A discordance in rosiglitazone mediated insulin sensitization and skeletal muscle mitochondrial content/activity in Type 2 diabetes mellitus

Skeletal muscle mitochondrial dysfunction is hypothesized to contribute to the pathophysiology of insulin resistance and Type 2 diabetes. Whether thiazolidinedione therapy enhances skeletal muscle mitochondrial function as a component of its insulin-sensitizing effect is unknown. To test this, we evaluated skeletal muscle mitochondria and exercise capacity in Type 2 diabetic subjects with otherwise normal cardiopulmonary function in response to rosiglitazone therapy. Twenty-three subjects were treated for 12 wk and underwent pre- and posttherapy metabolic stress testing and skeletal muscle biopsies. Rosiglitazone significantly ameliorated fasting glucose, insulin, and free fatty acid levels but did not augment the subjects' maximal oxygen consumption (Vo(2max)) or their skeletal muscle mitochondrial copy number. The baseline Vo(2max) correlated strongly with muscle mitochondrial copy number (r = 0.56, P = 0.018, n = 17) and inversely with the duration of diabetes (r = -0.67, P = 0.004, n = 23). Despite the global lack of effect of rosiglitazone-mediated insulin sensitization on skeletal muscle mitochondria, subjects with the most preserved functional capacity demonstrated some plasticity in their mitochondria biology as evidenced by an upregulation of electron transfer chain proteins and in citrate synthase activity. This study demonstrates that the augmentation of skeletal muscle mitochondrial electron transfer chain content and/or bioenergetics is not a prerequisite for rosiglitazone-mediated improved insulin sensitivity. Moreover, in diabetic subjects, Vo(2max) reflects the duration of diabetes and skeletal muscle mitochondrial content. It remains to be determined whether longer-term insulin sensitization therapy with rosiglitazone will augment skeletal muscle mitochondrial bioenergetics in those diabetic subjects with relatively preserved basal aerobic capacity.     

Effects of high-fat overfeeding on mitochondrial function, glucose and fat metabolism, and adipokine levels in low-birth-weight subjects

Low birth weight (LBW) is associated with an increased risk of insulin resistance and downregulation of oxidative phosphorylation (OXPHOS) genes when exposed to a metabolic challenge of high-fat overfeeding (HFO). To elaborate further on the differential effects of HFO in LBW subjects, we measured in vivo mitochondrial function, insulin secretion, hepatic glucose production, and plasma levels of key regulatory hormones before and after 5 days of HFO in 20 young LBW and 26 normal-birth-weight (NBW) men. The LBW subjects developed peripheral insulin resistance after HFO due to impaired endogenous glucose storage (9.42 ± 4.19 vs. 5.91 ± 4.42 mg·kg FFM(-1)·min(-1), P = 0.01). Resting muscle phosphorcreatine and total ATP in muscle increased significantly after HFO in LBW subjects only, whereas additional measurements of mitochondrial function remained unaffected. Despite similar plasma FFA levels, LBW subjects displayed increased fat oxidation during insulin infusion compared with normal-birth-weight (NBW) subjects after HFO (0.37 ± 0.35 vs. 0.17 ± 0.33 mg·kg FFM(-1)·min(-1), P = 0.02). In contrast to NBW subjects, the plasma leptin levels of LBW subjects did not increase, and the plasma gastric inhibitory polypeptide (GIP) as well as pancreatic polypeptide (PP) levels increased less in LBW compared with NBW subjects during HFO. In conclusion, HFO unmasks dissociation between insulin resistance and mitochondrial dysfunction in LBW subjects, suggesting that insulin resistance may be a cause, rather than an effect, of impaired muscle OXPHOS gene expression and mitochondrial dysfunction. Reduced increments in response to HFO of fasting plasma leptin, PP, and GIP levels may contribute to insulin resistance, lower satiety, and impaired insulin secretion in LBW subjects.     

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.     

High-dose oral vitamin C partially replenishes vitamin C levels in patients with Type 2 diabetes and low vitamin C levels but does not improve endothelial dysfunction or insulin resistance

