Role of Insulin and Free Fatty Acids in the Regulation of ob Gene Expression and Plasma Leptin in Normal Rats

Objective: It is under debate whether free fatty acids (FFAs) play an independent role in the regulation of adipose cell functions. In this study, we evaluated whether leptin secretion induced by FFA is due directly to an increased FFA availability or whether it is mediated by insulin levels. Research Methods and Procedures: To test this hypothesis, we compared the effects of six different experimental designs, with different FFA and insulin levels, on plasma leptin: euglycemic clamp, euglycemic clamp + FFA infusion, FFA infusion alone, FFA + somatostatin infusion, somatostatin infusion alone, and saline infusion. Results: Our results showed that euglycemic clamp, FFA infusion, or both in combination induced a similar increment of circulating leptin (3.31 ± 0.30, 3.40 ± 0.90, and 3.35 ± 0.80 ng/mL, respectively). Moreover, the inhibition of FFA‐induced insulin increase by means of somatostatin infusion completely abolished the rise of leptin in response to FFA (1.05 ± 0.30 vs. 3.40 ± 0.90 ng/mL, p < 0.001). Discussion: In conclusion, our data showed that the effects of high FFA levels on plasma leptin were mediated by the rise of insulin concentration. These data confirm a major role for insulin in the regulation of leptin secretion from rat adipose tissue and support the hypothesis that leptin secretion is coupled to net triglyceride synthesis in adipose tissue.     

Insulin secretion and sensitivity in acromegaly

To examine the effect of excess growth hormones on carbohydrate metabolism, we studied glucose-stimulated insulin secretion and glucose utilization in 6 patients with acromegaly and 6 age-, sex- and weight-matched normal subjects. The levels of plasma glucose and serum insulin were determined during fasting and every 30 min up to 180 min after 75 g of oral glucose loading. In addition, plasma glucose, serum insulin and serum C-peptide were measured during euglycemic glucose clamp with insulin infusion of 40 mU/m2,min-1. The acromegalic patients had significantly higher mean levels of fasting plasma glucose (p less than 0.05) and insulin (p less than 0.01). After glucose loading for 3 h, the acromegalic patients also had a higher incremental area under the curve of plasma glucose (p less than 0.05) and serum insulin (p less than 0.05). However, no significant difference in the fasting molar ratio of C-peptide/IRI was noted between these two groups. During euglycemic clamp studies, the steady-state serum insulin levels were identical between the two groups. The glucose disposal rate was lower in acromegalics than in normal subjects (p less than 0.01). The results demonstrated that glucose intolerance, hyperinsulinemia and insulin resistance are present in acromegalic patients.     

Ethnic Differences in the Responsiveness of Adipocyte Lipolytic Activity to Insulin

Objective: The goal of this study was to quantify differences in lipid metabolism and insulin sensitivity in black and white subjects to explain ethnic clinicopathological differences in type 2 diabetes. Research Methods and Procedures: The in vitro lipolytic activity of adipocytes isolated from obese black and white women was measured in the presence of insulin and isoproterenol. Insulin resistance was assessed in vivo using the euglycemic hyperinsulinemic clamp technique. Results: Fasting plasma levels of insulin and nonesterified fatty acid (NEFA) in black and white women were 67 ± 5 pM vs. 152 ± 20 pM (p < 0.01) and 863 ± 93 μM vs. 412 ± 34 μM (p < 0.01), respectively. Euglycemic hyperinsulinemic clamp studies showed that obese black subjects were more insulin‐resistant than their white counterparts (glucose infusion rates: 1.3 ± 0.2 vs. 2.2 ± 0.3 mg/kg per min; p < 0.05). Isolated adipocytes from white women were more responsive to insulin than those from black women with 0.7 nM insulin causing a 55 ± 4% inhibition of isoproterenol‐stimulated lipolysis compared with 27 ± 10% in black women (p < 0.05). Discussion: The low responsiveness of adipocyte lipolytic activity to insulin in black women in the presence of a relative insulinopenia may account for the high plasma NEFA levels seen in these women, which may, in turn, account for their higher in vivo insulin resistance. High NEFA levels may also contribute to the low insulin secretory activity observed in the obese black females. These data suggest that the pathogenesis of insulin resistance and type 2 diabetes within the black obese community is strongly influenced by their adipocyte metabolism.     

