果糖诱导大鼠胰岛素抵抗和高血压的机制及人组织型激肽释放酶基因的治疗作用

目的 研究果糖诱导高血压合并高胰岛素血症、胰岛素抵抗时内皮素1(ET-1)及其受体(ETA)、血管紧张素Ⅱ受体1(AT1)的变化和人组织型激肽释放酶(HK)基因的治疗作用。方法 雄性Sprague—Dawley(SD)大鼠饮用10％果糖水诱导形成高血压和胰岛素抵抗动物模型后，静脉注入含HKcDNA的重组质粒(pcDNA-HK)。监测血压，血清胰岛素及血糖等。2周时处死动物，测定脏器中HK的表达及ET-1、ETA和AT1的表达情况。结果 饮高果糖水2周后，动物血压、血胰岛素水平明显高于正常饮水组，静脉注射pcDNA-HK后第3天即可明显降低果糖诱导高血压大鼠的血压，最大降压效应在导入基因后第14天(达15mmHg)，其降压效应持续3周以上。HK基因导入后第2周，HK治疗组动物ET-1、ETA及AT1明显低于对照组，与正常组接近。并且在血压下降的同时，血胰岛素浓度亦明显降低。结论动物高果糖饮水可导致高血压及高胰岛素血症，其机理可能与ET-1、ETA和AT1表达增加有关，HK基因的导入可能通过降低血管ET-1、ETA和AT1表达而降低血压，并纠正高血压伴随的高胰岛素血症，结果提示HK基因治疗高血压病尤其是合并糖尿病或胰岛素对抗者的可行性。     

Chronic insulin infusion normalizes blood pressure and the gene expressions of angiotensin II type 1 receptor in fructose-fed rats.

It remains open to debate whether hyperinsulinemia leads to the development of hypertension. We addressed this issue by investigating the effect of chronic insulin infusion on blood pressure and related parameters in hypertensive fructose-fed rats. Rats were given either normal chow or a fructose-rich diet, and insulin or saline was infused through mini-pumps in the same animals for 14 days. The chronic insulin infusion exerted no effect on the blood pressure of the chow-fed rats. Fructose feeding increased the blood pressure and levels of insulin, triglyceride and fatty acid. Insulin infusion augmented the hyperinsulinemia but normalized the blood pressure and plasma lipids. Plasma angiotensin II was elevated in the fructose-fed rats, while insulin infusion left it unchanged. The expression of angiotensin II type 1 receptor (AT1R) mRNA was doubled in both the aorta and epididymal fat of the fructose-fed rats, while that of angiotensin II type 2 receptor (AT2R) was unaltered. Insulin infusion completely rectified the over-expression of the AT1R gene. Our findings indicate that chronic insulin infusion exacerbates hyperinsulinemia while normalizing blood pressure and the gene expressions of AT1R in insulin-resistant fructose-fed rats, suggesting that endogenous hyperinsulinemia caused by insulin resistance is associated with the development of hypertension, whereas exogenous hyperinsulinemia attenuates hypertension probably due to amelioration of insulin resistance.     

Increased vascular endothelin-1 gene expression with unaltered nitric oxide synthase levels in fructose-induced hypertensive rats

The present study aimed to investigate whether altered expression levels of endothelin-1 (ET-1) and nitric oxide synthase (NOS) are related to the development of insulin-resistant hypertension. Male Sprague-Dawley rats were fed a fructose-rich diet for 5 weeks. Systolic blood pressure significantly increased in fructose-fed rats. While serum free fatty acid (FFA) and plasma nitrite/nitrate (NOx) levels did not significantly differ between the fructose-fed and control groups, plasma insulin and serum triglyceride (TG) concentrations significantly increased in the former. ET-1 mRNA expression in the aorta increased to 195% in fructose-fed rats. Neither the protein expression of constitutive NOS (cNOS) nor that of inducible NOS (iNOS) were significantly affected by fructose feeding. However, NOx levels in the aorta were significantly increased. These results indicate that an increased expression of vascular ET-1 may be causally related to the development of hypertension in fructose-fed rats. However, an altered role of the vascular nitric oxide (NO) pathway may not be primarily involved in the development of fructose-induced hypertension.     

