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.     

Insulin impairs endothelium-dependent vasodilation independent of insulin sensitivity or lipid profile

Insulin resistance is a risk factor for atherosclerosis and is associated with hyperinsulinemia, abnormal lipid profile, and hypertension. Whether hyperinsulinemia affects vascular function independent of insulin resistance or other metabolic risk factors is unknown. This investigation aimed to assess the effects of hyperinsulinemia on endothelial function in subjects with a spectrum of insulin sensitivity and lipid profile. Endothelium-dependent (flow-mediated dilation, FMD) and -independent (nitroglycerin) responses of the brachial artery were studied by high-resolution ultrasound before and during hyperinsulinemia (euglycemic clamp) in 25 normoglycemic, normotensive subjects. Participants were divided into an insulin-sensitive and an insulin-resistant subgroup based on their sensitivity index values, with a cutoff of 8, and into a normal-cholesterol and a high-cholesterol subgroup based on their total cholesterol levels, with a cutoff of 5.2 mmol/l (200 mg/dl). In the whole population, FMD was lower during hyperinsulinemia compared with baseline (2.3 +/- 0.6% vs. 6 +/- 0.6%; P < 0.001). Resting FMD was lower in the insulin-resistant subgroup compared with the insulin-sensitive subgroup (4.2 +/- 0.9% vs. 7.4 +/- 0.8%; P = 0.014) and in the high-cholesterol subjects compared with the normal-cholesterol subjects (4.4 +/- 0.7% vs. 8 +/- 0.7%; P = 0.002). Hyperinsulinemia decreased FMD in both the insulin-sensitive (from 7.4 +/- 0.8% to 3.6 +/- 0.4%; P < 0.001) and insulin-resistant (from 4.2% to 1.22%; P = 0.012) subgroups and in both the normal-cholesterol (from 8 +/- 0.7% to 3.9 +/- 0.4%; P < 0.001) and high-cholesterol (from 4.4 +/- 0.7% to 1.1 +/- 0.8%; P = 0.01) participants. Acute hyperinsulinemia impairs conduit vessel endothelial function independent of insulin sensitivity and lipid profile. Insulin may trigger endothelial dysfunction and promote atherosclerosis.     

Digitalis-like factor response to hyperinsulinemia accompanying a euglycemic hyperinsulinemic clamp or oral glucose tolerance test

Many studies of essential hypertension find evidence of insulin resistance in the same individuals, leading some to postulate a hypertensive role for insulin. However, the mechanisms by which insulin might exert a hypertensive effect are not fully resolved. An endogenous sodium pump inhibitor or digitalis-like factor (DLF) has been proposed as a hypertensive agent and its plasma concentrations are elevated in hypertension and in Type II diabetes, where insulin levels are elevated. Hence, we studied the effect of insulin on DLF using two approaches to achieve hyperinsulinemia. Normotensive men and women underwent a hyperinsulinemic, euglycemic clamp (40 mU/m2/min insulin, 40 mU = 1.6 x 10(-6) g) in which plasma insulin concentration was kept at high, but physiologic levels. Serum DLF (measured as inhibition of [Na,K]ATPase activity) and insulin levels were measured at baseline and every 30 min throughout the 2 hr clamp. Additionally, other subjects underwent an oral glucose tolerance test (OGTT) as a second means of increasing insulin levels. Insulin and DLF levels were measured prior to and hourly for 3 hours after receiving 100 gm of oral glucose. Serum DLF increased significantly during the clamp from a baseline of 4.6 +/- 0.81 to a peak of 8.7 +/- 1.2% inhibition (p=0.001). Comparison of the baseline and peak DLF levels with concomitant plasma insulin levels revealed a significant correlation (R=0.60, p=0.003). During the OGTT, DLF levels rose from a baseline of 2.4 +/- 1.0 to a peak level of 5.0 +/- 0.4%, p = 0.04. These results suggest that DLF, a factor that can cause vascular smooth muscle contraction and potentially influence blood pressure, is increased by hyperinsulinemia and provides a mechanism by which insulin may increase blood pressure.     

