GLUT4 localisation with the plasma membrane is unaffected by an increase in plasma free fatty acid availability.

BACKGROUND: Insulin-stimulated glucose uptake into skeletal muscle occurs via translocation of GLUT4 from intracellular storage vesicles to the plasma membrane. Elevated free fatty acid (FFA) availability via a lipid infusion reduces glucose disposal, but this occurs in the absence of impaired proximal insulin signalling. Whether GLUT4 localisation to the plasma membrane is subsequently affected by elevated FFA availability is not known. METHODS: Trained (n = 11) and sedentary (n = 10) individuals, matched for age, sex and body mass index, received either a 6 h lipid or glycerol infusion in the setting of a concurrent hyperinsulinaemic-euglycaemic clamp. Sequential muscle biopsies (0, 2 and 6 h) were analysed for GLUT4 membrane localisation and microvesicle size and distribution using immunofluorescence microscopy. RESULTS: At baseline, trained individuals had more small GLUT4 spots at the plasma membrane, whereas sedentary individuals had larger GLUT4 spots. GLUT4 localisation with the plasma membrane increased at 2 h (P = 0.04) of the hyperinsulinemic-euglycemic clamp, and remained elevated until 6 h, with no differences between groups or infusion type. The number of GLUT4 spots was unchanged at 2 h of infusion. However, from 2 to 6 h there was a decrease in the number of small GLUT4 spots at the plasma membrane (P = 0.047), with no differences between groups or infusion type. CONCLUSION: GLUT4 localisation with the plasma membrane increases during a hyperinsulinemic-euglycemic clamp, but this is not altered by elevated FFA availability. GLUT4 appears to disperse from small GLUT4 clusters located at the plasma membrane to support glucose uptake during a hyperinsulinaemic-euglycaemic clamp.

High intramuscular triglyceride turnover rates and the link to insulin sensitivity: influence of obesity, type 2 diabetes and physical activity.

Large intramuscular triglyceride (IMTG) stores in sedentary, obese individuals have been linked to insulin resistance, yet well-trained athletes exhibit high IMTG levels whilst maintaining insulin sensitivity. Contrary to previous assumptions, it is now known that IMTG content per se does not result in insulin resistance. Rather, insulin resistance is caused, at least in part, by the presence of high concentrations of harmful lipid metabolites, such as diacylglycerols and ceramides in muscle. Several mechanistic differences between obese sedentary individuals and their highly trained counterparts have been identified, which determine the differential capacity for IMTG synthesis and breakdown in these populations. In this review, we first describe the most up-to-date mechanisms by which a low IMTG turnover rate (both breakdown and synthesis) leads to the accumulation of lipid metabolites and results in skeletal muscle insulin resistance. We then explore current and potential exercise and nutritional strategies that target IMTG turnover in sedentary obese individuals, to improve insulin sensitivity. Overall, improving IMTG turnover should be an important component of successful interventions that aim to prevent the development of insulin resistance in the ever-expanding sedentary, overweight and obese populations. Novelty: A description of the most up-to-date mechanisms regulating turnover of the IMTG pool. An exploration of current and potential exercise/nutritional strategies to target and enhance IMTG turnover in obese individuals. Overall, highlights the importance of improving IMTG turnover to prevent the development of insulin resistance.

A 7-day high-fat, high-calorie diet induces fibre-specific increases in intramuscular triglyceride and perilipin protein expression in human skeletal muscle.

