Role of nitric oxide in the early renal changes induced by high fructose diet in rats

Salt sensitivity of nitric oxide generation and blood pressure in mice with targeted knockout of the insulin receptor from the renal tubule

To elucidate the role of the insulin receptor (IR) on kidney nitric oxide generation and blood pressure (BP) control, we generated mice with targeted deletion of renal tubule IR using loxP recombination driven by a Ksp-cadherin promoter. Male knockout (KO) and wild-type (WT) littermates (~4 mo old) were transitioned through three 1-wk treatments: 1) low-NaCl diet (0.085%); 2) high-NaCl diet (HS; 5%); and 3) HS diet plus 3 mM tempol, a superoxide dismutase mimetic, in the drinking water. Mice were then switched to medium-NaCl (0.5%) diet for 5 days and kidneys harvested under pentobarbital anesthesia. Twenty-four-hour urinary nitrates plus nitrites were significantly higher in the WT mice under HS (2,067 ± 280 vs. 1,550 ± 230 nmol/day in WT and KO, respectively, P < 0.05). Tempol attenuated genotype differences in urinary nitrates plus nitrites. A rise in BP with HS was observed only in KO mice and not affected by tempol (mean arterial pressure, dark period, HS, 106 ± 5 vs. 119 ± 4 mmHg, for WT and KO, respectively, P < 0.05). Renal outer medullary protein levels of nitric oxide synthase (NOS) isoforms by Western blot (NOS1-3 and phosphorylated-S1177-NOS3) revealed significantly lower band density for NOS1 (130-kDa isoform) in the KO mice. A second study, when mice were euthanized under HS conditions, confirmed significantly lower NOS1 (130 kDa) in the KO, with an even more substantial (>50%) reduction of the 160-kDa NOS1 isoform. These studies suggest that the loss of renal IR signaling impairs renal nitric oxide production. This may be important in BP control, especially in insulin-resistant states, such as the metabolic syndrome.

The Relation between Fructose-Induced Metabolic Syndrome and Altered Renal Haemodynamic and Excretory Function in the Rat

This paper explores the possible relationships between dietary fructose and altered neurohumoral regulation of renal haemodynamic and excretory function in this model of metabolic syndrome. Fructose consumption induces hyperinsulinemia, hypertriglyceridaemia, insulin resistance, and hypertension. The pathogenesis of fructose-induced hypertension is dubious and involves numerous pathways acting both singly and together. In addition, hyperinsulinemia and hypertension contribute significantly to progressive renal disease in fructose-fed rats. Moreover, increased activity of the renin-angiotensin and sympathetic nervous systems leading to downregulation of receptors may be responsible for the blunted vascular sensitivity to angiotensin II and catecholamines, respectively. Various approaches have been suggested to prevent the development of fructose-induced hypertension and/or metabolic alteration. In this paper, we address the role played by the renin-angiotensin and sympathetic nervous systems in the haemodynamic alterations that occur due to prolonged consumption of fructose.

Effect of genistein, a soy isoflavone, on whole body insulin sensitivity and renal damage induced by a high-fructose diet

The study evaluates the effect of genistein, a soy isoflavone, on insulin sensitivity and renal functional and structural injury in rats rendered insulin-resistant by feeding on a high-fructose diet for 60 days. Fructose-fed animals (60 g /100 g) displayed insulin resistance as indicated by the measures of insulin sensitivity [insulin sensitivity index (ISI(0,120)), quantitative insulin check index (QUICKI), and homeostatic model assessment (HOMA)]. Alterations in body weight, kidney weight, urine volume, plasma, and urine electrolytes accompanied by significant increases in plasma and urinary levels of urea, uric acid, creatinine, total protein, and albumin were observed in fructose-fed rats. Oxidative stress in kidney was noted by an elevation in lipid peroxides and a decline in glutathione (GSH). Insulin sensitivity and renal function were improved in fructose-fed rats administered genistein. Histological changes such as fatty infiltration and thickening of glomeruli observed in fructose-fed rats were also ameliorated when genistein was co-administered. The study shows that genistein improves insulin sensitivity and kidney function in a dietary model of insulin resistance. We suggest that genistein may have benefits for patients suffering from kidney disease associated with insulin resistance.