Overexpression of TRB3 gene in adipose tissue of rats with high fructose-induced metabolic syndrome

Insulin resistance is the physiopathologic foundation of metabolic syndrome. TRB3 has been revealed to be involved in insulin resistance in the liver by interacting directly with Akt and blocking its activation. Our investigation aims at exploring the relationship between metabolic syndrome and TRB3 mRNA expression in adipose tissue of rats. Two groups were studied as follows: the control group (Control, n = 12) was fed a standard rodent chow, and the experimental group (Fructose n = 9) was fed a high-fructose diet. Body weight and systolic blood pressure were measured per 4 weeks. At the end of 38 weeks, levels of tribbles mRNAs in adipose tissue were determined by quantitative real-time polymerase chain reaction (PCR), and Akt/phospho-Akt expression was assessed by Western blot. Results show that levels of TRB1-3 mRNAs were expressed in adipose tissue of rats of both groups, and tribbles mRNAs were TRB1 (Control: 0.00515, Fructose: 0.00497), TRB2 (Control: 0.02104, Fructose: 0.01988), and TRB3 (Control: 0.00457, Fructose: 0.00822), respectively. Of the three, TRB3 mRNA alone significantly increased by 94% in adipose tissue of fructose-fed rats compared with those in adipose tissue of the controls (P<0.05), and there was significant positive correlation between TRB3 mRNA levels and HOMA-R in fructose group (r = 0.68, P<0.05). Western blot analysis showed that phospho-Akt (Ser-473) expression was significantly decreased in adipose tissue of fructose-fed rats compared with controls (P<0.001). The present study suggests that TRB3 may be involved in metabolic syndrome by inhibiting activation of Akt in adipose tissue.     

At1 Receptor Density Changes During Development of Hypertension in Hyperinsulinemic Rats

In a previous study we showed that the renin-angiotensin system (RAS) plays a role in the etiology of fructose-induced hypertension. To our knowledge, no previous study has evaluated changes in angiotensin II (Ang II) type I receptor (AT₁) density in fructose-fed rats that are insulin resistant and hypertensive. The purpose of this study was to determine the changes in plasma Ang II and AT₁ density associated with the elevation of blood pressure in fructose-treated rats. Male Sprague-Dawley rats were divided into two groups and were fed either normal rat chow or a 60% fructose-enriched diet for four weeks. Plasma Ang II and serum insulin levels of the fructose-treated rats were significantly elevated (p<0.01) by the end of the second week of fructose treatment. Plasma Ang II levels of the fructose-fed rats returned to basal levels by the end of the fourth week of dietary treatment, whereas the serum insulin levels consistently remained elevated. Blood pressure was significantly elevated in the fructose-fed rats within two weeks of fructose treatment. Elevation of blood pressure was associated with left ventricular hypertrophy. Furthermore, there was a significant increase in AT₁ receptor density in the ventricles and a significant decrease in AT₁ receptor density in the aortas of fructose-fed rats at the end of fourth week. There were no significant changes in receptor density in the hypothalami or adrenal glands of fructose-treated rats. These results suggest that chronic fructose treatment activates the renin-angiotensin system, which is manifested by an increase in plasma Ang II, elevation of blood pressure, cardiac hypertrophy, and changes in AT₁ receptor density.     

Renin-angiotensin system and fibronectin gene expression in Dahl Iwai salt-sensitive and salt-resistant rats

