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.     

Homeostasis model assessment closely mirrors the glucose clamp technique in the assessment of insulin sensitivity: studies in subjects with various degrees of glucose tolerance and insulin sensitivity

OBJECTIVE: To evaluate whether the homeostasis model assessment (HOMA) is a reliable surrogate measure of in vivo insulin sensitivity in humans. RESEARCH DESIGN AND METHODS: In the present study, we compared insulin sensitivity as assessed by a 4-h euglycemic (approximately 5 mmol/l) hyperinsulinemic (approximately 300 pmol/l) clamp with HOMA in 115 subjects with various degrees of glucose tolerance and insulin sensitivity. RESULTS: We found a strong correlation between clamp-measured total glucose disposal and HOMA-estimated insulin sensitivity (r = -0.820, P<0.0001), with no substantial differences between men (r = -0.800) and women (r = -0.796), younger (aged <50 years, r = -0.832) and older (r = -0.800) subjects, nonobese (BMI <27 kg/m2, r = -0.800) and obese (r = -0.765) subjects, nondiabetic (r = -0.754) and diabetic (r = -0.695) subjects, and normotensive ( r = -0.786) and hypertensive (r = -0.762) subjects. Also, we found good agreement between the two methods in the categorization of subjects according to insulin sensitivity (weighted k = 0.63). CONCLUSIONS: We conclude that the HOMA can be reliably used in large-scale or epidemiological studies in which only a fasting blood sample is available to assess insulin sensitivity     

In vivo stimulation of the insulin receptor kinase in human skeletal muscle. Correlation with insulin-stimulated glucose disposal during euglycemic clamp studies.

To assess the relationship between insulin receptor (IR) kinase activity and insulin action in vivo in humans, we measured glucose disposal rates (GDR) during a series of euglycemic clamp studies. Simultaneously, we measured IR kinase activity in IRs extracted from skeletal muscle obtained by needle biopsy at the end of each clamp. By preserving the phosphorylation state of the receptors as it existed in vivo at the time of biopsy, we could correlate GDR and IR kinase in skeletal muscle. Eight nondiabetic, nonobese male subjects underwent studies at insulin infusion rates of 0, 40, 120, and 1,200 mU/m2 per min. Kinase activity, determined with receptors immobilized on insulin agarose beads, was measured at 0.5 microM ATP, with 1 mg/ml histone, followed by SDS-PAGE. Insulin increased GDR approximately sevenfold with a half-maximal effect at approximately 100 microU/ml insulin and a maximal effect by approximately 400 microU/ml. Insulin also increased IR kinase activity; the half-maximal effect occurred at approximately 500-600 microU/ml insulin with a maximal 10-fold stimulation over basal. Within the physiologic range of insulin concentrations, GDR increased linearly with kinase activation (P less than 0.0006); at supraphysiologic insulin levels, this relationship became curvilinear. Half-maximal and maximal insulin-stimulated GDR occurred at approximately 20 and approximately 50% maximal kinase activation, respectively. These results are consistent with a role of the kinase in insulin action in vivo. Furthermore, they demonstrate the presence of a large amount of "spare kinase" for glucose disposal.     

Maximizing the success rate of minimal model insulin sensitivity measurement in humans: the importance of basal glucose levels.

