Clenbuterol-Induced Insulin Resistance in Calves Measured by Hyperinsulinemic,Euglycemic Clamp Technique

Hyperinsulinemic, euglycemic clamp tests were performed on calves before and after clenbuterol treatment. Clenbuterol was given as 2 intramuscular injections with an interval of about 12 h. The dose used was 1 μg/kg b.w. The treatment resulted in increased plasma levels of insulin and glucose. The results of the clamp tests showed that clenbuterol induced a transient decrease in insulin sensitivity. Both insulin mediated glucose disposal (M), expressed as μmol/kg live b.w./min. and the M/I-index (M divided by the average insulin concentration at steady state) were significantly reduced after treatment. The effect of clenbuterol on carbohydrate metabolism seemed to be rather short-lived, since significant changes occurred only in animals treated 5-6 h prior to the test. According to the literature, the metabolic effects of clenbuterol have been studied only after the high doses used for growth promoting purposes. The results from the present study showed that similar changes occur also after doses at the therapeutic level. The hyperinsulinemic, euglycemic clamp test was considered to be a valuable tool for the study of insulin sensitivity in cattle.     

Glucose uptake after resistance training of different intensities but of equal work volume

High-intensity (HI) resistance exercise augments postexercise glucose uptake to a greater degree than low-intensity (LO) resistance exercise; however, few studies have equated the work volumes between intensity levels. The purpose of this study was to compare the effect of acute HI and LO resistance exercise of equal work volume on glucose uptake in resistant-trained men. Fifteen healthy male (22.9 +/- 3.8 years old), resistance-trained (6.7 +/- 3.9 years) subjects completed three treatment sessions: CON (no-exercise control), HI (3 x 8, 85% 10-RM), and LO (3 x 15, 45% 10-RM). HI and LO sessions consisted of eight exercises. Glucose uptake was measured the following morning by using the hyperinsulinemic euglycemic clamp technique. Glucose disposal was measured by analyzing the glucose infusion rate during the final 30 minutes of steady-state blood glucose concentrations. Insulin sensitivity was calculated by dividing the glucose infusion rate by the average insulin infusion. Results indicate that fasting blood glucose levels were not significantly different among treatment sessions (CON = 80.5 +/- 5.3 versus HI = 77.0 +/- 4.9 versus LO = 77.1 +/- 6.0 mg.dL). Glucose uptake was not significantly different among treatment sessions (CON = 11.3 +/- 3.0 versus HI = 11.7 +/- 2.7 versus LO = 11.4 +/- 2.8 mg.kg FFM.min). Insulin sensitivity did not change among treatment sessions (CON = 0.26 +/- 0.09 versus HI = 0.28 +/- 0.07 versus LO = 0.27 +/- 0.06 (mg.kg FFM.min)/(uU.mL)). The data indicate that the resistance training sessions did not modify acute insulin sensitivity. This may have been because of the high levels of fitness of the subjects, which allowed for the cellular adaptations for enhanced insulin sensitivity and glucose uptake that are unaffected by this volume of acute exercise.     

LXRβ deficient mice have reduced hepatic insulin clearance during hyperinsulinemic euglucemic clamp

The present study addresses the insulin sensitivity in mice deficient in LXRβ (LXRβ^−/−) as well as in wild type (wt) mice assessed by hyperinsulinemic euglycemic clamp. Wt and LXRβ^−/− mice were fed either a normal chow diet or a high fat and high cholesterol diet (HFCD), and insulin sensitivity was assessed by hyperinsulinemic euglycemic clamps. We show that LXRβ^−/− mice have reduced insulin clearance during hyperinsulinemic clamps upon feeding both HFCD and a regular chow diet. Moreover we also observed reduced hepatic inflammation in LXRβ^−/− mice compared to wt mice upon feeding an HFCD, despite equal levels of hepatic steatosis. In summary, our results indicate that LXRβ^−/− mice have reduced insulin clearance during hyperinsulinemic euglycemic clamps and also reduced hepatic inflammation upon feeding an HFCD for 26 weeks.     

Long-term consumption of fermented soybean-derived Chungkookjang attenuates hepatic insulin resistance in 90% pancreatectomized diabetic rats.

