Mechanisms of fatty acid-induced inhibition of glucose uptake

Free fatty acids, insulin resistance, and type 2 diabetes mellitus

Evidence is presented that shows that free fatty acids (FFA) are one important link between obesity, insulin resistance, and type 2 diabetes. Plasma FFA levels are elevated in most obese subjects, and physiological elevations of plasma FFA inhibit insulin-stimulated glucose uptake into muscle. This peripheral insulin resistance is caused by an FFA-induced defect, which develops 3-4 hr after raising plasma FFA, in insulin-stimulated glucose transport or phosphorylation, or both. This resistance is also caused by a second defect, which develops after 4-6 hr, consisting of inhibition of glycogen synthase activity. Whether elevated plasma FFA levels inhibit insulin action on endogenous glucose production (EGP), that is, cause central insulin resistance, is more difficult to demonstrate. On the one hand, FFA increase gluconeogenesis, which enhances EGP; on the other hand, FFA increase insulin secretion, which decreases EGP. Basal plasma FFA support approximately one third of basal insulin secretion in diabetic and nondiabetic subjects and, hence, are responsible for some of the hyperinsulinemia in obese, normoglycemic patients. In addition, elevated plasma FFA levels potentiate glucose-stimulated insulin secretion acutely and during prolonged exposure (48 hr). It is hypothesized that obese subjects who are genetically predisposed to develop type 2 diabetes will become partially "lipid blind," that is, unable to compensate for their FFA-induced insulin resistance with FFA-induced insulin oversecretion. The resulting insulin resistance/secretion deficit will then have to be compensated for with glucose-induced insulin secretion, which, because of their partial "glucose blindness," will result in hyperglycemia and eventually in type 2 diabetes.

High prevalence of impaired fasting glucose and type 2 diabetes mellitus in Penghu Islets, Taiwan: evidence of a rapidly emerging epidemic?

The purpose of this study was to estimate the prevalence of type 2 diabetes and impaired fasting glucose (IFG) in Penghu, Taiwan and compare these estimates with those of the US (NHANES III). Diabetes and IFG (American Diabetes Association criteria, 1997) were assessed among a stratified random sample of 2500 residents of Penghu Islands, Taiwan. The prevalence (age-adjusted to world adult population) of diabetes and IFG were 16.8% (95% CI 15.0-18.6) and 21.0% (95% CI 19.0-23.0), respectively, among Penghu Islanders in Taiwan. Age sex-specific diabetes prevalence ranged from 10.0% in men aged 40-49 years to 29.4% in women aged 60-69 years. Prevalence of IFG ranged from 14.7% in women aged 40-49 years to 30.7% in men aged 50-59 years. Age, body mass index (BMI), and family history of diabetes were each independently associated with both diabetes and IFG. In addition, female gender, apolipoprotein B and triglyceride concentrations were associated with diabetes, and hypertension and apolipoprotein B concentration with IFG. Among persons > or = 40 years in Penghu, Taiwan, the prevalence of diabetes is up to a third higher and the prevalence of IFG is up to three times higher than comparably aged Americans, despite their having a mean BMI 2.2-3.2 kg/m2 lower than Americans. The alarmingly high prevalence of IFG in Taiwan may indicate an emerging diabetes epidemic.

Applications of NMR spectroscopy to study muscle glycogen metabolism in man

Prior to the advent of nuclear magnetic resonance (NMR) spectroscopy, human glucose metabolism was studied through tracer and tissue biopsy methodology. NMR spectroscopy now provides a noninvasive means to monitor metabolic flux and intracellular metabolite concentrations continuously. 13C NMR spectroscopy has shown that muscle glycogen synthesis accounts for the majority of insulin-stimulated muscle glucose uptake in normal volunteers and that defects in this process are chiefly responsible for insulin resistance in type 1 and type 2 diabetes mellitus, as well as in other insulin resistant states (obesity, insulin-resistant offspring of type 2 diabetic parents, elevation of plasma FFA concentrations). Furthermore, using 31P NMR spectroscopy to measure intracellular glucose-6-phosphate, it has been shown that defects in insulin-stimulated glucose transport/phosphorylation activity are primarily responsible for the insulin resistance in these states.

Whole body glucose metabolism

This review describes major factors that, singly or together, influence the concentration and distribution of D-glucose in mammals, particularly in humans, with emphasis on rest, physical activity, and alimentation. It identifies areas of uncertainty: distribution and concentrations of glucose in interstitial fluid, kinetics and mechanism of transcapillary glucose transport, kinetics and mechanism of glucose transport via its transporters into cells, detailed mechanisms by which hormones, exercise, and hypoxia affect glucose movement across cell membranes, whether translocation of glucose transporters to the cell membrane accounts completely, or even mainly, for insulin-stimulated glucose uptake, whether exercise stimulates release of a circulating insulinomimetic factor, and the relation between muscle glucose uptake and muscle blood flow. The review points out that there is no compartment of glucose in the body at which all glucose is at the same concentration, and that models of glucose metabolism, including effects of insulin on glucose metabolism based on assumptions of concentration homogeneity, cannot be entirely correct. A fresh approach to modeling is needed.

Effect of hyperglycemia-hyperinsulinemia on whole body and regional fatty acid metabolism

The effects of combined hyperglycemia-hyperinsulinemia on whole body, splanchnic, and leg fatty acid metabolism were determined in five volunteers. Catheters were placed in a femoral artery and vein and a hepatic vein. U-13C-labeled fatty acids were infused, once in the basal state and, on a different occasion, during infusion of dextrose (clamp; arterial glucose 8.8 +/- 0.5 mmol/l). Lipids and heparin were infused together with the dextrose to maintain plasma fatty acid concentrations at basal levels. Fatty acid availability in plasma and fatty acid uptake across the splanchnic region and the leg were similar during the basal and clamp experiments. Dextrose infusion decreased fatty acid oxidation by 51.8% (whole body), 47.4% (splanchnic), and 64.3% (leg). Similarly, the percent fatty acid uptake oxidized decreased at the whole body level (53 to 29%), across the splanchnic region (30 to 13%), and in the leg (48 to 22%) during the clamp. We conclude that, in healthy men, combined hyperglycemia-hyperinsulinemia inhibits fatty acid oxidation to a similar extent at the whole body level, across the leg, and across the splanchnic region, even when fatty acid availability is constant.

Insulin resistance, impaired glucose tolerance and non-insulin-dependent diabetes, pathologic mechanisms and treatment: current status and therapeutic possibilities

Impaired glucose tolerance and non-insulin-dependent diabetes (NIDDM) are the pathologic consequence of two co-incident and interacting conditions, namely insulin resistance and relative insulin deficiency. Recognised by the World Health Authority as a global health problem there are at 1995 estimates at least 110 million diagnosed diabetics world wide with at least the same number undiagnosed. Diabetes is the 4th leading cause of death in developed countries and its management exerts a vast economic and social burden. Insulin resistance is established as the characteristic pathologic feature of patients with glucose intolerance and NIDDM describing a state in which insulin stimulated glucose uptake and utilisation in liver, skeletal muscle and adipose tissue is impaired and coupled to impaired suppression of hepatic glucose output. Although the biochemical mechanisms underpinning both defects are becoming better understood, the genetic and molecular causes remain elusive; and whether insulin resistance or relative insulin deficiency represents the primary defect in patients with NIDDM is the matter of some debate. In this article we review the biochemical and molecular nature of the defects in insulin sensitivity and glucose uptake, and discuss some of the potential causative mechanisms. The genetic and environmental basis of insulin resistance is reviewed and presented, and potential therapeutic targets including thiazolidinediones are discussed.

Increased dependence of glucose production on peripheral insulin in diabetic depancreatized dogs

We have recently found that in nondiabetic dogs and humans, suppression of glucose production (GP) is mediated by both peripheral and hepatic effects of insulin. We have also found that both nonesterified fatty acids (NEFA) and glucagon are important determinants of the peripheral effect of insulin on GP. However, in moderately hyperglycemic depancreatized dogs, suppression of GP appeared to be mediated by peripheral but not hepatic insulin. In this latter study, insulin concentrations were in the high postprandial range (approximately 300 pmol/L) and suppression of GP may have been close to maximum. The aim of the present study was to determine whether GP can be regulated by hepatic insulin in depancreatized dogs at low insulin concentrations in the postabsorptive range. Depancreatized dogs were maintained at moderately hyperglycemic levels (approximately 10 mmol/L) by subbasal insulin infusions. In paired experiments, additional low-dose equimolar insulin infusions (0.75 pmol/kg x min) were administered peripherally (PER, n = 6) or portally (POR, n = 6) during glucose clamps. This resulted in a minimal increase in peripheral insulin levels, which was greater in PER versus POR, 29.0 +/- 3.7 versus 11.7 +/- 2.2 pmol/L. Also, we infused insulin peripherally at half this rate (1/2 PER, n = 6) to match the increase in peripheral insulin levels in POR (1/2 PER, 14.6 +/- 2.2) and thus obtain a selective POR versus 1/2 PER difference in hepatic sinusoidal insulin levels. PER suppressed GP more than POR (45.4% +/- 4.0% v 35.3% +/- 6.8%, P < .001), whereas POR did not suppress GP more than 1/2 PER (35.6% +/- 6.3%). Therefore, suppression of GP was proportional to peripheral rather than hepatic sinusoidal insulin levels, as in our previous study at higher insulin concentrations. In conclusion, during glucose clamps in moderately hyperglycemic depancreatized dogs, (1) suppression of GP was dominated by insulin's peripheral effects not only at postprandial but also postabsorptive insulin levels, and (2) we found no evidence for a hepatic effect of insulin in suppressing GP. We hypothesize that this effect is reduced in the depancreatized dog model of diabetes due to hepatic insulin resistance and/or hyperglycemia.