Endothelial dysfunction is a hallmark of Type 2 diabetes related to hyperglycemia and oxidative stress. Nitric oxide-dependent vasodilator actions of insulin may augment glucose disposal. Thus endothelial dysfunction may worsen insulin resistance. Intra-arterial administration of vitamin C improves endothelial dysfunction in diabetes. In the present study, we investigated effects of high-dose oral vitamin C to alter endothelial dysfunction and insulin resistance in Type 2 diabetes. Plasma vitamin C levels in 109 diabetic subjects were lower than healthy (36 +/- 2 microM) levels. Thirty-two diabetic subjects with low plasma vitamin C (<40 microM) were subsequently enrolled in a randomized, double-blind, placebo-controlled study of vitamin C (800 mg/day for 4 wk). Insulin sensitivity (determined by glucose clamp) and forearm blood flow in response to ACh, sodium nitroprusside (SNP), or insulin (determined by plethysmography) were assessed before and after 4 wk of treatment. In the placebo group (n = 17 subjects), plasma vitamin C (22 +/- 3 microM), fasting glucose (159 +/- 12 mg/dl), insulin (19 +/- 7 microU/ml), and SI(Clamp) [2.06 +/- 0.29 x 10(-4) dl x kg(-1) x min(-1)/(microU/ml)] did not change significantly after placebo treatment. In the vitamin C group (n = 15 subjects), basal plasma vitamin C (23 +/- 2 microM) increased to 48 +/- 6 microM (P < 0.01) after treatment, but this was significantly less than that expected for healthy subjects (>80 microM). No significant changes in fasting glucose (156 +/- 11 mg/dl), insulin (14 +/- 2 microU/ml), SI(Clamp) [2.71 +/- 0.46 x 10(-4) dl x kg(-1) x min(-1)/(microU/ml)], or forearm blood flow in response to ACh, SNP, or insulin were observed after vitamin C treatment. We conclude that high-dose oral vitamin C therapy, resulting in incomplete replenishment of vitamin C levels, is ineffective at improving endothelial dysfunction and insulin resistance in Type 2 diabetes.     

Relationship between various surrogate indices of insulin resistance and mitochondrial DNA content in the peripheral blood of 18 healthy volunteers

Mutations or deletions of mitochondrial DNA (mtDNA) are associated with diabetes mellitus. In this study, we investigated the relationships between the mtDNA content in peripheral blood and surrogate indices of insulin resistance in 18 healthy young women (mean age 20.8 +/- 1.5 years). The mtDNA content was significantly correlated with the area under the curve of insulin during an oral glucose tolerance test (r = -0.622), the homeostasis model assessment for insulin resistance (r = -0.616), the ratio of fasting glucose to insulin concentration (r = 0.586) and the fasting insulin level (r = -0.552). Further study is warranted to elucidate the mechanism by which the mtDNA content is associated with insulin resistance.     

Effects of glucagon-like peptide-1 on endothelial function in type 2 diabetes patients with stable coronary artery disease

GLP-1 stimulates insulin secretion, suppresses glucagon secretion, delays gastric emptying, and inhibits small bowel motility, all actions contributing to the anti-diabetogenic peptide effect. Endothelial dysfunction is strongly associated with insulin resistance and type 2 diabetes mellitus and may cause the angiopathy typifying this debilitating disease. Therefore, interventions affecting both endothelial dysfunction and insulin resistance may prove useful in improving survival in type 2 diabetes patients. We investigated GLP-1's effect on endothelial function and insulin sensitivity (S(I)) in two groups: 1) 12 type 2 diabetes patients with stable coronary artery disease and 2) 10 healthy subjects with normal endothelial function and S(I). Subjects underwent infusion of recombinant GLP-1 or saline in a random crossover study. Endothelial function was measured by postischemic FMD of brachial artery, using ultrasonography. S(I) [in (10(-4) dl.kg(-1).min(-1))/(muU/ml)] was measured by hyperinsulinemic isoglycemic clamp technique. In type 2 diabetic subjects, GLP-1 infusion significantly increased relative changes in brachial artery diameter from baseline FMD(%) (3.1 +/- 0.6 vs. 6.6 +/- 1.0%, P < 0.05), with no significant effects on S(I) (4.5 +/- 0.8 vs. 5.2 +/- 0.9, P = NS). In healthy subjects, GLP-1 infusion affected neither FMD(%) (11.9 +/- 0.9 vs. 10.3 +/- 1.0%, P = NS) nor S(I) (14.8 +/- 1.8 vs. 11.6 +/- 2.0, P = NS). We conclude that GLP-1 improves endothelial dysfunction but not insulin resistance in type 2 diabetic patients with coronary heart disease. This beneficial vascular effect of GLP-1 adds yet another salutary property of the peptide useful in diabetes treatment.     

Effect of training on insulin sensitivity of glucose uptake and lipolysis in human adipose tissue