Effect of nitric oxide-cGMP-dependent protein kinase activation on advanced glycation end-product-induced proliferation in renal fibroblasts

Renal interstitial fibrosis is believed to play a key role in the development of diabetic nephropathy (DN), and advanced glycation end-products (AGE) may contribute importantly to this. Recent reports have shown that nitric oxide (NO) is closely linked to the renal interstitial fibrosis of DN. In this study, the mechanisms by which NO and its downstream signals mediate the AGE-induced proliferative response in normal rat kidney fibroblasts (NRK-49F) are examined. AGE decreased NO production, cyclic guanosine 5'monophosphate (cGMP) synthesis, and cGMP-dependent protein kinase (PKG) activation time- and dose-dependently. These effects were not observed when cells were treated with nonglycated BSA. NO and inducible nitric oxide synthase (iNOS) stimulated by NO donors S-nitroso-N-acetylpenicillamine (SNAP)/sodium nitroprusside (SNP) and PKG activator 8-para-chlorophenylthio-cGMP (8-pCPT-cGMP) prevented both AGE-induced proliferation and Janus kinase 2 (JAK2)-signal transducers and activators of transcription 5 (STAT5) activation but not p42/p44 mitogen-activated protein kinase (MAPK) activation. The ability of NO-PKG to inhibit AGE-induced cell cycle progression was verified by the observation that SNAP, SNP, and 8-pCPT-cGMP inhibited both cyclin D1 and cdk4 activation. Furthermore, induction of NO-PKG significantly increased p21Waf1/Cip1 expression in AGE-treated NRK-49F cells. The data suggest that the NO-PKG pathway inhibits AGE-induced proliferation by suppressing activation of JAK2-STAT5 and cyclin D1/cdk4 and induction of p21Waf1/Cip1.

Effects of dietary fat, NaCl, and fructose on renal sodium and water transporter abundances and systemic blood pressure

Dietary fructose, NaCl, and/or saturated fat have been correlated with mean arterial pressure (MAP) rises in sensitive strains of rats. Dysregulation of sodium and/or water reabsorption by the kidney may contribute. Using radiotelemetry and parallel semiquantitative immunoblotting, we examined the effects of various diets on MAP and the regulation of abundance of the major renal sodium and water transport proteins in male Sprague-Dawley rats. In study 1, rats (∼275 g) were fed one of four diets for 4 wk ( n = 6/group): 1) control, 2) 65% fructose, 3) control + added NaCl (2.59%), or 4) fructose + NaCl. In study 2, 5% butter (fat) was added to the above four diets. Both fat and NaCl, but not fructose, caused modest rises in MAP (5–10 mmHg) and increased the day-to-night ratio in diastolic blood pressure. NaCl or fructose increased kidney size. Creatinine clearance was increased by salt or fat, and fractional excretion of sodium was decreased by fat. In study 1, high NaCl markedly reduced plasma renin and aldosterone and its regulated proteins in whole kidney, i.e., the thiazide-sensitive Na-Cl cotransporter and the α- and γ (70-kDa band)-subunits of the epithelial sodium channel. These effects were blunted by fat. Fructose increased the abundance of the sodium phosphate cotransporter, whereas it decreased the bumetanide-sensitive Na-K-2Cl cotransporter and aquaporin-2. Overall, doubling of dietary fat appeared to impair dietary sodium adaptation, i.e., blunt the downregulation of aldosterone-mediated effects, thus allowing blood pressure to rise at an accelerated rate.

Role of nitric oxide in diabetic nephropathy

Diabetic nephropathy is the leading cause of end-stage renal disease in the Western hemisphere. Endothelial dysfunction is the central pathophysiologic denominator for all cardiovascular complications of diabetes including nephropathy. Abnormalities of nitric oxide (NO) production modulate renal structure and function in diabetes but, despite the vast literature, major gaps exist in our understanding in this field because the published studies mostly are confusing and contradictory. In this review, we attempt to review the existing literature, discuss the controversies, and reach some general conclusions as to the role of NO production in the diabetic kidney. The complex metabolic milieu in diabetes triggers several pathophysiologic mechanisms that simultaneously stimulate and suppress NO production. The net effect on renal NO production depends on the mechanisms that prevail in a given stage of the disease. Based on the current evidence, it is reasonable to conclude that early nephropathy in diabetes is associated with increased intrarenal NO production mediated primarily by constitutively released NO (endothelial nitric oxide synthase [eNOS] and neuronal nitric oxide synthase [nNOS]). The enhanced NO production may contribute to hyperfiltration and microalbuminuria that characterizes early diabetic nephropathy. On the other hand, a majority of the studies indicate that advanced nephropathy leading to severe proteinuria, declining renal function, and hypertension is associated with a state of progressive NO deficiency. Several factors including hyperglycemia, advanced glycosylation end products, increased oxidant stress, as well as activation of protein kinase C and transforming growth factor (TGF)-beta contribute to decreased NO production and/or availability. These effects are mediated through multiple mechanisms such as glucose quenching, and inhibition and/or posttranslational modification of NOS activity of both endothelial and inducible isoforms. Finally, genetic polymorphisms of the NOS enzyme also may play a role in the NO abnormalities that contribute to the development and progression of diabetic nephropathy.