Fructose-induced hypertension in Wistar–Kyoto rats: interaction with moderately high dietary saltThis paper is one of a selection of papers published in this Special Issue, entitled The Cellular and Molecular Basis of Cardiovascular Dysfunction, Dhalla 70th Birthday Tribute

Multiphoton imaging of the effect of monosaccharide diffusion and formation of fluorescent advanced end products in porcine aorta

Prolonged exposure of tissues to elevated blood sugar levels lead to the formation of advanced glycation end products (AGEs), thus contributing to diabetic complications. Since the vascular system is in immediate contact with blood, diabetic effects on aorta is a major health concern. However, the relative effect of the diffusion of sugar molecular through the vascular wall and the rate of AGE formation is not known. In this study, we aim to address this issue by incubating excised porcine aorta in D-glucose, D-galactose, and D-fructose solutions for different periods. The tissue specimens were then excised for multiphoton imaging of autofluorescence intensity profiles across the aorta wall. We found that for Days 4 to 48 incubation, autofluorescence is constant along the radial direction of the aorta sections, suggesting that monosaccharide diffusion is rapid in comparison to the rate of formation of fluorescent AGEs (fAGEs). Moreover, we found that in porcine aorta, the rate of fAGE formation of D-fructose and D-glucose are factors 2.08 and 1.14 that of D-galactose. Our results suggest that for prolonged exposure of the cardiovascular system to elevated monosaccharides 4 days or longer, damage to the aorta is uniform throughout the tissues.

Metabolites and Hypertension: Insights into Hypertension as a Metabolic Disorder: 2019 Harriet Dustan Award

For over 100 years, essential hypertension has been researched from different perspectives ranging from genetics, physiology, and immunology to more recent ones encompassing microbiology (microbiota) as a previously underappreciated field of study contributing to the cause of hypertension. Each field of study in isolation has uniquely contributed to a variety of underlying mechanisms of blood pressure regulation. Even so, clinical management of essential hypertension has remained somewhat static. We, therefore, asked if there are any converging lines of evidence from these individual fields that could be amenable for a better clinical prognosis. Accordingly, here we present converging evidence which support the view that metabolic dysfunction underlies essential hypertension.

The effects of fruit consumption in patients with hyperuricaemia or gout

The consumption of fructose has gained increased attention as a potential cause of hyperuricaemia since fructose metabolism produces urate as a byproduct. In addition to sucrose and high fructose corn syrup, fresh fruits also contain fructose, suggesting that patients with hyperuricaemia or gout might also avoid fresh fruit. However, the effect of fruits is complex. Some studies reported that fruit intake was associated with gout flares while other studies showed that fruits rather lowered the risk for gout. Thus, fruits should not be simply viewed as a source of fructose. The complexity of fruits is accounted for by several nutrients existing in fruits. Vitamin C, epicatechin, flavonols, potassium and fibre are all nutrients in fruits, and these factors could modify fructose and urate effects. In this review, we discuss clinical studies evaluating the effect of fruit and fruit juice intake on hyperuricaemia and gout, and propose potential mechanisms for how fruit may influence urate levels.

Limonin alleviates macro- and micro-vascular complications of metabolic syndrome in rats: A comparative study with azelnidipine

BACKGROUND

Hypertension is a serious component of metabolic syndrome (MetS).

HYPOTHESIS

This research investigates the potential protective effect of limonin against MetS-associated hypertension in comparison with azelnidipine, a common calcium channel blocker.

STUDY DESIGN

MetS was induced in rats by 10% fructose in water and 3% salt in diet over a 16-week period. Limonin (50 mg/kg) and azelnidipine (5 mg/kg) were administered daily in the last four weeks METHODS: Non-invasive blood pressure (BP) was recorded in conscious animals. Concentration-response curves for phenylephrine (PE) and acetylcholine (ACh) were analysed in thoracic aorta (macrovessels) and kidney microvessels. Blood glucose level, serum insulin level, advanced glycation end products (AGEs), tumor necrosis factor-α (TNF-α), malondialdehyde (MDA) and transforming growth factor-β1 (TGF-β1) were determined.

