A quantization method based on threshold optimization for microarray short time series

BACKGROUND

Reconstructing regulatory networks from gene expression profiles is a challenging problem of functional genomics. In microarray studies the number of samples is often very limited compared to the number of genes, thus the use of discrete data may help reducing the probability of finding random associations between genes.

RESULTS

A quantization method, based on a model of the experimental error and on a significance level able to compromise between false positive and false negative classifications, is presented, which can be used as a preliminary step in discrete reverse engineering methods. The method is tested on continuous synthetic data with two discrete reverse engineering methods: Reveal and Dynamic Bayesian Networks.

CONCLUSION

The quantization method, evaluated in comparison with two standard methods, 5% threshold based on experimental error and rank sorting, improves the ability of Reveal and Dynamic Bayesian Networks to identify relations among genes.

Measurement of selective effect of insulin on glucose disposal from labeled glucose oral test minimal model

The oral glucose minimal model (OMM) measures insulin sensitivity (S(I)) and the glucose rate of appearance (R(a)) of ingested glucose in the presence of physiological changes of insulin and glucose concentrations. However, S(I) of OMM measures the overall effect of insulin on glucose utilization and glucose production. In this study we show that, by adding a tracer to the oral dose, e.g., of a meal, and by using the labeled version of OMM, OMM* to interpret the data, one can measure the selective effect of insulin on glucose disposal, S(I)*. Eighty-eight individuals underwent both a triple-tracer meal with the tracer-to-tracee clamp technique, providing a model-independent reference of the R(a) of ingested glucose (R(a meal)(ref)) and an insulin-modified labeled intravenous glucose tolerance test (IVGTT*). We show that OMM* provides not only a reliable means of tracing the R(a) of ingested glucose (R(a meal)) but also accurately measures S(I)*. We do so by comparing OMM* R(a meal) with the model-independent R(a meal)(ref) provided by the tracer-to-tracee clamp technique, while OMM* S(I)* is compared with both S(I)(* ref), obtained by using as known input R(a meal)(ref), and with S(I)* measured during IVGTT*.

Two-hour seven-sample oral glucose tolerance test and meal protocol: minimal model assessment of beta-cell responsivity and insulin sensitivity in nondiabetic individuals

Highly informative yet simple protocols to assess insulin secretion and action would considerably enhance the quality of epidemiological and large-scale clinical trials. In an effort to develop such protocols, a 5-h, 11-sample oral glucose tolerance test (OGTT) was performed in 100 individuals and a 7-h, 21-sample meal in another 100. Plasma glucose, insulin, and C-peptide concentrations were measured. We show that virtually the same minimal model assessment of beta-cell responsivity (dynamic [Phi(d)] and static [Phi(s)]), insulin sensitivity (Si), and disposition index (DI) can be obtained with a reduced seven-sample 2-h protocol: Phi(d), reduced versus full: 871.50 vs. 873.32, r = 0.98 in OGTT and 494.88 vs. 477.99 10(-9), r = 0.91 in meal; Phi(s): 42.36 vs. 44.35, r = 0.88 in OGTT and 35.31 vs. 35.37 10(-9) min(-1), r = 0.90 in meal; Si: 24.33 vs. 22.77 10(-5) dl x kg(-1) x min(-1) per pmol/l, r = 0.89 in OGTT and 19.03 vs. 19.77 10(-5) dl x kg(-1) x min(-1) per pmol/l, r = 0.85 in meal; and DI: 1,282.26 vs. 1,273.23, r = 0.84 in OGTT and 726.92 vs. 776.97 10(-14) dl . kg(-1) x min(-2) per pmol/l, r = 0.84 in meal. This reduced protocol will facilitate the study of insulin secretion and action under physiological conditions in nondiabetic humans.

Effects of dietary macronutrient intake on insulin sensitivity and secretion and glucose and lipid metabolism in healthy, obese adolescents

CONTEXT

Adolescent obesity is a serious public health concern.

OBJECTIVE

The aim of the study was to determine whether obese adolescents can adapt metabolically to changes in dietary macronutrient intake.

PATIENTS AND DESIGN

Using a random cross-over design, 13 healthy obese volunteers (six boys and seven girls; age, 14.7 +/- 0.3 yr; body mass index, 34 +/- 1 kg/m2; body fat, 42 +/- 1%) were studied twice after 7 d of isocaloric, isonitrogenous diets with 60% carbohydrate (CHO) and 25% fat (high CHO), or 30% CHO and 55% fat (low CHO).

MAIN OUTCOME MEASURES AND METHODS

Glucose metabolism, insulin sensitivity, and first- and second-phase insulin secretory indices were measured by stable isotope techniques and the stable labeled iv glucose tolerance test. The results were compared with those of previously studied lean adolescents.

