Whole-body Glucose Disposal Research Articles

Subcutaneous adipose tissue expression of plasminogen activator inhibitor-1 (PAI-1) in nondiabetic and Type 2 diabetic subjects.

Increased plasma levels of plasminogen activator inhibitor-1 (PAI-1) have been suggested to be a part of the insulin resistance syndrome, and recent data suggest that adipose tissue participates in the production of PAI-1. We examined the expression and insulin regulation of subcutaneous adipose tissue PAI-1 mRNA and its relationship to insulin sensitivity. A cross-sectional study involving five lean (60.0+/-3.1 years, BMI 23.5+/-0.5 kg/m(2)) and six obese nondiabetic men (56.0+/-3.1 years, BMI 30.4+/-0.7 kg/m(2)), and six obese Type 2 diabetic men (61.4+/-3.2 years, BMI 31.8+/-1.0 kg/m(2)). Subcutaneous adipose tissue PAI-1 mRNA and insulin sensitivity were quantified using RT-competitive PCR and euglycemic hyperinsulinemic clamp technique, respectively. Subcutaneous adipose tissue PAI-1 mRNA levels were higher in obese nondiabetic and Type 2 diabetic men than in lean nondiabetic men. PAI-1 mRNA levels decreased in the three groups during a 240-min euglycemic hyperinsulinemic clamp (P<0.05 for all groups), and a similar reduction was observed during a 240-min saline control study indicating that adipose tissue PAI-1 gene expression has diurnal variation and is not acutely controlled by hyperinsulinemia. The basal PAI-1 mRNA levels correlated positively with BMI, and waist-to-hip ratio; and negatively with whole-body glucose disposal rate in nondiabetic men. Subcutaneous adipose tissue PAI-1 mRNA expression is increased in obese nondiabetic or in Type 2 diabetic men. Subcutaneous adipose tissue PAI-1 mRNA expression is increased in proportion to visceral obesity and to the level of whole-body insulin resistance. Subcutaneous adipose tissue PAI-1 mRNA expression is not acutely regulated by insulin, and it is subject to a diurnal variation.

Plasma leptin level is associated with myocardial wall thickness in hypertensive insulin-resistant men.

Leptin, the product of the ob gene, has been shown to increase heart rate and blood pressure through a stimulation of cardiac sympathetic nervous system activity, a phenomenon also involved in the pathogenesis of left ventricular hypertrophy in hypertensives. Thus, we hypothesize that plasma leptin concentration is associated with left ventricular hypertrophy. Forty hypertensive males and 15 healthy male subjects underwent anthropometric and echocardiographic evaluations, assessment of insulin sensitivity through euglycemic glucose clamp combined with indirect calorimetry, and determination of fasting plasma leptin concentration. Fasting plasma leptin levels were higher in hypertensives than in controls (6.48+/-2.9 versus 4. 62+/-1.5 ng/mL, P<0.05); these results were unchanged after adjustment for body mass index (P<0.05). In the whole group of patients (n=55), fasting plasma leptin concentration was correlated with body mass index (r=0.46, P<0.001) and waist/hip ratio (r=0.50, P<0.001); independent of body mass index and waist/hip ratio, fasting plasma leptin concentration was correlated (n=55) with whole-body glucose disposal (r=-0.27, P<0.04), interventricular septum thickness (r=0.34, P<0.001), posterior wall thickness (r=0.38, P<0.003), and the sum of wall thicknesses (r=0.68, P<0.001). In a multivariate analysis (n=55), age, body mass index, fasting plasma leptin concentration, plasma Na(+) concentration, whole-body glucose disposal, and diastolic blood pressure explained 68% of the variability of the sum of wall thicknesses with fasting plasma leptin concentration (P<0.03), whole body glucose disposal (P<0.002), and diastolic blood pressure (P<0.001), which were significantly and independently associated with the sum of wall thicknesses. In conclusion, our study demonstrates that fasting plasma leptin levels are associated with increased myocardial wall thickness independent of body composition and blood pressure levels in hypertensives.

Aging changes tissue-specific glucose metabolism in rats.

This study defines the tissue-specific changes in glucose metabolic flux that occur over time prior to the onset of whole-body insulin resistance in rats. Rats at 6 weeks of age were maintained on a high-carbohydrate diet for either 12 or 26 weeks, at which time euglycemic clamps were performed at basal and midphysiological plasma insulin concentrations. Following death, insulin-sensitive tissues were excised and frozen until assayed for the rate of glucose uptake, glycogenesis, and lipogenesis. Glucose metabolic flux, particularly through glycogenesis, was reduced between 18 and 32 weeks of age in all tissues except the adipose tissues. For example, the rate of glycogenesis in liver at 18 weeks (117+/-10 nmol glucose incorporated/min/g) was more than double that observed at 32 weeks (54+/-8 nmol glucose incorporated/min/g, P < .01). Despite this, animals in the 32-week group displayed no impairment in whole-body glucose disposal, due to compensatory glucose uptake in white adipose tissue (WAT) and increased glucose flux through lipogenesis in brown adipose tissue (BAT). At 32 weeks, the rate of glucose uptake in WAT (85.0+/-5.6 nmol 2-deoxy-D-glucose phosphate accumulated/min/g) was approximately double that at 18 weeks (46.6+/-5.6 nmol 2-deoxy-D-glucose phosphate accumulated/min/g) was approximately double that at 18 weeks (46.6+/-5.6 nmol 2-deoxy-D-glucose phosphate accumulated/min/g, P < .01). These changes in insulin responsiveness in adipose tissue of older animals may underlie the increased adiposity that is currently thought to be the causative factor in the development of age-related insulin resistance.