RESULTS

Limonin alleviated elevations in systolic and diastolic BP associated with MetS similar to levels associated with azelnidipine. Limonin prevented the MetS induced exaggerated macro- and micro-vascular contractility to PE and the impaired dilatation to ACh. However, in vitro incubation with limonin partially alleviated the deteriorated vascular reactivity of aorta isolated from MetS animals or AGEs injured aorta. Limonin did not have direct relaxant effect on the isolated vessel. On the other hand, limonin reduced the elevated serum levels of AGEs, TNF-α and MDA. Limonin suppressed the vascular fibrosis through reducing the elevated serum level of TGF-β1 and excessive aortic collagen deposition. Limonin decreased the elevated HOMA-IR in MetS animals.

CONCLUSION

Limonin offsets the hypertensive and vascular impairment associated with MetS via attenuation of inflammation and fibrosis. Its impact is comparable to that of azelnidipine.

Dietary Fructose Enhances the Ability of Low Concentrations of Angiotensin II to Stimulate Proximal Tubule Na⁺ Reabsorption

Fructose-enriched diets cause salt-sensitive hypertension. Proximal tubules (PTs) reabsorb 70% of the water and salt filtered through the glomerulus. Angiotensin II (Ang II) regulates this process. Normally, dietary salt reduces Ang II allowing the kidney to excrete more salt, thereby preventing hypertension. We hypothesized that fructose-enriched diets enhance the ability of low concentrations of Ang II to stimulate PT transport. We measured the effects of a low concentration of Ang II (10⁻¹² mol/L) on transport-related oxygen consumption (QO₂), and Na/K-ATPase and Na/H-exchange (NHE) activities and expression in PTs from rats consuming tap water (Control) or 20% fructose (FRUC). In FRUC-treated PTs, Ang II increased QO₂ by 14.9 ± 1.3 nmol/mg/min (p < 0.01) but had no effect in Controls. FRUC elevated NHE3 expression by 19 ± 3% (p < 0.004) but not Na/K-ATPase expression. Ang II stimulated NHE activity in FRUC PT (Δ + 0.7 ± 0.1 Arbitrary Fluorescent units (AFU)/s, p < 0.01) but not in Controls. Na/K-ATPase activity was not affected. The PKC inhibitor Gö6976 blocked the ability of FRUC to augment the actions of Ang II. FRUC did not alter the inhibitory effect of dopamine on NHE activity. We conclude that dietary fructose increases the ability of low concentrations of Ang II to stimulate PT Na reabsorption via effects on NHE.

Oxidative stress causes hypertension and activation of nuclear factor-κB after high-fructose and salt treatments

There is evidence that diets rich in salt or simple sugars as fructose are associated with abnormalities in blood pressure regulation. However, the mechanisms underlying pathogenesis of salt- and fructose-induced kidney damage and/or consequent hypertension yet remain largely unexplored. Here, we tested the role of oxidative state as an essential factor along with high salt and fructose treatment in causing hypertension. Fischer male rats were supplemented with a high-fructose diet (20% in water) for 20 weeks and maintained on high-salt diet (8%) associate in the last 10 weeks. Fructose-fed rats exhibited a salt-dependent hypertension accompanied by decrease in renal superoxide dismutase activity, which is the first footprint of antioxidant inactivation by reactive oxygen species (ROS). Metabolic changes and the hypertensive effect of the combined fructose-salt diet (20 weeks) were markedly reversed by a superoxide scavenger, Tempol (10 mg/kg, gavage); moreover, Tempol (50 mM) potentially reduced ROS production and abolished nuclear factor-kappa B (NF-κB) activation in human embryonic kidney HEK293 cells incubated with L-fructose (30 mM) and NaCl (500 mosmol/kg added). Taken together, our data suggested a possible role of oxygen radicals and ROS-induced activation of NF-κB in the fructose- and salt-induced hypertension associated with the progression of the renal disease.