Free fatty acid-induced peripheral insulin resistance augments splanchnic glucose uptake in healthy humans

To investigate the effect of elevated plasma free fatty acid (FFA) concentrations on splanchnic glucose uptake (SGU), we measured SGU in nine healthy subjects (age, 44 +/- 4 yr; body mass index, 27.4 +/- 1.2 kg/m(2); fasting plasma glucose, 5.2 +/- 0.1 mmol/l) during an Intralipid-heparin (LIP) infusion and during a saline (Sal) infusion. SGU was estimated by the oral glucose load (OGL)-insulin clamp method: subjects received a 7-h euglycemic insulin (100 mU x m(-2) x min(-1)) clamp, and a 75-g OGL was ingested 3 h after the insulin clamp was started. After glucose ingestion, the steady-state glucose infusion rate (GIR) during the insulin clamp was decreased to maintain euglycemia. SGU was calculated by subtracting the integrated decrease in GIR during the period after glucose ingestion from the ingested glucose load. [3-(3)H]glucose was infused during the initial 3 h of the insulin clamp to determine rates of endogenous glucose production (EGP) and glucose disappearance (R(d)). During the 3-h euglycemic insulin clamp before glucose ingestion, R(d) was decreased (8.8 +/- 0.5 vs. 7.6 +/- 0.5 mg x kg(-1) x min(-1), P < 0.01), and suppression of EGP was impaired (0.2 +/- 0.04 vs. 0.07 +/- 0.03 mg x kg(-1) x min(-1), P < 0.01). During the 4-h period after glucose ingestion, SGU was significantly increased during the LIP vs. Sal infusion study (30 +/- 2 vs. 20 +/- 2%, P < 0.005). In conclusion, an elevation in plasma FFA concentration impairs whole body glucose R(d) and insulin-mediated suppression of EGP in healthy subjects but augments SGU.     

The pancreatic glucagon and C-peptide secretion during hyperinsulinemia in euglycemic glucose clamp with or without somatostatin infusion in normal man

6 normal subjects received two times of 2 hr euglycemic glucose clamp studies (insulin infusion rate 40 mU/M2/min) one with and the other without somatostatin (SRIF) infusion (500 microgram/hr). Serum C-peptide and glucagon levels were measured during clamp to study the sensitivity of pancreatic alpha and beta cells to the suppressive effects of exogenous hyperinsulinemia during normoglycemia in normal subjects and to find whether SRIF had any modulative effects on endocrine pancreas secretion at the status of hyperinsulinemia. The results showed that in normal man the degree of suppression of pancreatic glucagon secretion by hyperinsulinemia (approximately 100 uU/ml) during euglycemic glucose clamp without SRIF infusion was less than that of C-peptide with mean value of 62 +/- 4% of basal glucagon remained at the end of clamp study; while only about 30 +/- 2% of basal C-peptide concentrations remained. But during SRIF infused glucose clamp studies (SRIF was infused from 60 to 120 min), 32 +/- 2% of mean basal C-peptide concentrations and 38 +/- 6% of mean basal glucagon concentrations left at the end of 2 hr clamp studies when serum insulin level was about 100 uU/ml. For the glucose infusion rate (M value), it was significantly greater in our normal subjects in response to insulin + SRIF as compared to insulin alone (12.0 + 0.9 vs 8.8 +/- 1.4; P less than 0.01). We concluded: during hyperinsulinemia (100 uU/ml), the sensitivity of pancreatic alpha cells to insulin seems less than that of beta cells in normal man at normoglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Weight Loss Reduces Liver Fat and Improves Hepatic and Skeletal Muscle Insulin Sensitivity in Obese Adolescents