OBJECTIVE: The tissue renin-angiotensin system and extracellular matrix are involved in the cardiovascular hypertrophy and remodeling induced by hypertension. In this study, we examined the gene expression of the tissue renin-angiotensin system and fibronectin in inbred Dahl Iwai salt-sensitive and salt-resistant rats. MATERIALS AND METHODS: Eight pairs of 6-week-old male Dahl Iwai salt-sensitive and salt-resistant rats were fed either a low- or high-salt diet (0.3% or 8% NaCl, respectively) for 4 weeks. Activities of the circulating renin-angiotensin system were measured by radioimmunoassay and the gene expression of tissue angiotensinogen, the angiotensin II type 1 receptor (AT1) and fibronectin were analyzed by Northern blot analysis. RESULTS: Salt loading significantly increased blood pressure and produced cardiovascular hypertrophy and nephrosclerosis in the salt-sensitive rats. Activities of the circulating renin-angiotensin system were lower in salt-sensitive rats than in salt-resistant rats fed the low-salt diet, and salt loading lowered these activities in salt-resistant rats but not in salt-sensitive rats. In salt-resistant rats, salt loading increased renal, cardiac and aortic angiotensinogen, AT1 and fibronectin messenger (m)RNA expression except for aortic fibronectin mRNA expression. In contrast, in the salt-sensitive rats, salt loading stimulated the expression of cardiac fibronectin and aortic angiotensinogen, AT1 and fibronectin mRNAs. Furthermore, the cardiac and aortic fibronectin mRNA levels in salt-sensitive rats were higher than those in salt-resistant rats when both strains were fed the high-salt diet. CONCLUSIONS: These results demonstrate that the expression of tissue angiotensinogen, AT1 and fibronectin mRNAs is regulated differently in Dahl Iwai salt-sensitive and salt-resistant rats, and indicate that salt-mediated hypertension activates the cardiac fibronectin gene independently of the tissue renin-angiotensin system and stimulates the aortic fibronectin gene with activation of the tissue renin-angiotensin system.     

High-fructose diet decreases catalase mRNA levels in rat tissues

Insulin resistance and hyperinsulinemia have recently been identified as independent determinants of several risk factors for cardiovascular disease. The generation of reactive oxygen species (ROS) may play an important role as a final common mediator by which glucose and insulin resistance might contribute to development of cardiovascular disease and hypertension. The aim of the present study was to evaluate changes on mRNA expression of antioxidant enzymes [catalase, Cu-Zn superoxide dismutase (Cu-ZnSOD), MnSOD], blood pressure and metabolic parameters in insulin resistance that follow feeding normotensive Wistar rats a high-fructose-enriched diet. In our investigation 26 normal male Wistar rats were fed a high-fructose diet for 2 weeks (no.=14) or normal chow to serve as a control group (no.=12). In vivo insulin resistance was verified in a subgroup of control and fructose-fed rats by the euglycemic hyperinsulinemic clamp technique at 2 different insulin infusion rates, 29 (submaximal stimulation) and 290 (maximal stimulation) pmol/kg/min respectively. The glucose infusion rate (GIR) was not significantly different in the two groups during the submaximal infusion of insulin (1.4 +/- 0.8 mmol/kg/min in fructose-fed rats vs 1.6 +/- 0.7 mmol/kg/min in control rats, NS) while in fructose-fed rats it was significantly lower (-29.8%) than in control rats during maximal infusion of insulin (2.6 +/- 0.3 mmol/kg/min vs 3.7 +/- 0.3 mmol/kg/min, p<0.05). Fructose feeding markedly reduced the expression of catalase mRNA and Cu-ZnSOD mRNA in the liver, catalase mRNA in the heart (p<0.05). A tendency of fructose feeding to reduce the expression of antioxidant enzymes in skeletal muscle and adipose tissue was also observed (NS). Fructose feeding also increased plasma uric acid (119.9 +/- 30.4 vs 42.1 +/- 10 pmol/l, p<0.05) and systemic blood pressure (128 +/- 4 vs 109 +/- 5 mmHg, p<0.05) respect to control animals. No significant changes were observed in plasma levels of glycemia and tryglycerides. Our study suggests that in non-hyperglycemic, fructose-fed insulin-resistant rats the expression of catalase is inhibited in liver and heart. This condition might lead to higher susceptibility to oxidative stress in insulin resistance. However, an adaptive cellular response to maintain the effectiveness of intracellular signaling pathways mediated by insulin-activated hydrogen peroxide generating systems may also be hypothesized.     

