Serum leptin concentrations in young insulin-sensitive and insulin-resistant volunteers.

The aim of this study was to compare metabolic profiles and serum leptin concentrations between young insulin-sensitive and insulin-resistant subjects. A cross-sectional study was performed in 32 healthy, non-obese, young volunteers. Assessing of insulin sensitivity, serum leptin concentration, serum uric acid, creatinine levels and lipid profile were done on all subjects. An insulin suppression test modified with octreotide was performed to assess insulin sensitivity. Steady state glucose (SSG) and steady state insulin concentrations were calculated. Based on the SSG data, the volunteers were divided into four quartiles, considering as insulin-sensitive individuals those from quartile 1 to quartile 3, and insulin-resistant subjects those in quartile 4. Characteristics of both groups were compared, including metabolic profile and leptin levels. After dividing SSG into quartiles, 24 subjects were considered as insulin-sensitive individuals, and 8 were assessed as insulin-resistant subjects. Total cholesterol and low-density lipoprotein (LDL) cholesterol were significantly higher in the insulin-resistant group than in the insulin-sensitive group. Serum leptin concentration was significantly higher (p=0.05) in insulin-resistant women (6.1 +/- 3.1 ng/ml) than those considered as insulin-sensitive (3.7 +/- 2.3 ng/ml). In conclusion, insulin-resistant subjects had higher concentrations of total cholesterol and LDL-cholesterol compared to insulin-sensitive individuals. Serum leptin level was higher in insulin-resistant women than those considered as insulin-sensitive.     

Tetrahydrobiopterin increases insulin sensitivity in patients with type 2 diabetes and coronary heart disease

Tetrahydrobiopterin (BH(4)) is an essential cofactor of nitric oxide synthase that improves endothelial function in diabetics, smokers, and patients with hypercholesterolemia. Insulin resistance has been suggested as a contributing factor in the development of endothelial dysfunction via an abnormal pteridine metabolism. We hypothesized that BH(4) would restore flow-mediated vasodilation (FMD, endothelial-dependent vasodilation), which may affect insulin resistance in type 2 diabetic patients. Thirty-two subjects (12 type 2 diabetic subjects, 10 matched nondiabetic subjects, and 10 healthy unmatched subjects) underwent infusion of BH(4) or saline in a random crossover study. Insulin sensitivity index (S(I)) was measured by hyperinsulinemic isoglycemic clamp. FMD was measured using ultrasonography. BH(4) significantly increased S(I) in the type 2 diabetics [3.6 +/- 0.6 vs. 4.9 +/- 0.7 x 10(-4) dl.kg(-1).min(-1)/(microU/ml), P < 0.05], while having no effects in nondiabetics [8.9 +/- 1.1 vs. 9.0 +/- 0.9 x 10(-4) dl.kg(-1).min(-1)/(microU/ml), P = 0.92] or in healthy subjects [17.5 +/- 1.6 vs. 18 +/- 1.8 x 10(-4) dl.kg(-1).min(-1)/(microU/ml), P = 0.87]. BH(4) did not affect the relative changes in brachial artery diameter from baseline FMD (%) in type 2 diabetic subjects (2.3 +/- 0.8 vs. 1.8 +/- 1.0%, P = 0.42), nondiabetic subjects (5.3 +/- 1.1 vs. 6.6 +/- 0.9%, P = 0.32), or healthy subjects (11.9 +/- 0.6 vs. 11.0 +/- 1.0%, P = 0.48). In conclusion, BH(4) significantly increases insulin sensitivity in type 2 diabetic patients without any discernible improvement in endothelial function.     

Postprandial endothelial function does not differ in women by race: an insulin resistance paradox?

