Plasma resistin concentration, hepatic fat content, and hepatic and peripheral insulin resistance in pioglitazone-treated type II diabetic patients

OBJECTIVES

To study the effect of pioglitazone (PIO) on plasma resistin concentration, endogenous glucose production (EGP), and hepatic fat content (HFC) in patients with type II diabetes (T2DM).

SUBJECTS

A total of 13 T2DM patients (age=51+/-2 y, BMI=29.7+/-1.1 kg/m(2), HbA(1c)=8.0+/-0.5%).

METHODS

HFC (magnetic resonance spectroscopy) and basal plasma resistin concentration were quantitated before and after PIO treatment (45 mg/day) for 16 weeks. Subjects received a 3 h euglycemic insulin (100 mU/m(2)/min) clamp with 3-[(3)H] glucose to determine rates of EGP and tissue glucose disappearance (Rd) before and after PIO.

RESULTS

PIO reduced fasting plasma glucose (10.3+/-0.7 to 7.6+/-0.6 mmol/l, P<0.001) and HbA(1c) (8.0+/-0.4 to 6.8+/-0.3%, P<0.001) despite increased body weight (83.2+/-3.4 to 86.3+/-3.4 kg, P<0.001). PIO improved Rd (4.9+/-0.4 to 6.6+/-0.5 mg/kg/min, P<0.005) and reduced EGP (0.22+/-0.04 to 0.06+/-0.02 mg/kg/min, P<0.01) during the insulin clamp. Following PIO, HFC decreased from 21.1+/-3.5 to 11.2+/-2.1% (P<0.005), and plasma resistin decreased from 5.3+/-0.6 to 3.5+/-0.3 ng/ml (P<0.01). Plasma resistin concentration correlated positively with HFC before (r=0.58, P<0.05) and after (r=0.55, P<0.05) PIO treatment. Taken collectively, plasma resistin concentration, before and after PIO treatment, correlated positively with hepatic fat content (r=0.66, P<0.001) and EGP during the insulin clamp (r=0.41, P<0.05). However, the plasma resistin concentration did not correlate with whole body glucose disposal (Rd) during the insulin clamp either before (r=-0.18, P=NS) or after (r=-0.13, P=NS) PIO treatment.

CONCLUSIONS

PIO treatment in T2DM causes a significant decrease in plasma resistin concentration. The decrease in plasma resistin is positively correlated with the decrease in hepatic fat content and improvement in hepatic insulin sensitivity.

Adiponectin receptors gene expression and insulin sensitivity in non-diabetic Mexican Americans with or without a family history of Type 2 diabetes

AIMS/HYPOTHESIS

The recent discovery of two adiponectin receptors (AdipoR1 and AdipoR2) will improve our understanding of the molecular mechanisms underlying the insulin-sensitising effect of adiponectin. The aim of this study was to determine for the first time whether skeletal muscle AdipoR1 and/or AdipoR2 gene expression levels are associated with insulin resistance.

METHODS

Using RT-PCR and northern analysis we measured AdipoR1 and AdipoR2 gene expression in skeletal muscle from healthy Mexican Americans with normal glucose tolerance who had (n=8) or did not have (n=10) a family history of Type 2 diabetes.

RESULTS

Gene expression profiling indicated that the AdipoR1 and AdipoR2 isoforms are highly expressed in human skeletal muscle, unlike in mice where AdipoR2 expression was highest in the liver, and AdipoR1 was highest in skeletal muscle. In the study subjects, the expression levels of AdipoR1 (p=0.004) and AdipoR2 (p=0.04), as well as plasma adiponectin concentration (p=0.03) were lower in people with a family history of Type 2 diabetes than in those with no family history of the disease. Importantly, the expression levels of both receptors correlated positively with insulin sensitivity (r=0.64, p=0.004 and r=0.47, p=0.048 respectively).

CONCLUSIONS/INTERPRETATION

Collectively, these data indicate that both isoforms of the adiponectin receptor play a role in the insulin-sensitising effect of adiponectin.

Beta-cell dysfunction and glucose intolerance: results from the San Antonio metabolism (SAM) study

AIMS/HYPOTHESIS

Both insulin resistance and beta-cell dysfunction play a role in the transition from normal glucose tolerance (NGT) to Type 2 diabetes (T2DM) through impaired glucose tolerance (IGT). The aim of the study was to define the level of glycaemia at which beta-cell dysfunction becomes evident in the context of existing insulin resistance.

METHODS

Insulin response (OGTT) and insulin sensitivity (euglycaemic insulin clamp) were evaluated in 388 subjects in the San Antonio Metabolism (SAM) study (138 NGT, 49 IGT and 201 T2DM). In all subjects the insulin secretion/insulin resistance index (DeltaI/DeltaG/IR) was calculated as the ratio of the increment in plasma insulin to the increment in plasma glucose during the OGTT divided by insulin resistance, as measured during the clamp.

RESULTS

In lean NGTs with a 2-h plasma glucose concentration (2-h PG) between 5.6 and 6.6 and between 6.7 and 7.7 mmol/l, there was a progressive decline in DeltaI/DeltaG/IR compared with NGTs with a 2-h PG less than 5.6 mmol/l. There was a further decline in DeltaI/DeltaG/IR in IGTs with a 2-h PG between 7.8 and 9.3 and between 9.4 and 11.0 mmol/l, and in Type 2 diabetic patients with a 2-h PG greater than 11.1 mmol/l. Lean and obese subjects showed coincident patterns of relation of 2-h PG to DeltaI/DeltaG/IR.

CONCLUSION/INTERPRETATION

When the plasma insulin response to oral glucose is related to the glycaemic stimulus and severity of insulin resistance, there is a progressive decline in beta-cell function that begins in "normal" glucose tolerant individuals.

Predominant role of reduced beta-cell sensitivity to glucose over insulin resistance in impaired glucose tolerance

AIMS/HYPOTHESIS

Impaired glucose tolerance (IGT) is an insulin-resistant state and a risk factor for Type 2 diabetes. The relative roles of insulin resistance and insulin deficiency in IGT have been disputed.