Fructose feeding increases insulin resistance but not blood pressure in Sprague-Dawley rats

Fructose feeding has been widely reported to cause hypertension in rats, as assessed indirectly by tail cuff plethysmography. Because there are potentially significant drawbacks associated with plethysmography, we determined whether blood pressure changes could be detected by long-term monitoring with telemetry in age-matched male Sprague-Dawley rats fed either a normal or high-fructose diet for 8 weeks. Fasting plasma glucose (171+/-10 versus 120+/-10 mg/dL), plasma insulin (1.8+/-0.5 versus 0.7+/-0.1 microg/L), and plasma triglycerides (39+/-2 versus 30+/-2 mg/dL) were modestly but significantly elevated in fructose-fed animals. Using the hyperinsulinemic euglycemic clamp technique, the rate of glucose infusion necessary to maintain equivalent plasma glucose was significantly reduced in fructose-fed compared with control animals (22.9+/-3.6 versus 41.5+/-2.9 mg/kg per minute; P<0.05). However, mean arterial pressure (24-hour) did not change in the fructose-fed animals over the 8-week period (111+/-1 versus 114+/-2 mm Hg; week 0 versus 8), nor was it different from that in control animals (109+/-2 mm Hg). Conversely, systolic blood pressure measured by tail cuff plethysmography at the end of the 8-week period was significantly greater in fructose-fed versus control animals (162+/-5 versus 139+/-1 mm Hg; P<0.001). Together, these data demonstrate that long-term fructose feeding induces mild insulin resistance but does not elevate blood pressure. We propose that previous reports of fructose-induced hypertension reflect a heightened stress response by fructose-fed rats associated with restraint and tail cuff inflation.     

Androgens are necessary for the development of fructose-induced hypertension

Hyperinsulinemia and insulin resistance are closely associated with hypertension in humans and in animal models. Gender differences have been found in the development of hypertension in fructose-fed rats. The objectives of the present study were, first, to clarify whether androgens are required in the development of hyperinsulinemia, insulin resistance, and hypertension in fructose-fed rats, and second, to determine if cyclooxygenase-1 and cyclooxygenase-2 are also increased in the arteries of these rats. Male rats were gonadectomized or sham-operated and fed a 60% fructose diet beginning at age 7 weeks. Blood pressure was measured by a tail-cuff method, and an oral glucose tolerance test was performed to assess insulin sensitivity after 8 weeks of fructose feeding. Cyclooxygenase-1 and cyclooxygenase-2 mRNA expression was also assessed in the thoracic aortae and mesenteric arteries. Gonadectomy prevented hypertension from developing in the fructose-fed rats, but hyperinsulinemia and insulin resistance developed. There was an increase in cyclooxygenase-2 expression in the thoracic aortae and mesenteric arteries of the fructose-fed sham-operated rats while the expression of cyclooxygenase-1 remained unchanged. Gonadectomy prevented the mRNA overexpression of vascular cyclooxygenase-2 in the fructose-fed rats. These results suggest that the presence of androgens is necessary for the development of fructose-induced hypertension. Androgens apparently act as a link between hyperinsulinemia/insulin resistance and hypertension in fructose-hypertensive rats. Furthermore, an increase in the expression of cyclooxygenase-2 is implicated in the development of hypertension. The mechanisms involved require further study.     

Gene therapy attenuates the elevated blood pressure and glucose intolerance in an insulin-resistant model of hypertension