RESULTS

Obese adolescents increased first- and second-phase insulin secretory indices by 18 (P = 0.05) and 36% (P = 0.05), respectively, to maintain normoglycemia during the high-CHO diet because they failed to increase insulin sensitivity as did the lean adolescents. Regardless of diet, in obese adolescents, insulin sensitivity was half (P < 0.05) and first- and second-phase insulin secretory indices twice (P < 0.01), compared with the the corresponding values in lean subjects. In obese adolescents, gluconeogenesis increased by 32% during the low-CHO (high-fat diet) (P < 0.01).

CONCLUSION

In obese adolescents, insulin secretory demands were increased regardless of diet. Failure to increase insulin sensitivity while receiving a high-CHO diet required a further increase in insulin secretion, which may lead to earlier beta-cell failure. A low-CHO/high-fat diet resulted in increased gluconeogenesis, which may be a prelude to the increased glucose production and hyperglycemia observed in type 2 diabetics.

Obesity and type 2 diabetes do not alter splanchnic cortisol production in humans

CONTEXT

Cortisol is a potent regulator of carbohydrate, fat, and protein metabolism.

OBJECTIVE

The objective of the study was to determine whether obesity alone or in combination with type 2 diabetes increases splanchnic and/or leg cortisol production.

DESIGN

Splanchnic and leg cortisol production were measured using the hepatic and leg catheterization technique combined with infusion of D4-cortisol.

SETTING

The study was conducted in a General Clinical Research Center.

PARTICIPANTS

Nine lean nondiabetic, 10 obese nondiabetic, and 11 obese diabetic subjects were studied.

INTERVENTIONS

Diabetic volunteers were withdrawn from their glucose-lowering medications before study.

MAIN OUTCOME MEASURES

Rates of total body, splanchnic and leg cortisol, and D3-cortisol production were measured.

RESULTS

Rates of splanchnic cortisol production equaled or exceeded those occurring in extrasplanchnic tissues (e.g. the adrenals) in all three groups. However, because concurrent splanchnic cortisol uptake also occurred, net splanchnic cortisol release was minimal. Splanchnic cortisol production and splanchnic D3-cortisol production (an index of splanchnic 11beta-hydroxysteroid dehydrogenase type 1 activity) did not differ among the three groups. In addition, splanchnic cortisol production did not correlate with either visceral fat or endogenous glucose production. On the other hand, splanchnic cortisol uptake was greater in the obese diabetic than lean nondiabetic subjects (25 +/- 2.9 vs. 15.3 +/- 2.5 microg/min; P < 0.05). Splanchnic, but not leg, D3-cortisol production was correlated with total body D3-cortisol production (r = 0.70; P < 0.001).

CONCLUSIONS

Although large amounts of cortisol are produced within the splanchnic bed, implying high intrahepatic glucocorticoid concentrations, rates do not differ in lean and obese nondiabetic humans and are not influenced by the presence of type 2 diabetes mellitus. On the other hand, obesity but not diabetes increases splanchnic cortisol uptake.

Ovarian cancer identification based on dimensionality reduction for high-throughput mass spectrometry data

MOTIVATION

High-throughput and high-resolution mass spectrometry instruments are increasingly used for disease classification and therapeutic guidance. However, the analysis of immense amount of data poses considerable challenges. We have therefore developed a novel method for dimensionality reduction and tested on a published ovarian high-resolution SELDI-TOF dataset.

RESULTS

We have developed a four-step strategy for data preprocessing based on: (1) binning, (2) Kolmogorov-Smirnov test, (3) restriction of coefficient of variation and (4) wavelet analysis. Subsequently, support vector machines were used for classification. The developed method achieves an average sensitivity of 97.38% (sd = 0.0125) and an average specificity of 93.30% (sd = 0.0174) in 1000 independent k-fold cross-validations, where k = 2, ..., 10.

AVAILABILITY

The software is available for academic and non-commercial institutions.