Intravenous estrogens increase insulin clearance and action in postmenopausal women

To test the hypothesis that estrogens alter insulin action, we evaluated the effects of intravenous conjugated estrogens (CE) on insulin-stimulated steady-state glucose infusion rate (SSGIR) and suppression of plasma glycerol in postmenopausal women (mean +/- SD; 56 +/- 4 yr; n = 12) not using hormone replacement. SSGIR and glycerol were measured during a two-stage (8 and 40 mU. m-2. min-1) hyperinsulinemic euglycemic clamp on 2 days, with or without a 2.5-mg intravenous CE bolus. Serum estradiol concentrations were increased approximately 200% on the estrogen (EST) compared with the control (CON) days. Serum insulin was reduced (P < 0.01) during stage 2 of the clamp for EST (63.3 +/- 12.8 micro U/ml) vs. CON (78.2 +/- 15.8 micro U/ml). Mean SSGIR and plasma glycerol did not differ between CON and EST days. With adjustment for differences in insulin concentration between conditions, stage 2 glucose disposals were significantly higher (8.63 vs. 7.20 mg. kg-1. min-1) and plasma glycerol concentrations were significantly lower (29.4 vs. 35.0 micro mol/l) for EST vs. CON. Our findings suggest that acute CE administration increases insulin clearance and action in postmenopausal women.     

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.     

The pancreatic glucagon and C-peptide secretion during hyperinsulinemia in euglycemic glucose clamp with or without somatostatin infusion in normal man

6 normal subjects received two times of 2 hr euglycemic glucose clamp studies (insulin infusion rate 40 mU/M2/min) one with and the other without somatostatin (SRIF) infusion (500 microgram/hr). Serum C-peptide and glucagon levels were measured during clamp to study the sensitivity of pancreatic alpha and beta cells to the suppressive effects of exogenous hyperinsulinemia during normoglycemia in normal subjects and to find whether SRIF had any modulative effects on endocrine pancreas secretion at the status of hyperinsulinemia. The results showed that in normal man the degree of suppression of pancreatic glucagon secretion by hyperinsulinemia (approximately 100 uU/ml) during euglycemic glucose clamp without SRIF infusion was less than that of C-peptide with mean value of 62 +/- 4% of basal glucagon remained at the end of clamp study; while only about 30 +/- 2% of basal C-peptide concentrations remained. But during SRIF infused glucose clamp studies (SRIF was infused from 60 to 120 min), 32 +/- 2% of mean basal C-peptide concentrations and 38 +/- 6% of mean basal glucagon concentrations left at the end of 2 hr clamp studies when serum insulin level was about 100 uU/ml. For the glucose infusion rate (M value), it was significantly greater in our normal subjects in response to insulin + SRIF as compared to insulin alone (12.0 + 0.9 vs 8.8 +/- 1.4; P less than 0.01). We concluded: during hyperinsulinemia (100 uU/ml), the sensitivity of pancreatic alpha cells to insulin seems less than that of beta cells in normal man at normoglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Previous exercise attenuates muscle sympathetic activity and increases blood flow during acute euglycemic hyperinsulinemia.

Insulin infusion causes muscle vasodilation, despite the increase in sympathetic nerve activity. In contrast, a single bout of exercise decreases sympathetic activity and increases muscle blood flow during the postexercise period. We tested the hypothesis that muscle sympathetic activity would be lower and muscle vasodilation would be higher during hyperinsulinemia performed after a single bout of dynamic exercise. Twenty-one healthy young men randomly underwent two hyperinsulinemic euglycemic clamps performed after 45 min of seated rest (control) or bicycle exercise (50% of peak oxygen uptake). Muscle sympathetic nerve activity (MSNA, microneurography), forearm blood flow (FBF, plethysmography), blood pressure (BP, oscillometric method), and heart rate (HR, ECG) were measured at baseline (90 min after exercise or seated rest) and during hyperinsulinemic euglycemic clamps. Baseline glucose and insulin concentrations were similar in the exercise and control sessions. Insulin sensitivity was unchanged by previous exercise. During the clamp, insulin levels increased similarly in both sessions. As expected, insulin infusion increased MSNA, FBF, BP, and HR in both sessions (23 +/- 1 vs. 36 +/- 2 bursts/min, 1.8 +/- 0.1 vs. 2.2 +/- 0.2 ml.min(-1).100 ml(-1), 89 +/- 2 vs. 92 +/- 2 mmHg, and 58 +/- 1 vs. 62 +/- 1 beats/min, respectively, P < 0.05). BP and HR were similar between sessions. However, MSNA was significantly lower (27 +/- 2 vs. 31 +/- 2 bursts/min), and FBF was significantly higher (2.2 +/- 0.2 vs. 1.8 +/- 0.1 ml.min(-1).100 ml(-1), P < 0.05) in the exercise session compared with the control session. In conclusion, in healthy men, a prolonged bout of dynamic exercise decreases MSNA and increases FBF. These effects persist during acute hyperinsulinemia performed after exercise.     

