Rosiglitazone reduces glucose-stimulated insulin secretion rate and increases insulin clearance in nondiabetic, insulin-resistant individuals

Rosiglitazone Ameliorates Cardiac and Skeletal Muscle Dysfunction by Correction of Energetics in Huntington’s Disease

Huntington’s disease (HD) is a rare neurodegenerative disease that is accompanied by skeletal muscle atrophy and cardiomyopathy. Tissues affected by HD (central nervous system [CNS], skeletal muscle, and heart) are known to suffer from deteriorated cellular energy metabolism that manifests already at presymptomatic stages. This work aimed to test the effects of peroxisome proliferator-activated receptor (PPAR)-γ agonist—rosiglitazone on grip strength and heart function in an experimental HD model—on R6/1 mice and to address the mechanisms. We noted that rosiglitazone treatment lead to improvement of R6/1 mice grip strength and cardiac mechanical function. It was accompanied by an enhancement of the total adenine nucleotides pool, increased glucose oxidation, changes in mitochondrial number (indicated as increased citric synthase activity), and reduction in mitochondrial complex I activity. These metabolic changes were supported by increased total antioxidant status in HD mice injected with rosiglitazone. Correction of energy deficits with rosiglitazone was further indicated by decreased accumulation of nucleotide catabolites in HD mice serum. Thus, rosiglitazone treatment may not only delay neurodegeneration but also may ameliorate cardio- and myopathy linked to HD by improvement of cellular energetics.

Statins Are Associated With Increased Insulin Resistance and Secretion

Objective

Statin treatment reduces the risk of atherosclerotic cardiovascular disease but is associated with a modest increased risk of type 2 diabetes, especially in those with insulin resistance or prediabetes. Our objective was to determine the physiological mechanism for the increased type 2 diabetes risk.

Approach and Results

We conducted an open-label clinical trial of atorvastatin 40 mg daily in adults without known atherosclerotic cardiovascular disease or type 2 diabetes at baseline. The co-primary outcomes were changes at 10 weeks versus baseline in insulin resistance as assessed by steady-state plasma glucose during the insulin suppression test and insulin secretion as assessed by insulin secretion rate area under the curve (ISRAUC) during the graded-glucose infusion test. Secondary outcomes included glucose and insulin, both fasting and during oral glucose tolerance test. Of 75 participants who enrolled, 71 completed the study (median age 61 years, 37% women, 65% non-Hispanic White, median body mass index, 27.8 kg/m2). Atorvastatin reduced LDL (low-density lipoprotein)-cholesterol (median decrease 53%, P<0.001) but did not change body weight. Compared with baseline, atorvastatin increased insulin resistance (steady-state plasma glucose) by a median of 8% (P=0.01) and insulin secretion (ISRAUC) by a median of 9% (P<0.001). There were small increases in oral glucose tolerance test glucoseAUC (median increase, 0.05%; P=0.03) and fasting insulin (median increase, 7%; P=0.01).

Conclusions

In individuals without type 2 diabetes, high-intensity atorvastatin for 10 weeks increases insulin resistance and insulin secretion. Over time, the risk of new-onset diabetes with statin use may increase in individuals who become more insulin resistant but are unable to maintain compensatory increases in insulin secretion.

Defining the Relative Role of Insulin Clearance in Early Dysglycemia in Relation to Insulin Sensitivity and Insulin Secretion: The Microbiome and Insulin Longitudinal Evaluation Study (MILES)

Insulin resistance and insufficient insulin secretion are well-recognized contributors to type 2 diabetes. A potential role of reduced insulin clearance has been suggested, but few studies have investigated the contribution of insulin clearance while simultaneously examining decreased insulin sensitivity and secretion. The goal of this study was to conduct such an investigation in a cohort of 353 non-Hispanic White and African American individuals recruited in the Microbiome and Insulin Longitudinal Evaluation Study (MILES). Participants underwent oral glucose tolerance tests from which insulin sensitivity, insulin secretion, insulin clearance, and disposition index were calculated. Regression models examined the individual and joint contributions of these traits to early dysglycemia (prediabetes or newly diagnosed diabetes). In separate models, reduced insulin sensitivity, reduced disposition index, and reduced insulin clearance were associated with dysglycemia. In a joint model, only insulin resistance and reduced insulin secretion were associated with dysglycemia. Models with insulin sensitivity, disposition index, or three insulin traits had the highest discriminative value for dysglycemia (area under the receiver operating characteristics curve of 0.82 to 0.89). These results suggest that in the race groups studied, insulin resistance and compromised insulin secretion are the main independent underlying defects leading to early dysglycemia.

Extending Arms of Insulin Resistance from Diabetes to Alzheimer's Disease: Identification of Potential Therapeutic Targets

BACKGROUND & OBJECTIVE

Type 3 diabetes (T3D) is chronic insulin resistant state of brain which shares pathology with sporadic Alzheimer's disease (sAD). Insulin signaling is a highly conserved pathway in the living systems that orchestrate cell growth, repair, maintenance, energy homeostasis and reproduction. Although insulin is primarily studied as a key molecule in diabetes mellitus, its role has recently been implicated in the development of Alzheimer's disease (AD). Severe complications in brain of diabetic patients and metabolically compromised status is evident in brain of AD patients. Underlying shared pathology of two disorders draws a trajectory from peripheral insulin resistance to insulin unresponsiveness in the central nervous system (CNS). As insulin has a pivotal role in AD, it is not an overreach to address diabetic condition in AD brain as T3D. Insulin signaling is indispensable to nervous system and it is vital for neuronal growth, repair, and maintenance of chemical milieu at synapses. Downstream mediators of insulin signaling pathway work as a regulatory hub for aggregation and clearance of unfolded proteins like Aβ and tau.

CONCLUSION

In this review, we discuss the regulatory roles of insulin as a pivotal molecule in brain with the understanding of defective insulin signaling as a key pathological mechanism in sAD. This article also highlights ongoing trials of targeting insulin signaling as a therapeutic manifestation to treat diabetic condition in brain.

Adapting to insulin resistance in obesity: role of insulin secretion and clearance

AIMS/HYPOTHESIS

The aim of this study was to quantify the relative contributions of increased insulin secretion rate (ISR) and decreased insulin clearance rate (ICR) in the compensatory hyperinsulinaemia characteristic of insulin-resistant individuals without diabetes.

METHODS

Obese (BMI ≥30 kg/m²) individuals without diabetes (n = 91) were identified from a registry of volunteers. Volunteers underwent the following measurements: oral glucose tolerance; insulin resistance (steady-state plasma glucose [SSPG] concentration during the insulin suppression test [IST]); ISR (using the graded glucose infusion test [GGIT]); and ICR (using the IST and GGIT). Participants were stratified into tertiles based on SSPG concentration: SSPG-1(insulin-sensitive); SSPG-2 (intermediate); and SSPG-3 (insulin-resistant).