Insulin resistance is associated with endothelial dysfunction. Because African-American women are more insulin-resistant than white women, it is assumed that African-American women have impaired endothelial function. However, racial differences in postprandial endothelial function have not been examined. In this study, we test the hypothesis that African-American women have impaired postprandial endothelial function compared with white women. Postprandial endothelial function following a breakfast (20% protein, 40% fat, and 40% carbohydrate) was evaluated in 36 (18 African-American women, 18 white women) age- and body mass index (BMI)-matched (age: 37 ± 11 yr; BMI: 30 ± 6 kg/m(2)) women. Endothelial function, defined by percent change in brachial artery flow-mediated dilation (FMD), was measured at 0, 2, 4, and 6 h following a meal. There were no significant differences between the groups in baseline FMD, total body fat, abdominal visceral fat, and fasting levels of glucose, insulin, total cholesterol, low-density lipoprotein cholesterol, or serum estradiol. Although African-American women were less insulin-sensitive [insulin sensitivity index (mean ± SD): 3.6 ± 1.5 vs. 5.2 ± 2.6, P = 0.02], both fasting triglyceride (TG: 56 ± 37 vs. 97 ± 49 mg/dl, P = 0.007) and incremental TG area under the curve (AUC(0-6hr): 279 ± 190 vs. 492 ± 255 mg·dl(-1)·min(-1)·10(-2), P = 0.008) were lower in African-American than white women. Breakfast was associated with a significant increase in FMD in whites and African-Americans, and there was no significant difference in postprandial FMD between the groups (P > 0.1 for group × time interactions). Despite being insulin-resistant, postprandial endothelial function in African-American women was comparable to white women. These results imply that insulin sensitivity may not be an important determinant of racial differences in endothelial function.     

Effects of hyperinsulinemia on hepatic metalloproteinases and their tissue inhibitors

To gain insight into the pathogenesis of hepatic fibrosis related to insulin resistance, we have examined the effects of euglycemic hyperinsulinemia on three matrix metalloproteinases (MMP-2, MMP-9, and MT1-MMP) and on two major tissue inhibitors of MMPs (TIMP-1 and TIMP-2) in liver of insulin-sensitive and insulin-resistant rats. Four hours of insulin infusion (4.8 mU.kg(-1).min(-1)) without or with lipid-heparin infusion (to produce insulin resistance) decreased hepatic MMP-2 mRNA (by RT-PCR), pro-MMP-2, MMP-2, MMP-9, and MT1-MMP (all by Western blots) and the gelatinolytic activity of MMP-2 (by gelatin zymography) by approximately 60-80%. Hyperinsulinemia ( approximately 1.6 mmol/l) increased TIMP-1 and TIMP-2 concentrations (by ELISA) in insulin-sensitive and insulin-resistant rats. Phosphoinositide 3-kinase was activated by insulin in insulin-sensitive rats and inhibited in insulin-resistant rats. Extracellular signal-regulated kinases 1/2 (ERK1/2) were activated by insulin in insulin-sensitive rats and partially inhibited in insulin-resistant rats; c-jun NH(2)-terminal kinase-1 (JNK1), JNK2/3, or p38 MAPK were only activated by lipid but not by insulin. We conclude that hyperinsulinemia, whether or not associated with insulin resistance, shifts the MMP/TIMP balance toward reduction of extracellular matrix degradation and thus may promote the development of hepatic fibrosis.     

Skeletal muscle cells from insulin-resistant (non-diabetic) individuals are susceptible to insulin desensitization by palmitate.

We recently demonstrated that in vivo insulin resistance is not retained in cultured skeletal muscle cells. In the present study, we tested the hypothesis that treating cultured skeletal muscle cells with fatty acids has an effect on insulin action which differs between insulin-sensitive and insulin-resistant subjects. Insulin effects were examined in myotubes from 8 normoglycemic non-obese insulin-resistant and 8 carefully matched insulin-sensitive subjects after preincubation with or without palmitate, linoleate, and 2-bromo-palmitate. Insulin-stimulated glycogen synthesis decreased by 27 +/- 5 % after palmitate treatment in myotubes from insulin-resistant, but not from insulin-sensitive subjects (1.50 +/- 0.08-fold over basal vs. 1.81 +/- 0.09-fold, p = 0.042). Despite this observation, we did not find any impairment in the PI 3-kinase/PKB/GSK-3 pathway. Furthermore, insulin action was not affected by linoleate and 2-bromo-palmitate. In conclusion, our data provide preliminary evidence that insulin resistance of skeletal muscle does not necessarily involve primary defects in insulin action, but could represent susceptibility to the desensitizing effect of fatty acids and possibly other environmental or adipose tissue-derived factors.     