Codon 54 polymorphism of the fatty acid binding protein gene and insulin resistance in the Japanese population

AIM

To determine the relationship of the polymorphism at codon 54 of the intestinal fatty acid binding protein gene (FABP2) with insulin resistance and susceptibility to Type 2 diabetes mellitus (DM) in the Japanese population.

METHODS

We evaluated the polymorphism by the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in 150 Type 2 DM patients and 147 healthy control subjects. The frequency of alleles encoding threonine (Thr54) and alanine (Ala54) at codon 54 of FABP2 in Type 2 DM patients was compared with that of healthy controls. Insulin sensitivity was assessed by the hyperinsulinaemic euglycaemic clamp in Type 2 DM patients with Ala54 homozygotes, Ala54/Thr54 heterozygotes and Thr54 homozygotes and by homeostasis model assessment (HOMA) in the nondiabetic group.

RESULTS

The frequency of alleles encoding Ala54 and Thr54 was 0.59 and 0.41 in Type 2 DM patients, respectively, similar to that observed in nondiabetic controls (0.64 for Ala54 and 0.36 for Thr54). Insulin sensitivity was not significantly different between subjects with and without Thr54 allele either within the DM group or healthy controls.

CONCLUSIONS

The allele encoding threonine in the FABP2 does not predispose to Type 2 DM or insulin resistance in the Japanese population.

Effects of free fatty acids on glucose transport and IRS-1-associated phosphatidylinositol 3-kinase activity

To examine the mechanism by which free fatty acids (FFA) induce insulin resistance in human skeletal muscle, glycogen, glucose-6-phosphate, and intracellular glucose concentrations were measured using carbon-13 and phosphorous-31 nuclear magnetic resonance spectroscopy in seven healthy subjects before and after a hyperinsulinemic-euglycemic clamp following a five-hour infusion of either lipid/heparin or glycerol/heparin. IRS-1-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity was also measured in muscle biopsy samples obtained from seven additional subjects before and after an identical protocol. Rates of insulin stimulated whole-body glucose uptake. Glucose oxidation and muscle glycogen synthesis were 50%-60% lower following the lipid infusion compared with the glycerol infusion and were associated with a approximately 90% decrease in the increment in intramuscular glucose-6-phosphate concentration, implying diminished glucose transport or phosphorylation activity. To distinguish between these two possibilities, intracellular glucose concentration was measured and found to be significantly lower in the lipid infusion studies, implying that glucose transport is the rate-controlling step. Insulin stimulation, during the glycerol infusion, resulted in a fourfold increase in PI 3-kinase activity over basal that was abolished during the lipid infusion. Taken together, these data suggest that increased concentrations of plasma FFA induce insulin resistance in humans through inhibition of glucose transport activity; this may be a consequence of decreased IRS-1-associated PI 3-kinase activity.

Current views on the mechanism of action of thiazolidinedione insulin sensitizers

Resistance to the action of insulin in its target tissues in a major predisposing factor for the development of type 2 diabetes and is also tightly associated with a common pattern of cardiovascular risk factors that characterize the "insulin resistance syndrome." The thiazolidinediones are a new class of drugs that act as insulin sensitizers with well-documented-efficacy in the control of hyperglycemia in patients with overt diabetes. A growing body of evidence also suggests that thiazolidinediones may preserve beta-cell function and protect cardiovascular and renal function in patients with type 2 diabetes. This review will summarize our current notions of the mechanism of action of thiazolidinediones, which appears to involve a fascinating interplay between the partitioning of triglyceride stores, circulating free fatty acids and insulin signaling pathways. A detailed understanding of the action of thiazolidinediones will provide new insights into the pathogenesis of insulin resistance, diabetes and some of the causes of increased cardiovascular mortality in these conditions.

Non-esterified fatty acid metabolism and postprandial lipaemia

Non-esterified fatty acids (NEFA, or free fatty acids) are an important metabolic fuel. Both the concentration of NEFA and their flux through the circulation vary widely from hour to hour, reflecting nutritional state and physical activity. Inappropriately elevated plasma NEFA concentrations may have a number of adverse effects on both carbohydrate and lipid metabolism. These adverse effects are likely to be most marked in the postprandial period, when NEFA release from adipose tissue is usually suppressed. Although the regulation of NEFA release in the postabsorptive state is well understood in molecular terms, the predominant pathway for release of NEFA in the postprandial state is the action of lipoprotein lipase (LPL) in adipose tissue capillaries on chylomicron-triacylglycerol (TG). Fatty acids released by LPL may either be sequestered in the adipocytes by esterification, or released as NEFA into the plasma. The regulation of this branch-point, which may be of crucial significance for postprandial metabolism, is not well understood. Factors stimulating tissue retention of fatty acids include insulin and acylation stimulating protein. There is considerable indirect evidence that impaired regulation of this step occurs in insulin resistance and other conditions collectively recognised by an elevated concentration of apolipoprotein B (hyper-apo B). Inappropriate release of NEFA in the postprandial period is likely both to reduce the sensitivity of glucose metabolism to insulin and to accentuate postprandial lipaemia. Further study of the regulation of this pathway is much needed.

Complementary measures for promoting insulin sensitivity in skeletal muscle

Insulin resistance of skeletal muscle is fundamental to both syndrome X and its frequent sequel, type II diabetes. In these disorders, excessive exposure of muscle to free fatty acids (FFAs) and their metabolic derivatives appears to play a prominent role in the induction of insulin resistance. Recent evidence suggests that activation of novel isoforms of protein kinase C (PKC) by diacylglycerol may mediate at least part of the adverse impact of FFAs on muscle insulin sensitivity. Vitamin E and fish oil omega-3s, by promoting the activity of diacylglycerol kinase and inhibiting that of phosphatidate phosphohydrolase, should reduce diacylglycerol levels, thus accounting for their documented favorable impact on insulin sensitivity. Thiazolidinediones such as troglitazone, on the other hand, appear to intervene in the signaling pathway whereby PKC down-regulates insulin function. The insulin-sensitizing activity of chromium picolinate may be attributable, at least in part, to increased expression of insulin receptors. In combination with lifestyle modifications which reduce FFA exposure--weight loss, very-low-fat eating, excessive training--these measures can be expected to work in a complementary way to promote increased numbers of insulin receptors that are more functionally competent. As these measures appear to be safe and well-tolerated, they may have utility for the prevention of diabetes as well as its therapy. When they do not prove sufficient to achieve optimal glycemic control, excessive hepatic glucose output and impaired cell response to glucose can be addressed with metformin and sulfonylureas, respectively. The prospects for a rational medical management of type II diabetes, obviating the need for injectible insulin, have never been brighter.

Exaggerated stress-induced release of nonesterified fatty acids in JCR:LA-corpulent rats

Studies were performed to test the hypothesis that the stress response is exaggerated in obesity and to identify which component of the response is modified. Chronically cannulated lean (+/+) and obese (cp/cp) JCR:LA rats were subjected to mild restraint stress for 15 minutes. Blood pressure and serum glucose, insulin, and corticosterone responses did not differ significantly between genotypes before, during, or after restraint. Lean rats had a significantly greater plasma epinephrine (EPI) response but a similar norepinephrine (NE) response compared with obese rats. Serum nonesterified fatty acid (NEFA) concentrations were unchanged in lean rats, but increased from 0.86 to a mean of 1.48 mmol/L in obese rats within 10 minutes of restraint. All animals recovered to prestress values by 45 minutes postrestraint. In obese rats, handling increased NEFAs to greater than 2 mmol/L before or at 165 minutes after restraint. In lean rats, NEFAs increased when handling occurred at 165 minutes after restraint, but there was no significant NEFA response at the prerestraint point. The sensitivity of adipose tissue to NE-induced lipolysis was not significantly different between genotypes. It is concluded that the exaggerated accumulation of NEFAs in the blood of obese rats results from increased adipose tissue mass. These increases in NEFAs in obese rats resulting from mild stress reached levels normally associated with gross pathology such as ketotic diabetes.