Training increases insulin sensitivity of both whole body and muscle in humans. To investigate whether training also increases insulin sensitivity of adipose tissue, we performed a three-step hyperinsulinemic, euglycemic clamp in eight endurance-trained (T) and eight sedentary (S) young men [insulin infusion rates: 10,000 (step I), 20,000 (step II), and 150,000 (step III) microU x min(-1) x m(-2)]. Glucose and glycerol concentrations were measured in arterial blood and also by microdialysis in interstitial fluid in periumbilical, subcutaneous adipose tissue and in quadriceps femoris muscle (glucose only). Adipose tissue blood flow was measured by (133)Xe washout. In the basal state, adipose tissue blood flow tended to be higher in T compared with S subjects, and in both groups blood flow was constant during the clamp. The change from basal in arterial-interstitial glucose concentration difference was increased in T during the clamp but not in S subjects in both adipose tissue and muscle [adipose tissue: step I (n = 8), 0.48 +/- 0.18 mM (T), 0.23 +/- 0.11 mM (S); step II (n = 8), 0.19 +/- 0.09 (T), -0.09 +/- 0.24 (S); step III (n = 5), 0.47 +/- 0.24 (T), 0.06 +/- 0.28 (S); (T: P < 0.001, S: P > 0.05); muscle: step I (n = 4), 1. 40 +/- 0.46 (T), 0.31 +/- 0.21 (S); step II (n = 4), 1.14 +/- 0.54 (T), -0.08 +/- 0.14 (S); step III (n = 4), 1.23 +/- 0.34 (T), 0.24 +/- 0.09 (S); (T: P < 0.01, S: P > 0.05)]. Interstitial glycerol concentration decreased faster in T than in S subjects [half-time: T, 44 +/- 9 min (n = 7); S, 102 +/- 23 min (n = 5); P < 0.05]. In conclusion, training enhances insulin sensitivity of glucose uptake in subcutaneous adipose tissue and in skeletal muscle. Furthermore, interstitial glycerol data suggest that training also increases insulin sensitivity of lipolysis in subcutaneous adipose tissue. Insulin per se does not influence subcutaneous adipose tissue blood flow.     

Contribution of defects in glucose uptake to carbohydrate intolerance in liver cirrhosis: assessment during physiological glucose and insulin concentrations

It is well established that subjects with liver cirrhosis are insulin resistant, but the contribution of defects in insulin secretion and/or action to glucose intolerance remains unresolved. Healthy individuals and subjects with liver cirrhosis were studied on two occasions: 1) an oral glucose tolerance test was performed, and 2) insulin secretion was inhibited and glucose was infused in a pattern and amount mimicking the systemic delivery rate of glucose after a carbohydrate meal. Insulin was concurrently infused to mimic a healthy postprandial insulin profile. Postabsorptive glucose concentrations were equal (5.36 +/- 0.12 vs. 5.40 +/- 0.25 mmol/l, P = 0.89), despite higher insulin (P < 0.01), C-peptide (P < 0.01), and free fatty acid (P = 0.05) concentrations in cirrhotic than in control subjects. Endogenous glucose release (EGR; 11.50 +/- 0.50 vs. 11.73 +/- 1.00 mumol.kg(-1).min(-1), P = 0.84) and the contribution of gluconeogenesis to EGR (6.60 +/- 0.47 vs. 6.28 +/- 0.64 mumol.kg(-1).min(-1), P = 0.70) were unaltered by cirrhosis. A minimal model recently developed for the oral glucose tolerance test demonstrated an impaired insulin sensitivity index (P < 0.05), whereas the beta-cell response to glucose was unaltered (P = 0.72). During prandial glucose and insulin infusions, the integrated glycemic response was greater in cirrhotic than in control subjects (P < 0.05). EGR decreased promptly and comparably in both groups, but glucose disappearance was insufficient at the prevailing glucose concentration (P < 0.05). Moreover, identical rates of [3-(3)H]glucose infusion produced higher tracer concentrations in cirrhotic than in control subjects (P < 0.05), implying a defect in glucose uptake. In conclusion, carbohydrate intolerance in liver cirrhosis is determined by insulin resistance and the ability of glucose to stimulate insulin secretion. During prandial glucose and insulin concentrations, EGR suppression was unaltered, but glucose uptake was impaired, which demonstrates that intolerance can be ascribed to a defect in glucose uptake, rather than abnormalities in glucose production or beta-cell function. Although insulin secretion ameliorates glucose intolerance, impaired glucose uptake during physiological glucose and insulin concentrations produces marked and sustained hyperglycemia, despite concurrent abnormalities in glucose production or insulin secretion.     

Acute effects of valsartan on insulin sensitivity in obese, non-hypertensive subjects with and without type 2 diabetes.