Insulin effects on acetate metabolism

Acetate metabolism was studied in patients with insulin resistance. To evaluate the interaction between glucose and acetate metabolism, we measured acetate and glucose turnover with a hyperinsulinemic euglycemic clamp (hot clamp) in obese and diabetic patients with insulin resistance (n = 8) and in a control group with normal insulin sensitivity (n = 6). At baseline, acetate turnover and plasma concentrations were similar between the two groups (group means: 4.3 +/- 0.4 micromol x kg-1 x min-1 and 128.2 +/- 11.1 micromol/l). Acetate concentrations decreased in both groups with hyperinsulinemia but were significantly lower in the insulin-resistant group (20% vs. 12%, P < 0.05). After the hot clamp treatment, acetate turnover increased for the two groups and was higher in the group with normal insulin sensitivity: 8.1 +/- 0.7 vs. 5.5 +/- 0.5 micromol x kg-1 x min-1 (P < 0.001). No change related to insulin action was observed in either group in the percentage of acetate oxidation. This was approximately 70% of overall utilization at baseline and during the clamp. No correlation between glucose and acetate utilization was observed. Our results support the hypothesis that, like glucose metabolism, acetate metabolism is sensitive to insulin.     

Relationship between changes in brachial artery flow-mediated dilation and basal release of nitric oxide in subjects with Type 2 diabetes

Assessment of flow-mediated dilation (FMD) after forearm ischemia is widely used as a noninvasive bioassay of stimulated nitric oxide (NO)-mediated conduit artery vasodilator function in vivo. Whether this stimulated endothelial NO function reflects basal endothelial NO function is unknown. To test this hypothesis, retrospective analysis of randomized crossover studies was undertaken in 17 subjects with Type 2 diabetes; 9 subjects undertook an exercise training or control period, whereas the remaining 8 subjects were administered an angiotensin II receptor blocker or placebo. FMD was assessed by using wall tracking of high-resolution brachial artery ultrasound images in response to reactive hyperemia. Resistance vessel basal endothelium-dependent NO function was assessed by using intrabrachial administration of NG-monomethyl-L-arginine (L-NMMA) and plethysmographic assessment of forearm blood flow (FBF). FMD was higher after intervention compared with control/placebo (6.15+/-0.53 vs. 3.81+/-0.72%, P<0.001). There were no significant changes in the FBF responses to L-NMMA. Regression analysis between FMD and L-NMMA responses at entry to the study revealed an insignificant correlation (r=-0.10, P=0.7), and improvements in FMD with the interventions were not associated with changes in the L-NMMA responses (r=-0.04, P=0.9). We conclude that conduit artery-stimulated endothelial NO function (FMD) does not reflect basal resistance vessel endothelial NO function in subjects with Type 2 diabetes.     

Long-term high-fat feeding leads to severe insulin resistance but not diabetes in Wistar rats

Although lipid excess can impair beta-cell function in vitro, short-term high-fat feeding in normal rats produces insulin resistance but not hyperglycemia. This study examines the effect of long-term (10-mo) high polyunsaturated fat feeding on glucose tolerance in Wistar rats. The high fat-fed compared with the chow-fed group was 30% heavier and 60% fatter, with approximately doubled fasting hyperinsulinemia (P < 0.001) but only marginal fasting hyperglycemia (7.5 +/- 0.1 vs. 7.2 +/- 0.1 mmol/l, P < 0.01). Insulin sensitivity was approximately 67% lower in the high-fat group (P < 0.01). The acute insulin response to intravenous arginine was approximately double in the insulin-resistant high-fat group (P < 0.001), but that to intravenous glucose was similar in the two groups. After the intravenous glucose bolus, plasma glucose decline was slower in the high fat-fed group, confirming mild glucose intolerance. Therefore, despite severe insulin resistance, there was only a mildly elevated fasting glucose level and a relative deficiency in glucose-stimulated insulin secretion; this suggests that a genetic or congenital susceptibility to beta-cell impairment is required for overt hyperglycemia to develop in the presence of severe insulin resistance.     

Increased insulin receptor signaling and glycogen synthase activity contribute to the synergistic effect of exercise on insulin action.