An overview of muscle glucose uptake during exercise. Sites of regulation

The uptake of blood glucose by skeletal muscle is a complex process. In order to be metabolized, glucose must travel the path from blood to interstitium to intracellular space and then be phosphorylated to glucose 6-phosphate (G6P). Movement of glucose from blood to interstitium is determined by skeletal muscle blood flow, capillary recruitment and the endothelial permeability to glucose. The influx of glucose from the interstitium to intracellular space is determined by the number of glucose transporters in the sarcolemma and the glucose gradient across the sarcolemma. The capacity to phosphorylate glucose is determined by the amount of skeletal muscle hexokinase II, hexokinase II compartmentalization within the cell, and the concentration of the hexokinase II inhibitor G6P. Any change in glucose uptake occurs due to an alteration in one or more of these steps. Based on the low calculated intracellular glucose levels and the higher affinity of glucose for phosphorylation relative to transport, glucose transport is generally considered rate-determining for basal muscle glucose uptake. Exercise increases both the movement of glucose from blood to sarcolemma and the permeability of the sarcolemma to glucose. Whether the ability to phosphorylate glucose is increased in the working muscle remains to be clearly shown. It is possible that the accelerated glucose delivery and transport rates during exercise bias regulation so that muscle glucose phosphorylation exerts more control on muscle glucose uptake. Conditions that alter glucose uptake during exercise, such as increased NEFA concentrations, decreased oxygen availability and adrenergic stimulation, must work by altering one or more of the three steps involved in glucose uptake. This review describes the regulation of glucose uptake during exercise at each of these sites under a number of conditions, as well as describing muscle glucose uptake in the post-exercise state.

Insulin sensitivity, muscle fibre types, and membrane lipids

One of the key abnormalities of non-insulin-dependent diabetes mellitus (NIDDM) and related diseases of the "Metabolic Syndrome" is impaired insulin action (insulin resistance). Since skeletal muscle plays a major role in insulin-stimulated glucose uptake and whole-body energy expenditure, it is a central player in carbohydrate and lipid metabolism, and hence in the balance between health and disease. This manuscript seeks to describe the evidence both for involvement in insulin resistance of three major muscle variables: membrane lipid composition, storage triacylglycerol and fibre type mixture; and for the interrelationships between these variables. Taken with results provided in other chapters in this volume, the literature described gives insights into the role that certain dietary fats and physical inactivity may play in the development of insulin resistance and hence the disease cluster of the Metabolic Syndrome.

Five-hour fatty acid elevation increases muscle lipids and impairs glycogen synthesis in the rat

Insulin-mediated muscle glycogen synthesis is impaired after several weeks of high-fat feeding in rats, but not by short-term (2-hour) nonesterified fatty acids (NEFA) elevation induced by intravenous triglyceride/heparin infusion (TG/H). We examined whether a longer TG/H infusion induces defective glycogen synthesis. Five-hour hyperinsulinemic (700 pmol/L) euglycemic clamps with either TG/H or saline infusion were performed. TG/H-infused rats developed insulin resistance, but only after 2 to 3 hours. Red gastrocnemius glycogen synthesis rate decreased by 50% (P < .01 v saline) associated with decreased glycogen synthase activity (GSa; assessed at several glucose-6-phosphate [G-6-P] levels; two-way ANOVA, P=.02) and increased muscle TG and total long-chain acyl coenzyme A (LCAC) content (twofold; P < .05 v saline). Thus a 3- to 5-hour NEFA elevation in the rat produced significant impairment of insulin-stimulated muscle glycogen synthesis, associated with muscle lipid accumulation. These effects were similar to those observed after several weeks of fat feeding. The 5-hour TG/H-infused rat is a useful model for studying lipid-induced muscle insulin resistance.

Regulation of endogenous glucose production by glucose per se is impaired in type 2 diabetes mellitus

We examined the ability of an equivalent increase in circulating glucose concentrations to inhibit endogenous glucose production (EGP) and to stimulate glucose metabolism in patients with Type 2 diabetes mellitus (DM2). Somatostatin was infused in the presence of basal replacements of glucoregulatory hormones and plasma glucose was maintained either at 90 or 180 mg/dl. Overnight low-dose insulin was used to normalize the plasma glucose levels in DM2 before initiation of the study protocol. In the presence of identical and constant plasma insulin, glucagon, and growth hormone concentrations, a doubling of the plasma glucose levels inhibited EGP by 42% and stimulated peripheral glucose uptake by 69% in nondiabetic subjects. However, the same increment in the plasma glucose concentrations failed to lower EGP, and stimulated glucose uptake by only 49% in patients with DM2. The rate of glucose infusion required to maintain the same hyperglycemic plateau was 58% lower in DM2 than in nondiabetic individuals. Despite diminished rates of total glucose uptake during hyperglycemia, the ability of glucose per se (at basal insulin) to stimulate whole body glycogen synthesis (glucose uptake minus glycolysis) was comparable in DM2 and in nondiabetic subjects. To examine the mechanisms responsible for the lack of inhibition of EGP by hyperglycemia in DM2 we also assessed the rates of total glucose output (TGO), i.e., flux through glucose-6-phosphatase, and the rate of glucose cycling in a subgroup of the study subjects. In the nondiabetic group, hyperglycemia inhibited TGO by 35%, while glucose cycling did not change significantly. In DM2, neither TGO or glucose cycling was affected by hyperglycemia. The lack of increase in glucose cycling in the face of a doubling in circulating glucose concentrations suggested that hyperglycemia at basal insulin inhibits glucose-6-phosphatase activity in vivo. Conversely, the lack of increase in glucose cycling in the presence of hyperglycemia and unchanged TGO suggest that the increase in the plasma glucose concentration failed to enhance the flux through glucokinase in DM2. In summary, both lack of inhibition of EGP and diminished stimulation of glucose uptake contribute to impaired glucose effectiveness in DM2. The abilities of glucose at basal insulin to both increase the flux through glucokinase and to inhibit the flux through glucose-6-phosphatase are impaired in DM2. Conversely, glycogen synthesis is exquisitely sensitive to changes in plasma glucose in patients with DM2.

Effects of FFA on insulin-stimulated glucose fluxes and muscle glycogen synthase activity in rats

To examine effects of free fatty acids (FFA) on insulin-stimulated glucose fluxes, euglycemic hyperinsulinemic (86 pmol . kg-1 . min-1) clamps were performed for 5 h in conscious rats with (n = 8) or without (n = 8) lipid-heparin infusion. Glucose infusion rate required to maintain euglycemia was not different between the two groups during the first 2 h of clamps but became significantly lower with lipid-heparin infusion in the 3rd h and thereafter. To investigate changes in intracellular glucose metabolism during lipid-heparin infusion, additional clamps (n = 8 each) were performed for 1, 2, 3, or 5 h with an infusion of [3-3H]glucose. Insulin-stimulated whole body glucose utilization (Rd), glycolysis, and glycogen synthesis were estimated on the basis of tracer concentrations in plasma during the final 40 min of each clamp. Similar to changes in glucose infusion rate, Rd was not different between the two groups in the 1st and 2nd h but was significantly lower with lipid-heparin infusion in the 3rd h and thereafter. Whole body glycolysis was significantly lower with lipid-heparin infusion in all time periods, i.e., 1st, 2nd, 3rd, and 5th h of clamps. In contrast, whole body glycogen synthesis was higher with lipid-heparin infusion in the 1st and 2nd h but lower in the 5th h. Similarly, accumulation of [3H]glycogen radioactivity in muscle glycogen was significantly higher with lipid-heparin during the 1st and 2nd h but lower during the 3rd and 5th h. Glucose 6-phosphate (G-6-P) concentrations in gastrocnemius muscles were significantly higher with lipid-heparin infusion throughout the clamps. Muscle glycogen synthase (GS) activity was not altered with lipid-heparin infusion at 1, 2, and 3 h but was significantly lower at 5 h. Thus increased availability of FFA significantly reduced whole body glycolysis, but compensatory increase in skeletal muscle glycogen synthesis in association with accumulation of G-6-P masked this effect, and Rd was not affected in the early phase (within 2 h) of lipid-heparin infusion. Rd was reduced in the later phase (>2 h) of lipid-heparin infusion, when glycogen synthesis was reduced in association with reduced skeletal muscle GS activity.