Review of a causal role of fructose-containing sugars in myocardial susceptibility to ischemia/reperfusion injury

In 2012, the World Health Organization Global Status Report on noncommunicable diseases showed that 7.4 million deaths were due to ischemic heart disease. Consequently, cardiovascular disease is a significant health burden, especially when partnered with comorbidities such as obesity, metabolic syndrome, and type 2 diabetes mellitus. Of note, these diseases can all be induced or exacerbated by diet. Carbohydrates, in particular, fructose and glucose, generally form the largest part of the human diet. Accumulating evidence from animal studies suggests that if large amounts of fructose are consumed either in isolation or in combination with glucose (fructose-containing sugars), myocardial susceptibility to ischemia/reperfusion (I/R) injury increases. However, the underlying mechanisms that predisposes the myocardium to I/R injury in the fructose model are not elucidated, and no single mechanistic pathway has been described. Based on all available data on this topic, this review describes previously investigated mechanisms and highlights 3 main mechanistic pathways whereby fructose has shown to increase myocardial susceptibility to I/R injury. These pathways include (1) increased reactive oxygen species, resulting in reduced nitric oxide synthase and coronary flow; (2) elevated plasma fatty acids and insulin, leading to increased cardiac triglyceride content and lipotoxicity; and (3) disrupted myocardial calcium handling/homeostasis. Moreover, we highlight various factors that should be taken into account when the fructose animal model is used, such as rat strain, treatment periods, and doses. We argue that failure to do so would result in erratic inferences drawn from the existing body of evidence on fructose animal models.

The pathophysiology of hypertension in patients with obesity

The combination of obesity and hypertension is associated with high morbidity and mortality because it leads to cardiovascular and kidney disease. Potential mechanisms linking obesity to hypertension include dietary factors, metabolic, endothelial and vascular dysfunction, neuroendocrine imbalances, sodium retention, glomerular hyperfiltration, proteinuria, and maladaptive immune and inflammatory responses. Visceral adipose tissue also becomes resistant to insulin and leptin and is the site of altered secretion of molecules and hormones such as adiponectin, leptin, resistin, TNF and IL-6, which exacerbate obesity-associated cardiovascular disease. Accumulating evidence also suggests that the gut microbiome is important for modulating these mechanisms. Uric acid and altered incretin or dipeptidyl peptidase 4 activity further contribute to the development of hypertension in obesity. The pathophysiology of obesity-related hypertension is especially relevant to premenopausal women with obesity and type 2 diabetes mellitus who are at high risk of developing arterial stiffness and endothelial dysfunction. In this Review we discuss the relationship between obesity and hypertension with special emphasis on potential mechanisms and therapeutic targeting that might be used in a clinical setting.

High dietary salt decreases antioxidant defenses in the liver of fructose-fed insulin-resistant rats

In this study we investigated the hypothesis that a high-salt diet to hyperinsulinemic rats might impair antioxidant defense owing to its involvement in the activation of sodium reabsorption to lead to higher oxidative stress. Rats were fed a standard (CON), a high-salt (HS), or a high-fructose (HF) diet for 10 weeks after which, 50% of the animals belonging to the HF group were switched to a regimen of high-fructose and high-salt diet (HFS) for 10 more weeks, while the other groups were fed with their respective diets. Animals were then euthanized and their blood and liver were examined. Fasting plasma glucose was found to be significantly higher (approximately 50%) in fructose-fed rats than in the control and HS rats, whereas fat liver also differed in these animals, producing steatosis. Feeding fructose-fed rats with the high-salt diet triggered hyperinsulinemia and lowered insulin sensitivity, which led to increased levels of serum sodium compared to the HS group. This resulted in membrane perturbation, which in the presence of steatosis potentially enhanced hepatic lipid peroxidation, thereby decreasing the level of antioxidant defenses, as shown by GSH/GSSG ratio (HFS rats, 7.098±2.1 versus CON rats, 13.2±6.1) and superoxide dismutase (HFS rats, 2.1±0.05 versus CON rats, 2.3±0.1%), and catalase (HFS rats, 526.6±88.6 versus CON rats, 745.8±228.7 U/mg ptn) activities. Our results indicate that consumption of a salt-rich diet by insulin-resistant rats may lead to regulation of sodium reabsorption, worsening hepatic lipid peroxidation associated with impaired antioxidant defenses.