Minimal model analysis of glucose and insulin concentrations in the intravenous glucose tolerance test (IVGTT) has been widely used to obtain a measure of insulin sensitivity in humans. Issues of model validity and IVGTT protocol have been explored extensively. Less attention has been paid, however, to the computer programming protocol for estimating the model parameters (programming implementation). Minimal model analysis of data from an IVGTT protocol involving a high glucose dose (0.5 g/kg) and a reduced sample schedule, employed in healthy pre- or post-menopausal women, healthy men or men with coronary heart disease or chronic heart failure (20 in each group), was undertaken according to 12 different programming implementations using a commercially available model-equation-solving program. The ability of the program to arrive at an acceptable solution to the model equations gave a success rate of between 39% and 96%, depending on the implementation. Variation in basal glucose assignment significantly affected the magnitude of estimates of insulin sensitivity. The maximum modelling success rate was achieved by introduction of an imputed glucose measurement at 360 min from the glucose injection, taking the basal glucose level as the fasting glucose concentration, and overweighting the initial glucose measurement after a delay for mixing. Use of this implementation to analyse data from a study comparing insulin sensitivities obtained using the minimal model and a euglycaemic clamp reference gave a correlation of 0.80 (P<0.001) between the two methods. Straightforward variations in programming implementation, involving appropriate assignment of the basal glucose concentration and use of an imputed glucose measurement signifying re-establishment of basal glucose levels following the IVGTT, can considerably improve modelling success rate.     

Bayesian hierarchical approach to estimate insulin sensitivity by minimal model

We adopted Bayesian analysis in combination with hierarchical (population) modelling to estimate simultaneously population and individual insulin sensitivity (SI) and glucose effectiveness (SG) with the minimal model of glucose kinetics using data collected during insulin-modified intravenous glucose tolerance test (IVGTT) and made comparison with the standard non-linear regression analysis. After fasting overnight, subjects with newly presenting Type II diabetes according to World Health Organization criteria (n =65; 53 males, 12 females; age, 54 +/- 9 years; body mass index, 30.4 +/- 5.2 kg/m2; means+/-S.D.) underwent IVGTT consisting of a 0.3 g of glucose bolus/kg of body weight given at time zero for 2 min, followed by 0.05 unit of insulin/kg of body weight at 20 min. Bayesian inference was carried out using vague prior distributions and log-normal distributions to guarantee non-negativity and, thus, physiological plausibility of model parameters and associated credible intervals. Bayesian analysis gave estimates of SI in all subjects. Non-linear regression analysis failed in four cases, where Bayesian analysis-derived SI was located in the lower quartile and was estimated with lower precision. The population means of SI and SG provided by Bayesian analysis and non-linear regression were identical, but the interquartile range given by Bayesian analysis was tighter by approx. 20% for SI and by approx. 15% for SG. Individual insulin sensitivities estimated by the two methods were highly correlated ( rS=0.98; P <0.001). However, the correlation in the lower 20% centile of the insulin-sensitivity range was significantly lower than the correlation in the upper 80% centile ( rS=0.71 compared with rS=0.99; P <0.001). We conclude that the Bayesian hierarchical analysis is an appealing method to estimate SI and SG, as it avoids parameter estimation failures, and should be considered when investigating insulin-resistant subjects.     

Physiologic evaluation of factors controlling glucose tolerance in man: measurement of insulin sensitivity and beta-cell glucose sensitivity from the response to intravenous glucose.

The quantitative contributions of pancreatic responsiveness and insulin sensitivity to glucose tolerance were measured using the "minimal modeling technique" in 18 lean and obese subjects (88-206% ideal body wt). The individual contributions of insulin secretion and action were measured by interpreting the dynamics of plasma glucose and insulin during the intravenous glucose tolerance test in terms of two mathematical models. One, the insulin kinetics model, yields parameters of first-phase (phi 1) and second-phase (phi 2) responsivity of the beta-cells to glucose. The other glucose kinetics model yields the insulin sensitivity parameters, SI. Lean and obese subjects were subdivided into good (KG greater than 1.5) and lower (KG less than 1.5) glucose tolerance groups. The etiology of lower glucose tolerance was entirely different in lean and obese subjects. Lean, lower tolerance was related to pancreatic insufficiency (phi 2 77% lower than in good tolerance controls [P less than 0.03]), but insulin sensitivity was normal (P greater than 0.5). In contrast, obese lower tolerance was entirely due to insulin resistance (SI diminished 60% [P less than 0.01]); pancreatic responsiveness was not different from lean, good tolerance controls (phi 1: P greater than 0.06; phi 2: P greater than 0.40). Subjects (regardless of weight) could be segregated into good and lower tolerance by the product of second-phase beta-cell responsivity and insulin sensitivity (phi 2 . SI). Thus, these two factors were primarily responsible for overall determination of glucose tolerance. The effect of phi 1 was to modulate the KG value within those groups whose overall tolerance was determined by phi 2 . SI. This phi 1 modulating influence was more pronounced among insulin sensitive (phi 1 vs. KG, r = 0.79) than insulin resistant (obese, low tolerance; phi 1 vs. KG, r = 0.91) subjects. This study demonstrates the feasibility of the minimal model technique to determine the etiology of impaired glucose tolerance.     