Obesity in adolescents is associated with metabolic risk factors for type 2 diabetes, particularly insulin resistance and excessive accumulation of intrahepatic triglyceride (IHTG). The purpose of this study was to evaluate the effect of moderate weight loss on IHTG content and insulin sensitivity in obese adolescents who had normal oral glucose tolerance. Insulin sensitivity, assessed by using the hyperinsulinemic–euglycemic clamp technique in conjunction with stable isotopically labeled tracer infusion, and IHTG content, assessed by using magnetic resonance spectroscopy, were evaluated in eight obese adolescents (BMI ≥95th percentile for age and sex; age 15.3 ± 0.6 years) before and after moderate diet‐induced weight loss (8.2 ± 2.0% of initial body weight). Weight loss caused a 61.6 ± 8.5% decrease in IHTG content (P = 0.01), and improved both hepatic (56 ± 18% increase in hepatic insulin sensitivity index, P = 0.01) and skeletal muscle (97 ± 45% increase in insulin‐mediated glucose disposal, P = 0.01) insulin sensitivity. Moderate diet‐induced weight loss decreases IHTG content and improves insulin sensitivity in the liver and skeletal muscle in obese adolescents who have normal glucose tolerance. These results support the benefits of weight loss therapy in obese adolescents who do not have evidence of obesity‐related metabolic complications during a standard medical evaluation.     

The euglycemic clamp in patients with thalassaemia intermedia

In order to evaluate the influence of haemosiderosis on the glucose metabolism we studied tissue sensitivity to insulin and the metabolic clearance rate (M.C.R.) of this hormone by means of euglycemic clamp technique using an artificial endocrine pancreas in 8 patients with thalassaemia intermedia and 8 control subjects. During the steady-state of euglycemic-hyperinsulinemic clamp (40 mU/m2/min) plasma insulin values were significantly lower and the insulin M.C.R. was significantly higher in thalassaemic patients compared to the controls. To achieve a comparable steady-state insulin concentration to the controls, we performed for a second time the euglycemic clamp in the thalassaemic patients increasing the insulin infusion rate to 80/mU/m2/min. The insulin M.C.R., the M index and the M/IRIs-s ratio were significantly higher in the thalassaemic patients compared to the controls. These results are indicative of an increased tissue peripheral sensitivity to insulin as well as the metabolic clearance rate of this hormone.     

Insulin secretion and glucose uptake in hypomagnesemic sheep fed a low magnesium, high potassium diet

Hyperglycemic and euglycemic clamp experiments were conducted to evaluate insulin secretion and glucose uptake in the hypomagnesemic sheep fed a low magnesium (Mg), high potassium (K) diet. Five mature sheep were fed a semipurified diet containing 0.24% Mg and 0.56% K (control diet) and five were fed 0.04% Mg and 3.78% K (low Mg/high K diet) for at least 2 weeks. In the hyperglycemic clamp experiment, plasma glucose concentrations were raised and maintained at a hyperglycemic steady-state (approximately 130 mg/100 ml) by variable rates of glucose infusion during the experimental period (120 minutes). The insulin response in the sheep fed the low Mg/high K diet (31.0 microU/ml) were significantly (P < 0.01) lower than those (111.7 microU/ml) of the sheep fed the control diet. In the euglycemic clamp experiment, insulin was infused at rates of 5, 10, 15, or 20 mU/kg/min, each followed by variable rates of glucose infusion to maintain a euglycemic steady-state (basal fasting levels). Hypomagnesemic sheep fed the low Mg/high K diet had significantly (P < 0.01) lower mean glucose disposal (3.72 mg/kg/min) across the insulin infusion rates compared with those of the sheep fed the control diet (5.37 mg/kg/min). These results suggest that glucose-induced insulin secretion and insulin-induced glucose uptake would be depressed in hypomagnesemic sheep and are caused by feeding the low Mg/high K diet.     

