Female rats are protected against fructose-induced changes in metabolism and blood pressure

The objective of this study was to determine whether the effects of a fructose diet, which causes hyperinsulinemia, insulin resistance, and hypertension in male rats, are dependent on sex. Blood pressure was measured via the tail-cuff method, and oral glucose tolerance tests were performed to assess insulin sensitivity. Blood pressure in female rats did not differ between fructose-fed and control rats at any time point (126 +/- 5 and 125 +/- 3 mmHg at week 9 for fructose-fed and control rats, respectively) nor was there a difference in any metabolic parameter measured. Furthermore, the vascular insulin resistance that is present in male fructose-fed rats was not observed. After ovariectomy, fructose caused a significant change in systolic blood pressure from baseline compared with fructose-fed ovary-intact rats (change of 21 +/- 5 vs. -2 +/- 4 mmHg). The results demonstrate that females do not develop hypertension or hyperinsulinemia upon fructose feeding except after ovariectomy, suggesting that female sex hormones may confer protection against the effects of a fructose diet.     

Mechanisms of hepatic very low density lipoprotein overproduction in insulin resistance. Evidence for enhanced lipoprotein assembly, reduced intracellular ApoB degradation, and increased microsomal triglyceride transfer protein in a fructose-fed hamster model

A novel animal model of insulin resistance, the fructose-fed Syrian golden hamster, was employed to investigate the mechanisms mediating the overproduction of very low density lipoprotein (VLDL) in the insulin resistant state. Fructose feeding for a 2-week period induced significant hypertriglyceridemia and hyperinsulinemia, and the development of whole body insulin resistance was documented using the euglycemic-hyperinsulinemic clamp technique. In vivo Triton WR-1339 studies showed evidence of VLDL-apoB overproduction in the fructose-fed hamster. Fructose feeding induced a significant increase in cellular synthesis and secretion of total triglyceride (TG) as well as VLDL-TG by primary hamster hepatocytes. Increased TG secretion was accompanied by a 4.6-fold increase in VLDL-apoB secretion. Enhanced stability of nascent apoB in fructose-fed hepatocytes was evident in intact cells as well as in a permeabilized cell system. Analysis of newly formed lipoprotein particles in hepatic microsomes revealed significant differences in the pattern and density of lipoproteins, with hepatocytes derived from fructose-fed hamsters having higher levels of luminal lipoproteins at a density of VLDL versus controls. Immunoblot analysis of the intracellular mass of microsomal triglyceride transfer protein, a key enzyme involved in VLDL assembly, showed a striking 2.1-fold elevation in hepatocytes derived from fructose-fed versus control hamsters. Direct incubation of hamster hepatocytes with various concentrations of fructose failed to show any direct stimulation of its intracellular stability or extracellular secretion, further supporting the notion that the apoB overproduction in the fructose-fed hamster may be related to the fructose-induced insulin resistance in this animal model. In summary, hepatic VLDL-apoB overproduction in fructose-fed hamsters appears to result from increased intracellular stability of nascent apoB and an enhanced expression of MTP, which act to facilitate the assembly and secretion of apoB-containing lipoprotein particles.     

Taurine modulates kallikrein activity and glucose metabolism in insulin resistant rats

Summary. Taurine, a potent antioxidant has been reported to show an antidiabetic effect in streptozotocin-induced diabetes mellitus in which the development of hyperglycemia results from the damage to β cells of pancreas by reactive oxygen species. In addition, taurine also increases the excretion of nitrite and enhances the formation of kinins and would be expected to improve insulin resistance. The effect of taurine on insulin sensitivity was examined in the high fructose-fed rats, an animal model of insulin resistance. Male Wistar rats of body weight 170–190 g were divided into 4 groups: a control group and taurine-supplemented control group, taurine supplemented and unsupplemented fructose-fed group. An intravenous glucose tolerance test (IVGTT) and a steady state plasma glucose level (SSPG) were performed before the sacrifice. The fructose-fed rats displayed hyperglycemia and insulin resistance and they had a greater accumulation of glycogen than did control rats. Hyperglycemia and insulin resistance were significantly lower in the taurine supplemented fructose-fed group than in the unsupplemented fructose-fed group. Urinary kallikrein activity was higher in taurine-treated animals than in the rats fed only fructose. The activity of membrane bound ATPases were significantly lower in fructose-fed rats than in the control rats and were significantly higher in the taurine supplemented group than in the fructose-fed group. Taurine effectively improves glucose metabolism in fructose-fed rats presumably via improved insulin action and glucose tolerance. Received January 5, 2001 Accepted August 21, 2001     