Acute regulation of human lymphocyte insulin receptors. Analysis by the glucose clamp.

The T lymphocyte insulin receptor model has been used to explore the regulation of insulin receptor appearance in that lymphocytes do not bear the insulin receptor in the circulation and thus are not amenable to regulation by virtue of ligand binding. Such cells synthesize insulin receptors when stimulated by antigen in vivo or in vitro. In these studies, the glucose clamp technique was employed to isolate perturbations in plasma glucose and plasma insulin as potential mediators of the regulation of the mitogen-induced T lymphocyte insulin receptor. Nondiabetic, normal weight individuals volunteered for 10 hyperglycemic clamp studies and nine euglycemic clamp studies with five individuals studied by both protocols. Hyperglycemia and hyperinsulinemia were created by the hyperglycemic clamp (basal plasma glucose was increased from 89 +/- 2 mg/dl to 230 +/- 2 mg/dl and an insulin of 99 +/- 8 microU/ml was reached). Blood was removed for isolation of T lymphocytes at 0, 1, 3, and 4 h of the clamped condition. After 1 h of hyperglycemia accompanied by an elevated plasma insulin, T cell insulin binding fell from 9.9 +/- 0.9 pg/10(6) lymphocytes to 8.5 +/- 0.9 pg/10(6), and reached a nadir of 19 +/- 4% at the conclusion of the clamp. Scatchard analysis of binding data from two of the subjects who underwent the hyperglycemic clamp demonstrated a reduction of the number of binding sites per cell without a change in the affinity of ligand for receptor. To separate the effects of glucose and insulin and the manner in which insulin is provided, the 4-h euglycemic clamp was performed in which fasting plasma glucose was maintained (95 +/- 2 mg/dl) while constant hyperinsulinemia was created (80 +/- 3 microU/ml). Insulin binding to activated, cultured T lymphocytes demonstrated a similar fall in insulin binding. Scatchard analysis of three additional studies again revealed a reduction in receptor number to approximately 40% of base line. These studies reveal that T cell insulin receptor regulation is achieved by hyperinsulinemia independent of the glucose level achieved. The reduction in insulin binding and receptor number could not be accounted for by variations in the strength of lectin stimulation, the time course of lectin response, or by the stress of the clamp itself. The effect of the clamp was specific for the lymphocyte insulin receptor in that the clamp had little effect on the interleukin II receptor activation marker. Acute changes in plasma insulin by the glucose clamp technique are perceived by the T lymphocyte and displayed in tissue culture by an alteration in lectin-induced insulin receptors. One can conclude that rapid changes in ambient in vivo insulin concentrations can regulate the synthesis of T lymphocyte insulin receptors generated in vitro.     

The effect of insulin dose on the measurement of insulin sensitivity by the minimal model technique. Evidence for saturable insulin transport in humans.