Discordant effects of glucosamine on insulin-stimulated glucose metabolism and phosphatidylinositol 3-kinase activity.

The impact of increased GlcN availability on insulin-stimulated p85/p110 phosphatidylinositol 3-kinase (PI3K) activity in skeletal muscle was examined in relation to GlcN-induced defects in peripheral insulin action. Primed continuous GlcN infusion (750 micromol/kg bolus; 30 micromol/kg.min) in conscious rats limited both maximal stimulation of muscle PI3K by acute insulin (I) (1 unit/kg) bolus (I + GlcN = 1.9-fold versus saline = 3.3-fold above fasting levels; p < 0.01) and chronic activation of PI3K following 3-h euglycemic, hyperinsulinemic (18 milliunits/kg.min) clamp studies (I + GlcN = 1.2-fold versus saline = 2.6-fold stimulation; p < 0.01). To determine the time course of GlcN-induced defects in insulin-stimulated PI3K activity and peripheral insulin action, GlcN was administered for 30, 60, 90, or 120 min during 2-h euglycemic, hyperinsulinemic clamp studies. Activation of muscle PI3K by insulin was attenuated following only 30 min of GlcN infusion (GlcN 30 min = 1.5-fold versus saline = 2.5-fold stimulation; p < 0.05). In contrast, the first impairment in insulin-mediated glucose uptake (Rd) developed following 110 min of GlcN infusion (110 min = 39.9 +/- 1.8 versus 30 min = 42.8 +/- 1.4 mg/kg.min, p < 0.05). However, the ability of insulin to stimulate phosphatidylinositol 3,4, 5-trisphosphate production and to activate glycogen synthase in skeletal muscle was preserved following up to 180 min of GlcN infusion. Thus, increased GlcN availability induced (a) profound and early inhibition of proximal insulin signaling at the level of PI3K and (b) delayed effects on insulin-mediated glucose uptake, yet (c) complete sparing of insulin-mediated glycogen synthase activation. The pattern and time sequence of GlcN-induced defects suggest that the etiology of peripheral insulin resistance may be distinct from the rapid and marked impairment in insulin signaling.     

Association of the -514C-->T polymorphism in the hepatic lipase gene (LIPC) promoter with elevated fasting insulin concentrations, but not insulin resistance, in non-diabetic Germans.

Hepatic lipase hydrolyses triglycerides and phospholipids in all major classes of lipoproteins. The -514C-->T genetic variation in the hepatic lipase gene promoter was found to be associated with diminished lipase activity, dyslipidemia, and atherosclerosis. We investigated whether this polymorphism associates with hyperinsulinemia and insulin resistance in 535 normal glucose-tolerant Germans. Only in homozygous individuals (22 subjects), the T allele (frequency: 18.1 %) was significantly associated with elevated glucose concentrations after 120 min of oral glucose tolerance test (p = 0.05) and with elevated fasting concentrations of insulin (p = 0.03), triglycerides (p < 0.01), total and HDL-cholesterol (p = 0.02), as determined by multivariate linear regression analysis. In a recessive model (C/C+C/T vs. T/T), T/T was associated with decreased insulin sensitivity index (p = 0.03) as calculated from oral glucose tolerance test data (n = 535), but not with the glucose infusion rate during hyperinsulinemic euglycemic clamp (n = 218). In conclusion, we have provided evidence that, among the metabolic parameters tested, the hepatic lipase -514C-->T gene polymorphism correlates with elevated fasting insulin concentrations in a German population. Since no corresponding difference in insulin sensitivity was seen in the clamp-subgroup, an effect of this polymorphism on insulin clearance has to be considered.     