Soy protein and isoflavonoids in soybeans exhibit the improvement of insulin resistance. Our previous IN VITRO study showed that Chungkookjang (CKJ), fermented unsalted soybeans, had better antidiabetic actions than cooked unfermented soybeans (CSB) by increasing isoflavones aglycones and small peptides. We investigated whether 40% fat diets with different protein sources such as CSB, CKJ, and casein modulated peripheral insulin resistance in 90% pancreatectomized (Px) diabetic rats. The Px rats weighing 209+/-14 g were freely provided casein, CSB, or CKJ diets for 8 weeks. Both CKJ and CSB increased whole body glucose disposal rates and glucose uptake into skeletal muscles of Px rats as much as rosiglitazone plus casein treated rats during euglycemic hyperinsulinemic clamp. In addition, CKJ and CSB decreased hepatic glucose output at hyperinsulinemic clamped states, compared to the Casein group. The reduction of hepatic glucose output was greater in CKJ than CSB. This reduction was associated with enhanced tyrosine phosphorylation of IRS2 and serine (473) phosphporylation of Akt, indicating improved hepatic insulin signaling. This improved signaling led to decreased phosphoenolpyruvate carboxykinase expression to reduce hepatic glucose output. In conclusion, fermented soybeans mainly with BACILLUS SUBTILIS improved hepatic insulin sensitivity better than unfermented soybeans by enhancing hepatic insulin signaling cascade in diabetic rats.     

Insulin decreases human adiponectin plasma levels.

Insulin resistance and hyperinsulinemia are known atherosclerosis risk factors. The association between adiponectin plasma levels and obesity, insulinemia, and atherosclerosis has been shown. Thus, adiponectin may be a link between hyperinsulinemia and vascular disease. In vitro data demonstrated a reduction of adiponectin expression by insulin. However, it is still unclear whether insulin regulates adiponectinemia in vivo in humans. Five healthy male volunteers were studied. Circulating adiponectin levels were determined before and during hyperinsulinemic euglycemic clamp. Adiponectin was measured by radioimmunoassay. Hyperinsulinemia (85.0 +/- 33.2 at baseline vs. 482.8 +/- 64.4 pmol/l during steady state; p < 0.01) was achieved using a euglycemic hyperinsulinemic clamp, keeping blood glucose levels basically unchanged during the intervention (4.6 +/- 0.14 vs. 4.37 +/- 0.15 mmol/l, respectively; ns). We found a significant decrease of adiponectin plasma levels during the steady state of hyperinsulinemic euglycemic clamp (26.7 +/- 3.5 micro g/ml) compared to baseline levels (30.4 +/- 5 micro g/ml; p < 0.05). Hyperinsulinemia caused a significant decrease of adiponectin plasma levels under euglycemic conditions. Considering existing data about adiponectin dependent effects, hypoadiponectinemia might at least partly be a link between hyperinsulinemia and vascular disease in metabolic syndrome.     

A steroidal glycoside from Polygonatum odoratum (Mill.) Druce. improves insulin resistance but does not alter insulin secretion in 90% pancreatectomized rats

The aim of this study was to investigate the effects of three steroidal glycosides (SG-100, SG-280, and SG-460) obtained from Polygonatum odoratum (Mill.) Druce. on insulin secretion, insulin action, and relative glucose uptake in various tissues of 90% pancreatectomized male Sprague-Dawley rats. One of the compounds (30 mg/kg body weight daily) with a 40%-fat diet was orally administered to a group of such rats for 13 weeks. On the day after a hyperglycemic clamp, euglycemic hyperinsulinemic clamp with 1 microCi of [1-(14)C]2-deoxyglucose per 100 g body weight was used. Serum glucose levels were lowest in the rats receiving SG-100. Insulin secretion from pancreatic beta-cells did not change with SG administration. Whole-body glucose disposal rates increased with SG-100 administration by 39%. SG-100 increased the glycogen contents and glycogen synthase activity in the soleus muscle of pancreatectomized rats. Uptake of [1-(14)C]2-deoxyglucose into the soleus muscle was higher in such rats receiving SG-100 than in rats receiving other compounds. In conclusion, SG-100 has an antihyperglycemic effect by promoting peripheral insulin sensitivity without changing insulin secretion.     

