Chinese medicine, Jiang-Tang-Ke-Li, improves insulin resistance by modulating muscle fiber composition and muscle tumor necrosis factor-alpha in fructose-fed rats.

Insulin resistance and hyperinsulinemia are common findings in patients with essential hypertension. These impairments in glucose metabolism are commonly associated with diabetes mellitus, hypertension, and dyslipidemia, which are high risk factors of cardiovascular diseases, and recent evidence indicates that they may play a role in the development of coronary artery disease. The aim of this study was to determine the effect of Jiang-Tang-Ke-Li (JTKL), a traditional Chinese medicine used to treat diabetes mellitus in China, on insulin resistance and hypertension in fructose-fed rats (FFR). Systolic blood pressures in the FFR groups were significantly higher than that in the control group, although JTKL had no effect on systolic blood pressure for the last 2 weeks of treatment with the medicine. The average rate of glucose infusion during a glucose clamp, as an index of insulin sensitivity (M value), was significantly lower in the FFR than in the control rats, and treatment with JTKL for 2 weeks significantly increased the M value to that of the control. Treatment with Panax ginseng (PG), a component of JTKL, for 2 weeks also significantly increased the M value of FFR to the control level. The composite ratio of type I fibers in soleus muscle decreased significantly in the FFR compared to that in the control, and treatment with JTKL led to recovery of the composite ratio of type I fibers to the same level as that of the control group. The M value showed a significant positive correlation with the composite ratio of type I fibers and a significant negative correlation with the composite ratio of type II fibers. Tumor necrosis factor (TNF)-alpha levels were significantly higher in the soleus and extensor digitorum longus (EDL) muscles of the FFR than in those of the control rats. Treatment with JTKL for 2 weeks significantly lowered TNF-alpha levels to the control levels. M values showed a significant negative correlation with TNF-alpha in both the soleus and EDL muscles. The results suggest that the Chinese medicine JTKL, which contains PG as one of its valid components, improves insulin resistance by modulating muscle fiber composition and TNF-alpha in skeletal muscles in hypertensive and insulin-resistant FFR.     

The contribution of skeletal muscle tumor necrosis factor-alpha to insulin resistance and hypertension in fructose-fed rats

OBJECTIVE: The aim of this study was to determine the role of tumor necrosis factor-alpha (TNF-alpha) in skeletal muscle tissue in insulin resistance and hypertension and the effect of anti-hypertensive medicine on skeletal muscle TNF-alpha in fructose-induced insulin-resistant and hypertensive rats (fructose-fed rats: FFR). DESIGN AND METHODS: Six-week-old male Sprague-Dawley rats were fed either normal rat chow or fructose-rich chow. For the last 2 weeks of a 6-week period of either diet, the rats were treated with a vehicle (control or FFR); temocapril, an angiotensin converting enzyme inhibitor (ACEI); or CS-866, an angiotensin II type 1 receptor blocker (ARB). The euglycemic hyperinsulinemic glucose clamp technique was performed to evaluate insulin sensitivity (M value). TNF-alpha levels in soleus and extensor digitorum longus (EDL) muscles and epididymal fat pads were measured. We also measured the TNF-alpha concentration in an incubated medium secreted from soleus muscle strips with or without angiotensin II. RESULTS: TNF-alpha levels were significantly higher in the soleus and EDL muscles, but not in the epididymal fat, in the FFRs compared with the control rats. Temocapril and CS-866 lowered systolic blood pressure, improved insulin resistance, and reduced TNF-alpha in both skeletal muscles. There were significant negative correlations between M values and TNF-alpha levels in both soleus and EDL muscles. Also, the soleus muscle strip incubation with 10(-7) mol/l angiotensin II increased TNF-alpha secreted into the incubation medium compared to the incubation without angiotensin II. These results suggest that skeletal muscle TNF-alpha is linked to insulin resistance and hypertension and that angiotensin II may be one of the factors that regulate skeletal muscle TNF-alpha.     

Effects of the Chinese medicine, TSJN on insulin resistance and hypertension in fructose-fed rats.