Prior exercise and the response to insulin-induced hypoglycemia in the dog

To test whether hepatic insulin action and the response to an insulin-induced decrement in blood glucose are enhanced in the immediate postexercise state as they are during exercise, dogs had sampling (artery, portal vein, and hepatic vein) catheters and flow probes (portal vein and hepatic artery) implanted 16 days before a study. After 150 min of moderate treadmill exercise or rest, dogs were studied during a 150-min hyperinsulinemic (1 mU.kg(-1).min(-1)) euglycemic (n = 5 exercised and n = 9 sedentary) or hypoglycemic (65 mg/dl; n = 8 exercised and n = 9 sedentary) clamp. Net hepatic glucose output (NHGO) and endogenous glucose appearance (R(a)) and utilization (R(d)) were assessed with arteriovenous and isotopic ([3-(3)H]glucose) methods. Results show that, immediately after prolonged, moderate exercise, in relation to sedentary controls: 1) the glucose infusion rate required to maintain euglycemia, but not hypoglycemia, was higher; 2) R(d) was greater under euglycemic, but not hypoglycemic conditions; 3) NHGO, but not R(a), was suppressed more by a hyperinsulinemic euglycemic clamp, suggesting that hepatic glucose uptake was increased; 4) a decrement in glucose completely reversed the enhanced suppression of NHGO by insulin that followed exercise; and 5) arterial glucagon and cortisol were transiently higher in the presence of a decrement in glucose. In summary, an increase in insulin action that was readily evident under euglycemic conditions after exercise was abolished by moderate hypoglycemia. The means by which the glucoregulatory system is able to overcome the increase in insulin action during moderate hypoglycemia is related not to an increase in R(a) but to a reduction in insulin-stimulated R(d). The primary site of this reduction is the liver.     

Different effects of human and porcine insulin on hypoglycemia-induced abnormalities of brainstem sensory function

Following a switch from porcine insulin (PI) to human insulin (HI), a subgroup of diabetic patients complained of unawareness of hypoglycemia. In the present study, a glucose clamp technique was used to assess changes in auditory evoked brainstem responses (ABR) during infusion of HI and PI (0.015 IU/kg/min) under conditions of euglycemia (about 5.00 mM) and of hypoglycemia (3.40 and 2.60 mM) in 9 healthy volunteers. Serum insulin and plasma glucose did not differ between HI and PI conditions. ABR components remained unchanged during the euglycemic clamp, but increased in latency during hypoglycemia in all subjects. At mean glucose levels of 2.60 mM, the increase in latency of ABR wave V ranged between 50 and 400 microseconds during PI infusion, and between 100 and 2,460 microseconds during HI infusion. Thus, compared to the PI condition, changes during HI infusion were significantly more variable (p less than 0.01) due to some subjects displaying extremely prolonged ABR latencies. These findings suggest that hypoglycemia induced by HI can be more detrimental to early sensory processing in humans as compared to PI.     

Improvement of impaired postoperative insulin action by bradykinin

The effect of bradykinin on insulin-stimulated glucose metabolism was studied in 5 operated patients using the euglycemic insulin clamp technique and the forearm catheter technique. Insulin infusion [1.0 mU/(kg b.w. X min)] raised plasma insulin levels up to 73 muU/ml. Euglycemia was maintained by a computerized glucose infusion rate, amounting to 2.9 mg/(kg b.w. X min). Addition of bradykinin [1.5 micrograms/(kg b.w. X h)] resulted in a significant increase of the glucose infusion rate [+ 1.0 mg/(kg b.w. X min)] indicating elevated whole body glucose uptake. This was related to an enhanced forearm glucose uptake [+ 1.16 mumol/(100 g X min)]. Forearm blood flow remained stable.     