Bezafibrate, an anti-hypertriglyceridemic drug, attenuates vascular hyperresponsiveness and elevated blood pressure in fructose-induced hypertensive rats

A high fructose diet induces hypertension, hyperinsulinemia - insulin resistance, and hypertriglyceridemia (syndrome X). In this study, we investigated the role of an abnormal lipid profile in mediating fructose-induced hypertension. We hypothesized that bezafibrate, a lipid-lowering drug, would reduce elevated blood pressure and inhibit increased vascular reactivity in fructose-fed rats. Male rats were placed on four different diets: group 1 was fed standard chow (n = 6); group 2 was fed 60% fructose (n = 5); group 3 was fed fructose plus bezafibrate (30 mg x kg(-1) x day(-1); drinking water; n = 5); and group 4 was fed standard chow plus bezafibrate (n = 6). In addition, the direct effects of very low density lipoprotein (VLDL) on vascular reactivity were examined. Bezafibrate treatment lowered blood pressure, free fatty acids, and triglycerides in the fructose-fed group, suggesting that lipid abnormalities play a role in the elevation of blood pressure in the fructose-induced hypertensive rat. Aortae from fructose-fed rats were hyperresponsive to the calcium channel agonist Bay K 8644, which was normalized with bezafibrate treatment. Incubation of aortae in a VLDL medium resulted in increased responsiveness to Bay K 8644, lending further support to lipid abnormalities altering vascular reactivity. An altered lipid profile evidenced by elevated triglycerides and free fatty acids is causally related to the development of high blood pressure and increased vascular reactivity in the fructose-induced hypertensive rat.     

Effect of galangin supplementation on oxidative damage and inflammatory changes in fructose-fed rat liver

The study examined the effects of galangin (GA) on oxidative stress, inflammatory cytokine levels and nuclear factor-kappa B (NF-κB) activation in fructose-fed rat liver. Adult male albino Wistar rats were divided into 4 groups. Groups 1 and 4 received the control diet containing starch as the source of carbohydrate while groups 2 and 3 were fed a diet containing fructose. Groups 3 and 4 additionally received GA (100 μg/kg, p.o) from the 15th day. At the end of 60 days, the levels of plasma glucose, insulin and triglycerides, insulin sensitivity indices and oxidative stress markers in the liver were determined. Cytokines of interest were assayed by ELISA and RT-PCR and NF-κB p65 nuclear translocation by Western blot and RT-PCR. Compared to control diet-fed animals, fructose-fed animals developed hyperglycemia, hyperinsulinemia, hypertriglyceridemia and insulin resistance (IR) (all p < 0.01). GA prevented the rise in plasma glucose, insulin and triglycerides and improved insulin sensitivity. Tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) levels in plasma and the mRNA and protein levels of TNF-α and transforming growth factor-β1(TGF-β₁) in liver were significantly higher in fructose-fed rats than control rats. However, treatment with GA downregulated the expression of these cytokines. Translocation of NF-κB into the nucleus was also increased in fructose diet-fed animals, which was prevented by GA. These results suggest that GA prevents oxidative damage and has a downregulatory effect on the inflammatory pathway in liver of fructose-fed rats.     

Differential regulation of insulin resistance and hypertension by sex hormones in fructose-fed male rats