Minimal model estimation of glucose absorption and insulin sensitivity from oral test: validation with a tracer method

Measuring insulin sensitivity during the physiological milieu of oral glucose perturbation, e.g., a meal or an oral glucose tolerance test, would be extremely valuable but difficult since the rate of appearance of absorbed glucose is unknown. The reference method is a tracer two-step one: first, the rate of appearance of glucose (R(a meal)(ref)) is reconstructed by employing the tracer-to-tracee ratio clamp technique with two tracers and a model of non-steady-state glucose kinetics; next, this R(a meal)(ref) is used as the known input of a model describing insulin action on glucose kinetics to estimate insulin sensitivity (SI(ref)). Recently, a nontracer method based on the oral minimal model (OMM) has been proposed to estimate simultaneously the above quantities, denoted R(a meal) and SI, respectively, from plasma glucose and insulin concentrations measured after an oral glucose perturbation. This last method has obvious advantages over the tracer method, but its domain of validity has never been assessed against a reference method. It is thus important to establish whether or not the "nontracer" R(a meal) and SI compare well with the "tracer" R(a meal)(ref) and SI(ref). We do this comparison on a database of 88 subjects, and it is very satisfactory: R(a meal) profiles agree well with the R(a meal)(ref) and correlation of SI(ref) with SI is r = 0.86 (P < 0.0001). We conclude that OMM candidates as a reliable tool to measure both the rate of glucose absorption and insulin sensitivity from oral glucose tests without employing tracers.

Impaired beta-cell compensation to dexamethasone-induced hyperglycemia in women with polycystic ovary syndrome

Deterioration in glucose tolerance occurs rapidly in women with polycystic ovary syndrone (PCOS), suggesting that pancreatic beta-cell dysfunction may supervene early. To determine whether the compensatory insulin secretory response to an increase in insulin resistance induced by the glucocorticoid dexamethasone differs in women with PCOS and control subjects, we studied 10 PCOS and 6 control subjects with normal glucose tolerance. An oral glucose tolerance test (OGTT) and a graded glucose infusion protocol were performed at baseline and after subjects took 2.0 mg of dexamethasone orally. Basal (Phi(b)), static (Phi(s)), dynamic (Phi(d)), and global (Phi) indexes of beta-cell sensitivity to glucose were derived. Insulin sensitivity (S(i)) was calculated using the minimal model; a disposition index (DI) was calculated as the product of S(i) and Phi. PCOS and control subjects had nearly identical fasting and 2-h glucose levels at baseline. Phi(b) was higher, although not significantly so, in the PCOS subjects. The Phi(d), Phi(s), and Phi indexes were 28, 19, and 20% higher, respectively, in PCOS subjects. The DI was significantly lower in PCOS (30.01 +/- 5.33 vs. 59.24 +/- 7.59) at baseline. After dexamethasone, control subjects averaged a 9% increase (to 131 +/- 12 mg/dl) in 2-h glucose levels; women with PCOS had a significantly greater 26% increase to 155 +/- 6 mg/dl. The C-peptide-to-glucose ratios on OGTT increased by 44% in control subjects and by only 15% in PCOS subjects. The accelerated deterioration in glucose tolerance in PCOS may result, in part, from a relative attenuation in the response of the beta-cell to the demand placed on it by factors exacerbating insulin resistance.

Splanchnic cortisol production occurs in humans: evidence for conversion of cortisone to cortisol via the 11-beta hydroxysteroid dehydrogenase (11beta-hsd) type 1 pathway

Glucocorticoids are potent regulators of protein, fat, and carbohydrate metabolism. To determine if cortisol production occurs within the splanchnic bed in humans, 11 nondiabetic subjects were studied using the hepatic/leg catheterization method along with an infusion of [9,11,12,12-2H4] cortisol (D4-cortisol) as proposed by Andrews et al. In the fasting state, there was net release (P < 0.05) of cortisol from the splanchnic bed (6.1 +/- 2.6 microg/min) and net uptake (P < 0.05) by the leg (1.7 +/- 0.7 microg/min). This, along with cortisol production by other tissues (e.g., the adrenals), resulted in a total-body cortisol appearance rate of 18.1 +/- 1.9 microg/min. Fractional splanchnic D4-cortisol extraction averaged 12.9 +/- 1.3% (P < 0.001), splanchnic cortisol uptake 14.8 +/- 2.0 microg/min (P < 0.001), and splanchnic cortisol production 22.2 +/- 3.3 microg/min (P < 0.001). On the other hand, fractional leg D4-cortisol extraction averaged 5.6 +/- 1.8% (P < 0.02), leg cortisol uptake 2.3 +/- 0.7 microg/min (P < 0.01), and leg cortisol production 0.4 +/- 0.4 microg/min, which did not differ from zero. Because D4-cortisol loses a deuterium during conversion to [9,12,12-2H3] cortisone (D3-cortisone), which in turn generates [9,12,12(2)H3] cortisol (D3-cortisol) via 11-beta hydroxysteroid dehydrogenase (11beta-HSD) type 1, D3-cortisol production can be used as an index of 11beta-HSD type 1 activity. Net splanchnic D3-cortisol release (3.9 +/- 0.4 microg/min) and splanchnic D3-cortisol production (7.1 +/- 0.7 microg/min) occurred (P < 0.01) in all subjects. In contrast, there was minimal leg D3-cortisol production (0.04 +/- 0.01 microg/min), resulting in a strong correlation between splanchnic D3-cortisol production and total-body 3D-cortisol production in both the fasting state (r = 0.84; P < 0.02) and during an infusion of insulin (r = 0.97; P < 0.01). Thus, splanchnic production of cortisol occurs in nondiabetic humans at rates approximating that which occurs in the remainder of the body. These data support the possibility that alterations in splanchnic cortisol production contribute to visceral fat accumulation and the hepatic insulin resistance of obesity or type 2 diabetes.

