Exercise training increases intramyocellular lipid and oxidative capacity in older adults

Intramyocellular lipid (IMCL) has been associated with insulin resistance. However, an association between IMCL and insulin resistance might be modulated by oxidative capacity in skeletal muscle. We examined the hypothesis that 12 wk of exercise training would increase both IMCL and the oxidative capacity of skeletal muscle in older (67.3 +/- 0.7 yr), previously sedentary subjects (n = 13; 5 men and 8 women). Maximal aerobic capacity (Vo(2 max)) increased from 1.65 +/- 0.20 to 1.85 +/- 0.14 l/min (P < 0.05), and systemic fat oxidation induced by 1 h of cycle exercise at 45% of Vo(2 max) increased (P < 0.05) from 15.03 +/- 40 to 19.29 +/- 0.80 (micromol.min(-1).kg fat-free mass(-1)). IMCL, determined by quantitative histological staining in vastus lateralis biopsies, increased (P < 0.05) from 22.9 +/- 1.9 to 25.9 +/- 2.6 arbitrary units (AU). The oxidative capacity of muscle, determined by succinate dehydrogenase staining intensity, significantly increased (P < 0.05) from 75.2 +/- 5.2 to 83.9 +/- 3.6 AU. The percentage of type I fibers significantly increased (P < 0.05) from 35.4 +/- 2.1 to 40.1 +/- 2.3%. In conclusion, exercise training increases IMCL in older persons in parallel with an enhanced capacity for fat oxidation.     

Intramyocellular lipid content in human skeletal muscle

Fat can be stored not only in adipose tissue but also in other tissues such as skeletal muscle. Fat droplets accumulated in skeletal muscle [intramyocellular lipids (IMCLs)] can be quantified by different methods, all with advantages and drawbacks. Here, we briefly review IMCL quantification methods that use biopsy specimens (biochemical quantification, electron microscopy, and histochemistry) and non-invasive alternatives (magnetic resonance spectroscopy, magnetic resonance imaging, and computed tomography). Regarding the physiological role, it has been suggested that IMCL serves as an intracellular source of energy during exercise. Indeed, IMCL content decreases during prolonged submaximal exercise, and analogously to glycogen, IMCL content is increased in the trained state. In addition, IMCL content is highest in oxidative, type 1 muscle fibers. Together, this, indeed, suggests that the IMCL content is increased in the trained state to optimally match fat oxidative capacity and that it serves as readily available fuel. However, elevation of plasma fatty acid levels or dietary fat content also increases IMCL content, suggesting that skeletal muscle also stores fat simply if the availability of fatty acids is high. Under these conditions, the uptake into skeletal muscle may have negative consequences on insulin sensitivity. Besides the evaluation of the various methods to quantify IMCLs, this perspective describes IMCLs as valuable energy stores during prolonged exercise, which, however, in the absence of regular physical activity and with overconsumption of fat, can have detrimental effects on muscular insulin sensitivity.     

GH receptor signaling in skeletal muscle and adipose tissue in human subjects following exposure to an intravenous GH bolus

Growth hormone (GH) regulates muscle and fat metabolism, which impacts on body composition and insulin sensitivity, but the underlying GH signaling pathways have not been studied in vivo in humans. We investigated GH signaling in biopsies from muscle and abdominal fat obtained 30 (n = 3) or 60 (n = 3) min after an intravenous bolus of GH (0.5 mg) vs. saline in conjunction with serum sampling in six healthy males after an overnight fast. Expression of the following signal proteins were assayed by Western blotting: STAT5/p-STAT5, MAPK, and Akt/PKB. IRS-1-associated PI 3-kinase activity was measured by in vitro phosphorylation of PI. STAT5 DNA binding activity was assessed with EMSA, and the expression of IGF-I and SOCS mRNA was measured by real-time RT-PCR. GH induced a 52% increase in circulating FFA levels with peak values after 155 min (P = 0.03). Tyrosine-phosphorylated STAT5 was detected in muscle and fat of all subjects after GH. Activation of MAPK was observed in several lysates but without GH dependency. Neither PKB/Akt nor PI 3-kinase activity was affected by GH. GH-induced STAT5 DNA binding and expression of IGF-I mRNA were detected in fat, whereas expression of SOCS-1 and -3 tended to increase after GH in muscle and fat, respectively. We conclude that 1) STAT5 is acutely activated in human muscle and fat after a GH bolus, but additional downstream GH signaling was significant only in fat; 2) the direct GH effects in muscle need further characterization; and 3) this human in vivo model may be used to study the mechanisms subserving the actions of GH on substrate metabolism and insulin sensitivity in muscle and fat.     