Uric acid and fructose: potential biological mechanisms

Excessive fructose consumption is associated with the development of metabolic syndrome and type II diabetes. Both conditions are well-known risk factors for cardiovascular and renal diseases. Uric acid synthesis is linked biochemically to fructose metabolism, thus the widespread consumption of this monosaccharide has been related to steady increasing levels of serum uric acid during the past few decades. Recent evidence has suggested that uric acid may act as a cardiorenal toxin. In this regard, experimental studies have suggested that the primary noxious effect of uric acid occurs inside the cell and is likely the stimulation of oxidative stress. More studies to disclose the harmful mechanisms associated with increasing intracellular uric acid levels after a fructose load are warranted.

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.

Dietary fructose and hypertension

The association between fructose and increased blood pressure is still incompletely defined, because experimental studies have produced dissimilar conclusions. Amplified vasopressor responses to minimal stimuli and differing responses to fructose in peripheral versus central sites may explain the controversy. Fructose induces systemic hypertension through several mechanisms mainly associated with deleterious effects on target organs (kidney, endothelium, heart) exerted by the byproducts of its metabolism, such as uric acid. The kidney is particularly sensitive to the effects of fructose because high loads of this sugar reach renal tissue. In addition, fructose increases reabsorption of salt and water in the small intestine and kidney; thus the combination of salt and fructose has a synergistic effect in the development of hypertension. Clinical and epidemiologic studies have also linked fructose consumption with hypertension. Further studies are warranted in order to understand the role of fructose in the development of hypertension.

Fructose and moderately high dietary salt-induced hypertension: prevention by a combination of N-acetylcysteine and L-arginine

Diets containing 8% salt or 4% fructose (FR) cause insulin resistance and increase tissue methylglyoxal and advanced glycation end products (AGEs), platelet cytosolic-free calcium, and systolic blood pressure (SBP) in rats. In WKY rats, we have shown that moderately high salt, 4% NaCl (MHS) alone in diet does not cause hypertension, and when given along with 4% FR it does not have an additive effect. N-acetylcysteine (NAC) or L-arginine (ARG), treatment alone does not prevent hypertension in this model. The objectives of this study were to investigate the effect of NAC plus ARG in diet on SBP, platelet cytosolic-free calcium in a MHS + FR model, and to measure the plasma levels of methylglyoxal and the AGE, methylglyoxal-derived hydroimidazolone (MGH). At 7 weeks of age, WKY rats were divided into three groups: control group was given regular rat chow (0.7% NaCl) and water; MHS + FR group, diet containing 4% NaCl and 4% FR in drinking water; and MHS + FR + NAC + ARG group, MHS diet supplemented with 1.5% N-acetylcysteine (NAC) and 1.5% L-arginine (ARG), and 4% FR in drinking water, and followed for 6 weeks. NAC + ARG prevented the increase in platelet cytosolic-free calcium and SBP in MHS + FR treated rats. There was no difference in mean values of plasma methylglyoxal and MGH among the groups. In conclusion, NAC + ARG treatment is effective in preventing hypertension in a moderately high salt + FR-induced animal model. Plasma methylglyoxal and MGH may not represent tissue modification or, alternatively, other tissue AGEs, derived from methylglyoxal or other aldehydes, may be involved in hypertension in this model.

Hypothesis: could excessive fructose intake and uric acid cause type 2 diabetes?

We propose that excessive fructose intake (>50 g/d) may be one of the underlying etiologies of metabolic syndrome and type 2 diabetes. The primary sources of fructose are sugar (sucrose) and high fructose corn syrup. First, fructose intake correlates closely with the rate of diabetes worldwide. Second, unlike other sugars, the ingestion of excessive fructose induces features of metabolic syndrome in both laboratory animals and humans. Third, fructose appears to mediate the metabolic syndrome in part by raising uric acid, and there are now extensive experimental and clinical data supporting uric acid in the pathogenesis of metabolic syndrome. Fourth, environmental and genetic considerations provide a potential explanation of why certain groups might be more susceptible to developing diabetes. Finally, we discuss the counterarguments associated with the hypothesis and a potential explanation for these findings. If diabetes might result from excessive intake of fructose, then simple public health measures could have a major impact on improving the overall health of our populace.