BACKGROUND: Two studies were designed to determine whether a single dose (80 mg) of the angiotensin II receptor blocker (ARB), valsartan, alters insulin sensitivity in obese, non-hypertensive subjects with and without Type 2 diabetes. METHODS: Insulin sensitivity (S(I)), glucose effectiveness (S(G)), and acute insulin response (AIR(0-10 min)) were measured by means of a 3-hour insulin-modified frequently sampled intravenous glucose tolerance test (FSIVGTT) before and after a single dose of valsartan. Study 1: obese, normotensive non-diabetic male subjects (n = 12), mean (SD) age 37.2 +/- 11.2 years, BMI 32.8 +/- 6.8 kg/m (2); Study 2: obese, normotensive Type 2 diabetic patients (n = 12), mean age 55.7 +/- 6.9 years, BMI 35.0 +/- 6.8 kg/m (2)/l. Both studies were randomised, double-blind, placebo-controlled, single-dose crossover group studies involving subjects in two study days, two weeks apart. After fasting samples were taken, a 300 mg/kg iv glucose bolus was injected at 0 min, and 0.05 U/kg iv insulin was given 20 min later. Blood samples for analysis of glucose and insulin were taken throughout the 3-hour study period. RESULTS: Study 1 (non-diabetic subjects) S(I) 2.81 vs. 2.63 x 10 (-4) min (-1) per microU/ml (p = 0.54), S(G) 0.020 vs. 0.020 min (-1) (p = 0.90), AIR(0-10) min 3305 vs. 3450 microU/min/ml (p = 0.71); Study 2 (patients with type 2 diabetes) S(I) 0.59 vs. 0.85 x 10 (-4) min (-1) per microU/ml (p = 0.15), S(G) 0.013 vs. 0.014 min (-1) (p = 0.71), AIR(0-10) min 65 vs. 119 microU/min/ml (p = 0.14), placebo vs. valsartan, respectively. CONCLUSION: In obese, non-hypertensive non-diabetic and Type 2 diabetic subjects a single dose of valsartan does not alter insulin sensitivity.     

Fetal and neonatal exposure to AZT and low-protein diet affects glucose homeostasis: a model with implications for AIDS prevention

Zidovudine (AZT) lowers the perinatal transmission of HIV but can impair mitochondrial function by depleting mitochondrial DNA (mtDNA). AZT therapy and perinatal nutritional deprivation affect the body fat distribution, which influences glucose tolerance. We sought to model intrauterine exposure to AZT in humans to determine whether it interacts with low-protein diet (LPD) to impact on birth weight and glucose homeostasis in the offspring. Pregnant dams and their offspring were given AZT, an LPD, or AZT and an LPD (LPD + AZT). AZT reduced mtDNA copy number in liver and birth weight in the offspring and increased their fasting glucose and insulin (P = 0.021, 0.03, 0.001, and 0.011 respectively) at 6-8 wk of age. LPD decreased litter size and birth weight (P = 0.01 and 0.012). In the LPD + AZT group, birth weight and litter size were reduced compared with untreated controls, and fasting blood glucose and insulin were raised. There was a significant interaction between LPD and AZT on fasting insulin levels (P = 0.025). Islet size was not significantly affected, but the mean beta-cell area/islet was reduced in the LPD + AZT group compared with controls (P < 0.05). Early exposure to AZT interacts with LPD to impair fetal development in this model. This combination appeared to impair the supply of insulin and, hence, glucose homeostasis, perhaps as a result of impaired mitochondrial function. Although it is not certain that this can be extrapolated to humans, maternal nutritional deprivation combined with AIDS therapy could influence both birth weight and onset of diabetes.     

Twelve-week monotherapy with the DPP-4 inhibitor vildagliptin improves glycemic control in subjects with type 2 diabetes.

Inhibition of dipeptidyl peptidase-4 enhances the activity of incretin hormones, improving glycemic control in subjects with type 2 diabetes. This twelve-week randomized, double-masked, placebo-controlled study assessed the efficacy and tolerability of the specific and potent oral dipeptidyl peptidase-4 inhibitor, vildagliptin (25 mg, bid, n=70) VS. placebo (bid, n=28) in previously diet-treated subjects with type 2 diabetes. Standardized meal tests were performed at baseline and endpoint. The between-group difference in adjusted mean change in HbA1c from baseline to endpoint was - 0.6 +/- 0.2 % (p=0.0012) for the whole cohort (baseline 8.0 %) and -1.2 % for subjects with baseline HbA1c 8.0 - 9.5 %. Fasting glucose and mean prandial glucose were reduced by 1.1 +/- 0.4 (p=0.0043) and 1.9 +/- 0.5 mmol/l (p <0.0001), respectively. The between-group differences in corrected insulin response at peak glucose and mean prandial C-peptide were + 0.06 +/- 0.02 (p=0.0258) and + 0.10 +/- 0.03 nmol/l (p=0.0031), respectively. Vildagliptin had no effect on fasting lipid levels or body weight. The incidence of adverse events was similar in subjects receiving placebo (71.4 %) and vildagliptin (55.7 %). CONCLUSION: monotherapy with vildagliptin is well tolerated and improves glycemic control in diet-treated subjects with type 2 diabetes. Concomitant improvements in beta-cell function were also observed. Subjects with higher baseline HbA1c levels showed greater response.     