METHODS

In 40 IGT subjects and 63 sex-, age-, and weight-matched controls with normal glucose tolerance (NGT), we measured (i) indices of insulin sensitivity of fasting glucose production (by tracer glucose) and glucose disposal (M value on a 240 pmol x min(-1) x m(-2) insulin clamp) and (ii) indices of beta-cell function (glucose sensitivity, rate sensitivity, and potentiation) derived from model analysis (Am J Physiol 283:E1159-E1166, 2002) of the insulin secretory response (by C-peptide deconvolution) to oral glucose.

RESULTS

In comparison with NGT, IGT were modestly insulin resistant (M=29+/-2 vs 35+/-2 micromol x min(-1) x kg(FFM)(-1), p=0.01); insulin sensitivity of glucose production also was reduced, in approximate proportion to M. Despite higher baseline insulin secretion rates, IGT was characterized by a 50% reduction in glucose sensitivity [53 (36) vs 102 (123) pmol x min(-1) x m(-2) x mM(-1), median (interquartile range), p=0.001] and impaired potentiation [1.6 (0.8) vs 2.0 (1.5) units, p<0.04] of insulin release, whereas rate sensitivity [1.15 (1.15) vs 1.38 (1.28) nmol x m(-2) x mM(-1)] was not significantly reduced. Glucose sensitivity made the single largest contribution (approximately 50%) to the observed variability of glucose tolerance.

CONCLUSION/INTERPRETATION

In IGT the defect in glucose sensitivity of insulin release quantitatively predominates over insulin resistance in the genesis of the reduced tolerance to oral glucose.

Effect of misoprostol (PGE1) on glucose metabolism in type-2-diabetic and control subjects

In vitro and in vivo studies have demonstrated that prostaglandins of the E series enhance muscle glucose uptake. We examined the effect of acute misoprostol (PGE1) administration on whole body insulin-mediated glucose disposal, as well as the major intracellular pathways of glucose metabolism in type 2 diabetic (n = 10) and non-diabetic (n = 4) subjects. Each subject received two 240-min euglycaemic insulin (40 mU/m2/min) clamp studies with tritiated glucose and indirect calorimetry. During one of the insulin clamp studies, 200 microg of misoprostol was ingested at 90 and 150 min after the start of the insulin infusion. Insulin-mediated total body glucose disposal, glycolysis, glycogenesis and glucose oxidation were similar during the insulin clamp studies performed without and with misoprostol in both the diabetic and non-diabetic groups. These results demonstrate that the acute administration of misoprostol does not enhance insulin-mediated glucose disposal in either type-2-diabetic or non-diabetic subjects.

Effect of rosiglitazone on glucose and non-esterified fatty acid metabolism in Type II diabetic patients

AIMS/HYPOTHESIS

We aimed to examine the mechanisms by which rosiglitazone improves glycaemic control in Type II (non-insulin-dependent) diabetic patients.

METHODS

Altogether 29 diet-treated diabetic patients were assigned at random to rosiglitazone, 8 mg/day (n = 15), or placebo (n = 14) for 12 weeks. Patients received 75 g OGTT and two-step euglycaemic insulin (40 and 160 mU/m(2)min) clamp with 3-(3)H-glucose, (14)C-palmitate and indirect calorimetry.

RESULTS

After 12 weeks, rosiglitazone reduced fasting plasma glucose (195 +/- 11 to 150 +/- 7 mg/dl, p < 0.01), mean plasma glucose (PG) during OGTT (293 +/- 12 to 236 +/- 9 mg/dl, p < 0.01), and HbA1 c (8.7 +/- 0.4 to 7.4 +/- 0.3 %, p < 0.01) without changes in plasma insulin concentration. Basal endogenous glucose production (EGP) declined (3.3 +/- 0.1 to 2.9 +/- 0.1 mg/kg FFM. min, p < 0.05) and whole body glucose metabolic clearance rate increased after rosiglitazone (first clamp step: 2.8 +/- 0.2 to 3.5 +/- 0.2 ml/kg FFM. min, p < 0.01; second clamp step: 6.7 +/- 0.6 to 9.2 +/- 0.8, p < 0.05) despite increased body weight (86 +/- 4 to 90 +/- 4 kg, p < 0.01) and fat mass (33 +/- 3 to 37 +/- 3 kg, p < 0.01). Fasting plasma non-esterified fatty acid (NEFA) (735 +/- 52 to 579 +/- 49 microEq/l, p < 0.01), mean plasma NEFA during OGTT (561 +/- 33 to 424 +/- 35, p < 0.01), and basal NEFA turnover (18.3 +/- 1.5 to 15.5 +/- 1.2 microEq/kg FM. min, p < 0.05) decreased after rosiglitazone. Changes in EPG and mean plasma glucose (PG) during OGTT correlated with changes in basal EGP (r = 0.54; r = 0.58), first EGP (r = 0.36; r = 0.41), first MCR (r = -0.66; r = -0.68), second MCR (r = -0.49; r = -0.54), fasting plasma NEFA (r = 0.53; r = 0.49), and NEFA during OGTT (r = 0.66; r = 0.66).

CONCLUSION/INTERPRETATION

Rosiglitazone increases hepatic and peripheral (muscle) tissue insulin sensitivity and reduces NEFA turnover despite increased total body fat mass. These results suggest that the beneficial effects of rosiglitazone on glycaemic control are mediated, in part, by the drug's effect on NEFA metabolism.

