucose at 12.5 +/- 1.3 pmol/mm.min. After 10(-10) M angiotensin was added to the bath, tubules absorbed fluid at 0.82 +/- 0.06 nL/mm.min and glucose at 16.0 +/- 2.4 pmol/mm.min--a stimulation of both parameters by 30%. Time controls showed no significant change in the rate of glucose or fluid absorption. This stimulation was due to an increase in the maximum rate of transport. The maximum rate of glucose absorption with 5.5 mM in bath and perfusate increased from 14.5 +/- 1.6 pmol/mm.min to 18.5 +/- 1.4 pmol/mm.min after 10(-10) M angiotensin was added to the bath. In contrast, the Km for glucose did not change, since a solution containing 0.55 mM glucose (approximately Km) supported 63 +/- 6% of the maximum rate during the control period and 60 +/- 5% of the maximum rate after angiotensin was added to the bath. Angiotensin also had no effect on glucose permeability, which was (3.7 +/- 0.4) x 10(-6) cm/s before treatment and (3.8 +/- 0.4) x 10(-6) cm/s after treatment. From these data, it is concluded that: (1) angiotensin stimulates glucose absorption in the rat proximal straight tubule; (2) this stimulation is the result of an increase in the maximum rate of transport rather than a change in Km or permeability; and (3) stimulation of Na/glucose absorption by angiotensin accounts for a significant portion of the increase in fluid absorption caused by angiotensin.     

Reabsorption of horseradish peroxidase by proximal tubules in rats with Heymann nephritis

The distribution of the histochemical protein tracer, horseradish peroxidase, was studied in proximal tubules of rats with Heymann nephritis. Peroxidase reabsorption was substantially reduced in stage 2 of Heymann nephritis, a period during which the brush border of proximal tubules is severely damaged by specific antibodies. Impairment of the reabsorption function could not be attributed either to proteinuria or disturbances of proximal tubule metabolism and appeared to result from loss of microvilli. Recovery of brush border membrane morphology in stage 4 of Heymann nephritis was not accompanied by recovery of the normal capacity to reabsorb peroxidase. Functional deficits resulting from immunologic injury to proximal tubules in Heymann nephritis may persist despite waning of the anti-brush border antibody response and regeneration of the brush border of proximal tubule cells.     

Insulinlike effects of sodium selenate in streptozocin-induced diabetic rats.

Treatment of streptozocin (STZ)-induced diabetic rats with sodium selenate (10-15 mumol.kg-1.day-1) for 7 wk resulted in a decrease in plasma glucose, food intake, and water intake to control or near control levels. Plasma insulin was reduced in control rats given sodium selenate to the level found in the diabetic and treated diabetic group. Treatment did not affect control rats with regard to the other measurements cited. Sodium selenate enhanced weight gain in responding diabetic rats to that seen in controls; sodium selenate's actions thus resembled those of insulin. Thus selenate, like vanadium, appears to have insulinlike effects when administered in vivo.     

Electrophysiology of ammonia transport in renal straight proximal tubules.

To test for electrogenic transport of ammonium ions in straight proximal renal tubules, isolated perfused tubules have been exposed to peritubular ammonium ions during continuous recording of cell membrane potential. As a result, 20 mmol/liter NH4+ leads to a rapid, reversible depolarization of the cell membrane by 9.0 +/- 0.3 mV (N = 86). This depolarization is not significantly affected by 10 mmol/liter barium or 0.1 mmol/liter amiloride on both sides of the epithelium, but is significantly blunted by omission of extracellular bicarbonate and CO2 (3.8 +/- 0.4 mV, N = 9), by 1 mmol/liter acetazolamide (4.3 +/- 0.3 mV, N = 11), by 1 mmol/liter peritubular amiloride (4.3 +/- 1.1 mV, N = 7), by 1 mmol/liter SITS (5.7 +/- 0.4 mV, N = 6), and by replacement of extracellular sodium with choline (4.7 +/- 0.5 mV, N = 8). In the presence of both amiloride (1 mmol/liter) and acetazolamide (1 mmol/liter) in the bath, the NH4+ induced depolarization is completely abolished. Furthermore, the combined omission of bicarbonate and addition of 10 mmol/liter barium eliminates the NH4+ induced depolarization. About 50% of the depolarization can be explained by enhanced electrogenic bicarbonate exit due to the intracellular alkalosis. The other 50% is explained by amiloride and barium sensitive electrogenic entry of NH4+ into the cell.     