Administration of exogenous insulin during an intravenous glucose tolerance test allows the use of the minimal model technique to determine the insulin sensitivity index in subjects with reduced endogenous insulin responses. To study the effect of different insulin administration protocols, we performed three intravenous glucose tolerance tests in each of seven obese subjects (age, 20-41 yr; body mass index, 30-43 kg/m2). Three different insulin administration protocols were used: a low-dose (0.025 U/kg) infusion given over 10 min, a low-dose (0.025 U/kg) bolus injection, and a high-dose (0.050 U/kg) bolus injection, resulting in peak insulin concentrations of 1,167 +/- 156, 3,014 +/- 483, and 6,596 +/- 547 pM, respectively. The mean insulin sensitivity index was 4.80 +/- 0.95 x 10(-5), 3.56 +/- 0.53 x 10(-5), and 2.42 +/- 0.40 x 10(-5) min-1/pM respectively (chi +/- SEM; P = 0.01). The association of higher peak insulin concentrations with lower measured insulin sensitivity values suggested the presence of a saturable process. Because results were not consistent with the known saturation characteristics of insulin action on tissue, a second saturable site involving the transport of insulin from plasma to interstitium was introduced, leading to a calculated Km of 807 +/- 165 pM for this site, a value near the 1/Kd of the insulin receptor. Thus, the kinetics of insulin action in humans in these studies is consistent with two saturable sites, and supports the hypothesis for transport of insulin to the interstitial space. Saturation may have an impact on minimal model results when high doses of exogenous insulin are given as a bolus, but can be minimized by infusing insulin at a low dose.     

A mathematical model for the glucose induced insulin release in man

Abstract. The dynamics of the insulin response to intravenously administered glucose were studied in man. It was shown that (a) insulin response to prolonged stimulation is biphasic; (b) if the glucose stimulus is repeated with short intervals, inhibition of the second response occurs; (c) if longer time-intervals are used, enhancement of the response is noted at the second stimulation. These findings suggest that when the pancreatic islets are exposed to hyperglycaemia, three kinetically distinct phenomena are initiated. Glucose induces almost instantaneous initiation of insulin release. Shortly thereafter, the pancreas enters a refractory phase. Thirdly, and at a later stage, a state of potentiation is built up in the islets. The effect of glucose on insulin synthesis is not considered here.—Against this background, and based on an earlier model, a mathematical model for the analysis of the glucose-insulin interplay during glucose infusions was constructed. The model describes the eventual occurrence of glucosuria, changes in the concentration of glucose in its pool, and mimics the effects of regulatory hormones when hypo-glycaemia appears. Insulin secretion is assumed to be controlled, in a multiplicative manner, by an immediate glucose function, a hypothetical potentiator that is slowly generated by glucose, and a negative factor with a shorter time-course which corresponds to the refractory phase of the pancreas. A three compartment model is used in the simulation of the metabolism and distribution of insulin after its release. Finally, glucose utilization is described as a multiplicative function, related to the prevailing concentrations of glucose in blood and insulin in the extracellular space.—This model is able to simulate all the experimental situations described in this report, both in normal man and in the diabetic syndrome, in which insulin secretion shows varying degrees of impairment. The results of the simulation of individual experiments are given either as a set of theoretical parameter values, or described as the insulin response of the model to a standard, hypothetical glucose stimulus. The latter alternative is an attractive method for objectively evaluating the insulin response to a standard glucose load in clinical materials.     

13C- and 2H-labelled glucose compared for minimal model estimates of glucose metabolism in man

In the present study, we have investigated the use of 1-[(13)C]glucose and GC/combustion/isotope-ratio MS as an alternative to 6,6-[(2)H(2)]glucose and GC/MS in the determination of parameters of glucose metabolism using the IVGTT (intravenous glucose tolerance test) interpreted by labelled (hot) minimal models. The study has been done in four populations, normoglycaemics (subdivided into lean and obese individuals), subjects with impaired glucose tolerance and those with diabetes mellitus. Although the use of carbon label may in some circumstances be compromised by substrate recycling, our hypothesis was that this would not be an issue under the condition of suppression of hepatic glucose production during the short timescale of an IVGTT. In all four groups, we found that the methodology employing the carbon label gave equivalent results to those obtained using the conventional deuterated material, but the sensitivity of the measurement technique in the new approach was sufficient to allow an approx. 15-fold reduction in the quantity of isotope administered. In addition to the clear cost advantages, this represents a significant scientific advance in that true tracer status is more nearly attained in these measurements with near-physiological tracee loads.     