Model to assess muscle protein turnover: domain of validity using amino acyl-tRNA vs. surrogate measures of precursor pool

Current models to measure protein turnover across muscle bed are based on many surrogate measures of amino acyl-tRNA. We measured muscle protein turnover based on tracer-to-tracee ratios of the stable isotopes of leucine, phenylalanine, and ketoisocaproate (KIC) in artery and vein and muscle amino acyl-tRNA and muscle tissue fluid (TF) in 26 healthy subjects. A three-compartment model calculation based on arteriovenous and tRNA measurements was first performed and its domain of validity assessed. The results were then compared with those using simpler approaches based on surrogate measures of tRNA such as those of TF and KIC and a one-compartment model based on arteriovenous amino acids. In 96% of cases, the model using tRNA was applicable, but only in a lower percentage of cases were the results using surrogate measures applicable. Protein breakdown, protein synthesis, and shunting of amino acids from artery to vein were consistently underestimated, and fluxes of amino acid from artery to intracellular compartment and from intracellular compartment to vein were overestimated, when surrogate measures were used. The one-compartment model also underestimated protein breakdown and synthesis. Measurements using tissue fluid gave results closer to those based on tRNA. In conclusion, a three-compartment model using arteriovenous samples and amino acyl-tRNA provides measurements of muscle protein turnover of acceptable precision in 96% of cases. The precision was unacceptable in a substantial percentage of cases, and the accuracy of the estimation of protein fluxes was significantly affected when surrogate measures were used.

Iron and zinc absorption from two bean (Phaseolus vulgaris L.) genotypes in young women

Extrinsic and intrinsic iron and zinc labels were used to test iron and zinc absorption from two bean (Phaseolus vulgaris) genotypes, containing normal (common beans, CB) or higher (HFeZnB) iron and zinc concentrations, fed in single meals to young women with low iron reserves. The women were divided into two groups, with one receiving a CB test meal (n = 12) and the other, an HFeZnB test meal (n = 11). The beans were intrinsically labeled hydroponically with (55)Fe (CB and HFeZnB) and with (70)Zn (HFeZnB). Concentrations of zinc and iron were 98 and 65% higher, respectively, in HFeZnB as compared to CB, but phytic acid contents were similar. Extrinsic labels were (59)Fe (CB and HFeZnB), (67)Zn (CB), and (68)Zn (HFeZnB). Iron and zinc percent absorption levels were calculated from radio-iron activity in red blood cells and from urinary excretion of zinc isotopes. Intrinsic and extrinsic iron absorption measures were highly correlated (R (2) = 0.986) (average extrinsic/intrinsic ratio was 1.00). Iron absorption was low (geometric mean < 2%) in both bean types, and total iron absorbed was not different between types. Intrinsic zinc absorption from the HFeZn beans was higher than extrinsic absorption (15.2% vs 13.4%, p < 0.05) (average extrinsic/intrinsic was 0.90). The correlation between intrinsic and extrinsic zinc measures was not as high as that for iron (R (2) = 0.719). Percent zinc absorption levels were similar in both bean types, but total extrinsic zinc absorbed was 90% higher (p < 0.05) from the HFeZnB meal. Thus, the less expensive and time-consuming extrinsic labeling may be used to screen various varieties of beans for iron bioavailability in humans, but it underestimates zinc absorption by approximately 10%. Selective breeding for high-zinc bean genotypes may improve zinc status. However, high-iron genotypes appear to have little effect on iron status when fed alone in single meals to women with low iron reserves.

