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.

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 N^G-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.

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.

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.