Differences in gender are in part responsible for the development of insulin resistance (IR) and associated hypertension. Currently, it is unclear whether these differences are dictated by gender itself or by the relative changes in plasma estrogen and/or testosterone. We investigated the interrelationships between testosterone and estrogen in the progression of IR and hypertension in vivo in intact and gonadectomized fructose-fed male rats. Treatment with estrogen significantly reduced the testosterone levels in both normal chow-fed and fructose-fed rats. Interestingly, fructose feeding induced a relative increase in estradiol levels, which did not affect IR in both intact and gonadectomized fructose-fed rats. However, increasing the estrogen levels improved insulin sensitivity in both intact and gonadectomized fructose-fed rats. In intact males, fructose feeding increased the blood pressure (140 +/- 2 mmHg), which was prevented by estrogen treatment. However, the blood pressure in the fructose-fed estrogen rats (125 +/- 1 mmHg) was significantly higher than that of normal chow-fed (113 +/- 1 mmHg) and fructose-fed gonadectomized rats. Estrogen treatment did not affect the blood pressure in gonadectomized fructose-fed rats (105 +/- 2 mmHg). These data suggest the existence of a threshold value for estrogen below which insulin sensitivity is unaffected. The development of hypertension in this model is dictated solely by the presence or absence of testosterone. In summary, the development of IR and hypertension is governed not by gender per se but by the interactions of specific sex hormones such as estrogen and testosterone.     

Nutritional and hormonal control of lung and liver fatty acid synthesis.

During starvation and in streptozotocin-induced diabetes, the total activities of rat lung acetyl CoA carboxylase and fatty acid synthetase are reduced to one-third of the normal values. Refeeding of the starved animals or administration of insulin to diabetic animals restores the levels to the original values. The insulin effect is dose and time dependent. These data contrast with those in the liver, where a 30- to 50-fold depression of these enzymes is observed in the diabetic state and administration of insulin is actually followed by doubling of the activity over normal controls. Fat-free high-fructose diet (containing 60% fructose by weight) enhances the activities of liver enzymes 3- to 6-fold over the values of controls on laboratory diet but has no effect on the lung enzymes. Long-term feeding of fructose diet also increases the activities of liver enzymes from diabetic animals to twice the value of normal controls on laboratory diet. Insulin administration to fructose-fed diabetic animals restores the enzyme activities to those obtained with fructose-fed normal controls. However, the stimulation of lung enzymes of diabetic animals can be effected either by fructose or by insulin. Antigen-antibody titrations and measurements of the rate of protein synthesis show that the increased activity of the lung and liver fatty acid synthetase is due to enhanced content rather than increased specific activity. These data suggest that insulin or fructose effects on fatty acid-synthesizing enzymes are mediated through intermediate(s) whose concentration is affected in the experimental diabetes. Furthermore, all tissues may not have stringent insulin requirements since the lung enzymes can be stimulated by fructose alone.     

Hepatic very low density lipoprotein-ApoB overproduction is associated with attenuated hepatic insulin signaling and overexpression of protein-tyrosine phosphatase 1B in a fructose-fed hamster model of insulin resistance

A fructose-fed hamster model of insulin resistance was previously documented to exhibit marked hepatic very low density lipoprotein (VLDL) overproduction. Here, we investigated whether VLDL overproduction was associated with down-regulation of hepatic insulin signaling and insulin resistance. Hepatocytes isolated from fructose-fed hamsters exhibited significantly reduced tyrosine phosphorylation of the insulin receptor and insulin receptor substrates 1 and 2. Phosphatidylinositol 3-kinase activity as well as insulin-stimulated Akt-Ser473 and Akt-Thr308 phosphorylation were also significantly reduced with fructose feeding. Interestingly, the protein mass and activity of protein-tyrosine phosphatase-1B (PTP-1B) were significantly higher in fructose-fed hamster hepatocytes. Chronic ex vivo exposure of control hamster hepatocytes to high insulin also appeared to attenuate insulin signaling and increase PTP-1B. Elevation in PTP-1B coincided with marked suppression of ER-60, a cysteine protease postulated to play a role in intracellular apoB degradation, and an increase in the synthesis and secretion of apoB. Sodium orthovanadate, a general phosphatase inhibitor, partially restored insulin receptor phosphorylation and significantly reduced apoB secretion. In summary, we hypothesize that fructose feeding induces hepatic insulin resistance at least in part via an increase in expression of PTP-1B. Induction of hepatic insulin resistance may then contribute to reduced apoB degradation and enhanced VLDL particle assembly and secretion.     