OBJECTIVE : Fructose feeding in male Sprague-Dawley (SD) rats results in a mild hypertension and glucose intolerance. Although the mechanism of this glucose intolerance and hypertension is not completely understood, a role for the renin-angiotensin system (RAS) has been proposed. In the current study our aim was to test the hypothesis that intervention of the RAS with a gene therapy approach would be effective in preventing the development of hypertension and glucose intolerance in this animal model. DESIGN AND METHODS : Five-day-old SD rats were administered either an empty retroviral vector (LNSV) or retroviral vector containing AT1 receptor antisense DNA (AT1R-AS). The virus (25 microl, 8 x 10(9) CFU/ml) was injected into the heart and the animals were returned to their mothers. After weaning, half the animals from each group were placed on breeder's chow or a 60% fructose diet. Indirect blood pressures (BP) were determined and an oral glucose tolerance test (OGTT) was performed when the animals had been on the respective diets for 2 months. RESULTS : Fructose-fed animals developed mild hypertension (145 +/- 3 versus 132 +/- 4 mmHg) by 6 weeks of dietary intervention. This increase in BP was prevented by AT1R-AS treatment (125 +/- 3 mmHg). At 2 months of age, fasting blood glucose was comparable among the four groups; however, the glucose excursion during the OGTT was significantly greater and more prolonged in the LNSV-treated, fructose-fed group than the other three groups. AT1R-AS treatment significantly prevented glucose intolerance in the fructose rat to levels observed in the controls. CONCLUSIONS : Early fructose dietary treatment results in moderate hypertension and glucose intolerance, which is prevented by a single neonatal treatment with AT1R-AS. These results suggest that the RAS is involved in the glucose intolerance associated with fructose feeding and that genetic intervention is effective in this rat model.     

Cellular mechanisms of insulin resistance in rats with Fructose-Induced hypertension

BackgroundFeeding a high-fructose diet induces hypertension and insulin-resistance in Sprague-Dawley rats.MethodsTo investigate whether insulin receptors contribute to abnormal glucose metabolism and whether their regulation is differentially regulated in different tissues, we evaluated the glycemic and insulinemic response to an oral glucose load, insulin receptor binding, and insulin receptor messengerRNA (mRNA) levels in tissues of rats that were fed either standard rat chow or a diet containing 66% fructose for 2 weeks.ResultsBlood pressure and plasma triglycerides increased significantly in the fructose-fed rats, whereas body weight, fasting plasma glucose, and plasma insulin did not differ significantly from controls. Plasma glucose and insulin responses to oral glucose were significantly greater in fructose-fed than in control rats. Insulin receptor-binding characteristics were determined by an in situ autoradiographic technique associated with computerized microdensitometry. The insulin receptor number was significantly lower in both skeletal muscle and liver of fructose-fed rats as compared to controls, whereas no difference was observed in the kidney. No significant differences were found in binding affinity. Insulin receptor mRNA levels were determined by slot-blot hybridization with a cRNA probe encoding the 5′ end of the rat insulin receptor cDNA. Consistent with binding data, mRNA levels were significantly lower in skeletal muscle and liver of fructose-fed rats as compared to controls, but not in the kidney.ConclusionsDecreased number of insulin receptors occurring at the level of gene expression is present in skeletal muscle and liver of fructose-fed rats and might contribute to insulin resistance in this model.     

Effect of telmisartan, angiotensin II receptor antagonist, on metabolic profile in fructose-induced hypertensive, hyperinsulinemic, hyperlipidemic rats.

The metabolic syndrome (MS) is a common risk factor for cardiovascular disease and type-2 diabetes. Recently, telmisartan, an angiotensin II receptor antagonist that has an antihypertensive effect, has been reported to be a partial peroxisome proliferator-activated receptor gamma (PPARgamma) agonist. The anti-diabetic hormone adiponectin has been recognized as a marker of in vivo PPARgamma activation. Therefore, we studied telmisartan's effect on the metabolic profile and adiponectin levels in a fructose-induced hypertensive, hyperinsulinemic, hyperlipidemic rat model. Twenty-four male Sprague-Dawley rats were divided into three groups (eight in each). One group of control rats was fed standard chow for 5 weeks while a second was fed a fructose-enriched diet. A third group was fed a fructose-enriched diet for 5 weeks and treated with telmisartan 5 mg/kg/day during the last 2 weeks. Fructose feeding increased systolic blood pressure (mean+/-SEM), from 130+/-1 to 148+/-2 mmHg, insulin from 0.26+/-0.03 to 0.68+/-0.08 ng/mL, and triglycerides from 102+/-6 to 285+/-23 mg/dL (p<0.05 for all variables). Telmisartan treatment reversed these effects and reduced blood pressure to 125+/-2 mmHg, insulin levels to 0.41+/-0.07 ng/mL, and triglycerides to 146+/-18 mg/dL (p<0.05 for all variables), while attenuating the increase in body weight during weeks 3 to 5. In contrast, telmisartan did not affect plasma adiponectin levels. In conclusion, although telmisartan is considered a partial PPARgamma agonist, its beneficial effect in the fructose-induced hypertension, hypertriglyceridemia, and hyperinsulinemia rat model is apparently not mediated by adiponectin elevation but rather by direct inhibition of AT1 receptor.     