Glomerular filtration rate in streptozocin-induced diabetic rats. Role of exchangeable sodium, vasoactive hormones, and insulin therapy

The interrelationships of sodium and volume status, atrial natriuretic peptide (ANP), plasma renin activity (PRA), insulinlike growth factor I (IGF-I), and kidney weight and their influence on glomerular filtration rate (GFR) were investigated in rats during the first 4 wk of streptozocin-induced diabetes (STZ-D). In each of three experiments, untreated diabetic rats were compared with nondiabetic control rats and rats with varying degrees of glycemic control during insulin therapy. The first experiment evaluated exchangeable sodium, plasma volume, and GFR. In untreated diabetic rats, exchangeable sodium and plasma volume, but not GFR, were increased by approximately 25% compared with control rats. Insulin-treated diabetic rats with plasma glucose levels ranging from 12 to 30 mM had increased GFR, whereas exchangeable sodium and plasma volume were reduced toward control values. Daily insulin therapy, titrated to maintain euglycemia, further reduced exchangeable sodium and plasma volume and decreased but did not normalize GFR. The second experiment evaluated the relationship between vasoactive hormones and GFR. In untreated diabetic rats, plasma ANP levels increased 89% and urinary cyclic GMP (cGMP) excretion increased 94%, with an 85% decrease in PRA, whereas GFR was unchanged. Moderate hyperglycemia (plasma glucose 12-30 mM) was associated with normalized plasma ANP levels and urinary cGMP excretion, a 52% decrease in PRA, and a 13% increase in GFR. The third experiment studied serial changes in food and water intake and vasoactive hormones and end-point measurement of kidney weight, GFR, and plasma IGF-I. In the untreated diabetic group, urinary cGMP excretion was significantly elevated after 3 wk, whereas the reduction in PRA levels was apparent after 1 wk.(ABSTRACT TRUNCATED AT 250 WORDS)     

黄腐酸钠对实验性糖尿病大鼠肾脏的保护作用

目的研究黄腐酸钠对糖尿病大鼠肾脏病变的保护作用及其机制。方法观察黄腐酸钠对肾小球形态，肾小球基底膜超微结构，尿白蛋白排出率的影响及血浆组织型纤溶酶原激活剂（ｔＰＡ）、纤溶酶原激活剂抑制物（ＰＡＩ１）活性改变。结果糖尿病大鼠血浆ｔＰＡ活性明显降低，而ＰＡＩ１活性升高。黄腐酸钠明显降低糖尿病大鼠尿白蛋白，抑制肾小球肥大，延缓肾小球基底膜增厚及足突融合，显著提高血浆ｔＰＡ活性而降低ＰＡＩ１活性。结论黄腐酸钠对糖尿病肾病具有保护作用。此作用可能与提高血浆纤溶活性有关。     

阿魏酸钠在糖尿病肾病治疗中的作用

本研究探讨阿魏酸钠在糖尿病肾病(DN)中的作用及其机制,为中西医结合治疗DN提供理论依据.     

Elemental microanalysis of organelles in proximal tubules. II. Effects of oxygen deprivation

This communication describes the effects of anoxia on rabbit proximal renal tubule element (ion) content by using high-resolution electron probe x-ray microanalytical imaging to obtain quantitative elemental data from subcellular compartments not previously resolvable with low-resolution imaging. These organelles and regions include the heterochromatin and euchromatin of the nucleus and the microvilli of the apical brush border, in addition to mitochondria, lysosomes, and cytoplasm. Anoxia of 40-min duration caused the expected decrease in K and increase in Na and Cl concentrations in the tubules with the cytoplasmic K:Na ratio declining to 0.13:1. These changes were accompanied by decreases in ATP and total K contents, and an increase in lactate dehydrogenase release. Swelling occurred in some cells as evidenced by ultrastructural changes. No alterations were evident after oxygen deprivation in Ca content of cytoplasm (control, 6.7 +/- 0.6 versus anoxia, 7.6 +/- 0.7 nmol/mg dry wt) or mitochondria (control, 4.0 +/- 0.4 versus anoxia, 4.9 +/- 0.6 nmol/mg dry wt) or in S content of recognizable lysosomes (control, 314 +/- 11 versus anoxia, 325 +/- 12 nmol/mg dry wt). Brush border (microvillus) Ca content was higher than cytoplasmic Ca content during normoxia (10.7 +/- 0.9 nmol/mg dry wt) and increased further during anoxia (17.0 +/- 1.0 nmol of Ca/mg dry wt). The finding of higher Ca content within the brush border region during normoxia is unexpected and novel, because such results suggest that Ca homeostasis in the apical elaboration of the proximal cell may be different from that in the cytoplasm. The results also raise the possibility that an increase in Ca content in the brush border membrane region may be involved in the pathogenesis of renal cell injury.     

