Kinetics of insulin-mediated and non-insulin-mediated glucose uptake in humans

Distinct Roles for Brain and Pancreas in Basal and Postprandial Glucose Homeostasis

The glucose homeostasis system ensures that the circulating glucose level is maintained within narrow physiological limits both in the fasting (or basal) state and following a nutrient challenge. Although glucose homeostasis is traditionally conceptualized as a single overarching system, evidence reviewed here suggests that basal glycemia and glucose tolerance are governed by distinct control systems. Specifically, whereas glucose tolerance appears to be determined largely by interactions between insulin secretion and insulin sensitivity, basal-state glucose homeostasis is predominated by insulin-independent mechanisms governed largely by the brain. In addition to a new perspective on how glucose homeostasis is achieved, this "dual control system" hypothesis offers a feasible and testable explanation for observations that are otherwise difficult to reconcile and sheds new light on the integration of central and peripheral metabolic control mechanisms. The implications of this model for the pathogenesis and treatment of impaired fasting glucose, impaired glucose tolerance, and type 2 diabetes are also discussed.

The Effect of Glucagon-Like Peptide 1 Receptor Blockade on Glucagon-Induced Stimulation of Insulin Secretion

Data from transgenic rodent models suggest that glucagon acts as an insulin secretagogue by signaling through the glucagon-like peptide 1 receptor (GLP-1R) present on β-cells. However, its net contribution to physiologic insulin secretion in humans is unknown. To address this question, we studied individuals without diabetes in two separate experiments. Each subject was studied on two occasions in random order. In the first experiment, during a hyperglycemic clamp, glucagon was infused at 0.4 ng/kg/min, increasing by 0.2 ng/kg/min every hour for 5 h. On one day, exendin-9,39 (300 pmol/kg/min) was infused to block GLP-1R, while on the other, saline was infused. The insulin secretion rate (ISR) was calculated by nonparametric deconvolution from plasma concentrations of C-peptide. Endogenous glucose production and glucose disappearance were measured using the tracer-dilution technique. Glucagon concentrations, by design, did not differ between study days. Integrated ISR was lower during exendin-9,39 infusion (213 ± 26 vs. 191 ± 22 nmol/5 h, saline vs. exendin-9,39, respectively; P = 0.02). In the separate experiment, exendin-9,39 infusion, compared with saline infusion, also decreased the β-cell secretory response to a 1-mg glucagon bolus. These data show that, in humans without diabetes, glucagon partially stimulates the β-cell through GLP-1R.

Relationship Between β-cell Response and Insulin Sensitivity in Horses based on the Oral Sugar Test and the Euglycemic Hyperinsulinemic Clamp

Background

A hyperbolic relationship between β‐cell response and insulin sensitivity (IS) has been described in several species including rodents, dogs, and humans. This relationship has not been elucidated in the horse.

Hypothesis/Objectives

To determine whether the hyperbolic relationship between β‐cell response and IS exists in horses by using indices of β‐cell response from the oral sugar test (OST) and IS measurements from the euglycemic hyperinsulinemic clamp (EHC). A second aim was to compare how well IS estimates from the OST and EHC correlate.

Animals

Forty‐nine horses with different degrees of insulin regulation (normal‐to‐severe insulin dysregulation).

Methods

Cross‐sectional study. Horses were examined with an OST and an EHC.

Results

Decreased IS was associated with increased β‐cell response in the horses. Nine of 12 comparisons between indices of β‐cell response and IS measures fulfilled the criteria for a hyperbolic relationship. Indices of IS calculated from the OST correlated highly with the insulin‐dependent glucose disposal rate (M) and the insulin‐dependent glucose disposal rate per unit of insulin (M/I) determined from the EHC (r = 0.81–0.87).

Conclusions and Clinical Importance

A hyperbolic relationship between β‐cell response and IS exists in horses, which suggest that horses with insulin dysregulation respond not only with postprandial hyperinsulinemia but are also insulin resistant. The OST is primarily a test for β‐cell response rather than a test for IS, but calculated indices of IS from the OST may be useful to estimate IS in horses, especially when the horse is insulin resistant.

Sitagliptin Attenuates Endothelial Dysfunction of Zucker Diabetic Fatty Rats: Implication of the Antiperoxynitrite and Autophagy

Although the contributions of sitagliptin to endothelial function in diabetes mellitus were previously reported, the potential mechanisms still remain undefined. Our research was intended to explore the underlying mechanisms of protective effects of sitagliptin treatment on endothelial dysfunction in Zucker diabetic fatty (ZDF) rats. Male lean nondiabetic Zucker rats were used as control and male obese ZDF rats were randomly divided into ZDF and ZDF + sitagliptin groups. The significant decrease in endothelium-dependent relaxation induced by acetylcholine was observed in mesenteric arteries and thoracic aorta rings of ZDF rats. The administration of sitagliptin restored the vascular function effectively. The morphology study showed severe endothelial injuries in thoracic aortas of ZDF rats, and sitagliptin treatment attenuated these changes. The increased malondialdehyde levels and decreased superoxide dismutase activities in serum of ZDF rats were reversed by sitagliptin treatment. Sitagliptin also increased the expression of endothelial nitric oxide synthase and microtubule-associated protein 1 light chain 3 (LC3) and decreased the expression of inducible nitric oxide synthase, 3-nitrotyrosine, and p62 in ZDF rats. After giving Fe (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride porphyrin pentachloride (FeTMPyP, a peroxynitrite [ONOO^-] scavenger) or sitagliptin to high-glucose (30 mmol/L, 48 hours) cultured human umbilical vein endothelial cells (HUVECs), the increased levels of Beclin-1 and lysosome-associated membrane protein type 2 were detected. Both FeTMPyP and sitagliptin also significantly increased the number of mRFP-GFP-LC3 dots per cell, suggesting that autophagic flux was increased in HUVECs. Our study indicated that sitagliptin treatment can improve the endothelium-dependent relaxation and attenuate the endothelial impairment of ZDF rats. The protective effects of sitagliptin are possibly related to antiperoxynitrite and promoting autophagy.

Protein Ingestion Induces Muscle Insulin Resistance Independent of Leucine-Mediated mTOR Activation

Increased plasma branched-chain amino acid concentrations are associated with insulin resistance, and intravenous amino acid infusion blunts insulin-mediated glucose disposal. We tested the hypothesis that protein ingestion impairs insulin-mediated glucose disposal by leucine-mediated mTOR signaling, which can inhibit AKT. We measured glucose disposal and muscle p-mTOR(Ser2448), p-AKT(Ser473), and p-AKT(Thr308) in 22 women during a hyperinsulinemic-euglycemic clamp procedure with and without concomitant ingestion of whey protein (0.6 g/kg fat-free mass; n = 11) or leucine that matched the amount given with whey protein (n = 11). Both whey protein and leucine ingestion raised plasma leucine concentration by approximately twofold and muscle p-mTOR(Ser2448) by ∼30% above the values observed in the control (no amino acid ingestion) studies; p-AKT(Ser473) and p-AKT(Thr308) were not affected by whey protein or leucine ingestion. Whey protein ingestion decreased insulin-mediated glucose disposal (median 38.8 [quartiles 30.8, 61.8] vs. 51.9 [41.0, 77.3] µmol glucose/µU insulin · mL(-1) · min(-1); P < 0.01), whereas ingestion of leucine did not (52.3 [43.3, 65.4] vs. 52.3 [43.9, 73.2]). These results indicate that 1) protein ingestion causes insulin resistance and could be an important regulator of postprandial glucose homeostasis and 2) the insulin-desensitizing effect of protein ingestion is not due to inhibition of AKT by leucine-mediated mTOR signaling.