Abdominal obesity, impaired nonesterified fatty acid suppression, and insulin-mediated glucose disposal are early metabolic abnormalities in families with premature myocardial infarction

British Indian Asian men aged <40 years have a twofold to threefold increased risk of death from coronary heart disease (CHD) compared with British whites. Epidemiological studies have suggested an association between glucose intolerance and hyperinsulinemia with premature CHD in Indian Asians. We tested the association of insulin action with myocardial infarction (MI) by using the hyperinsulinemic-euglycemic clamp in 17 MI patients: 8 Punjabi Sikhs (PSMIs), 9 British whites (BWMIs), and 17 control subjects (9 PSCs and 8 BWCs). Metabolic factors associated with insulin resistance were investigated in 51 MI patients (24 PSMIs and 27 BWMIs) and 53 control subjects (28 PSCs and 25 BWCs). Familial aggregation of defective insulin action was examined by studying five pedigrees of Sikh survivors of MI. Sikh survivors of premature MI demonstrated impaired insulin-mediated glucose uptake (P<.001) by use of the clamp technique and nonesterified fatty acid (NEFA) suppression (P<.05) by using both clamp techniques and the oral glucose tolerance test, as compared with Sikh control subjects. White patients had impaired insulin-mediated glucose uptake but normal NEFA suppression. Metabolic factors usually associated with insulin resistance, including increased 2-hour post-oral glucose tolerance test triglycerides, smaller low density lipoprotein particle size, and increased plasminogen activator inhibitor-1, were present in white (all P<.05) but surprisingly absent in Sikh (all P>.05) MI patients compared with respective ethnic control subjects. Fasting glucose and total cholesterol levels did not differ between patients and control subjects. Abdominal obesity, impaired NEFA suppression after oral glucose, and fasting hyperinsulinemia were present in Sikh MI patients and their nondiabetic first-degree relatives compared with Sikh control subjects. PS survivors of premature MI demonstrated impaired insulin-mediated glucose disposal and NEFA suppression compared with ethnic control subjects. BWMI patients showed abnormalities of carbohydrate, but not of NEFA, metabolism compared with white control subjects. Defects of insulin action manifested as abdominal obesity, impaired NEFA suppression, and fasting hyperinsulinemia are present in Sikh MI patients and their asymptomatic, nondiabetic, first-degree relatives. We suggest that these defects may be early metabolic markers that predict risk of premature MI among PSs.

Insulin resistance in hypertension is associated with body fat rather than blood pressure

The insulin resistance syndrome has been characterized by hypertension, upper body obesity, insulin resistance, hyperinsulinemia, glucose intolerance, and hypertriglyceridemia. Previous studies are inconsistent regarding the relationship between blood pressure and insulin resistance. We therefore compared the metabolic profile in 60 hypertensive subjects (mean+/-SD arterial pressure, 116+/-7 mm Hg) and 60 normotensive subjects (mean arterial pressure, 88+/-5 mm Hg) matched for age, gender, and body mass index. Hypertensives had significantly higher waist-to-hip ratio than normotensives (P=0.002). The groups did not differ in fasting plasma glucose (0.2 mmol/L, P=0.09), insulin (6 pmol/L, P=0.14), insulin sensitivity index (-0.01 micromol x kg(-1) x min(-1) x pmol/L(-1), P=0.7), and suppression of nonesterified fatty acids during a hyperglycemic clamp (1%, P=0.40). There were significant differences in fasting levels of C-peptide (50 pmol/L, P=0.004) and proinsulin (2 pmol/L, P=0.01), 2-hour postload levels of glucose (0.8 mmol/L, P=0.01) and insulin (84 pmol/L, P=0.01) after oral glucose challenge, and hepatic glucose production during the clamp (2.87 micromol x kg(-1) x min(-1), P=0.02). These differences were not significant when controlling for waist-to-hip ratio. Body mass index and waist-to-hip ratio were similarly associated with the insulin sensitivity index in the hypertensive (r=-0.59, P=0.0001 and r=-0.32, P=0.05) and normotensive (r=-0.58, P=0.0001 and r=-0.39, P=0.05) groups. Hypertension per se is not associated with insulin resistance. However, even small increments in both body mass index and waist-to-hip ratio, as often seen in hypertension, may lead to impairment in insulin sensitivity, probably mediated through altered lipid metabolism.

Free fatty acids and insulin resistance during pregnancy

The purpose of this study was to determine whether elevation of plasma free fatty acids (FFA) in early pregnancy would cause alterations in insulin-stimulated glucose disposal similar to those occurring in late gestation. Seven glucose-tolerant women underwent 4-h euglycemic hyperinsulinemic (1 mU/kg.min) clamping during the early second trimester of pregnancy (14-17 weeks) on 2 consecutive days, receiving either lipid (Liposyn II; 1.5 mL/min) and heparin (0.4 U/kg.min; L/H) or saline/glycerol (2.25 g/h; S/G) infusions. Rates of total body glucose disposal (6,6-2H2 glucose) and of carbohydrate and fat oxidation (indirect calorimetry) were determined at hourly intervals. Blood glucose was clamped at about 85 mg/dL. Plasma FFA increased from 290 +/- 50 to 1000 +/- 139 mumol/L during L/H infusion and decreased from 351 +/- 60 to 35 +/- 11 mumol/L during S/G infusion. L/H infusion inhibited insulin stimulation of total body glucose disposal by 28% compared with S/G infusion (from 6.7 +/- 0.7 to 4.9 +/- 0.6 mg/kg.min; P < 0.01). L/H infusion increased fat oxidation from 0.73 +/- 0.04 to 1.26 +/- 0.2 mg/kg.min (P < 0.05) and decreased carbohydrate oxidation from 2.0 +/- 0.2 to 1.6 +/- 0.2 mg/kg.min (P < 0.05). Endogenous glucose production decreased equally by approximately 70% during L/H and S/G infusions. These data showed that elevating plasma FFA levels during early pregnancy inhibits total body glucose uptake and oxidation. We conclude that elevation of plasma FFA can contribute to the peripheral insulin resistance commonly observed during late pregnancy.

Role of muscle in triglyceride metabolism

It has long been recognized that skeletal muscle can contain modest stores of triglyceride and that this depot of fuel can make a major contribution to energy production during exercise. More recently, an adverse effect of muscle triglyceride has begun to be defined within the context of insulin resistance. Animal and clinical investigations have revealed a significant relation between increased muscle triglyceride and insulin resistance, at least among mostly sedentary individuals. These observations have stimulated the development, or at least the refinement, of new methodologies to assess this aspect of 'regional' fat deposition. In parallel, there has also been important new work designed to enable better understanding of the factors that regulate muscle triglyceride and to determine whether fatty acids taken up by skeletal muscle are oxidized or stored, and how these pathways might be either altered by the presence of insulin resistance or, in turn, contribute to the pathogenesis of insulin resistance.

A study of insulin resistance using the minimal model in nondiabetic familial combined hyperlipidemic patients

The presence of insulin resistance in 20 male nondiabetic patients with familial combined hyperlipidemia (FCH) and 20 controls of similar age and body mass index (BMI) was investigated using the minimal model method modified by the administration of insulin and an oral glucose tolerance test. The peripheral sensitivity of insulin, expressed as the insulin sensitivity index (Si), was 1.91+/-1.05 and 2.86+/-1.19 x 10(-4) x min(-1) x mU/L in FCH patients and controls, respectively (P < .01), and the corresponding value for the peripheral utilization of glucose independently of insulin (Sg) was 1.70+/-1.13 in FCH patients and 2.35+/-0.60 x 10(-2) x min(-1) in controls (P < .02). In the FCH group, the Si value correlated significantly (P < .05) with the waist to hip ratio (WHR), plasma triglycerides (TG), free fatty acids (FFA), and the area under the curve of glucose (AUCg) and insulin (AUCi). In the control group, the correlation also reached statistical significance (P < .05) with age, BMI, WHR, blood pressure, TG, AUCg, and AUCi. Subgrouping the subjects with respect to central obesity defined as a WHR of 0.95 or greater, we observed lower Si values in obese and non-obese FCH subjects relative to controls (P < .02). The mean Si value in obese subjects was significantly lower than in non-obese FCH subgroups (1.40+/-0.79 v 2.68+/-0.95 x 10(-4) x min(-1) x mU/L, respectively, P < .01). In conclusion, a higher degree of insulin resistance relative to control values appears to be an integral part of the metabolic derangements observed in FCH, and central-trunk obesity exacerbates the insulin resistance syndrome.

TNF-alpha and insulin resistance: summary and future prospects

While the causes of obesity remain elusive, the relationship between obesity and insulin resistance is a well-established fact [1]. Insulin resistance is defined as a smaller than normal response to a certain dose of insulin, and contributes to several pathological problems of obese patients such as hyperlipidemia, arteriosclerosis and hypertension. Several pieces of evidence indicate that the cytokine tumor necrosis factor a (TNF-alpha) is an important player in the state of insulin resistance observed during obesity. In this review we will try to summarize what is known about the function of TNF-alpha in insulin resistance during obesity and how TNF-alpha interferes with insulin signaling.

Effect of insulin on glycerol production in obese adolescents

Impaired stimulation of glucose metabolism and reduced suppression of lipolytic activity have both been suggested as important defects related to the insulin resistance of adolescent obesity. To further explore the relationship between these abnormalities, we studied seven obese [body mass index (BMI) 35 +/- 2 kg/m2] and seven lean (BMI 21 +/- 1 kg/m2) adolescents aged 13-15 yr and compared them with nine lean adults (aged 21-27 yr, BMI 23 +/- 1 kg/m2) during a two-step euglycemic-hyperinsulinemic clamp in combination with 1) a constant [2H5]glycerol (1.2 mg.m-2.min-1) infusion to quantify glycerol turnover and 2) indirect calorimetry to estimate glucose and net lipid oxidation rates. In absolute terms, basal glycerol turnover was increased and suppression by insulin was impaired in obese adolescents compared with both groups of lean subjects (P < 0.01). However, when the rates of glycerol turnover were adjusted for differences in body fat mass, the rates were similar in all three groups. Basal plasma free fatty acid (FFA) concentrations were significantly elevated, and the suppression by physiological increments in plasma insulin was impaired in obese adolescents compared with lean adults (P < 0.05). In parallel with the high circulating FFA levels, net lipid oxidation in the basal state and during the clamp was also elevated in the obese group compared with lean adults. Net lipid oxidation was inversely correlated with glucose oxidation (r = -0.50, P < 0.01). In conclusion, these data suggest that lipolysis is increased in obese adolescents (vs. lean adolescents and adults) as a consequence of an enlarged adipose mass rather than altered sensitivity of adipocytes to the suppressing action of insulin.