Paradoxical phosphorylation of skeletal muscle glycogen synthase by in vivo insulin in very lean young adult rhesus monkeys.

Calorie restriction (CR) has previously been shown to unexpectedly induce a reversal of in vivo insulin action (phosphorylation instead of dephosphorylation) on skeletal muscle glycogen synthase (GS) in four out of six long-term calorie-restricted (CR) monkeys. The purpose of the present study was to determine whether this increase in Ka (concentration of glucose 6-phosphate [G6P] at which GS activity is half-maximal) during insulin is also present in very lean (VL) young adult monkeys maintained on a controlled feeding regimen. Muscle samples from 10 VL monkeys (10 +/- 2% body fat; 7 years old) were obtained before and during a euglycemic hyperinsulinemic clamp and the Ka was determined and compared to the Ka of two other groups of monkeys, one matched in age but fully ad libitum (AL)-fed (n = 9.8 +/- 1 years old, 20 +/- 3% body fat, p = 0.01 vs. VL monkeys), and the other our previously described weight-clamped long-term CR monkeys (n = 6.20 +/- 1 years old, 21 +/- 2% body fat, p = 0.01 vs. VL monkeys). All of the AL monkeys had the expected decrease in Ka with insulin; however, similar to the 4 out of 6 CR monkeys, 7 out of 10 VL monkeys had an increase in Ka with insulin. The 11 monkeys with an increase in Ka (+Ka) (7 VL + 4 CR) were compared to the 14 monkeys with a decrease in Ka with insulin (-Ka) (3 VL + 2 CR + 9 AL). The +Ka monkeys had lower basal Ka (p = 0.0001), higher basal GS fractional activity (p = 0.0003), lower basal G6P content (p = 0.002), lower glycogen phosphorylase fractional activity (p = 0.01), and lower whole-body insulin-mediated glucose disposal rate (p < 0.05) than the -Ka monkeys. We conclude that the condition of steady-state restrained calorie intake (as in the CR monkeys and in the controlled feeding VL monkeys) produces the paradoxical action of in vivo insulin to phosphorylate muscle GS, and raises the possibility that the presence of the unusual response to insulin may serve as a marker in calorie-restrained individuals for the genotype of obesity, insulin resistance and/or Type 2 diabetes.

Effects of Vitamin E and Glutathione on Glucose Metabolism

Vitamin E is an antioxidant that has been demonstrated to improve insulin action. Glutathione, another natural antioxidant, may also be important in blood pressure and glucose homeostasis, consistent with the involvement of free radicals in both essential hypertension and diabetes mellitus. Our group has recently suggested that the effects of reduced glutathione on glucose metabolism may be mediated, at least in part, by intracellular magnesium levels (Mg([i])). Recent evidence suggests that vitamin E enhances glutathione levels and may play a protective role in magnesium deficiency-induced cardiac lesions. To directly investigate the effects of vitamin E supplementation on insulin sensitivity in hypertension, in relation to the effects on circulating levels of reduced (GSH) and oxidized (GSSG) glutathione and on Mg([i]), we performed a 4-week, double-blind, randomized study of vitamin E administration (600 mg/d) versus placebo in 24 hypertensive patients and measured whole-body glucose disposal (WBGD) by euglycemic glucose clamp, GSH/GSSG ratios, and Mg([i]) before and after intervention. The relationships among WBGD, GSH/GSSG, and Mg([i]) in both groups were evaluated. In hypertensive subjects, vitamin E administration significantly increased WBGD (25.56+/-0.61 to 31.75+/-0.53 micromol/kg of fat-free mass per minute; P<0.01), GSH/GSSG ratio (1.10+/-0.07 to 1.65+/-0.11; P<0.01), and Mg([i]) (1.71+/-0.042 to 1.99+/-0.049 mmol/L; P<0.01). In basal conditions, WBGD was significantly related to both GSH/GSSG ratios (r=0.58, P=0.047) and Mg([i]) (r=0.78, P=0.003). These data show a clinical link between vitamin E administration, cellular magnesium, GSH/GSSG ratio, and tissue glucose metabolism. Further studies are needed to explore the cellular mechanism(s) of this association.

Effects of glutathione on red blood cell intracellular magnesium: relation to glucose metabolism.