KEY POINTS: We have recently shown that a high-fat, high-calorie (HFHC) diet decreases whole body glucose clearance without impairing skeletal muscle insulin signalling, in healthy lean individuals. These diets are also known to increase skeletal muscle IMTG stores, but the effect on lipid metabolites leading to skeletal muscle insulin resistance has not been investigated. This study measured the effect of 7 days' HFHC diet on (1) skeletal muscle concentration of lipid metabolites, and (2) potential changes in the perilipin (PLIN) content of the lipid droplets storing intramuscular triglyceride (IMTG). The HFHC diet increased PLIN3 protein expression and redistributed PLIN2 to lipid droplet stores in type I fibres. The HFHC diet increased IMTG content in type I fibres, while lipid metabolite concentrations remained the same. The data suggest that the increases in IMTG stores assists in reducing the accumulation of lipid metabolites known to contribute to skeletal muscle insulin resistance. ABSTRACT: A high-fat, high-calorie (HFHC) diet reduces whole body glucose clearance without impairing skeletal muscle insulin signalling in healthy lean individuals. HFHC diets also increase skeletal muscle lipid stores. However, unlike certain lipid metabolites, intramuscular triglyceride (IMTG) stored within lipid droplets (LDs) does not directly contribute to skeletal muscle insulin resistance. Increased expression of perilipin (PLIN) proteins and colocalisation to LDs has been shown to assist in IMTG storage. We aimed to test the hypothesis that 7 days on a HFHC diet increases IMTG content while minimising accumulation of lipid metabolites known to disrupt skeletal muscle insulin signalling in sedentary and obese individuals. We also aimed to identify changes in expression and subcellular distribution of proteins involved in IMTG storage. Muscle biopsies were obtained from the m. vastus lateralis of 13 (11 males, 2 females) healthy lean individuals (age: 23 ± 2.5 years; body mass index: 24.5 ± 2.4 kg m-2 ), following an overnight fast, before and after consuming a high-fat (64% energy), high-calorie (+47% kcal) diet for 7 days. After the HFHC diet, IMTG content increased in type I fibres only (+101%; P < 0.001), whereas there was no change in the concentration of either total diacylglycerol (P = 0.123) or total ceramides (P = 0.150). Of the PLINs investigated, only PLIN3 content increased (+50%; P < 0.01) solely in type I fibres. LDs labelled with PLIN2 increased (+80%; P < 0.01), also in type I fibres only. We propose that these adaptations of LDs support IMTG storage and minimise accumulation of lipid metabolites to protect skeletal muscle insulin signalling following 7 days' HFHC diet.

Lipid droplet remodelling and reduced muscle ceramides following sprint interval and moderate-intensity continuous exercise training in obese males.

BACKGROUND/OBJECTIVES: In obesity, improved muscle insulin sensitivity following exercise training has been linked to the lowering of diacylglycerol (DAG) and ceramide concentrations. Little is known, however, about how improved insulin action with exercise training in obese individuals relates to lipid droplet (LD) adaptations in skeletal muscle. In this study we investigated the hypothesis that short-term sprint interval training (SIT) and moderate-intensity continuous training (MICT) in obese individuals would increase perilipin (PLIN) expression, increase the proportion of LDs in contact with mitochondria and reduce muscle concentrations of DAGs and ceramides. METHODS: Sixteen sedentary obese males performed 4 weeks of either SIT (4-7 × 30 s sprints at 200% Wmax, 3 days week) or MICT (40-60 min cycling at ~65% VO2peak, 5 days per week), and muscle biopsies were obtained pre- and post-training. RESULTS: Training increased PLIN2 (SIT 90%, MICT 68%) and PLIN5 (SIT 47%, MICT 34%) expression in type I fibres only, and increased PLIN3 expression in both type I (SIT 63%, MICT 67%) and type II fibres (SIT 70%, MICT 160%) (all P<0.05). Training did not change LD content but increased the proportion of LD in contact with mitochondria (SIT 12%, MICT 21%, P<0.01). Ceramides were reduced following training (SIT -10%, MICT -7%, P<0.05), but DAG was unchanged. No training × group interactions were observed for any variables. CONCLUSIONS: These results confirm the hypothesis that SIT and MICT results in remodelling of LDs and lowers ceramide concentrations in skeletal muscle of sedentary obese males.

Vascular Health in Patients in Remission of Cushing's Syndrome Is Comparable With That in BMI-Matched Controls.

CONTEXT: In active Cushing's syndrome (CS), patients suffer from endothelial dysfunction and premature atherosclerosis. However, it is uncertain to what extent vascular health recovers after long-term remission. This is highly relevant because this topic relates to future development of cardiovascular disease. OBJECTIVE: The objective of the study was to investigate whether micro- and macrovascular health is impaired after long-term remission of CS in patients with no or adequately treated comorbidities. DESIGN AND SETTING: This was a cross-sectional case-control study in two tertiary referral centers. PATIENTS AND MAIN OUTCOME MEASURES: Sixty-three patients (remission of CS for ≥ 4 y) and 63 healthy, well-matched controls were compared. In group A (58 patients and 58 controls), serum biomarkers associated with endothelial dysfunction, intima media thickness, pulse wave velocity, and pulse wave analysis were studied. In group B (14 patients and 14 controls), endothelium-dependent and -independent vasodilatation was studied in conduit arteries (flow mediated dilation of the brachial artery) and forearm skeletal muscle resistance arteries (vasodilator response to intraarterial acetylcholine, sodium-nitroprusside, and NG-monomethyl-L-arginine using venous occlusion plethysmography). RESULTS: There were no significant differences between the outcome measures of vascular health of patients and controls in groups A and B. CONCLUSION: The vascular health of patients in long-term remission of CS seems to be comparable with that of healthy gender-, age-, and body mass index-matched controls, provided that the patients have no, or adequately controlled, comorbidities. Therefore, the effects of hypercortisolism per se on the vasculature may be reversible. This accentuates the need for the stringent treatment of metabolic comorbidities in these patients.