Intramuscular lipid metabolism, insulin action, and obesity

With the increasing prevalence of obesity, research has focused on the molecular mechanism(s) linking obesity and skeletal muscle insulin resistance. Metabolic alterations within muscle, such as changes in the cellular location of fatty acid transporter proteins, decreased mitochondrial enzyme activity, and defects in mitochondrial morphology, likely contribute to obesity and insulin resistance. These defects are thought to play a role in the reduced skeletal muscle fatty acid oxidation and increased intramuscular lipid (IMCL) accumulation that is apparent with obesity and other insulin-resistant states such as type 2 diabetes. Intramuscular triacylglycerol does not appear to be a ubiquitous marker of insulin resistance, although specific IMCL intermediates such as long-chain fatty acyl-CoAs, ceramide, and diacylglycerol may inhibit insulin signal transduction. In this review, we will briefly summarize the defects in skeletal muscle lipid metabolism associated with obesity, and discuss the proposed mechanisms by which these defects may contribute to insulin resistance.     

Myofibrillar distribution of succinate dehydrogenase activity and lipid stores differs in skeletal muscle tissue of paraplegic subjects

Lack of physical activity has been related to an increased risk of developing insulin resistance. This study aimed to assess the impact of chronic muscle deconditioning on whole body insulin sensitivity, muscle oxidative capacity, and intramyocellular lipid (IMCL) content in subjects with paraplegia. Nine subjects with paraplegia and nine able-bodied, lean controls were recruited. An oral glucose tolerance test was performed to assess whole body insulin sensitivity. IMCL content was determined both in vivo and in vitro using (1)H-magnetic resonance spectroscopy and fluorescence microscopy, respectively. Muscle biopsy samples were stained for succinate dehydrogenase (SDH) activity to measure muscle fiber oxidative capacity. Subcellular distributions of IMCL and SDH activity were determined by defining subsarcolemmal and intermyofibrillar areas on histological samples. SDH activity was 57 ± 14% lower in muscle fibers derived from subjects with paraplegia when compared with controls (P < 0.05), but IMCL content and whole body insulin sensitivity did not differ between groups. In muscle fibers taken from controls, both SDH activity and IMCL content were higher in the subsarcolemmal region than in the intermyofibrillar area. This typical subcellular SDH and IMCL distribution pattern was lost in muscle fibers collected from subjects with paraplegia and had changed toward a more uniform distribution. In conclusion, the lower metabolic demand in deconditioned muscle of subjects with paraplegia results in a significant decline in muscle fiber oxidative capacity and is accompanied by changes in the subcellular distribution patterns of SDH activity and IMCL. However, loss of muscle activity due to paraplegia is not associated with substantial lipid accumulation in skeletal muscle tissue.     

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.     