Recent evidence suggests that the endogenous antioxidant glutathione may play a protective role in cardiovascular disease. To directly investigate the role of glutathione in the regulation of glucose metabolism in hypertension, we studied the acute effects of in vivo infusions of this antioxidant (alone or in combination with insulin) on whole body glucose disposal (WBGD) using euglycemic glucose clamp and the effects on total red blood cell intracellular magnesium (RBC-Mg) in hypertensive (n=20) and normotensive (n=30) subjects. The relationships among WBGD, circulating reduced/oxidized glutathione (GSH/GSSG) levels, and RBC-Mg in both groups were evaluated. The in vitro effects of glutathione (100 micromol/L) on RBC free cytosolic magnesium (Mg(i)) were also studied. In vivo infusions of glutathione (15 mg/minx120 minutes) increased RBC-Mg in both normotensives and hypertensives (1.99+/-0.02 to 2.13+/-0.03 mmol/L, P<0.01, and 1.69+/-0.03 to 1.81+/-0.03 mmol/L, P<0.01, respectively). In vitro GSH but not GSSG increased Mg(i) (179+/-3 to 214+/-5 micromol/L, P<0.01). In basal conditions, RBC-Mg values were related to GSH/GSSG ratios (r=0.84, P<0.0001), and WBGD was directly, significantly, and independently related to both GSH/GSSG ratios (r=0.79, P<0.0001) and RBC-Mg (r=0.89, P<0.0001). This was also true when hypertensive and control groups were analyzed separately. On multivariate analysis, basal RBC-Mg (t=6.81, P<0.001), GSH/GSSG (t=3. 67, P<0.02), and blood pressure (t=2.89, P<0.05) were each independent determinants of WBGD, with RBC-Mg explaining 31% of the variability of WBGD. These data demonstrate a direct action of glutathione both in vivo and in vitro to enhance intracellular magnesium and a clinical linkage between cellular magnesium, GSH/GSSG ratios, and tissue glucose metabolism.

The impact of increased dietary lipid on the regulation of glucose uptake and oxidation by insulin in brown- and a range of white-adipose-tissue depots in vivo.

To determine: (a) whether active pyruvate dehydrogenase (PDHa) activity in interscapular brown adipose tissue (IBAT) is acutely regulated by altered insulin status at euglycaemia; (b) the relationship between glucose uptake/phosphorylation and PDHa activities in IBAT in vivo; and (c) the impact of increased dietary lipid on the regulation of glucose uptake and oxidation by insulin in IBAT in comparison with that exerted on various white adipose tissue depots. Rats were provided with either a standard diet (8% fat, 72% carbohydrate, by energy) or a diet moderately high in saturated fat (47%, by energy) and low in carbohydrate (33%, by energy) for four weeks. Rats were studied in the absorptive state, in the post-absorptive state or after 2.5 h euglycaemic hyperinsulinaemia. Tissues sampled included IBAT and four white adipose tissue depots, two abdominal (parametrial (PM) and perirenal (PR)) and two superficial (subcutaneous (SC) and interscapular (IS)). Whole-body glucose disposal was estimated using [3-3H]glucose. Glucose uptake/phosphorylation in vivo was estimated using 2-deoxy[1-3H]-D-glucose. Insulin action was evaluated using the euglycaemic-hyperinsulinaemic clamp technique. PDHa activity was assayed spectrophotometrically in freeze-clamped tissue extracts. PDH kinase (PDK) activities were assayed in mitochondrial extracts by rates of ATP-dependent PDHa inactivation. Whole-body glucose disposal was decreased by high-fat-feeding in the post-absorptive state (P < 0.05) and during euglycaemic hyperinsulinaemia (P < 0.01). Glucose transport/phosphorylation in IBAT was decreased by high-fat feeding in the absorptive (P < 0.001) and post-absorptive states (by 84%, P < 0.05) and during steady-state euglycaemic hyperinsulinaemia (by 73%, P < 0.001). IBAT PDHa activities were suppressed by high-fat feeding. Euglycaemic hyperinsulinaemia in post-absorptive control rats increased PDHa activities in IBAT (P < 0.001) to values comparable to those found in the absorptive state. Although IBAT PDHa activities were increased by euglycaemic hyperinsulinaemia in post-absorptive high-fat-fed rats, they remained lower (by 55%; P < 0.01) than those of controls. The failure of hyperinsulinaemia to normalise IBAT pyruvate dehydrogenase complex (PDHC) activities in high-fat-fed rats was associated with a stable 1.4-fold increase in IBAT PDK activity. High-fat feeding decreased glucose utilisation rates in the post-absorptive state in IS, but not in SC, PM or PR white adipose tissue depots. Euglycaemic hyperinsulinaemia significantly increased glucose utilisation in three out of the four depots of the control rats, but did not elicit statistically-significant changes in high-fat-fed rats. High-fat feeding influenced PDHa activity in PR, but was without significant effect on PDHa activity in PM, SC and IS. The results demonstrate that PDHa activity in the IBAT of rats maintained on standard diet responds to changes in insulin concentrations over the low physiological range, and that inactivation of PDHa in IBAT after feeding a diet moderately high in saturated fat is a consequence of the induction of tissue insulin resistance. Effects of this dietary regime on PDHa activity are paralleled by changes in insulin-stimulated glucose uptake/phosphorylation by IBAT in vivo, and IBAT is specifically targeted, with only moderate effects in white adipose tissue. The finding of impaired activation of BAT glucose transport/phosphorylation and PDHa activity in response to insulin may contribute to impaired thermogenesis in rats maintained on diets containing a relatively high proportion of saturated fat.