Modifying effect of obesity on the association between sitting and incident diabetes in post-menopausal women

OBJECTIVE

To evaluate the association between self-reported daily sitting time and the incidence of type 2 diabetes in a cohort of postmenopausal women.

METHODS

Women (N=88,829) without diagnosed diabetes reported the number of hours spent sitting over a typical day. Incident cases of diabetes were identified annually by self-reported initiation of using oral medications or insulin for diabetes > 14.4 years follow-up.

RESULTS

Each hour of sitting time was positively associated with increased risk of diabetes [risk ratio (RR): 1.05; 95% confidence interval (CI): 1.02-1.08]. However, sitting time was only positively associated with incident diabetes in obese women. Obese women reporting sitting 8-11 (RR: 1.08; 95% CI 1.0-1.1), 12-15 (OR: 1.13; 95% CI 1.0-1.2), and ≥16 hours (OR: 1.25; 95% CI 1.0-1.5) hours per day had an increased risk of diabetes compared to women sitting ≤7 hours per day. These associations were adjusted for demographics, health conditions, behaviors (smoking, diet, and alcohol intake), and family history of diabetes. Time performing moderate to vigorous intensity physical activity did not modify these associations.

CONCLUSIONS

Time spent sitting was independently associated with increased risk of diabetes diagnosis among obese women-a population already at high risk of the disease.

Strain-dependent differences for suppression of insulin-stimulated glucose uptake in skeletal and cardiac muscle by ethanol

BACKGROUND

Chronic ethanol (EtOH) consumption impairs the ability of insulin to suppress hepatic glucose production in a strain-dependent manner, with hepatic insulin resistance being greater in Long-Evans (LE) than Sprague-Dawley (SD) rats. We assessed whether strain differences exist for whole-body and tissue glucose uptake under basal and insulin-stimulated conditions and whether they were associated with coordinate strain-dependent elevations in muscle cytokines.

METHODS

Male rats (160 g) were provided the Lieber-DeCarli EtOH-containing (36% total energy) diet or pair-fed a control diet for 8 weeks. Rats were studied in the basal state or during a euglycemic hyperinsulinemic clamp, and whole-body glucose flux assessed using (3) H-glucose and in vivo tissue glucose uptake by (14) C-2-deoxyglucose.

RESULTS

EtOH impaired whole-body insulin-mediated glucose uptake (IMGU) more in SD than LE rats. This difference was due to impaired IMGU by gastrocnemius and heart in EtOH-fed SD versus LE rats. However, decreased IMGU in adipose tissue (epididymal and perirenal) produced by EtOH was comparable between strains. EtOH-induced insulin resistance in muscle from SD rats was associated with reduced AKT and AS160 phosphorylation and plasma membrane-localized GLUT4 protein as well as enhanced phosphorylation of c-Jun N-terminal kinase (JNK) and IRS-1 (S307), changes which were absent in muscle from LE rats. EtOH increased tumor necrosis factor alpha (TNFα) mRNA in gastrocnemius and fat under basal conditions in both SD and LE rats; however, hyperinsulinemia decreased TNFα in skeletal muscle from LE, but not SD rats. Interleukin (IL)-6 mRNA in gastrocnemius was increased under basal conditions and increased further in response to insulin in SD rats, but no EtOH- or insulin-induced change was detected in muscle IL-6 of LE rats.

CONCLUSIONS

These data indicate strain-dependent differences in EtOH-induced IMGU in skeletal and cardiac muscle, but not fat, associated with sustained increases in TNFα and IL-6 mRNA and JNK activation and decreased plasma membrane GLUT4 in response to insulin.

Blood glucose levels, insulin concentrations, and insulin resistance in healthy women and women with premenstrual syndrome: a comparative study

Objective

To compare the blood glucose levels, insulin concentrations, and insulin resistance during the two phases of the menstrual cycle between healthy women and patients with premenstrual syndrome (PMS).

Methods

From January of 2011 to the August of 2012, a descriptive cross-sectional study was performed among students in the School of Medicine of Jahrom University of Medical Sciences. We included 30 students with the most severe symptoms of PMS and 30 age frequency-matched healthy controls. We analyzed the serum concentrations of glucose, insulin, and insulin resistance by using the glucose oxidase method, radioimmunometric assay, and homeostasis model assessment of insulin resistance equation, respectively.

Results

No significant differences between the demographic data of the control and PMS groups were observed. The mean concentrations of glucose of the two study groups were significantly different during the follicular and luteal phases (p=0.011 vs. p<0.0001, respectively). The amounts of homeostasis model assessment of insulin resistance of the two study groups were significantly different in the luteal phase (p=0.0005).

Conclusion

The level of blood glucose and insulin resistance was lower during the two phases of the menstrual cycle of the PMS group than that of the controls.

Nonlinear model predictive control of glucose concentration in subjects with type 1 diabetes

A nonlinear model predictive controller has been developed to maintain normoglycemia in subjects with type 1 diabetes during fasting conditions such as during overnight fast. The controller employs a compartment model, which represents the glucoregulatory system and includes submodels representing absorption of subcutaneously administered short-acting insulin Lispro and gut absorption. The controller uses Bayesian parameter estimation to determine time-varying model parameters. Moving target trajectory facilitates slow, controlled normalization of elevated glucose levels and faster normalization of low glucose values. The predictive capabilities of the model have been evaluated using data from 15 clinical experiments in subjects with type 1 diabetes. The experiments employed intravenous glucose sampling (every 15 min) and subcutaneous infusion of insulin Lispro by insulin pump (modified also every 15 min). The model gave glucose predictions with a mean square error proportionally related to the prediction horizon with the value of 0.2 mmol L(-1) per 15 min. The assessment of clinical utility of model-based glucose predictions using Clarke error grid analysis gave 95% of values in zone A and the remaining 5% of values in zone B for glucose predictions up to 60 min (n = 1674). In conclusion, adaptive nonlinear model predictive control is promising for the control of glucose concentration during fasting conditions in subjects with type 1 diabetes.

Postprandial leg uptake of triglyceride is greater in women than in men

The postprandial excursion of plasma triglyceride (TG) concentration is greater in men than in women. In this study, the disposition of dietary fat was examined in lean healthy men and women (n = 8/group) in either the overnight-fasted or fed (4.5 h after breakfast) states. A [14C]oleate tracer was incorporated into a test meal, providing 30% of total daily energy requirements. After ingestion of the test meal, measures of arteriovenous differences in TG and 14C across the leg were combined with needle biopsies of skeletal muscle and adipose tissue and respiratory gas collections to define the role of skeletal muscle in the clearance of dietary fat. The postprandial plasma TG and 14C tracer excursions were lower (P = 0.04) in women than in men in the overnight-fasted and fed states. Women, however, had significantly greater limb uptake of total TG compared with men on both the fasted (3,849 +/- 846 vs. 528 +/- 221 total micro mol over 6 h) and fed (4,847 +/- 979 vs. 1,571 +/- 334 total micromol over 6 h) days. This was also true for meal-derived 14C lipid uptake. 14C content of skeletal muscle tissue (micro Ci/g tissue) was significantly greater in women than in men 6 h after ingestion of the test meal. In contrast, 14C content of adipose tissue was not significantly different between men and women at 6 h. The main effect of nutritional state, fed vs. fasted, was to increase the postmeal glucose (P = 0.01) excursion (increase from baseline) and decrease the postmeal TG excursion (P = 0.02). These results support the notion that enhanced skeletal muscle clearance of lipoprotein TG in women contributes to their reduced postprandial TG excursion. Questions remain as to the mechanisms causing these sex-based differences in skeletal muscle TG uptake and metabolism. Furthermore, nutritional state can significantly impact postprandial metabolism in both men and women.