RESULTS

There were no differences in BMI and waist circumference among the SSPG tertiles. Serum alanine aminotransferase concentrations were higher in the SSPG-2 and SSPG-3 groups compared with the SSPG-1 group (p = 0.02). Following an oral glucose challenge, there was a progressive increase in the total integrated insulin response from the most insulin-sensitive to the most insulin-resistant tertiles (p < 0.001). Following intravenous glucose, the SSPG-3 group had significantly greater integrated glucose (median [interquartile range], 32.9 [30.8-36.3] mmol/l × h) and insulin responses (1711 [1476-2223] mmol/l × h) compared with the SSPG-1 group (30.3 [28.8-32.9] mmol/l × h, p = 0.04, and 851 [600-1057] pmol/l × h, p < 0.001, respectively). Furthermore, only the SSPG-3 group had significant changes in both ISR and ICR (p < 0.001). In the SSPG-2 group, only the ICR was significantly decreased compared with the SSPG-1 group. Therefore, ICR progressively declined during the IST with increasing insulin resistance (SSPG-1, 0.48 [0.41-0.59]; SSPG-2, 0.43 [0.39-0.50]; SSPG-3, 0.34 [0.31-0.40]).

CONCLUSIONS/INTERPRETATION

While both increases in ISR and decreases in ICR compensate for insulin resistance, decreases in ICR may provide the first adaptation to decreased insulin sensitivity.

Mathematical investigation of diabetically impaired ultradian oscillations in the glucose-insulin regulation

We study the effect of diabetic deficiencies on the production of an oscillatory ultradian regime using a deterministic nonlinear model which incorporates two physiological delays. It is shown that insulin resistance impairs the production of oscillations by dampening the ultradian cycles. Four strategies for restoring healthy regulation are explored. Through the introduction of an instantaneous glucose-dependent insulin response, explicit conditions for the existence of periodic solutions in the linearised model are formulated, significantly reducing the complexity of identifying an oscillatory regime. The model is thus shown to be suitable for representing the effect of diabetes on the oscillatory regulation and for investigating pathways to reinstating a physiological healthy regime.

Characterization of Metabolic Status in Nonhuman Primates with the Intravenous Glucose Tolerance Test

The intravenous glucose tolerance test (IVGTT) plays a key role in the characterization of glucose homeostasis. When taken together with serum biochemical profiles, inclusive of blood glucose levels in both the fed and fasted state, HbA1c, insulin levels, clinical history of diet, body composition, and body weight status, an assessment of normal and abnormal glycemic control can be made. Interpretation of an IVGTT is done through measurement of changes in glucose and insulin levels over time in relation to the dextrose challenge. Critical components to be considered are: peak glucose and insulin levels reached in relation to T0 (end of glucose infusion), the glucose clearance rate K derived from the slope of rapid glucose clearance in the first 20 min (T1 to T20), the time to return to glucose baseline, and the area under the curve (AUC). These IVGTT measures will show characteristic changes as glucose homeostasis moves from a healthy to a diseased metabolic state5. Herein we will describe the characterization of nonhuman primates (Rhesus and Cynomolgus macaques), which are the most relevant animal model of Type II diabetes (T2D) in humans and the IVGTT and clinical profiles of these animals from a lean healthy, to obese dysmetabolic, and T2D state 8, 10, 11.

Mechanisms by which the thiazolidinedione troglitazone protects against sucrose-induced hepatic fat accumulation and hyperinsulinaemia

BACKGROUND AND PURPOSE

Thiazolidinediones (TZD) are known to ameliorate fatty liver in type 2 diabetes. To date, the underlying mechanisms of their hepatic actions remain unclear.

EXPERIMENTAL APPROACH

Hepatic triglyceride content and export rates were assessed in 2 week high-sucrose-fed Wistar rats treated with troglitazone and compared with untreated high-sucrose rodent controls. Fractional de novo lipogenesis (DNL) contributions to hepatic triglyceride were quantified by analysis of triglyceride enrichment from deuterated water. Hepatic insulin clearance and NO status during a meal tolerance test were also evaluated.

KEY RESULTS

TZD significantly reduced hepatic triglyceride (P < 0.01) by 48%, decreased DNL contribution to hepatic triglyceride (P < 0.01) and increased postprandial non-esterified fatty acids clearance rates (P < 0.01) in comparison with the high-sucrose rodent control group. During a meal tolerance test, plasma insulin AUC was significantly lower (P < 0.01), while blood glucose and plasma C-peptide levels were not different. Insulin clearance was increased (P < 0.001) by 24% and was associated with a 22% augmentation of hepatic insulin-degrading enzyme activity (P < 0.05). Finally, hepatic NO was decreased by 24% (P < 0.05).

CONCLUSIONS

Overall, TZD show direct actions on liver by reducing hepatic DNL and increasing hepatic insulin clearance. The alterations in hepatic insulin clearance were associated with changes in insulin-degrading enzyme activity, with possible modulation of NO levels.

Relationship between insulin sensitivity and insulin secretion rate: not necessarily hyperbolic

AIMS

There is general acceptance that the physiological relationship between insulin sensitivity and insulin secretion is hyperbolic. This conclusion has evolved from studies in which one test assessed both variables, and changes in plasma insulin concentration were used as a surrogate measure for insulin secretion rate. The aim of this study was to see if a hyperbolic relationship would also emerge when separate and direct measures were used to quantify both insulin sensitivity and insulin secretion rate.

METHODS

Steady-state plasma glucose (SSPG) was determined in 146 individuals without diabetes using the insulin suppression test, with 1/SSPG used to quantify insulin sensitivity. The graded-glucose infusion test was used to quantify insulin secretion rate. Plasma glucose and insulin concentrations obtained during an oral glucose tolerance test (OGTT) were used to calculate surrogate estimates of insulin action and insulin secretion rate. A hyperbolic relationship was assumed if the β coefficient was near -1 using the following model: log (insulin secretion measure) = constant + β × log (insulin sensitivity measure).

RESULTS

OGTT calculations of insulin sensitivity (Matsuda) and plasma insulin response [ratio of insulin/glucose area-under-the-curve (AUC) or insulin total AUC] provided the expected hyperbolic relationship [β = -0.95, 95% CI (-1.09, -0.82); -1.06 (-1.14, -0.98)]. Direct measurements of insulin sensitivity and insulin secretion rate did not yield the same curvilinear relationship [β = -1.97 (-3.19, -1.36)].

CONCLUSIONS

These findings demonstrate that the physiological relationship between insulin sensitivity and insulin secretion rate is not necessarily hyperbolic, but will vary with the method(s) by which it is determined.