Partitioning glucose distribution/transport, disposal, and endogenous production during IVGTT

We have separated the effect of insulin on glucose distribution/transport, glucose disposal, and endogenous production (EGP) during an intravenous glucose tolerance test (IVGTT) by use of a dual-tracer dilution methodology. Six healthy lean male subjects (age 33 +/- 3 yr, body mass index 22.7 +/- 0.6 kg/m(2)) underwent a 4-h IVGTT (0.3 g/kg glucose enriched with 3-6% D-[U-(13)C]glucose and 5-10% 3-O-methyl-D-glucose) preceded by a 2-h investigation under basal conditions (5 mg/kg of D-[U-(13)C]glucose and 8 mg/kg of 3-O-methyl-D-glucose). A new model described the kinetics of the two glucose tracers and native glucose with the use of a two-compartment structure for glucose and a one-compartment structure for insulin effects. Insulin sensitivities of distribution/transport, disposal, and EGP were similar (11.5 +/- 3.8 vs. 10.4 +/- 3.9 vs. 11.1 +/- 2.7 x 10(-2) ml small middle dot kg(-1) small middle dot min(-1) per mU/l; P = nonsignificant, ANOVA). When expressed in terms of ability to lower glucose concentration, stimulation of disposal and stimulation of distribution/transport accounted each independently for 25 and 30%, respectively, of the overall effect. Suppression of EGP was more effective (P < 0.01, ANOVA) and accounted for 50% of the overall effect. EGP was suppressed by 70% (52-82%) (95% confidence interval relative to basal) within 60 min of the IVGTT; glucose distribution/transport was least responsive to insulin and was maximally activated by 62% (34-96%) above basal at 80 min compared with maximum 279% (116-565%) activation of glucose disposal at 20 min. The deactivation of glucose distribution/transport was slower than that of glucose disposal and EGP (P < 0.02) with half-times of 207 (84-510), 12 (7-22), and 29 (16-54) min, respectively. The minimal-model insulin sensitivity was tightly correlated with and linearly related to sensitivity of EGP (r = 0.96, P < 0.005) and correlated positively but nonsignificantly with distribution/transport sensitivity (r = 0.73, P = 0.10) and disposal sensitivity (r = 0.55, P = 0.26). We conclude that, in healthy subjects during an IVGTT, the two peripheral insulin effects account jointly for approximately one-half of the overall insulin-stimulated glucose lowering, each effect contributing equally. Suppression of EGP matches the effect in the periphery.     

Thiazolidinediones improve beta-cell function in type 2 diabetic patients

Thiazolidinediones (TZDs) improve glycemic control and insulin sensitivity in patients with type 2 diabetes mellitus (T2DM). There is growing evidence from in vivo and in vitro studies that TZDs improve pancreatic beta-cell function. The aim of this study was to determine whether TZD-induced improvement in glycemic control is associated with improved beta-cell function. We studied 11 normal glucose-tolerant and 53 T2DM subjects [age 53+/-2 yr; BMI 29.4+/-0.8 kg/m2; fasting plasma glucose (FPG) 10.3+/-0.4 mM; Hb A1c 8.2+/-0.3%]. Diabetic patients were randomized to receive placebo or TZD for 4 mo. Subjects received 1) 2-h OGTT with determination of plasma glucose, insulin, and C-peptide concentrations and 2) two-step euglycemic insulin (40 and 160 mU.m-2.min-1) clamp with [3-(3)H]glucose. T2DM patients were then randomized to receive 4 mo of treatment with pioglitazone (45 mg/day), rosiglitazone (8 mg/day), or placebo. Pioglitazone and rosiglitazone similarly improved FPG, mean plasma glucose during OGTT, Hb A1c, and insulin-mediated total body glucose disposal (Rd) and decreased mean plasma FFA during OGTT (all P<0.01, ANOVA). The insulin secretion/insulin resistance (disposition) index [DeltaISR(AUC)/Deltaglucose(AUC)/IR] was significantly improved in all TZD-treated groups: +1.8+/-0.7 (PIO+drug-na?ve diabetics), +0.7+/-0.3 (PIO+sulfonylurea-treated diabetics), and 0.7+/-0.2 (ROSI+sulfonylurea-withdrawn diabetics) vs. -0.2+/-0.3 in the two placebo groups (P<0.01, all TZDs vs. placebo, ANOVA). Improved insulin secretion correlated positively with increased body weight, fat mass, and Rd and inversely with decreased plasma glucose and FFA during the OGTT. In T2DM patients, TZD treatment leads to improved beta-cell function, which correlates strongly with improved glycemic control.     

Retinol-binding protein 4 (RBP-4) levels do not change after oral glucose tolerance test and after dexamethasone, but correlate with some indices of insulin resistance in humans