Glucose response to an oral glucose tolerance test predicts weight change in non-diabetic subjects

Objective: Glucose exerts a dual action in the regulation of energy balance, consisting of inhibition of energy intake and stimulation of energy expenditure. Whether blood glucose affects long‐term regulation of body weight in humans remains to be established. We sought to test the hypothesis that the post‐challenge glucose response is a predictor of weight change. Research Methods and Procedures: We performed a prospective analysis of the impact of glucose response to an oral glucose tolerance test (OGTT) and a mixed‐meal test (MT) on subsequent changes in body weight (BW) on 253 Pima Indians (166 men and 87 women) with normal glucose regulation at baseline and follow‐up (follow‐up: 7 ± 4 years). Main outcome measures included BW change (total, percent, and annual), plasma glucose and insulin concentrations during OGTT and MT [total and incremental areas under the curve (AUCs)], resting metabolic rate (RMR; indirect calorimetry), and insulin action (euglycemic‐hyperinsulinemic clamp). Results: Total and incremental glucose AUCs during the OGTT (but not the MT) were negatively associated with BW change (total, percent, and annual), both before and after adjusting for sex, age, initial BW, follow‐up time, insulin action, RMR, fasting plasma glucose and insulin concentrations, and insulin response. Total and incremental glucose AUCs during the OGTT were independent determinants of final BW with age, initial BW, follow‐up time, fasting plasma insulin concentrations, and RMR. Discussion: Higher post‐challenge glucose response protects against BW gain in subjects with normal glucose regulation. We propose that this action may be because of the effect of glucose on food intake and/or thermogenesis.     

Long-term mild jogging increases insulin action despite no influence on body mass index or VO2 max

Physical training has been shown to improve glucose tolerance and insulin sensitivity. In the present study, insulin action was determined using the euglycemic clamp technique in six untrained nonobese subjects before, during, and after long-term mild regular jogging. After 1 yr of jogging, steady-state plasma insulin levels (I) decreased significantly, and the metabolic clearance rate of insulin was increased by 87%, although insulin infusion rate during the clamp was constant for each individual. The amount of glucose infused (glucose metabolism, M) tended to increase from 6.16 +/- 0.94 to 8.15 +/- 1.94 mg.kg-1.min-1 after regular jogging for 1 yr, although that was not statistically significant. However, M/I increases significantly from 0.060 +/- 0.012 to 0.184 +/- 0.056 (P less than 0.05) after 1 yr. The concentrations of plasma free fatty acids during the hyperinsulinemic clamp decreased more significantly after 1 yr of jogging (P less than 0.05). The concentrations of plasma glycerol decreased gradually before and after long-term regular jogging, showing only a 50-60% reduction in 120 min. Therefore, long-term mild regular jogging, which did not influence either body mass index or maximal O2 uptake, appears to improve insulin action in both carbohydrate and lipid metabolism and to increase the metabolic clearance rate of insulin.     

Urinary C‐Peptide Excretion: A Novel Alternate Measure of Insulin Sensitivity in Physiological Conditions

Insulin sensitivity (IS) is measured by the euglycemic–hyperinsulinemic clamp under a nonphysiological condition. Daily C‐peptide urinary excretion may be a physiological index of IS, because C‐peptide is co‐secreted with insulin as a function of nutrient intake and IS. The amount of ²H₂O released from glycolytic glucose metabolism after [6,6‐²H₂]‐glucose ingestion was recently proposed as a physiological measure of IS. We compared these IS surrogates to the gold standard (euglycemic–hyperinsulinemic clamp). Thirty (15 male/15 female) sedentary, nondiabetic participants (27.2 ± 4.0 (s.d.) kg/m², 35 ± 12 years) were admitted for 3 days to our in‐patient unit. After a 10‐h fast, they received 60 g of glucose and 15 g of [6,6‐²H₂]‐glucose. Before glucose ingestion and hourly thereafter for 4 h, plasma glucose and insulin concentrations, and plasma deuterium enrichment were determined. Plasma ²H₂O production divided by insulin response was used as the glycolytic index. On day 2, subjects spent 23 h in a metabolic chamber (eucaloric diet, 50% carbohydrate, 30% fat). Urinary C‐peptide excretion was divided by energy intake yielding the C‐peptide to energy intake ratio (CPEP/EI). After leaving the chamber (day 3, 10‐h fast), IS was measured by a 2‐h clamp (120 mU/m²/min). Average IS (clamp) was 7.3 ± 2.6 mg glucose/kg estimated metabolic body size/min (range: 3.6–13.2). These values were inversely correlated with CPEP/EI (r = ?0.62; P < 0.01) and positively with the glycolytic rate (r = 0.60; P < 0.01). In nondiabetic subjects, two novel estimates of IS—daily urinary C‐peptide urinary excretion and glycolytic rate during an oral glucose tolerance test —were related to IS by a clamp.     