Insulin clearance: an underappreciated modulator of plasma insulin concentration

Plasma glucose concentrations are tightly regulated and maintained within a narrow range in non-diabetic individuals. Maintenance of this physiological state is primarily a function of the ability of the pancreatic β-cells to modify insulin secretion rate (ISR), thus preventing wide-swings in plasma glucose concentrations. As a consequence, and in contrast to plasma glucose concentrations, plasma insulin concentrations vary substantially in non-diabetic individuals. Although differences in ISR are primarily responsible for the variability in plasma insulin concentration, there is increasing evidence that differences in insulin clearance rate (ICR) also play a role in regulation of plasma insulin concentration. The goal of this mini-review is to highlight situations that demonstrate the important role of ICR in both insulin and glucose homeostasis.

Circulating microRNA-320a and microRNA-486 predict thiazolidinedione response: Moving towards precision health for diabetes prevention

INTRODUCTION

The aims of this study were to compare microRNA (miR) expression between individuals with and without insulin resistance and to determine whether miRs predict response to thiazolidinedione treatment.

MATERIALS AND METHODS

In a sample of 93 healthy adults, insulin resistance was defined as steady state plasma glucose (SSPG)≥180 mg/dL and insulin sensitive as <120 mg/dL. Response to thiazolidinedione therapy was defined as ≥10% decrease in SSPG. We selected a panel of microRNAs based on prior evidence for a role in insulin or glucose metabolism. Fold change and Wilcoxon rank sum tests were calculated for the 25 miRs measured.

RESULTS

At baseline, 81% (n=75) of participants were insulin resistant. Five miRs were differentially expressed between the insulin resistant and sensitive groups: miR-193b (1.45 fold change (FC)), miR-22-3p (1.15 FC), miR-320a (1.36 FC), miR-375 (0.59 FC), and miR-486 (1.21 FC) (all p<0.05). In the subset who were insulin resistant at baseline and received thiazolidinediones (n=47), 77% (n=36) showed improved insulin sensitivity. Six miRs were differentially expressed between responders compared to non-responders: miR-20b-5p (1.20 FC), miR-21-5p, (0.92 FC), miR-214-3p (1.13 FC), miR-22-3p (1.14 FC), miR-320a (0.98 FC), and miR-486-5p (1.25 FC) (all p<0.05).

DISCUSSION

This study is the first to report miRs associated with response to a pharmacologic intervention for insulin resistance. MiR-320a and miR-486-5p identified responders to thiazolidinedione therapy among the insulin resistant group.

Effect of salsalate on insulin action, secretion, and clearance in nondiabetic, insulin-resistant individuals: a randomized, placebo-controlled study

OBJECTIVE

Salsalate treatment has been shown to improve glucose homeostasis, but the mechanism remains unclear. The aim of this study was to evaluate the effect of salsalate treatment on insulin action, secretion, and clearance rate in nondiabetic individuals with insulin resistance.

RESEARCH DESIGN AND METHODS

This was a randomized (2:1), single-blind, placebo-controlled study of salsalate (3.5 g daily for 4 weeks) in nondiabetic individuals with insulin resistance. All individuals had measurement of glucose tolerance (75-g oral glucose tolerance test), steady-state plasma glucose (SSPG; insulin suppression test), and insulin secretion and clearance rate (graded-glucose infusion test) before and after treatment.

RESULTS

Forty-one individuals were randomized to salsalate (n = 27) and placebo (n = 14). One individual from each group discontinued the study. Salsalate improved fasting (% mean change -7% [95% CI -10 to -14] vs. 1% [-3 to 5], P = 0.005) but not postprandial glucose concentration compared with placebo. Salsalate also lowered fasting triglyceride concentration (-25% [-34 to -15] vs. -6% [-26 to 14], P = 0.04). Salsalate had no effect on SSPG concentration or insulin secretion rate but significantly decreased insulin clearance rate compared with placebo (-23% [-30 to -16] vs. 3% [-10 to 15], P < 0.001). Salsalate was well tolerated, but four individuals needed a dose reduction due to symptoms.

CONCLUSIONS

Salsalate treatment in nondiabetic, insulin-resistant individuals improved fasting, but not postprandial, glucose and triglyceride concentration. These improvements were associated with a decrease in insulin clearance rate without change in insulin action or insulin secretion.

Pancreatic beta cell function following liraglutide-augmented weight loss in individuals with prediabetes: analysis of a randomised, placebo-controlled study

AIMS/HYPOTHESIS

Liraglutide can modulate insulin secretion by directly stimulating beta cells or indirectly through weight loss and enhanced insulin sensitivity. Recently, we showed that liraglutide treatment in overweight individuals with prediabetes (impaired fasting glucose and/or impaired glucose tolerance) led to greater weight loss (-7.7% vs -3.9%) and improvement in insulin resistance compared with placebo. The current study evaluates the effects on beta cell function of weight loss augmented by liraglutide compared with weight loss alone.

METHODS

This was a parallel, randomised study conducted in a single academic centre. Both participants and study administrators were blinded to treatment assignment. Individuals who were 40-70 years old, overweight (BMI 27-40 kg/m(2)) and with prediabetes were randomised (via a computerised system) to receive liraglutide (n = 35) or matching placebo (n = 33), and 49 participants were analysed. All were instructed to follow an energy-restricted diet. Primary outcome was insulin secretory function, which was evaluated in response to graded infusions of glucose and day-long mixed meals.

RESULTS

Liraglutide treatment (n = 24) significantly (p ≤ 0.03) increased the insulin secretion rate (% mean change [95% CI]; 21% [12, 31] vs -4% [-11, 3]) and pancreatic beta cell sensitivity to intravenous glucose (229% [161, 276] vs -0.5% (-15, 14]), and decreased insulin clearance rate (-3.5% [-11, 4] vs 8.2 [0.2, 16]) as compared with placebo (n = 25). The liraglutide-treated group also had significantly (p ≤ 0.03) lower day-long glucose (-8.2% [-11, -6] vs -0.1 [-3, 2]) and NEFA concentrations (-14 [-20, -8] vs -2.1 [-10, 6]) following mixed meals, whereas day-long insulin concentrations did not significantly differ as compared with placebo. In a multivariate regression analysis, weight loss was associated with a decrease in insulin secretion rate and day-long glucose and insulin concentrations in the placebo group (p ≤ 0.05), but there was no association with weight loss in the liraglutide group. The most common side effect of liraglutide was nausea.

CONCLUSIONS/INTERPRETATION

A direct stimulatory effect on beta cell function was the predominant change in liraglutide-augmented weight loss. These changes appear to be independent of weight loss.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01784965 FUNDING: The study was funded by the ADA.