The purpose of this study was to determine the factors contributing to the ability of exercise to enhance insulin-stimulated glucose disposal. Sixteen insulin-resistant nondiabetic and seven Type 2 diabetic subjects underwent two hyperinsulinemic (40 mU x m-2 x min-1) clamps, once without and once with concomitant exercise at 70% peak O2 consumption. Exercise was begun at the start of insulin infusion and was performed for 30 min. Biopsies of the vastus lateralis were performed before and after 30 min of insulin infusion (immediately after cessation of exercise). Exercise synergistically increased insulin-stimulated glucose disposal in nondiabetic [from 4.6 +/- 0.4 to 9.5 +/- 0.8 mg x kg fat-free mass (FFM)-1x min-1] and diabetic subjects (from 4.3 +/- 1.0 to 7.9 +/- 0.7 mg. kg FFM-1x min-1) subjects. The rate of glucose disposal also was significantly greater in each group after cessation of exercise. Exercise enhanced insulin-stimulated increases in glycogen synthase fractional velocity in control (from 0.07 +/- 0.02 to 0.22 +/- 0.05, P < 0.05) and diabetic (from 0.08 +/- 0.03 to 0.15 +/- 0.03, P < 0.01) subjects. Exercise also enhanced insulin-stimulated glucose storage (glycogen synthesis) in nondiabetic (2.9 +/- 0.9 vs. 4.9 +/- 1.1 mg x kg FFM-1x min-1) and diabetic (1.7 +/- 0.5 vs. 4.2 +/- 0.8 mg x kg FFM-1. min-1) subjects. Increased glucose storage accounted for the increase in whole body glucose disposal when exercise was performed during insulin stimulation in both groups; effects of exercise were correlated with enhancement of glucose disposal and glucose storage (r = 0.93, P < 0.001). Exercise synergistically enhanced insulin-stimulated insulin receptor substrate 1-associated phosphatidylinositol 3-kinase activity (P < 0.05) and Akt Ser473 phosphorylation (P < 0.05) in nondiabetic subjects but had little effect in diabetic subjects. The data indicate that exercise, performed in conjunction with insulin infusion, synergistically increases insulin-stimulated glucose disposal compared with insulin alone. In nondiabetic and diabetic subjects, increased glycogen synthase activation is likely to be involved, in part, in this effect. In nondiabetic, but not diabetic, subjects, exercise-induced enhancement of insulin stimulation of the phosphatidylinositol 3-kinase pathway is also likely to be involved in the exercise-induced synergistic enhancement of glucose disposal.     

Effects of glucagon-like peptide-1 on endothelial function in type 2 diabetes patients with stable coronary artery disease

GLP-1 stimulates insulin secretion, suppresses glucagon secretion, delays gastric emptying, and inhibits small bowel motility, all actions contributing to the anti-diabetogenic peptide effect. Endothelial dysfunction is strongly associated with insulin resistance and type 2 diabetes mellitus and may cause the angiopathy typifying this debilitating disease. Therefore, interventions affecting both endothelial dysfunction and insulin resistance may prove useful in improving survival in type 2 diabetes patients. We investigated GLP-1's effect on endothelial function and insulin sensitivity (S(I)) in two groups: 1) 12 type 2 diabetes patients with stable coronary artery disease and 2) 10 healthy subjects with normal endothelial function and S(I). Subjects underwent infusion of recombinant GLP-1 or saline in a random crossover study. Endothelial function was measured by postischemic FMD of brachial artery, using ultrasonography. S(I) [in (10(-4) dl.kg(-1).min(-1))/(muU/ml)] was measured by hyperinsulinemic isoglycemic clamp technique. In type 2 diabetic subjects, GLP-1 infusion significantly increased relative changes in brachial artery diameter from baseline FMD(%) (3.1 +/- 0.6 vs. 6.6 +/- 1.0%, P < 0.05), with no significant effects on S(I) (4.5 +/- 0.8 vs. 5.2 +/- 0.9, P = NS). In healthy subjects, GLP-1 infusion affected neither FMD(%) (11.9 +/- 0.9 vs. 10.3 +/- 1.0%, P = NS) nor S(I) (14.8 +/- 1.8 vs. 11.6 +/- 2.0, P = NS). We conclude that GLP-1 improves endothelial dysfunction but not insulin resistance in type 2 diabetic patients with coronary heart disease. This beneficial vascular effect of GLP-1 adds yet another salutary property of the peptide useful in diabetes treatment.     

Postprandial dyslipidemia in men with visceral obesity: an effect of reduced LDL receptor expression?