Mechanisms of the age-associated deterioration in glucose tolerance: contribution of alterations in insulin secretion, action, and clearance

Glucose tolerance decreases with age. For determining the cause of this decrease, 67 elderly and 21 young (70.1 +/- 0.7 vs. 23.7 +/- 0.8 years) participants ingested a mixed meal and received an intravenous injection of glucose. Fasting glucose and the glycemic response above basal were higher in the elderly than in the young participants after either meal ingestion (P < 0.001) or glucose injection (P < 0.01). Insulin action (Si), measured with the meal and intravenous glucose tolerance test models, was highly correlated (r = 0.72; P < 0.001) and lower (P <or= 0.002) in the elderly than in the young participants. However, when adjusted for differences in percentage body fat and visceral fat, Si no longer differed between groups. When considered in light of the degree of insulin resistance, all indexes of insulin secretion were lower (P < 0.01) in the elderly participants, indicating impaired beta-cell function. Hepatic insulin clearance was increased (P < 0.002), whereas total insulin clearance was decreased (P < 0.002) in the elderly subjects. Multivariate analysis (r = 0.70; P < 0.001) indicated that indexes of insulin action (Si) and secretion (Phi(total)) but not age, peak oxygen uptake, fasting glucose, degree of fatness, or hepatic insulin clearance predicted the postprandial glycemic response. We conclude that the deterioration in glucose tolerance that occurs in healthy elderly subjects is due to a decrease in both insulin secretion and action with the severity of the defect in insulin action being explained by the degree of fatness rather than age per se.

L-arginine-nitric oxide kinetics in normal and type 2 diabetic subjects: a stable-labelled 15N arginine approach

Defective endothelium is a key event in the development of atherosclerosis in diabetes: alteration of the L-arginine-nitric oxide (NO) pathway has been suggested. We propose a modeling approach of the L-arginine-NO pathway in vivo in both control and type 2 diabetic subjects based on the intravenous bolus injection of L-[(15)N]arginine and subsequent noncompartmental and compartmental model analysis of L-[(15)N] arginine in plasma and [(15)N]nitrate in the urine. No differences in arginine kinetics were observed between normal subjects and diabetic patients. [(15)N]nitrates were detectable up to 48 h from the L-(15)[N]arginine administration; no differences were found in the tracer-to-tracee ratio in each urine collection. However, the NO synthesis in plasma from arginine was lower (P = 0.05 for the noncompartmental and 0.1208 for the compartmental analysis, by Mann-Whitney test) in diabetic patients than in control subjects when expressed both in absolute terms (50% decrease) and as percentage of NO turnover (30% decrease). This new modeling approach of L-arginine-NO pathway provides a detailed picture of arginine kinetics and nitrate metabolism. From our data, it appears that noncomplicated type 2 diabetic patients have a decreased conversion of arginine to NO.

The hot IVGTT two-compartment minimal model: an improved version

The two-compartment minimal model (2CMM) interpretation of a labeled intravenous glucose tolerance test (IVGTT) is a powerful tool to assess glucose metabolism in a single individual. It has been reported that a derived 2CMM parameter describing the proportional effect of glucose on insulin-independent glucose disposal can take physiologically unplausible negative values. In addition, precision of 2CMM parameter estimates is sometimes not satisfactory. Here we resolve the above issues by presenting an improved version of 2CMM that relies on a new assumption on the constant component R(d0) of insulin-independent glucose disposal. Here R(d0) is not fixed to 1 mg x kg(-1) x min(-1) but instead is expressed as a fraction of steady-state glucose disposal. The new 2CMM is identified on the same stable labeled IVGTT data base on which the original 2CMM was formulated. A more reliable insulin-independent glucose disposal portrait is obtained while that of insulin action remains unchanged. The new 2CMM also improves the precision with which model parameters and metabolic indexes are estimated.

Use of a novel triple-tracer approach to assess postprandial glucose metabolism

Numerous studies have used the dual-tracer method to assess postprandial glucose metabolism. The present experiments were undertaken to determine whether the marked tracer nonsteady state that occurs with the dual-tracer approach after food ingestion introduces error when it is used to simultaneously measure both meal glucose appearance (R(a meal)) and endogenous glucose production (EGP). To do so, a novel triple-tracer approach was designed: 12 subjects ingested a mixed meal containing [1-(13)C]glucose while [6-(3)H]glucose and [6,6-(2)H(2)]glucose were infused intravenously in patterns that minimized the change in the plasma ratios of [6-(3)H]glucose to [1-(13)C]glucose and of [6,6-(2)H(2)]glucose to endogenous glucose, respectively. R(a meal) and EGP measured with this approach were essentially model independent, since non-steady-state error was minimized by the protocol. Initial splanchnic glucose extraction (ISE) was 12.9% +/- 3.4%, and suppression of EGP (EGPS) was 40.3% +/- 4.1%. In contrast, when calculated with the dual-tracer one-compartment model, ISE was higher (P < 0.05) and EGPS was lower (P < 0.005) than observed with the triple-tracer approach. These errors could only be prevented by using time-varying volumes different for R(a meal) and EGP. Analysis of the dual-tracer data with a two-compartment model reduced but did not totally avoid the problems associated with marked postprandial changes in the tracer-to-tracee ratios. We conclude that results from previous studies that have used the dual-tracer one-compartment model to measure postprandial carbohydrate metabolism need to be reevaluated and that the triple-tracer technique may provide a useful approach for doing so.