Evidence against a role for insulin-signaling proteins PI 3-kinase and Akt in insulin resistance in human skeletal muscle induced by short-term GH infusion

Prolonged growth hormone (GH) excess is known to be associated with insulin resistance, but the underlying mechanisms remain unknown. The aim of this study was to assess the impact of GH on insulin-stimulated glucose metabolism and insulin signaling in human skeletal muscle. In a cross-over design, eight healthy male subjects (age 26.0 +/- 0.8 yr and body mass index 24.1 +/- 0.5 kg/m2) were infused for 360 min with either GH (Norditropin, 45 ng.kg(-1).min(-1)) or saline. During the final 180 min of the infusion, a hyperinsulinemic euglycemic clamp was performed (insulin infusion rate: 1.2 mU.kg(-1).min(-1)). Muscle biopsies from vastus lateralis were taken before GH/saline administration and after 60 min of hyperinsulinemia. GLUT4 content and insulin signaling, as assessed by insulin receptor substrate (IRS)-1-associated phosphatidylinositol 3-kinase and Akt activity were determined. GH levels increased to a mean (+/-SE) level of 20.0 +/- 2.3 vs. 0.5 +/- 0.2 microg/l after saline infusion (P < 0.01). During GH infusion, the glucose infusion rate during hyperinsulinemia was reduced by 38% (P < 0.01). In both conditions, free fatty acids were markedly suppressed during hyperinsulinemia. Despite skeletal muscle insulin resistance, insulin still induced a similar approximately 3-fold rise in IRS-1-associated PI 3-kinase activity (269 +/- 105 and 311 +/- 71% compared with baseline, GH vs. saline). GH infusion did not change Akt protein expression, and insulin caused an approximately 13-fold increase in Akt activity (1,309 +/- 327 and 1,287 +/- 173%) after both GH and saline infusion. No difference in total GLUT4 content was noted (114.7 +/- 7.4 and 107.6 +/- 16.7 arbitrary units, GH vs. saline, compared with baseline). In conclusion, insulin resistance in skeletal muscle induced by short-term GH administration is not associated with detectable changes in the upstream insulin-signaling cascade or reduction in total GLUT4. Yet unknown mechanisms in insulin signaling downstream of Akt may be responsible.     

Intramyocellular lipid content and insulin sensitivity are increased following a short-term low-glycemic index diet and exercise intervention

The relationship between intramyocellular (IMCL) and extramyocellular lipid (EMCL) accumulation and skeletal muscle insulin resistance is complex and dynamic. We examined the effect of a short-term (7-day) low-glycemic index (LGI) diet and aerobic exercise training intervention (EX) on IMCL and insulin sensitivity in older, insulin-resistant humans. Participants (66 ± 1 yr, BMI 33 ± 1 kg/m(2)) were randomly assigned to a parallel, controlled feeding trial [either an LGI (LGI/EX, n = 7) or high GI (HGI/EX, n = 8) eucaloric diet] combined with supervised exercise (60 min/day, 85% HR(max)). Insulin sensitivity was determined via 40 mU·m(-2)·min(-1) hyperinsulinemic euglycemic clamp and soleus IMCL and EMCL content was assessed by (1)H-MR spectroscopy with correction for fiber orientation. BMI decreased (kg/m(2) -0.6 ± 0.2, LGI/EX; -0.7 ± 0.2, HGI/EX P < 0.0004) after both interventions with no interaction effect of diet composition. Clamp-derived insulin sensitivity increased by 0.91 ± 0.21 (LGI/EX) and 0.17 ± 0.55 mg·kg(-1)·min(-1) (HGI/EX), P = 0.04 (effect of time). HOMA-IR was reduced by -1.1 ± 0.4 (LGI/EX) and -0.1 ± 0.2 (HGI/EX), P = 0.007 (effect of time), P = 0.02 (time × trial). Although both interventions increased IMCL content, (Δ: 2.3 ± 1.3, LGI/EX; 1.4 ± 0.9, HGI/EX, P = 0.03), diet composition did not significantly effect the increase. However, the LGI/EX group showed a robust increase in the [IMCL]/[EMCL] ratio compared with the HGI/EX group (Δ: 0.5 ± 0.2 LGI/EX vs. 0.07 ± 0.1, P = 0.03). The LGI/EX group also demonstrated greater reductions in [EMCL] than the HGI/EX group (Δ: -5.8 ± 3.4, LGI/EX; 2.3 ± 1.1, HGI/EX, P = 0.03). Changes in muscle lipids and insulin sensitivity were not correlated; however, the change in [IMCL]/[EMCL] was negatively associated with the change in FPI (r = -0.78, P = 0.002) and HOMA-IR (r = -0.61, P = 0.03). These data suggest that increases in the IMCL pool following a low glycemic diet and exercise intervention may represent lipid repartitioning from EMCL. The lower systemic glucose levels that prevail while eating a low glycemic diet may promote redistribution of lipid stores in the muscle.     