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

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

Effects of clenbuterol on insulin resistance in conscious obese Zucker rats

The present study was conducted to determine the effect of chronic administration of the long-acting beta(2)-adrenergic agonist clenbuterol on rats that are genetically prone to insulin resistance and impaired glucose tolerance. Obese Zucker rats (fa/fa) were given 1 mg/kg of clenbuterol by oral intubation daily for 5 wk. Controls received an equivalent volume of water according to the same schedule. At the end of the treatment, rats were catheterized for euglycemic-hyperinsulinemic (15 mU insulin. kg(-1). min(-1)) clamping. Clenbuterol did not change body weight compared with the control group but caused a redistribution of body weight: leg muscle weights increased, and abdominal fat weight decreased. The glucose infusion rate needed to maintain euglycemia and the rate of glucose disappearance were greater in the clenbuterol-treated rats. Furthermore, plasma insulin levels were decreased, and the rate of glucose uptake into hindlimb muscles and abdominal fat was increased in the clenbuterol-treated rats. This increased rate of glucose uptake was accompanied by a parallel increase in the rate of glycogen synthesis. The increase in muscle glucose uptake could not be ascribed to an increase in the glucose transport protein GLUT-4 in clenbuterol-treated rats. We conclude that chronic clenbuterol treatment reduces the insulin resistance of the obese Zucker rat by increasing insulin-stimulated muscle and adipose tissue glucose uptake. The improvements noted may be related to the repartitioning of body weight between tissues.

Evidence for physiological coupling of insulin-mediated glucose metabolism and limb blood flow

We hypothesized that the vasodilation observed during insulin stimulation is closely coupled to the rate of glucose metabolism. Lean (L, n = 13), obese nondiabetic (OB, n = 13), and obese type 2 diabetic subjects (Type 2 DM, n = 16) were studied. Leg blood flow (LBF) was examined under conditions of euglycemic hyperinsulinemia (EH) and hyperglycemic hyperinsulinemia (HH), which produced a steady-state whole body glucose disposal rate (GDR) of approximately 2,000 micromol. m(-2). min(-1). At this GDR, under both conditions, subjects across the range of insulin sensitivity exhibited equivalent LBF (l/min EH: L, 0.42 +/- 0.03; OB, 0.43 +/- 0. 03; Type 2 DM, 0.38 +/- 0.07; P = 0.72 by ANOVA. HH: L, 0.44 +/- 0. 04; OB, 0.39 +/- 0.05; Type 2 DM, 0.41 +/- 0.04; P = 0.71). The continuous relationship between LBF and GDR did not differ across subject groups [slope x 10(-5) l/(micromol. m(-2). min(-1)) by ANOVA. EH: L, 8.6; OB, 9.2; Type 2 DM, 7.9; P = 0.91. HH: L, 4.2; OB, 2.5; Type 2 DM, 4.1; P = 0.77], although this relationship did differ between the EH and HH conditions (P = 0.001). These findings support a physiological coupling of LBF and insulin-mediated glucose metabolism. The mechanism(s) linking substrate delivery and metabolism appears to be intact in insulin-resistant states.

Endothelial activation in patients with cardiac syndrome X

BACKGROUND

The presence of endothelial dysfunction with increased endothelin-1 plasma concentrations in patients with cardiac syndrome X is still under debate. The aim of the present study was to evaluate the presence of endothelial dysfunction in patients with cardiac syndrome X.

METHODS AND RESULTS

++Endothelin-1 levels were evaluated with a sensitive radioimmunoassay with previous purification through reverse phase HPLC in 24 patients (3 men and 21 women, mean age 54+/-7 years) with typical angina, instrumental evidence of ischemia, and normal coronary angiograms both under baseline conditions and after oral glucose load (75 g D-glucose). We also measured plasma nitrite-plus-nitrate levels, a sharp index of endothelial nitric oxide production, and circulating concentrations of the soluble fraction of the endothelial adhesion molecule vascular cell adhesion molecule-1, a well-recognized marker of early endothelial dysfunction. Fourteen healthy subjects (1 man and 13 women, mean age 47+/-15 years) served as controls. There were no significant differences in baseline plasma endothelin-1 concentrations between patients and control subjects (0.55+/-0.34 versus 0.48+/-0.22 pg/mL, P=0.503). Plasma nitrite-plus-nitrate and soluble vascular cell adhesion molecule-1 concentrations were also similar between the 2 groups. After glucose ingestion, circulating endothelin-1 concentrations were significantly higher in patients with cardiac syndrome X than in control subjects (P<0.03 at 60, 90, and 120 minutes).

CONCLUSIONS

Our findings show that no basal endothelial damage is present in patients with cardiac syndrome X. Nevertheless, increased responsiveness of endothelin-1 to glucose loading suggests that patients with cardiac syndrome X present an increased susceptibility to releasing endothelin-1 under stressful circumstances.

Limitations to basal and insulin-stimulated skeletal muscle glucose uptake in the high-fat-fed rat

Rats fed a high-fat diet display blunted insulin-stimulated skeletal muscle glucose uptake. It is not clear whether this is due solely to a defect in glucose transport, or if glucose delivery and phosphorylation are also impaired. To determine this, rats were fed standard chow (control rats) or a high-fat diet (HF rats) for 4 wk. Experiments were then performed on conscious rats under basal conditions or during hyperinsulinemic euglycemic clamps. Rats received primed constant infusions of 3-O-methyl-[(3)H]glucose (3-O-MG) and [1-(14)C]mannitol. Total muscle glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-MG concentration [which distributes based on the transsarcolemmal glucose gradient (TSGG)] were used to calculate glucose concentrations at the inner and outer sarcolemmal surfaces ([G](im) and [G](om), respectively) in soleus. Total muscle glucose was also measured in two fast-twitch muscles. Muscle glucose uptake was markedly decreased in HF rats. In control rats, hyperinsulinemia resulted in a decrease in soleus TSGG compared with basal, due to increased [G](im). In HF rats during hyperinsulinemia, [G](im) also exceeded zero. Hyperinsulinemia also decreased muscle glucose in HF rats, implicating impaired glucose delivery. In conclusion, defects in extracellular and intracellular components of muscle glucose uptake are of major functional significance in this model of insulin resistance.