Skeletal muscle insulin resistance in normoglycemic subjects with a strong family history of type 2 diabetes is associated with decreased insulin-stimulated insulin receptor substrate-1 tyrosine phosphorylation

Normoglycemic subjects with a strong family history of type 2 diabetes are insulin resistant, but the mechanism of insulin resistance in skeletal muscle of such individuals is unknown. The present study was undertaken to determine whether abnormalities in insulin-signaling events are present in normoglycemic, nonobese subjects with a strong family history of type 2 diabetes. Hyperinsulinemic-euglycemic clamps with percutaneous muscle biopsies were performed in eight normoglycemic relatives of type 2 diabetic patients (FH(+)) and eight control subjects who had no family history of diabetes (FH(-)), with each group matched for age, sex, body composition, and ethnicity. The FH(+) group had decreased insulin-stimulated glucose disposal (6.64 +/- 0.52 vs. 8.45 +/- 0.54 mg. kg(-1) fat-free mass. min(-1); P < 0.05 vs. FH(-)). In skeletal muscle, the FH(+) and FH(-) groups had equivalent insulin stimulation of insulin receptor tyrosine phosphorylation. In contrast, the FH(+) group had decreased insulin stimulation of insulin receptor substrate (IRS)-1 tyrosine phosphorylation (0.522 +/- 0.077 vs. 1.328 +/- 0.115 density units; P < 0.01) and association of PI 3-kinase activity with IRS-1 (0.299 +/- 0.053 vs. 0.466 +/- 0.098 activity units; P < 0.05). PI 3-kinase activity was correlated with the glucose disposal rate (r = 0.567, P = 0.02). In five subjects with sufficient biopsy material for further study, phosphorylation of Akt was 0.266 +/- 0.061 vs. 0.404 +/- 0.078 density units (P < 0.10) and glycogen synthase activity was 0.31 +/- 0.06 vs. 0.50 +/- 0.12 ng. min(-1). mg(-1) (P < 0.10) for FH(+) and FH(-) subjects, respectively. Therefore, despite normal insulin receptor phosphorylation, postreceptor signaling was reduced and was correlated with glucose disposal in muscle of individuals with a strong genetic background for type 2 diabetes.

Physiological hyperinsulinemia impairs insulin-stimulated glycogen synthase activity and glycogen synthesis

Although chronic hyperinsulinemia has been shown to induce insulin resistance, the basic cellular mechanisms responsible for this phenomenon are unknown. The present study was performed 1) to determine the time-related effect of physiological hyperinsulinemia on glycogen synthase (GS) activity, hexokinase II (HKII) activity and mRNA content, and GLUT-4 protein in muscle from healthy subjects, and 2) to relate hyperinsulinemia-induced alterations in these parameters to changes in glucose metabolism in vivo. Twenty healthy subjects had a 240-min euglycemic insulin clamp study with muscle biopsies and then received a low-dose insulin infusion for 24 (n = 6) or 72 h (n = 14) (plasma insulin concentration = 121 +/- 9 or 143 +/- 25 pmol/l, respectively). During the baseline insulin clamp, GS fractional velocity (0.075 +/- 0.008 to 0.229 +/- 0.02, P < 0.01), HKII mRNA content (0.179 +/- 0.034 to 0.354 +/- 0.087, P < 0.05), and HKII activity (2.41 +/- 0.63 to 3.35 +/- 0.54 pmol x min(-1) x ng(-1), P < 0.05), as well as whole body glucose disposal and nonoxidative glucose disposal, increased. During the insulin clamp performed after 24 and 72 h of sustained physiological hyperinsulinemia, the ability of insulin to increase muscle GS fractional velocity, total body glucose disposal, and nonoxidative glucose disposal was impaired (all P < 0.01), whereas the effect of insulin on muscle HKII mRNA, HKII activity, GLUT-4 protein content, and whole body rates of glucose oxidation and glycolysis remained unchanged. Muscle glycogen concentration did not change [116 +/- 28 vs. 126 +/- 29 micromol/kg muscle, P = nonsignificant (NS)] and was not correlated with the change in nonoxidative glucose disposal (r = 0.074, P = NS). In summary, modest chronic hyperinsulinemia may contribute directly (independent of change in muscle glycogen concentration) to the development of insulin resistance by its impact on the GS pathway.

Improved glycemic control and enhanced insulin sensitivity in type 2 diabetic subjects treated with pioglitazone

OBJECTIVE

To elucidate the effects of pioglitazone treatment on glucose and lipid metabolism in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

A total of 23 diabetic patients (age 30-70 years BMI < 36 kg/m2) who being treated with a stable dose of sulfonylurea were randomly assigned to receive either placebo (n = 11) or pioglitazone (45 mg/day) (n = 12) for 16 weeks. Before and after 16 weeks of treatment, all subjects received a 75-g oral glucose tolerance test (OGTT) and hepatic peripheral insulin sensitivity was measured with a two-step euglycemic insulin (40 and 160 mU x min(-1) x m(-2) clamp performed with 3-[3H]glucose and indirect calorimetry HbA1c measured monthly throughout the study period.

RESULTS

After 16 weeks of pioglitazone treatment, the fasting plasma glucose (FPG; 184 +/- 15 to 135 +/- 11 mg/dl, P < 0.01), mean plasma glucose during OGTT(293 +/- 12 to 225 +/- 14 mg/dl, P < 0.01), and HbA1c (8.9 +/- 0.3 to 7.2 +/- 0.5%, P < 0.01 ) decreased significantly without change in fasting or glucose-stimulated insulin/C-peptide concentrations. Fasting plasma free fatty acid (FFA; 647 +/- 39 to 478 +/- 49) microEq/l, P < 0.01) and mean plasma FFA during OGTT (485 +/- 30 to 347 +/- 33 microEq/l, P < 0.01) decreased significantly after pioglitazone treatment. Before and after pioglitazone treatment, basal endogenous glucose prodution (EGP) and FPG were strongly correlated (r = 0.67, P < 0.01). EGP during the first insulin clamp step was significantly decreased after pioglitazone treatment (P < 0.05) whereas insulin-stimulated total and nonoxidative glucose disposal during the second insulin clamp was increased (P < 0.01). The change in FPG was related to the change in basal EGP, EGP during the first insulin clamp step, and total glucose disposal during the second insulin clamp step. The change in mean plasma glucose concentration during the OGGTT was strongly related to the change in total body glucose disposl during the second insulin clamp step.

CONCLUSIONS

These results suggest that pioglitazone therapy in type 2 diabetic patients decreases lasting and postprandial plasma glucose levels by improving hepatic and peripheral (muscle) tissue sensitivity to insulin.