Effects of one-year low-dose growth hormone (GH) therapy on body composition, lipid profile and carbohydrate metabolism in young adults with childhood-onset severe GH deficiency confirmed after completion of growth promotion

Introduction: The symptoms of GH deficiency (GHD) in adults include: abnormalities in body composition, unfavourable lipid profile, early atherosclerosis and impaired quality of life. The aim of the study was the selection of patients with confirmed severe GHD from among all the children treated due to GHD, who could benefit from GH therapy continuation in adulthood and the optimization of GH dosage in young adults with severe GHD. Material and methods: The study group consisted of 54 young adults (38 male), age 17.6 +/- 1.5 years, with childhood-onset GHD, who had reached final height. At least 1 month after the GH therapy withdrawal, the second evaluation of GH secretion was performed in all the patients. In 24% of patients, permanent severe GHD (PSGHD) was confirmed, but a group of 9 patients (4 male) was involved in renewed GH therapy. Results: The renewed GH therapy gave positive effects, including a significant increase in fat-free mass and a decrease in fat mass, and a significant decrease in LDL-cholesterol, but connected with an insignificant decrease of HDL-cholesterol serum concentration and improved results of quality of life (QoL) assessment. During the therapy, an insignificant increase of fasting insulin was observed, with no change in fasting glucose and only a slight increase in HbA(1c) percentage. A decrease of insulin sensitivity was also observed, but both insulin secretion and the values of insulin resistance indices still remained within the reference range. Conclusions: The observed positive effects on body composition, lipid metabolism and QoL, together with the absence of adverse events, confirm the indications for GH therapy in young adults with severe GHD.     

Overexpression of PLIN5 in skeletal muscle promotes oxidative gene expression and intramyocellular lipid content without compromising insulin sensitivity

Aims/hypothesis: While lipid deposition in the skeletal muscle is considered to be involved in obesity-associated insulin resistance, neutral intramyocellular lipid (IMCL) accumulation per se does not necessarily induce insulin resistance. We previously demonstrated that overexpression of the lipid droplet coat protein perilipin 2 augments intramyocellular lipid content while improving insulin sensitivity. Another member of the perilipin family, perilipin 5 (PLIN5), is predominantly expressed in oxidative tissues like the skeletal muscle. Here we investigated the effects of PLIN5 overexpression – in comparison with the effects of PLIN2 – on skeletal muscle lipid levels, gene expression profiles and insulin sensitivity. Methods: Gene electroporation was used to overexpress PLIN5 in tibialis anterior muscle of rats fed a high fat diet. Eight days after electroporation, insulin-mediated glucose uptake in the skeletal muscle was measured by means of a hyperinsulinemic euglycemic clamp. Electron microscopy, fluorescence microscopy and lipid extractions were performed to investigate IMCL accumulation. Gene expression profiles were obtained using microarrays. Results: TAG storage and lipid droplet size increased upon PLIN5 overexpression. Despite the higher IMCL content, insulin sensitivity was not impaired and DAG and acylcarnitine levels were unaffected. In contrast to the effects of PLIN2 overexpression, microarray data analysis revealed a gene expression profile favoring FA oxidation and improved mitochondrial function. Conclusions/interpretation: Both PLIN2 and PLIN5 increase neutral IMCL content without impeding insulin-mediated glucose uptake. As opposed to the effects of PLIN2 overexpression, overexpression of PLIN5 in the skeletal muscle promoted expression of a cluster of genes under control of PPARα and PGC1α involved in FA catabolism and mitochondrial oxidation.     