Supranutritional selenium intake from enriched milk casein impairs hepatic insulin sensitivity via attenuated IRS/PI3K/AKT signaling and decreased PGC-1α expression in male Sprague–Dawley rats

Selenium (Se)-enriched milk provides antioxidant benefits and has therapeutic potential against cancer. However, both antidiabetic and prodiabetic effects have been attributed to Se. Our objective was to evaluate the effect of Se-enriched milk casein on insulin sensitivity in rats when given at the requirement of 0.25 ppm Se and supranutritionally on both low- and high-fat diets. Two hundred sixteen male Sprague–Dawley rats were fed low- or high-fat diets containing one, two or eight times the Se requirement in a randomized block design. After 7 weeks, 72 rats were subjected to the hyperinsulinemic–euglycemic clamp with [3-³H]glucose infusion to estimate glucose fluxes. Tissues were collected from the remaining 144 rats 8 min after ip saline or insulin injection. During hyperinsulinemic–euglycemic clamps, glucose infusion rate was 22% lower (P=.058), and endogenous glucose production was 76% higher (P=.054) when Se content increased from one to eight times the requirement on low-fat diets, indicating impaired hepatic insulin sensitivity. Se also decreased the ability for insulin to stimulate Akt phosphorylation at Thr308. Hepatic oxidation state and expression of selenoprotein P and glutathione peroxidase-1 were unaffected while expression of insulin receptor substrate (IRS)-1 and−2 and PPARγ coactivator-1α (PGC-1α) decreased with supranutritional Se and high-fat intake. In addition, hepatic expression of regulatory and catalytic subunits of phosphatidylinositol 3-kinase (PI3K) decreased with supranutritional intake of Se. Se intake from enriched casein up to eight times the requirement impairs hepatic insulin sensitivity in a mechanism similar to fat feeding, via attenuated IRS/PI3K/Akt signaling and decreased PGC-1α expression.     

Effect of leptin on insulin sensitivity in the Otsuka Long-Evans Tokushima Fatty rat

Leptin has been proposed to be a sensor of energy storage in adipose tissues, and is capable of mediating a feedback signal to the hypothalamus, which is involved in the regulation of energy homeostasis and body weight. In order to investigate the issue of whether resistance to the activity of leptin on insulin sensitivity is observed in young Otsuka Long-Evans Tokushima Fatty (OLETF) rats at 8 weeks of age, leptin (50 nmol/kg/h) was administered intravenously for 16 h to OLETF and Long-Evans Tokushima Otsuka (LETO) (lean controls) rats, followed by a measurement of insulin-stimulated glucose uptake in hindlimb muscles during hyperinsulinemic euglycemic clamp technique. In the case of LETO rats, the administration of leptin significantly decreased plasma insulin levels prior to the clamp test, but did not change plasma glucose levels. Furthermore, leptin led to an increase in insulin-stimulated glucose uptake in hindlimb muscles. However, in the case of OLETF rats, leptin administration changed neither plasma insulin levels nor insulin-stimulated glucose uptake. These data demonstrate that OLETF rats at 8 weeks of age have already become resistant to high concentration of peripheral leptin.     

Acute insulin infusion decreases plasma ghrelin levels in uncomplicated obesity

BACKGROUND: Plasma ghrelin levels have been shown to decrease after insulin infusion in lean subjects. Nevertheless, the mechanism of the suggested inhibitory effect of insulin on ghrelin is still unclear and no data about the effect of acute insulin infusion on plasma ghrelin concentration in obese subjects are available. OBJECTIVE: We sight to evaluate plasma ghrelin concentration during an hyperinsulinemic euglycemic clamp in uncomplicated obese subjects. METHODS: 35 uncomplicated obese subjects, body mass index (BMI) 43.3+/-10.1 kg/m(2), 33 women and 2 men, mean age 34.9+/-10, with a history of excess fat of at least 10 years underwent euglycemic hyperinsulinemic clamp. Blood samples for ghrelin were performed at baseline and steady state of euglycemic insulin clamp. RESULTS: Ghrelin concentrations decreased over time to 10.6+/-15% (range 2-39%) of baseline, from a mean of 205.53+/-93.79 pg/ml to 179.03+/-70.43 pg/ml during the clamp (95% CI, 10.69 to 36.44, P<0.01). In a univariate linear regression analysis baseline plasma ghrelin levels were inversely correlated to BMI (r=-0.564, P=0.04). A linear positive trend between whole body glucose utilization (M(FFMkg) index) and ghrelin reduction during the clamp was found (chi(2) 3.05, p=0.05). CONCLUSIONS: Our data seem to suggest that hyperinsulinemia during a euglycemic clamp is able to suppress plasma ghrelin concentrations in uncomplicated obesity. This effect appears to be positively related to insulin sensitivity.     