Sulfonylurea therapy fails to diminish insulin resistance in type I-diabetic subjects

To assess whether extrapancreatic effects of sulfonylureas in vivo are detectable in the absence of endogenous insulin secretion, insulin sensitivity was determined in six insulin-deficient type 1-diabetic subjects. Peripheral uptake and hepatic production of glucose and lipolysis were measured during hyperinsulinemia using the euglycemic clamp technique and 3-3H-glucose infusions twice, once during a period with glibornuride treatment (50 mg b.i.d.), and once without. Hepatic glucose production decreased in diabetic subjects during hyperinsulinemia (insulin infusion of 20 mU/m2 X min; plasma free insulin levels of 40 +/- 4 mU/l) from 2.9 +/- 0.6 mg/kg min to 0.2 +/- 0.1 mg/kg X min after 120 min, and plasma free fatty acid (FFA) concentrations decreased from 1.33 +/- 0.29 to 0.38 +/- 0.08 mmol/l. Hepatic production, peripheral uptake of glucose and plasma FFA concentrations before and during hyperinsulinemia were not influenced by pretreatment with glibornuride. Compared to 8 non-diabetic subjects, type 1-diabetics demonstrated a diminished effect of hyperinsulinemia on peripheral glucose clearance (2.4 +/- 0.04 vs 4.2 +/- 0.5 ml/kg X min, P less than 0.01), whereas hepatic glucose production and plasma FFA levels were similarly suppressed by insulin. The data indicate that sulfonylurea treatment did not improve the diminished insulin sensitivity of peripheral glucose clearance in type 1-diabetic subjects; insulin action on hepatic glucose production and lipolysis was unimpaired in diabetics and remained uninfluenced by glibornuride. Thus, extrapancreatic effects of sulfonylureas in vivo are dependent on the presence of functioning beta-cells.     

Hyperglycemia prevents the suppressive effect of hyperinsulinemia on plasma adiponectin levels in healthy humans

Adiponectin is a fat-derived hormone with insulin-sensitizing properties. In patients with type 2 diabetes plasma adiponectin levels are decreased. Since these patients are characterized by high plasma insulin and glucose concentrations, hyperinsulinemia and hyperglycemia could be responsible for the downregulation of adiponectin. Insulin decreases adiponectin levels in humans. The effect of hyperglycemia is unknown. To determine the selective effects of insulin, glucose, or their combination on plasma adiponectin, clamps were performed in six healthy males on four occasions in a crossover design: 1) lower insulinemic-euglycemic clamp (100 pmol/l insulin, 5 mmol/l glucose) (reference clamp); 2) hyperinsulinemic-euglycemic clamp (400 pmol/l insulin, 5 mmol/l glucose); 3) lower insulinemic-hyperglycemic clamp (100 pmol/l insulin, 12 mmol/l glucose); and 4) hyperinsulinemic-hyperglycemic clamp (400 pmol/l insulin, 12 mmol/l glucose). Adiponectin concentrations and high-molecular-weight (HMW)-to-total adiponectin ratio were measured at the start and end of the 6-h clamps. After the 6-h study period, total plasma adiponectin levels were significantly (P = 0.045) decreased by 0.63 microg/ml in the lower insulinemic-euglycemic clamp (clamp 1). In both euglycemic groups (clamps 1 and 2) adiponectin concentrations significantly declined (P = 0.016) over time by 0.56 microg/ml, whereas there was no change in both hyperglycemic groups (clamps 3 and 4) (P = 0.420). In none of the clamps did the ratio of HMW to total adiponectin change. We conclude that insulin suppresses plasma adiponectin levels already at a plasma insulin concentration of 100 pmol/l. Hyperglycemia prevents the suppressive effect of insulin. This suggests that, in contrast to glucose, insulin could be involved in the downregulation of plasma adiponectin in insulin-resistant patients.     