Introduction: Secretory products from adipocytes may contribute to deterioration in glycaemic control and increased insulin resistance (IR). Retinol-binding protein 4 (RBP-4) may increase IR in mice, with elevated levels in insulin-resistant mice and humans with obesity and type 2 diabetes. However, the mechanisms regulating RBP-4 synthesis remain not fully understood. It is not clear whether short-term glucose-induced hyperglycaemia and hyperinsulinaemia as well as glucocorticosteroid-induced increase in IR might be reflected in alterations in serum RBP-4 levels in humans. In order to investigate this, we measured serum RBP-4, glucose and insulin concentrations during 75.0 gram oral glucose tolerance test (OGTT) - Study 1, as well as before and after oral administration of dexamethasone - Study 2. Material and methods: Both studies included 35 subjects (8 males), age (mean +/- SD) 39.1 +/- 15.6 years, BMI 35.8 +/- 8.7 kg/m(2). Twenty-four of those subjects (5 males), age 38.7 +/- 15.1 years, BMI 34.4 +/- 8.3 kg/m(2), had 75 gram oral glucose tolerance test (OGTT) - Study 1. Blood samples were taken before (0 minutes), and at 60 and 120 minutes of OGTT. 17 subjects (3 males, 4 subjects with type 2 diabetes), age 43.1 +/- 18.1 years, BMI 36.7 +/- 9.0 kg/m(2) underwent screening for Cushing's disease/syndrome (Study 2). Dexamethasone was administered in a dose of 0.5 mg every 6 hours for 48 hours. Fasting serum concentrations of RBP-4, glucose and insulin were assessed before (D0) and after 48 hours of dexamethasone administration (D2). IR was assessed by HOMA in all non-diabetic subjects and in subjects participating in study 1 also by Insulin Resistance Index (IRI), which takes into account glucose and insulin levels during OGTT. Results: Glucose administration resulted in significant increases in insulin and glucose (p < 0.0001). There was, however, no change in RBP-4 concentrations (124.1 +/- 32 mg/ml at 0 minutes, 123 +/- 35 mg/ml at 60 minutes and 126.5 +/- 37.5 mg/ml at 120 minutes of OGTT, p = ns). All subjects in Study 2 achieved suppression of cortisol below 50 nmo/l. Dexamethasone administration resulted in an increase in fasting insulin (from 11.6 +/- 6.8 to 17.1 +/- 7.2 muU/ml; p = 0.003), and an increase in HOMA (from 2.73 +/- 1.74 to 4.02 +/- 2.27; p = 0.015), although without a significant change in RBP-4 levels (119 +/- 26.8 vs. 117.5 +/- 24.8 mg/ml, p = ns). RBP-4 correlated with fasting insulin (r = 0.40, p = 0.025), fasting glucose (r = 0.41, p = 0.02) and HOMA (r = 0.43, p = 0.015), but not with IRI (r = 0.19, p = 0.31). There was, however, only a moderate correlation between HOMA and IRI (r = 0.49 [r(2) = 0.24]; p = 0.006, Spearman rank correlation), while the best correlation was obtained between the product of glucose and insulin levels at 60 min of OGTT and IRI in a non-linear model (r = 0.94 [r(2) = 0.88]; p<0.00001). In subjects who received dexamethasone, a positive correlation between RBP-4 and HOMA (p = 0.01) was lost after two days of dexamethasone administration (p = 0.61). Conclusions: RBP-4 levels do not change during oral glucose tolerance test or after a dexamethasone-induced increase in insulin resistance. This implies that it is highly unlikely that RBP-4 is involved in short-term regulation of glucose homeostasis in humans and that it responds to short-term changes in insulin resistance. A moderate correlation between RBP-4 and some insulin resistance indices (HOMA) does not exclude the fact that RBP-4 might be one of many factors that can influence insulin sensitivity in humans.     

Characterization of beta-cell function impairment in first-degree relatives of type 2 diabetic subjects: modeling analysis of 24-h triple-meal tests

To investigate early secretory defects in prediabetes, we evaluated beta-Cell function and insulin sensitivity (M value, by euglycemic clamp) in 26 normotolerant first-degree relatives of type 2 diabetic patients (FDR) and 17 age- and weight-matched control subjects. beta-Cell function was assessed by modeling analysis of glucose and C-peptide concentrations measured during 24 h of standardized living conditions. Fasting and total insulin secretion (ISR) were increased in FDR, as was ISR at a reference 5 mM glucose level (ISR5, 107 +/- 6 vs. 87 +/- 6 pmol x min(-1) x m(-2), P < 0.05). ISR5 was inversely related to M in controls (ISR5 = k/M1.23, rho = -0.74, P < 0.005) but not in FDR; when M was accounted for (by calculating a compensation index ISR5 x M1.23), compensation for insulin resistance was impaired in FDR (10.8 +/- 1.0 vs. 13.4 +/- 0.6 units, P < 0.05). Potentiation of ISR, expressing relative transient increases in glucose-stimulated ISR during meals, was impaired in FDR (1.29 +/- 0.08 vs. 1.62 +/- 0.08 during 1st meal, P < 0.02). Moreover, the potentiation time course was related to glucose-dependent insulin-releasing polypeptide (GIP) concentrations in both groups, and the sensitivity of potentiation to GIP derived from this relationship tended to be impaired in FDR. Compensation index, potentiation, and sensitivity to GIP were interrelated parameters (P < 0.05 or less). beta-Cell function parameters were also related to mean 24-h glucose levels (r2 = 0.63, P < 0.0001, multivariate model). In conclusion, although in absolute terms ISR is increased in insulin-resistant FDR, beta-cell function shows a cluster of interrelated abnormalities involving compensation for insulin resistance, potentiation, and sensitivity to GIP, suggesting a beta-cell defect in the amplifying pathway of insulin secretion.     