PPARgamma activators improve glucose homeostasis by stimulating fatty acid uptake in the adipocytes

It is currently thought that the effects of PPARgamma activation on glucose homeostasis may be due to the effect of this nuclear receptor on the production of adipocyte-derived signalling molecules, which affect muscle glucose metabolism. Potential signalling molecules derived from adipocytes and modified by PPARgamma activation include TNFalpha and leptin, which both interfere with glucose homeostasis. In addition to its effects on these proteins, PPARgamma also profoundly affects fatty acid metabolism. Activation of PPARgamma will selectively induce the expression of several genes involved in fatty acid uptake, such as lipoprotein lipase, fatty acid transport protein and acyl-CoA synthetase, in adipose tissue without changing their expression in muscle tissue. This co-ordinate regulation of fatty acid partitioning by PPARgamma results in an adipocyte 'FFA steal' causing a relative depletion of fatty acids in the muscle. Based on the well established interference of muscle fatty acid and glucose metabolism it is hypothesized that reversal of muscle fatty acid accumulation will contribute to the improvement in whole body glucose homeostasis.

Caloric restriction reverses hepatic insulin resistance in aging rats by decreasing visceral fat

Hyperinsulinemia and increased visceral/abdominal fat (VF) are common features of human aging. To examine the relationships among VF, peripheral, and hepatic insulin sensitivity, we studied 4- and 18-mo-old male Sprague-Dawley rats (n = 42) fed ad libitum (4 AL and 18 AL) or moderately calorie restricted (18 CR) up to 18 mo of age. Total fat mass (FM) and VF were decreased in 18 CR to approximately one-third of that of 18 AL (P < 0.001), while lean body mass (LBM) was unchanged. Most important, 18 CR had more FM (65+/-6 vs. 45+/-6 g) but less VF (7.8+/-0.6 vs. 12.3+/-3.3 g) compared with 4 AL (P < 0.01 for both). Thus, the effects of variable VF on HIS could be assessed, independent of FM and age. Marked hepatic insulin resistance ensued with aging (18 AL) and CR restored hepatic insulin sensitivity to the levels of young rats, while peripheral insulin sensitivity remained unchanged (by insulin clamp of 18 mU/kg/min). In fact, the rates of insulin infusion required to maintain basal hepatic glucose production in the presence of pancreatic clamp were 0.75+/-0.10, 1.41+/-0.13, and 0.51+/-0.12 mU/kg . min, in 4 AL, 18 AL, and 18 CR, respectively (P < 0.01 between all groups), and in 18 CR rats infused with insulin at similar rates as in the 18 AL (1.4 mU/kg/min) hepatic glucose production was decreased by 32% (P < 0. 005). Furthermore, when 18 CR rats were fed AL for 14 d, VF rapidly and selectively increased and severe hepatic insulin resistance was induced. We propose that in this animal model the age-associated decrease in hepatic (rather than peripheral) insulin action is the major determinant of fasting hyperinsulinemia and that increased visceral adiposity plays the major role in inducing hepatic insulin resistance. Thus, interventions designed to prevent the accumulation of VF are likely to represent an effective mean to improve carbohydrate metabolism in aging.

Mechanisms of insulin resistance and new pharmacological approaches to metabolism and diabetic complications

1. Resistance to insulin-mediated glucose transport and metabolism has been identified as a primary mechanism in the pathogenesis of non-insulin-dependent diabetes mellitus (NIDDM) and as a target for drug development. The aetiology of insulin resistance is likely to be multifactorial, but the present review focuses on candidate post-receptor mechanisms of insulin resistance, particularly protein kinase C (PKC), and the metabolic and genetic significance of beta3-adrenoceptors (beta3-AR) in adipose tissue. 2. Multiple lines of evidence suggest that isoform-selective activation of PKC phosphorylates and down-regulates one or more substrates involved in glucose transport and metabolism (e.g. glycogen synthase and the insulin receptor) and recent studies have shown increased expression of calcium-independent isozymes (PKC-epsilon and PKC-theta) in the membrane fraction of skeletal muscle in fructose- and fat-fed rat models of insulin resistance. In addition, there is separate evidence that glucose-induced PKC activation plays an important role in the micro- and macrovascular complications of diabetes. 3. New pharmacological approaches to NIDDM and obesity have focused on insulin-sensitizing agents (e.g. troglitazone), beta3-AR agonists, anti-lipolytic drugs (e.g. the adenosine A1 receptor agonist GR79236) and selective inhibitors of PKC isoforms (e.g. the inhibitor of PKC-beta LY333531). Experimental studies with GR79236 show that this drug ameliorates the hypertriglyceridaemia induced by fructose feeding and that the reduction in fatty acid levels is associated with secondary improvements in glucose tolerance. 4. Recent insights into the pathogenesis of NIDDM and its associated complications have been used to develop a range of new therapeutic agents that are currently showing promise in clinical and preclinical development.

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.

Short-term metabolic and haemodynamic effects of GR79236 in normal and fructose-fed rats

The adenosine (A1) receptor agonist, GR79236 (N-[(1S,trans)-2-hydroxycyclopentyl]adenosine), inhibits catecholamine-induced lipolysis in vitro, but the short-term metabolic and haemodynamic effects have not been previously reported in the fructose fed model of insulin resistance, dyslipidaemia and hypertension. This study reports the effects of GR79236 (1 mg/kg/day for 8 days) on nonesterified free fatty acid and triglyceride metabolism, oral and i.v. glucose tolerance, blood pressure and heart rate, and insulin sensitivity, in normal rats and rats fed a fructose-enriched diet. In normal rats, GR79236 significantly reduced fasting glucose (25%), free fatty acid (50%) and triglyceride (55%) concentrations, and improved glucose tolerance (AUC[glu] 21.2 +/- 1.3 vs. 16.5 +/- 1.1 mmol h/l, p < 0.05). Fructose feeding induced a state of insulin resistance and dyslipidaemia, as shown by an increase in steady-state plasma glucose levels (7.1 vs. 6.1 mmol/l), impaired i.v. glucose tolerance and a 3-fold rise in fasting triglyceride levels; fructose-fed rats also developed a significant increase in blood pressure. GR79236 ameliorated the effects of fructose feeding on fatty acid and triglyceride levels, and blood pressure, and improved i.v. glucose tolerance in fructose-fed rats. The hypotriglyceridaemic effect was due to a reduction in triglyceride secretion rate (17.3 +/- 1.7 vs. 30.2 +/- 1.1). Thus, in normal rats and in a dietary-induced rodent model of insulin resistance, dyslipidaemia and hypertension, GR79236 has lipid-lowering and glucose-lowering activity, as well as haemodynamic effects, which are potentially useful for treating both the metabolic and haemodynamic features of insulin resistance and NIDDM in humans.

Interaction of equal increments in arterial and portal vein insulin on hepatic glucose production in the dog

We have previously shown that a selective increase of 84 pmol/l in either arterial or portal vein insulin (independent of a change in insulin in the other vessel) can suppress tracer-determined glucose production (TDGP) and net hepatic glucose output (NHGO) by approximately 50%. In the present study we investigated the interaction between equal increments in arterial and portal vein insulin in the suppression of TDGP and NHGO. Isotopic ([3-3H]glucose) and arteriovenous difference methods were used in conscious overnight fasted dogs. A pancreatic clamp was used to control the endocrine pancreas. A 40-min basal period was followed by a 180-min test period, during which arterial and portal vein insulin levels were simulataneously and equally increased 102 pmol/l. Hepatic sinusoidal glucagon levels remained unchanged, and euglycemia was maintained by peripheral glucose infusion. TDGP was suppressed approximately 60% by the last 30 min of the experimental period. In contrast, NHGO was suppressed 100% by that time. Coincidentally, hepatic glucose uptake (net hepatic [3H]glucose balance) increased significantly (approximately 4 mumol.kg-1.min-1). The effects of simultaneous equal increases in peripheral and portal venous insulin were not additive in the suppression of TDGP. However, they were additive in decreasing NHGO as a result of an increase in the uptake of glucose by the liver.