Short-term oral α-lipoic acid does not prevent lipid-induced dysregulation of glucose homeostasis in obese and overweight nondiabetic men

Prolonged elevation of plasma free fatty acids (FFAs) induces insulin resistance and impairs pancreatic β-cell adaptation to insulin resistance. The mechanisms whereby lipid induces these impairments are not fully defined but may involve oxidative stress, inflammation, and endoplasmic reticulum stress. α-Lipoic acid (ALA), a commonly used health supplement with antioxidant, anti-inflammatory, and AMPK-activating properties, has been shown to have therapeutic value in type 2 diabetes and its complications. Here we examined the effects of ALA on insulin sensitivity and secretion in humans under the conditions of 24-h iv lipid infusion to elevate plasma FFAs. Eight overweight and obese male subjects underwent four randomized studies each, 4-6 wk apart: 1) SAL, 2-wk oral placebo followed by 24-h iv infusion of saline; 2) IH, 2-wk placebo followed by 24-h iv infusion of intralipid plus heparin to raise plasma FFAs approximately twofold; 3) IH + ALA, 2-wk ALA (1,800 mg/day) followed by 24-h infusion of intralipid plus heparin; and 4) ALA, 2-wk ALA followed by 24-h infusion of saline. Insulin secretion rates (ISR) and insulin sensitivity were assessed with a 2-h, 20-mmol/l hyperglycemic clamp and a hyperinsulinemic euglycemic clamp, respectively. ISR was not significantly different between treatments. Lipid infusion impaired insulin sensitivity with and without ALA pretreatment. These results indicate that ALA, administered orally at this dose for 2 wk, does not protect against lipid-induced insulin resistance in overweight and obese humans.     

Insulin effects on acetate metabolism

Acetate metabolism was studied in patients with insulin resistance. To evaluate the interaction between glucose and acetate metabolism, we measured acetate and glucose turnover with a hyperinsulinemic euglycemic clamp (hot clamp) in obese and diabetic patients with insulin resistance (n = 8) and in a control group with normal insulin sensitivity (n = 6). At baseline, acetate turnover and plasma concentrations were similar between the two groups (group means: 4.3 +/- 0.4 micromol x kg-1 x min-1 and 128.2 +/- 11.1 micromol/l). Acetate concentrations decreased in both groups with hyperinsulinemia but were significantly lower in the insulin-resistant group (20% vs. 12%, P < 0.05). After the hot clamp treatment, acetate turnover increased for the two groups and was higher in the group with normal insulin sensitivity: 8.1 +/- 0.7 vs. 5.5 +/- 0.5 micromol x kg-1 x min-1 (P < 0.001). No change related to insulin action was observed in either group in the percentage of acetate oxidation. This was approximately 70% of overall utilization at baseline and during the clamp. No correlation between glucose and acetate utilization was observed. Our results support the hypothesis that, like glucose metabolism, acetate metabolism is sensitive to insulin.     

Insulin secretion and glucose uptake in hypomagnesemic sheep fed a low magnesium, high potassium diet

Hyperglycemic and euglycemic clamp experiments were conducted to evaluate insulin secretion and glucose uptake in the hypomagnesemic sheep fed a low magnesium (Mg), high potassium (K) diet. Five mature sheep were fed a semipurified diet containing 0.24% Mg and 0.56% K (control diet) and five were fed 0.04% Mg and 3.78% K (low Mg/high K diet) for at least 2 weeks. In the hyperglycemic clamp experiment, plasma glucose concentrations were raised and maintained at a hyperglycemic steady-state (approximately 130 mg/100 ml) by variable rates of glucose infusion during the experimental period (120 minutes). The insulin response in the sheep fed the low Mg/high K diet (31.0 microU/ml) were significantly (P < 0.01) lower than those (111.7 microU/ml) of the sheep fed the control diet. In the euglycemic clamp experiment, insulin was infused at rates of 5, 10, 15, or 20 mU/kg/min, each followed by variable rates of glucose infusion to maintain a euglycemic steady-state (basal fasting levels). Hypomagnesemic sheep fed the low Mg/high K diet had significantly (P < 0.01) lower mean glucose disposal (3.72 mg/kg/min) across the insulin infusion rates compared with those of the sheep fed the control diet (5.37 mg/kg/min). These results suggest that glucose-induced insulin secretion and insulin-induced glucose uptake would be depressed in hypomagnesemic sheep and are caused by feeding the low Mg/high K diet.     