Effects of dietary macronutrient content on glucose metabolism in children

Effects of carbohydrate, fat, and fructose intake on substrate and hormone concentrations, glucose production, gluconeogenesis, and insulin sensitivity were determined in healthy, nonobese prepubertal children (n = 12) and adolescents (n = 24) using a cross-over design. In one group (12 prepubertal children and 12 adolescents), subjects were studied after 7 d of isocaloric, isonitrogenous diets providing either 60% carbohydrate and 25% fat [high carbohydrate (H(CHO))/low fat (L(F))] or 30% carbohydrate and 55% fat [low carbohydrate (L(CHO))/high fat (H(F))], and in a second group (12 adolescents) H(CHO)/L(F) diets containing either 40% or 10% fructose was used. All subjects adapted to changes in carbohydrate and fat intakes primarily by appropriately adjusting their substrate oxidation rates to match the intakes, with only minor changes in parameters of glucose metabolism. Changing from a L(CHO)/H(F) to H(CHO)/L(F) diet resulted in increased insulin sensitivity (stable labeled iv glucose tolerance test) in adolescents [from 3.2 +/- 0.7 x 10(-4) to 5.0 +/- 1.4 x 10(-4) (min(-1))/( micro U.ml(-1)) (mean +/- SE)] but not in prepubertal children [9.4 +/- 2.5 x 10(-4) to 9.9 +/- 1.5 x 10(-4) (min(-1))/( micro U.ml(-1))], whereas beta-cell sensitivity was unaffected in both groups. Insulin sensitivity was higher in prepubertal children than in adolescents (P < 0.05). The dietary fructose content did not affect any measured parameter. We conclude that in the short term, dramatic changes in fat and carbohydrate intakes (regardless of fructose content) did not adversely affect glucose and lipid metabolism in healthy nonobese children. In the adolescents, the high carbohydrate diet resulted in increased insulin sensitivity, thus facilitating insulin-mediated glucose uptake.

Insulin secretory responses to rising and falling glucose concentrations are delayed in subjects with impaired glucose tolerance

AIMS/HYPOTHESIS

We hypothesized that beta-cell responses to changes in glucose would not be normal in subjects with impaired glucose tolerance (IGT).

METHODS

Three groups of 6 subjects were studied: normal weight with normal glucose tolerance (control subjects); obese with normal glucose tolerance (Obese-NGT); and obese with IGT (Obese-IGT). All subjects had a graded glucose infusion protocol to increase (step-up) and then decrease (step-down) plasma glucose. We obtained average insulin-secretion rates (ISR) over the glucose range common to all three groups during step-up and step-down phases, minimal model indices of beta-cell function (f(b), f(d), f(s), T(up), T(down) ), and insulin sensitivity (Si).

RESULTS

ISR differed significantly between step-up and -down phases only in Obese-IGT individuals. Basal (f(b)) and stimulated (f(d), f(s)) beta-cell sensitivity to glucose were similar in the three groups. Delays between glucose stimulus and beta-cell response during both step-up (T(up)) and -down (T(down)) phases were higher in Obese-IGT compared to Controls and Obese-NGT individuals. The product ISR x Si (10(-5.)min(-2) x l) was lower in Obese-IGT compared to Controls, both during step-up (919 +/- 851 vs 3192 +/- 1185, p < 0.05) and step-down (1455 +/- 1203 vs 3625 +/- 691, p < 0.05) phases. Consistently, the product f(s) x Si (10(-14.)min(-2). pmol(-1) x l) was lower in Obese-IGT than in control subjects (27.6 +/- 25.4 vs 103.1 +/- 20.2, p < 0.05).

CONCLUSION/INTERPRETATION

Subjects with IGT are not able to secrete insulin to compensate adequately for insulin resistance. They also show delays in the timing of the beta-cell response to glucose when glucose levels are either rising or falling.