Phosphoproteomic analysis of AT1 receptor-mediated signaling responses in proximal tubules of angiotensin II-induced hypertensive rats

The signaling mechanisms underlying the effects of angiotensin II in proximal tubules of the kidney are not completely understood. Here we measured signal protein phosphorylation in isolated proximal tubules using pathway-specific proteomic analysis in rats continuously infused with pressor or non-pressor doses of angiotensin II over a 2-week period. Of the 38 phosphoproteins profiled, 14 were significantly altered by the pressor dose. This included increased phosphorylation of the protein kinase C isoenzymes, PKCα and PKCβII, and the glycogen synthase kinases, GSK3α and GSK3β. Phosphorylation of the cAMP-response element binding protein 1 and PKCδ were decreased, whereas PKC? remained unchanged. By contrast, the phosphorylation of only seven proteins was altered by the non-pressor dose, which increased that of PKCα, PKCδ, and GSKα. Phosphorylation of MAP kinases, ERK1/2, was not increased in proximal tubules in vivo by the pressor dose, but was in proximal tubule cells in vitro. Infusion of the pressor dose decreased, whereas the non-pressor dose of angiotensin II increased the phosphorylation of the sodium and hydrogen exchanger 3 (NHE-3) in membrane fractions of proximal tubules. Losartan largely blocked the signaling responses induced by the pressor dose. Thus, PKCα and PKCβII, GSK3α and GSK3β, and cAMP-dependent signaling pathways may have important roles in regulating proximal tubular sodium and fluid transport in Ang II-induced hypertensive rats.     

Indomethacin blocks enhanced paracellular backflux in proximal tubules

Renal interstitial hydrostatic pressure (RIHP) is a link between increased arterial BP and natriuresis. The mechanism whereby increases in RIHP inhibits sodium and water transport across the mammalian proximal tubule epithelium may involve changes in flux across the tight junction of the proximal tubule. The purpose of this study was to determine the effects of increases in RIHP and inhibition of cyclooxygenase on paracellular backflux of an extracellular marker from the renal interstitium into the proximal tubule of the rat. During in vivo microperfusion of proximal tubules, the extracellular tracer of paracellular flux, lanthanum (La), was infused directly into the renal interstitium via a chronically implanted matrix. The net paracellular interstitium-to-lumen lanthanum backflux was measured before and after direct renal interstitial volume expansion (DRIVE) in the absence and presence of indomethacin. DRIVE significantly increased RIHP by 37% (Delta1.8 +/- 0.2 mmHg) and interstitium-to-lumen La backflux by 32% (Delta40.2 +/- 16.6 pg/min per mm), and it significantly decreased proximal reabsorption by 27% (Delta-7.7 +/- 3.8 nl/min; n = 6). In indomethacin-treated rats (n = 6), DRIVE again significantly increased RIHP by 40% (Delta1.9 +/- 0.2 mmHg), but it did not increase La backflux (Delta-39.0 +/- 24.4 pg/min per mm) or significantly decrease proximal reabsorption (Delta1.2 +/- 2.3 nl/min). These results demonstrate that increased RIHP increases paracellular backflux of lanthanum from the renal interstitium to the proximal tubule lumen in association with decreases in proximal reabsorption. Furthermore, indomethacin blocks the effects of increased RIHP on proximal reabsorption and paracellular backflux of lanthanum through the intercellular tight junctions of the proximal tubule epithelium.     

NHE3 and NHERF are targeted to the basolateral membrane in proximal tubules of colchicine-treated rats

BACKGROUND: Depolymerization of microtubules in proximal tubule (PT) cells of colchicine-treated rats causes disruption of vesicle recycling and redistribution of some brush-border membrane (BBM) transporters into cytoplasmic vesicles. NHE3, an isoform of the Na+/H+ exchanger in the PT cell BBM, is acutely regulated by a variety of mechanisms, including protein trafficking and interaction with the PDZ protein, NHERF. The effects of microtubule disruption by colchicine on NHE3 trafficking in PT and the potential role of NHERF in this process have not been studied. METHODS: Immunofluorescence and immunogold cytochemistry were performed on cryosections of kidney tissue, and immunoblotting of BBM isolated from the renal cortex and outer stripe of control and colchicine-treated (3.2 mg/kg, IP, a single dose 12 hours before sacrifice) rats. RESULTS: In cells of the convoluted PT (S1/S2 segments) of control rats, NHE3 was located mainly in the BBM; subapical endosomes were weakly stained. In cells of the straight PT (S3 segment), NHE3 was present in the BBM and in lysosomes. In colchicine-treated rats, there was a marked redistribution of NHE3 from the BBM into intracellular vesicles and the basolateral plasma membrane in the S1/S2 segments. In the S3 segment, the abundance of BBM NHE3 was not visibly changed, but NHE3-positive intracellular organelles largely disappeared, and the antigen was detectable in the basolateral plasma membrane. The PDZ protein NHERF followed a similar pattern: in control animals, it was strong in the BBM and negative in the basolateral membrane in cells along the PT. After colchicine treatment, expression of NHERF in the basolateral membrane strongly increased in all PT segments, where it colocalized with NHE3. CONCLUSIONS: The data indicate that: (a) microtubules are involved in the apical targeting of NHE3 and NHERF in renal PT cells, and (b) the parallel basolateral insertion of NHE3 and NHERF may represent an indirect targeting pathway that involves transient, microtubule-independent basolateral insertion of these proteins, followed by microtubule-dependent, vesicle-mediated transcytosis to the BBM.