Limitations to exercise- and maximal insulin-stimulated muscle glucose uptake

The hypothesis of this investigation was that insulin and muscle contraction, by increasing the rate of skeletal muscle glucose transport, would bias control so that glucose delivery to the sarcolemma (and t tubule) and phosphorylation of glucose intracellularly would exert more influence over glucose uptake. Because of the substantial increases in blood flow (and hence glucose delivery) that accompany exercise, we predicted that glucose phosphorylation would become more rate determining during exercise. The transsarcolemmal glucose gradient (TSGG; the glucose concentration difference across the membrane) is inversely related to the degree to which glucose transport determines the rate of glucose uptake. The TSGG was determined by using isotopic methods in conscious rats during euglycemic hyperinsulinemia [Ins; 20 mU/(kg. min); n = 7], during treadmill exercise (Ex, n = 6), and in sedentary, saline-infused rats (Bas, n = 13). Rats received primed, constant intravenous infusions of trace 3-O-[3H]methyl-D-glucose and [U-14C]mannitol. Then 2-deoxy-[3H]glucose was infused for the calculation of a glucose metabolic index (Rg). At the end of experiments, rats were anesthetized, and soleus muscles were excised. Total soleus glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-[3H]methyl-D-glucose (which distributes on the basis of the TSGG) were used to calculate ranges of possible glucose concentrations ([G]) at the inner and outer sarcolemmal surfaces ([G]im and [G]om, respectively). Soleus Rg was increased in Ins and further increased in Ex. In Ins, total soleus glucose, [G]om, and the TSGG were decreased compared with Bas, while [G]im remained near 0. In Ex, total soleus glucose and [G]im were increased compared with Bas, and there was not a decrease in [G]om as was observed in Ins. In addition, accumulation of intracellular free 2-deoxy-[3H]glucose occurred in soleus in both Ex and Ins. Taken together, these data indicate that, in Ex, glucose phosphorylation becomes an important limitation to soleus glucose uptake. In Ins, both glucose delivery and glucose phosphorylation influence the rate of soleus glucose uptake more than under basal conditions.

Relationship between blood pressure, metabolic variables and blood flow in obese subjects with or without non-insulin-dependent diabetes mellitus

BACKGROUND

To assess the relationship between systemic blood pressure, metabolic variables and adipose tissue blood flow, we studied 55 subjects before and 36 subjects after an oral glucose load (100 g).

METHODS

The subjects were divided into four different groups: (a) young lean control subjects [age 31 +/- 1 years, mean +/- SE, BMI (body mass index) 22.7 +/- 0.4 kg m-2]; (b) young obese subjects (age 29 +/- 2 years, BMI 37.8 +/- 1.8 kg m-2); (c) middle-aged obese subjects (age 50 +/- 2 years, BMI 30.2 +/- 0.9 kg m-2); and (d) middle-aged obese non-insulin-dependent diabetic (NIDDM) subjects (age 54 +/- 2 years, BMI 30.0 +/- 0.7 kg m-2).

RESULTS

Groups 2-4 demonstrated a low fasting adipose tissue blood flow (ATBF) and the increase in ATBF after oral glucose was impaired. A further impairment was present in NIDDM subjects. Systolic and diastolic blood pressure were also increased in groups 2-4 and further so in group 4. Fasting glucose, lactate and free fatty acid (FFA) levels correlated positively with the systolic blood pressure, whereas ATBF correlated negatively with the diastolic blood pressure. Furthermore, in the NIDDM subjects fasting lactate correlated closely with both the systolic (r = 0.649, P = 0.01) and diastolic (r = 0.626, P = 0.013) blood pressure.

CONCLUSION

These data suggest a close relationship between insulin resistance and regulation of adipose tissue blood flow as well as blood pressure.

Analysis of insulin-stimulated skeletal muscle glucose uptake in conscious rat using isotopic glucose analogs

An isotopic method was used in conscious rats to determine the roles of glucose transport and the transsarcolemmal glucose gradient (TSGG) in control of basal and insulin-stimulated muscle glucose uptake. Rats received an intravenous 3-O-[3H]methylglucose (3-O-[3H]MG) infusion from -100 to 40 min and a 2-deoxy-[3H]glucose infusion from 0 to 40 min to calculate a glucose metabolic index (Rg). Insulin was infused from -100 to 40 min at rates of 0.0, 0.6, 1.0, and 4.0 mU.kg-1.min-1, and glucose was clamped at basal concentrations. The ratios of soleus intracellular to extracellular 3-O-[3H]MG concentration and soleus glucose concentrations were used to estimate the TSGG using principles of glucose counter-transport. Tissue glucose concentrations were compared in well-perfused, slow-twitch muscle (soleus) and poorly perfused, fast-twitch muscle (vastus lateralis, gastrocnemius). Data show that 1) small increases in insulin increase soleus Rg without decreasing TSGG, suggesting that muscle glucose delivery and phosphorylation can accommodate the increased flux; 2) due to a limitation in soleus glucose phosphorylation and possibly delivery, insulin at high physiological levels decreases TSGG, and at supraphysiological insulin levels the TSGG is not significantly different from 0; 3) maximum Rg is maintained even though TSGG decreases with increasing insulin levels, indicating that glucose transport continues to increase and is not rate limiting for maximal insulin-stimulated glucose uptake; and 4) muscle consisting of fast-twitch fibers that are poorly perfused exhibits a 35-45% fall in tissue glucose with insulin, suggesting that glucose delivery is a major limitation in sustaining the TSGG. In conclusion, control of glucose uptake is distributed between glucose transport and factors that determine the TSGG. Insulin stimulation of glucose transport increases the demands on the factors that maintain glucose delivery to the muscle membrane and glucose phosphorylation inside the muscle.

Haemodynamic actions of insulin

Several lines of evidence indicate a significant association between insulin and cardiovascular disease. This association might be explained by direct (cardio) vascular effects of insulin. Two hemodynamic actions of insulin are discussed in this review; it induces direct vasodilation in skeletal muscle and stimulation of the sympathetic nervous system. These closely linked effects normally offset each other. Although more insight has been obtained into responses in insulin-resistant individuals and possible mechanisms, direct evidence to support a causative role for insulin is not yet available.

Preexercise muscle glycogen content affects metabolism during exercise despite maintenance of hyperglycemia

Trained cyclists with low muscle glycogen (LGH; n = 8) or normal glycogen (NGH; n = 5) exercised for 145 min at 70% of maximal oxygen uptake during a hyperglycemic clamp. Respiratory exchange ratio was higher in NGH than LGH, and free fatty acid concentrations were lower in NGH than LGH. Areas under the curve for insulin and lactate were lower in LGH than NGH. Total glucose infusion and total glucose oxidation were not different between NGH and LGH, and total glucose oxidation amounted to 65 and 66% of total glucose infusion in NGH and LGH, respectively. Rates of glucose oxidation rose during exercise, reaching peaks of 9.2 +/- 1.7 and 8.3 +/- 1.1 mmol/min in NGH and LGH, respectively. Muscle glycogen disappearance was greater in NGH than LGH. Thus 1) low muscle glycogen content does not cause increased glucose oxidation, even during hyperglycemia; instead there is an increase in fat oxidation, 2) there is an upper limit to the rate of glucose oxidation during exercise with hyperglycemia irrespective of muscle glycogen status, and 3) net muscle glycogen utilization is determined by muscle glycogen content at the start of exercise, even during hyperglycemia.