Chronic leptin administration increases insulin-stimulated skeletal muscle glucose uptake and transport

Leptin, the product of the ob gene, has been shown to reduce fat mass, food intake, hyperglycemia, and hyperinsulinemia and to increase whole-body glucose disposal. However, it is unknown if leptin improves insulin action in skeletal muscle. Therefore, the purpose of this investigation was to determine if chronic leptin administration increases insulin-stimulated skeletal muscle glucose uptake and transport. Sixty-nine female Sprague-Dawley rats (240 to 250 g) were randomly assigned to one of three groups: (1) control, (2) pair-fed, and (3) leptin. All animals were subcutaneously implanted with miniosmotic pumps that delivered 0.5 mg leptin/kg/d to the leptin animals and vehicle to the control and pair-fed animals for 14 days. Following this 14-day period, all animals were subjected to hindlimb perfusion to determine the rates of skeletal muscle glucose uptake and 3- O-methyl- d-glucose (3-MG) transport under basal, submaximal (500 μU/mL), and maximal (10,000 μU/mL) insulin concentrations. Chronic leptin treatment significantly increased ( P < .05) the rate of glucose uptake across the hindlimb by 27%, 32%, and 47% under basal, submaximal, and maximal insulin, respectively, compared with the control and pair-fed condition. However, when the submaximal rate of glucose uptake was expressed as a percentage of maximal insulin-stimulated glucose uptake, no differences existed among the groups, indicating that leptin treatment does not increase insulin sensitivity. Rates of 3-MG transport in the soleus, plantaris, and white and red portions of the gastrocnemius (WG and RG) were significantly increased ( P < .05) in leptin animals under all perfusion conditions. 3-MG transport was not different between control and pair-fed animals. Collectively, these findings suggest that improvements in insulin-stimulated skeletal muscle glucose uptake and transport following chronic leptin treatment result from increased insulin responsiveness.

The insulin resistance syndrome

Insulin resistance is a frequently occurring abnormality. Although there can be insensitivity to any of insulin's actions, insulin resistance par excellence is a decreased insulin-mediated whole-body glucose disposal rate. A distinction is made between primary and secondary insulin resistance. Primary insulin resistance is of unknown origin, is only partially experimentally reproducible, and is essentially irreversible (spontaneously or by treatment). In addition, it is both pathway-specific (ie, glucose storage) and organ-specific (mostly skeletal muscle), and is compatible with a postreceptor defect in insulin action. Primary insulin resistance is found in a proportion (approximately 25%) of otherwise healthy people, in non-insulin-dependent diabetes mellitus, essential hypertension, and some forms of dyslipidemia. The idea of an insulin resistance syndrome derives from the striking pattern of overlap among these clinical conditions. Their tendency to cluster in the same individuals is evident from both cross-sectional and longitudinal observations. It is proposed that the insulin resistance syndrome is a large constellation of interrelated changes in metabolic, anthropometric, and hemodynamic variables centered around insulin resistance or hyperinsulinemia. There is a significant genetic component, a predisposing influence for non-insulin-dependent diabetes mellitus, hypertension, dyslipidemia, and possibly, a distinct atherogenic potential.

Preoperative oral carbohydrates and postoperative insulinresistance

Infusions of carbohydrates before surgery have been shown to reduce postoperative insulin resistance. Presently, we investigated the effects of a carbohydrate drink, given shortly before surgery, on postoperative insulin sensitivity. Methods: Insulin sensitivity and glucose turnover ([6, 6, 2H Z]-D-glucose) were measured using hyperinsulinemic,normoglycemic clamps before and after elective surgery. Sixteen patients undergoing total hip replacement were randomly assigned to preoperative oral carbohydrate administration (CHO-H, n = 8) or the same amount of a placebo drink (placebo, n = 8) before surgery. Insulin sensitivity was measured before and immediately after surgery. Patients undergoing elective colorectal surgery were studied before surgery and 24 h postoperatively (CHO-C (n = 7), and fasted (n = 7), groups). The fasted group underwent surgery after an overnight fast. In both studies, the CHO groups received 800 ml of an isoosmolar carbohydrate rich beverage the evening before the operation (100g carbohydrates), as well as another 400 ml (50g carbohydrates) 2 h before the initiation of anesthesia. Results: Immediately after surgery, insulin sensitivity was reduced 37% in the placebo group ( P < 0.05 vs. preoperatively) while no significant change was found in the CHO-H group (-16%, p = NS). During clamps performed 24h postoperatively, insulin sensitivity and whole-body glucose disposal was reduced in both groups, but the reduction was greater compared to that in the CHO-C group (−49 ± 6% vs. −26 ± 8%, P < 0.05 fasted vs. CHO-C). Conclusions: Patients given a carbohydrate drink shortly before elective surgery displayed less reducedinsulin sensitivity after surgery as compared to patients undergoing surgery after an overnight fast.