Vanadyl sulfate improves hepatic and muscle insulin sensitivity in type 2 diabetes

Vanadyl sulfate (VOSO(4)) is an oxidative form of vanadium that in vitro and in animal models of diabetes has been shown to reduce hyperglycemia and insulin resistance. Small clinical studies of 2- to 4-week duration in type 2 diabetes (T2DM) have led to inconsistent results. To define its efficacy and mechanism of action, 11 type 2 diabetic patients were treated with VOSO(4) at a higher dose (150 mg/day) and for a longer period of time (6 weeks) than in previous studies. Before and after treatment we measured insulin secretion during an oral glucose tolerance test, and endogenous glucose production (EGP) and whole body insulin-mediated glucose disposal using the euglycemic insulin clamp technique combined [3-(3)H]glucose infusion. Treatment significantly improved glycemic control: fasting plasma glucose (FPG) decreased from 194 +/- 16 to 155 +/- 15 mg/dL, hemoglobin A(1c) decreased from 8.1 +/- 0.4 to 7.6 +/- 0.4%, and fructosamine decreased from 348 +/- 26 to 293 +/- 12 micromol/L (all P < 0.01) without any change in body weight. Diabetics had an increased rate of EGP compared with nondiabetic controls (4.1 +/- 0.2 vs. 2.7 +/- 0.2 mg/kg lean body mass.min; P< 0.001), which was closely correlated with FPG (r = 0.56; P< 0.006). Vanadyl sulfate reduced EGP by about 20% (P< 0.01), and the decline in EGP was correlated with the reduction in FPG (r = 0.60; P< 0.05). Vanadyl sulfate also caused a modest increase in insulin-mediated glucose disposal (from 4.3 +/- 0.4 to 5.1 +/- 0.6 mg/kg lean body mass x min; P< 0.03), although the improvement in insulin sensitivity did not correlate with the decline in FPG after treatment (r = -0.16; P = NS). Vanadyl sulfate treatment lowered the plasma total cholesterol (223 +/- 14 vs. 202 +/- 16 mg/dL; P < 0.01) and low density lipoprotein cholesterol (141 +/- 14 vs. 129 +/- 14 mg/dL; P < 0.05), whereas 24-h ambulatory blood pressure was unaltered. We conclude that VOSO(4) at maximal tolerated doses for 6 weeks improves hepatic and muscle insulin sensitivity in T2DM. The glucose-lowering effect of VOSO(4) correlated well with the reduction in EGP, but not with insulin-mediated glucose disposal, suggesting that liver, rather than muscle, is the primary target of VOSO(4) action at therapeutic doses in T2DM.

Bromocriptine: a novel approach to the treatment of type 2 diabetes

OBJECTIVE

In vertebrates, body fat stores and insulin action are controlled by the temporal interaction of circadian neuroendocrine oscillations. Bromocriptine modulates neurotransmitter action in the brain and has been shown to improve glucose tolerance and insulin resistance in animal models of obesity and diabetes. We studied the effect of a quick-release bromocriptine formulation on glucose homeostasis and insulin sensitivity in obese type 2 diabetic subjects.

RESEARCH DESIGN AND METHODS

There were 22 obese subjects with type 2 diabetes randomized to receive a quick-release formulation of bromocriptine (n = 15) or placebo (n = 7) in a 16-week double-blind study. Subjects were prescribed a weight-maintaining diet to exclude any effect of changes in body weight on the primary outcome measurements. Fasting plasma glucose concentration and HbA(1c) were measured at 2- to 4-week intervals during treatment. Body composition (underwater weighing), body fat distribution (magnetic resonance imaging), oral glucose tolerance (oral glucose tolerance test [OGTT]), insulin-mediated glucose disposal, and endogenous glucose production (2-step euglycemic insulin clamp, 40 and 160 mU x min(-1) x m(-2)) were measured before and after treatment.

RESULTS

No changes in body weight or body composition occurred during the study in either placebo- or bromocriptine-treated subjects. Bromocriptine significantly reduced HbA(1c) (from 8.7 to 8.1%, P = 0.009) and fasting plasma glucose (from 190 to 172 mg/dl, P = 0.02) levels, whereas these variables increased during placebo treatment (from 8.5 to 9.1%, NS, and from 187 to 223 mg/dl, P = 0.02, respectively). The differences in HbA(1c) (delta = 1.2%, P = 0.01) and fasting glucose (delta = 54 mg/dl, P < 0.001) levels between the bromocriptine and placebo group at 16 weeks were highly significant. The mean plasma glucose concentration during OGTT was significantly reduced by bromocriptine (from 294 to 272 mg/dl, P = 0.005), whereas it increased in the placebo group. No change in glucose disposal occurred during the first step of the insulin clamp in either the bromocriptine- or placebo-treated group. During the second insulin clamp step, bromocriptine improved total glucose disposal from 6.8 to 8.4 mg x min(-1) kg(-1) fat-free mass (FFM) (P = 0.01) and nonoxidative glucose disposal from 3.3 to 4.3 mg min(-1) x kg(-1) FFM (P < 0.05), whereas both of these variables deteriorated significantly (P < or = 0.02) in the placebo group.

CONCLUSIONS

Bromocriptine improves glycemic control and glucose tolerance in obese type 2 diabetic patients. Both reductions in fasting and postprandial plasma glucose levels appear to contribute to the improvement in glucose tolerance. The bromocriptine-induced improvement in glycemic control is associated with enhanced maximally stimulated insulin-mediated glucose disposal.