Increased hypothalamic-pituitary-adrenal axis activity and hepatic insulin resistance in low-birth-weight rats

Individuals born with a low birth weight (LBW) have an increased prevalence of type 2 diabetes, but the mechanisms responsible for this association are unknown. Given the important role of insulin resistance in the pathogenesis of type 2 diabetes, we examined insulin sensitivity in a rat model of LBW due to intrauterine fetal stress. During the last 7 days of gestation, rat dams were treated with dexamethasone and insulin sensitivity was assessed in the LBW offspring by a hyperinsulinemic euglycemic clamp. The LBW group had liver-specific insulin resistance associated with increased levels of PEPCK expression. These changes were associated with pituitary hyperplasia of the ACTH-secreting cells, increased morning plasma ACTH concentrations, elevated corticosterone secretion during restraint stress, and an approximately 70% increase in 24-h urine corticosterone excretion. These data support the hypothesis that prenatal stress can result in chronic hyperactivity of the hypothalamic-pituitary-adrenal axis, resulting in increased plasma corticosterone concentrations, upregulation of hepatic gluconeogenesis, and hepatic insulin resistance.     

Changes in insulin sensitivity precede changes in body composition during 14 days of step reduction combined with overfeeding in healthy young men

A lifestyle characterized by inactivity and a high-calorie diet is a known risk factor for impaired insulin sensitivity and development of Type 2 diabetes mellitus. To investigate possible links, nine young healthy men (24 ± 3 yr; body mass index of 21.6 ± 2.5 kg/m(2)) completed 14 days of step reduction (10,000 to 1,500 steps/day) and overfeeding (+50% kcal). Body composition (dual X-ray absorptiometry, MRI), aerobic fitness (maximal O(2) consumption), systemic inflammation and insulin sensitivity [oral glucose tolerance test (OGTT), hyperinsulinemic euglycemic clamp] were assessed before (day 0), during (days 3 and 7), and immediately after the intervention (day 14), with follow-up tests (day 30). Body weight had increased at days 7 and 14 (P < 0.05). The amount of visceral fat had increased at day 14 compared with day 0 (P < 0.05). The insulin response to the OGTT had increased at days 7 and 14 (P < 0.05). Insulin sensitivity, estimated using the Matsuda index, had decreased at days 3 and 7 (P < 0.01). At day 14, glucose infusion rates had decreased by ～44% during the euglycemic clamps (P < 0.05). Also, plasma levels of leptin and adiponectin had increased (P < 0.05), whereas no changes were seen in inflammatory markers. At day 30, body weight and whole body adiposity were still elevated compared with day 0 (P < 0.05), whereas the insulin sensitivity as well as the insulin response to the OGTT did not differ from baseline. The glucose response to the OGTT was only affected at day 30, with a decrease compared with day 0. Our data show that insulin sensitivity was impaired after 3 days of inactivity and overfeeding. Impairments in insulin sensitivity occurred before changes in body composition, supporting the notion that the initial steps in impairment of insulin sensitivity may be linked directly to the effects of inactivity and a high calorie intake.     

Insulin action and secretion in endurance-trained and untrained humans

To evaluate insulin sensitivity and responsiveness, a two-stage hyperinsulinemic euglycemic clamp procedure (insulin infusions of 40 and 400 mU.m-2.min-1) was performed on 11 endurance-trained and 11 untrained volunteers. A 3-h hyperglycemic clamp procedure (plasma glucose approximately 180 mg/dl) was used to study the insulin response to a fixed glycemic stimulus in 15 trained and 12 untrained subjects. During the 40-mU.m-2.min-1 insulin infusion, the glucose disposal rate was 10.2 +/- 0.5 mg.kg fat-free mass (FFM)-1.min-1 in the trained group compared with 8.0 +/- 0.6 mg.kg FFM-1.min-1 in the untrained group (P less than 0.01). In contrast, there was no significant difference in maximally stimulated glucose disposal: 17.7 +/- 0.6 in the trained vs. 16.7 +/- 0.7 mg.kg FFM-1.min-1 in the untrained group. During the hyperglycemic clamp procedure, the incremental area for plasma insulin was lower in the trained subjects for both early (0-10 min: 140 +/- 18 vs. 223 +/- 23 microU.ml-1.min; P less than 0.005) and late (10-180 min: 4,582 +/- 689 vs. 8,895 +/- 1,316 microU.ml-1.min; P less than 0.005) insulin secretory phases. These data demonstrate that 1) the improved insulin action in healthy trained subjects is due to increased sensitivity to insulin, with no change in responsiveness to insulin, and 2) trained subjects have a smaller plasma insulin response to an identical glucose stimulus than untrained individuals.     