Postprandial lipemia after an oral fat challenge was studied in middle-aged men with visceral obesity. The two groups had similar plasma cholesterol levels, but obese subjects had higher levels of plasma triglyceride and reduced amounts of high-density cholesterol. Fasting plasma insulin was fourfold greater in obese subjects because of concomitant insulin resistance, with a calculated HOMA score of 3.1 +/- 0.6 vs. 0.8 +/- 0.2, respectively. Plasma apolipoprotein B(48) (apoB(48)) and retinyl palmitate (RP) after an oral fat challenge were used to monitor chylomicron metabolism. Compared with lean subjects, the fasting concentration of apoB(48) was more than twofold greater in obese individuals, suggestive of an accumulation of posthydrolyzed particles. After the oral lipid load, the incremental areas under the apoB(48) and RP curves (IAUC) were both significantly greater in obese subjects (apoB(48): 97 +/- 17 vs. 44 +/- 12 microg.ml(-1). h; RP: 3,120 +/- 511 vs. 1,308 +/- 177 U. ml(-1). h, respectively). A delay in the conversion of chylomicrons to remnants probably contributed to postprandial dyslipidemia in viscerally obese subjects. The triglyceride IAUC was 68% greater in obese subjects (4.7 +/- 0.6 vs. 2.8 +/- 0.8 mM. h, P < 0.06). Moreover, peak postprandial triglyceride was delayed by approximately 2 h in obese subjects. The reduction in triglyceride lipolysis in vivo did not appear to reflect changes in hydrolytic enzyme activities. Postheparin plasma lipase rates were found to be similar for lean and obese subjects. In this study, low-density lipoprotein (LDL) receptor expression on monunuclear cells was used as a surrogate marker of hepatic activity. We found that, in obese subjects, the binding of LDL was reduced by one-half compared with lean controls (70.9 +/- 15.07 vs. 38.9 +/- 4.6 ng LDL bound/microg cell protein, P = 0.02). Because the LDL receptor is involved in the removal of proatherogenic chylomicron remnants, we suggest that the hepatic clearance of these particles might be compromised in insulin-resistant obese subjects. Premature and accelerated atherogenesis in viscerally obese, insulin-resistant subjects may in part reflect delayed clearance of postprandial lipoprotein remnants.     

Estimations of muscle interstitial insulin, glucose, and lactate in type 2 diabetic subjects

Previous measurement of insulin in human muscle has shown that interstitial muscle insulin and glucose concentrations are approximately 30-50% lower than in plasma during hyperinsulinemia in normal subjects. The aims of this study were to measure interstitial muscle insulin and glucose in patients with type 2 diabetes to evaluate whether transcapillary transport is part of the peripheral insulin resistance. Ten patients with type 2 diabetes and ten healthy controls matched for sex, age, and body mass index were investigated. Plasma and interstitial insulin, glucose, and lactate (measured by intramuscular in situ-calibrated microdialysis) in the medial quadriceps femoris muscle were analyzed during a hyperinsulinemic euglycemic clamp. Blood flow in the contralateral calf was measured by vein plethysmography. At steady-state clamping, at 60-120 min, the interstitial insulin concentration was significantly lower than arterial insulin in both groups (409 +/- 86 vs. 1,071 +/- 99 pmol/l, P < 0.05, in controls and 584 +/- 165 vs. 1, 253 +/- 82 pmol/l, P < 0.05, in diabetic subjects, respectively). Interstitial insulin concentrations did not differ significantly between diabetic subjects and controls. Leg blood flow was significantly higher in controls (8.1 +/- 1.2 vs. 4.4 +/- 0.7 ml. 100 g(-1).min(-1) in diabetics, P < 0.05). Calculated glucose uptake was less in diabetic patients compared with controls (7.0 +/- 1.2 vs. 10.8 +/- 1.2 micromol. 100 g(-1).min(-1), P < 0.05, respectively). Arterial and interstitial lactate concentrations were both higher in the control group (1.7 +/- 0.1 vs. 1.2 +/- 0.1, P < 0. 01, and 1.8 +/- 0.1 vs. 1.2 +/- 0.2 mmol/l, P < 0.05, in controls and diabetics, respectively). We conclude that, during hyperinsulinemia, muscle interstitial insulin and glucose concentrations did not differ between patients with type 2 diabetes and healthy controls despite a significantly lower leg blood flow in diabetic subjects. It is suggested that decreased glucose uptake in type 2 diabetes is caused by insulin resistance at the cellular level rather than by a deficient access of insulin and glucose surrounding the muscle cell.     