Replacing dietary glucose with fructose increases ChREBP activity and SREBP-1 protein in rat liver nucleus

Diets high in fructose cause hypertriglyceridemia and insulin resistance in part due to simultaneous induction of gluconeogenic and lipogenic genes in liver. We investigated the mechanism underlying the unique pattern of gene induction by dietary fructose. Male Sprague-Dawley rats (n = 6 per group) were meal-fed (4 h/d) either 63% (w/w) glucose or 63% fructose diet. After two weeks, animals were killed at the end of the last meal. Nuclear SREBP-1 was 2.2 times higher in fructose-fed rats than glucose-fed rats. Nuclear FoxO1 was elevated 1.7 times in fructose group, but did not reach significance (P = 0.08). Unexpectedly, no difference was observed in nuclear ChREBP between two groups. However, ChREBP DNA binding was 3.9× higher in fructose-fed animals without an increase in xylulose-5-phospate, a proposed ChREBP activator. In conclusion, the gene induction by dietary fructose is likely to be mediated in part by simultaneously increased ChREBP activity, SREBP-1 and possibly FoxO1 protein in nucleus.     

Influence of 6-week exercise training on erythrocyte and liver antioxidant defense in hyperinsulinemic rats

High dosage of fructose in rats causes insulin resistance and hyperinsulinemia. This study investigates the effect of physical exercise on oxidant-antioxidant balance in rats fed a high fructose diet, which show characteristic features of insulin resistance. Products of lipid peroxidation and the activity of enzymic antioxidants namely superoxide dismutase, catalase, glutathione peroxidase, glutathione-S-transferase and glutathione reductase, in red blood cells (RBCs) and liver were assayed. Levels of non-enzymic antioxidants alpha-tocopherol and ascorbic acid and of protein and non-protein thiols were also determined. The levels of lipid peroxides, diene conjugates, lipofuscin and hydroperoxides were significantly higher in the liver of fructose-fed rats. The RBCs showed significantly higher susceptibility to H(2)O(2)-induced stress compared to control rats. Inadequate antioxidant system was noted in high fructose-fed rats. Physical training to these rats reversed the adverse effects, which could be important in alleviating the pathological consequences of insulin resistance.     

Ameliorated hepatic insulin resistance is associated with normalization of microsomal triglyceride transfer protein expression and reduction in very low density lipoprotein assembly and secretion in the fructose-fed hamster

To determine whether reduction of insulin resistance could ameliorate fructose-induced very low density lipoprotein (VLDL) oversecretion and to explore the mechanism of this effect, fructose-fed hamsters received placebo or rosiglitazone for 3 weeks. Rosiglitazone treatment led to normalization of the blunted insulin-mediated suppression of the glucose production rate and to a approximately 2-fold increase in whole body insulin-mediated glucose disappearance rate (p < 0.001). Rosiglitazone ameliorated the defect in hepatocyte insulin-stimulated tyrosine phosphorylation of the insulin receptor, IRS-1, and IRS-2 and the reduced protein mass of IRS-1 and IRS-2 induced by fructose feeding. Protein-tyrosine phosphatase 1B levels were increased with fructose feeding and were markedly reduced by rosiglitazone. Rosiglitazone treatment led to a approximately 50% reduction of VLDL secretion rates (p < 0.05) in vivo and ex vivo. VLDL clearance assessed directly in vivo was not significantly different in the FR (fructose-fed + rosiglitazone-treated) versus F (fructose-fed + placebo-treated) hamsters, although there was a trend toward a lower clearance with rosiglitazone. Enhanced stability of nascent apolipoprotein B (apoB) in fructose-fed hepatocytes was evident, and rosiglitazone treatment resulted in a significant reduction in apoB stability. The increase in intracellular mass of microsomal triglyceride transfer protein seen with fructose feeding was reduced by treatment with rosiglitazone. In conclusion, improvement of hepatic insulin signaling with rosiglitazone, a peroxisome proliferator-activated receptor gamma agonist, is associated with reduced hepatic VLDL assembly and secretion due to reduced intracellular apoB stability.     