Is leptin concentration associated with the insulin resistance syndrome in nondiabetic men?

Insulin resistance has been strongly associated with cardiovascular risk. Recently, leptin, a hormone that regulates appetite, has been associated with both obesity and insulin resistance. However, the possible relation of leptin to the insulin resistance syndrome has been controversial. To explore this issue, we examined the relation of leptin to blood pressure, lipid levels, low density lipoprotein (LDL) size, and glucose levels in 87 normoglycemic men. Leptin levels were significantly correlated with body mass index (BMI) (r = 0.494), fasting insulin (r = 0.576), whole-body glucose disposal rate (GDR) (r = -0.566), fasting glucose (r = 0.510) total triglycerides (r = 0.294), apolipoprotein B (r = 0.223), systolic blood pressure (r = 0.223), and LDL size (r = -0.244). After adjustment for BMI and GDR, leptin levels remained significantly correlated with fasting insulin, fasting glucose, triglyceride, apolipoprotein B, and systolic blood pressure. Leptin levels were also correlated with the number of metabolic risk factors (dyslipidemia, systolic blood pressure, and fasting glucose). We conclude that leptin concentrations may be associated with several cardiovascular risk factors related to insulin resistance syndrome. These associations are only partly explained by leptin's relationship with BMI and GDR.

Rapid impairment of skeletal muscle glucose transport/phosphorylation by free fatty acids in humans.

The initial effects of free fatty acids (FFAs) on glucose transport/phosphorylation were studied in seven healthy men in the presence of elevated (1.44 +/- 0.16 mmol/l), basal (0.35 +/- 0.06 mmol/l), and low (<0.01 mmol/l; control) plasma FFA concentrations (P < 0.05 between all groups) during euglycemic-hyperinsulinemic clamps. Concentrations of glucose-6-phosphate (G-6-P), inorganic phosphate (Pi), phosphocreatine, ADP, and pH in calf muscle were measured every 3.2 min for 180 min by using 31P nuclear magnetic resonance spectroscopy. Rates of whole-body glucose uptake increased similarly until 140 min but thereafter declined by approximately 20% in the presence of basal and high FFAs (42.8 +/- 3.6 and 41.6 +/- 3.3 vs. control: 52.7 +/- 3.3 micromol x kg(-1) x min(-1), P < 0.05). The rise of intramuscular G-6-P concentrations was already blunted at 45 min of high FFA exposure (184 +/- 17 vs. control: 238 +/- 17 micromol/l, P = 0.008). At 180 min, G-6-P was lower in the presence of both high and basal FFAs (197 +/- 21 and 213 +/- 18 vs. control: 286 +/- 19 micromol/l, P < 0.05). Intramuscular pH decreased by -0.013 +/- 0.001 (P < 0.005) during control but increased by +0.008 +/- 0.002 (P < 0.05) during high FFA exposure, while Pi rose by approximately 0.39 mmol/l (P < 0.005) within 70 min and then slowly decreased in all studies. In conclusion, the lack of an initial peak and the early decline of muscle G-6-P concentrations suggest that even at physiological concentrations, FFAs primarily inhibit glucose transport/phosphorylation, preceding the reduction of whole-body glucose disposal by up to 120 min in humans.

Insulin unexpectedly increases the glucose 6-phosphate Ka of skeletal muscle glycogen synthase in calorie-restricted monkeys.

In skeletal muscle of normal subjects, the concentration of glucose 6-phosphate (G6P) at which the activity of glycogen synthase (GS) is half maximal (Ka) is decreased by in vivo insulin, and the fractional activity is increased without a change in GS maximal activity (Vmax). We have shown that moderate chronic calorie restriction, previously shown in rodents to be effective in slowing aging, resulted in the prevention of obesity and type 2 diabetes in primates (rhesus monkeys, Macaca mulatta). However, unexpectedly, in a subgroup of calorie-restricted monkeys, insulin during a euglycemic hyperinsulinemic clamp caused an unanticipated decrease in skeletal muscle GS fractional activity. These same monkeys had the lowest whole-body glucose disposal rate (M), the greatest increase in skeletal muscle G6P content and the greatest increase in skeletal muscle glycogen phosphorylase activity during the euglycemic hyperinsulinemic clamp compared to the remaining calorie-restricted monkeys with normal insulin action. To determine whether this highly unusual insulin-mediated decrease in GS fractional activity was due to increased phosphorylation (increased Ka), we measured the activity of skeletal muscle GS at 9 different G6P concentrations before and during the euglycemic hyperinsulinemic clamp in 6 calorie-restricted monkeys. G6P Ka increased (n = 4) and Vmax decreased (n = 5) during the clamp. Basal G6P Ka was inversely related to basal GSfv (r = -0.94, p < 0.002). G6P Ka and skeletal muscle G6P content were positively related under insulin-stimulated conditions (r = 0.93, p < 0.005). The change in G6P Ka (insulin-stimulated minus basal) was inversely related to M (r = -0.94, p < 0.002) and positively related to the change in skeletal muscle G6P content (r = 0.93, p < 0.005). We conclude that moderate calorie restriction results in a reversal of normal insulin action at the skeletal muscle with inactivation of glycogen synthase which is likely to be due to an increase in phosphorylation of GS together with a decrease in Vmax of GS during a euglycemic hyperinsulinemic clamp in most of the calorie-restricted monkeys. These alterations are likely to be involved in the anti-diabetogenic effects of calorie restriction.