The aim of this study was to determine the effect of Tang-Shen-Jiao-Nang (TSJN), a Chinese medicine used to treat diabetes mellitus, on insulin resistance and hypertension in fructose-fed rats (FFR). Six-week-old male Sprague-Dawley rats were fed either normal rat chow (control) or a fructose-rich chow (FFR) for 6 wk. For the last 2 or 4 wk of a 6-wk period of either diet, the rats were treated by gavage with gum arabic solution as a vehicle (control or FFR) or TSJN (800 mg/kg/d; FFR+TS), and then we performed the euglycemic hyperinsulinemic glucose clamp technique to estimate insulin sensitivity. Systolic blood pressure was measured weekly for 6 wk. At the end of the glucose clamp, the soleus muscle was dissected out for determination of muscle fiber composition by ATPase methods. Systolic blood pressure was elevated at 2 wk after the start of the fructose-rich chow feeding and persisted thereafter throughout the study. Systolic blood pressure during the glucose clamp in the FFR group was significantly higher than that in the control group. Although there was no effect on systolic blood pressure in rats treated with TSJN for the last 2 wk of their 6-wk diet, those treated with TSJN for the last 4 wk of their 6-wk diet had lower systolic blood pressure than did the rats in the FFR group. The average rate of glucose infusion during the glucose clamp, as a measure of insulin sensitivity (M value), was significantly lower in the FFR than in the controls (10.9 +/- 0.6 and 15.4 +/- 0.4, mg/kg/min, for FFR and controls, respectively; p< 0.01). Treatment with TSJN for 2 wk significantly improved the M value compared to that of the control level (15.1 +/- 0.5 mg/kg/min). The composite ratio of type I fibers in the soleus muscle was significantly decreased in the FFR compared to controls (75.0 +/- 1.7 and 81.7 +/- 1.5%, for FFR and controls, respectively; p< 0.01), and treatment with TSJN for 2 wk led to a recovery composite ratio of type I fiber to the same level as that of the control group (78.7 +/- 1.7% in FFR + TS). The M value was significantly correlated with the compositions of type I and type II fibers (for type I fibers, r= 0.45, p < 0.01, for type II fibers, r= -0.44, p< 0.05). These results suggest that the Chinese medicine TSJN may improve insulin resistance, lower the systolic blood pressure, and modulate muscle fiber composition in hypertensive and insulin-resistant fructose-fed rats.     

Alteration of muscle fiber composition linking to insulin resistance and hypertension in fructose-fed rats.

The aim of this study was to examine the role of muscle fiber composition in insulin resistance and the effect of a calcium channel antagonist on insulin sensitivity in fructose-induced insulin resistant and hypertensive rats. Six-week-old male Sprague-Dawley rats were fed either normal rat chow (control) or fructose-rich diet (FFR). For the last 2 weeks of a 6-week period of either diet, the rats were treated, by gavage, with gum arabic solution (control or FFR) or a dihydropyridine calcium channel antagonist, benidipine hydrochloride (3 mg/kg/day: FFR + Ca), then the euglycemic hyperinsulinemic glucose clamp technique was performed to evaluate insulin sensitivity. Blood pressure was measured weekly for 6 weeks. At the end of the glucose clamp, the soleus muscle was dissected out for determination of muscle fiber composition by ATPase methods. Blood pressure was elevated at 2 weeks after the start of fructose-rich chow feeding and persisted thereafter throughout the study. Blood pressure at the glucose clamp in the FFR was significantly higher than that in the control group (142 +/- 2 v 155 +/- 2 mm Hg, P < .01) and the calcium antagonist significantly lowered blood pressure of FFR (136 +/- 6 mm Hg for FFR +/- Ca, P < .05). The average rate of glucose infusion during glucose clamp, as a measure of insulin sensitivity (M value), was significantly lower in the FFR than in the control (15.4 +/- 0.4 v 10.9 +/- 0.6 mg/kg/min, P < .01). The calcium channel antagonist partially improved the M value compared to that of FFR (13.4 +/- 0.7 mg/kg/min in FFR +/- Ca, P < .01 compared to FFR, P < .05 compared to control). The composite ratio of type I fiber in soleus muscle was significantly decreased in FFR compared to control (81.7 +/- 1.5% v 75.0 +/- 1.7%, P < .01), and the composite ratio of type I fiber in rats treated with the calcium channel antagonist (FFR +/- Ca) recovered to the control level (79.9 +/- 1.1%, P < .05 compared to FFR). The M value was significantly correlated with the compositions of type I and type II fibers (for type I fibers, r = 0.80, P < .01; for type II fibers, r = -0.81, P < .01). These results suggest that fiber composition of skeletal muscle links insulin resistance and that a calcium channel antagonist may modulate muscle fiber composition in hypertensive animal model, fructose-fed rats.     