Insulin resistance differentially affects the PI 3-kinase- and MAP kinase-mediated signaling in human muscle

The broad nature of insulin resistant glucose metabolism in skeletal muscle of patients with type 2 diabetes suggests a defect in the proximal part of the insulin signaling network. We sought to identify the pathways compromised in insulin resistance and to test the effect of moderate exercise on whole-body and cellular insulin action. We conducted euglycemic clamps and muscle biopsies on type 2 diabetic patients, obese nondiabetics and lean controls, with and without a single bout of exercise. Insulin stimulation of the phosphatidylinositol 3-kinase (PI 3-kinase) pathway, as measured by phosphorylation of the insulin receptor and IRS-1 and by IRS protein association with p85 and with PI 3-kinase, was dramatically reduced in obese nondiabetics and virtually absent in type 2 diabetic patients. Insulin stimulation of the MAP kinase pathway was normal in obese and diabetic subjects. Insulin stimulation of glucose-disposal correlated with association of p85 with IRS-1. Exercise 24 hours before the euglycemic clamp increased phosphorylation of insulin receptor and IRS-1 in obese and diabetic subjects but did not increase glucose uptake or PI 3-kinase association with IRS-1 upon insulin stimulation. Thus, insulin resistance differentially affects the PI 3-kinase and MAP kinase signaling pathways, and insulin-stimulated IRS-1-association with PI 3-kinase defines a key step in insulin resistance.

Altered hypothalamic function in response to glucose ingestion in obese humans

The hypothalamus plays a central role in the regulation of energy intake and feeding behavior. However, the presence of a functional abnormality in the hypothalamus in humans that may be related to excess energy intake and obesity has yet to be demonstrated in vivo. We, therefore, used functional magnetic resonance imaging (fMRI) to monitor hypothalamic function after oral glucose intake. The 10 obese (34 +/- 2 years of age, BMI 34.2 +/- 1.3 kg/m2) and 10 lean (32 +/- 4 years of age, BMI 22.0 +/- 0.9 kg/m2) subjects with normal glucose tolerance ingested 75 g of glucose while a midsagittal slice through the hypothalamus was continuously imaged for 50 min using a conventional T2*-weighted gradient-echo pulse sequence. After glucose ingestion, lean subjects demonstrated an inhibition of the fMRI signal in the areas corresponding to the paraventricular and ventromedial nuclei. In obese subjects, this inhibitory response was markedly attenuated (4.8 +/- 1.3 vs. 7.0 +/- 0.6% inhibition, P < 0.05) and delayed (9.4 +/- 0.5 vs. 6.4 +/- 0.5 min, P < 0.05) compared with that observed in lean subjects. The time taken to reach the maximum inhibitory response correlated with the fasting plasma glucose (r = 0.75, P < 0.001) and insulin (r = 0.47, P < 0.05) concentrations in both lean and obese subjects. These results demonstrate in vivo, for the first time, the existence of differential hypothalamic function in lean and obese humans that may be secondary to obesity.

Insulin sensitivity indices obtained from oral glucose tolerance testing: comparison with the euglycemic insulin clamp

OBJECTIVE

Several methods have been proposed to evaluate insulin sensitivity from the data obtained from the oral glucose tolerance test (OGTT). However, the validity of these indices has not been rigorously evaluated by comparing them with the direct measurement of insulin sensitivity obtained with the euglycemic insulin clamp technique. In this study, we compare various insulin sensitivity indices derived from the OGTT with whole-body insulin sensitivity measured by the euglycemic insulin clamp technique.

RESEARCH DESIGN AND METHODS

In this study, 153 subjects (66 men and 87 women, aged 18-71 years, BMI 20-65 kg/m2) with varying degrees of glucose tolerance (62 subjects with normal glucose tolerance, 31 subjects with impaired glucose tolerance, and 60 subjects with type 2 diabetes) were studied. After a 10-h overnight fast, all subjects underwent, in random order, a 75-g OGTT and a euglycemic insulin clamp, which was performed with the infusion of [3-3H]glucose. The indices of insulin sensitivity derived from OGTT data and the euglycemic insulin clamp were compared by correlation analysis.

RESULTS

The mean plasma glucose concentration divided by the mean plasma insulin concentration during the OGTT displayed no correlation with the rate of whole-body glucose disposal during the euglycemic insulin clamp (r = -0.02, NS). From the OGTT, we developed an index of whole-body insulin sensitivity (10,000/square root of [fasting glucose x fasting insulin] x [mean glucose x mean insulin during OGTT]), which is highly correlated (r = 0.73, P < 0.0001) with the rate of whole-body glucose disposal during the euglycemic insulin clamp.

CONCLUSIONS

Previous methods used to derive an index of insulin sensitivity from the OGTT have relied on the ratio of plasma glucose to insulin concentration during the OGTT. Our results demonstrate the limitations of such an approach. We have derived a novel estimate of insulin sensitivity that is simple to calculate and provides a reasonable approximation of whole-body insulin sensitivity from the OGTT.

Pharmacologic therapy for type 2 diabetes mellitus

Type 2 diabetes mellitus is a chronic metabolic disorder that results from defects in both insulin secretion and insulin action. An elevated rate of basal hepatic glucose production in the presence of hyperinsulinemia is the primary cause of fasting hyperglycemia; after a meal, impaired suppression of hepatic glucose production by insulin and decreased insulin-mediated glucose uptake by muscle contribute almost equally to postprandial hyperglycemia. In the United States, five classes of oral agents, each of which works through a different mechanism of action, are currently available to improve glycemic control in patients with type 2 diabetes. The recently completed United Kingdom Prospective Diabetes Study (UKPDS) has shown that type 2 diabetes mellitus is a progressive disorder that can be treated initially with oral agent monotherapy but will eventually require the addition of other oral agents, and that in many patients, insulin therapy will be needed to achieve targeted glycemic levels. In the UKPDS, improved glycemic control, irrespective of the agent used (sulfonylureas, metformin, or insulin), decreased the incidence of microvascular complications (retinopathy, neuropathy, and nephropathy). This review examines the goals of antihyperglycemic therapy and reviews the mechanism of action, efficacy, nonglycemic benefits, cost, and safety profile of each of the five approved classes of oral agents. A rationale for the use of these oral agents as monotherapy, in combination with each other, and in combination with insulin is provided.