Insulin resistance in cirrhosis: prolonged reduction of hyperinsulinemia normalizes insulin sensitivity.

Insulin resistance is present in nearly all patients with cirrhosis, but its etiology remains unknown. Chronic hyperinsulinemia has been suspected as a potential candidate, and we therefore tested the hypothesis that, in cirrhosis, prolonged reduction of the hyperinsulinemia restores insulin sensitivity. Whole-body insulin sensitivity (euglycemic insulin-clamp technique), glucose turnover (6,6-2H2-glucose isotope dilution), glucose oxidation (indirect calorimetry), non-oxidative glucose disposal, and fractional glycogen synthase activity in muscle (biopsies) were measured in eight clinically stable patients with cirrhosis before and at the end of a 4-day continuous subcutaneous infusion of the somatostatin-analogue octreotide (200 microg/24 h) designed to continuously reduce plasma insulin levels. Baseline data were compared with results obtained in healthy individuals matched for sex, age, and weight (n = 8). During the baseline (pre-octreotide) study, patients demonstrated a significant decrease in insulin-mediated glucose uptake compared with controls (5.75 +/- 0.21 vs. 7.98 +/- 0.84 mg/kg/min; P < .03), which was entirely accounted for by an impairment in non-oxidative glucose disposal (P < .04). Four-day infusion of octreotide to cirrhotic patients: 1) reduced postabsorptive and meal-stimulated plasma insulin levels by approximately 35% to 45% without significantly affecting glucose tolerance; 2) did not significantly alter plasma free fatty acids (FFA), growth hormone, and glucagon levels in the postabsorptive state and during the meal test; 3) normalized insulin-mediated whole-body glucose disposal (7.63 +/- 0.72 mg/kg/min post-octreotide; P = not significant vs. control). Restoration of insulin-mediated glucose utilization was entirely caused by normalization of non-oxidative glucose disposal; 4) was associated with a considerably more pronounced stimulation by insulin of the fractional glycogen synthase in muscle compared with pre-octreotide results (increment above baseline pre: 0.035 +/- 0.010 vs. post: 0.060 +/- 0.023 nmol/min/mg protein; P < .04). Fractional glycogen activity significantly correlated with non-oxidative glucose disposal during insulin infusion (r = .69; P < .03). Prolonged reduction of hyperinsulinemia for 96 hours in cirrhotic patients normalizes insulin-mediated glucose uptake and glycogen synthesis in muscle. We conclude that chronic hyperinsulinemia causes insulin resistance in cirrhosis.

Bidirectional modulation of insulin action by amino acids.

Amino acids have been shown to stimulate protein synthesis, inhibit proteolysis, and decrease whole-body and forearm glucose disposal. Using cultured hepatoma and myotube cells, we demonstrate that amino acids act as novel signaling elements in insulin target tissues. Exposure of cells to high physiologic concentrations of amino acids activates intermediates important in the initiation of protein synthesis, including p70 S6 kinase and PHAS-I, in synergy with insulin. This stimulatory effect is largely due to branched chain amino acids, particularly leucine, and can be reproduced by its transamination product, ketoisocaproic acid. Concurrently, amino acids inhibit early steps in insulin action critical for glucose transport and inhibition of gluconeogenesis, including decreased insulin-stimulated tyrosine phosphorylation of IRS-1 and IRS-2, decreased binding of grb 2 and the p85 subunit of phosphatidylinositol 3-kinase to IRS-1 and IRS-2, and a marked inhibition of insulin-stimulated phosphatidylinositol 3-kinase. Taken together, these data support the hypothesis that amino acids act as specific positive signals for maintenance of protein stores, while inhibiting other actions of insulin at multiple levels. This bidirectional modulation of insulin action indicates crosstalk between hormonal and nutritional signals and demonstrates a novel mechanism by which nutritional factors contribute to insulin resistance.

The impact of genetic hypertension on insulin secretion and glucoregulatory control in vivo: studies with the TGR(mRen2)27 transgenic rat.