Proton magnetic resonance spectroscopy shows lower intramyocellular lipid accumulation in middle-aged subjects predisposed to familial longevity

Families predisposed to longevity show enhanced glucose tolerance and skeletal muscle insulin sensitivity compared with controls, independent of body composition and physical activity. Intramyocellular lipid (IMCL) accumulation in skeletal muscle has been associated with insulin resistance. Here, we assessed whether subjects enriched for familial longevity have lower IMCL levels. We determined IMCL levels in 48 subjects from the Leiden Longevity Study, comprising 24 offspring of nonagenarian siblings and 24 partners thereof as control subjects. IMCL levels were assessed noninvasively using short echo time proton magnetic resonance spectroscopy ((1)H-MRS) of the tibialis anterior muscle with a 7 Tesla human MR scanner. IMCL levels were calculated relative to the total creatine (tCr) CH3 signal. Physical activity was assessed using the International Physical Activity Questionnaire (IPAQ). After correction for age, sex, BMI, and physical activity, offspring of long-lived nonagenarian siblings tended to show lower IMCL levels compared with controls (IMCL/tCr: 3.1 ± 0.5 vs. 4.5 ± 0.5, respectively, P = 0.051). In a pairwise comparison, this difference reached statistical significance (P = 0.038). We conclude that offspring of nonagenarian siblings predisposed to longevity show lower IMCL levels compared with environmentally matched control subjects. Future research should focus on assessing what mechanisms may explain the lower IMCL levels in familial longevity.     

Effect of diet-induced obesity and metabolic syndrome on skeletal muscles of Ossabaw miniature swine

Ossabaw swine fed excess kilocalorie diet develop metabolic syndrome (MS) characterized by obesity, hypertension, insulin resistance, and glucose intolerance with/without dyslipidemia. The purpose of this study was to test the hypothesis that MS would have a detrimental effect on skeletal muscle structure and cause changes in the expression of myosin heavy chains (MHCs). Adult male Ossabaw swine were fed for 24 wk high-fructose or high-fat/cholesterol/fructose diets to induce normolipidemic MS (MetS) or dyslipidemic MS (DMetS), respectively, and were compared with the lean swine on control diet. MetS swine showed mild MS, lacking increases in total and low density lipoprotein (LDL) cholesterol, both of which were highly upregulated in DMetS swine. There was an ～1.2-fold increase in the cross-sectional areas of muscle fibers in MetS and DMetS groups compared with control for biceps femoris and plantaris muscles. In plantaris muscles, DMetS diet caused an ～2-fold decrease in slow MHC mRNA and protein expression and an ～1.2- to 1.8-fold increase in the number of intramyocellular lipid (IMCL) droplets without large changes in the size of the droplets. There was a trend to the decrease in slow MHC expression in muscles of swine on MetS diet. The number of IMCL droplets in muscle fibers of the MetS group was comparable to controls. These data correlate well with the data on total plasma cholesterol (control = 60, MetS = 70, and DMetS = 298 mg/dl) and LDL (control = 29, MetS = 30, and DMetS = 232 mg/dl). We conclude that structural changes observed in skeletal muscle of obese Ossabaw swine correlate with those previously reported for obese humans.     

Metformin and exercise reduce muscle FAT/CD36 and lipid accumulation and blunt the progression of high-fat diet-induced hyperglycemia