Improvement of insulin resistance and insulin secretion by water extracts of Cordyceps militaris, Phellinus linteus, and Paecilomyces tenuipes in 90% pancreatectomized rats

The effect of supplementation with Phellinus linteus (P. linteus), Paecilomyces tenuipes (P. tenuipes), and Cordyceps militaris (C. militaris) mushroom water extracts on the insulin secretion and insulin resistance of 90% pancreatectomized (Px) male Sprague Dawley rats was investigated. Px rats were daily administered 0.5 g of P. linteus, P. tenuipes, and C. militaris aqueous extracts or a placebo per 1 kg body weight with a 40% fat diet for 8 weeks. Fasting serum glucose levels were lower in rats receiving C. militaris than in the control group. Insulin secretion at the elevated serum glucose levels was lowest in rats that consumed P. tenuipes in hyperglycemic clamp. Whole body glucose disposal rates increased in C. militaris but decreased in P. tenuipes compared to those in the control group in euglycemic hyperinsulinemic clamp. The GLUT4 content and fraction velocity of glycogen synthase in the soleus and quadriceps muscles increased in the rats treated with C. militaris, but P. tenuipes decreased both. In sum, a water extract of C. militaris ameliorates insulin resistance by enhancing glucose utilization in skeletal muscles.     

Central visfatin potentiates glucose-stimulated insulin secretion and β-cell mass without increasing serum visfatin levels in diabetic rats

IntroductionOur previous study revealed that plasma visfatin levels were lower in pregnant women with gestational diabetes (GDM) than non-GDM independent of prepreganacy BMI. We examined whether central visfatin modulates energy and glucose homeostasis via altering insulin resistance, insulin secretion or islet morphometry in diabetic rats.MethodsPartial pancreatectomized, type 2 diabetic, rats were interacerbroventricularly infused with visfatin (100 ng/rat/day, Px-VIS), visfatin + visfatin antagonist, CHS-828 (100 μg/rat/day, Px-VIS-ANT), or saline (control, Px-Saline) via osmotic pump, respectively, for 4 weeks.ResultsCentral visfatin improved insulin signaling (pAkt → pFOXO-1) but not pSTAT3 in the hypothalamus. Central visfatin did not alter serum visfatin levels in diabetic rats whereas the levels were higher in non-diabetic rats than diabetic rats. Body weight at the 2nd week was lowered in the Px-VIS group due to decreased food intake in the first two weeks compared to the Px-Saline group and energy expenditure was not significantly different among the treatment groups of diabetic rats. Visfatin antagonist treatment nullified the central visfatin effect. Px-VIS increased whole body glucose disposal rates in euglycemic hyperinsulinemic clamp compared to Px-Saline and lowered hepatic glucose output, whereas Px-VIS-ANT blocked the visfatin effect on insulin resistance (P < 0.05). In hyperglycemic clamp study, the area under the curve of insulin in first and second phase were significantly higher in the Px-VIS group than the Px-Saline group without modifying insulin sensitivity at the hyperglycemic state, whereas the increase in serum insulin levels was blocked in the Px-VIS-ANT group. Central visfatin also increased β-cell mass by increasing β-cell proliferation.ConclusionsCentral visfatin improved glucose homeostasis by increasing insulin secretion and insulin sensitivity at euglycemia through the hypothalamus in diabetic rats. Therefore, visfatin is a positive modulator of glucose homeostasis by delivering the hypothalamic signals into the peripheries.     

12/15-Lipoxygenase Is Required for the Early Onset of High Fat Diet-Induced Adipose Tissue Inflammation and Insulin Resistance in Mice

Background

Recent understanding that insulin resistance is an inflammatory condition necessitates searching for genes that regulate inflammation in insulin sensitive tissues. 12/15-lipoxygenase (12/15LO) regulates the expression of proinflammatory cytokines and chemokines and is implicated in the early development of diet-induced atherosclerosis. Thus, we tested the hypothesis that 12/15LO is involved in the onset of high fat diet (HFD)-induced insulin resistance.