To examine the associations among hypertension, insulin secretion, glucose tolerance and insulin resistance in vivo. Glucose tolerance and insulin secretion during an intravenous glucose tolerance test and action of insulin on whole-body glucose kinetics in the post-absorptive state and during hyperinsulinaemia were examined in conscious, unrestrained TGR(mRen2)27 rats and age-matched transgene-negative controls. Glucose tolerance and insulin secretion were examined after intravenous administration of 500 mg glucose/kg body weight. Endogenous glucose production and whole-body glucose disposal were estimated using [3-3H]-glucose during euglycaemic-hyperinsulinaemic clamping. Muscle glucose utilization was estimated using 2-deoxy-[1-3H]-glucose. Despite there being higher insulin levels, whole-body glucose turnover was significantly lower in post-absorptive TGR(mRen2)27 rats than it was in transgene-negative controls. This was associated with significant suppression of glucose uptake/phosphorylation by oxidative skeletal muscles. TGR(mRen2)27 rats also exhibited significantly lower blood glucose levels, higher plasma insulin levels and higher rates of disappearance of glucose after intravenous administration of glucose. During hyperinsulinaemia, steady-state glucose infusion rates required to maintain euglycaemia in TGR(mRen2)27 rats were significantly greater, indicating that an increase in whole-body action of insulin had occurred. This was due to significantly greater suppression of endogenous production of glucose: insulin-stimulated glucose disposal rates did not differ significantly between the two groups. The results indicate that TGR(mRen2)27 rats have an enhanced and sensitized insulin-secretory response to glucose, together with a greater than normal hepatic action of insulin. Insulin-mediated glucose disposal was not impaired. The results therefore do not support the hypothesis that hypertension plays a primary role in the development of insulin resistance.

Selective impairment in GLUT4 translocation to transverse tubules in skeletal muscle of streptozotocin-induced diabetic rats.

We previously reported that insulin induces the translocation of GLUT4 to both the plasma membrane and the transverse tubules (T-tubules) in rat skeletal muscle (Am J Physiol 270:E667-E676, 1996). The aim of the present study was to investigate whether the insulin-resistant glucose utilization of skeletal muscle from streptozotocin (STZ)-induced diabetic rats is linked to an impaired translocation of GLUT4 to the plasma membrane, the T-tubules, or both surface compartments. Whole-body insulin-mediated glucose disposal, assessed during a hyperinsulinemic-euglycemic clamp, was reduced by 48% (P < 0.01) in diabetic rats as compared with controls. Subcellular membrane fractions enriched with plasma membranes, T-tubules, or GLUT4-enriched intracellular membranes were isolated from hindlimb muscles of control and insulin-stimulated rats, and GLUT4 content was measured by Western blot analysis. In the absence of insulin (unstimulated), GLUT4 content in muscle of diabetic rats was markedly lower (by approximately 40%) in both the T-tubules and the intracellular membrane fraction as compared with controls. In contrast, the transporter protein levels were similar in the plasma membrane fraction. In skeletal muscle of control animals, the hyperinsulinemic clamp induced GLUT4 translocation from the intracellular membrane pool to both the plasma membrane and the T-tubule-enriched fractions (approximately 2.2-fold to approximately 2.5-fold). Surprisingly, insulin increased plasma membrane GLUT4 content to comparable levels in control and diabetic rat skeletal muscle. However, insulin-mediated GLUT4 translocation to the T-tubules was significantly reduced in the same muscle. Whole-body insulin action was significantly correlated with GLUT4 protein levels in the T-tubules, but not with the transporter content in either plasma membranes or intracellular membranes. These results strongly suggest that peripheral resistance to insulin action on glucose disposal in STZ-induced diabetic rats is caused by a selective impairment of GLUT4 translocation to skeletal muscle T-tubules.

Insulin resistance and advancing age: What role for dehydroepiandrosterone sulfate?

The relationship between insulin resistance and aging is still debated. This study aims to investigate the role that age-related differences in plasma dehydroepiandrosterone sulfate (DHEAS) concentration may have on insulin action. For this reason, 75 subjects (42 men and 33 women) with a wide age range (21 to 106 years) were studies. In all subjects, plasma DHEAS and total testosterone concentrations were measured and a euglycemic clamp was used, but substrate oxidation was not determined in centenarians (n = 15). Plasma DHEAS correlated with age ( r = −.77, P < .001) and whole-body glucose disposal (WBGD) ( r = .57, P < .001). After controlling for age, sex, body fat, and waist to hip ratio (WHR), the association between plasma DHEAS and WBGD was still observed ( r = .31, P < .005). Comparing subjects at the third tertile versus those at the first and second tertiles of plasma age-adjusted DHEAS concentration, the former group showed a weaker association between WBGD and age ( r = −.38, P < .05) that the latter group ( r = −.43, P < .002). The difference between the two regression lines was also significant ( P < .03). After controlling for sex, body fat, and WHR, the association between plasma DHEAS and WBGD was dependent on the age of the subjects, being strong in adults (n = 30, age < 50 years, r = .69, P < .001), weak in old subjects throughout the different age groups, the univariate association between plasma DHEAS and WBGD was present in females ( r = .43, P < .01) but not in males ( r = .17, P < .32). Plasma total testosterone and insulin-like growth factor-1 (IGF-1) concentrations declined with advancing age and were significantly correlated with DHEAS and WBGD. In a multivariate analysis with WBGD as the dependent variable, a model including age, sex, body fat, WHR, DHEAS, total testosterone, and IGF-1 explained 66% of WBGD variability, with DHEAS significantly and independently associated with WBGD ( P < .004). In conclusion, the negative relationship between advancing age and insulin action seems related to plasma DHEAS concentration. Differences in plasma total testosterone and IGF-1 concentrations may provide a further contribution to the relationship between DHEAS and WBGD.