Fatty acid-induced insulin resistance in adipocytes

Elevated serum-free fatty acid (FFA) levels induce insulin resistance in whole animals and humans. To understand the direct mechanism by which FFAs impact insulin-responsive tissue, we have used our previously developed in vitro model of long-chain saturated fatty acids (LCSFA)-induced insulin resistance in adipocytes. In addition to explanted rat adipocytes, we now demonstrate that overnight exposure of 3T3-L1 adipocytes to 1 mM individually of the LCSFA palmitate, myristate, and stearate, leads to an approximately 50% inhibition of insulin-induced glucose transport. Insulin resistance can be accomplished at 0.3 mM palmitate, which is within the range ofpalmitate found in diabetic and obese individuals. This inhibition was noted within 4 h of exposure to FFA, which is comparable to in vivo lipid infusion studies. Initial LCSFA-induced resistance is specific to glucose transport and does not affect insulin stimulation of glucose incorporation into glycogen. In 3T3-L1 adipocytes overexpressing the EGF receptor, LCSFA exposure also specifically inhibited EGF-induced GLUT4-mediated glucose transport, but not EGF-induced glycogen synthesis. We find that LCSFA treatment did not impair insulin stimulation of GLUT4 translocation or exofacial presentation on the cell surface as determined by trypsin accessibility. Our results suggest that the initial direct effect of elevated LCSFA is to impair activation of GLUT4 transporter activity and that this effect is specific for glucose transport.

Fat accretion and the regulation of insulin-mediated glycogen synthesis after puberty in rats

Peripheral insulin sensitivity decreases after puberty in both humans and rodents and can be explained mostly by a reduction in insulin-mediated glycogen synthesis. We tested the hypothesis that the increase in postpubertal fat mass (FM), reflecting an alternative energy store, regulates a decrease in the capacity to store muscle glycogen. We studied Sprague-Dawley rats (n = 21) before puberty (Pre) or after puberty (at 4 mo of age) in groups that were either ad libitum fed (Post) or moderately caloric restricted (CR). FM (by 3H2O isotope dilution technique) was decreased by >40% in CR compared with Post. Glucose uptake (Rd, by 18 mU x kg(-1) x min(-1) hyperinsulinemic clamp) was 63 +/- 8 mg x kg(-1) x min(-1) in Pre and decreased to 39 +/- 2 mg x kg(-1) x min(-1) in Post (P < 0.001). However, it increased in CR to 53 +/- 2 mg x kg(-1) x min(-1) (P < 0.001 vs. Post). This increase in Rd was mainly accounted for by an increase in glycogen synthesis (Rd glycolysis determined by the rate of conversion of 3H-labeled glucose to 3H2O) from 23 +/- 2 in Post to 33 +/- 2 mg x kg(-1) x min(-1) in CR (P < 0.001; 38 +/- 7 mg x kg(-1) x min(-1) in Pre). Correction of glycogen synthesis in CR to near-prepubertal levels was further supported by directly assayed muscle glycogen content after insulin stimulation that was 45% higher and by a 35% enhanced accumulation of [3H]glucose into glycogen. No changes in the enzyme kinetics of glycogen synthase or phosphorylase were observed. An additional group of 2-mo-old postpubertal ad libitum-fed rats was matched with CR for lean body mass but had more FM. This group demonstrated 25% lower rates of insulin-mediated glycogen synthesis compared with CR, further supporting the notion that a moderate reduction of FM prevents the decline in insulin responsiveness and glycogen synthesis occurring after puberty. These data suggest a cause-effect relationship between the increased deposition of fat and the reduced ability to store glucose in skeletal muscle after puberty.

Peripheral but not hepatic insulin resistance in mice with one disrupted allele of the glucose transporter type 4 (GLUT4) gene

Glucose transporter type 4 (GLUT4) is insulin responsive and is expressed in striated muscle and adipose tissue. To investigate the impact of a partial deficiency in the level of GLUT4 on in vivo insulin action, we examined glucose disposal and hepatic glucose production (HGP) during hyperinsulinemic clamp studies in 4-5-mo-old conscious mice with one disrupted GLUT4 allele [GLUT4 (+/-)], compared with wild-type control mice [WT (+/+)]. GLUT4 (+/-) mice were studied before the onset of hyperglycemia and had normal plasma glucose levels and a 50% increase in the fasting (6 h) plasma insulin concentrations. GLUT4 protein in muscle was approximately 45% less in GLUT4 (+/-) than in WT (+/+). Euglycemic hyperinsulinemic clamp studies were performed in combination with [3-3H]glucose to measure the rate of appearance of glucose and HGP, with [U-14C]-2-deoxyglucose to estimate muscle glucose transport in vivo, and with [U-14C]lactate to assess hepatic glucose fluxes. During the clamp studies, the rates of glucose infusion, glucose disappearance, glycolysis, glycogen synthesis, and muscle glucose uptake were approximately 55% decreased in GLUT4 (+/-), compared with WT (+/+) mice. The decreased rate of in vivo glycogen synthesis was due to decreased stimulation of glucose transport since insulin's activation of muscle glycogen synthase was similar in GLUT4 (+/-) and in WT (+/+) mice. By contrast, the ability of hyperinsulinemia to inhibit HGP was unaffected in GLUT4 (+/-). The normal regulation of hepatic glucose metabolism in GLUT4 (+/-) mice was further supported by the similar intrahepatic distribution of liver glucose fluxes through glucose cycling, gluconeogenesis, and glycogenolysis. We conclude that the disruption of one allele of the GLUT4 gene leads to severe peripheral but not hepatic insulin resistance. Thus, varying levels of GLUT4 protein in striated muscle and adipose tissue can markedly alter whole body glucose disposal. These differences most likely account for the interindividual variations in peripheral insulin action.

Role of the glucosamine pathway in fat-induced insulin resistance

To examine whether the hexosamine biosynthetic pathway might play a role in fat-induced insulin resistance, we monitored the effects of prolonged elevations in FFA availability both on skeletal muscle levels of UDP-N-acetyl-hexosamines and on peripheral glucose disposal during 7-h euglycemic-hyperinsulinemic (approximately 500 microU/ml) clamp studies. When the insulin-induced decrease in the plasma FFA levels (to approximately 0.3 mM) was prevented by infusion of a lipid emulsion in 15 conscious rats (plasma FFA approximately 1.4 mM), glucose uptake (5-7 h = 32.5+/-1.7 vs 0-2 h = 45.2+/-2.8 mg/kg per min; P < 0.01) and glycogen synthesis (P < 0.01) were markedly decreased. During lipid infusion, muscle UDP-N-acetyl-glucosamine (UDP-GlcNAc) increased by twofold (to 53.4+/-1.1 at 3 h and to 55.5+/-1.1 nmol/gram at 7 h vs 20.4+/-1.7 at 0 h, P < 0.01) while glucose-6-phosphate (Glc-6-P) levels were increased at 3 h (475+/-49 nmol/gram) and decreased at 7 h (133+/-7 vs 337+/-28 nmol/gram at 0 h, P < 0.01). To discern whether such an increase in the skeletal muscle UDP-GlcNAc concentration could account for the development of insulin resistance, we generated similar increases in muscle UDP-GlcNAc using three alternate experimental approaches. Euglycemic clamps were performed after prolonged hyperglycemia (18 mM, n = 10), or increased availability of either glucosamine (3 micromol/kg per min; n = 10) or uridine (30 micromol/kg per min; n = 4). These conditions all resulted in very similar increases in the skeletal muscle UDP-GlcNAc (to approximately 55 nmol/gram) and markedly impaired glucose uptake and glycogen synthesis. Thus, fat-induced insulin resistance is associated with: (a) decreased skeletal muscle Glc-6-P levels indicating defective transport/phosphorylation of glucose; (b) marked accumulation of the endproducts of the hexosamine biosynthetic pathway preceding the onset of insulin resistance. Most important, the same degree of insulin resistance can be reproduced in the absence of increased FFA availability by a similar increase in skeletal muscle UDP-N-acetyl-hexosamines. In conclusion, our results support the hypothesis that increased FFA availability induces skeletal muscle insulin resistance by increasing the flux of fructose-6-phosphate into the hexosamine pathway.