血清单核细胞趋化蛋白1浓度与葡萄糖钳夹试验中胰岛素敏感性的关系

目的评价单核细胞趋化蛋白1(MCP-1)与正常糖耐量者、2型糖尿病患者葡萄糖钳夹试验中胰岛素敏感性的关系。方法分别对15例正常糖耐量者和20例2型糖尿病患者进行高胰岛素-正葡萄糖钳夹试验,以钳夹稳态期胰岛素介导的葡萄糖利用率(Rd)来判定周围组织胰岛素敏感性。同时用酶联免疫吸附测定(ELISA)法测定血清MCP-1水平,并测定其他临床指标,分析各指标之间的相关性及与胰岛素敏感性的关系。结果与正常糖耐量者相比,2型糖尿病患者外周血MCP-1水平明显升高(P<0.01),Rd值明显降低(P<0.01),其中MCP-1(65.64±17.67)ng/Lvs(144.39±61.63)ng/L,Rd(10.05±2.24)mg·kg-1.min-1vs(6.33±2.41)mg·kg-1.min-1,两者呈负相关(P<0.001)。Rd值、MCP-1均与体质量指数、腰围呈中度相关。偏相关分析和多元逐步回归分析显示,MCP-1是胰岛素敏感性的独立影响因素。结论外周血MCP-1与胰岛素抵抗、2型糖尿病关系密切。既独立于体质量、腰围、血脂等因素影响胰岛素抵抗的发生,又与之紧密相关,促进胰岛素抵抗的进展。     

Comparison of pharmacokinetic and pharmacodynamic properties of single-dose oral insulin spray and subcutaneous insulin injection in healthy subjects using the euglycemic clamp technique

BACKGROUND: Oral insulin spray is a new, noninjectable method of insulin delivery. This system delivers an aerosol of uniform-sized droplets containing regular human insulin at a high velocity into the oropharyngeal cavity for local transmucosal absorption. OBJECTIVE: The purpose of this study was to compare the pharmacokinetic and pharmacodynamic properties of single-dose oral insulin spray and SC insulin injection in healthy subjects. METHODS: Healthy male volunteers aged 21 to 25 years participated in this open-label study conducted at the Diabetes Unit, Hadassah-Hebrew University Hospital, Jerusalem, Israel. Subjects presented at 2 visits separated by 7 to 14 days. At both visits, the euglycemic clamp technique was used to maintain a constant blood glucose level. At one visit, subjects received regular human insulin 0.1 U/kg by SC injection. At the other visit, subjects received 15 puffs (150 U) of oral insulin spray. The pharmacokinetic (insulin absorption) and pharmacodynamic (glucose uptake) properties of the drugs were evaluated using blood analyses over the subsequent 360 minutes. RESULTS: Six volunteers were enrolled (mean [SD] age, 22.8 [1.2] years; mean [SD] body mass index, 23.2 [2.2] kg/m(2)). The mean (SD) baseline-corrected C(max) was significantly higher with oral insulin spray compared with SC insulin (54.0 [20.3] vs 30.8 [6.1] microU/mL; P = 0.028). Mean (SD) T(max) was significantly shorter with oral insulin spray compared with SC insulin (23.3 [5.2] vs 83.3 [42.2] minutes; P = 0.027). The mean (SD) time to maximal metabolic effect (maximum glucose infusion rate [GIR(max)]) (44.2 [8.6] vs 100.0 [35.6] minutes) and late time to half-maximal effect (101.0 [41.0] vs 257.2 [27.8] minutes) were shorter with oral insulin spray compared with SC insulin (both, P = 0.028). The baseline-corrected GIR(max) (6.8 [3.3] vs 6.2 [2.3] mg/kg . min) and glucose consumption (396.7 [178.0] vs 432.1 [226.0] mg/kg) during the 120 minutes after study drug administration were comparable between oral and SC insulin, respectively. CONCLUSIONS: In this study in a small, selected population of healthy male subjects under euglycemic conditions, oral insulin spray was associated with a higher C(max), shorter T(max), and faster time to peak glucose uptake compared with SC insulin. The short T(max) and the 120-minute duration of effect of oral insulin spray suggest it may be a promising alternative for fulfilling meal-related insulin requirements in persons with diabetes.     