Muscle Type‐Dependent Responses to Insulin in Intramyocellular Triglyceride Turnover in Obese Rats

Objective: To understand the role of hyperinsulinemia in intramyocellular (imc) triglyceride (TG) accumulation and in regulating imcTG turnover. Research Methods and Procedures: imcTG was first prelabeled by continuous infusion of [U‐¹⁴C]glycerol (pulse), and then the rate of label loss from the prelabeled imcTG pool (turnover) in gastrocnemius, tibialis anterior, and soleus muscle of awake, high‐fat‐fed obese rats during the subsequent hyperinsulinemic‐euglycemic clamp experiments (chase) was determined. Results: Post‐absorptive basal fractional imcTG turnover rate in soleus was 0.010 ± 0.001/min, significantly lower than that in gastrocnemius (0.026 ± 0.002/min, p < 0.001) or tibialis anterior (0.030 ± 0.002/min, p < 0.0001), a pattern reciprocal to their imcTG pool size. Insulin infusion at 25 pmol/kg per minute resulted in pathophysiological hyperinsulinemia (5‐fold increase over the baseline value). This caused an increase in imcTG turnover by 3‐fold in soleus (0.029 ± 0.006/min, p = 0.002) but a decrease in gastrocnemius (0.012 ± 0.003/min, p = 0.001) and in tibialis anterior (0.0064 ± 0.001/min, p < 0.0001). Pathophysiological hyperinsulinemia suppressed hormone‐sensitive lipase activity in heart (p = 0.01) and mesenteric fat (p = 0.05) but not in skeletal muscle (p > 0.05). The pool size of imcTG was not affected by hyperinsulinemia. Discussion: The results demonstrated muscle‐type dependence in the response of imcTG turnover to hyperinsulinemia in the obesity model. The reciprocal insulin effects on imcTG turnover in oxidative vs. oxidative‐glycolytic muscle indicated a possibility that oxidative muscle contributes more to insulin resistance under hyperinsulinemia if imcTG‐fatty acid oxidation is a function of turnover. imcTG turnover does not seem to regulate imcTG pool size acutely.     

Altered Intramuscular Lipid Metabolism Relates to Diminished Insulin Action in Men,but Not Women,in Progression to Diabetes