Methodology/Principal Findings

Cells over-expressing 12/15LO secreted two potent chemokines, MCP-1 and osteopontin, implicated in the development of insulin resistance. We assessed adipose tissue inflammation and whole body insulin resistance in wild type (WT) and 12/15LO knockout (KO) mice after 2–4 weeks on HFD. In adipose tissue from WT mice, HFD resulted in recruitment of CD11b⁺, F4/80⁺ macrophages and elevated protein levels of the inflammatory markers IL-1β, IL-6, IL-10, IL-12, IFNγ, Cxcl1 and TNFα. Remarkably, adipose tissue from HFD-fed 12/15LO KO mice was not infiltrated by macrophages and did not display any increase in the inflammatory markers compared to adipose tissue from normal chow-fed mice. WT mice developed severe whole body (hepatic and skeletal muscle) insulin resistance after HFD, as measured by hyperinsulinemic euglycemic clamp. In contrast, 12/15LO KO mice exhibited no HFD-induced change in insulin-stimulated glucose disposal rate or hepatic glucose output during clamp studies. Insulin-stimulated Akt phosphorylation in muscle tissue from HFD-fed mice was significantly greater in 12/15LO KO mice than in WT mice.

Conclusions

These results demonstrate that 12/15LO mediates early stages of adipose tissue inflammation and whole body insulin resistance induced by high fat feeding.     

Effect of supplemental calcium propionate on insulin action to blood glucose metabolism in adult sheep

An experiment combining a hyperinsulinemic euglycemic clamp procedure of four sequential 2-h periods and an isotope dilution method of [U-13C]glucose determined the effect of supplemental calcium propionate on blood glucose metabolism during insulin and glucose infusions in adult sheep. They were fed lucerne hay cubes and commercial concentrate with and without supplementary calcium propionate (Prop and Cont diets, respectively) in a crossover design for each 21-day period. At the preinfusion period, blood glucose turnover rate (GTR) was greater (P < 0.05) for the Prop diet than for the Cont diet. Blood GTR, endogenous glucose production rate (EGPR) and the ratio of EGPR to blood GTR were greater (P < 0.01, P < 0.05 and P < 0.05, respectively) for the Prop diet than for the Cont diet. Blood GTR and glucose infusion rate (GIR) increased (P < 0.001) and the ratio of EGPR to blood GTR was reduced (P < 0.01) with increased insulin infusion rates. The maximal GIR tended to be (P < 0.10) greater for the Prop diet than for the Cont diet but plasma insulin concentration at half maximal GIR did not differ between diets. It is suggested that in adult sheep, dietary propionate supplementation enhances insulin action on glucose metabolism, however, changes in measures of tissue responsiveness and sensitivity were not significant.     

Effect of metformin on the vascular and glucose metabolic actions of insulin in hypertensive rats

We investigated the long-term effect of metformin treatment on blood pressure, insulin sensitivity, and vascular responses to insulin in conscious spontaneously hypertensive rats (SHR). The rats were instrumented with intravascular catheters and pulsed Doppler flow probes to measure blood pressure, heart rate, and blood flow. Insulin sensitivity was assessed by the euglycemic hyperinsulinemic clamp technique. Two groups of SHR received metformin (100 or 300 mg x kg(-1) x day(-1)) for 3 wk while another group of SHR and a group of Wistar Kyoto (WKY) rats were left untreated. We found that vasodilation of skeletal muscle and renal vasculatures by insulin is impaired in SHR. Moreover, a reduced insulin sensitivity was detected in vivo and in vitro in isolated soleus and extensor digitorum longus muscles from SHR compared with WKY rats. Three weeks of treatment with metformin improves the whole-body insulin-mediated glucose disposal in SHR but has no blood pressure-lowering effect and no influence on vascular responses to insulin (4 mU x kg(-1) x min(-1)). An improvement in insulin-mediated glucose transport activity was detected in isolated muscles from metformin-treated SHR, but in the absence of insulin no changes in basal glucose transport activity were observed. It is suggested that part of the beneficial effect of metformin on insulin resistance results from a potentiation of the hormone-stimulating effect on glucose transport in peripheral tissues (mainly skeletal muscle). The results argue against a significant antihypertensive or vascular effect of metformin in SHR.