Resistance training increases skeletal muscle oxidative capacity and net intramuscular triglyceride breakdown in type I and II fibres of sedentary males.

Recent in vitro and in vivo experimental observations suggest that improvements in insulin sensitivity following endurance training are mechanistically linked to increases in muscle oxidative capacity, intramuscular triglyceride (IMTG) utilization during endurance exercise and increases in the content of the lipid droplet-associated perilipin 2 (PLIN2) and perilipin 5 (PLIN5). This study investigated the hypothesis that similar adaptations may also underlie the resistance training (RT)-induced improvements in insulin sensitivity. Thirteen sedentary men (20 ± 1 years old; body mass index 24.8 ± 0.8 kg m(-2)) performed 6 weeks of whole-body RT (three times per week), and changes in peak O2 uptake (in millilitres per minute per kilogram) and insulin sensitivity were assessed. Muscle biopsies (n = 8) were obtained before and after 60 min steady-state cycling at ~65% peak O2 uptake. Immunofluorescence microscopy was used to assess changes in oxidative capacity (measured as cytochrome c oxidase protein content), IMTG and PLIN2 and PLIN5 protein content. Resistance training increased peak O2 uptake (by 8 ± 3%), COX protein content (by 46 ± 13 and 61 ± 13% in type I and II fibres, respectively) and the Matsuda insulin sensitivity index (by 47 ± 6%; all P < 0.05). In type I fibres, IMTG (by 52 ± 11%; P < 0.05) and PLIN2 content (by 107 ± 19%; P < 0.05) were increased and PLIN5 content tended to increase (by 54 ± 22%; P = 0.054) post-training. In type II fibres, PLIN2 content increased (by 57 ± 20%; P < 0.05) and IMTG (by 46 ± 17%; P = 0.1) and PLIN5 content (by 44 ± 24%; P = 0.054) tended to increase post-training. Breakdown of IMTG during moderate-intensity exercise was greater in both type I and type II fibres (by 43 ± 5 and 37 ± 5%, respectively; P < 0.05) post-RT. The results confirm the hypothesis that RT enhances muscle oxidative capacity and increases IMTG breakdown and the content of PLIN2 and PLIN5 in both type I and type II fibres during endurance-type exercise.

Sprint interval and traditional endurance training increase net intramuscular triglyceride breakdown and expression of perilipin 2 and 5.

Intramuscular triglyceride (IMTG) utilization is enhanced by endurance training (ET) and is linked to improved insulin sensitivity. This study first investigated the hypothesis that ET-induced increases in net IMTG breakdown and insulin sensitivity are related to increased expression of perilipin 2 (PLIN2) and perilipin 5 (PLIN5). Second, we hypothesized that sprint interval training (SIT) also promotes increases in IMTG utilization and insulin sensitivity. Sixteen sedentary males performed 6 weeks of either SIT (4-6, 30 s Wingate tests per session, 3 days week(-1)) or ET (40-60 min moderate-intensity cycling, 5 days week(-1)). Training increased resting IMTG content (SIT 1.7-fold, ET 2.4-fold; P < 0.05), concomitant with parallel increases in PLIN2 (SIT 2.3-fold, ET 2.8-fold; P < 0.01) and PLIN5 expression (SIT 2.2-fold, ET 3.1-fold; P < 0.01). Pre-training, 60 min cycling at ∼65% pre-training decreased IMTG content in type I fibres (SIT 17 ± 10%, ET 15 ± 12%; P < 0.05). Following training, a significantly greater breakdown of IMTG in type I fibres occurred during exercise (SIT 27 ± 13%, ET 43 ± 6%; P < 0.05), with preferential breakdown of PLIN2- and particularly PLIN5-associated lipid droplets. Training increased the Matsuda insulin sensitivity index (SIT 56 ± 15%, ET 29 ± 12%; main effect P < 0.05). No training × group interactions were observed for any variables. In conclusion, SIT and ET both increase net IMTG breakdown during exercise and increase in PLIN2 and PLIN5 protein expression. The data are consistent with the hypothesis that increases in PLIN2 and PLIN5 are related to the mechanisms that promote increased IMTG utilization during exercise and improve insulin sensitivity following 6 weeks of SIT and ET.