The effects of an angiotensin-converting enzyme inhibitor and an angiotensin II receptor antagonist on insulin resistance in fructose-fed rats

The aim of this study was to compare the effects of an angiotensin-converting enzyme (ACE) inhibitor and an angiotensin II receptor (AT) antagonist on insulin resistance, especially on muscle fiber composition in fructose-induced insulin-resistant and hypertensive rats. Six-week-old male Sprague-Dawley rats were fed either normal rat chow (control) or a fructose-rich diet (FFR). For the last two weeks of a six-week period of either diet, the rats were treated with gum arabic solution as a vehicle (control or FFR), angiotensin-converting enzyme inhibitor (FFR+ACE), temocapril (1 mg/kg/ day) or an angiotensin II receptor antagonist (FFR+AT), CS-866 (0.3 mg/kg/day), by gavage, and then the euglycemic hyperinsulinemic glucose clamp technique was performed to evaluate insulin sensitivity. At the end of the glucose clamp, the soleus muscle was dissected for determination of the muscle fiber composition by ATPase methods. Blood pressure at the glucose clamp in the FFR group was significantly higher than that of the control group, and both temocapril and CS-866 significantly lowered the blood pressure of the FFR group. The average rate of glucose infusion during the glucose clamp, as a measure of insulin sensitivity (M value), was significantly lower in the FFR rats compared to the controls (15.4 +/- 0.4, 10.9 +/- 0.6 mg/kg/min, for control and FFR, respectively, P < .01). Both temocapril and CS-866 partially improved the M values compared to FFR (13.2 +/- 0.7, 12.8 +/- 0.5 mg/kg/min, for FFR+ACE, FFR+AT, respectively, P < .01 compared with FFR, P < .05 compared with control). The composite ratio of type I fibers of the soleus muscle was decreased significantly in the FFR rats compared with the controls (82% +/- 2%, 75% +/- 2%, for control and FFR, respectively, P < .01), and both temocapril and CS-866 restored a composite ratio of type I fibers to the same level as that of the controls (81% +/- 1%, 80% +/- 1% for FFR+ACE and FFR+AT, respectively). The M value was significantly correlated with the composition of type I and type II fibers. These results suggest that the fiber composition of skeletal muscle is correlated to insulin resistance, and that both ACE inhibitors and AT antagonists may modulate the muscle fiber composition in a hypertensive and insulin-resistant animal model, fructose-fed rats, to the same extent.     

Angiotensin II inhibits glucose uptake of skeletal muscle via the adenosine monophosphate-activated protein kinase pathway