Prevention of diabetes with pioglitazone in ACT NOW: physiologic correlates

We examined the metabolic characteristics that attend the development of type 2 diabetes (T2DM) in 441 impaired glucose tolerance (IGT) subjects who participated in the ACT NOW Study and had complete end-of-study metabolic measurements. Subjects were randomized to receive pioglitazone (PGZ; 45 mg/day) or placebo and were observed for a median of 2.4 years. Indices of insulin sensitivity (Matsuda index [MI]), insulin secretion (IS)/insulin resistance (IR; ΔI0-120/ΔG0-120, ΔIS rate [ISR]0-120/ΔG0-120), and β-cell function (ΔI/ΔG × MI and ΔISR/ΔG × MI) were calculated from plasma glucose, insulin, and C-peptide concentrations during oral glucose tolerance tests at baseline and study end. Diabetes developed in 45 placebo-treated vs. 15 PGZ-treated subjects (odds ratio [OR] 0.28 [95% CI 0.15-0.49]; P < 0.0001); 48% of PGZ-treated subjects reverted to normal glucose tolerance (NGT) versus 28% of placebo-treated subjects (P < 0.005). Higher final glucose tolerance status (NGT > IGT > T2DM) was associated with improvements in insulin sensitivity (OR 0.61 [95% CI 0.54-0.80]), IS (OR 0.61 [95% CI 0.50-0.75]), and β-cell function (ln IS/IR index and ln ISR/IR index) (OR 0.26 [95% CI 0.19-0.37]; all P < 0.0001). Of the factors measured, improved β-cell function was most closely associated with final glucose tolerance status.

Quantification of β-cell insulin secretory function using a graded glucose infusion with C-peptide deconvolution in dysmetabolic, and diabetic cynomolgus monkeys

BACKGROUND

Quantitation of β-cell function is critical in better understanding of the dynamic interactions of insulin secretion, clearance and action at different phases in the progression of diabetes. The present study aimed to quantify β-cell secretory function independently of insulin sensitivity in the context of differential metabolic clearance rates of insulin (MCRI) in nonhuman primates (NHPs).

METHODS

Insulin secretion rate (ISR) was derived from deconvolution of serial C-peptide concentrations measured during a 5 stage graded glucose infusion (GGI) in 12 nondiabetic (N), 8 prediabetic or dysmetabolic (DYS) and 4 overtly diabetic (DM) cynomolgus monkeys. The characterization of the monkeys was based on the fasting glucose and insulin concentrations, glucose clearance rate measured by intravenous glucose tolerance test, and insulin resistance indices measured in separate experiments. The molar ratio of C-peptide/insulin (C/I) was used as a surrogate index of hepatic MCRI.

RESULTS

Compared to the N monkeys, the DYS with normal glycemia and hyperinsulinemia had significantly higher basal and GGI-induced elevation of insulin and C-peptide concentrations and lower C/I, however, each unit of glucose-stimulated ISR increment was not significantly different from that in the N monkeys. In contrast, the DM monkeys with β-cell failure and hyperglycemia had a depressed GGI-stimulated ISR response and elevated C/I.

CONCLUSIONS

The present data demonstrated that in addition to β-cell hypersecretion of insulin, reduced hepatic MCRI may also contribute to the development of hyperinsulinemia in the DYS monkeys. On the other hand, hyperinsulinemia may cause the saturation of hepatic insulin extraction capacity, which in turn reduced MCRI in the DYS monkeys. The differential contribution of ISR and MCRI in causing hyperinsulinemia provides a new insight into the trajectory of β-cell dysfunction in the development of diabetes. The present study was the first to use the GGI and C-peptide deconvolution method to quantify the β-cell function in NHPs.

Vildagliptin and pioglitazone in patients with impaired glucose tolerance after kidney transplantation: a randomized, placebo-controlled clinical trial

BACKGROUND

New-onset diabetes after transplantation (NODAT) is a serious complication after kidney transplantation affecting graft and patient survival. Currently, no guidelines exist for the management of renal transplant patients with impaired glucose tolerance (IGT), a risk factor for the development of NODAT and an independent predictor of death.

METHODS

In a population of 48 stable renal transplant recipients at least 6 months from time of transplantation with newly diagnosed IGT, we tested the dipeptidylpeptidase-4 inhibitor vildagliptin, the thiazolidinedione pioglitazone, or placebo for 3 months in addition to lifestyle counseling. Outcome measures were difference in change in oral glucose tolerance test between the groups and between baseline and end of study as well as change in HbA1c, serum lipids, and renal and hepatic function.

RESULTS

In both treatment groups, 2-hr plasma glucose at 3 months was significantly reduced compared with baseline (vildagliptin: -20±24 mg/dL; P=0.002 and pioglitazone: -23±29 mg/dL; P=0.004), and pioglitazone also significantly improved fasting plasma glucose (-11±14 mg/dL; P=0.003), although the primary outcome (difference in change in 2-hr plasma glucose among the three groups) did not reach statistical significance. Furthermore, HbA1c was decreased in both treatment arms (vildagliptin: -0.1%±0.3%; P=0.046 and pioglitazone: -0.2%±0.3%; P=0.029). In the placebo group, no significant changes in these parameters were observed. Only mild adverse events occurred and at a similar rate in all three groups.

CONCLUSIONS

These data demonstrate that both vildagliptin and pioglitazone are of potential benefit in patients with IGT after renal transplantation in addition to lifestyle modification.

Pioglitazone acutely reduces energy metabolism and insulin secretion in rats

Our objective was to determine if the insulin-sensitizing drug pioglitazone acutely reduces insulin secretion and causes metabolic deceleration in vivo independently of change in insulin sensitivity. We assessed glucose homeostasis by hyperinsulinemic-euglycemic and hyperglycemic clamp studies and energy expenditure by indirect calorimetry and biotelemetry in male Wistar and obese hyperinsulinemic Zucker diabetic fatty (ZDF) rats 45 min after a single oral dose of pioglitazone (30 mg/kg). In vivo insulin secretion during clamped hyperglycemia was reduced in both Wistar and ZDF rats after pioglitazone administration. Insulin clearance was slightly increased in Wistar but not in ZDF rats. Insulin sensitivity in Wistar rats assessed by the hyperinsulinemic-euglycemic clamp was minimally affected by pioglitazone at this early time point. Pioglitazone also reduced energy expenditure in Wistar rats without altering respiratory exchange ratio or core body temperature. Glucose-induced insulin secretion (GIIS) and oxygen consumption were reduced by pioglitazone in isolated islets and INS832/13 cells. In conclusion, pioglitazone acutely induces whole-body metabolic slowing down and reduces GIIS, the latter being largely independent of the insulin-sensitizing action of the drug. The results suggest that pioglitazone has direct metabolic deceleration effects on the β-cell that may contribute to its capacity to lower insulinemia and antidiabetic action.