Whether sex differences in intramuscular triglyceride (IMTG) metabolism underlie sex differences in the progression to diabetes are unknown. Therefore, the current study examined IMTG concentration and fractional synthesis rate (FSR) in obese men and women with normal glucose tolerance (NGT) vs. those with prediabetes (PD). PD (n = 13 men and 7 women) and NGT (n = 7 men and 12 women) groups were matched for age and anthropometry. Insulin action was quantified using a hyperinsulinemic‐euglycemic clamp with infusion of [6,6^?2H₂]‐glucose. IMTG concentration was measured by gas chromatography/mass spectrometry (GC/MS) and FSR by GC/combustion isotope ratio MS (C‐IRMS), from muscle biopsies taken after infusion of [U^?13C]palmitate during 4 h of rest. In PD men, the metabolic clearance rate (MCR) of glucose was lower during the clamp (4.71 ± 0.77 vs. 8.62 ± 1.26 ml/kg fat‐free mass (FFM)/min, P = 0.04; with a trend for lower glucose rate of disappearance (Rd), P = 0.07), in addition to higher IMTG concentration (41.2 ± 5.0 vs. 21.2 ± 3.4 µg/mg dry weight, P ≤ 0.01), lower FSR (0.21 ± 0.03 vs. 0.42 ± 0.06 %/h, P ≤ 0.01), and lower oxidative capacity (P = 0.03) compared to NGT men. In contrast, no difference in Rd, IMTG concentration, or FSR was seen in PD vs. NGT women. Surprisingly, glucose Rd during the clamp was not different between NGT men and women (P = 0.25) despite IMTG concentration being higher (42.6 ± 6.1 vs. 21.2 ± 3.4 µg/mg dry weight, P = 0.03) and FSR being lower (0.23 ± 0.04 vs. 0.42 ± 0.06 %/h, P = 0.02) in women. Alterations in IMTG metabolism relate to diminished insulin action in men, but not women, in the progression toward diabetes.     

Lipid‐induced Insulin Resistance Is Associated With Increased Monocyte Expression of Scavenger Receptor CD36 and Internalization of Oxidized LDL

Elevated free fatty acids (FFAs) contribute to the development of insulin resistance, type 2 diabetes mellitus (T2DM), and may be atherogenic. We tested the relationship among lipid‐induced insulin resistance, endothelial dysfunction, and monocyte capacity to form foam cells through scavenger receptor A (SRA) and CD36. Ten healthy subjects underwent 24‐h infusion of Intralipid/heparin and saline (0.5 ml/min) on two separate occasions followed by brachial artery reactivity testing and a euglycemic hyperinsulinemic (80 mU/(kg·min)) clamp study to determine insulin sensitivity. Isolation of blood monocytes was performed 24 h after infusion. Surface expression and function of CD36 and SRA to take up oxidized low‐density lipoprotein (oxLDL) was determined by flow cytometry and quantitative confocal imaging. Lipid infusion resulted in a twofold increase in serum FFA levels, reduced whole‐body glucose disposal by ～20% (P < 0.05), and possibly impaired endothelial‐dependent vasodilation (P = 0.1). Blood monocytes obtained during lipid infusion demonstrated a ～25% increase in cell surface expression of CD36 (P < 0.05) but no change in SRA expression. Enhanced CD36 expression was associated with a 50% increase in internalization of oxLDL (P < 0.05). The increase in CD36 surface expression during lipid infusion correlated inversely with glucose disposal (P < 0.05) and not with FFA levels or brachial artery dilation. These data support a role for FFAs in induction of insulin resistance and provide a link to atherogenic mechanisms mediated by expression of scavenger receptor CD36.     

A high-fructose diet impairs Akt and PKCzeta phosphorylation and GLUT4 translocation in rat skeletal muscle.

The molecular mechanism of insulin resistance induced by high-fructose feeding is not fully understood. The present study investigated the role of downstream signaling molecules of phosphatidylinositol 3-kinase (PI3K) in the insulin-stimulated skeletal muscle of high-fructose-fed rats. Rats were divided into chow-fed and fructose-fed groups. The results of the euglycemic clamp study (insulin infusion rates: 6 mU/kg BW/min) showed a significant decrease in the glucose infusion rate (GIR) and the metabolic clearance rate of glucose (MCR) in fructose-fed rats compared with chow-fed rats. In skeletal muscle removed immediately after the clamp procedure, high-fructose feeding did not alter protein levels of protein kinase B (PKB/Akt), protein kinase C zeta (PKCzeta), or glucose transporter 4 (GLUT4). However, insulin-stimulated phosphorylation of Akt and PKCzeta and GLUT4 translocation to the plasma membrane were reduced. Our findings suggest that insulin resistance in fructose-fed rats is associated with impaired Akt and PKCzeta activation and GLUT4 translocation in skeletal muscle.