Tissue-dependent activation of protein kinase C in fructose-induced insulin resistance

Rats fed a fructose-enriched diet develop increases in blood pressure and resistance to insulin-mediated glucose disposal, but the underlying biochemical alterations have not been clearly defined. Since protein kinase C (PKC) has been implicated in the pathogenesis of insulin resistance, as well as blood pressure (BP) regulation, the present study was initiated to see whether changes in PKC signaling are present in rats with fructose-induced insulin resistance and hypertension. Consequently, liver, muscle, and adipose tissues were collected from fructose (n = 13) and chow (n = 12) fed Sprague-Dawley rats. PKC enzyme activity, and expression of classical PKC isozymes, were measured in cytosol and membrane fractions, and 1, 2-diacylglycerol (DAG), an endogenous stimulator of PKC, was measured by radio-enzymatic assay. Fructose feeding was associated with significant increases in fasting plasma insulin (140%) and triglyceride (400%) levels, and increased BP (20 mmHg). PKC activity was increased in the membrane fraction of adipose tissue (234 ± 38 (SE)vs 85 ± 30 pmol/min/mg protein,P< 0.007), without evidence of increased translocation or activation by DAG. Thus, fructose-induced insulin resistance has no effect on conventional PKC activity and subcellular distribution in liver and muscle, but the 3-fold increase in membraneassociated kinase activity in fat may be relevant to the mechanism of hypertriglyceridemia associated with fructose feeding.     

Myosin isoenzyme distribution and Ca+2-activated myosin ATPase activity in the rat heart is influenced by fructose feeding and triiodothyronine

Studies were conducted to determine if the level of cardiac Ca+2-activated myosin ATPase activity and ventricular myosin isoenzyme distribution are influenced by both T3 administration and fructose feeding. Previous studies have shown that in the cardiac ventricle of hypothyroid rats, only myosin V3 is present, and the Ca+2-activated myosin ATPase activity is markedly decreased. Hypothyroid [thyroidectomized (Tx)] rats were fed a diet containing 60% fructose or a regular diet (47% complex carbohydrates) for 4 weeks. Fructose feeding of hypothyroid rats led to a significant increase in Ca+2-activated myosin ATPase activity (Tx regular diet, 0.33 +/- 0.02 mumol Pi/mg protein X min; Tx fructose diet, 0.54 +/- 0.04 mumol Pi/mg protein X min). In addition, myosin V1 was detectable in the heart of fructose-fed Tx rats, but was absent in Tx rats on the regular diet. To determine if fructose had an effect of similar magnitude in animals of different thyroid states, Tx rats were injected with 0.075, 0.150, 0.225, and 0.300 micrograms T3/100 g BW daily and placed on fructose or regular diets. The fructose-induced increase in Ca+2-myosin ATPase activity was between 24-27% in Tx rats receiving 0-0.15 micrograms T3/100 g BW daily. In animals receiving 0.225 and 0.300 micrograms T3/100 g BW daily, fructose feeding did not induce a significant increase in myosin ATPase activity. This is due to the fact that the Ca+2-activated myosin ATPase activities of euthyroid and hyperthyroid animals are not significantly different from each other. In hypothyroid rats receiving a 60% glucose diet, Ca+2-myosin ATPase activity showed a significant 20% increase above the value in regular diet-fed Tx rats. Fructose- and glucose-induced changes in Ca+2-myosin ATPase activity occurred in the absence of changes in thyroid hormone or insulin levels. Our findings may indicate that cardiac carbohydrate consumption influences the predominance of ventricular myosin isoenzymes in the rat heart.     