Adenosine monophosphate-activated protein kinase (AMPK) mediates metabolic responses of muscle to exercise and is involved in improvement of insulin resistance by endurance exercise. Recent studies have suggested that the renin-angiotensin system might negatively modulate insulin-mediated actions, but there has been little investigation of the correlation between the renin-angiotensin system and AMPK. To determine this correlation, we performed studies with glucose clamp in vivo, and glucose uptake by skeletal muscle ex vivo using 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). Six-week-old male Sprague-Dawley rats were fed standard chow (standard-diet rats; SD) or fructose-rich chow (fructose-fed rats; FFR) for 6 weeks. At the age of 12 weeks, SD and FFR were treated by oral gavage, either with angiotensin II (Ang II) receptor blockade (ARB; valsartan 30 mg/kg) or vehicle. Thirty minutes after the treatment, we performed glucose clamp studies to measure glucose infusion rates during infusion of insulin (GIR_I) and of AICAR (GIR_A), which stimulates AMPK, and studied the effect of ARB on either GIR_I or GIR_A. In an ex vivo study, we used bilateral fresh soleus muscles from 3-week-old male Sprague-Dawley rats to examine the glucose uptake (measured by ³H-2-deoxyglucose uptake) of one side of soleus muscle incubated with AICAR with or without Ang II, or with tumor necrosis factor-α, in comparison with that of the other (untreated control) side of the muscle. Blood pressure of FFR was significantly higher than that of SD rats. GIR_I was significantly lower in FFR than in SD, and treatment with ARB did not change GIR_I. GIR_A of FFR was significantly lower than that of SD, but GIR_A of FFR treated with ARB was significantly increased compared with that of FFR treated with vehicle. In the ex vivo study, incubation with AICAR significantly increased glucose uptake of soleus muscles, Ang II significantly decreased AICAR-activated glucose uptake in a dose-dependent manner, and ARB canceled the effect of Ang II. The results suggest that acute inhibition of the angiotensin 1 receptor improves glucose metabolism via not insulin but AMPK pathway through the angiotensin 1 receptor in FFR.     

Blockade of the renin-angiotensin system decreases adipocyte size with improvement in insulin sensitivity

OBJECTIVE: Based on results of in vitro studies, it has been hypothesized that blockade of the renin-angiotensin system (RAS) promotes the recruitment and differentiation of pre-adipocytes and that increased formation of small insulin-sensitive adipocytes counteracts ectopic deposition of lipids, thereby improving insulin sensitivity. We investigated the effect of RAS blockade on insulin sensitivity, adipocyte size, and intramuscular lipid content in fructose-fed rats (FFR) as a model of insulin-resistant hypertension. DESIGN AND METHODS: Six-week-old male Sprague-Dawley rats were divided into two groups: those fed a standard chow (control) and those fed a fructose-rich chow for 6 weeks. FFR were treated with a vehicle or with 1 mg/kg per day of temocapril, an angiotensin-converting enzyme inhibitor, or 0.1 mg/kg per day of olmesartan, an angiotensin II type 1 receptor blocker, for the last 2 weeks. Insulin sensitivity (M value: mg/kg per min) was estimated by the euglycemic hyperinsulinemic glucose clamp method. Sizes of adipocytes derived from epididymal fat and triglyceride content in the soleus muscle were determined. RESULTS: FFR had lower M value, higher blood pressure, larger adipocyte size, higher ratio of epididymal fat pads over body weight (%fat pads), and higher intramuscular triglyceride than did the control rats. Both temocapril and olmesartan significantly improved the M value and decreased blood pressure and adipocyte size without change in %fat pads in FFR. Adipocyte size was negatively correlated with the M value. Treatment for 2 weeks decreased, but not significantly, intramuscular triglyceride. CONCLUSIONS: RAS blockade decreases adipocyte size without change in epididymal %fat pads accompanied by improvement in insulin sensitivity.     

Effects of cilnidipine on muscle fiber composition, capillary density and muscle blood flow in fructose-fed rats.

The aim of this study was to examine the roles of muscle fiber composition, capillary density and muscle blood flow in insulin resistance (IR) and the effect of cilnidipine, a calcium channel blocker in fructose-fed rats (FFR). Six-week-old male Sprague-Dawley rats were fed either normal rat chow or fructose-rich chow for 6 weeks. For the last 2 weeks, the rats were treated by gavage with a vehicle (Control and FFR groups) or with cilnidipine (FFR+Cil group). Blood pressure (BP) and insulin sensitivity were assessed in the sixth week. Muscle fiber composition, capillary density and blood flow in the soleus muscle were evaluated. BP of FFR was significantly higher than that of the controls. Cilnidipine significantly lowered BP in FFR. Insulin sensitivity was significantly lower in FFR than in the controls. Cilnidipine significantly improved IR in FFR. The composite ratio of type I fibers in the soleus muscle was significantly lower in FFR than in the controls, but that of type II fibers was significantly higher in FFR. Treatment with cilnidipine resulted in recovery of this ratio to that of the controls. Insulin sensitivity was found to be significantly correlated with the composite ratio of either type I fibers or type II fibers. There were no intergroup differences in capillary density. Muscle blood flow in the FFR+Cil group was higher than that in the Control or FFR groups. These results suggest that muscle fiber composition is linked to IR and that cilnidipine may improve IR in FFR either by modulating muscle fiber composition or by increasing muscle blood flow.     