Derangements in skeletal muscle fatty acid (FA) metabolism associated with insulin resistance in obesity appear to involve decreased FA oxidation and increased accumulation of lipids such as ceramides and diacylglycerol (DAG). We investigated potential lipid-related mechanisms of metformin (Met) and/or exercise for blunting the progression of hyperglycemia/hyperinsulinemia and skeletal muscle insulin resistance in female Zucker diabetic fatty rats (ZDF), a high-fat (HF) diet-induced model of diabetes. Lean and ZDF rats consumed control or HF diet (48 kcal %fat) alone or with Met (500 mg/kg), with treadmill exercise, or with both exercise and Met interventions for 8 wk. HF-fed ZDF rats developed hyperglycemia (mean: 24.4 +/- 2.1 mM), impairments in muscle insulin-stimulated glucose transport, increases in the FA transporter FAT/CD36, and increases in total ceramide and DAG content. The development of hyperglycemia was significantly attenuated with all interventions, as was skeletal muscle FAT/CD36 abundance and ceramide and DAG content. Interestingly, improvements in insulin-stimulated glucose transport and increased GLUT4 transporter expression in isolated muscle were seen only in conditions that included exercise training. Reduced FA oxidation and increased triacylglycerol synthesis in isolated muscle were observed with all ZDF rats compared with lean rats (P < 0.01) and were unaltered by therapeutic intervention. However, exercise did induce modest increases in peroxisome proliferator-activated receptor-gamma coactivator-1alpha, citrate synthase, and beta-hydroxyacyl-CoA dehydrogenase activity. Thus reduction of skeletal muscle FAT/CD36 and content of ceramide and DAG may be important mechanisms by which exercise training blunts the progression of diet-induced insulin resistance in skeletal muscle.     

Ursolic acid increases skeletal muscle and brown fat and decreases diet-induced obesity, glucose intolerance and fatty liver disease

Skeletal muscle Akt activity stimulates muscle growth and imparts resistance to obesity, glucose intolerance and fatty liver disease. We recently found that ursolic acid increases skeletal muscle Akt activity and stimulates muscle growth in non-obese mice. Here, we tested the hypothesis that ursolic acid might increase skeletal muscle Akt activity in a mouse model of diet-induced obesity. We studied mice that consumed a high fat diet lacking or containing ursolic acid. In skeletal muscle, ursolic acid increased Akt activity, as well as downstream mRNAs that promote glucose utilization (hexokinase-II), blood vessel recruitment (Vegfa) and autocrine/paracrine IGF-I signaling (Igf1). As a result, ursolic acid increased skeletal muscle mass, fast and slow muscle fiber size, grip strength and exercise capacity. Interestingly, ursolic acid also increased brown fat, a tissue that shares developmental origins with skeletal muscle. Consistent with increased skeletal muscle and brown fat, ursolic acid increased energy expenditure, leading to reduced obesity, improved glucose tolerance and decreased hepatic steatosis. These data support a model in which ursolic acid reduces obesity, glucose intolerance and fatty liver disease by increasing skeletal muscle and brown fat, and suggest ursolic acid as a potential therapeutic approach for obesity and obesity-related illness.     

Intramyocellular Lipid Content and Molecular Adaptations in Response to a 1‐Week High‐Fat Diet

Objective: To investigate molecular adaptations that accompany the elevation of intramyocellular lipid (IMCL) content on a high‐fat (HF) diet for 1 week. Research Methods and Procedures: Ten subjects consumed a normal‐fat (NF) diet for 1 week, followed by an HF diet for another week. After both dietary periods, we determined the IMCL content by proton magnetic resonance spectroscopy in the vastus lateralis muscle and quantified changes in gene expression, protein content, and activity in biopsy samples. We investigated genes involved in carbohydrate and fatty acid handling [lipoprotein lipase, acetyl‐coenzyme A carboxylase (ACC) 2, hormone‐sensitive lipase, hexokinase II, and glucose transporter 4] and measured protein levels of CD36 and phosphorylated and unphosphorylated ACC2 and the activity of adenosine monophosphate‐activated kinase. Results: IMCL content was increased by 54% after the HF period. Lipoprotein lipase mRNA concentration was increased by 33%, whereas ACC2 mRNA concentration tended to be increased after the HF diet. Hexokinase II, glucose transporter 4, and hormone‐sensitive lipase mRNA were unchanged after the HF diet. ACC2 and CD36 protein levels, phosphorylation status of ACC2, and adenosine monophosphate‐activated kinase activity did not change in response to the HF diet. Discussion: We found that IMCL content in skeletal muscle increased after 1 week of HF feeding, accompanied by molecular adaptations that favor fat storage in muscle rather than oxidation.     

