Marked heterogeneity of human skeletal muscle lipolysis at rest

Intranasal orexin A modulates sympathetic vascular tone - a pilot study in healthy male humans

INTRODUCTION

Previous research suggests that the neuropeptide orexin A contributes to sympathetic blood pressure (BP) control inasmuch as hypothalamic injection of orexin A increases sympathetic vasomotor tone and arterial BP in rodents. In humans with narcolepsy, a disorder associated with loss of orexin-producing neurons, vasoconstrictive muscle sympathetic nerve activity (MSNA) is reduced. Since intranasally administered oligopeptides like orexin are known to modulate brain function, we investigated the effect of intranasal orexin A on vascular sympathetic baroreflex function in healthy humans.

METHODS

In a balanced, double-blind cross-over study, orexin A (500 nmol) and placebo, respectively, were intranasally administered to 10 lean healthy males (age, 25.8±4.6 years). MSNA was assessed microneurographically before and 30-45 minutes after either substance administration. Additionally, baroreflex was challenged via graded infusions of vasoactive drugs before and after substance administration. Baroreflex function was defined as the correlation of BP with MSNA and heart rate.

RESULTS

Intranasal orexin A compared to placebo induced a significant increase in resting MSNA from prior to post administration (Δ-burst rate, orexin A vs. placebo: +5.8±0.8 vs. +2.1±0.6; p=0.007; total activity (169±11.5% vs. 115±5.0%; p=0.002). BP, heart rate and sympathovagal balance to the heart, as represented by HRV, as well as baroreflex sensitivity during the vasoactive challenge were not altered.

CONCLUSION

Intranasally administered orexin A acutely induced vasoconstrictory sympathoactivation in healthy male humans. This result suggests that orexin A mediates upward resetting of the vascular baroreflex setpoint at centers superordinate to the mere baroreflex-feedback-loop.

Combined effects of a ketogenic diet and exercise training alter mitochondrial and peroxisomal substrate oxidative capacity in skeletal muscle

Ketogenic diets (KDs) are reported to improve body weight, fat mass, and exercise performance in humans. Unfortunately, most rodent studies have used a low-protein KD, which does not recapitulate diets used by humans. Since skeletal muscle plays a critical role in responding to macronutrient perturbations induced by diet and exercise, the purpose of this study was to test if a normal-protein KD (NPKD) impacts shifts in skeletal muscle substrate oxidative capacity in response to exercise training (ExTr). A high fat, carbohydrate-deficient NPKD (16.1% protein, 83.9% fat, 0% carbohydrate) was given to C57BL/6J male mice for 6 wk, whereas controls (Con) received a low-fat diet with similar protein (15.9% protein, 11.9% fat, 72.2% carbohydrate). After 3 wk on the diet, mice began treadmill training 5 days/wk, 60 min/day for 3 wks. The NPKD increased body weight and fat mass, whereas ExTr negated a continued rise in adiposity. ExTr increased intramuscular glycogen, whereas the NPKD increased intramuscular triglycerides. Neither the NPKD nor ExTr alone altered mitochondrial content; however, in combination, the NPKD-ExTr group showed increases in PGC-1α and markers of mitochondrial fission/fusion. Pyruvate oxidative capacity was unchanged by either intervention, whereas ExTr increased leucine oxidation in NPKD-fed mice. Lipid metabolism pathways had the most notable changes as the NPKD and ExTr interventions both enhanced mitochondrial and peroxisomal lipid oxidation and many adaptations were additive or synergistic. Overall, these results suggest that a combination of a NPKD and ExTr induces additive and/or synergistic adaptations in skeletal muscle oxidative capacity.NEW & NOTEWORTHY A ketogenic diet with normal protein content (NPKD) increases body weight and fat mass, increases intramuscular triglyceride storage, and upregulates pathways related to protein metabolism. In combination with exercise training, a NPKD induces additive and/or synergistic activation of AMPK, PGC-1α, mitochondrial fission/fusion genes, mitochondrial fatty acid oxidation, and peroxisomal adaptations in skeletal muscle. Collectively, results from this study provide mechanistic insight into adaptations in skeletal muscle relevant to keto-adaptation.