Role of exercise training in the prevention and treatment of insulin resistance and non-insulin-dependent diabetes mellitus

Recent epidemiological studies indicate that individuals who maintain a physically active lifestyle are much less likely to develop impaired glucose tolerance and non-insulin-dependent diabetes mellitus (NIDDM). Moreover, it was found that the protective effect of physical activity was strongest for individuals at highest risk of developing NIDDM. Reducing the risk of insulin resistance and NIDDM by regularly performed exercise is also supported by several aging studies. It has been found that older individuals who vigorously train on a regular basis exhibit a greater glucose tolerance and a lower insulin response to a glucose challenge than sedentary individuals of similar age and weight. While the evidence is substantial that aerobic exercise training can reduce the risk of impaired glucose tolerance and NIDDM, the evidence that exercise training is beneficial in the treatment of NIDDM is not particularly strong. Many of the early studies investigating the effects of exercise training on NIDDM could not demonstrate improvements in fasting plasma glucose and insulin levels, or glucose tolerance. The adequacy of the training programmes in many of these studies, however, is questionable. More recent studies using prolonged, vigorous exercise-training protocols have produced more favourable results. There are several important adaptations to exercise training that may be beneficial in the prevention and treatment of insulin resistance, impaired glucose tolerance and NIDDM. An increase in abdominal fat accumulation and loss of muscle mass are highly associated with the development of insulin resistance. Exercise training results in preferential loss of fat from the central regions of the body and should therefore contribute significantly in preventing or alleviating insulin resistance due to its development. Likewise, exercise training can prevent muscle atrophy and stimulate muscle development. Several months of weight training has been found to significantly lower the insulin response to a glucose challenge without affecting glucose tolerance, and to increase the rate of glucose clearance during a euglycaemic clamp. Muscle glucose uptake is equal to the product of the arteriovenous glucose difference and the rate of glucose delivery or muscle blood flow. While it has been known for many years that insulin will accelerate blood glucose extraction by insulin-sensitive peripheral tissues, recent evidence suggests that it can also acutely vasodilate skeletal muscle and increase muscle blood flow in a dose-dependent manner. A reduced ability of insulin to stimulate muscle blood flow is a characteristic of insulin-resistant obese individuals and individuals with NIDDM. Exercise training, however, has been found to help alleviate this problem, and substantially improve the control of insulin over blood glucose. Improvements in insulin resistance and glucose tolerance with exercise training are highly related to an increased skeletal muscle insulin action. This increased insulin action is associated with an increase in the insulin-regulatable glucose transporters, GLUT4, and enzymes responsible for the phosphorylation, storage and oxidation of glucose. Changes in muscle morphology may also be important following training. With exercise training there is an increase in the conversion of fast twitch glycolytic IIb fibres to fast twitch oxidative IIa fibres, as well as an increase in capillary density. IIa fibres have a greater capillary density and are more insulin-sensitive and -responsive than IIb fibres. Evidence has been provided that morphological changes in muscle, particularly the capillary density of the muscle, are associated with changes in fasting insulin levels and glucose tolerance. Furthermore, significant correlations between glucose clearance, muscle capillary density and fibre type have been found in humans during a euglycaemic clamp. Exercise training may also improve control over hepatic glucose production by increasin

Blood flow, lipid oxidation, and muscle glycogen synthesis after glycogen depletion by strenuous exercise

We studied the interrelationship between blood flow, glycogen synthesis, and glucose and lipid utilization in 14 healthy men. A 4-h euglycemic insulin clamp with indirect calorimetry and muscle biopsies were done after a glycogen depletion (exercise) and after a resting day (control). In spite of the exercise induced decrease in leg muscle glycogen content (28% in the basal state, 22% after hyperinsulinemia, P < 0.05 in both as compared with the control study), basal or insulin stimulated glycogen synthase activity remained unchanged. In the basal state, glucose oxidation was 54% lower (P < 0.001) and lipid oxidation 108% higher (P < 0.001) after the glycogen depletion as compared with that in the control study. During the post-depletion insulin clamp, the glucose oxidation rate was 17% lower (P < 0.02) and lipid oxidation 169% higher (P < 0.01), while the whole body total glucose disposal was similar in both studies. Baseline forearm blood flow was similar and increased equally by over 40% during both insulin clamp studies (P < 0.05). Basal glucose extraction after glycogen depletion study was one third of that in the control study (P < 0.05). Both basal and insulin stimulated leg muscle glycogen content correlated inversely with basal forearm blood flow (r = -0.69, P < 0.01 and r = -0.82, P < 0.001, respectively) and basal lipid oxidation (r = -0.54, P < 0.05 and r = -0.64, P < 0.01, respectively) after glycogen depletion. Basal glycogen synthase fractional activity correlated positively with forearm blood flow (r = 0.78, P < 0.001) and forearm glucose uptake (r = 0.71, P < 0.05) during the insulin infusion.

IN CONCLUSION

1) the unchanged insulin sensitivity in the face of glycogen depletion is probably a result of increased lipid oxidation, and 2) blood flow is related inversely to muscle glycogen content and directly to glycogen synthase activity.

Effects of insulin and the combination of insulin plus metformin (glucophage) on microvascular reactivity in control and diabetic hamsters

The purpose of this study was to determine the in vivo microvascular reactivity of arterioles (mean internal diameter range: 16.0 to 106.4 microm) and venules (mean internal diameter range: 24.0 to 117.3 microm) in the hamster cheek pouch to insulin and to the mixture insulin + metformin. Experiments were performed using an intravital microscope coupled to a closed-circuit TV system and a videotape. The TV monitor display was used to obtain arteriolar and venular internal diameter measurements by an image-shearing device. The studied drugs were applied topically, added to the superfusion solution, to avoid systemic effects that would complicate the analysis of the results. In control animals (glycemia 7.7 +/- 0.4 mmol/L), application of insulin (10 to 500 microU/mL/min) evoked vasodilatation in a dose-dependent fashion in arterioles (4.9 +/- 3.2% to 50.9 +/- 6.5%, smallest and largest concentration, respectively, values expressed in percent of the initial diameter as mean +/- SE) and venules (-2.1 +/- 3.1% to 14.3 +/- 5.1%), decreased and finally abolished the spontaneous vasomotion frequency (from 9.5 +/- 0.3 cycles per minute [cpm] to 0.0 +/- 0.0 cpm) and amplitude (from 8.6 +/- 0.3 to 0.0 +/- 0.0 microm). Addition of metformin, 0.2 mg/mL/min, did not significantly change either the observed vasodilatation in arterioles and venules or the vasomotion frequency and amplitude curves. Two types of diabetic hamsters were studied: severely diabetic, induced with three intraperitoneal injections of streptozotocin, diluted in physiological saline, 50 mg/kg/dose, given in three consecutive days, and mildly diabetic, induced by a single dose of streptozotocin. All diabetic animals were studied four weeks after the onset of diabetes and no specific treatment for diabetes was given. In severely diabetic hamsters (glycemia 18.0 +/- 2.2 mmol/L), application of insulin, in the same concentration range, evoked a significantly reduced vasodilatation in arterioles as compared with control animals (5.9 +/- 1.3% to 18.9 +/- 3.5%) and did not change the vasodilatation observed in the venules (5.9 +/- 1.4% to 21.3 +/- 2.5%). In these preparations no spontaneous arteriolar vasomotion could be detected. Addition of metformin did not significantly improve the impaired vasodilatation. In mildly diabetic hamsters (glycemia 12.1 +/- 0.8 mmol/L), application of insulin, in the same concentration range, evoked vasodilatation, in a dose-dependent fashion, equivalent to the one observed in control animals, in arterioles (3.1 +/- 2.5% to 53.4 +/- 10.0%) and venules (7.1 +/- 3.0% to 29.9 +/- 4.8%) and also reduced the vasomotion frequency (from 10.1 +/- 0.3 to 0.1 +/- 0.1 cpm) and amplitude (from 9.2 +/- 0.6 to 0.2 +/- 0.2 microm). Addition of metformin tended to increase the observed arteriolar dilatation (6.6 +/- 3.0% to 67.8 +/- 5.5%), did not change the venular dilatation (6.7 +/- 4.8% to 28.0 +/- 3.3%), and tended to preserve vasomotion frequency and amplitude. These experiments show that (1) insulin has a direct dilatatory effect on arterioles and venules; (2) the vasodilatation evoked by insulin is impaired in severe diabetes, and (3) no significant abnormality could be detected on microvascular reactivity in mild diabetes. Further addition of metformin helped to maintain the spontaneous arteriolar vasomotion even during moderate vasodilatation and tended to augment the arteriolar dilatation evoked by insulin in mildly diabetic animals.