Olmesartan ameliorates insulin sensitivity by modulating tumor necrosis factor-alpha and cyclic AMP in skeletal muscle.

We have reported that tumor necrosis factor (TNF)-alpha in skeletal muscle is one of the determinants of insulin resistance and that the renin-angiotensin system may be related to the regulation of TNF-a in skeletal muscle. Recent studies have suggested the involvement of cyclic adenosine monophosphate (cAMP) in the regulation of TNF-a in vascular smooth muscle cells or monocytes. The aim of this study was to determine the relationship between cAMP and TNF-a in skeletal muscle in connection with the renin-angiotensin system. Six-week-old male Sprague-Dawley rats were fed either normal rat chow or fructose-rich chow for 6 weeks. For the last 2 weeks of a 6-week period, the rats were treated with a vehicle or with an angiotensin II type 1 receptor antagonist (olmesartan medoxomil, 0.1 mg/kg/day). TNF-alpha levels in the soleus muscle were significantly higher and cAMP levels in the soleus muscle were significantly lower in fructose-fed rats than in control rats. Olmesartan increased cAMP and reduced TNF-a simultaneously in fructose-fed rats. There was a significant negative correlation between levels of cAMP and TNF-alpha. Moreover, a cAMP analogue reduced TNF-a levels in the soleus muscle. These results indicate that the increase in TNF-alpha via suppression of cAMP may affect the induction of insulin resistance. In addition, the facts that olmesartan increased cAMP and decreased TNF-alpha suggest that a part of the TNF-alpha regulation by angiotensin II might consist of modulation of cAMP through Gi protein activation in skeletal muscle.     

Losartan modulates muscular capillary density and reverses thiazide diuretic-exacerbated insulin resistance in fructose-fed rats

The renin-angiotensin system (RAS) is involved in the pathogenesis of insulin sensitivity (IS). The role of RAS in insulin resistance and muscular circulation has yet to be elucidated. Therefore, this study sought to determine the mechanisms of angiotensin II receptor blockers (ARBs) and/or diuretics on IS and capillary density (CD) in fructose-fed rats (FFRs). Sprague-Dawley rats were fed either normal chow (control group) or fructose-rich chow for 8 weeks. For the last 4 weeks, FFRs were allocated to four groups: an FFR group and groups treated with the thiazide diuretic hydrochlorothiazide (HCTZ), with the ARB losartan, or both. IS was evaluated by the euglycemic hyperinsulinemic glucose clamp technique at week 8. In addition, CD in the extensor digitorum longus muscle was evaluated. Blood pressure was significantly higher in the FFRs than in the controls. HCTZ, losartan and their combination significantly lowered blood pressure. IS was significantly lower in the FFR group than in the controls and was even lower in the HCTZ group. Losartan alone or combined with HCTZ significantly increased IS. In all cases, IS was associated with muscular CD, but not with plasma adiponectin or lipids. These results indicate that losartan reverses HCTZ-exacerbated insulin resistance, which can be mediated through the modulation of muscular circulation in rats with impaired glucose metabolism.     

Tissue-specific impairment of insulin signaling in vasculature and skeletal muscle of fructose-fed rats.