Phase variation in Mycobacterium tuberculosis glpK produces transiently heritable drug tolerance

The length and complexity of tuberculosis (TB) therapy, as well as the propensity of Mycobacterium tuberculosis to develop drug resistance, are major barriers to global TB control efforts. M. tuberculosis is known to have the ability to enter into a drug-tolerant state, which may explain many of these impediments to TB treatment. We have identified a mechanism of genetically encoded but rapidly reversible drug tolerance in M. tuberculosis caused by transient frameshift mutations in a homopolymeric tract (HT) of 7 cytosines (7C) in the glpK gene. Inactivating frameshift mutations associated with the 7C HT in glpK produce small colonies that exhibit heritable multidrug increases in minimal inhibitory concentrations and decreases in drug-dependent killing; however, reversion back to a fully drug-susceptible large-colony phenotype occurs rapidly through the introduction of additional insertions or deletions in the same glpK HT region. These reversible frameshift mutations in the 7C HT of M. tuberculosis glpK occur in clinical isolates, accumulate in M. tuberculosis-infected mice with further accumulation during drug treatment, and exhibit a reversible transcriptional profile including induction of dosR and sigH and repression of kstR regulons, similar to that observed in other in vitro models of M. tuberculosis tolerance. These results suggest that GlpK phase variation may contribute to drug tolerance, treatment failure, and relapse in human TB. Drugs effective against phase-variant M. tuberculosis may hasten TB treatment and improve cure rates.

Sprint interval exercise versus continuous moderate intensity exercise: acute effects on tissue oxygenation, blood pressure and enjoyment in 18-30 year old inactive men

BACKGROUND

Sprint interval training (SIT) can be as effective, or more effective, than continuous moderate intensity exercise (CMIE) for improving a primary risk factor for cardiometabolic disease, low cardiorespiratory fitness (CRF). However, there has been no direct comparison in inactive individuals, of the acute effects of a session of SIT with a work-matched session of CMIE on local oxygen utilisation, which is a primary stimulus for increasing CRF. Furthermore, post-exercise blood pressure (BP) and enjoyment, if symptomatic and low, respectively, have implications for safety and adherence to exercise and have not been compared between these specific conditions. It was hypothesised that in young inactive men, local oxygen utilisation would be higher, while post-exercise BP and enjoyment would be lower for SIT, when compared to CMIE.

METHODS

A total of 11 inactive men (mean ± SD; age 23 ± 4 years) completed a maximal ramp-incremental exercise test followed by two experiment conditions: (1) SIT and (2) work-matched CMIE on a cycle ergometer on separate days. Deoxygenated haemoglobin (∆HHb) in the pre-frontal cortex (FH), gastrocnemius (GN), left vastus lateralis (LVL) and the right vastus lateralis (RVL) muscles, systemic oxygen utilisation (VO2), systolic (SBP) and diastolic (DBP) blood pressure and physical activity enjoyment scale (PACES) were measured during the experiment conditions.

RESULTS

During SIT, compared to CMIE, ∆HHb in FH (p = 0.016) and GN (p = 0.001) was higher, while PACES (p = 0.032) and DBP (p = 0.043) were lower. No differences in SBP and ∆HHb in LVL and RVL were found between conditions.

CONCLUSIONS

In young inactive men, higher levels of physiological stress occurred during SIT, which potentially contributed to lower levels of post-exercise DBP and enjoyment, when compared to CMIE.

The effect of running versus cycling high-intensity intermittent exercise on local tissue oxygenation and perceived enjoyment in 18-30-year-old sedentary men

BACKGROUND

High-intensity interval training (HIIT) has been proposed as a time-efficient exercise format to improve exercise adherence, thereby targeting the chronic disease burden associated with sedentary behaviour. Exercise mode (cycling, running), if self-selected, will likely affect the physiological and enjoyment responses to HIIT in sedentary individuals. Differences in physiological and enjoyment responses, associated with the mode of exercise, could potentially influence the uptake and continued adherence to HIIT. It was hypothesised that in young sedentary men, local and systemic oxygen utilisation and enjoyment would be higher during a session of running HIIT, compared to a session of cycling HIIT.