Glucose effectiveness assessed under dynamic and steady state conditions. Comparability of uptake versus production components

Glucose tolerance is determined by both insulin action and insulin-independent effects, or "glucose effectiveness," which includes glucose-mediated stimulation of glucose uptake (Rd) and suppression of hepatic glucose output (HGO). Despite its importance to tolerance, controversy surrounds accurate assessment of glucose effectiveness. Furthermore, the relative contributions of glucose's actions on Rd and HGO under steady state and dynamic conditions are unclear. We performed hyperglycemic clamps and intravenous glucose tolerance tests in eight normal dogs, and assessed glucose effectiveness by two independent methods. During clamps, glucose was raised to three successive 90-min hyperglycemic plateaus by variable labeled glucose infusion rate; glucose effectiveness (GE) was quantified as the slope of the dose-response relationship between steady state glucose and glucose infusion rate (GE[CLAMP(total)]), Rd (GE[CLAMP(uptake)]) or HGO (GE[CLAMP(HGO)]). During intravenous glucose tolerance tests, tritiated glucose (1.2 microCi/kg) was injected with cold glucose (0.3 g/kg); glucose and tracer dynamics were analyzed using a two-compartment model of glucose kinetics to obtain Rd and HGO components of glucose effectiveness. All experiments were performed during somatostatin inhibition of islet secretion, and basal insulin and glucagon replacement. During clamps, Rd rose from basal (2.54+/-0.20) to 3.95+/-0.54, 6.76+/-1.21, and 9.48+/-1.27 mg/min per kg during stepwise hyperglycemia; conversely, HGO declined to 2.06+/-0.17, 1.17+/-0.19, and 0.52+/-0.33 mg/min per kg. Clamp-based glucose effectiveness was 0.0451+/-0.0061, 0.0337+/-0.0060, and 0.0102+/-0.0009 dl/min per kg for GE[CLAMP(total)], GE[CLAMP(uptake)], and GE[CLAMP(HGO)], respectively. Glucose's action on Rd dominated overall glucose effectiveness (72.2+/-3.3% of total), a result virtually identical to that obtained during intravenous glucose tolerance tests (71.6+/-6.1% of total). Both methods yielded similar estimates of glucose effectiveness. These results provide strong support that glucose effectiveness can be reliably estimated, and that glucose-stimulated Rd is the dominant component during both steady state and dynamic conditions.

Influence of a diet regimen on glucose homeostasis and serum lipid levels of male elite athletes

Physical training affects carbohydrate metabolism and results in an increased insulin-stimulated glucose disposal. To investigate if carbohydrate and lipid metabolism would be affected by nutritional factors in optimally trained elite athletes, during a 1-year period we studies elite ice-hockey players on two Swedish top-performance teams. Players on one team were subjected to extensive dietary monitoring and intervention, whereas players on the second team continued their ordinary diet. Blood levels of insulin, C-peptide, glucose, hemoglobin A1C (HbA1c), lipids, and lipoproteins were measured repeatedly. Basal insulin levels and insulin resistance (IR) were significantly lower among athletes on both teams compared with a sedentary group, and muscle weight and body mass index were significantly higher. During the course of the study in the intervention group, insulin levels decreased (3.6 +/- 0.3 v 6.2 +/- 0.6 [mean +/- SEM], P <.05) in conjunction with a decreased relative fat energy content, but returned toward baseline levels when relative fat energy content increased. IR decreased in parallel (0.59 +/- 0.05 v l.12 +/- 0.12, P <.05) and followed a similar pattern, reverting toward baseline levels. Also, levels of HbA1c changed during dietary manipulation. No changes in these parameters were observed among the elite players from the team not participating in the diet regimen. In contrast to the parameters for glucose homeostasis, no significant changes were found in serum lipid or lipoprotein levels in either team during the course of the study. The results verify the presence of an improved carbohydrate metabolism in elite athletes. The observed changes in glycemic control and glucose homeostasis as a consequence of dietary modification demonstrate further that nutritional factors may affect carbohydrate metabolism also in well-trained athletes.

Basal plasma insulin levels exert a qualitative but not quantitative effect on glucose-mediated glucose uptake

We assessed the effect of hyperglycemia on glucose uptake in the presence of normal basal insulin levels or somatostatin-induced hypoinsulinemia in seven normal volunteers during a 200-min hyperglycemic clamp (+ 9 mmol/l) carried out with [3-3H]glucose and indirect calorimetry. Hyperglycemia increased glucose uptake to 22.4 +/- 2.6 and 21.3 +/- 1.6 mumol.kg-1.min-1 with and without insulin replacement, respectively. Normoinsulinemia increased glucose oxidation (delta = + 4.5 +/- 0.6 mumol.kg-1.min-1) and nonoxidative glucose metabolism (delta = + 5.2 +/- 1.7 mumol.kg-1.min-1), whereas with insulinopenia, glucose oxidation did not change (delta = -0.3 +/- 0.6 mumol.kg-1.min-1), and nonoxidative glucose metabolism increased (delta = + 48.7 +/- 0.8 mumol.kg-1.min-1). Nonoxidative glucose metabolism was higher during insulinopenic (13.5 +/- 1.8 mumol.kg-1.min-1) than normoinsulinemic hyperglycemia (9.8 +/- 2.7 mumol.kg-1.min-1; P < 0.01). Plasma FFA concentration and lipid oxidation were higher with insulinopenia. Blood lactate and alanine concentrations were greater with normoinsulinemia.

IN CONCLUSION

1) hyperglycemia promotes glucose uptake by stimulating both nonoxidative and oxidative glucose disposal; 2) the ability of hyperglycemia to enhance total body glucose uptake is similar with and without normoinsulinemia; 3) although acute insulinopenia does not impair the ability of hyperglycemia to stimulate glucose uptake, it plays a critical role in determining the intracellular metabolic fate of glucose taken up in response to hyperglycemia.

Insulinopenia and hyperglycemia influence the in vivo partitioning of GE and SI

We determined the impact of variable insulinemia and glycemia on the in vivo partitioning of glucose effectiveness (GE) and insulin sensitivity (SI) and the in vitro intracellular processing of glucose metabolism. Six somatostatin- and [3-3H]glucose-infused dogs underwent euglycemic and hyperglycemic clamps at four physiological insulin (Ins) levels before a muscle biopsy. From the rates of glucose infusion (GINF), total glucose disposal (Rd), total glycolysis (GF), and glucose storage (GS), plots of delta GINF, delta Rd, delta GS vs. delta log Ins concentration were found to be linear for each dog, allowing calculation of the partitioning of GE and SI into their major in vivo sites (periphery vs. liver) and intracellular metabolic pathways (GS vs. GF). Insulinopenia induced a significant reduction in total GE. From insulinopenia to high insulinemia, the 2.3-fold increase in total GE was due to the increased peripheral glucose responsiveness of the GS pathway. Hyperglycemia induced a significant reduction in total SI, with approximately one-half of this reduction due to the decreased peripheral insulin responsiveness of the GF pathway. In skeletal muscle, both glycogen content and glycogen synthase fractional activity were positively correlated with log Ins concentration, Rd, and GS but negatively correlated with glucose 6-phosphate concentration. Moreover, both Rd and GS were negatively correlated with lactate concentration. We conclude that 1) the inhibition of GE and SI induced by insulinopenia and hyperglycemia, respectively, is due mainly to the reduced peripheral responsiveness of contrasting intracellular metabolic pathways; and 2) hyperinsulinemia and/or hyperglycemia stimulates glycogen synthesis and GF but not nonoxidative glycolysis.