Effects of exercise and insulin on insulin signaling proteins in human skeletal muscle

Insulin and exercise independently increase glucose metabolism in muscle. Moreover, exercise training or a prior bout of exercise increases insulin-stimulated glucose uptake in resting skeletal muscle. The present study was undertaken to compare how physiological hyperinsulinemia and moderate intensity aerobic exercise affect the tyrosine phosphorylation state and activity of insulin signaling molecules in healthy, physically inactive volunteers. Subjects had biopsies of the vastus lateralis muscle before and immediately after 30 min of either hyperinsulinemia (euglycemic insulin clamp) or moderate-intensity exercise on a cycle ergometer (approximately 60% of VO2max). Insulin receptor and IRS-1 tyrosine phosphorylation, association of the p85 regulatory subunit of PI 3-kinase with IRS-1, IRS-1 associated PI 3-kinase activity, and glycogen synthase activity were determined in muscle biopsy specimens taken from healthy subjects before and after insulin or exercise. Physiological hyperinsulinemia increased the rate of glucose disposal from 11.4 +/- 1.5 to 25.6 +/- 6.7 micromol x kg(-1) x min(-1) (P < 0.01), insulin receptor and IRS-1 tyrosine phosphorylation (173 +/- 19% and 159 +/- 35% of basal values, respectively, P < 0.05), association of the p85 regulatory subunit of PI 3-kinase with IRS-1 (159 +/- 10%, P < 0.05), and glycogen synthase fractional velocity (136 +/- 11%, P < 0.01). Exercise also increased glucose disposal, from 10.4 +/- 0.5 to 15.6 +/- 1.7 micromol x kg(-1) x min(-1) (P < 0.01) and glycogen synthase fractional velocity (253 +/- 35% of basal, P < 0.01). The exercise-induced increase in glycogen synthase was greater than that due to insulin (P < 0.05). In contrast to insulin, exercise decreased tyrosine phosphorylation of the insulin receptor to 72 +/- 10% of basal values (P < 0.05 vs basal and P < 0.05 vs insulin) and had no effect on IRS-1 tyrosine phosphorylation, or association of p85 with IRS-1. The exercise-induced decreased insulin receptor tyrosine phosphorylation could explain the well-known effect of exercise to enhance the sensitivity of muscle to insulin.

Synergistic interaction of magnesium and vanadate on glucose metabolism in diabetic rats

The effect of vanadate (V) alone, magnesium (Mg) alone, and the combination of Mg plus V (MgV) on insulin-mediated glucose disposal and glucose tolerance was investigated in normal and streptozotocin-induced diabetic rats. MgV, magnesium sulfate (MgSO4) and sodium metavanadate (NaV) were added to the drinking water of normal or diabetic rats (approximately 300 g) for 3 weeks. After 3 weeks of V treatment (both MgV and NaV), diabetic rats demonstrated a normal meal tolerance test without any increase in the plasma insulin response. Rats also received a euglycemic insulin clamp (12 mU/kg x min for 120 minutes) with 3-3H-glucose infusion to quantify total body glucose disposal, glycolysis (3H2O production), and glycogen synthesis (total body glucose disposal minus glycolysis). Total glucose disposal was decreased in diabetic versus control rats (29 +/- 2 v 35 +/- 2 mg/kg x min, P < .01) and returned to levels greater than the nondiabetic control values after MgV (41 +/- 2, P < .01). Supersensitivity to insulin was not observed in diabetic rats treated with NaV (34 +/- 1). Glycogen synthesis was increased by both MgV and NaV treatment (23 +/- 21, P < .01 and 18 +/- 1, P < .05 v 14 +/- 2 mg/kg x min) in diabetic rats. A small increase in glycolysis was observed in MgSO4 and MgV rats (18 +/- 1 and 18 +/- 1 v 16 +/- 1, P < .05). NaV alone had no effect on glycolysis. Thus, Mg has a synergistic effect with V to increase muscle glycogen synthesis in diabetic rats. In normal rats, neither MgSO4 nor NaV had any effect on glucose utilization. However, MgV increased glucose disposal to rates that were significantly higher than the rate in untreated control rats (P < .05). Based on these results, MgV is superior to either V alone or Mg alone in improving insulin sensitivity and glycogen synthesis in diabetic rats.

Urinary platelet-activating factor excretion is elevated in non-insulin dependent diabetes mellitus

Proteinuria is currently considered a very sensitive predictor of diabetic nephropathy, but 20-25% of all diabetic patients with negative Albustix reaction excrete higher than normal (< 20 mg/24 h) amounts of albumin in their urine. It is our hypothesis that platelet-activating factor (PAF), a potent glycerophospholipid that acts as a chemical mediator for a wide spectrum of biological activities, including increased vascular permeability, may be produced in significant amounts during periods preceding microalbuminuria. In this study, we compared urinary PAF excretion in Mexican-American subjects who were diagnosed with non-insulin dependent diabetes mellitus (NIDDM) with their healthy control counterparts. The age of the NIDDM subjects (45.9 +/- 2.1 years) was not significantly different from the healthy control group, which was 39.4 +/- 2.7 years (P < 0.0672). The NIDDM subjects (body mass index, 29.9 +/- 1.1 compared to 26.1 +/- 0.9 kg/m2 in healthy controls) were characterized by significantly increased (P < 0.05) fasting plasma glucose (192 +/- 11 vs. 97 +/- 4 mg/dl in healthy controls), fasting insulin (20.9 +/- 2.4 vs. 12.3 +/- 1.6 microU/ml), fasting C-peptide (2.93 +/- 1.26 vs. 1.48 +/- 0.51 ng/ml), and hemoglobin A1c (10.3 +/- 0.7 vs. 5.6 +/- 0.3%), respectively. The urine output for the NIDDM and control subjects were 1942 +/- 191 ml/24 h and 1032 +/- 94 ml/24 h, respectively, and urinary albumin excretion (UAE) rates were estimated to be 38 +/- 7 micrograms/min and 11 +/- 1 micrograms/min, respectively. The NIDDM subjects produced significantly increased levels of urinary PAF (2606.3 +/- 513.1 ng/24 h compared with 77.9 +/- 14.1 ng/24 h in controls (or 1706.3 +/- 420.8 ng/ml compared with 85.4 +/- 17.8 pg/ml of urine, in NIDDM and control subjects, respectively). We found that urinary PAF excretion was significantly correlated with microalbumin excretion (r = 0.7) especially at UAE rates greater than 30 mg/day and more importantly, some NIDDM patients with negative Albustix reaction (i.e. normal UAE) produced significantly more PAF, suggesting that PAF excretion may precede microalbuminuria and that subtle injury to the kidneys are present in NIDDM long before overt albuminuria ensues, urinary PAF measurements could potentially therefore serve as a sensitive indicator of renal injury in diabetes mellitus. These results lend further credence to our hypothesis that PAF may be the biochemical compound linking the various members of the insulin resistance syndrome.