Evidence for a causal role of the renin-angiotensin system in vascular dysfunction associated with insulin resistance

Excess production of superoxide anion in response to angiotensin II plays a central role in the transduction of signal molecules and the regulation of vascular tone. We examined the ability of insulin resistance to stimulate superoxide anion production and investigated the identity of the oxidases responsible for its production. Rats were fed diets containing 60% fructose (fructose-fed rats) or 60% starch (control rats) for 8 weeks. In aortic homogenates from fructose-fed rats, the superoxide anion generated in response to NAD(P)H was more than 2-fold higher than that of control rats. Pretreatment of the aorta from fructose-fed rats with inhibitors of NADPH oxidase significantly reduced superoxide anion production. In the isolated aorta, contraction induced by angiotensin II was more potent in fructose-fed rats compared with control rats. Losartan normalized blood pressure, NAD(P)H oxidase activity, endothelial function, and angiotensin II-induced vasoconstriction in fructose-fed rats. To elucidate the molecular mechanisms of the enhanced constrictor response to angiotensin II, expressions of angiotensin II receptor and subunits of NADPH oxidase were examined with the use of angiotensin II type 1a receptor knockout (AT1a KO) mice. Expression of AT1a receptor mRNA was enhanced in fructose-fed mice, whereas expression of either AT1b or AT2 was unaltered. In addition, protein expression of each subunit of NADPH oxidase was increased in fructose-fed mice, whereas the expression was significantly decreased in fructose-fed AT1a KO mice. The novel observation of insulin resistance-induced upregulation of AT1 receptor expression could explain the association of insulin resistance with endothelial dysfunction and hypertension.     

Low carbohydrate/high-fat diet attenuates cardiac hypertrophy, remodeling, and altered gene expression in hypertension

The effects of dietary fat intake on the development of left ventricular hypertrophy and accompanying structural and molecular remodeling in response to hypertension are not understood. The present study compared the effects of a high-fat versus a low-fat diet on development of left ventricular hypertrophy, remodeling, contractile dysfunction, and induction of molecular markers of hypertrophy (ie, expression of mRNA for atrial natriuretic factor and myosin heavy chain beta). Dahl salt-sensitive rats were fed either a low-fat (10% of total energy from fat) or a high-fat (60% of total energy from fat) diet on either low-salt or high-salt (6% NaCl) chow for 12 weeks. Hearts were analyzed for mRNA markers of ventricular remodeling and activities of the mitochondrial enzymes citrate synthase and medium chain acyl-coenzyme A dehydrogenase. Similar levels of hypertension were achieved with high-salt feeding in both diet groups (systolic pressure of approximately 190 mm Hg). In hypertensive rats fed low-fat chow, left ventricular mass, myocyte cross-sectional area, and end-diastolic volume were increased, and ejection fraction was decreased; however, these effects were not observed with the high-fat diet. Hypertensive animals on low-fat chow had increased atrial natriuretic factor mRNA, myosin heavy chain isoform switching (alpha to beta), and decreased activity of citrate synthase and medium chain acyl-coenzyme A dehydrogenase, which were all attenuated by high-fat feeding. In conclusion, increased dietary lipid intake can reduce cardiac growth, left ventricular remodeling, contractile dysfunction, and alterations in gene expression in response to hypertension.     

Mechanism of insulin resistance in fructose-fed rats

Previous results from our laboratory demonstrated that chronic administration of fructose to normal rats led to both hyperinsulinemia and in vivo insulin resistance. To localize the major tissue site of insulin resistance in fructose-fed animals, we compared glucose uptake by perfused hindlimb skeletal muscle and liver from rats fed either a 60% fructose diet or laboratory chow. Glucose uptake by perfused muscle from chow and fructose-fed rats was comparable at perfusate insulin levels of 0 μU/ml (15.2 versus 15.5 μl/min/g muscle), 100 μU/ml (18.3 versus 19.8), and >500 μU/ml (35.5 versus 33.4). In contrast, glucose outflow from livers of fructose-fed rats was significantly greater (p < .02) than chow-fed animals perfused in the absence of added insulin (52.1 versus 36.5 μmol/g). Furthermore, the ability of insulin to suppress glucose outflow was less in livers from fructose-fed rats at perfusate insulin levels of 165 μU/ml (13.2 versus 41.4% as well as at insulin concentration >900 μU/ml, (32.5% versus 62.2%). These findings suggest that the insulin resistance resulting from chronic fructose feeding is due to the diminished ability of insulin to suppress hepatic glucose output, and not to a decrease in insulin-stimulated glucose uptake by muscle.     