Evidence against a role for insulin-signaling proteins PI 3-kinase and Akt in insulin resistance in human skeletal muscle induced by short-term GH infusion

Prolonged growth hormone (GH) excess is known to be associated with insulin resistance, but the underlying mechanisms remain unknown. The aim of this study was to assess the impact of GH on insulin-stimulated glucose metabolism and insulin signaling in human skeletal muscle. In a cross-over design, eight healthy male subjects (age 26.0 +/- 0.8 yr and body mass index 24.1 +/- 0.5 kg/m2) were infused for 360 min with either GH (Norditropin, 45 ng.kg(-1).min(-1)) or saline. During the final 180 min of the infusion, a hyperinsulinemic euglycemic clamp was performed (insulin infusion rate: 1.2 mU.kg(-1).min(-1)). Muscle biopsies from vastus lateralis were taken before GH/saline administration and after 60 min of hyperinsulinemia. GLUT4 content and insulin signaling, as assessed by insulin receptor substrate (IRS)-1-associated phosphatidylinositol 3-kinase and Akt activity were determined. GH levels increased to a mean (+/-SE) level of 20.0 +/- 2.3 vs. 0.5 +/- 0.2 microg/l after saline infusion (P < 0.01). During GH infusion, the glucose infusion rate during hyperinsulinemia was reduced by 38% (P < 0.01). In both conditions, free fatty acids were markedly suppressed during hyperinsulinemia. Despite skeletal muscle insulin resistance, insulin still induced a similar approximately 3-fold rise in IRS-1-associated PI 3-kinase activity (269 +/- 105 and 311 +/- 71% compared with baseline, GH vs. saline). GH infusion did not change Akt protein expression, and insulin caused an approximately 13-fold increase in Akt activity (1,309 +/- 327 and 1,287 +/- 173%) after both GH and saline infusion. No difference in total GLUT4 content was noted (114.7 +/- 7.4 and 107.6 +/- 16.7 arbitrary units, GH vs. saline, compared with baseline). In conclusion, insulin resistance in skeletal muscle induced by short-term GH administration is not associated with detectable changes in the upstream insulin-signaling cascade or reduction in total GLUT4. Yet unknown mechanisms in insulin signaling downstream of Akt may be responsible.     

Estimations of muscle interstitial insulin, glucose, and lactate in type 2 diabetic subjects