METHODS

A total of 12 sedentary men (mean ± SD; age 24 ± 3 years) completed three exercise sessions: a maximal incremental exercise test on a treadmill (MAX) followed by two experiment conditions, (1) free-paced cycling HIIT on a bicycle ergometer (HIITCYC) and (2) constant-paced running HIIT on a treadmill ergometer (HIITRUN). Deoxygenated haemoglobin (HHb) in the gastrocnemius (GN), the left vastus lateralis (LVL) and the right vastus lateralis (RVL) muscles, oxygen consumption (VO2), heart rate (HR), ratings of perceived exertion (RPE) and physical activity enjoyment (PACES) were measured during HIITCYC and HIITRUN.

RESULTS

There was a higher HHb in the LVL (p = 0.001) and RVL (p = 0.002) sites and a higher VO2 (p = 0.017) and HR (p < 0.001) during HIITCYC, compared to HIITRUN. RPE was higher (p < 0.001) and PACES lower (p = 0.032) during HIITCYC compared to HIITRUN.

DISCUSSION

In sedentary individuals, free-paced cycling HIIT produces higher levels of physiological stress when compared to constant-paced running HIIT. Participants perceived running HIIT to be more enjoyable than cycling HIIT. These findings have implications for selection of mode of HIIT for physical stress, exercise enjoyment and compliance.

Triglyceride metabolism in exercising muscle

Triglycerides are stored within lipid droplets in skeletal muscle and can be hydrolyzed to produce fatty acids for energy production through β-oxidation and oxidative phosphorylation. While there was some controversy regarding the quantitative importance of intramyocellular triglyceride (IMTG) as a metabolic substrate, recent advances in proton magnetic resonance spectroscopy and confocal microscopy support earlier tracer and biopsy studies demonstrating a substantial contribution of IMTG to energy production, particularly during moderate-intensity endurance exercise. This review provides an update on the understanding of IMTG utilization during exercise, with a focus on describing the key regulatory proteins that control IMTG breakdown and how these proteins respond to acute exercise and in the adaptation to exercise training. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

The Effect of Active versus Passive Recovery Periods during High Intensity Intermittent Exercise on Local Tissue Oxygenation in 18 - 30 Year Old Sedentary Men

PURPOSE

High intensity interval training (HIIT) has been proposed as a time-efficient format of exercise to reduce the chronic disease burden associated with sedentary behaviour. Changes in oxygen utilisation at the local tissue level during an acute session of HIIT could be the primary stimulus for the health benefits associated with this format of exercise. The recovery periods of HIIT effect the physiological responses that occur during the session. It was hypothesised that in sedentary individuals, local and systemic oxygen utilisation would be higher during HIIT interspersed with active recovery periods, when compared to passive recovery periods.

METHODS

Twelve sedentary males (mean ± SD; age 23 ± 3 yr) completed three conditions on a cycle ergometer: 1) HIIT with passive recovery periods between four bouts (HIITPASS) 2) HIIT with active recovery periods between four bouts (HIITACT) 3) HIITACT with four HIIT bouts replaced with passive periods (REC). Deoxygenated haemoglobin (HHb) in the vastus lateralis (VL) and gastrocnemius (GN) muscles and the pre-frontal cortex (FH), oxygen consumption (VO2), power output and heart rate (HR) were measured continuously during the three conditions.

RESULTS

There was a significant increase in HHb at VL during bouts 2 (p = 0.017), 3 (p = 0.035) and 4 (p = 0.035) in HIITACT, compared to HIITPASS. Mean power output was significantly lower in HIITACT, compared to HIITPASS (p < 0.001). There was a significant main effect for site in both HIITPASS (p = 0.029) and HIITACT (p = 0.005). There were no significant differences in VO2 and HR between HIITPASS and HIITACT.

CONCLUSIONS

The increase in HHb at VL and the lower mean power output during HIITACT could indicate that a higher level of deoxygenation contributes to decreased mechanical power in sedentary participants. The significant differences in HHb between sites indicates the specificity of oxygen utilisation.