Myocardial Wall Thickness and Left Ventricular Geometry in Hypertensives: Relationship With Insulin

In hypertensive patients the presence of left ventricular (LV) hypertrophy has been associated with a more severe degree of insulin resistance. Whether myocardial wall thickness or LV geometry are associated with a different degree of insulin resistance is still unknown in essential hypertensives. For this reason 26 men with new diagnosed essential hypertension were enrolled. All patients underwent echocardiographic examination and euglycemic hyperinsulinemic glucose clamp combined with indirect calorimetry. According to LV mass and relative wall thickness data, all patients were categorized in four groups: 1) patients with a normal geometric LV pattern (n = 8) (PAT = 0); 2) patients with concentric remodeling LV mass (n = 8) (PAT = 1); 3) patients with eccentric LV hypertrophy (n = 3) (PAT = 2); and 4) patients with concentric LV hypertrophy (n = 7) (PAT = 3). All groups were similar for anthropometric characteristics. Patients with normal echocardiographic LV pattern (PAT = 0) had higher whole body glucose disposal (WBGD), oxidative and nonoxidative glucose metabolism, and lower lipid oxidation than patients with abnormal echocardiographic LV patterns (PAT = 1 to 3). Nevertheless, no significant differences among the groups with abnormal echocardiographic patterns were found. After controlling for age, body mass index (BMI), waist/hip ratio (WHR), and mean arterial blood pressure, only sum of the wall thickness was significantly correlated with fasting plasma insulin (r = −0.38, P < .05), WBGD (r = − 0.50, P < .009), and NOGM (r = − 0.48, P < .02). In multivariate analysis, a model made by age, BMI, WHR, systolic and diastolic blood pressure, and WBGD explained 38% of the echocardiographic pattern variability. In this model, WBGD ( P < .02) was significantly and independently associated with echocardiographic patterns explaining 19% of the echocardiographic pattern variability. In conclusion, our data demonstrate that in arterial hypertension hyperinsulinemia/insulin resistance mainly affects myocardial wall thickness, whereas only a trivial association with LV geometry occurs.

L-arginine but not d-arginine stimulates insulin-mediated glucose uptake

Our study aims at investigating a possible role for l-arginine and d-arginine in insulin-mediated glucose uptake. Twelve lean healthy subjects volunteered for the study and were submitted to three euglycemic-hyperinsulinemic glucose clamps to investigate the effect of l-arginine (0.5 g/min in the last 60 minutes of the clamp), d-arginine (0.5 g/min in the last 60 minutes of the clamp), and saline 0.9% NaCl on insulin-mediated glucose uptake. All tests were made in random order. In study 1, l-arginine versus saline infusion was associated with a significant increase in blood flow (131% ± 7% v 87% ± 5%, P < .001) and whole-body glucose disposal ([WBGD] 61.4 ± 4.4 v 41.3 ± 3.5 μmol/kg fat-free mass [FFM] · min, P < .001). Analysis of substrate oxidation demonstrated that both oxidative and nonoxidative glucose metabolism was improved by l-arginine delivery. After adjustment for the change in blood flow, WBGD was still greater after l-arginine than after saline infusion. Along with l-arginine infusion and independently of the change in blood flow, the percent change in WBGD correlated with the percent change in plasma cGMP ( r = .56, P < .05). d-Arginine infusion did not affect insulin-mediated glucose uptake. In particular, WBGD (42.1 ± 3.4 v 41.3 ± 3.5 μmol/kg FFM · min, P = NS) was similar in both experimental conditions. Basal levels (2.8 ± 0.2 v 2.7 ± 0.3 nmol/L, P = NS) and the insulin-mediated increase (43% ± 5% v 39% ± 4%, P = NS) in plasma cGMP were also superimposable along with insulin plus d-arginine and insulin alone, respectively. Finally, blood flow (224 ± 29 v 230 ± 35 mL/min, P = NS) was not different at baseline and was similarly stimulated (84% ± 4% v 87% ± 5%, P = NS) by insulin infusion. In conclusion, l-arginine but not d-arginine stimulates insulin-mediated glucose uptake. Nitric oxide (NO), the metabolic mediator for l-arginine, potentiates insulin-mediated glucose uptake through the increase in blood flow. Nevertheless, an independent effect of intracellular cGMP on WBGD cannot be ruled out.