Previous measurement of insulin in human muscle has shown that interstitial muscle insulin and glucose concentrations are approximately 30-50% lower than in plasma during hyperinsulinemia in normal subjects. The aims of this study were to measure interstitial muscle insulin and glucose in patients with type 2 diabetes to evaluate whether transcapillary transport is part of the peripheral insulin resistance. Ten patients with type 2 diabetes and ten healthy controls matched for sex, age, and body mass index were investigated. Plasma and interstitial insulin, glucose, and lactate (measured by intramuscular in situ-calibrated microdialysis) in the medial quadriceps femoris muscle were analyzed during a hyperinsulinemic euglycemic clamp. Blood flow in the contralateral calf was measured by vein plethysmography. At steady-state clamping, at 60-120 min, the interstitial insulin concentration was significantly lower than arterial insulin in both groups (409 +/- 86 vs. 1,071 +/- 99 pmol/l, P < 0.05, in controls and 584 +/- 165 vs. 1, 253 +/- 82 pmol/l, P < 0.05, in diabetic subjects, respectively). Interstitial insulin concentrations did not differ significantly between diabetic subjects and controls. Leg blood flow was significantly higher in controls (8.1 +/- 1.2 vs. 4.4 +/- 0.7 ml. 100 g(-1).min(-1) in diabetics, P < 0.05). Calculated glucose uptake was less in diabetic patients compared with controls (7.0 +/- 1.2 vs. 10.8 +/- 1.2 micromol. 100 g(-1).min(-1), P < 0.05, respectively). Arterial and interstitial lactate concentrations were both higher in the control group (1.7 +/- 0.1 vs. 1.2 +/- 0.1, P < 0. 01, and 1.8 +/- 0.1 vs. 1.2 +/- 0.2 mmol/l, P < 0.05, in controls and diabetics, respectively). We conclude that, during hyperinsulinemia, muscle interstitial insulin and glucose concentrations did not differ between patients with type 2 diabetes and healthy controls despite a significantly lower leg blood flow in diabetic subjects. It is suggested that decreased glucose uptake in type 2 diabetes is caused by insulin resistance at the cellular level rather than by a deficient access of insulin and glucose surrounding the muscle cell.     

Insulin action during late pregnancy in the conscious dog

Our aim was to assess the magnitude of peripheral insulin resistance and whether changes in hepatic insulin action were evident in a canine model of late (3rd trimester) pregnancy. A 3-h hyperinsulinemic (5 mU.kg(-1).min(-1)) euglycemic clamp was conducted using conscious, 18-h-fasted pregnant (P; n = 6) and nonpregnant (NP; n = 6) female dogs in which catheters for intraportal insulin infusion and assessment of hepatic substrate balances were implanted approximately 17 days before experimentation. Arterial plasma insulin rose from 11 +/- 2 to 192 +/- 24 and 4 +/- 2 to 178 +/- 5 microU/ml in the 3rd h in NP and P, respectively. Glucagon fell equivalently in both groups. Basal net hepatic glucose output was lower in NP (1.9 +/- 0.1 vs. 2.4 +/- 0.2 mg.kg(-1).min(-1), P < 0.05). Hyperinsulinemia completely suppressed hepatic glucose release in both groups (-0.4 +/- 0.2 and -0.1 +/- 0.2 mg.kg(-1).min(-1) in NP and P, respectively). More exogenous glucose was required to maintain euglycemia in NP (15.2 +/- 1.3 vs. 11.5 +/- 1.1 mg.kg(-1).min(-1), P < 0.05). Nonesterified fatty acids fell similarly in both groups. Net hepatic gluconeogenic amino acid uptake with high insulin did not differ in NP and P. Peripheral insulin action is markedly impaired in this canine model of pregnancy, whereas hepatic glucose production is completely suppressed by high circulating insulin levels.     

Alterations in liver, muscle, and adipose tissue insulin sensitivity in men with HIV infection and dyslipidemia

Dyslipidemia is common in patients with HIV infection. In this study, a two-stage euglycemic hyperinsulinemic clamp, with infusion of stable isotopically labeled tracers, was used to evaluate insulin action in skeletal muscle, liver, and adipose tissue in HIV-infected men with dyslipidemia (HIV-DL; plasma triglyceride >250 mg/dl and HDL <45 mg/dl; n=12), HIV-infected men without dyslipidemia (HIV w/o DL; n=12), and healthy men (n=6). Basal rates of glucose production (glucose R(a)), glucose disposal (glucose R(d)), and lipolysis (palmitate R(a)) were similar between groups. The relative suppression of glucose R(a) (63+/- 4, 77+/- 2, and 78+/- 3%, P=0.008) and palmitate R(a) (49+/-4, 63+/-3, and 68+/-3%, P=0.005) during ow-dose insulin infusion (plasma insulin approximately 30 microU/ml), and the relative stimulation of glucose R(d) (214+/-21, 390+/-25, and 393+/-46%, P=0.001) during high-dose insulin infusion (plasma insulin approximately 75 microU/ml) were lower in HIV-DL than in HIV w/o DL and healthy volunteers, respectively. Suppression of basal glucose R(a) correlated with plasma adiponectin (r=0.44, P=0.02) and inversely with plasma IL-6 (r=-0.49, P<0.001). Stimulation of glucose R(d) correlated directly with adiponectin (r=0.48, P<0.01) and inversely with IL-6 (r=-0.49, P=0.02). We conclude that dyslipidemia in HIV-infected men is indicative of multiorgan insulin resistance, and circulating adipokines may be important in the pathogenesis of impaired insulin action.     