FRUCTOSE PERFUSION IN RAT MESENTERIC ARTERIES IMPAIRS ENDOTHELIUM-DEPENDENT VASODILATION

We demonstrated that the fructose-induced hypertensive rat, representative of the principal metabolic abnormalities found in a majority of hypertensive patients, i.e. hypertriglyceridemia, hyperinsulinemia and insulin resistance (Syndrome X), is associated with an impaired response to endothelium-dependent vasodilators and that fructose may directly contribute to this impairment. Twelve male Wistar rats were divided into two groups, one given 10% fructose (n=6); the other no fructose (n=6) for 40 days in the drinking water. Systolic blood pressure was measured via the tail cuff method. Perfusion pressure responses to acetylcholine, were measured in the isolated perfused mesenteric vascular bed. Constrictor or dilator responses were measured as increases or decreases, respectively, of the perfusion pressure at a constant flow (4 ml/min). Fructose-fed rats had significantly higher blood pressure, insulin and triglyceride levels than control animals. In phenylephrine constricted beds, the endothelium-dependent dilatation to acetylcholine (0.001 to 1 μmol) was attenuated in the fructose-fed group compared to control animals. Whether this abnormality results from the syndromes (hyperinsulinemia, hypertension and hypertriglyceridemia) associated with the fructose-fed animal model is unknown. We therefore hypothesized that fructose can impair the endothelium-dependent vasodilator response. This was evaluated by perfusing mesenteric arteries from normal rats with control mannitol (40 mM) or fructose (40 mM). Endothelium-dependent dilation to acetylcholine was impaired in fructose-perfused mesenteric arteries. Indomethacin restored the vasodilator response to acetylcholine, suggesting that a cyclooxygenase derivative mediates the impaired response. Thus, we conclude that fructose can contribute to the impaired endothelium-dependent response in the fructose-induced hypertensive rat model. Published by Elsevier Science Inc.     

O3-induced mucosa-linked airway muscle hyperresponsiveness in the guinea pig.

We investigated the effects of ozone exposure (3.0 ppm, 2 h) on the responsiveness of guinea pig airway muscle in vitro from animals developing bronchial hyperreactivity. Muscarinic reactivity in vivo was determined by measuring specific airway resistance (sRaw) in response to increasing concentrations of aerosolized acetylcholine (ACh) administered before and 30 min after exposure. Immediately after reactivity testing, multiple tracheal rings from ozone- and air-exposed animals were prepared and the contractile responses to increasing concentrations of substance P, ACh, or KCl were assessed in the presence of 10 microM indomethacin with or without 1 microM phosphoramidon, an inhibitor of neutral endopeptidase. Isometric force generation in vitro was measured on stimulation by cumulative concentrations of the agonists, and force generation (in g/cm2) was calculated after determination of muscle cross-sectional area. The smooth muscle of mucosa-intact airways from guinea pigs with ozone-induced bronchial hyper-reactivity proved to be hyperresponsive in vitro to substance P and ACh but not to KCl. Pretreatment with phosphoramidon abolished the increase in substance P responsiveness but had no effect on muscarinic hyperresponsiveness after ozone exposure. Furthermore, substance P responsiveness was not augmented in ozone-exposed airways in which the mucosa had been removed before testing in vitro. Likewise, muscarinic hyperresponsiveness was not present in ozone-exposed airways without mucosa. Our data indicate that airway smooth muscle responsiveness is increased in guinea pigs with ozone-induced bronchial hyperreactivity and suggest that this hyperresponsiveness may be linked to non-cyclooxygenase mucosa-derived factors.     

Proteomic profiling of hepatic endoplasmic reticulum-associated proteins in an animal model of insulin resistance and metabolic dyslipidemia

Hepatic insulin resistance and lipoprotein overproduction are common features of the metabolic syndrome and insulin-resistant states. A fructose-fed, insulin-resistant hamster model was recently developed to investigate mechanisms linking the development of hepatic insulin resistance and overproduction of atherogenic lipoproteins. Here we report a systematic analysis of protein expression profiles in the endoplasmic reticulum (ER) fractions isolated from livers of fructose-fed hamsters with the intention of identifying new candidate proteins involved in hepatic complications of insulin resistance and lipoprotein dysregulation. We have profiled hepatic ER-associated proteins from chow-fed (control) and fructose-fed (insulin-resistant) hamsters using two-dimensional gel electrophoresis and mass spectrometry. A total of 26 large scale two-dimensional gels of hepatic ER were used to identify 34 differentially expressed hepatic ER protein spots observed to be at least 2-fold differentially expressed with fructose feeding and the onset of insulin resistance. Differentially expressed proteins were identified by matrix-assisted laser desorption ionization-quadrupole time of flight (MALDI-Q-TOF), MALDI-TOF-postsource decay, and database mining using ProteinProspector MS-fit and MS-tag or the PROWL ProFound search engine using a focused rodent or mammalian search. Hepatic ER proteins ER60, ERp46, ERp29, glutamate dehydrogenase, and TAP1 were shown to be more than 2-fold down-regulated, whereas alpha-glucosidase, P-glycoprotein, fibrinogen, protein disulfide isomerase, GRP94, and apolipoprotein E were all found to be up-regulated in the hepatic ER of the fructose-fed hamster. Seven isoforms of ER60 in the hepatic ER were all shown to be down-regulated at least 2-fold in hepatocytes from fructosefed/insulin-resistant hamsters. Implications of the differential expression of positively identified protein factors in the development of hepatic insulin resistance and lipoprotein abnormalities are discussed.     