13C and 31P NMR studies on the effects of increased plasma free fatty acids on intramuscular glucose metabolism in the awake rat

The effects of increased plasma free fatty acids (FFA) on insulin-dependent whole body glucose disposal, skeletal muscle glycolysis, glycogen synthesis, pyruvate versus FFA/ketone oxidation, and glucose 6-phosphate (Glu-6-P) were investigated in the awake rat. A control group (glycerol-infused) and high plasma FFA group (Liposyn-infused) were clamped at euglycemia (approximately 6 mM)-hyperinsulinemia (10 milliunits/kg/min) throughout the experiment (180-240 min). In the initial experiment, 13C NMR was used to observe [1-13C]glucose incorporation into [1-13C]glycogen in the rat hindlimb for glycogen synthesis calculations and into [3-13C]lactate and [3-13C]alanine for glycolytic flux calculations. These experiments were followed by 31P NMR measurements of Glu-6-P changes under identical conditions of the initial experiment. Plasma FFA concentrations were 2.25 +/- 0.36 and 0.20 +/- 0.03 mM in the high plasma FFA and control groups respectively (p < 0.0005). Glucose infusion rates (Ginf) decreased significantly in the Liposyn-infused rats (29.5 +/- 0.7 and 27.2 +/- 1.2 mg/kg/min for control and high plasma FFA group, respectively, at 15 min to 30.7 +/- 2.3 and 17.7 +/- 1.3 mg/kg/min, respectively, at the end of the experiment, p < 0.002). Glycogen synthesis rates were 163 +/- 32 and 104 +/- 17 nmol/g/min, and glycolytic rates were 57.9 +/- 8.0 and 19. 5 +/- 3.6 nmol/g/min (p < 0.002) in the control and high plasma FFA groups, respectively. The relative flux of pyruvate versus free fatty acids and ketones entering the tricarboxylic acid cycle was greater in the control (57 +/- 9%) versus high plasma FFA group (25 +/- 4%) (p < 0.005) as assessed by [4-13C]glutamate/[3-13C]lactate steady state isotopic enrichment measurements. Finally, Glu-6-P concentrations increased by 29.8 +/- 7.0 and 52.8 +/- 12.3% (p < 0. 05) in the control and high plasma FFA groups, respectively, above their basal concentrations by 180 min. In conclusion, we have demonstrated the ability to use in vivo NMR to elucidate the metabolic fate of glucose within skeletal muscle of an awake rat during a euglycemic-hyperinsulinemic clamp and increased levels of plasma FFA. These data suggest that increased concentrations of plasma FFA inhibit insulin-stimulated muscle glucose metabolism in the rat through inhibition of glycolysis.

Glucose-fatty acid interactions in prepubertal and pubertal children: effects of lipid infusion

This investigation examined whether puberty differs from prepuberty in regard to the effects of increased free fatty acid (FFA) on in vivo glucose metabolism. Nine prepubertal and 13 pubertal healthy children were studied. Each subject was studied twice, once with 0.9% sodium chloride solution (control study) and once with 20% Intralipid infusion in the basal state and during a 3-h hyperinsulinemic-euglycemic clamp, with [6,6-2H2]glucose tracer. During control studies, prepubertal children had lower basal fat oxidation and higher insulin-mediated glucose disposal than pubertal adolescents. During Intralipid infusion, basal glucose uptake increased in prepubertal children but did not change in pubertal adolescents. Insulin-stimulated whole body glucose disposal did not change in prepubertal children (control 77.6 +/- 8.9, Intralipid 84.5 +/- 13.3 micromol x kg(-1) x min(-1)) but decreased in pubertal adolescents (control 55.0 +/- 3.6, Intralipid 46.7 +/- 3.4 micromol x kg(-1) x min(-1), P = 0.01) despite comparable decrements in glucose oxidaion. We conclude that in prepubertal children lipids exert effects in the basal state by stimulating hepatic glucose production and glucose disposal, whereas in pubertal adolescents they induce peripheral tissue insulin resistance by decreasing insulin-stimulated glucose uptake. This differential response could be due to developmental-maturational changes in tissue sensitivity and/or specificity to the glucose-FFA interaction.

Solution structure of human intestinal fatty acid binding protein: implications for ligand entry and exit

The human intestinal fatty acid binding protein (I-FABP) is a small (131 amino acids) protein which binds dietary long-chain fatty acids in the cytosol of enterocytes. Recently, an alanine to threonine substitution at position 54 in I-FABP has been identified which affects fatty acid binding and transport, and is associated with the development of insulin resistance in several populations including Mexican-Americans and Pima Indians. To investigate the molecular basis of the binding properties of I-FABP, the 3D solution structure of the more common form of human I-FABP (Ala54) was studied by multidimensional NMR spectroscopy. Recombinant I-FABP was expressed from E. coli in the presence and absence of 15N-enriched media. The sequential assignments for non-delipidated I-FABP were completed by using 2D homonuclear spectra (COSY, TOCSY and NOESY) and 3D heteronuclear spectra (NOESY-HMQC and TOCSY-HMQC). The tertiary structure of human I-FABP was calculated by using the distance geometry program DIANA based on 2519 distance constraints obtained from the NMR data. Subsequent energy minimization was carried out by using the program SYBYL in the presence of distance constraints. The conformation of human I-FABP consists of 10 antiparallel beta-strands which form two nearly orthogonal beta-sheets of five strands each, and two short alpha-helices that connect the beta-strands A and B. The interior of the protein consists of a water-filled cavity between the two beta-sheets. The NMR solution structure of human I-FABP is similar to the crystal structure of rat I-FABP. The NMR results show significant conformational variability of certain backbone segments around the postulated portal region for the entry and exit of fatty acid ligand.

Intracellular pathways of insulin-mediated glucose uptake before and after puberty in conscious rats

Studies in humans and animals indicate that peripheral insulin sensitivity is decreased after puberty. Although glucose, after its uptake and phosphorylation, will be diverted to either the glycolytic or glycogen synthesis pathway, these pathways have not been characterized after the transition to puberty. Thus, we examined the changes in the pathways of glucose utilization in conscious (n = 22) prepuberty (81 +/- 3 g), and postpuberty (258 +/- 9 g) Sprague-Dawley rats. Insulin stimulated (by insulin clamp 18 mU/kg/min) glucose uptake [rate of glucose disappearance (Rd)] was decreased by approximately 30% postpuberty (from 339 +/- 22 to 239 +/- 28 mumol/kg/min; p < 0.001). Although glycolysis (estimated by the rate of conversion of [3H]glucose to 3H2O) decreased by approximately 15% (p < 0.05), glycogen synthesis decreased by approximately 40% (from 200 +/- 17 prepuberty to 122 +/- 22 mumol/kg/min postpuberty; p < 0.001), and accounted for approximately 80% of the decrease in Rd postpuberty. Decrease in the capacity to store glycogen in response to insulin was also confirmed by approximately 40% decrease in both glycogen levels, and in 3H accumulation into glycogen (from 3H-glucose) at the end of the clamp study. This occurred in the absence of any changes in either the K(m) or the Vmax of glycogen synthase nor in the activity of glycogen phosphorylase. We conclude that the postpubertal decrease in insulin responsiveness is characterized by decreased ability to store muscle glycogen. We propose that high capacity for muscle glycogen synthesis may be required to sustain the increased metabolic requirements during peripubertal growth.

Prolonged suppression of glucose metabolism causes insulin resistance in rat skeletal muscle

To determine whether an impairment of intracellular glucose metabolism causes insulin resistance, we examined the effects of suppression of glycolysis or glycogen synthesis on whole body and skeletal muscle insulin-stimulated glucose uptake during 450-min hyperinsulinemic euglycemic clamps in conscious rats. After the initial 150 min to attain steady-state insulin action, animals received an additional infusion of saline, Intralipid and heparin (to suppress glycolysis), or amylin (to suppress glycogen synthesis) for up to 300 min. Insulin-stimulated whole body glucose fluxes were constant with saline infusion (n = 7). In contrast, Intralipid infusion (n = 7) suppressed glycolysis by approximately 32%, and amylin infusion (n = 7) suppressed glycogen synthesis by approximately 45% within 30 min after the start of the infusions (P < 0.05). The suppression of metabolic fluxes increased muscle glucose 6-phosphate levels (P < 0.05), but this did not immediately affect insulin-stimulated glucose uptake due to compensatory increases in other metabolic fluxes. Insulin-stimulated whole body glucose uptake started to decrease at approximately 60 min and was significantly decreased by approximately 30% at the end of clamps (P < 0.05). Similar patterns of changes in insulin-stimulated glucose fluxes were observed in individual skeletal muscles. Thus the suppression of intracellular glucose metabolism caused decreases in insulin-stimulated glucose uptake through a cellular adaptive mechanism in response to a prolonged elevation of glucose 6-phosphate rather than the classic mechanism involving glucose 6-phosphate inhibition of hexokinase.

Glucose plus insulin regulate fat oxidation by controlling the rate of fatty acid entry into the mitochondria

We tested the hypothesis that glucose plus insulin determine the rate of fat oxidation in humans by controlling the rate of fatty acid entrance into the mitochondria. We gave constant infusions of [1-13C]oleate, a long-chain fatty acid, and [1-14C]octanoate, a medium-chain fatty acid, for 3 h in seven volunteers (basal). Immediately after the basal period, a hyperinsulinemic (insulin infusion = 120 mU x m(-2) min(-1)), hyperglycemic (plasma glucose = 140 mg/dl) clamp was started and continued for 5 h. During the last 3 h of the clamp, the infusions of [1-13C]oleate and [1-14C]octanoate were repeated. Intracellular acylcarnitine concentrations were measured in muscle biopsies obtained before and after the clamp. Plasma oleate enrichment and FFA concentration were kept constant by means of variable infusions of lipids and heparin. Oleate, but not octanoate, requires carnitine binding to gain access to the mitochondrial matrix; hence, if glucose and/or insulin limit long-chain fatty acid entrance into the mitochondria, then, during the clamp, long-chain acylcarnitine formation should be decreased, causing a decrease in oleate, but not octanoate, oxidation. Oleate oxidation decreased from the basal value of 0.7+/-0.1 to 0.4+/-0.1 micromol x kg(-1) x min(-1) (P < 0.05). In contrast, octanoate oxidation remained unchanged. Long-chain acylcarnitine concentration decreased from 855+/-271 in the basal state to 376+/-83 nmol/gram dry weight during the clamp (P < 0.05). We conclude that glucose and/or insulin determine fatty acid oxidation by controlling the rate of long-chain fatty acid entrance into the mitochondria.