Insulin regulation of MCP-1 in human adipose tissue of obese and lean women

CCL2 (MCP-1, monocyte chemoattractant protein 1) and CCL3 (MIP-1alpha, macrophage inflammatory protein 1alpha) are required for macrophage infiltration in adipose tissue. Insulin increases CCL2 expression in adipose tissue and in serum more in insulin-resistant obese than in insulin-sensitive lean mice, but whether this is true in humans is unknown. We compared basal expression and insulin regulation of CCL2 and CCL3 in adipose tissue and MCP-1 and MIP-1alpha in serum between insulin-resistant and insulin-sensitive human subjects. Subcutaneous adipose tissue biopsies and blood samples were obtained before and at the end of 6 h of in vivo euglycemic hyperinsulinemia (maintained by the insulin clamp technique) in 11 lean insulin-sensitive and 10 obese insulin-resistant women, and before and after a 6-h saline infusion in 8 women. Adipose tissue mRNA concentrations of monocyte/macrophage markers CD68, EMR1, ITGAM, ADAM8, chemokines CCL2 and CCL3, and housekeeping gene ribosomal protein large P0 (RPLP0) were measured by means of real-time PCR at baseline. In addition, mRNA concentrations of CCL2, CCL3, and RPLP0 were measured after insulin infusion. Levels of MCP-1 and MIP-1alpha were determined in serum, and protein concentration of MCP-1 was determined in adipose tissue at baseline and after insulin infusion. Basally, expression of the macrophage markers CD68 and EMR1 were increased in adipose tissue of insulin-resistant subjects. Insulin increased MCP-1 gene and protein expression significantly more in the insulin-resistant than in the insulin-sensitive subjects. Basally expression of CCL2 and CCL3 and expression of macrophage markers CD68 and ITGAM were significantly correlated. In serum, MCP-1 decreased significantly in insulin-sensitive but not insulin-resistant subjects. MIP-1alpha was undetectable in serum. Insulin regulation of CCL2 differs between insulin-sensitive and -resistant subjects in a direction that could exacerbate adipose tissue inflammation.     

The impact of baseline diameter on flow-mediated dilation differs in young and older humans

Flow-mediated dilation (FMD) has become a commonly applied approach for the assessment of vascular function and health, but methods used to calculate FMD differ between studies. For example, the baseline diameter used as a benchmark is sometimes assessed before cuff inflation, whereas others use the diameter during cuff inflation. Therefore, we compared the brachial artery diameter before and during cuff inflation and calculated the resulting FMD in healthy children (n=45; 10+/-1 yr), adults (n=31; 28+/-6 yr), and older subjects (n=22; 58+/-5 yr). Brachial artery FMD was examined after 5 min of distal ischemia. Diameter was determined from either 30 s before cuff inflation or from the last 30 s during cuff inflation. Edge detection and wall tracking of high resolution B-mode arterial ultrasound images was used to calculate conduit artery diameter. Brachial artery diameter during cuff inflation was significantly larger than before inflation in children (P=0.02) and adults (P<0.001) but not in older subjects (P=0.59). Accordingly, FMD values significantly differed in children (11.2+/-5.1% vs. 9.4+/-5.2%; P=0.02) and adults (7.3+/-3.2% vs. 4.6+/-3.3%; P<0.001) but not in older subjects (6.3+/-3.4% vs. 6.0+/-4.2%; P=0.77). When the diameter before cuff inflation was used, an age-dependent decline was evident in FMD, whereas FMD calculated using the diameter during inflation was associated with higher FMD values in older than younger adults. In summary, the inflation of the cuff significantly increases brachial artery diameter, which results in a lower FMD response. This effect was found to be age dependent, which emphasizes the importance of using appropriate methodology to calculate the FMD.     