Insulin clearance: confirmation as a highly heritable trait, and genome-wide linkage analysis

AIMS/HYPOTHESIS

We have previously documented a high heritability of insulin clearance in a Hispanic cohort. Here, our goal was to confirm the high heritability in a second cohort and search for genetic loci contributing to insulin clearance.

METHODS

Hyperinsulinaemic-euglycaemic clamps were performed in 513 participants from 140 Hispanic families. Heritability was estimated for clamp-derived insulin clearance and a two-phase genome-wide linkage scan was conducted using a variance components approach. Linkage peaks were further investigated by candidate gene association analysis in two cohorts.

RESULTS

The covariate-adjusted heritability of insulin clearance was 73%, indicating that the majority of the phenotypic variance is due to genetic factors. In the Phase 1 linkage scan, no signals with a logarithm of odds (LOD) score >2 were detected. In the Phase 2 scan, two linkage peaks with an LOD >2 for insulin clearance were identified on chromosomes 15 (LOD 3.62) and 20 (LOD 2.43). These loci harbour several promising candidate genes for insulin clearance, with 12 single nucleotide polymorphisms (SNPs) on chromosome 15 and six SNPs on chromosome 20 being associated with insulin clearance in both Hispanic cohorts.

CONCLUSIONS/INTERPRETATION

In a second Hispanic cohort, we confirmed that insulin clearance is a highly heritable trait and identified chromosomal loci that harbour genes regulating insulin clearance. The identification of such genes may improve our understanding of how the body clears insulin, thus leading to improved risk assessment, diagnosis, prevention and therapy of diabetes, as well as of other hyperinsulinaemic disorders, such as the metabolic syndrome and polycystic ovary syndrome.

Use of a two-stage insulin infusion study to assess the relationship between insulin suppression of lipolysis and insulin-mediated glucose uptake in overweight/obese, nondiabetic women

Differences in insulin regulation of free fatty acids (FFAs) are not readily apparent at the same insulin concentrations used to differentiate relative insulin-mediated glucose disposal. Resistance to insulin-mediated glucose disposal and higher daylong FFA concentrations occur more commonly in obese individuals. However, the relationship between the ability of insulin to suppress FFA release from adipose tissue and stimulate glucose disposal in muscle has not been clearly defined in this population. The current study was initiated to test the hypothesis that these 2 facets of insulin action are related, with greater defects in insulin-mediated glucose disposal associated with less effective insulin inhibition of FFA release from adipose tissue. Subjects included 56 healthy nondiabetic overweight/moderately obese women classified as insulin resistant or insulin sensitive based on whole-body glucose disposal. All underwent a modified 240-minute 2-stage insulin infusion with basal (∼15 µU/mL) and physiologically elevated (∼80 µU/mL) steady-state insulin concentrations. Plasma glucose, insulin, FFA, and glycerol were measured throughout. Whereas plasma glucose differed most during physiological hyperinsulinemia in insulin-resistant vs insulin-sensitive subjects, plasma FFA/glycerol differed most during basal insulin concentrations. The FFA concentrations during the basal insulin steady state correlated highly (r = 0.85, P < .001) with glucose concentrations during the hyperinsulinemic steady state. Overweight/moderately obese women exhibit dramatic differences in the ability of insulin to suppress plasma FFA, which correlate highly with differences in insulin-mediated glucose disposal. Variability in insulin regulation of FFA is most apparent at basal insulin concentrations, whereas differences in glucose disposal are most apparent during physiologic hyperinsulinemia. Both can be quantified using a simple 2-stage insulin infusion study, with first-stage FFA concentrations and second-stage glucose concentrations being most informative.

Effect of lifestyle intervention plus rosiglitazone or placebo therapy on left ventricular mass assessed with cardiovascular magnetic resonance in the metabolic syndrome

BACKGROUND

To evaluate the effect of lifestyle intervention in conjunction with rosiglitazone or placebo therapy on left ventricular (LV) mass, using cardiovascular magnetic resonance (CMR) in the metabolic syndrome.

METHODS

The present study was a pre-specified substudy of a double-blind randomized controlled trial evaluating the effect of lifestyle intervention in conjunction with rosiglitazone or placebo therapy on carotid artery atherosclerosis in the metabolic syndrome. From this original study population, 10 subjects from the placebo group and 10 from the rosiglitazone group were randomly selected. At baseline and follow-up (52 weeks), clinical and laboratory measurements were assessed and a CMR-examination was performed to evaluate LV mass indexed for body surface area (LV mass-I). Subsequently, the effect of therapy (rosiglitazone vs. placebo) and clinical and laboratory variables on LV mass-I was evaluated.

RESULTS

In both groups, body mass index, waist circumference, systolic and diastolic blood pressure significantly decreased during follow-up. Interestingly, LV mass-I significantly decreased in the placebo group (48.9 ± 5.3 g/m2 vs. 44.3 ± 5.6 g/m2, p < 0.001) indicating reverse remodeling, whereas LV mass-I remained unchanged in the rosiglitazone group (54.7 ± 9.9 g/m2 vs. 53.7 ± 9.2 g/m2, p = 0.3). After correction for systolic and diastolic blood pressure and triglyceride, the kind of therapy (rosiglitazone vs. placebo) remained the only significant predictor of LV mass-I reduction.

CONCLUSIONS

Lifestyle intervention resulted in a reduction of LV mass-I in the metabolic syndrome, indicating reverse remodeling. However, rosiglitazone therapy may have inhibited this positive reverse remodeling.

TRIAL REGISTRATION

Current Controlled Trials ISRCTN54951661.

What is the effect of rosiglitazone treatment on insulin secretory function in insulin-resistant individuals? It depends on how you measure it

The goal of this study was to compare methods used to quantify the effect of rosiglitazone (RSG) on insulin secretory function, particularly estimates based on changes in fasting plasma glucose and insulin concentration vs daylong insulin responses to meals. To do this, we compared these measures of insulin secretion before and 3 months after RSG treatment in insulin-resistant individuals, subdivided into nondiabetic subjects (n = 29) and patients with type 2 diabetes mellitus (2DM) (n = 22). Insulin resistance was quantified by determining the steady-state plasma glucose concentration during the insulin suppression test and insulin secretory function by homeostasis model assessment of β-cell function (HOMA-β) and the total integrated daylong plasma insulin responses to mixed meals (insulin area under the curve). Baseline fasting and daylong plasma glucose concentrations were higher (P < .001) in patients with 2DM, associated with lower HOMA-β values (P < .001). However, neither fasting nor daylong insulin concentrations after mixed meals differed in the 2 groups. Insulin sensitivity improved (P < .001) after RSG administration, with decreases of 31% ± 23% and 21% ± 14% in steady-state plasma glucose concentration in nondiabetic and diabetic subjects, respectively. Although fasting and daylong plasma glucose and insulin concentrations fell (P < .001) in both groups of RSG-treated individuals, HOMA-β decreased in nondiabetic subjects and did not change in those with 2DM. In conclusion, RSG administration improved insulin sensitivity in both groups, associated with lower fasting and daylong glucose concentrations. Fasting and daylong insulin concentrations were also lower in both groups of RSG-treated subjects, but the values of HOMA-β indicated either a decrease (nondiabetics) or no change (diabetics) in insulin secretory function. These results suggest that measurements of HOMA-β may not provide a complete view of insulin secretory function, either when comparing diabetic with nondiabetic individuals or when assessing the response to RSG treatment in insulin-resistant individuals.