IGF-I and IGF binding protein-3 levels during initial GH dosage step-up are indicators of GH sensitivity in GH-deficient children and short children born small for gestational age

BACKGROUND: A stepwise increment of the GH dose is an approach aimed at avoiding adverse events. We investigated GH sensitivity by studying IGF-I and IGFBP-3 concentrations during the initial phase of GH treatment. METHODS: Our investigation was part of the regular follow-up of prepubertal children with GH deficiency (GHD) (n = 31) and small for gestational age (SGA) (n = 23). Dosage was increased in three steps: one-third at the start, two-thirds after 14 days, and the full dose after 28 days (full dose: GHD = 28 microg/kg body weight (BW)/day; SGA = 60 microg/kg BW/day). Blood samples were taken on days 0, 14 and 28, as well as in conjunction with anthropometrical examinations after 3, 6 and 12 months. IGF-I and IGFBP-3 were measured by means of published in-house RIAs and age-related references were used to calculate standard deviation scores (SDS). Height velocity (cm/year) and Delta HT SDS were taken as growth response parameters. RESULTS: Before GH treatment (GHD vs. SGA; median and p values): age (years) (6.6 vs. 6.0; n.s.), HT SDS (-2.6 vs. -3.2; p < 0.05); GH amount after stepping up (mug/kg BW/day) (28 vs. 60; p < 0.01); BW SDS (-0.5 vs. -2.9; p < 0.01); max. GH stimulated (microg/l) (5.6 vs. 10.8; p < 0.01); IGF-I SDS (-3.5 vs. -1.8; p < 0.01); IGFBP-3 SDS (-2.0 vs. 0.8; p < 0.01). After 1 year of GH therapy: HT velocity (cm/year) (9.8 vs. 9.6; n.s.), Delta HT SDS (0.9 vs. 0.9; n.s.); WT velocity (kg/year) (3.3 vs. 3.5; n.s.). Our results show that changes in growth similar to GHD could be induced in SGA by a dosage that was twice as high as the replacement dose given in GHD. GH dose and HT velocity did not correlate in both groups. IGF-I and IGFBP-3 increased as follows in GHD and SGA during stepping up of the dosage (ng/ml, GHD vs. SGA): at start, 54 vs. 89; at day 14, 78 vs. 132; at day 28, 90 vs. 167; at 3 months, 118 vs. 218. There was the same relationship between dose levels and absolute IGF-I concentrations in both groups. In terms of IGF-I SDS, the dose-response curve in SGA showed a shift to the right in comparison to GHD, thus indicating lower sensitivity to GH. The dynamics of IGF-I and IGFBP-3 differed, as IGFBP-3 peaked earlier (on day 28). In GHD, IGF-I SDS at 3 months was -0.7 vs. +0.9 in SGA. Near-identical levels were found for Delta IGF-I SDS and IGFBP-3 SDS above basal levels for each time-point investigated. First year HT velocity in GHD correlated negatively with basal IGF-I SDS (R(2) = 0.33; p <0.001) and basal IGFBP-3 (R(2) = 0.17; p <0.05) but did not correlate with the IGF-I increment during the 0- to 3-month period. Conversely, first year HT velocity correlated (+) in SGA with the IGF SDS increment during the 0- to 3-month period (R(2) = 0.26; p = <0.05). Height velocity in SGA, however, correlated neither with basal IGF-I and IGFBP-3 nor with the 0- to 3-month increments of IGFBP-3 SDS. CONCLUSIONS: IGFs increase during initial GH therapy, thus raising questions about short-term IGF generation tests. (I) In terms of IGF generation, substantially lower sensitivity to GH was observable in SGA. (II) Higher GH sensitivity during first year catch-up growth is associated with GHD, but in SGA it is attributable to increases in IGF. A wider range of GH dosages needs to be explored in order to gain further insight into the relationship between GH dose, IGF levels, and growth. Monitoring IGFs is a practical means for exploring GH sensitivity during dosage stepping up.     