The relation between insulin resistance/hyperinsulinemia and cardiovascular diseases has attracted much attention. Insulin affects not only glucose metabolism, but also protein synthesis and cell growth. Insulin stimulates both the phosphatidylinositol 3-kinase (PI3-K) and mitogen-activated protein kinase (MAPK) pathways, but the relationship between cardiovascular disease and selective insulin signal pathways is unclear. We investigated the tissue specificity and intracellular signal transduction selectivity of insulin resistance in the vasculature and skeletal muscle of fructose-fed rats (FFR). Sprague-Dawley rats were fed either normal rat chow (control rats) or fructose-rich chow. Normal saline with or without 1,000 (microg/kg) insulin was injected, and then the thoracic aorta or soleus muscle was removed under anesthetization. Insulin-induced tyrosine phosphorylation of insulin receptor beta subunit (IRbeta) and insulin receptor substrate-1 (IRS-1) and tyrosine/threonine phosphorylation of p44/42 MAPK (ERK-1/2) were evaluated. There were no significant differences in the degree of phosphorylation of IRbeta or ERK-1/2 in the thoracic aorta or in the soleus muscle between FFR and controls. However, tyrosine phosphorylation of IRS-1 in the soleus muscle of FFR was significantly reduced to 80% (p<0.001) of that in controls. The results suggest that PI3-K pathway in skeletal muscle is selectively impaired in FFR, and this impairment may induce hyperinsulinemia, which in turn may stimulate the MAPK pathway and lead to atherosclerosis. Thus PI3-K pathway may be one of the factors underlying the onset of cardiovascular disease in patients with insulin resistance.     

Fenofibrate improves insulin sensitivity in connection with intramuscular lipid content,muscle fatty acid-binding protein,and beta-oxidation in skeletal muscle

We investigated the effect of fenofibrate, a peroxisome proliferator-activated receptor-alpha agonist, on insulin sensitivity including lipid metabolism in skeletal muscle. Six-week-old male Sprague-Dawley rats were divided into two groups: those fed a standard chow (control) or a fructose-rich chow (fructose-fed rats (FFRs)) for 6 weeks. FFRs were treated either with a vehicle or with 30 mg/kg per day of fenofibrate for the last 2 weeks. Insulin sensitivity (M-value) was estimated by the euglycemic hyperinsulinemic glucose clamp method. Fatty acid-binding protein (FABP) in skeletal muscle was measured by ELISA, and the expression of FABP mRNA was analyzed by semi-quantitative RT-PCR. The serum and muscle triglyceride (sTG and mTG) levels and the activity of 3-hydroxyacyl-CoA dehydrogenase (HADH), a beta-oxidation enzyme, in muscle were also determined. FFRs showed a lower M-value and higher blood pressure, sTG and mTG than did the control group. The mTG was correlated positively with sTG and negatively with the M-value. Fenofibrate treatment for 2 weeks did not change blood pressure but significantly improved the M-value, sTG and mTG. FABP content and mRNA in the soleus muscle were significantly elevated in FFRs compared with those in the control group. Fenofibrate treatment further increased FABP. The HADH activity was comparable between the control group and FFRs, but significantly increased by fenofibrate treatment. These results suggest that fenofibrate improves insulin sensitivity not only by lowering serum lipids and subsequent influx of fatty acids into muscles but also by reducing intramuscular lipid content via further induction of FABP and stimulation of beta-oxidation in muscles.     

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.     

Reduction of endothelial NOS and bradykinin-induced extravasation of macromolecules in skeletal muscle of the fructose-fed rat model