IL-6 and epinephrine have divergent fiber type effects on intramuscular lipolysis

IL-6 is an exercise-regulated myokine that has been suggested to increase lipolysis in fast-twitch skeletal muscle. However, it is not known if a similar effect is present in slow-twitch muscle. Furthermore, epinephrine increases IL-6 secretion from skeletal muscle, suggesting that IL-6 could play a role in mediating the lipolytic effects of catecholamines. The purpose of this study was to determine whether IL-6 stimulates skeletal muscle lipolysis in a fiber type dependent manner and is required for epinephrine-stimulated lipolysis in murine skeletal muscle. Soleus and extensor digitorum longus (EDL) muscles from male C57BL/6J wild-type and IL-6(-/-) mice were incubated with 1 μM (183 ng/ml) epinephrine or 75 ng/ml recombinant IL-6 (rIL-6) for 60 min. IL-6 treatment increased 5'-AMP-activated protein kinase and signal transducer and activator of transcription 3 phosphorylation and glycerol release in isolated EDL but not soleus muscles from C57BL/6J mice. Conversely, epinephrine increased glycerol release in soleus but not EDL muscles from C57BL/6J mice. Basal lipolysis was elevated in soleus muscle from IL-6(-/-) mice, and this was associated with increases in adipose triglyceride lipase (ATGL) and its coactivator comparative gene identification-58 (CGI-58). The increase in ATGL content does not appear to be due to a loss of IL-6's direct effects, because ex vivo treatment with IL-6 failed to alter the expression of ATGL mRNA in soleus muscle. In summary, IL-6 stimulates lipolysis in glycolytic but not oxidative muscle, whereas the opposite fiber type effect is seen with epinephrine. The absence of IL-6 indirectly upregulates lipolysis, and this is associated with increases in ATGL and its coactivator CGI-58.

Exercise training favors increased insulin-stimulated glucose uptake in skeletal muscle in contrast to adipose tissue: a randomized study using FDG PET imaging

Physical exercise increases peripheral insulin sensitivity, but regional differences are poorly elucidated in humans. We investigated the effect of aerobic exercise training on insulin-stimulated glucose uptake in five individual femoral muscle groups and four different adipose tissue regions, using dynamic (femoral region) and static (abdominal region) 2-deoxy-2-[¹⁸F]fluoro-d-glucose (FDG) PET/CT methodology during steady-state insulin infusion (40 mU·m⁻²·min⁻¹). Body composition was measured by dual X-ray absorptiometry and MRI. Sixty-one healthy, sedentary [V(O2max) 36(5) ml·kg⁻¹·min⁻¹; mean(SD)], moderately overweight [BMI 28.1(1.8) kg/m²], young [age: 30(6) yr] men were randomized to sedentary living (CON; n = 17 completers) or moderate (MOD; 300 kcal/day, n = 18) or high (HIGH; 600 kcal/day, n = 18) dose physical exercise for 11 wk. At baseline, insulin-stimulated glucose uptake was highest in femoral skeletal muscle followed by intraperitoneal visceral adipose tissue (VAT), retroperitoneal VAT, abdominal (anterior + posterior) subcutaneous adipose tissue (SAT), and femoral SAT (P < 0.0001 between tissues). Metabolic rate of glucose increased similarly (~30%) in the two exercise groups in femoral skeletal muscle (MOD 24[9, 39] μmol·kg⁻¹·min⁻¹, P = 0.004; HIGH 22[9, 35] μmol·kg⁻¹·min⁻¹, P = 0.003) (mean[95% CI]) and in five individual femoral muscle groups but not in femoral SAT. Standardized uptake value of FDG decreased ~24% in anterior abdominal SAT and ~20% in posterior abdominal SAT compared with CON but not in either intra- or retroperitoneal VAT. Total adipose tissue mass decreased in both exercise groups, and the decrease was distributed equally among subcutaneous and intra-abdominal depots. In conclusion, aerobic exercise training increases insulin-stimulated glucose uptake in skeletal muscle but not in adipose tissue, which demonstrates some interregional differences.