The influence of insulin on circulating ghrelin

Ghrelin is a novel peptide that acts on the growth hormone (GH) secretagogue receptor in the pituitary and hypothalamus. It may function as a third physiological regulator of GH secretion, along with GH-releasing hormone and somatostatin. In addition to the action of ghrelin on the GH axis, it appears to have a role in the determination of energy homeostasis. Although feeding suppresses ghrelin production and fasting stimulates ghrelin release, the underlying mechanisms controlling this process remain unclear. The purpose of this study was to test the hypotheses, by use of a stepped hyperinsulinemic eu- hypo- hyperglycemic glucose clamp, that either hyperinsulinemia or hypoglycemia may influence ghrelin production. Having been stable in the period before the clamp, ghrelin levels rapidly fell in response to insulin infusion during euglycemia (baseline ghrelin 207 +/- 12 vs. 169 +/- 10 fmol/ml at t = 30 min, P < 0.001). Ghrelin remained suppressed during subsequent periods of hypoglycemia (mean glucose 53 +/- 2 mg/dl) and hyperglycemia (mean glucose 163 +/- 6 mg/dl). Despite suppression of ghrelin, GH showed a significant rise during hypoglycemia (baseline 4.1 +/- 1.3 vs. 28.2 +/- 3.9 microg/l at t = 120 min, P < 0.001). Our data suggest that insulin may suppress circulating ghrelin independently of glucose, although glucose may have an additional effect. We conclude that the GH response seen during hypoglycemia is not regulated by circulating ghrelin.     

Adiponectin expression in adipose tissue is reduced in first-degree relatives of type 2 diabetic patients

Adiponectin is suggested to be an important mediator of insulin resistance. Therefore, we investigated the association between adiponectin and insulin sensitivity in 22 healthy first-degree relatives (FDR) to type 2 diabetic patients and 13 matched control subjects. Subcutaneous adipose tissue biopsies were taken before and after a hyperinsulinemic euglycemic clamp. FDR subjects were insulin resistant, as indicated by a reduced M value (4.44 vs. 6.09 mg x kg(-1) x min(-1), P < 0.05). Adiponectin mRNA expression was 45% lower in adipose tissue from FDR compared with controls (P < 0.01), whereas serum adiponectin was similar in the two groups (6.4 vs. 6.6 microg/ml, not significant). Insulin infusion reduced circulating levels of adiponectin moderately (11-13%) but significantly in both groups (P < 0.05). In the control group, adiponectin mRNA levels were negatively correlated with fasting insulin (P < 0.05) and positively correlated with insulin sensitivity (P < 0.05). In contrast, these associations were not found in the FDR group. In conclusion, FDR have reduced adiponectin mRNA in subcutaneous adipose tissue but normal levels of circulating adiponectin. Adiponectin mRNA levels are positively correlated with insulin sensitivity in control subjects but not in FDR. These findings indicate dysregulation of adiponectin gene expression in FDR.     