Insulin action on protein metabolism in acromegalic patients

Insulin resistance in acromegaly causes glucose intolerance and diabetes, but it is unknown whether it involves protein metabolism, since both insulin and growth hormone promote protein accretion. The effects of acromegaly and of its surgical cure on the insulin sensitivity of glucose and amino acid/protein metabolism were evaluated by infusing [6,6-(2)H(2)]glucose, [1-(13)C]leucine, and [2-(15)N]glutamine during a euglycemic insulin (1 mU x kg(-1) x min(-1)) clamp in 12 acromegalic patients, six studied again 6 mo after successful adenomectomy, and eight healthy controls. Acromegalic patients, compared with postsurgical and control subjects, had higher postabsorptive glucose concentration (5.5 +/- 0.3 vs. 4.9 +/- 0.2 micromol/l, P < 0.05, and 5.1 +/- 0.1 micromol/l) and flux (2.7 +/- 0.1 vs. 2.0 +/- 0.2 micromol x kg(-1) x min(-1), P < 0.01, and 2.2 +/- 0.1 micromol x kg(-1) x min(-1), P < 0.05) and reduced insulin-stimulated glucose disposal (+15 +/- 9 vs. +151 +/- 18%, P < 0.01, and 219 +/- 58%, P < 0.001 from basal). Postabsorptive leucine metabolism was similar among groups. In acromegalic and postsurgical subjects, insulin suppressed less than in controls the endogenous leucine flux (-9 +/- 1 and -12 +/- 2 vs. -18 +/- 2%, P < 0.001 and P < 0.05), the nonoxidative leucine disposal (-4 +/- 3 and -1 +/- 3 vs. -18 +/- 2%, P < 0.01 and P < 0.05), respectively, indexes of proteolysis and protein synthesis, and leucine oxidation (-17 +/- 6% in postsurgical patients vs. -26 +/- 6% in controls, P < 0.05). Within 6 mo, surgery reverses insulin resistance for glucose but not for protein metabolism. After adenomectomy, more leucine is oxidized during hyperinsulinemia.     

Exercise-induced alterations in intramyocellular lipids and insulin resistance: the athlete's paradox revisited

We previously reported an "athlete's paradox" in which endurance-trained athletes, who possess a high oxidative capacity and enhanced insulin sensitivity, also have higher intramyocellular lipid (IMCL) content. The purpose of this study was to determine whether moderate exercise training would increase IMCL, oxidative capacity of muscle, and insulin sensitivity in previously sedentary overweight to obese, insulin-resistant, older subjects. Twenty-five older (66.4 +/- 0.8 yr) obese (BMI = 30.3 +/- 0.7 kg/m2) men (n = 9) and women (n = 16) completed a 16-wk moderate but progressive exercise training program. Body weight and fat mass modestly but significantly (P < 0.01) decreased. Insulin sensitivity, measured using the euglycemic hyperinsulinemic clamp, was increased (21%, P = 0.02), with modest improvements (7%, P = 0.04) in aerobic fitness (Vo2peak). Histochemical analyses of IMCL (Oil Red O staining), oxidative capacity [succinate dehydrogenase activity (SDH)], glycogen content, capillary density, and fiber type were performed on skeletal muscle biopsies. Exercise training increased IMCL by 21%. In contrast, diacylglycerol and ceramide, measured by mass spectroscopy, were decreased (n = 13; -29% and -24%, respectively, P < 0.05) with exercise training. SDH (19%), glycogen content (15%), capillary density (7%), and the percentage of type I slow oxidative fibers (from 50.8 to 55.7%), all P < or = 0.05, were increased after exercise. In summary, these results extend the athlete's paradox by demonstrating that chronic exercise in overweight to obese older adults improves insulin sensitivity in conjunction with favorable alterations in lipid partitioning and an enhanced oxidative capacity within muscle. Therefore, several key deleterious effects of aging and/or obesity on the metabolic profile of skeletal muscle can be reversed with only moderate increases in physical activity.     

Exercise training improves conduit vessel function in patients with coronary artery disease.

It is well established that endothelial dysfunction is present in coronary artery disease (CAD), although few studies have determined the effect of training on peripheral conduit vessel function in patients with CAD. A randomized, crossover design determined the effect of 8 wk of predominantly lower limb, combined aerobic and resistance training, in 10 patients with treated CAD. Endothelium-dependent dilation of the brachial artery was determined, by using high-resolution vascular ultrasonography, from flow-mediated vasodilation (FMD) after ischemia. Endothelium-independent vasodilation was measured after administration of glyceryl trinitrate (GTN). Baseline function was compared with that of 10 control subjects. Compared with matched healthy control subjects, FMD and GTN responses were significantly impaired in the untrained CAD patients [3.0 +/- 0.8 (SE) vs. 5.8 +/- 0.8% and 14.5 +/- 1.9 vs. 20.4 +/- 1.5%, respectively; both P < 0.05]. Training significantly improved FMD in the CAD patients (from 3.0 +/- 0.8 to 5.7 +/- 1.1%; P < 0.05) but not responsiveness to GTN (14.5 +/- 1.9 vs. 12.1 +/- 1.4%; P = not significant). Exercise training improves endothelium-dependent conduit vessel dilation in subjects with CAD, and the effect, evident in the brachial artery, appears to be generalized rather than limited to vessels of exercising muscle beds. These results provide evidence for the benefit of exercise training, as an adjunct to routine therapy, in patients with a history of CAD.     