Endogenous angiotensin and pressure modulate brain angiotensinogen and AT1A mRNA expression

In the coarctation hypertension model, we showed both dissociation of plasma renin activity from cardiovascular-induced effects and the reversal of hypertension-induced responses by losartan. In this study, we investigated the effects of hypertension on the expression of brain renin-angiotensin system components and the simultaneous functional responses and effects of long-term angiotensin II (AT) receptor blockade on these responses. Rats were given vehicle or losartan for 9 days and subjected to subdiaphragmatic aortic constriction or sham surgery after 4 days of treatment. On the fifth postsurgical day, pressure and heart rate were measured in the conscious state; the brain was perfused and removed afterward. Sequential slices of brainstem were hybridized with 35S-oligodeoxynucleotide probes for angiotensinogen, AT1A, and AT1B receptors and processed for autoradiography and densitometry. In vehicle-treated rats, hypertension was accompanied by tachycardia and marked increments in angiotensinogen and AT1A mRNA expression in the cardiovascular system-controlling brainstem areas. In the nucleus tractus solitarii, AT1A density was correlated with both pressure and heart rate values (P<0.01), whereas angiotensinogen levels were correlated with pressure only (P<0.05). Losartan did not change the pressure of hypertensive rats (142+/-4 versus 146+/-2 mm Hg, losartan versus vehicle) and the hypertension-induced angiotensinogen mRNA expression but did block both tachycardic response and hypertension-induced AT1A mRNA expression. Hypertension and losartan did not change AT1B mRNA expression. The hypertension-induced positive feedback on angiotensinogen and AT1A mRNA expression supports the concept of a permissive role for brain angiotensin II in orchestrating circulatory responses during the development of hypertension. These data also explain the efficacy of long-term AT1 receptor blockade to reverse hypertension-induced effects.     

Androgen-dependent angiotensinogen and renin messenger RNA expression in hypertensive rats.

Our previous studies demonstrated that the sexually dimorphic pattern of hypertension in the spontaneously hypertensive rat is androgen dependent. Gonadectomy retards the development of hypertension in young males, but not in females, and administration of testosterone propionate to gonadectomized spontaneously hypertensive rats of both sexes confers a male pattern of blood pressure development. The current study tested the hypothesis that renal and hepatic renin and angiotensinogen gene expression are also androgen dependent in the spontaneously hypertensive rat. Male and female spontaneously hypertensive rats underwent gonadectomy or a sham operation at 4 weeks of age. Subgroups of gonadectomized rats of both sexes were implanted with a 15-mm or 30-mm Silastic capsule filled with testosterone at the same time the gonadectomy was performed; a third group received an empty Silastic capsule. Northern and slot blot analyses were used to characterize and quantitate renin and angiotensinogen messenger RNA (mRNA) in the kidney and liver 18 weeks after the gonadectomy. Blood pressure, plasma renin activity, and hepatic angiotensinogen mRNA levels were higher in intact males than in females. Orchidectomy retarded the development of hypertension and lowered plasma renin and renal and hepatic angiotensinogen mRNA levels, and testosterone replacement restored the male pattern of hypertension and plasma renin and increased renal and hepatic angiotensinogen mRNA. Ovariectomy did not alter blood pressure or plasma renin but did lower renal renin and renal and hepatic angiotensinogen mRNA; testosterone increased blood pressure, plasma renin, renal renin and angiotensinogen mRNA, and hepatic angiotensinogen mRNA levels in ovariectomized females.(ABSTRACT TRUNCATED AT 250 WORDS)