Prolonged exercise training increases intramuscular lipid content and perilipin 2 expression in type I muscle fibers of patients with type 2 diabetes.

The aim of the present study was to investigate changes in intramuscular triglyceride (IMTG) content and perilipin 2 expression in skeletal muscle tissue following 6 mo of endurance-type exercise training in type 2 diabetes patients. Ten obese male type 2 diabetes patients (age 62 ± 1 yr, body mass index BMI 31 ± 1 kg/m²) completed three exercise sessions/week consisting of 40 min of continuous endurance-type exercise at 75% V(O2 peak) for a period of 6 mo. Muscle biopsies collected at baseline and after 2 and 6 mo of intervention were analyzed for IMTG content and perilipin 2 expression using fiber type-specific immunofluorescence microscopy. Endurance-type exercise training reduced trunk body fat by 6 ± 2% and increased whole body oxygen uptake capacity by 13 ± 7% (P < 0.05). IMTG content increased twofold in response to the 6 mo of exercise training in both type I and type II muscle fibers (P < 0.05). A threefold increase in perilipin 2 expression was observed from baseline to 2 and 6 mo of intervention in the type I muscle fibers only (1.1 ± 0.3, 3.4 ± 0.6, and 3.6 ± 0.6% of fibers stained, respectively, P < 0.05). Exercise training induced a 1.6-fold increase in mitochondrial content after 6 mo of training in both type I and type II muscle fibers (P < 0.05). In conclusion, this is the first study to report that prolonged endurance-type exercise training increases the expression of perilipin 2 alongside increases in IMTG content in a type I muscle fiber-type specific manner in type 2 diabetes patients.

Preferential utilization of perilipin 2-associated intramuscular triglycerides during 1 h of moderate-intensity endurance-type exercise.

The lipid droplet (LD)-associated protein perilipin 2 (PLIN2) appears to colocalize with LDs in human skeletal muscle fibres, although the function of PLIN2 in the regulation of intramuscular triglyceride (IMTG) metabolism is currently unknown. Here we investigated the hypothesis that the presence of PLIN2 in skeletal muscle LDs is related to IMTG utilisation during exercise. We therefore measured exercise-induced changes in IMTG and PLIN2 distribution and changes in their colocalization. Muscle biopsies from the vastus lateralis were obtained from seven lean, untrained men (22 ± 2 years old, body mass index 24.2 ± 0.9 kg m(-2) and peak oxygen uptake 3.35 ± 0.13 l min(-1)) before and after 1 h of moderate-intensity cycling at ~65% peak oxygen uptake. Cryosections were stained for perilipin 2, IMTG and myosin heavy chain type I and viewed using wide-field and confocal fluorescence microscopy. Exercise induced a 50 ± 7% decrease in IMTG content in type I fibres only (P < 0.05), but no change in PLIN2 content. Colocalization analysis showed that the fraction of PLIN2 associated with IMTG was 0.67 ± 0.03 before exercise, which was reduced to 0.51 ± 0.01 postexercise (P < 0.05). Further analysis revealed that the number of PLIN2-associated LDs was reduced by 31 ± 10% after exercise (P < 0.05), whereas the number of PLIN2-null LDs was unchanged. No such changes were seen in type II fibres. In conclusion, this study shows that PLIN2 content in skeletal muscle is unchanged in response to a single bout of endurance exercise. Furthermore, the PLIN2 and IMTG association is reduced postexercise, apparently due to preferential utilization of PLIN2-associated LDs. These results confirm the hypothesis that the PLIN2 association with IMTG is related to the utilization of IMTG as a fuel during exercise.

Intravenous AICAR administration reduces hepatic glucose output and inhibits whole body lipolysis in type 2 diabetic patients.