Impaired non-esterified fatty acid suppression is associated with endothelial dysfunction in insulin resistant subjects.

In a recent study, we found a significant association between insulin resistance (IR) and disturbed flow-associated (endothelial-dependent) vasodilation in first-degree relatives of subjects with type 2 diabetes. However, the mechanisms linking insulin resistance and endothelial dysfunction (ED) have not been fully elucidated. Experimental data have pointed out that non-esterified fatty acids (NEFA) have a modulating effect on NO-synthase activity, and therefore on endothelial function. The aim of our study was to evaluate whether insulin resistance associated impaired NEFA suppression is present in subjects with ED. We examined 53 first-degree relatives (FDR) of patients with type 2 diabetes (32f, 21 m, mean age 35 years). Endothelial function was measured as flow-associated vasodilation (FAD%) of the brachial artery. Insulin sensitivity was evaluated with a standard hyperinsulinemic glucose clamp (insulin infusion rate of 1 mU/kg/min). While under fasting conditions, NEFA did not differ between groups with high or low FAD (0.415+/-0.033 vs. 0.394 +/- 0.040 mmol/l; p = n. s.), reduced FAD% was significantly associated with higher non-esterified fatty acids concentrations during steady state of the glucose clamp (0.072+/-0.022 vs. 0.039+/-0.016mmol/l; p=0.04). This association was independent of insulin levels under fasting conditions and during the glucose clamp. In conclusion, our results reveal a significant association between endothelial dysfunction and impaired non-esterified fatty acid suppression in insulin resistant subjects. As insulin resistance of lipolysis is a feature of the insulin resistance syndrome, these results suggest that elevated NEFA concentrations could play a role linking endothelial dysfunction and insulin resistance in vivo.     

Oral administration of RAC-alpha-lipoic acid modulates insulin sensitivity in patients with type-2 diabetes mellitus: a placebo-controlled pilot trial.

Alpha-lipoic acid (ALA), a naturally occuring compound and a radical scavenger was shown to enhance glucose transport and utilization in different experimental and animal models. Clinical studies described an increase of insulin sensitivity after acute and short-term (10 d) parenteral administration of ALA. The effects of a 4-week oral treatment with alpha-lipoic acid were evaluated in a placebo-controlled, multicenter pilot study to determine see whether oral treatment also improves insulin sensitivity. Seventy-four patients with type-2 diabetes were randomized to either placebo (n = 19); or active treatment in various doses of 600 mg once daily (n = 19), twice daily (1200 mg; n = 18), or thrice daily (1800 mg; n = 18) alpha-lipoic acid. An isoglycemic glucose-clamp was done on days 0 (pre) and 29 (post). In this explorative study, analysis was done according to the number of subjects showing an improvement of insulin sensitivity after treatment. Furthermore, the effects of active vs. placebo treatment on insulin sensitivity was compared. All four groups were comparable and had a similar degree of hyperglycemia and insulin sensitivity at baseline. When compared to placebo, significantly more subjects had an increase in insulin-stimulated glucose disposal (MCR) after ALA treatment in each group. As there was no dose effect seen in the three different alpha-lipoic acid groups, all subjects receiving ALA were combined in the "active" group and then compared to placebo. This revealed significantly different changes in MCR after treatment (+27% vs. placebo; p < .01). This placebo-controlled explorative study confirms previous observations of an increase of insulin sensitivity in type-2 diabetes after acute and chronic intravenous administration of ALA. The results suggest that oral administration of alpha-lipoic acid can improve insulin sensitivity in patients with type-2 diabetes. The encouraging findings of this pilot trial need to be substantiated by further investigations.     

Mitochondrial dysfunction and lipotoxicity

Mitochondrial dysfunction in skeletal muscle has been suggested to underlie the development of insulin resistance and type 2 diabetes mellitus. Reduced mitochondrial capacity will contribute to the accumulation of lipid intermediates, desensitizing insulin signaling and leading to insulin resistance. Why mitochondrial function is reduced in the (pre-)diabetic state is, however, so far unknown. Although it is tempting to suggest that skeletal muscle insulin resistance may result from an inherited or acquired reduction in mitochondrial function in the pre-diabetic state, it cannot be excluded that mitochondrial dysfunction may in fact be the consequence of the insulin-resistant/diabetic state. Lipotoxicity, the deleterious effects of accumulating fatty acids in skeletal muscle cells, may lie at the basis of mitochondrial dysfunction: next to producing energy, mitochondria are also the major source of reactive oxygen species (ROS). Fatty acids accumulating in the vicinity of mitochondria are vulnerable to ROS-induced lipid peroxidation. Subsequently, these lipid peroxides could have lipotoxic effects on mtDNA, RNA and proteins of the mitochondrial machinery, leading to mitochondrial dysfunction. Indeed, increased lipid peroxidation has been reported in insulin resistant skeletal muscle and the mitochondrial uncoupling protein-3, which has been suggested to prevent lipid-induced mitochondrial damage, is reduced in subjects with an impaired glucose tolerance and in type 2 diabetic patients. These findings support the hypothesis that fat accumulation in skeletal muscle may precede the reduction in mitochondrial function that is observed in type 2 diabetes mellitus.     