OBJECTIVE: Reduced capillary permeability of the skeletal muscle vascular bed has been suggested to play a role in fructose-fed rats, corroborating a long held view that insulin resistance might partially be explained by the lack of access of insulin and glucose to its target organs, mainly skeletal muscles. The goal of this study was to explore mechanisms underlying this vascular abnormality, and more specifically the role of bradykinin and nitric oxide (NO) on skeletal muscle microcirculation and the extravasation of macromolecules. METHODS: For that purpose, Sprague-Dawley rats were fed with either a fructose-enriched (F) or a normal chow (N) diet and extravasation of macromolecules was assessed at 4 weeks by measuring in vivo the extravasation of Evans Blue (EB) dye in the quadriceps muscles of both groups after the intravenous injection of the potent vasodilator bradykinin (150 microg/kg). RESULTS: As expected, fructose-fed rats had less extravasation of EB in skeletal muscle in the basal state as compared to controls (F 17.6 +/- 4.4 vs. N 43.6 +/- 6.9 microg/g dry tissue; P<0.01). In response to bradykinin, the EB dye extravasation in skeletal muscle was 89.4% higher in rats fed the normal chow diet compared to the basal state (P<0.03). In contrast, no significant increase in vasopermeability was observed in fructose-fed animals acutely injected with BK (17.6 +/- 4.4 microg/g in the basal state versus 24.6 +/- 3.1 microg/g after the injection of BK; P=NS). To distinguish a functional from an anatomical/structural defect, hematoxylin-eosin sections as well as electron micrographs of skeletal muscle microvessels were examined in both groups of animals: no obvious abnormalities were found. However, in homogenates of skeletal muscles (quadriceps) of fructose-fed rats, there was a marked reduction of NO synthase (NOS) activity (-33.8%; P<0.001) as well as endothelial NOS immunoreactive mass (-23.4%; P<0.04) as compared to control animals. CONCLUSION: There is unresponsiveness of the skeletal muscle capillary bed to bradykinin in insulin-resistant animals most probably due to a reduction in endothelial NOS (activity and mass). Our results indicate a functional defect possibly involving responsiveness of the precapillary resistance and/or the endothelial barrier to bradykinin in skeletal muscles. Since insulin must cross the endothelial monolayer to reach its target cells on the abluminal side, it is suggested that reduced endothelial NOS and consequent reduced extravasation of macromolecules could exacerbate insulin resistance in skeletal muscles and hypertension in the fructose-fed rat.     

Chronic thromboxane synthase inhibition prevents fructose-induced hypertension

To investigate the role of thromboxane A(2) in the development of hypertension in the fructose-fed rat, we treated male fructose-fed rats with dazmegrel (a thromboxane synthase inhibitor) and monitored blood pressure, fasting plasma parameters, and insulin sensitivity for 7 weeks. Systolic blood pressure was measured each week using tail plethysmography, and an oral glucose tolerance test was performed at the end of the study to assess insulin sensitivity. Treatment with a 60% fructose diet and dazmegrel (100 mg. kg(-1). d(-1) via oral gavage) was initiated on the same day. Plasma triglyceride levels increased 2-fold in both fructose- and fructose/dazmegrel-treated groups, and plasma insulin levels tended to be higher in these groups, although not significantly. Systolic blood pressure increased significantly throughout the study in the fructose-fed group only (132+/-3 versus 112+/-4 mm Hg in control rats, 118+/-2 mm Hg in control-treated rats, 116+/-2 mm Hg in fructose-treated rats). Both fructose groups demonstrated a higher peak insulin response to oral glucose challenge and had 40% to 60% lower insulin sensitivity index values. The results of this study show that treatment with a thromboxane synthase inhibitor, dazmegrel, can prevent the development of hypertension but does not improve insulin sensitivity or other fructose-induced metabolic impairments. Based on these data, we conclude that the potent vasoconstrictor thromboxane is involved in the link between hyperinsulinemia/insulin resistance and hypertension.     

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.     

Impaired endothelial alpha-2 adrenergic receptor-mediated vascular relaxation in the fructose-fed rat.

To investigate the vascular endothelial dysfunction in the insulin resistance syndrome, muscarinic and alpha2-adrenergic mediated relaxations were studied in the fructose-fed rat. Male Sprague-Dawley rats were fed either fructose-rich chow (FFR, n=14) or normal chow (CNT, n=13) for 8 weeks. Systolic blood pressure (SBP) was measured by the tail-cuff method. A 3 mm segment of mesenteric artery was cannulated and pressurized, pretreated with prazosin (10(-6) mol/l) and propranolol (3x10(-6) mol/l), then pre-contracted with serotonin (10(-6) mol/l). Endothelium-dependent relaxation was induced by addition of acetylcholine (ACh, 10(-9)-10(-4) mol/l) or a selective alpha2-agonist, B-HT 920 (10(-9)-10(-5) mol/l), with or without the nitric oxide (NO) synthase inhibitor, L-NAME (10(-4) mol/l). SBP was significantly elevated in FFR but not in CNT. Plasma triglyceride in FFT (241+/-115 mg/dl) was significantly (p<0.01) higher than in CNT (84+/-34 mg/dl). Insulin and insulin/glucose ratio were higher but not significantly. Plasma glucose was not different between the two groups. In the dose-response curves to ACh, maximum relaxation and ED50 were similar between FFR and CNT. Moreover, L-NAME shifted the dose-response curves similarly to the right in both groups. Dose-response curves to B-HT 920, however, showed less relaxation in FFR than in CNT (p<0.05). B-HT 920-induced relaxations were mostly abolished by L-NAME. It is concluded that endothelial alpha2-adrenergic relaxation, predominantly mediated by NO, is likely more sensitive to the development of insulin resistance than muscarinic receptor relaxation in this 8-weeks FFR model. This early impairment of endothelial alpha2-adrenergic relaxation may contribute to the development of hypertension and insulin resistance in the FFR.     