AIMS/HYPOTHESIS: The 5'-AMP-activated protein kinase (AMPK) pathway is intact in type 2 diabetic patients and is seen as a target for diabetes treatment. In this study, we aimed to assess the impact of the AMPK activator 5-aminoimidazole-4-carboxamide riboside (AICAR) on both glucose and fatty acid metabolism in vivo in type 2 diabetic patients. METHODS: Stable isotope methodology and blood and muscle biopsy sampling were applied to assess blood glucose and fatty acid kinetics following continuous i.v. infusion of AICAR (0.75 mg kg(-1) min(-1)) and/or NaCl (0.9%) in ten male type 2 diabetic patients (age 64 +/- 2 years; BMI 28 +/- 1 kg/m(2)). RESULTS: Plasma glucose rate of appearance (R (a)) was reduced following AICAR administration, while plasma glucose rate of disappearance (R (d)) was similar in the AICAR and control test. Consequently, blood glucose disposal (R (d) expressed as a percentage of R (a)) was increased following AICAR infusion (p < 0.001). Accordingly, a greater decline in plasma glucose concentration was observed following AICAR infusion (p < 0.001). Plasma NEFA R (a) and R (d) were both significantly reduced in response to AICAR infusion, and were accompanied by a significant decline in plasma NEFA concentration. Although AMPK phosphorylation in skeletal muscle was not increased, we observed a significant increase in acetyl-CoA carboxylase phosphorylation (p < 0.001). CONCLUSIONS/INTERPRETATION: The i.v. administration of AICAR reduces hepatic glucose output, thereby lowering blood glucose concentrations in vivo in type 2 diabetic patients. Furthermore, AICAR administration stimulates hepatic fatty acid oxidation and/or inhibits whole body lipolysis, thereby reducing plasma NEFA concentration.

Adipose tissue fatty acid metabolism in insulin-resistant men.

AIMS/HYPOTHESIS: Increased NEFA production and concentrations may underlie insulin resistance. We examined systemic and adipose tissue NEFA metabolism in insulin-resistant overweight men (BMI 25-35 kg/m2). METHODS: In a cohort study we examined NEFA concentrations in men in the upper quartile of fasting insulin (n = 124) and in men with fasting insulin below the median (n = 159). In a metabolic study we examined NEFA metabolism in the fasting and postprandial states, in ten insulin-resistant men and ten controls. RESULTS: In the cohort study, fasting NEFA concentrations were not significantly different between the two groups (median values: insulin-resistant men, 410 micromol/l; controls, 445 micromol/l). However, triacylglycerol concentrations differed markedly (1.84 vs 1.18 mmol/l respectively, p < 0.001). In the metabolic study, arterial NEFA concentrations again did not differ between groups, whereas triacylglycerol concentrations were significantly higher in insulin-resistant men. Systemic NEFA production and the release of NEFA from subcutaneous adipose tissue, expressed per unit of fat mass, were both reduced in insulin-resistant men compared with controls (fasting values by 32%, p = 0.02, and 44%, p = 0.04 respectively). 3-Hydroxybutyrate concentrations, an index of hepatic fat oxidation and ketogenesis, were lower (p = 0.03). CONCLUSIONS/INTERPRETATION: Adipose tissue NEFA output is not increased (per unit weight of tissue) in insulin resistance. On the contrary, it appears to be suppressed by high fasting insulin concentrations. Alterations in triacylglycerol metabolism are more marked than those in NEFA metabolism and are indicative of altered metabolic partitioning of fatty acids (decreased oxidation, increased esterification) in the liver.

Substrate source utilisation in long-term diagnosed type 2 diabetes patients at rest, and during exercise and subsequent recovery.

AIMS/HYPOTHESIS: Disturbances in substrate source metabolism and, more particularly, in fatty acid metabolism, play an important role in the aetiology and progression of type 2 diabetes. However, data on substrate source utilisation in type 2 diabetes are inconclusive. METHODS: [U-(13)C]palmitate and [6,6-(2)H(2)]glucose tracers were used to assess plasma NEFA and glucose oxidation rates and to estimate the use of muscle- and/or lipoprotein-derived triacylglycerol and muscle glycogen. Subjects were ten male patients who had a long-term (7 +/- 1 years) diagnosis of type 2 diabetes and were overweight, and ten matched healthy, male control subjects. Muscle biopsy samples were collected before and after exercise to assess muscle fibre type-specific intramyocellular lipid and glycogen content. RESULTS: At rest and during exercise, the diabetes patients had greater values than the controls for palmitate rate of appearance (Ra) (rest, 2.46 +/- 0.18 and 1.85 +/- 0.20 respectively; exercise, 3.71 +/- 0.36 and 2.84 +/- 0.20 micromol kg(-1) min(-1)) and rate of disappearance (Rd) (rest, 2.45 +/- 0.18 and 1.83 +/- 0.20; exercise, 3.64 +/- 0.35 and 2.80 +/- 0.20 micromol kg(-1) min(-1) respectively). This was accompanied by significantly higher fat oxidation rates at rest and during recovery in the diabetes patients (rest, 0.11 +/- 0.01 in diabetes patients and 0.09 +/- 0.01 in controls; recovery, 0.13 +/- 0.01 and 0.11 +/- 0.01 g/min respectively), despite significantly greater plasma glucose Ra, Rd and circulating plasma glucose concentrations. Furthermore, exercise significantly lowered plasma glucose concentrations in the diabetes patients, as a result of increased blood glucose disposal. CONCLUSION: This study demonstrates that substrate source utilisation in long-term-diagnosed type 2 diabetes patients, in whom compensatory hyperinsulinaemia is no longer present, shifts towards an increase in whole-body fat oxidation rate and is accompanied by disturbances in fat and carbohydrate handling.