Assessment of beta-cell function during the oral glucose tolerance test by a minimal model of insulin secretion

OBJECTIVE: To characterise the performance of beta-cell during a standard oral glucose tolerance test (OGTT). DESIGN: Fifty-six subjects were studied. A minimal analogic model of beta-cell secretion during the OGTT was applied to all OGTTs (see below). The amount of insulin secreted over 120' in response to oral glucose (OGTT-ISR; Insulin Units 120'-1 m-2 BSA) and an index of beta-cell secretory 'force' (beta-Index; pmol.min-2.m-2 BSA) were computed with the aid of the model. In protocol A, 10 healthy subjects underwent two repeat 75 g OGTT with frequent (every 10'-15') blood sampling for glucose and C-peptide to test the reproducibility of OGTT-ISR and beta-Index with a complete or a reduced data set. In protocol B, 7 healthy subjects underwent three OGTTs (50, 100 or 150 g), to test the stability of the beta-Index under different glucose loads. In protocol C, 29 subjects (15 with normal glucose tolerance, 7 with impaired glucose tolerance and 7 with newly diagnosed type 2 diabetes) underwent two repeat 75 g OGTT with reduced (every 30' for 120') blood sampling to compare the reproducibility and the discriminant ratio (DR) of OGTT-ISR and beta-index with the insulinogenic index (IG-Index: Delta Insulin 30' - Basal/Delta Glucose 30' - Basal). In protocol D, 20 subjects (14 with normal glucose tolerance, 5 with impaired glucose tolerance and 1 with newly-diagnosed type 2 diabetes) underwent a 75 g OGTT and an intravenous glucose tolerance test (IVGTT) on separate days to explore the relationships between acute (0'-10') insulin response (AIR) during the IVGTT and beta-index and OGTT-ISR during the OGTT. RESULTS: In all protocols, the minimal analogic model of C-peptide secretion achieved a reasonable fit of the experimental data. In protocol A, a good reproducibility of both beta-index and OGTT-ISR was observed with both complete and reduced (every 30') data sets. In protocol B, increasing the oral glucose load caused progressive increases in OGTT-ISR (from 2.63 +/- 0.70 to 5.11 +/- 0.91 Units.120'-1.m-2 BSA; P < 0.01), but the beta-index stayed the same (4.14 +/- 0.35 vs. 4.29 +/- 0.30 vs. 4.30 +/- 0.33 pmol.min-2.m-2 BSA). In protocol C, both OGTT-ISR and beta-index had lower day-to-day CVs (17.6 +/- 2.2 and 12.4 +/- 2.4%, respectively) and higher DRs (2.57 and 1.74, respectively) than the IG-index (CV: 35.5 +/- 6.3%; DR: 0.934). OGTT-ISR was positively correlated to BMI (P < 0.03), whereas beta-index was inversely related to both fasting and 2 h plasma glucose (P < 0.01 for both). In protocol D, beta-index, but not OGTT-ISR, was significantly correlated to AIR (r = 0.542, P < 0.02). CONCLUSIONS: Analogically modelling beta-cell function during the OGTT provides a simple, useful tool for the physiological assessment of beta-cell function.     

Relative contribution of glycogen synthesis and glycolysis to insulin-mediated glucose uptake. A dose-response euglycemic clamp study in normal and diabetic rats.