Phosphorylation of adipose triglyceride lipase Ser(404) is not related to 5'-AMPK activation during moderate-intensity exercise in humans

Intramyocellular triacylglycerol provides fatty acid substrate for ATP generation in contracting muscle. The protein adipose triglyceride lipase (ATGL) is a key regulator of triacylglycerol lipolysis and whole body energy metabolism at rest and during exercise, and ATGL activity is reported to be enhanced by 5'-AMP-activated protein kinase (AMPK)-mediated phosphorylation at Ser(406) in mice. This is a curious observation, because AMPK activation reduces lipolysis in several cell types. We investigated whether the phosphorylation of ATGL Ser(404) (corresponding to murine Ser(406)) was increased during exercise in human skeletal muscle and with pharmacological AMPK activation in myotubes in vitro. In human experiments, skeletal muscle and venous blood samples were obtained from recreationally active male subjects before and at 5 and 60 min during exercise. ATGL Ser(404) phosphorylation was not increased from rest during exercise, but ATGL Ser(404) phosphorylation correlated with myosin heavy chain 1 expression, suggesting a possible fiber type dependency. ATGL Ser(404) phosphorylation was not related to increases in AMPK activity, and immunoprecipitation experiments indicated no interaction between AMPK and ATGL. Rather, ATGL Ser(404) phosphorylation was associated with protein kinase A (PKA) signaling. ATGL Ser(406) phosphorylation in C(2)C(12) myotubes was unaffected by 5-aminoimidazole-4-carboxaminde-1-β-d-ribofuranoside, an AMPK activator, and the PKA activator forskolin. Our results demonstrate that ATGL Ser(404) phosphorylation is not increased in mixed skeletal muscle during moderate-intensity exercise and that AMPK does not appear to be an activating kinase for ATGL Ser(404/406) in skeletal muscle.

Perilipin family (PLIN) proteins in human skeletal muscle: the effect of sex, obesity, and endurance training

Proteins that coat the lipid droplets (also known as PAT proteins or perilipin (PLIN) family proteins) have diverse functions that are not well elucidated in many tissues. In skeletal muscle, there is even less known about the functions or characteristics of these proteins or how they might change in response to perturbations that alter both intramyocellular lipid (IMCL) content and fat utilization and oxidation. Therefore, the purpose of this study was to examine the human muscle content and gene expression of the four skeletal muscle PLIN proteins in both lean and obese men and women and how this was changed following a 12-week endurance training protocol. PLIN2–PLIN5 proteins were all more abundant in women than in men (p = 0.037 and p < 0.0001, respectively), consistent with higher IMCL content observed in female skeletal muscle. PLIN5 (previously known as OXPAT) is of particular interest because it has previously been associated primarily with oxidative tissues that rely heavily on fat oxidation for energy production. Although PLIN5 was not different between lean and obese subjects, it was the only PLIN protein to increase in response to endurance training in both sexes. PLIN5 correlated with IMCL volume (p < 0.0001), but in general, the other PLIN proteins did not correlate well with IMCL volume, suggesting that the relationship between lipid accumulation and PLIN family protein content is not a simple one. Although more work is necessary, it is clear that PLIN5 likely plays an important role in IMCL accumulation and oxidation, both of which increase with endurance training in human skeletal muscle.

Intranasal application of the melanocortin 4 receptor agonist MSH/ACTH(4-10) in humans causes lipolysis in white adipose tissue

OBJECTIVE

The melanocortin system has a highly significant role in the hypothalamic regulation of body weight and energy expenditure. In animals, intracerebroventricular infusion of melanocortin receptor 4 (MCR-4) agonists increases basal metabolic rate through activation of the sympathetic nervous system and subsequently reduces food intake. In humans, direct access of MCR-4 agonists to the central nervous system can be achieved by a transnasal route, which leads to weight loss with chronic administration. In the present study, we aimed at investigating the effects of intranasally administered MC4-R agonist MSH/ACTH(4-10) on lipolysis and sympathetic nervous system activity in healthy humans.

DESIGN

Healthy normal weight, male volunteers (n=10) received either 10 mg MSH/ACTH(4-10) or placebo intranasally in a double-blinded randomized crossover design. Interstitial glycerol release was assessed by microdialysis in abdominal white adipose tissue (WAT) and in skeletal muscle (SM) of the forearm. Local blood flow, systemic blood pressure, heart rate and muscle sympathetic nerve activity (MSNA) within the superficial peroneal nerve were recorded at rest and after nitroprusside infusion.