Ubiquitin-proteasome-dependent proteolytic activity remains elevated after zymosan-induced sepsis in rats while muscle mass recovers.

We studied the role of the ubiquitin-proteasome system in rat skeletal muscle during sepsis and subsequent recovery. Sepsis was induced with intraperitoneal zymosan injections. This model allows one to study a sustained and reversible catabolic phase and mimics the events that prevail in septic and subsequently recovering patients. In addition, the role of the ubiquitin-proteasome system during muscle recovery is poorly documented. There was a trend for increased ubiquitin-conjugate formation in the muscle wasting phase, which was abolished during the recovery phase. The trypsin- and chymotrypsin-like peptidase activities of the 20S proteasome peaked at day 6 following zymosan injection (i.e. when both muscle mass and muscle fiber cross-sectional area were reduced the most), but remained elevated when muscle mass and muscle fiber cross-sectional area were recovering (11 days). This clearly suggests a role for the ubiquitin-proteasome pathway in the muscle remodeling and/or recovery process. Protein levels of 19S complex and 20S proteasome subunits did not increase throughout the study, pointing to alternative mechanisms regulating proteasome activities. Overall these data support a role for ubiquitin-proteasome dependent proteolysis in the zymosan septic model, in both the catabolic and muscle recovery phases.

Skeletal Muscle wasting and contractile performance in septic rats.

We investigated the temporal effects of sepsis on muscle wasting and function in order to study the contribution of wasting to the decline in muscle function; we also studied the fiber-type specificity of this muscle wasting. Sepsis was induced by injecting rats intraperitoneally with a zymosan suspension. At 2 h and at 2, 6, and 11 days after injection, muscle function was measured using in situ electrical stimulation, Zymosan injection induced severe muscle wasting compared to pair-fed and ad libitum fed controls. At 6 days, isometric force-generating capacity was drastically reduced in zymosan-treated rats. We conclude that this was fully accounted fo by the reduction of muscle mas. At day 6, we also observed increased activity of the 20S proteasome in gastrocnemius but not soleus muscle from septic rats. In tibialis anterior but not in soleus, muscle wasting occurred in a fiber-type specific fashion, i.e., the reduction in cross-sectional area was significantly smaller in type 1 than type 2A and 2B/X fibers. These findings suggest that both the inherent function of a muscle and the muscle fiber-type distribution affect the responsiveness to catabolic signals.

Substrate utilization in non-obese Type II diabetic patients at rest and during exercise.

Recently, we observed that impairments exist in skeletal muscle free fatty acid (FFA) utilization during exercise in obese subjects with Type II diabetes. The main objective of the present study was to investigate whether plasma FFA oxidation is impaired during exercise in non-obese Type II diabetic patients. Stable isotope tracers of palmitate and glucose were infused for 2 h at rest and 1h of bicycle exercise at 40% peak oxygen consumption ( V*O(2)max) in volunteers with Type II diabetes and a healthy control group. At rest, plasma FFA oxidation was not significantly different between subjects with Type II diabetes and control subjects (2.13+/-0.51 versus 1.93+/-0.54 micromol.kg(-1).min(-1) respectively). During exercise, Type II diabetic patients and control subjects had similar rates of total fat [Type II diabetes, 9.62+/-1.84 micromol.kg(-1).min(-1); control, 12.08+/-4.59 micromol.kg(-1).min(-1); not significant (NS)] and glucose oxidation (Type II diabetes, 44.24+/-10.36 micromol.kg(-1).min(-1); control, 57.37+/-14.54 micromol.kg(-1).min(-1); NS). No aberrations were present in plasma FFA uptake [rate of disappearance ( Rd ); Type II diabetes, 11.78+/-4.82; control, 10.84+/-3.39; NS] and oxidation rates (Type II diabetes 8.10+/-1.44; control 8.00+/-3.12, NS) in Type II diabetic patients; triacylglycerol-derived fatty acid oxidation was 2.6-fold lower in Type II diabetic patients than in control subjects, but this difference was not statistically significant. Muscle glycogen oxidation was lower in diabetes patients than in control subjects (Type II diabetes, 25.16+/-13.82 micromol.kg(-1).min(-1); control, 42.04+/-10.58 micromol.kg(-1).min(-1); P <0.05) and plasma glucose contributed more to energy expenditure in Type II diabetes (26+/-3% in diabetic versus 15+/-2% in control, P <0.05). We conclude that plasma FFA oxidation is not impaired during exercise in non-obese Type II diabetic patients. The data confirm that Type II diabetes is a heterogeneous disease, and that the adaptation at the substrate level differs between obese and non-obese patients and may contribute to differences in the final appearance of the various phenotypes.