To examine the relationship between plasma insulin concentration and intracellular glucose metabolism in control and diabetic rats, we measured endogenous glucose production, glucose uptake, whole body glycolysis, muscle and liver glycogen synthesis, and rectus muscle glucose-6-phosphate (G-6-P) concentration basally and during the infusion of 2, 3, 4, 12, and 18 mU/kg.min of insulin. The contribution of glycolysis decreased and that of muscle glycogen synthesis increased as the insulin levels rose. Insulin-mediated glucose disposal was decreased by 20-30% throughout the insulin dose-response curve in diabetics compared with controls. While at low insulin infusions (2 and 3 mU/kg.min) reductions in both the glycolytic and glycogenic fluxes contributed to the defective tissue glucose uptake in diabetic rats, at the three higher insulin doses the impairment in muscle glycogen repletion accounted for all of the difference between diabetic and control rats. The muscle G-6-P concentration was decreased (208 +/- 11 vs. 267 +/- 18 nmol/g wet wt; P less than 0.01) compared with saline at the lower insulin infusion, but was gradually increased twofold (530 +/- 16; P less than 0.01 vs. basal) as the insulin concentration rose. The G-6-P concentration in diabetic rats was similar to control despite the reduction in glucose uptake. These data suggest that (a) glucose transport is the major determinant of glucose disposal at low insulin concentration, while the rate-limiting step shifts to an intracellular site at high physiological insulin concentration; and (b) prolonged moderate hyperglycemia and hypoinsulinemia determine two distinct cellular defects in skeletal muscle at the levels of glucose transport/phosphorylation and glycogen synthesis.     

Effect of insulin on the distribution and disposition of glucose in man.

Understanding the influence of insulin on glucose turnover is the key to interpreting a great number of metabolic situations. Little is known, however, about insulin's effect on the distribution and exchange of glucose in body pools. We developed a physiological compartmental model to describe the kinetics of plasma glucose in normal man in the basal state and under steady-state conditions of euglycemic hyperinsulinemia. A bolus of [3-3H]glucose was rapidly injected into a peripheral vein in six healthy volunteers, and the time-course of plasma radioactivity was monitored at very short time intervals for 150 min. A 1-mU/min kg insulin clamp was then started, thereby raising plasma insulin levels to a high physiological plateau (approximately 100 microU/ml). After 90 min of stable euglycemic hyperinsulinemia, a second bolus of [3-3H]glucose was given, and plasma radioactivity was again sampled frequently for 90 min more while the clamp was continued. Three exponential components were clearly identified in the plasma disappearance curves of tracer glucose of each subject studied, both before and after insulin. Based on stringent statistical criteria, the data in the basal state were fitted to a three-compartment model. The compartment of initial distribution was identical to the plasma pool (40 +/- 3 mg/kg); the other two compartments had similar size (91 +/- 12 and 96 +/- 9 mg/kg), but the former was in rapid exchange with plasma (at an average rate of 1.09 +/- 0.15 min-1), whereas the latter exchanged 10 times more slowly (0.12 +/- 0.01 min-1). The basal rate of glucose turnover averaged 2.15 +/- 0.12 mg/min kg, and the total distribution volume of glucose in the postabsorptive state was 26 +/- 1% of body weight. In view of current physiological information, it was assumed that the more rapidly exchanging pool represented the insulin-independent tissues of the body, while the slowly exchanging pool was assimilated to the insulin-dependent tissues. Insulin-independent glucose uptake was estimated (from published data) at 75% of basal glucose uptake, and was constrained not to change with euglycemic hyperinsulinemia. When the kinetic data obtained during insulin administration were fitted to this model, neither the size nor the exchange rates of the plasma or the rapid pool were appreciably changed. In contrast, the slow pool was markedly expanded (from 96 +/- 9 to 190 +/- 30 mg/kg, P less than 0.02) at the same time as total glucose disposal rose fourfold above basal (to 7.96 +/- 0.85 mg/min kg, P less than 0.001). Furthermore, a significant direct correlation was found to exist between the change in size of the slow pool and the insulin-stimulated rate of total glucose turnover (r=0.92, P<0.01). We conclude that hyperinsulinemia, independent of hyperglycemia, markedly increases the exchangeable mass of glucose in the body, presumably reflecting the accumulation of free, intracellular glucose in insulin-dependent tissues.