Effects of glycaemia on glucose transport in isolated skeletal muscle from patients with NIDDM: in vitro reversal of muscular insulin resistance

We investigated the influence of altered glucose levels on insulin-stimulated 3-0-methylglucose transport in isolated skeletal muscle obtained from NIDDM patients (n = 13) and non-diabetic subjects (n = 23). Whole body insulin sensitivity was 71% lower in the NIDDM patients compared to the non-diabetic subjects (p < 0.05), whereas, insulin-mediated peripheral glucose utilization in the NIDDM patients under hyperglycaemic conditions was comparable to that of the non-diabetic subjects at euglycaemia. Following a 30-min in vitro exposure to 4 mmol/l glucose, insulin-stimulated 3-0-methylglucose transport (600 pmol/l insulin) was 40% lower in isolated skeletal muscle strips from the NIDDM patients when compared to muscle strips from the non-diabetic subjects. The impaired capacity for insulin-stimulated 3-0-methylglucose transport in the NIDDM skeletal muscle was normalized following prolonged (2h) exposure to 4 mmol/l, but not to 8 mmol/l glucose. Insulin-stimulated 3-0-methylglucose transport in the NIDDM skeletal muscle exposed to 8 mmol/l glucose was similar to that of the non-diabetic muscle exposed to 5 mmol/l glucose, but was decreased by 43% (p < 0.01) when compared to non-diabetic muscle exposed to 8 mmol/l glucose. Despite the impaired insulin-stimulated 3-0-methylglucose transport capacity demonstrated by skeletal muscle from the NIDDM patients, skeletal muscle glycogen content was similar to that of the non-diabetic subjects. Kinetic studies revel a Km for 3-0-methylglucose transport of 9.7 and 8.8 mmol/l glucose for basal and insulin-stimulated conditions, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of enhanced insulin sensitivity in athletes. Increased blood flow, muscle glucose transport protein (GLUT-4) concentration, and glycogen synthase activity

We examined the mechanisms of enhanced insulin sensitivity in 9 male healthy athletes (age, 25 +/- 1 yr; maximal aerobic power [VO2max], 57.6 +/- 1.0 ml/kg per min) as compared with 10 sedentary control subjects (age, 28 +/- 2 yr; VO2max, 44.1 +/- 2.3 ml/kg per min). In the athletes, whole body glucose disposal (240-min insulin clamp) was 32% (P < 0.01) and nonoxidative glucose disposal (indirect calorimetry) was 62% higher (P < 0.01) than in the controls. Muscle glycogen content increased by 39% in the athletes (P < 0.05) but did not change in the controls during insulin clamp. VO2max correlated with whole body (r = 0.60, P < 0.01) and nonoxidative glucose disposal (r = 0.64, P < 0.001). In the athletes forearm blood flow was 64% greater (P < 0.05) than in the controls, whereas their muscle capillary density was normal. Basal blood flow was related to VO2max (r = 0.63, P < 0.05) and glucose disposal during insulin infusion (r = 0.65, P < 0.05). The forearm glucose uptake in the athletes was increased by 3.3-fold (P < 0.01) in the basal state and by 73% (P < 0.05) during insulin infusion. Muscle glucose transport protein (GLUT-4) concentration was 93% greater in the athletes than controls (P < 0.01) and it was related to VO2max (r = 0.61, P < 0.01) and to whole body glucose disposal (r = 0.60, P < 0.01). Muscle glycogen synthase activity was 33% greater in the athletes than in the controls (P < 0.05), and the basal glycogen synthase fractional activity was closely related to blood flow (r = 0.88, P < 0.001).

IN CONCLUSION

(a) athletes are characterized by enhanced muscle blood flow and glucose uptake. (b) The cellular mechanisms of glucose uptake are increased GLUT-4 protein content, glycogen synthase activity, and glucose storage as glycogen. (c) A close correlation between glycogen synthase fractional activity and blood flow suggests that they are causally related in promoting glucose disposal.

Insulin resistance improves in hyperandrogenic women treated with Lupron

OBJECTIVE

To examine if changes in insulin sensitivity and glucose effectiveness in women with polycystic ovarian disease (PCOD) occurred after ovarian androgen suppression with a GnRH agonist, leuprolide acetate (LA, Lupron; TAP Pharmaceuticals, Deerfield, IL) using the minimal model method.

DESIGN

Twelve patients with PCOD were tested in the untreated state (baseline) and after 6 weeks of LA treatment. Subjects were divided into two groups based on the degree of impairment of their baseline insulin sensitivity index (SI; (min-1) (microU/mL-1): mild insulin resistance (SI > 1) or severe insulin resistance (SI < 1).

RESULTS

In all patients, serum T was significantly decreased from elevated baseline levels to normal female concentrations after 6 weeks of LA therapy. Insulin sensitivity in PCOD patients with mild insulin resistance significantly improved from baseline after 6 weeks of LA therapy, whereas no change in SI on LA therapy was seen in PCOD women with severe insulin resistance. Glucose utilization independent of increased insulin secretion did not change as a function of LA treatment in either group.

CONCLUSION

These findings indicate a significant improvement in SI in mildly insulin-resistant women with PCOD after suppression of ovarian function with LA treatment.

Differential effects of hyperinsulinemia and carbohydrate metabolism on sympathetic nerve activity and muscle blood flow in humans

Euglycemic hyperinsulinemia evokes both sympathetic activation and vasodilation in skeletal muscle, but the mechanism remains unknown. To determine whether insulin per se or insulin-induced stimulation of carbohydrate metabolism is the main excitatory stimulus, we performed, in six healthy lean subjects, simultaneous microneurographic recordings of muscle sympathetic nerve activity, plethysmographic measurements of calf blood flow, and calorimetric determinations of carbohydrate oxidation rate. Measurements were made during 2 h of: (a) insulin/glucose infusion (hyperinsulinemic [6 pmol/kg per min] euglycemic clamp), (b) exogenous glucose infusion at a rate matched to that attained during protocol a, and (c) exogenous fructose infusion at the same rate as for glucose infusion in protocol b. For a comparable rise in carbohydrate oxidation, insulin/glucose infusion that resulted in twofold greater increases in plasma insulin concentrations than did glucose infusion alone, evoked twofold greater increases in both muscle sympathetic nerve activity and calf blood flow. Fructose infusion, which increased carbohydrate oxidation comparably, but had only a minor effect on insulinemia, did not stimulate either muscle sympathetic nerve activity or calf blood flow. These observations suggest that in humans hyperinsulinemia per se, rather than insulin-induced stimulation of carbohydrate metabolism, is the main mechanism that triggers both sympathetic activation and vasodilation in skeletal muscle.