A discordance in rosiglitazone mediated insulin sensitization and skeletal muscle mitochondrial content/activity in Type 2 diabetes mellitus

Skeletal muscle mitochondrial dysfunction is hypothesized to contribute to the pathophysiology of insulin resistance and Type 2 diabetes. Whether thiazolidinedione therapy enhances skeletal muscle mitochondrial function as a component of its insulin-sensitizing effect is unknown. To test this, we evaluated skeletal muscle mitochondria and exercise capacity in Type 2 diabetic subjects with otherwise normal cardiopulmonary function in response to rosiglitazone therapy. Twenty-three subjects were treated for 12 wk and underwent pre- and posttherapy metabolic stress testing and skeletal muscle biopsies. Rosiglitazone significantly ameliorated fasting glucose, insulin, and free fatty acid levels but did not augment the subjects' maximal oxygen consumption (Vo(2max)) or their skeletal muscle mitochondrial copy number. The baseline Vo(2max) correlated strongly with muscle mitochondrial copy number (r = 0.56, P = 0.018, n = 17) and inversely with the duration of diabetes (r = -0.67, P = 0.004, n = 23). Despite the global lack of effect of rosiglitazone-mediated insulin sensitization on skeletal muscle mitochondria, subjects with the most preserved functional capacity demonstrated some plasticity in their mitochondria biology as evidenced by an upregulation of electron transfer chain proteins and in citrate synthase activity. This study demonstrates that the augmentation of skeletal muscle mitochondrial electron transfer chain content and/or bioenergetics is not a prerequisite for rosiglitazone-mediated improved insulin sensitivity. Moreover, in diabetic subjects, Vo(2max) reflects the duration of diabetes and skeletal muscle mitochondrial content. It remains to be determined whether longer-term insulin sensitization therapy with rosiglitazone will augment skeletal muscle mitochondrial bioenergetics in those diabetic subjects with relatively preserved basal aerobic capacity.     

Relationship between visceral adiposity and intramyocellular lipid content in two rat models of insulin resistance

High visceral adiposity and intramyocellular lipid levels (IMCL) are both associated with the development of type 2 diabetes. The relationship between visceral adiposity and IMCL levels was explored in diet- and glucocorticoid-induced models of insulin resistance. In the diet-induced model, lean and fa/fa Zucker rats were fed either normal or high-fat (HF) chow over 4 wk. Fat distribution, IMCL content in the tibialis anterior (TA) muscle (IMCL(TA)), and whole body insulin resistance were measured before and after the 4-wk period. The HF diet-induced increase in IMCL(TA) was strongly correlated with visceral fat accumulation and greater glucose intolerance in both groups. The increase in IMCL(TA) to visceral fat accumulation was threefold greater for fa/fa rats. In the glucocorticoid-induced model, insulin sensitivity was impaired with dexamethasone. In vivo adiposity and IMCL(TA) content measurements were combined with ex vivo analysis of plasma and muscle tissue. Dexamethasone treatment had minimal effects on visceral fat accumulation while increasing IMCL(TA) levels approximately 30% (P < 0.05) compared with controls. Dexamethasone increased plasma glucose by twofold and increased the saturated fatty acid content of plasma lipids [fatty acid (CH2)n/omegaCH3 ratio +15%, P < 0.05]. The lipid composition of the TA muscle was unchanged by dexamethasone treatment, indicating that the relative increase in IMCL(TA) observed in vivo resulted from a decrease in lipid oxidation. Visceral adiposity may influence IMCL accumulation in the context of dietary manipulations; however, a "causal" relationship still remains to be determined. Dexamethasone-induced insulin resistance likely operates under a different mechanism, i.e., independently of visceral adiposity.