Insulin release in impaired glucose tolerance: oral minimal model predicts normal sensitivity to glucose but defective response times

The availability of quantitative indexes describing beta-cell function in normal life conditions is important for the characterization of impaired mechanisms of insulin secretion in pathophysiological states. Recently, an oral C-peptide minimal model has been proposed and applied to subjects with normal glucose tolerance (NGT) during graded up-and-down glucose infusion protocols (40-min periods at 4, 8, 16, 8, 4, and 0 mg.kg(-1).min(-1)) and oral glucose tolerance tests. These tests are characterized by slow glucose and C-peptide dynamics, which reproduce prandial conditions. In view of the importance of beta-cell dysfunction in the pathogenesis of type 2 diabetes, our aim was to test and use the oral minimal model in subjects with impaired glucose tolerance (IGT) to identify deranged mechanisms of beta-cell function. Plasma C-peptide and glucose data from graded up-and-down glucose infusions were analyzed in nine NGT and four IGT subjects using the classic deconvolution approach and the oral minimal model, and indexes of beta-cell function were derived. An index of insulin sensitivity was also obtained for each subject from minimal model analysis of glucose and insulin levels achieved during the test. Both deconvolution and minimal model analyses revealed that individuals with IGT have a relative defect in the ability to secrete enough insulin to adequately compensate for insulin resistance. Additionally, minimal model analysis suggests that insulin secretory defect in IGT arises from delays in the timing of the beta-cell response to glucose.

Glucose production, gluconeogenesis, and insulin sensitivity in children and adolescents: an evaluation of their reproducibility

The prevalence of overweight and obese children has doubled, and the incidence of type 2 diabetes in children (0-19 y) has increased 4-fold during the past several decades. As a result we can anticipate an increased number of metabolic studies in children. There are few data on measures of glucose metabolism in normal children, and virtually none relating to their reproducibility. The aims of this study were 1) to provide new data on energy expenditure and glucose, lipid, and protein metabolism in nonobese, healthy children and adolescents; 2) to evaluate their reproducibility; and 3) on the basis of these data, to perform power calculations for metabolic studies. Eight nonobese subjects (8-16 y) were studied on two occasions, preceded by 7 d of a diet with identical energy content and macronutrient distribution. Gluconeogenesis, measured by deuterium oxide, accounted for 50% of glucose production. Insulin sensitivity, measured by the labeled minimal model, averaged 4.9 x 10(-4) mL(mU x min)(-1). Glucose appearance rate was significantly higher (p < 0.01) in the children than in the adolescents. Furthermore, we demonstrated that for energy intake and expenditure, plasma concentrations of glucose and C-peptide, and rates of appearance of glucose and leucine, a 10% difference can be detected in fewer than five subjects with a power of 80% and a type I error of 5%. Insulin concentration, gluconeogenesis, insulin secretory indices, insulin sensitivity, and glucose effectiveness were more variable, but with the above power a difference of 25% could be detected in 7-11 subjects using a paired study design.

Impact of cyclosporine and low-dose steroid therapy on insulin sensitivity and beta-cell function in patients with long-term liver grafts

To examine whether factors controlling glucose tolerance, i.e., insulin sensitivity (SI) and first-(phi1) and second-phase insulin secretion (phi2), are impaired in after orthotopic liver transplantation (OLT), they were assesssed in patients that had undergone OLT for cirrhosis (n = 10) with cyclosporin A and low-dose steroid therapy (5 mg prednisone per day) and were compared with those of healthy matched control subjects (n = 10). These factors were determined by means of computer-based analysis of frequently sampled intravenous glucose tolerance tests (FSIGTT). Glucose and insulin profiles (posthepatic insulin) did not differ between both groups, whereas C-peptide levels (prehepatic insulin) were elevated in the transplant group after the FSIGTT, indicating an increased hepatic insulin degradation. SI and (phi1 did not differ between both groups. phi2, however, was significantly enhanced (23.94 +/- 2.63 vs 13.88 +/- 1.25 min(-1), P < 0.05). These results indicate that cyclosporine and low-dose steroid therapy do not impair SI and phi1. However, enhanced phi2 compensates the increased hepatic insulin clearance.

Quantitative indexes of beta-cell function during graded up&down glucose infusion from C-peptide minimal models