Characterization of cellular defects of insulin action in type 2 (non-insulin-dependent) diabetes mellitus

Seven non-insulin-dependent diabetes mellitus (NIDDM) patients participated in three clamp studies performed with [3-3H]- and [U-14C]glucose and indirect calorimetry: study I, euglycemic (5.2 +/- 0.1 mM) insulin (269 +/- 39 pM) clamp; study II, hyperglycemic (14.9 +/- 1.2 mM) insulin (259 +/- 19 pM) clamp; study III, euglycemic (5.5 +/- 0.3 mM) hyperinsulinemic (1650 +/- 529 pM) clamp. Seven control subjects received a euglycemic (5.1 +/- 0.2 mM) insulin (258 +/- 24 pM) clamp. Glycolysis and glucose oxidation were quantitated from the rate of appearance of 3H2O and 14CO2; glycogen synthesis was calculated as the difference between body glucose disposal and glycolysis. In study I, glucose uptake was decreased by 54% in NIDDM vs. controls. Glycolysis, glycogen synthesis, and glucose oxidation were reduced in NIDDM patients (P < 0.05-0.001). Nonoxidative glycolysis and lipid oxidation were higher. In studies II and III, glucose uptake in NIDDM was equal to controls (40.7 +/- 2.1 and 40.7 +/- 1.7 mumol/min.kg fat-free mass, respectively). In study II, glycolysis, but not glucose oxidation, was normal (P < 0.01 vs. controls). Nonoxidative glycolysis remained higher (P < 0.05). Glycogen deposition increased (P < 0.05 vs. study I), and lipid oxidation remained higher (P < 0.01). In study III, hyperinsulinemia normalized glycogen formation, glycolysis, and lipid oxidation but did not normalize the elevated nonoxidative glycolysis or the decreased glucose oxidation. Lipid oxidation and glycolysis (r = -0.65; P < 0.01), and glucose oxidation (r = -0.75; P < 0.01) were inversely correlated. In conclusion, in NIDDM: (a) insulin resistance involves glycolysis, glycogen synthesis, and glucose oxidation; (b) hyperglycemia and hyperinsulinemia can normalize total body glucose uptake; (c) marked hyperinsulinemia normalizes glycogen synthesis and total flux through glycolysis, but does not restore a normal distribution between oxidation and nonoxidative glycolysis; (d) hyperglycemia cannot overcome the defects in glucose oxidation and nonoxidative glycolysis; (e) lipid oxidation is elevated and is suppressed only with hyperinsulinemia.

Acute and chronic signals controlling glucose transport in skeletal muscle

Glucose transport into muscle cells occurs through facilitated diffusion mediated primarily by the GLUT1 and GLUT4 glucose transporters. These transporter proteins are controlled by acute and chronic exposure to insulin, glucose, muscle contraction, and hypoxia. We propose that acute responses occur through recruitment of pre-formed glucose transporters from an intracellular storage site to the plasma membrane. In contrast, chronic control is achieved by changes in transporter biosynthesis and protein stability. Using subcellular fractionation of rat skeletal muscle, recruitment of GLUT4 glucose transporters to the plasma membrane is demonstrated by acute exposure to insulin in vivo. The intracellular pool appears to arise from a unique organelle depleted of transverse tubule, plasma membrane, or sarcoplasmic reticulum markers. In diabetic rats, GLUT4 content in the plasma membranes and in the intracellular pool is reduced, and incomplete insulin-dependent GLUT4 recruitment is observed, possibly through a defective incorporation of transporters to the plasma membrane. The lower content of GLUT4 transporters in the muscle plasma membranes is reversed by restoration of normoglycemia with phlorizin treatment. In some muscle cells in culture, GLUT1 is the only transporter expressed yet they respond to insulin, suggesting that this transporter can also be regulated by acute mechanisms. In the L6 muscle cell line, GLUT1 transporter content diminishes during myogenesis and GLUT4 appears after cell fusion, reaching a molar ratio of about 1:1 in the plasma membrane. Prolonged exposure to high glucose diminishes the amount of GLUT1 protein in the plasma membrane by both endocytosis and reduced biosynthesis, and lowers GLUT4 protein content in the absence of changes in GLUT4 mRNA possibly through increased protein degradation. These studies suggest that the relative contribution of each transporter to transport activity, and the mechanisms by which glucose exerts control of the glucose transporters, will be key subjects of future investigations.

Effect of glycemia and nonesterified fatty acids on forearm glucose uptake in normal humans

The purpose of this study was to examine the effect of physiological plasma nonesterified fatty acid (NEFA) levels on insulin-stimulated forearm and whole body glucose uptake and substrate oxidation during euglycemia and hyperglycemia. Seven healthy men received Intralipid and heparin for 210 min in two studies, with saline as control in two further studies. Insulin (0.05 U.kg-1.h-1) was infused from 60 min, and euglycemia was maintained during lipid (EL) and control (EC) studies, and hyperglycemia was maintained in the other studies (HL and HC). Forearm NEFA uptake was comparable in the lipid studies (+61 +/- 10 and +52 +/- 8 nmol.100 ml forearm-1.min-1, EL and HL) and was suppressed in the controls. With Intralipid, forearm glucose uptake decreased during euglycemia but not during hyperglycemia (+3.85 +/- 0.34 vs. +3.34 +/- 0.25 mumol.100 ml forearm-1.min-1, EC vs. EL, P less than 0.02), with comparable changes in whole body glucose uptake. Glucose oxidation and forearm alanine release decreased with Intralipid at both blood glucose levels, with no significant change in the rates of nonoxidative glucose disposal. These observations support the operation of the glucose-fatty acid cycle at physiological plasma NEFA levels at both blood glucose concentrations, but this was associated with a decrease in peripheral insulin sensitivity only during euglycemia.

Hormonal and local factors in hypertension

The cause of primary (essential) hypertension remains unknown, but a number of circulating hormones and endothelium-derived factors are probably involved. This review summarizes recent evidence on the roles of hyperinsulinemia, the renin-angiotensin system, atrial natriuretic factor, and three endothelium-derived factors--prostacyclin, endothelium-derived relaxing factor, and endothelin.

Reduced capacity and affinity of skeletal muscle for insulin-mediated glucose uptake in noninsulin-dependent diabetic subjects. Effects of insulin therapy

We have estimated the capacity and affinity of insulin-mediated glucose uptake (IMGU) in whole body and in leg muscle of obese non-insulin-dependent diabetics (NIDDM, n = 6) with severe hyperglycemia, glycohemoglobin (GHb 14.4 +/- 1.2%), lean controls (ln, n = 7) and obese nondiabetic controls (ob, n = 7). Mean +/- SEM weight (kg) was 67 +/- 2 (ln), 100 +/- 7 (ob), and 114 +/- 11 (NIDDM), P = NS between obese groups. NIDDM were also studied after 3 wk of intensive insulin therapy, GHb post therapy was 10.1 +/- 0.9, P less than 0.01 vs. pretherapy. Insulin (120 mu/m2 per min) was infused and the arterial blood glucose (G) sequentially maintained at approximately 4, 7, 12, and 21 mmol/liter utilizing the G clamp technique. Leg glucose uptake (LGU) was calculated as the product of the femoral arteriovenous glucose difference (FAVGd) and leg blood flow measured by thermodilution. Compared to ln, ob and NIDDM had significantly lower rates of whole body IMGU and LGU at all G levels. Compared to ob, the NIDDM exhibited approximately 50% and approximately 40% lower rates of whole body IMGU over the first two G levels (P less than 0.02) but did not differ at the highest G, P = NS. LGU was 83% lower in NIDDM vs. ob, P less than 0.05 at the first G level only. After insulin therapy NIDDM were indistinguishable from ob with respect to whole body IMGU or LGU at all G levels. A significant correlation was noted between the percent GHb and the EG50 (G at which 1/2 maximal FAVGd occurs) r = 0.73, P less than 0.05. Thus, (a) insulin resistance in NIDDM and obese subjects are characterized by similar decreases in capacity for skeletal muscle IMGU, but differs in that poorly controlled NIDDM display a decrease in affinity for skeletal muscle IMGU, and (b) this affinity defect is related to the degree of antecedent glycemic control and is reversible with insulin therapy, suggesting that it is an acquired defect.