Chronic free fatty acid infusion in rats results in insulin resistance but no alteration in insulin-responsive glucose transporter levels in skeletal muscle

To investigate the mechanism by which free fatty acids (FFA) affect glucose uptake, we studied the effect of chronic elevation (24 h) of plasma FFA in rats on whole body glucose disposal and glucose utilization index (GUI) in the basal state and under a euglycemic hyperinsulinemic clamp in relation to the amount of insulin-responsive glucose transporter (IRGT, i.e., GLUTU) protein in different muscles (oxidative and glycolytic) and adipose tissue. Infusion of intralipid in the basal state led to a approximately 40% increase in whole body glucose uptake and a approximately 250% increase in GUI in adipose tissue as compared to control rats. There was no change in the amount of IRGT protein in any of the muscle types whereas in fat depots it was either unchanged or decreased. Under moderate of supraphysiological hyperinsulinemia, increment of whole body glucose disposal was significantly lower in intralipid perfused rats when compared to controls (approximately 110 microU/mL: 0.7 +/- 0.1 vs. 1.3 +/- 0.1 mg/min, P < 0.02; approximately 1000 microU/mL: 3.0 +/- 0.2 vs. 3.9 +/- 0.4 mg/min, P < 0.02). Under moderate hyperinsulinemia stimulation, GUI was significantly reduced in different muscles and adipose tissue as compared to controls. We conclude that peripheral insulin resistance which occurs after elevation of plasma FFA levels does not seem to be explained by changes in the amount of IRGT protein in either oxidative or glycolytic skeletal muscle. Thus fatty acid infusion appears to be associated with a defect in IRGT translocation to the plasma membrane, fusion with the membrane, or intrinsic activity.

Effects of vanadyl sulfate on carbohydrate and lipid metabolism in patients with non-insulin-dependent diabetes mellitus

The safety and efficacy of vanadyl sulfate (VS) was tested in a single-blind, placebo-controlled study. Eight patients (four men and four women) with non-insulin-dependent diabetes mellitus (NIDDM) received VS (50 mg twice daily orally) for 4 weeks. Six of these patients (four men and two women) continued in the study and were given a placebo for an additional 4 weeks. Euglycemic-hyperinsulinemic clamps were performed before and after the VS and placebo phases. VS was associated with gastrointestinal side effects in six of eight patients during the first week, but was well tolerated after that. VS administration was associated with a 20% decrease in fasting glucose concentration (from 9.3 +/- 1.8 to 7.4 +/- 1.4 mmol/L, P < .05) and a decrease in hepatic glucose output (HGO) during hyperinsulinemia (from 5.0 +/- 1.0 pre-VS to 3.1 +/- 0.9 micromol/kg x min post-VS, P < .02). The improvement in fasting plasma glucose and HGO that occurred during VS treatment was maintained during the placebo phase. VS had no significant effects on rates of total-body glucose uptake, glycogen synthesis, glycolysis, carbohydrate (CHO) oxidation, or lipolysis during euglycemic-hyperinsulinemic clamps. We conclude that VS at the dose used was well tolerated and resulted in modest reductions of fasting plasma glucose and hepatic insulin resistance. However, the safety of larger doses and use of vanadium salts for longer periods remains uncertain.

Causal linkage between insulin suppression of lipolysis and suppression of liver glucose output in dogs

Suppression of hepatic glucose output (HGO) has been shown to be primarily mediated by peripheral rather than portal insulin concentrations; however, the mechanism by which peripheral insulin suppresses HGO has not yet been determined. Previous findings by our group indicated a strong correlation between free fatty acids (FFA) and HGO, suggesting that insulin suppression of HGO is mediated via suppression of lipolysis. To directly test the hypothesis that insulin suppression of HGO is causally linked to the suppression of adipose tissue lipolysis, we performed euglycemic-hyperinsulinemic glucose clamps in conscious dogs (n = 8) in which FFA were either allowed to fall or were prevented from falling with Liposyn plus heparin infusion (LI; 0.5 ml/min 20% Liposyn plus 25 U/min heparin with a 250 U prime). Endogenous insulin and glucagon were suppressed with somatostatin (1 microgram/min/kg), and insulin was infused at a rate of either 0.125 or 0.5 mU/min/kg. Two additional experiments were performed at the 0.5 mU/min/kg insulin dose: a double Liposyn infusion (2 x LI; 1.0 ml/min 20% Liposyn, heparin as above), and a glycerol infusion (19 mg/min). With the 0.125 mU/min/kg insulin infusion, FFA fell 40% and HGO fell 33%; preventing the fall in FFA with LI entirely prevented this decline in HGO. With 0.5 mU/min/kg insulin infusion, FFA levels fell 64% while HGO declined 62%. Preventing the fall in FFA at this higher insulin dose largely prevented the fall in HGO; however, steady state HGO still declined by 18%. Doubling the LI infusion did not further affect HGO, suggesting that the effect of FFA on HGO is saturable. Elevating plasma glycerol levels did not alter insulin's ability to suppress HGO. These data directly support the concept that insulin suppression of HGO is not direct, but rather is mediated via insulin suppression of adipose tissue lipolysis. Thus, resistance to insulin control of hepatic glucose production in obesity and/or non-insulin-dependent diabetes mellitus may reflect resistance of the adipocyte to insulin suppression of lipolysis.

Increased lipid oxidation but normal muscle glycogen response to epinephrine in humans with IDDM

The effects of physiological increments in epinephrine and insulin on glucose production (GP), skeletal muscle glycogen metabolism, and substrate oxidation were studied in eight insulin-dependent diabetes mellitus (IDDM) and nine control subjects. Epinephrine was coinfused for the final 120 min of a 240-min euglycemic, hyperinsulinemic clamp. In both groups, insulin increased glucose uptake, glycogen synthesis, and whole body carbohydrate (CHO) oxidation and inhibited GP (by 70-80%) and lipid oxidation (by approximately 50%), whereas epinephrine antagonized the effect of insulin on glucose uptake and glycogen synthesis. In contrast, GP increased in IDDM subjects (P < 0.02) but remained suppressed by insulin in controls. CHO oxidation fell (1.37 +/- 0.25 vs. 2.08 +/- 0.32 mg.kg-1.min-1) and lipid oxidation increased to baseline in IDDM subjects, with increments in plasma free fatty acids (FFA) and glycerol. In contrast, in controls, plasma FFA and glycerol remained suppressed and lipid oxidation decreased further with epinephrine (P < 0.005). Epinephrine completely reversed insulin's activation of muscle glycogen synthase in both groups. Thus, during hyperinsulinemia, the hepatic response to epinephrine in IDDM subjects may be dependent on activation of lipid oxidation. Skeletal muscle glycogen metabolism is exquisitely sensitive to epinephrine despite the presence of hyperinsulinemia.

Insulin-resistant lipolysis in abdominally obese hypertensive individuals. Role of the renin-angiotensin system

Resistance to the capacity of insulin to suppress lipolysis may be an important link in the association between abdominal obesity and hypertension. Furthermore, a more active renin-angiotensin system in adipose tissue may contribute to insulin-resistant lipolysis in abdominally obese hypertensive subjects. We determined nonesterified fatty acid concentrations and turnover as well as lipid oxidation under basal conditions and during steady-state euglycemia with two levels of insulinemia (72 and 287 pmol/L) in lean normotensive, abdominally obese normotensive, and abdominally obese hypertensive subjects. To assess the role of the renin-angiotensin system in determining non-esterified fatty acid turnover, we repeated studies in the abdominally obese hypertensive subjects after double-blind random assignment to placebo or enalapril for 1 month each. The main findings were the following: (1) Nonesterified fatty acid flux was significantly higher in abdominally obese hypertensive subjects at both levels of insulinemia than in either abdominally obese normotensive or lean normotensive subjects and correlated significantly with both mean blood pressure and total systemic resistance during the higher level of insulinemia. (2) Enalapril significantly improved insulin-resistant lipolysis in the abdominally obese hypertensive subjects. The improvement in insulin suppressibility of nonesterified fatty acid flux at the high hormonal concentrations correlated positively with the magnitude of reduction in blood pressure. (3) Basal lipid oxidation and suppression in response to insulin were similarly impaired in both obese groups. Resistance to the antilipolytic actions of insulin is thus a characteristic feature in abdominally obese hypertensive subjects and may be linked to the elevated blood pressure in these individuals. A more active renin-angiotensin system may partly explain the insulin-resistant lipolysis in this form of hypertension.