RESULTS

At 45 min after MSH/ACTH(4-10) administration WAT glycerol concentrations increased by 53.4±19.3% compared with baseline conditions (P<0.05) and remained significantly higher throughout the experiment when compared with placebo (P<0.05) while local glycerol release in SM was not significantly affected. Resting MSNA was not altered by MSH/ACTH(4-10) administration; however, sympathoexcitation by intravenous nitroprusside was markedly elevated (MSH/ACTH(4-10) 569±69% increase to baseline; placebo: 315±64%; P<0.01).

CONCLUSION

Intranasally administered MCR-4 agonist MSH/ACTH 4-10 increases both subcutaneous WAT lipolysis and MSNA, which suggests a direct central nervous peptide effect in humans on key factors of human energy metabolism.

Greater systemic lipolysis in women compared with men during moderate-dose infusion of epinephrine and/or norepinephrine

Women have lower circulating catecholamine levels during metabolic perturbations, such as exercise or hypoglycemia, but similar rates of systemic lipolysis. This suggests women may be more sensitive to the lipolytic action of catecholamines, while maintaining similar glucoregulatory effects. The aim of the present study, therefore, was to determine whether women have higher rates of systemic lipolysis compared with men in response to matched peripheral infusion of catecholamines, but similar rates of glucose turnover. Healthy, nonobese women (n = 11) and men (n = 10) were recruited and studied on 3 separate days with the following infusions: epinephrine (Epi), norepinephrine (NE), or the two combined. Tracer infusions of glycerol and glucose were used to determine systemic lipolysis and glucose turnover, respectively. Following basal measurements of substrate kinetics, the catecholamine infusion commenced, and measures of substrate kinetics continued for 60 min. Catecholamine concentrations were similarly elevated in women and men during each infusion: Epi, 182-197 pg/ml and NE, 417-507 pg/ml. There was a significant sex difference in glycerol rate of appearance and rate of disappearance with the catecholamine infusions (P < 0.0001), mainly due to a significantly greater glycerol turnover during the first 30 min of each infusion: glycerol rate of appearance during Epi was only 268 +/- 18 vs. 206 +/- 21 micromol/min in women and men, respectively; during NE, only 173 +/- 13 vs. 153 +/- 17 micromol/min, and during Epi+NE, 303 +/- 24 vs. 257 +/- 21 micromol/min. No sex differences were observed in glucose kinetics under any condition. In conclusion, these data suggest that women are more sensitive to the lipolytic action of catecholamines, but have no difference in their glucoregulatory response. Thus the lower catcholamine levels observed in women vs. men during exercise and other metabolic perturbations may allow women to maintain a similar or greater level of lipid mobilization while minimizing changes in glucose turnover.

Stable isotope-labeled tracers for the investigation of fatty acid and triglyceride metabolism in humans in vivo

Understanding lipid metabolism and its regulation requires information on the rates at which lipids are produced within the body, absorbed (dietary lipids) into the body, transported within the body, and utilized by various tissues. This article focuses on the use of stable isotope-labeled tracers for the quantitative evaluation of major pathways of fatty acid and triglyceride metabolism in humans in vivo. Adipose tissue lipolysis and free fatty acid appearance in plasma, fatty acid tissue uptake and oxidation, and hepatic very low-density lipoprotein triglyceride secretion are among the metabolic pathways that can be studied by using stable isotope labeled tracers, and will be discussed in detail. The methodology has been in use for many years and is constantly being refined. A variety of tracers and analytical approaches are available and can be used; knowing the advantages, assumptions, and limitations of each is essential for the planning of studies and the interpretation of data, which can provide unique insights into human lipid metabolism.

Caveolin, GLUT4 and insulin receptor protein content in human arm and leg muscles