Insulin sensitivity by oral glucose minimal models: validation against clamp

Measuring insulin sensitivity in the presence of physiological changes in glucose and insulin concentrations, e.g., during a meal or OGTT, is important to better understand insulin resistance in a variety of metabolic conditions. Recently, two oral minimal models have been proposed to measure overall insulin sensitivity (S(I)) and its selective effect on glucose disposal (S(I)*) from oral tests. S(I) and S(I)* have been successfully validated against multiple tracer meal estimates, but validation against euglycemic hyperinsulinemic clamp estimates is lacking. Here, we do so in 21 subjects who underwent both a multiple-tracer OGTT and a labeled euglycemic hyperinsulinemic clamp. Correlation between minimal-model S(I), S(I) and corresponding clamp estimates S(I)(*clamp), S(I)(*clamp) was satisfactory, respectively r = 0.81, P < 0.001, and r = 0.71, P < 0.001. S(I) was significantly lower than S(I)(clamp) (8.08 +/- 0.89 vs. 13.66 +/- 1.69 10(-4) dl.kg(-1).min(-1) per microU/ml, P = 0.0002), whereas S(I) and S(I)(*clamp) were very similar (8.17 +/- 1.59 vs. 8.84 +/- 1.39 10(-4) dl.kg(-1).min(-1) per microU/ml, P = 0.52). These results add credibility to the oral minimal-model method as a simple and reliable physiological tool to estimate S(I) and S(I)*, also in large-scale clinical trials.     

Insulin prolongs the QTc interval in humans

Insulin hyperpolarizes plasma membranes; we tested whether insulin affects ventricular repolarization. In 35 healthy volunteers, we measured the Q-T interval during electrocardiographic monitoring in the resting state and in response to hyperinsulinemia (euglycemic 1-mU. min(-1). kg(-1) insulin clamp). A computerized algorithm was used to identify T waves; Bazett's formula was employed to correct Q-T (QTc) by heart rate (HR). In the resting state, QTc was inversely related to indexes of body size (e.g., body surface area, r = -0.53, P = 0.001) but not to indexes of body fatness. During the clamp, HR (67 +/- 1 to 71 +/- 1 beats/min, P < 0.0001) and plasma norepinephrine levels (161 +/- 12 to 184 +/- 10 pg/ml, P < 0.001) increased. QTc rose promptly and consistently, averaging 428 +/- 6 ms between 30 and 100 min (P = 0.014 vs. the resting value of 420 +/- 5 ms). Fasting serum potassium (3.76 +/- 0.03 mM) declined to 3. 44 +/- 0.03 mM during insulin. After adjustment for body size, resting QTc was directly related to fasting plasma insulin (partial r = 0.43, P = 0.01); furthermore, QTc was inversely related to serum potassium levels both in the fasting state (partial r = -0.16, P < 0. 04) and during insulin stimulation (partial r = -0.47, P = 0.003). Neither resting nor clamp-induced QTc was related to insulin sensitivity. Physiological hyperinsulinemia acutely prolongs ventricular repolarization independent of insulin sensitivity. Both insulin-induced hypokalemia and adrenergic activation contribute to this effect.     

Insulin resistance,intramyocellular lipid content,and plasma adiponectin in patients with type 1 diabetes

Insulin resistance is a key pathogenic factor of type 2 diabetes (T2DM); in contrast, in type 1 diabetes (T1DM) it is considered a secondary alteration. Increased intramyocellular lipid (IMCL) content accumulation and reduced plasma adiponectin were suggested to be pathogenic events of insulin resistance in T2DM. This study was designed to assess whether IMCL content and plasma adiponectin were also associated with the severity of insulin resistance in T1DM. We studied 18 patients with T1DM, 7 older and overweight/obese patients with T2DM, and 15 nondiabetic, insulin-resistant offspring of T2DM parents (OFF) and 15 healthy individuals (NOR) as appropriate control groups matched for anthropometric features with T1DM patients by means of the euglycemic hyperinsulinemic clamp combined with the infusion of [6,6-2H2]glucose and 1H magnetic resonance spectroscopy of the calf muscles. T1DM and T2DM patients showed reduced insulin-stimulated glucose metabolic clearance rate (MCR: 5.1 +/- 0.6 and 3.2 +/- 0.8 ml x kg(-1) min(-1)) similar to OFF (5.3 +/- 0.4 ml x kg(-1) x min(-1)) compared with NOR (8.5 +/- 0.5 ml x kg(-1) min(-1), P < 0.001). Soleus IMCL content was increased in T1DM (112 +/- 15 AU), T2DM (108 +/- 10 AU) and OFF (82 +/- 13 AU) compared with NOR (52 +/- 7 AU, P < 0.05) and the result was inversely proportional to the MCR (R2 = 0.27, P < 0.001); an association between IMCL content and Hb A1c was found only in T1DM (R2 = 0.57, P < 0.001). Fasting plasma adiponectin was reduced in T2DM (7 +/- 1 microg/ml, P = 0.01) and OFF (11 +/- 1 microg/ml, P = 0.03) but not in T1DM (25 +/- 6 microg/ml), whose plasma level was increased with respect to both OFF (P = 0.03) and NOR (16 +/- 2 microg/ml, P = 0.05). In conclusion, in T1DM, T2DM, and OFF, IMCL content was associated with insulin resistance, demonstrating that IMCL accretion is a marker of insulin resistance common to both primary genetically determined and secondary metabolic (chronic hyperglycemia) alterations. The increased adiponectin levels in insulin-resistant patients with T1DM, in contrast to the reduced levels found in patients with T2DM and in OFF, demonstrated that the relationship of adiponectin to insulin resistance in humans is still unclear.     