Hyperinsulinemia fails to augment ET-1 action in the skeletal muscle vascular bed in vivo in humans

Endogenous endothelin action is augmented in human obesity and type 2 diabetes and contributes to endothelial dysfunction and impairs insulin-mediated vasodilation in humans. We hypothesized that insulin resistance-associated hyperinsulinemia could preferentially drive endothelin-mediated vasoconstriction. We applied hyperinsulinemic-euglycemic clamps with higher insulin dosing in obese subjects than lean subjects (30 vs. 10 mU.m(-2).min(-1), respectively), with the goal of matching insulin's nitric oxide (NO)-mediated vascular effects. We predicted that, under these circumstances, insulin-stimulated endothelin-1 (ET-1) action (assessed with the type A endothelin receptor antagonist BQ-123) would be augmented in proportion to hyperinsulinemia. NO bioactivity was assessed using the nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine. Insulin-mediated vasodilation and insulin-stimulated NO bioavailability were well matched across groups by this approach. As expected, steady-state insulin levels were approximately threefold higher in obese than lean subjects (109.2 +/- 10.2 pmol/l vs. 518.4 +/- 84.0, P = 0.03). Despite this, the augmentation of insulin-mediated vasodilation by BQ-123 was not different between groups. ET-1 flux across the leg was not augmented by insulin alone but was increased with the addition of BQ-123 to insulin (P = 0.01 BQ-123 effect, P = not significant comparing groups). Endothelin antagonism augmented insulin-stimulated NO bioavailability and NOx flux, but not differently between groups and not proportional to hyperinsulinemia. These findings do not support the hypothesis that insulin resistance-associated hyperinsulinemia preferentially drives endothelin-mediated vasoconstriction.     

Plasma obestatin is lower at fasting and not suppressed by insulin in insulin-resistant humans

Obestatin, a recently discovered 23-amino acid peptide, is involved in the regulation of appetite and body weight in antagonistic fashion to ghrelin, both deriving from a common precursor peptide. Ghrelin was shown to be associated with insulin resistance, which may also affect obestatin. We investigated the association between insulin resistance and plasma concentrations of obestatin and ghrelin in nondiabetic individuals with high (IS; n = 18, 13 females and 5 males, age 47 +/- 2 yr, BMI = 25.5 +/- 0.9 kg/m(2)) and low (IR; n = 18, 12 females and 6 males, age 45 +/- 2 yr, P = 0.49, BMI = 27.5 +/- 1.1 kg/m(2), P = 0.17) insulin-stimulated glucose disposal (M), measured by 2-h hyperinsulinemic (40 mU.min(-1).m(-2)) isoglycemic clamp tests. M(100-120 min) was higher in IS (10.7 +/- 0.7) than in IR (4.4 +/- 0.2 mg.min(-1).kg(-1), P < 10(-9)), whereas insulin-dependent suppression of free fatty acids (FFA) in plasma was reduced in IR (71 +/- 6% vs. IS: 82 +/- 5%, P < 0.02). In both groups, plasma ghrelin concentrations were comparable at fasting and similarly reduced by 24-28% during insulin infusion. IR had lower fasting plasma obestatin levels (383 +/- 26 pg/ml vs. IS: 469 +/- 23 pg/ml, P < 0.02). Clamp insulin infusion reduced plasma obestatin to approximately 81% of basal values in IS (P < 0.00002), but not in IR. Fasting plasma obestatin was correlated positively with M (r = 0.34, P = 0.04), HDL cholesterol (r = 0.45, P = 0.01), and plasma ghrelin concentrations (r = 0.80, P < 0.000001) and negatively with measures of adiposity, plasma FFA during clamp (r = -0.42, P < 0.01), and systolic blood pressure (r = -0.33, P < 0.05). In conclusion, fasting plasma concentrations of obestatin, but not of ghrelin, are reduced in insulin resistance and are positively associated with whole body insulin sensitivity in nondiabetic humans. Furthermore, plasma obestatin is reduced by insulin in insulin-sensitive but not in insulin-resistant persons.