Availability of quantitative indexes of insulin secretion is important for definition of the alterations in beta-cell responsivity to glucose associated with different physiopathological states. This is presently possible by using the intravenous glucose tolerance test (IVGTT) in conjunction with the C-peptide minimal model. However, the secretory response to a more physiological slowly increasing/decreasing glucose stimulus may uncover novel features of beta-cell function. Therefore, plasma C-peptide and glucose data from a graded glucose infusion protocol (seven 40-min periods of 0, 4, 8, 16, 8, 4, and 0 mg. kg(-1). min(-1)) in eight normal subjects were analyzed by use of a new model of insulin secretion and kinetics. The model assumes a two-compartment description of C-peptide kinetics and describes the stimulatory effect on insulin secretion of both glucose concentration and the rate at which glucose increases. It provides in each individual the insulin secretion profile and three indexes of pancreatic sensitivity to glucose: Phi(s), Phi(d), and Phi(b), related, respectively, to the control of insulin secretion by the glucose level (static control), the rate at which glucose increases (dynamic control), and basal glucose. Indexes (means +/- SE) were Phi(s) = 18.8 +/- 1.8 (10(9) min(-1)), Phi(d) = 222 +/- 30 (10(9)), and Phi(b) = 5.2 +/- 0.4 (10(9) min(-1)). The model also allows one to quantify the beta-cell times of response to increasing and decreasing glucose stimulus, equal to 5.7 +/- 2.2 (min) and 17.8 +/- 2.0 (min), respectively. In conclusion, the graded glucose infusion protocol, interpreted with a minimal model of C-peptide secretion and kinetics, provides a quantitative assessment of pancreatic function in an individual. Its application to various physiopathological states should provide novel insights into the role of insulin secretion in the development of glucose intolerance.

Oral glucose tolerance test minimal model indexes of beta-cell function and insulin sensitivity

The simultaneous assessment of quantitative indexes of insulin secretion and action in a single individual is important when quantifying their relative role in the evolution of glucose tolerance in different physiopathological states. Available methods quantify these indexes in relatively nonphysiological conditions, e.g., during glucose clamps or intravenous glucose tolerance tests. Here, we present a method based on a physiological test applicable to large-scale genetic and epidemiologic studies-the oral glucose tolerance test (OGTT). Plasma C-peptide, insulin, and glucose data from a frequently sampled OGTT with 22 samples throughout 300 min (FSOGTT300-22) were analyzed in 11 subjects with various degrees of glucose tolerance. In each individual, two indexes of pancreatic sensitivity to glucose (phis [10(9) min(-1)] and phid [10(9)]) and the insulin sensitivity index (SI) (10(5) dl/kg per min per pmol/l) were estimated by using the minimal model of C-peptide secretion and kinetics originally proposed for intravenous graded glucose infusion and the minimal model approach recently proposed for meal/OGTTs. The indexes obtained from FSOGTT300-22 were used as a reference for internal validation of OGTT protocols with reduced sampling schedules. Our results show that 11 samples in a 300-min period (OGTT300-11) is the test of choice because the indexes it provides (phis = 36 +/- 3 [means +/- SE]; phid = 710 +/- 111; SI = 10.2 +/- 2.4) show excellent correlation and are not statistically different from those of FSOGTT300-22 (phis = 33 +/- 3; phid = 715 +/- 120; SI = 10.1 +/- 2.3). In conclusion, OGTT300-11, interpreted with C-peptide and glucose minimal models, provides a quantitative description of beta-cell function and insulin sensitivity in a single individual while preserving the important clinical classification of glucose tolerance provided by the standard 120-min OGTT.

Severity of HCV-induced liver damage alters glucose homeostasis in noncirrhotic patients with chronic HCV infection

BACKGROUND/AIMS

To investigate the link between hepatitis C infection and glucose intolerance, we measured insulin sensitivity, glucose effectiveness and beta-cell secretion in noncirrhotic HCV-infected patients with normal glucose tolerance according to WHO criteria as assessed by oral glucose tolerance tests.

METHODS

Glucose, insulin and C-peptide data from frequently sampled intravenous glucose tolerance tests were analyzed using the minimal modeling technique for glucose and C-peptide to determine insulin sensitivity, glucose effectiveness, first and second phase insulin secretion in noncirrhotic HCV-infected patients (n = 10) and in healthy control subjects (n = 10). Histological activity index (HAI) as well as the extent of fibrosis were evaluated by scoring liver biopsies.

RESULTS

Insulin sensitivity (2.72 +/- 1.63 vs. 6.84 +/- 1. 20 10(-4) min(-1) per microU/ml, p < 0.01) and glucose effectiveness (2.29 +/- 0.45 vs. 2.89 +/- 0.39 10(-2) min(-1), p < 0.05) ere significantly lower in patients with HCV-induced liver disease. Insulin sensitivity was negatively related to serum alanine aminotransferase (r = -0.47, p < 0.05) and aspartate aminotransferase concentrations (r = -0.65, p < 0.05). Multiple linear regression analysis revealed a strong relation of insulin sensitivity with fibrosis score and HAI (r = -0.82, p < 0.02 for both). Second phase insulin secretion was significantly enhanced in HCV-infected patients (14.30 +/- 2.04 vs. 8.29 +/- 1.65 min(-1), p < 0.05).

CONCLUSIONS

HCV-infected patients with normal glucose tolerance are insulin and glucose resistant. The impairment of glucose tolerance appears to be closely related with the severity of HCV-induced liver damage.