Glucose-stimulated insulin secretion in gastric bypass patients with hypoglycemic syndrome: no evidence for inappropriate pancreatic beta-cell function

BACKGROUND

Roux-en-Y gastric bypass surgery (RYGB) has been associated with a hypoglycemic syndrome characterized by postprandial hypoglycemia and hyperinsulinemia. The syndrome is believed to occur due to insulin hypersecretion from either pancreatic beta-cell hyperplasia or hyperfunction.

METHODS

Eight RYGB patients with hypoglycemic syndrome had insulin secretion rates determined during a 240-min graded intravenous glucose infusion. They were compared to 34 nondiabetic, nonsurgical individuals who were divided based on their insulin sensitivity status as measured by the insulin suppression test: insulin-sensitive (n = 8), insulin intermediate (n = 7), and insulin-resistant (n = 19).

RESULTS

RYGB patients had insulin concentrations and HOMA-IR similar to the insulin-sensitive reference group. In addition, integrated insulin secretion rates were comparable to the insulin-sensitive group and significantly lower than the insulin intermediate (p <or= 0.046) and insulin-resistant groups (p <or= 0.001). Pancreatic beta-cell sensitivity to glucose (slope relating glucose and ISR) was lowest in the RYGB group (p <or= 0.04).

CONCLUSIONS

Patients with hypoglycemic syndrome post-RYGB do not have generalized hypersecretion of insulin and appear to have appropriate insulin secretion rate in response to intravenous glucose.

Pioglitazone acutely reduces insulin secretion and causes metabolic deceleration of the pancreatic beta-cell at submaximal glucose concentrations

Thiazolidinediones (TZDs) have beneficial effects on glucose homeostasis via enhancement of insulin sensitivity and preservation of beta-cell function. How TZDs preserve beta-cells is uncertain, but it might involve direct effects via both peroxisome proliferator-activated receptor-gamma-dependent and -independent pathways. To gain insight into the independent pathway(s), we assessed the effects of short-term (<or=90 min) exposure to pioglitazone (Pio) (10 to 50 microM) on glucose-induced insulin secretion (GIIS), AMP-activated protein kinase (AMPK) activation, and beta-cell metabolism in INS 832/13 beta-cells and rat islets. Pio caused a right shift in the dose-dependence of GIIS, such that insulin release was reduced at intermediate glucose but unaffected at either basal or maximal glucose concentrations. This was associated in INS 832/13 cells with alterations in energy metabolism, characterized by reduced glucose oxidation, mitochondrial membrane polarization, and ATP levels. Pio caused AMPK phosphorylation and its action on GIIS was reversed by the AMPK inhibitor compound C. Pio also reduced palmitate esterification into complex lipids and inhibited lipolysis. As for insulin secretion, the alterations in beta-cell metabolic processes were mostly alleviated at elevated glucose. Similarly, the antidiabetic agents and AMPK activators metformin and berberine caused a right shift in the dose dependence of GIIS. In conclusion, Pio acutely reduces glucose oxidation, energy metabolism, and glycerolipid/fatty acid cycling of the beta-cell at intermediate glucose concentrations. We suggest that AMPK activation and the metabolic deceleration of the beta-cell caused by Pio contribute to its known effects to reduce hyperinsulinemia and preserve beta-cell function and act as an antidiabetic agent.

Neferine enhances insulin sensitivity in insulin resistant rats

ETHNOPHARMACOLOGICAL RELEVANCE

Neferine was isolated from green seed embryo of Nelumbo nucifera Gaertn which has been used as an anti-obesity agent in traditional Chinese herbal medicine.

AIM OF THE STUDY

This study was conducted to investigate the effects of neferine on enhancing insulin sensitivity in insulin resistant rats compared with rosiglitazone and to potentially reveal its role in mediating the anti-obesity properties of Nelumbo nucifera Gaertn.

MATERIALS AND METHODS

Fasting blood glucose (FBG), fasting blood insulin (FINS), triglycerides (TG) and tumor necrosis factor-alpha (TNF-alpha) were measured, and the oral glucose tolerance test for 2-h plasma glucose level (2-h PG) was carried out. The glucose infusion rate (GIR) was used to measure the insulin sensitivity by hyperinsulinemic euglycemic clamp technique.

RESULTS

The levels of FBG, FINS, TG, TNF-alpha and 2-h PG all decreased significantly in the rosiglitazone and neferine groups compared with the insulin resistance (IR) model group. Neferine diminished the 2-h PG more than did rosiglitazone treatment. Compared to the IR model group, the treatments of neferine and rosiglitazone remarkably increased GIRs but no difference between these two treatments themselves was evident.

CONCLUSIONS

These data demonstrate that neferine has effects similar to rosiglitazone in decreasing fasting blood glucose, insulin, TG, TNF-alpha and enhancing insulin sensitivity in insulin resistant rats.

Evaluation of rosiglitazone administration on cardiovascular function in severe obesity

BACKGROUND

Obese patients have myocardial structural and functional alterations related to insulin resistance.

HYPOTHESIS

The purpose of the study was to analyze the effects of rosiglitazone, an insulin sensitizer agent, on cardiac morphometry and functioning.

METHODS

In 2 groups of sex- and age-matched, nondiabetic, obese patients (5 men and 7 women, age 19-51 y; group A: body mass index [BMI] 40.6 +/- 3.4 kg/m(2); group B: BMI 42.6 +/- 2.7 kg/m(2)), we evaluated the basal insulin sensitivity index (HOMA[IS]), body composition by bioelectrical impedance analysis and 24-h blood pressure monitoring. Furthermore, all patients underwent conventional 2-Dimensional and color Doppler echocardiography, and pulsed-wave tissue Doppler imaging (TDI). After the baseline evaluation, all patients were put on a hypocaloric diet (70% basal metabolic rate) plus placebo if they were in group A, or plus rosiglitazone (4 mg twice daily; Avandia [GlaxoSmithKline plc., Brentford, Middlesex, United Kingdom]) if they were in group B, for 6 mo.