High-carbohydrate diets affect the size and composition of plasma lipoproteins in hamsters (Mesocricetus auratus)

High-carbohydrate diets reduce plasma low-density lipoprotein (LDL)-cholesterol but also provoke the appearance of an atherogenic lipoprotein profile (ALP). Characterized by high plasma triglyceride, small dense LDL, and reduced high-density lipoprotein (HDL) cholesterol, an ALP is associated with insulin resistance. Despite extensive use of the fructose-fed hamster as a model of insulin resistance, little is known about changes that occur in the physical properties of circulating lipoproteins. Therefore, we investigated the metabolic and physical properties of lipoproteins in hamsters fed high-carbohydrate diets of varying complexity (60% carbohydrate as chow, cornstarch, or fructose) for 2 wk. Hamsters fed the high-fructose diet showed significantly increased very- low-density lipoprotein (VLDL)-triglyceride (92.3%), free cholesterol (68.6%), and phospholipid (95%), whereas apolipoprotein B levels remained unchanged. Median diameter of circulating VLDL was larger in fructose-fed hamsters (63 nm) than in cornstarch-fed hamsters. Fructose feeding induced a 42.5% increase LDL-triglyceride concurrent with a 20% reduction in LDL-cholesteryl ester. Compositional changes were associated with reduced LDL diameter. In contrast, fructose feeding caused elevations in all HDL fractions. The physical properties of apolipoprotein-B-containing lipoprotein fractions are similar between fructose-fed hamsters and humans with ALP. However, metabolism of high-density lipoprotein appears to differ in the 2 species.     

Effects of fructose loading in streptozotocin-diabetic and nondiabetic rats.

The present study compares the cardiovascular consequences of a 6-week fructose feeding in nondiabetic and streptozotocin-diabetic rats. Myocardial performance of these animals was determined using the isolated perfused working heart preparation. Systolic blood pressure, pulse rate, ventricular weight/body weight ratio, and plasma levels of glucose, insulin, triglycerides, and cholesterol were measured. In nondiabetic rats, fructose drinking caused significant increases in blood pressure, pulse rate, and plasma concentrations of insulin and triglycerides. Streptozotocin-diabetic animals exhibited significantly less body weight growth, slower pulse rate, higher plasma levels of cholesterol and triglycerides, ventricular enlargement, and functional impairment of the myocardium. The fructose-loaded diabetic rats had larger increases in plasma cholesterol and triglycerides than did control fructose-fed rats, but the fructose-induced increases in blood pressure and pulse rate were attenuated significantly. However, plasma levels of glucose and insulin and the degree of ventricular enlargement and myocardial dysfunction were not significantly different from those of control diabetic rats. These results show that fructose loading for 6 weeks can cause increases in blood pressure, pulse rate, and plasma lipids in both nondiabetic and diabetic rats. However, fructose ingestion does not significantly alter glycemic control or affect the development of myocardial dysfunction in streptozotocin-diabetic rats.     