The effect of glutamate infusion on cardiac performance is independent of changes in metabolism in patients undergoing routine coronary artery bypass surgery.

In a double-blind randomized placebo-controlled study, the effects of intravenous glutamate infusion on myocardial haemodynamics and metabolism were studied in 22 patients undergoing routine coronary artery bypass graft (CABG) surgery. Immediately after aortic cross-clamp release, an intravenous infusion of a solution of glutamate (125 mmol x l(-1)) at a rate of 1.5 ml x h(-1) x kg(-1) was given over 1 h to 11 patients (G group). The other 11 patients received a placebo infusion (0.9% NaCl) (P group). Haemodynamic functions and rates of exchange of glucose, non-esterified fatty acids and lactic acid over the heart were measured before sternotomy (T1), 40 min after cross-clamp release (T2) and 4 h after cross-clamp release (T3). At T2, decreases were seen in comparison with T1 in systemic vascular resistance index, and increases were seen in cardiac index and coronary sinus flow. All of these changes were greater in the G group than in the P group (P<0.05). Myocardial glutamate consumption increased 2-fold after glutamate administration. No significant changes were observed in the myocardial utilization of glucose, lactate or non-esterified fatty acids between the P and the G groups at T1, T2 or T3. These data show that an intravenous glutamate infusion after routine CABG surgery significantly improved cardiac haemodynamic performance without direct effects on cardiac substrate metabolism. This suggests that a reduction of the afterload via a peripheral vasodilatory effect is the main mechanism leading to the observed changes in haemodynamics. Earlier claims that patients with post-operative cardiac failure show metabolic benefits from the glutamate infusion do not seem to apply to patients undergoing routine CABG surgery.

The effects of increasing exercise intensity on muscle fuel utilisation in humans.

1. Contemporary stable isotope methodology was applied in combination with muscle biopsy sampling to accurately quantify substrate utilisation and study the regulation of muscle fuel selection during exercise. 2. Eight cyclists were studied at rest and during three consecutive 30 min stages of exercise at intensities of 40, 55 and 75 % maximal workload (W(max)). A continuous infusion of [U-(13)C]palmitate and [6,6-(2)H(2)]glucose was administered to determine plasma free fatty acid (FFA) oxidation and estimate plasma glucose oxidation, respectively. Biopsy samples were collected before and after each exercise stage. 3. Muscle glycogen and plasma glucose oxidation rates increased with every increment in exercise intensity. Whole-body fat oxidation increased to 32 +/- 2 kJ min(-1) at 55 % W(max), but declined at 75 % W(max) (19 +/- 2 kJ min(-1)). This decline involved a decrease in the oxidation rate of both plasma FFA and triacylglycerol fat sources (sum of intramuscular plus lipoprotein-derived triacylglycerol), and was accompanied by increases in muscle pyruvate dehydrogenase complex activation and acetylation of the carnitine pool, resulting in a decline in muscle free carnitine concentration. 4. We conclude that the most likely mechanism for the reduction in fat oxidation during high-intensity exercise is a downregulation of carnitine palmitoyltransferase I, either by this marked decline in free carnitine availability or by a decrease in intracellular pH.

Plasma free Fatty Acid uptake and oxidation are already diminished in subjects at high risk for developing type 2 diabetes.