Dose-response characteristics of insulin action on glucose metabolism: a non-steady-state approach

The traditional methods for the assessment of insulin sensitivity yield only a single index, not the whole dose-response curve information. This curve is typically characterized by a maximally insulin-stimulated glucose clearance (Cl(max)) and an insulin concentration at half-maximal response (EC(50)). We developed an approach for estimating the whole dose-response curve with a single in vivo test, based on the use of tracer glucose and exogenous insulin administration (two steps of 20 and 200 mU x min(-1) x m(-2), 100 min each). The effect of insulin on plasma glucose clearance was calculated from non-steady-state data by use of a circulatory model of glucose kinetics and a model of insulin action in which glucose clearance is represented as a Michaelis-Menten function of insulin concentration with a delay (t(1/2)). In seven nondiabetic subjects, the model predicted adequately the tracer concentration: the model residuals were unbiased, and their coefficient of variation was similar to the expected measurement error (approximately 3%), indicating that the model did not introduce significant systematic errors. Lean (n = 4) and obese (n = 3) subjects had similar half-times for insulin action (t(1/2) = 25 +/- 9 vs. 25 +/- 8 min) and maximal responses (Cl(max) = 705 +/- 46 vs. 668 +/- 259 ml x min(-1) x m(-2), respectively), whereas EC(50) was 240 +/- 84 microU/ml in the lean vs. 364 +/- 229 microU/ml in the obese (P < 0.04). EC(50) and the insulin sensitivity index (ISI, initial slope of the dose-response curve), but not Cl(max), were related to body adiposity and fat distribution with r of 0.6-0.8 (P < 0.05). Thus, despite the small number of study subjects, we were able to reproduce information consistent with the literature. In addition, among the lean individuals, t(1/2) was positively related to the ISI (r = 0.72, P < 0.02). We conclude that the test here presented, based on a more elaborate representation of glucose kinetics and insulin action, allows a reliable quantitation of the insulin dose-response curve for whole body glucose utilization in a single session of relatively short duration.     

Abnormal renal, hepatic, and muscle glucose metabolism following glucose ingestion in type 2 diabetes

Recent studies indicate an important role of the kidney in postprandial glucose homeostasis in normal humans. To determine its role in the abnormal postprandial glucose metabolism in type 2 diabetes mellitus (T2DM), we used a combination of the dual-isotope technique and net balance measurements across kidney and skeletal muscle in 10 subjects with T2DM and 10 age-, weight-, and sex-matched nondiabetic volunteers after ingestion of 75 g of glucose. Over the 4.5-h postprandial period, diabetic subjects had increased mean blood glucose levels (14.1 +/- 1.1 vs. 6.2 +/- 0.2 mM, P < 0.001) and increased systemic glucose appearance (100.0 +/- 6.3 vs. 70.0 +/- 3.3 g, P < 0.001). The latter was mainly due to approximately 23 g greater endogenous glucose release (39.8 +/- 5.9 vs. 17.0 +/- 1.8 g, P < 0.002), since systemic appearance of the ingested glucose was increased by only approximately 7 g (60.2 +/- 1.4 vs. 53.0 +/- 2.2 g, P < 0.02). Approximately 40% of the diabetic subjects' increased endogenous glucose release was due to increased renal glucose release (19.6 +/- 3.1 vs. 10.6 +/- 2.4 g, P < 0.05). Postprandial systemic tissue glucose uptake was also increased in the diabetic subjects (82.3 +/- 4.7 vs. 69.8 +/- 3.5 g, P < 0.05), and its distribution was altered; renal glucose uptake was increased (21.0 +/- 3.5 vs. 9.8 +/- 2.3 g, P < 0.03), whereas muscle glucose uptake was normal (18.5 +/- 1.8 vs. 25.9 +/- 3.3 g, P = 0.16). We conclude that, in T2DM, 1) both liver and kidney contribute to postprandial overproduction of glucose, and 2) postprandial renal glucose uptake is increased, resulting in a shift in the relative importance of muscle and kidney for glucose disposal. The latter may provide an explanation for the renal glycogen accumulation characteristic of diabetes mellitus as well as a mechanism by which hyperglycemia may lead to diabetic nephropathy.