Chronic physiologic hyperinsulinemia impairs suppression of plasma free fatty acids and increases de novo lipogenesis but does not cause dyslipidemia in conscious normal rats

Type 2 diabetes mellitus and obesity are characterized by fasting hyperinsulinemia, insulin resistance with respect to glucose metabolism, elevated plasma free fatty acid (FFA) levels, hypertriglyceridemia, and decreased high-density lipoprotein (HDL) cholesterol. An association between hyperinsulinemia and dyslipidemia has been suggested, but the causality of the relationship remains uncertain. Therefore, we infused eight 12-week-old male catheterized conscious normal rats with insulin (1 mU/min) for 7 days while maintaining euglycemia using a modification of the glucose clamp technique. Control rats (n = 8) received vehicle infusion. Baseline FFAs were 1.07+/-0.13 mmol/L, decreased to 0.57+/-0.10 (P < .05) upon initiation of the insulin infusion, and gradually increased to 0.95+/-0.12 by day 7 (P = NS vbaseline). On day 7 after a 6-hour fast, plasma insulin, glucose, and FFA levels in control and chronically hyperinsulinemic rats were 32+/-5 versus 116+/-21 mU/L (P < .005), 122+/-4 versus 129+/-8 mg/dL (P = NS), and 1.13+/-0.18 versus 0.95+/-0.12 mmol/L (P = NS); total plasma triglyceride and cholesterol levels were 78+/-7 versus 66+/-9 mg/dL (P = NS) and 50+/-3 versus 47+/-2 mg/dL (P = NS), respectively. Very-low-density lipoprotein (VLDL) + intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and HDL2 and HDL3 subfractions of plasma triglyceride and cholesterol were similar in control and hyperinsulinemic rats. Plasma FFA correlated positively with total (r = .61, P < .005) triglycerides. On day 7 after an 8-hour fast, hyperinsulinemic-euglycemic clamps with 3-3H-glucose infusion were performed in all rats. Chronically hyperinsulinemic rats showed peripheral insulin resistance (glucose uptake, 15.8+/-0.8 v 19.3+/-1.4 mg/kg x min, P < .02) but normal suppression of hepatic glucose production (HGP) compared with control rats (4.3+/-1.0 v 5.6+/-1.4 mg/kg x min, P = NS). De novo tissue lipogenesis (3-3H-glucose incorporation into lipids) was increased in chronically hyperinsulinemic versus control rats (0.90+/-0.10 v 0.44+/-0.08 mg/kg x min, P < .005). In conclusion, chronic physiologic hyperinsulinemia (1) causes insulin resistance with regard to the suppression of plasma FFA levels and increases lipogenesis; (2) induces peripheral but not hepatic insulin resistance with respect to glucose metabolism; and (3) does not cause an elevation in VLDL-triglyceride or a reduction in HDL-cholesterol.

IGF-I increases forearm blood flow without increasing forearm glucose uptake

Decreased insulin-mediated muscle glucose uptake is a characteristic feature of non-insulin-dependent diabetes mellitus and other insulin-resistant states. It has been suggested that an impairment in the ability of insulin to augment limb blood flow, resulting in diminished glucose delivery to muscle, may contribute to this abnormality. In this study, we used human insulin-like growth factor (IGF) I in conjunction with the forearm balance technique to determine whether forearm glucose uptake could be stimulated by increasing blood flow without directly stimulating the intrinsic ability of the muscle to extract glucose. IGF-I was infused intra-arterially in healthy controls at a rate of either 0.4 microg . kg-1 . min-1 (high IGF) or 0.04 microg . kg-1 . min-1 (low IGF) for 140 min. With high IGF, forearm blood flow increased approximately twofold (34 +/- 3 vs. 64 +/- 8 ml . min-1 . l forearm volume-1, P < 0.01), and arteriovenous glucose concentration difference (a-v difference) increased modestly (0.19 +/- 0.05 vs. 0.31 +/- 0.08 mM, P = 0.32), resulting in an increased forearm glucose uptake (6.4 +/- 1.7 vs. 21.7 +/- 7.4 micromol . min-1 . l forearm volume-1, P = 0.09 vs. basal). With low IGF, forearm blood flow increased by 59% (29 +/- 4 vs. 46 +/- 9 ml . min-1 . l forearm volume-1, P < 0.05) and was associated with a proportional decrease in the a-v difference (0. 29 +/- 0.04 vs. 0.18 +/- 0.05 mM, P < 0.05). Forearm glucose uptake therefore was not significantly different from basal values (7.6 +/- 0.6 vs. 6.9 +/- 1.8 micromol . min-1 . kg-1). These data demonstrate that increasing blood flow without increasing the intrinsic ability of the muscle to extract glucose does not increase forearm muscle glucose uptake.

Role of tissue-specific blood flow and tissue recruitment in insulin-mediated glucose uptake of human skeletal muscle

BACKGROUND

Conflicting evidence exists concerning whether insulin-induced vasodilation plays a mechanistic role in the regulation of limb glucose uptake. It can be predicted that if insulin augments blood flow by causing tissue recruitment, this mechanism would enhance limb glucose uptake.