RESULTS

Significant decreases in body weight, total fat mass, BMI, and systolic blood pressure were registered in both groups. Rosiglitazone administration appeared more efficient in improving HOMA(IS) (mean difference: 0.30 +/- 0.19 versus 0.11 +/- 0.21, p < 0.05). Left ventricular (LV) diastolic diameter (49.4 +/- 7.7 versus 52.3 +/- 5.4 mm, p < 0.05) and E wave (0.89 +/- 0.18 versus 0.99 +/- 0.20 m/sec, p < 0.05) increased in the rosiglitazone group due to a rise in preload and water content without peripheral edema. The increase in systolic (Sa) wave velocity in both groups was probably a result of the general improvement in insulin metabolism and the decrease in blood pressure.

CONCLUSIONS

We confirmed the positive effect of rosiglitazone on glucose metabolism in obese, nondiabetic patients, but changes in insulin sensitivity did not explain the cardiac effects produced by further mechanisms.

Rosiglitazone is more effective than metformin in improving fasting indexes of glucose metabolism in severely obese, non-diabetic patients

AIM

In obese patients, the diet-induced weight loss markedly improves glucose tolerance with an increase in insulin sensitivity and a partial reduction of insulin secretion. The association with metformin treatment might potentiate the effect of diet alone.

METHODS

From patients admitted to our Nutritional Division for diet programme, we selected obese, non-diabetic, uncomplicated patients with age 18-65 years and body mass index 35-50 kg/m(2) and studied the effects of a 6-month pharmacological treatment with either metformin (850 mg twice daily) or rosiglitazone (4 mg twice daily) on possible changes in body weight, fat mass, glucose and lipids metabolism.

RESULTS

A significant weight loss and reduction of fat mass was demonstrated with metformin (-9.7 +/- 1.8 kg and -6.6 +/- 1.1 kg) and also with rosiglitazone (-11.0 +/- 1.9 kg and -7.2 +/- 1.8 kg), without fluid retention in either treatment group. Rosiglitazone administration induced a significant decrease in glucose concentration (4.7 +/- 0.1 vs. 4.4 +/- 0.1 mmol/l, p < 0.005) and insulin-circulating level (13.6 +/- 1.5 vs. 8.0 +/- 0.,7 microU/ml, p < 0.005), an increase in insulin sensitivity as measured by homeostatic model assessment (HOMA) of insulin sensitivity (68.9 +/- 8.8 vs. 109.9 +/- 10.3, p < 0.005) with a concomitant decrease in beta-cell function as measured by HOMA of beta-cell function (163.2 +/- 16.1 vs. 127.4 +/- 8.4, p < 0.005). In contrast, metformin did not produce any significant effect on blood glucose concentration, insulin level and HOMA2 indexes. No adverse events were registered with pharmacological treatments.

CONCLUSION

Our study shows that in severely obese, non-diabetic, hyperinsulinaemic patients undergoing a nutritional programme, rosiglitazone is more effective than metformin in producing favourable changes in fasting-based indexes of glucose metabolism, with a reduction of both insulin resistance and hyperinsulinaemia. In spite of previous studies reporting rosiglitazone-induced body weight gain, in our study the joint treatment with diet and rosiglitazone was accompanied by weight loss and fat mass reduction.

Rosiglitazone treatment improves cardiac efficiency in hearts from diabetic mice

Isolated perfused hearts from type 2 diabetic (db/db) mice show impaired ventricular function, as well as altered cardiac metabolism. Assessment of the relationship between myocardial oxygen consumption (MVO(2)) and ventricular pressure-volume area (PVA) has also demonstrated reduced cardiac efficiency in db/db hearts. We hypothesized that lowering the plasma fatty acid supply and subsequent normalization of altered cardiac metabolism by chronic treatment with a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist will improve cardiac efficiency in db/db hearts. Rosiglitazone (23 mg/kg body weight/day) was administered as a food admixture to db/db mice for five weeks. Ventricular function and PVA were assessed using a miniaturized (1.4 Fr) pressure-volume catheter; MVO(2) was measured using a fibre-optic oxygen sensor. Chronic rosiglitazone treatment of db/db mice normalized plasma glucose and lipid concentrations, restored rates of cardiac glucose and fatty acid oxidation, and improved cardiac efficiency. The improved cardiac efficiency was due to a significant decrease in unloaded MVO(2), while contractile efficiency was unchanged. Rosiglitazone treatment also improved functional recovery after low-flow ischemia. In conclusion, the present study demonstrates that in vivo PPARgamma-treatment restores cardiac efficiency and improves ventricular function in perfused hearts from type 2 diabetic mice.

Direct rosiglitazone-induced modifications in insulin secretion, action and clearance: a single-dose hyperglycaemic clamp study

AIMS/HYPOTHESIS

In addition to the improvement in insulin sensitivity, it has been shown that thiazolidinediones modulate beta cell function and insulin clearance in type 2 diabetic subjects. However, interactions between all these actions, and confounding factors due to co-morbidities and co-treatments in diabetic individuals, complicate the identification of specific effects. The aim of this pilot study was to investigate the potential acute effects of rosiglitazone on beta cell function and insulin sensitivity by the hyperglycaemic clamp technique in healthy volunteers.

SUBJECTS AND METHODS

Twelve healthy men were included in a randomised, double-blind crossover study. Rosiglitazone (8 mg) or placebo was given orally 45 min before the hyperglycaemic clamp (10 mmol/l for 2 h).

RESULTS

The second phase of the insulin response was significantly decreased by rosiglitazone: 13,066 +/- 1,531 vs 16,316 +/- 2,813 pmol l(-1) 110 min in controls (p < 0.05), without change in the first phase. Serum C-peptide was not modified. Rosiglitazone treatment significantly increased insulin clearance (molar ratio of the C-peptide to insulin AUCs: 12.80 +/- 1.34 vs 11.38 +/- .33, p < 0.05) and the insulin sensitivity index (12.0 +/- 1.5 vs 8.5 +/- 1.1 micromol m(-2) min(-1) pmol(-1)l, p < 0.01).

CONCLUSIONS/INTERPRETATION

The present results show that a single dose of rosiglitazone rapidly increases insulin clearance and insulin sensitivity index in healthy volunteers, with no direct effect on insulin secretion. The precise mechanisms mediating these actions remain to be determined.

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-naïve 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.