Hyperinsulinemia and oxidative stress

The aim of the study was to compare the effect of short-term hyperglycemia and short-term hyperinsulinemia on parameters of oxidative stress in Wistar rats. Twenty male rats (aged 3 months, average weight 325 g) were tested by hyperinsulinemic clamp (100 IU/l) at two different glycemia levels (6 and 12 mmol/l). Further 20 rats were used as a control group infused with normal saline (instead of insulin) and 30 % glucose simultaneously. Measured parameters of oxidative stress were malondialdehyd (MDA), reduced glutathion (GSH) and total antioxidant capacity (AOC). AOC remained unchanged during hyperglycemia and hyperinsulinemia. Malondialdehyde (as a marker of lipid peroxidation) decreased significantly (p<0.05) during the euglycemic hyperinsulinemic clamp, and increased significantly during isolated hyperglycemia without hyperinsulinemia. Reduced glutathion decreased significantly (p<0.05) during hyperglycemia without hyperinsulinemia. These results suggest that the short-term exogenous hyperinsulinemia reduced the production of reactive oxygen species (ROS) during hyperglycemia in an animal model compared with the control group.     

Diminution of hypertriglyceridemia-induced pressor effect under hyperinsulinemic condition in normal and fructose-induced insulin resistant rats

This study aimed to evaluate the effect of hyperinsulinemia on hypertriglyceridemia-induced pressor response in normal and fructose-induced insulin resistant rats. The rats were divided into six groups of eight rats and were fed a fructose-enriched diet (FINs, F(INS+TG)) or a regular chow diet (C, C(TG), C(INS), C(INS+TG)) for 8 wks. The acute experiment was conducted at the end of wk 8 and consisted of a 30-min basal period and followed by a 120-min test period. After the basal period, somatostatin (1.3 microg/kg/ min) combined with regular insulin (0.6 or 4 mU/kg/min) and variable glucose infusion were given to clamp euglycemia and euinsulinemia in C and C(TG) or euglycemia and hyperinsulinemia in CINs, C(INS+TG), F(INS) and F(INS+TG). During test period, lipofundin (a triglyceride emulsion) was infused into CTG, C(INS+TG), F(INS+TG) and saline instead was infused into C, C(INS), FINS. Plasma insulin and triglyceride levels were significantly higher in fructose-fed rats than in normal rats. During the test period, the lipofundin infusion (1.2 ml/kg/hr) increased plasma triglyceride levels by 368 +/- 39, 351 +/- 71 and 489 +/- 38 mg/dl compared with their baseline levels in lipid-infused groups. During the test period, low-dose insulin infusion kept plasma insulin at basal levels in C and C(TG) and high-dose insulin infusion increased plasma insulin levels about 6 times the baseline insulin level in C. Glucose infusion rate (GIR) was significantly higher in rats with high insulin infusion than those with low insulin infusion. The increase in GIR was lower in fructose-fed groups than in control groups under similar hyperinsulinemia. Rats with or without lipofundin infusion did not alter GIR during the test period. The present results demonstrated that hypertriglyceridemia-induced pressor response was diminished under hyperinsulinemic condition in both normal and fructose-induced insulin resistant rats.