METHODS AND RESULTS

Twenty healthy subjects were studied with the forearm perfusion technique in combination with the euglycemic insulin clamp technique. Ten subjects were studied at physiological insulin concentrations (approximately 400 pmol/L) and the other 10 at supraphysiological insulin concentrations (approximately 5600 pmol/L). Four additional subjects underwent a saline control study. Pulse injections of a nonmetabolizable extracellular marker (1-[3H]-L-glucose) were administered into the brachial artery, and its washout curves were measured in one ipsilateral deep forearm vein and used to estimate the extracellular volume of distribution and hence the amount of muscle tissue drained by the deep forearm vein. Both during saline infusion and at physiological levels of hyperinsulinemia we observed no changes in blood flow and/or muscle tissue drained by the deep forearm vein. However, supraphysiological hyperinsulinemia accelerated total forearm blood flow (45.0+/-1.8 versus 36.5+/-1.3 mL x min(-1) x kg(-1), P<0.01) and increased the amount of muscle tissue drained by the deep forearm vein (305+/-46 versus 229+/-32 g, P<0.05). The amount of tissue newly recruited by insulin was strongly correlated to the concomitant increase in tissue glucose uptake (r=0.789, P<0.01).

CONCLUSIONS

Acceleration of forearm blood flow mediated by supraphysiological hyperinsulinemia is accompanied by tissue recruitment, which may be a relevant determinant of forearm (muscle) glucose uptake.

Neonatal de-afferentation of capsaicin-sensitive sensory nerves increases in vivo insulin sensitivity in conscious adult rats

Sensory neuropeptides, released from the peripheral nervous system, might modulate glucose homeostasis by antagonizing insulin action. The effects of de-afferentation of functional small diameter unmyelinated C-fibres (sensory nerves) on in vivo insulin-mediated intracellular glucose metabolism were investigated by using euglycaemic insulin (6 and 18 mU/kg x min) clamps with [3-(3)H]-glucose infusion in 24 adult rats, treated neonatally with either capsaicin (CAP) (50 mg/kg) or vehicle (CON). Following the clamp, skeletal muscle groups, liver and adipose tissue were freeze-clamped. At plasma insulin levels of approximately 90 mU/l, CAP-rats showed a 21% increase in whole body glucose uptake compared with CON (24.4 +/- 1.6 vs 20.1 +/- 0.8 mg/kg min, p < 0.02), which was paralleled by a 20% increase in whole body glycolysis (12.6 +/- 0.8 vs 10.5 +/- 0.5 mg/ kg.min p < 0.05) (concentration of 3H2O in plasma). Whole body skeletal muscle glycogenesis was increased by 80% in CAP-rats (5.7 +/- 0.7 vs 3.1 +/- 0.7 mg/kg x min, p < 0.05) with increased muscle glycogen synthase activity. Whole body (muscle, liver and adipose tissue combined) de novo lipogenesis also was increased in CAP-rats compared with CON (0.69 +/- 0.10 vs 0.44 +/- 0.06 mg/kg x min, p < 0.05) (incorporation of [3-(3)H]-glucose counts into glycogen or fat). Hepatic glucose production was lower in CAP-rats compared with CON (0.6 +/- 0.6 vs 2.1 +/- 0.7 mg/kg x min, p < 0.05). Plasma glucagon, corticosterone, epinephrine and norepinephrine levels were reduced in CAP-rats: 43 +/- 2 compared with 70 +/- 6 pg/ml, 855 +/- 55 compared with 1131 +/- 138 nmol/l, 513 +/- 136 compared with 1048 +/- 164 pmol/l and 928 +/- 142 compared with 1472 +/- 331 pmol/l, respectively, p < 0.05. At plasma insulin levels of approximately 400 mU/l, CAP-rats showed no differences in peripheral and hepatic insulin action compared with CON. We conclude that the removal of endogenous sensory neuropeptides, by de-afferentation of capsaicin-sensitive sensory nerves, increases in vivo insulin sensitivity, but not responsiveness: 1) primarily through an increased sensitivity of skeletal muscle glycogen synthesis to insulin; 2) through a reduction in the levels of counter-regulatory hormones, thereby creating a milieu which favours overall in vivo insulin sensitivity with respect to glucose uptake, glucose production, glycolysis, glycogenesis and lipogenesis.

Effect of hyperinsulinemia on plasma leptin concentrations and food intake in rats

We investigated the dose- and time-dependent effect of insulin infusion on peripheral and portal plasma leptin concentrations in normal rats. Three groups were studied: group I: euglycemic (6 mmol/l) insulin (6 mU . kg-1 . min-1) clamps for 0, 2, 4, 12, and 24 h; group II: euglycemic insulin (18 mU . kg-1 . min-1) clamp for 2 h; and group III: euglycemic insulin (3 mU . kg-1 . min-1) clamp for 7 days. In group III, food intake was quantified during days 1-7. After an overnight fast, peripheral and portal plasma leptin levels were identical (1.5 +/- 0.2 and 1.6 +/- 0.2 ng/ml). Insulin infusion (6 mU . kg-1 . min-1) for 2 h had no effect on plasma leptin levels (1.5 +/- 0.2 ng/ml). After 4 h (2.0 +/- 0.2 ng/ml), 12 h (2.2 +/- 0. 4 ng/ml), and 24 h (2.7 +/- 0.6 ng/ml; all P < 0.05) of insulin infusion, a progressive time-related increase in plasma leptin concentration was observed; portal vein leptin levels rose in parallel and were similar to peripheral levels. When insulin (18 mU . kg-1 . min-1) was infused for 2 h, plasma leptin levels increased to 3.0 +/- 0.3 ng/ml (P < 0.01). Seven days of constant insulin infusion (3 mU . kg-1 . min-1) resulted in a progressive increase in fasting plasma leptin and a parallel decrease in food intake. A mean increase in plasma leptin concentration of 1 ng/ml during the 7-day insulin infusion period was associated with a mean decrease in food intake of 2.5 g/day (multivariate ANOVA, P < 0.05). We conclude that the insulin-induced rise in peripheral and portal vein leptin levels is similar and both dose and time dependent. The inverse relationship between plasma leptin concentration and food intake during prolonged hyperinsulinemia, but not during short-term hyperinsulinemia, supports the role of leptin in long-term food consumption.