The dual peroxisome proliferator-activated receptor alpha/gamma activator muraglitazar prevents the natural progression of diabetes in db/db mice

There are two major defects in type 2 diabetes: 1) insulin resistance and 2) insulin deficiency due to loss of beta-cell function. Here we demonstrated that treatment with muraglitazar (a dual peroxisome proliferator-activated receptor alpha/gamma activator), when initiated before or after the onset of diabetes in mice, is effective against both defects. In study 1, prediabetic db/db mice were treated for 12 weeks. The control mice developed diabetes, as evidenced by hyperglycemia, hyperinsulinemia, reduced insulin levels in the pancreas, blunted insulin response to glucose, and impaired glucose tolerance. The muraglitazar-treated mice had normal plasma glucose, and insulin levels, equivalent or higher pancreatic insulin content than normal mice, showed a robust insulin response to glucose and exhibited greater glucose tolerance. In study 2, diabetic db/db mice were treated for 4 weeks. The control mice displayed increased glucose levels, severe loss of islets, and their isolated islets secreted reduced amounts of insulin in response to glucose and exendin-4 compared with baseline. In muraglitazar-treated mice, glucose levels were reduced to normal. These mice showed reduced loss of islets, and their isolated islets secreted insulin at levels comparable to baseline. Thus, muraglitazar treatment decreased both insulin resistance and preserved beta-cell function. As a result, muraglitazar treatment, when initiated before the onset of diabetes, prevented development of diabetes and, when initiated after the onset of diabetes, prevented worsening of diabetes in db/db mice.

Thiazolidinediones increase hepatic insulin extraction in African Americans with impaired glucose tolerance and type 2 diabetes mellitus. A pilot study of rosiglitazone

Peripheral insulin levels are determined by beta-cell secretion, insulin sensitivity, and hepatic insulin extraction (HIE). We have previously shown that whereas sulfonylureas reduce insulin extraction, metformin enhances HIE. However, the effects of thiazolidinediones (TZDs) on HIE remain uncertain. Thus, we investigated the potential contribution of hepatic insulin clearance to peripheral insulin levels during rosiglitazone therapy in African Americans with impaired glucose tolerance (IGT) and type 2 diabetes mellitus (DM). The study was composed of 12 first-degree relatives with IGT and 17 patients with newly diagnosed type 2 DM. Nineteen healthy relatives with normal glucose tolerance served as controls. Serum glucose, insulin, and C-peptide, and HIE (C-peptide-insulin molar ratios) were measured at t = 0 and 120 minutes during oral glucose tolerance test (OGTT) in all the subjects. The OGTT was performed before and after 3 months of rosiglitazone therapy (4 mg/d x 4 weeks and >8 mg/d x 8 weeks) in patients with IGT and type 2 DM. Insulin resistance index and beta-cell function were calculated in each subject using homeostasis model assessment (HOMA). Rosiglitazone therapy improved but did not normalize the overall glycemic control in the IGT and type 2 DM groups. After rosiglitazone therapy, the mean serum insulin and C-peptide levels at fasting remained unchanged. However, the 2-hour serum glucose and insulin were lower, whereas serum C-peptide was unchanged during 3 months of rosiglitazone treatment. Mean insulin resistance index of HOMA was reduced by 30% (4.12 +/- 1.95 vs 6.33 +/- 3.54, P < .05) in the type 2 DM group and by 21% (3.78 +/- 2.45 vs 4.81 +/- 3.49, P = NS) in the IGT group. Mean HIE values were significantly lower (70%) in the type 2 DM and IGT groups when compared with the normal glucose tolerance group. At 3 months, basal HIE was not significantly changed by rosiglitazone therapy in IGT and type 2 DM groups when compared with the baseline (0 month). However, rosiglitazone therapy was associated with increased HIE at 2 hours during OGTT by 40% and 30% in the IGT and type 2 DM groups, respectively, from the baseline (0 month) values. Furthermore, HIE inversely correlated with the insulin resistance index of HOMA (r = -.46, P < .05). We conclude that rosiglitazone therapy improved overall glucose tolerance and enhanced insulin sensitivity in patients with IGT and type 2 DM. Although basal HIE remained unchanged, rosiglitazone therapy increased postglucose challenge HIE in African Americans with IGT and type 2 DM. We speculate that TZDs increase insulin clearance or HIE after oral glucose challenge. This study suggests that in addition to insulin sensitization, rosiglitazone may be involved in insulin metabolism. The significance of the increased insulin clearance by TZD therapy remains uncertain and deserves further investigation in patients with insulin resistance and glucose intolerance.

Effect of pioglitazone on insulin secretion in patients with both impaired fasting glucose and impaired glucose tolerance

AIM

To evaluate the effect of a thiazolidinedione, pioglitazone, on insulin secretion in patients with both impaired fasting glucose and impaired glucose tolerance.

METHODS

A randomized, double-blind, placebo-controlled clinical trial was carried out in 18 overweight or obese patients with both impaired fasting glucose and impaired glucose tolerance. Pharmacological intervention consisted of an oral morning administration of pioglitazone (30 mg) or a placebo with a similar presentation for 30 days. Before and after the intervention, glucose, creatinine, lipid profile and uric acid concentrations were measured. To evaluate insulin secretion (early, late and total phases) and insulin sensitivity, a hyperglycemic-hyperinsulinemic clamp was also performed.

RESULTS

There were significant reductions (p=0.008) in fasting insulin concentration (121 versus 45 pmol/l), late (565 versus 307 pmol/l) and total insulin secretion (474 versus 254 pmol/l), as well as, in 2h postload glucose levels (9.7 versus 6.9 mmol/l, p=0.028), with an increment in insulin sensitivity after pioglitazone administration (7.5 versus 9.9).

CONCLUSION

Pioglitazone administration during a period of 4 weeks decreased late and total insulin secretion phases, fasting insulin and 2h postload glucose levels, and improved insulin sensitivity in patients with both impaired fasting glucose and impaired glucose tolerance.

Pioglitazone acutely influences glucose-sensitive insulin secretion in normal and diabetic human islets

We have studied acute effects of the PPARgamma agonist pioglitazone in vitro on human islets from both non-diabetic and type 2 diabetic subjects. In 5 mM glucose, pioglitazone caused a transient increase in insulin secretion in non-diabetic, but not diabetic, islets. Continuous presence of the drug suppressed insulin release in both non-diabetic and diabetic islets. In islets from non-diabetic subjects, both high glucose and tolbutamide-stimulated insulin secretion was inhibited by pioglitazone. When islets were continuously perifused with 5 mM glucose, short-term pretreatment with pioglitazone caused approximately 2-fold increase in insulin secretion after drug withdrawal. Pioglitazone pretreatment of diabetic islets restored their glucose sensitivity. Examination of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in non-diabetic islets revealed slight Ca(2+) transient by pioglitazone at 3 mM glucose with no significant changes at high glucose. Our data suggest that short-term pretreatment with pioglitazone primes both healthy and diabetic human islets for enhanced glucose-sensitive insulin secretion.