肿瘤坏死因子α对慢性阻塞性肺疾病大鼠模型呼吸肌蛋白质分解代谢的影响

目的研究肿瘤坏死因子α（TNF-α）对慢性阻塞性肺疾病（COPD）模型大鼠呼吸肌蛋白质分解代谢率的影响。方法成年雄性Wistar大鼠90只，分为模型组70只，对照组20只，采用反复熏香烟和气管内注入猪胰弹性蛋白酶的方法建立COPD大鼠模型，模型建立成功2周后，以低于对照组大鼠平均体重的90％作为判断发生营养不良的标准，随机抽取已出现营养不良的大鼠10只，采用尾静脉注射TNF-α单克隆抗体0．1mg／kg进行干预，连续干预4d，继续饲养动物10d，处死全部动物。酶联免疫吸附测定法测定膈肌及肋间内肌匀浆中TNF-α含量；反相高效液相色谱荧光法测定膈肌和肋间内肌匀浆中3-甲基组氨酸（3-MH）及酪氨酸含量。结果营养不良组大鼠膈肌和肋间内肌的TNF-α含量[（125±11）和（119±11）pg／g]显著高于对照组[（64±5）和（59±5）pg／g]，营养不良组大鼠膈肌和肋间内肌匀浆中3-MH含量[（7．1±0．6）和（7．4±0．6）nmol／g]和酪氨酸含量[（639±24）和（660±25）nmol／g]均显著高于对照组[（4．0±0．5）、（4．2±0．3）和（557±24）、（579±26）nmol／g]，膈肌和肋间内肌TNF-α含量与蛋白质分解代谢率呈阴显正相关（r=0．854。P〈0．01）。TNF-α单克隆抗体干预后，蛋白质分解代谢率降低。结论COPD大鼠模型呼吸肌蛋白质分解代谢率增高，这种现象在COPD大鼠模型中发生营养不良的大鼠表现更为明显。TNF-α是引起COPD大鼠呼吸肌蛋白质分解代谢率增高的因素之一。     

Effects of insulin on regional blood flow and glucose uptake in Wistar and Sprague-Dawley rats

The euglycemic-hyperinsulinemic clamp technique in conscious Sprague-Dawley and Wistar rats chronically instrumented with intravascular catheters and pulsed Doppler flow probes was used to examine insulin's actions on regional blood flow and glucose metabolism. The effect of insulin on in vivo and in vitro glucose utilization in individual muscles was estimated using [3H]-2-deoxy-D-glucose. We found that in both strains, insulin (4, 32, and 64 mU x kg(-1) x min(-1)) causes similar cardiovascular changes characterized by slight increases in blood pressure (at high dose), vasodilation in renal and hindquarter vascular beds, and vasoconstriction (at high dose) in the superior mesenteric vascular bed. However, at the lowest dose of insulin tested, we found a smaller insulin sensitivity index and a lower insulin-stimulated in vivo glucose uptake in extensor digitorum longus (EDL) muscles of Wistar versus Sprague-Dawley rats. Higher insulin-stimulated glucose transport activity was found in isolated soleus muscle, while greater basal glucose transport was noted in isolated EDL muscle from Sprague-Dawley versus Wistar rats. These results provide further evidence for an insulin blood flow-regulatory effect and suggest that strain characteristics (differences in muscle perfusion, hindquarter composition, or fiber insulin sensitivity) constitute a major determinant in the variation in whole-body insulin sensitivity.