Enhanced in vivo sensitivity of vanadyl-treated diabetic rats to insulin

Vanadium has been reported to have insulin-like properties and has recently been demonstrated to be beneficial in the treatment of diabetic animals. In the present study, concentration dependence of the therapeutic effects of vanadium and the nature of interaction under in vivo conditions between vanadium and insulin were examined in streptozotocin-diabetic rats. During a 2-week period, blood glucose levels in all treated animals were decreased. At higher concentrations of vanadyl this decrease was greater and more rapid, and remained consistently lower for the entire treatment period. Daily intake of vanadyl, however, reached a similar steady state in all groups. Acute administration of submaximal doses of insulin, which had minimal effects in untreated diabetic rats, lowered blood glucose concentrations in vanadyl-treated and vanadyl-withdrawn animals to control levels. Chronic treatment of streptozotocin-diabetic rats with submaximal levels of vanadyl and insulin, ineffective alone, also produced significant decreases in blood glucose levels when used in combination. Finally, the insulin dosage required to maintain a nonglycosuric state in spontaneously diabetic (BB) rats was reduced in the presence of vanadyl. These studies indicate that chronic oral vanadyl treatment (a) produces a concentration-related lowering of blood glucose in diabetic rats, (b) potentiates the in vivo glucose lowering effects of acute and chronic administrations of insulin in streptozotocin-diabetic rats, and (c) substitutes for, or potentiates, the effects of chronic insulin therapy in spontaneously diabetic BB rats.     

Fructose diet treatment in mice induces fundamental disturbance of cardiomyocyte Ca2+ handling and myofilament responsiveness

High fructose intake has been linked to insulin resistance and cardiac pathology. Dietary fructose-induced myocardial signaling and morphological alterations have been described, but whether cardiomyocyte function is influenced by chronic high fructose intake is yet to be elucidated. The goal of this study was to evaluate the cardiomyocyte excitation-contraction coupling effects of high dietary fructose and determine the capacity for murine cardiomyocyte fructose transport. Male C57Bl/6J mice were fed a high fructose diet for 12 wk. Fructose- and control-fed mouse cardiomyocytes were isolated and loaded with the fura 2 Ca(2+) fluorescent dye for analysis of twitch and Ca(2+) transient characteristics (4 Hz stimulation, 37°C, 2 mM Ca(2+)). Myocardial Ca(2+)-handling protein expression was determined by Western blot. Gene expression of the fructose-specific transporter, GLUT5, in adult mouse cardiomyocytes was detected by real-time and conventional RT-PCR techniques. Diastolic Ca(2+) and Ca(2+) transient amplitude were decreased in isolated cardiomyocytes from fructose-fed mice relative to control (16 and 42%, respectively), coincident with an increase in the time constant of Ca(2+) transient decay (24%). Dietary fructose increased the myofilament response to Ca(2+) (as evidenced by a left shift in the shortening-Ca(2+) phase loop). Protein expression of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a), phosphorylated (P) phospholamban (Ser(16)), and P-phospholamban (Thr(17)) was reduced, and protein phosphatase 2A expression increased, in fructose-fed mouse hearts. Hypertension and cardiac hypertrophy were not evident. These findings demonstrate that fructose diet-associated myocardial insulin resistance induces profound disturbance of cardiomyocyte Ca(2+) handling and responsiveness in the absence of altered systemic loading conditions.     

Endothelin-1 modulates angiotensin II in the development of hypertension in fructose-fed rats

Two of the most potent vasoconstrictors, endothelin-1 (ET-1) and angiotensin II (Ang II), are upregulated in fructose hypertensive rats. It is unknown whether an interrelationship exists between these peptides that may contribute to the development of fructose-induced hypertension. The objective of this study was to investigate the existence of an interaction between the endothelin and renin angiotensin systems that may play a role in the development of fructose-induced hypertension. High fructose feeding and treatment with either bosentan, a dual endothelin receptor antagonist, or with L-158,809, an angiotensin type 1 receptor antagonist, were initiated simultaneously in male Wistar rats. Systolic blood pressure, fasted plasma parameters, insulin sensitivity, plasma Ang II, and vascular ET-1-immunoreactivity were determined following 6 weeks of high fructose feeding. Rats fed with a high fructose diet exhibited insulin resistance, hyperinsulinemia, hypertriglyceridemia, hypertension, and elevated plasma Ang II. Treatment with either bosentan or L-158,809 significantly attenuated the rise in blood pressure with no effect on insulin levels or insulin sensitivity in fructose-fed rats. Bosentan treatment significantly reduced plasma Ang II levels, while L-158,809 treatment significantly increased vascular ET-1-immunoreactivity in fructose-fed rats. Thus, treatment with the endothelin receptor antagonist prevented the development of fructose-induced hypertension and decreased plasma Ang II levels. These data suggest that ET-1 contributes to the development of fructose-induced hypertension through modulation of Ang II levels.