The objective of this study was to investigate to what extent disturbances in fatty acid metabolism found in type 2 diabetes are already present in subjects at high risk for developing diabetes (i.e., impaired glucose tolerance [IGT]). Components of fatty acid metabolism were measured in male subjects with IGT during postabsorptive conditions and during 60 min of exercise (50% VO(2max)) with the use of the stable isotope tracer [U-(13)C]palmitate in combination with indirect calorimetry, and those values were compared with previously published findings in male type 2 diabetic and male obese subjects. No differences were found between groups in energy expenditure and in total fat and carbohydrate oxidation. Rate of appearance and rate of disappearance of plasma free fatty acid (FFA) were lower in subjects with IGT and type 2 diabetes compared with obese subjects (P < 0.05). Plasma FFA oxidation was lower in subjects with IGT and type 2 diabetes compared with obese subjects at rest and tended to be lower during exercise (rest: 3.7 +/- 0.3, 4.4 +/- 0.6, and 6.9 +/- 1.0 micromol. kg fat-free mass [FFM](-1). min(-1), P < 0.01; exercise: 15.0 +/- 1.7, 14.1 +/- 1.9, and 19.6 +/- 1.5 micromol. kg FFM(-1). min(-1) for IGT, type 2 diabetic, and obese subjects, respectively, P = 0.07). Triglyceride-derived fatty acid oxidation, however, was elevated in subjects with IGT and type 2 diabetes during exercise (3.6 +/- 1.4, 1.4 +/- 1.4, and -4.0 +/- 2.0 micromol. kg FFM(-1). min(-1) for IGT, type 2 diabetic, and obese subjects, respectively; P < 0.05). These data demonstrate that male subjects with a prediabetic condition (IGT) have the same defects in fatty acid utilization as subjects with type 2 diabetes, suggesting that these disturbances may play an important role in the progression from IGT to type 2 diabetes.

Muscle function in critically ill patients.

UNLABELLED: Endotoxemia and inflammation (cytokines) lead to an acute decrease of the muscle resting membrane potential, loss of the sodium-potassium gradient and to an increase in cytosolic Ca(2+) in critically ill intensive care unit patients. As a consequence, muscle (and nerve) contractility is reduced. As a consequence also, amino acid gradients are reduced, proteolysis is increased, the mitochondrial density is reduced to levels as low as 10% of normal within 2-3 days and cellular substrate metabolism is impaired. The author of this paper proposes that treatment modalities in clinical nutrition should primarily aim at improving muscle function and restoring muscle membrane potential and that these variables should be used as the primary outcome variables of clinical trials. Attempts to modify these measurements such that they can be used routinely in the ICU setting are ongoing in our group. Muscle protein and substrate metabolism can only be normalized when these primary variables have successfully been restored. The use of muscle relaxants may lead to a functional denervation of the muscle, to changes in the molecular structure of the myofibrils and may postpone a successfull recovery. LEARNING OBJECTIVES: Causes of muscle weakness and loss of contractility in ICU patients; Relation between loss of contractility and impairments in muscle metabolism; Muscle function as an endpoint variable for clinical nutrition interventions.

Addition of protein and amino acids to carbohydrates does not enhance postexercise muscle glycogen synthesis.

Ingestion of a protein-amino acid mixture (Pro; wheat protein hydrolysate, leucine, and phenylalanine) in combination with carbohydrate (CHO; 0.8 g x kg(-1) x h(-1)) has been shown to increase muscle glycogen synthesis after exercise compared with the same amount of CHO without Pro. The aim of this study was to investigate whether coingestion of Pro also increases muscle glycogen synthesis when 1.2 g CHO. kg(-1). h(-1) is ingested. Eight male cyclists performed two experimental trials separated by 1 wk. After glycogen-depleting exercise, subjects received either CHO (1.2 g x kg(-1) x h(-1)) or CHO+Pro (1.2 g CHO x kg(-1) x h(-1) + 0.4 g Pro x kg(-1) x h(-1)) during a 3-h recovery period. Muscle biopsies were obtained immediately, 1 h, and 3 h after exercise. Blood samples were collected immediately after the exercise bout and every 30 min thereafter. Plasma insulin was significantly higher in the CHO+Pro trial compared with the CHO trial (P < 0.05). No difference was found in plasma glucose or in rate of muscle glycogen synthesis between the CHO and the CHO+Pro trials. Although coingestion of a protein amino acid mixture in combination with a large CHO intake (1.2 g x kg(-1) x h(-1)) increases insulin levels, this does not result in increased muscle glycogen synthesis.