Recent evidence suggests that insulin sensitivity is relatively better preserved in arm muscle than in leg muscle in both healthy controls and type 2 DM, based on measurements of basal and insulin-mediated glucose clearance performed simultaneously in the two sets of muscles. It has also been reported that glucose uptake rates are higher in arm compared to leg muscles in the fasted state during normo-insulinaemia. However, the mechanism(s) for this are unknown. Currently, no information is available on the content of glucose transport proteins between arm and leg muscles. Therefore, we compared four proteins, Caveolin-1 (Cav-1), Caveolin-3 (Cav-3), GLUT4, and IR-beta, each of which plays an important role in regulating glucose transport between arm and leg muscles using muscle samples that were obtained from the deltoid (DEL) and vastus lateralis (VAS) of 14 male college pentathlon athletes before and after two swimming trials performed over 100 and 1,500 m. In the present study, we have shown the levels of Cav-1, -3, GLUT4, and IR-beta measured together for the first time in human arm and leg muscles. There was no difference in the levels of these proteins between arm and leg muscles. Cav-3, GLUT4, and IR-beta were unchanged from the resting levels after both exercise trials in DEL, while Cav-1 was increased (17%) at the end of the longer swim trial. In contrast, all measurements of Cav-1, -3, GLUT4, and IR-beta after the 1,500 m swim trial in VAS were increased, by 120, 46, 123, and 60%, respectively. These data imply that there was no functional difference in glucose transport capacity between arm and leg muscles in highly trained pentathlon athletes in the resting state. Although Cav-3, GLUT4, and IR-beta were unchanged from the resting levels at the end of both exercise trials in DEL, all measures, including Cav-1, increased after the 1,500 m swim trial in VAS.

Atrial natriuretic peptide induces postprandial lipid oxidation in humans

OBJECTIVE

Atrial natriuretic peptide (ANP) regulates arterial blood pressure. In addition, ANP has recently been shown to promote human adipose tissue lipolysis through cGMP-mediated hormone-sensitive lipase activation. We hypothesized that ANP increases postprandial free fatty acid (FFA) availability and energy expenditure while decreasing arterial blood pressure.

RESEARCH DESIGN AND METHODS

We infused human ANP (25 ng . kg(-1) . min(-1)) in 12 men (age 32 +/- 0.8 years, BMI 23.3 +/- 0.4 kg/m(2)) before, during, and 2 h after ingestion of a standardized high-fat test meal in a randomized, double-blind, cross-over fashion. Cardiovascular changes were monitored by continuous electrocardiogram and beat-by-beat blood pressure recordings. Metabolism was monitored through venous blood sampling, intramuscular and subcutaneous abdominal adipose tissue microdialysis, and indirect calorimetry.

RESULTS

ANP infusion decreased mean arterial blood pressure by 4 mmHg during the postprandial phase (P < 0.01 vs. placebo). At the same time, ANP induced lipolysis systemically (P < 0.05 vs. placebo) and locally in subcutaneous abdominal adipose tissue (P < 0.0001 vs. placebo), leading to a 50% increase in venous glycerol (P < 0.01) and FFA (P < 0.05) concentrations compared with placebo. The increase in FFA availability with ANP was paralleled by a 15% increase in lipid oxidation rates (P < 0.05 vs. placebo), driving a substantial increase in postprandial energy expenditure (P < 0.05 vs. placebo).

CONCLUSIONS

Our data identify the ANP system as a novel pathway regulating postprandial lipid oxidation, energy expenditure, and concomitantly arterial blood pressure. The findings could have therapeutic implications.

Hormone-sensitive lipase serine phosphorylation and glycerol exchange across skeletal muscle in lean and obese subjects: effect of beta-adrenergic stimulation

OBJECTIVE

Increased intramuscular triacylglycerol (IMTG) storage is a characteristic of the obese insulin-resistant state. We aimed to investigate whether a blunted fasting or beta-adrenergically mediated lipolysis contributes to this increased IMTG storage in obesity.

RESEARCH DESIGN AND METHODS

Forearm skeletal muscle lipolysis was investigated in 13 lean and 10 obese men using [(2)H(5)]glycerol combined with the measurement of arteriovenous differences before and during beta-adrenergic stimulation using the nonselective beta-agonist isoprenaline (ISO). Muscle biopsies were taken from the vastus lateralis muscle before and during ISO to investigate hormone-sensitive lipase (HSL) protein expression and serine phosphorylation.

RESULTS

Baseline total glycerol release across the forearm was significantly blunted in obese compared with lean subjects (P = 0.045). This was accompanied by lower HSL protein expression (P = 0.004), HSL phosphorylation on PKA sites Ser(563) (P = 0.041) and Ser(659) (P = 0.09), and HSL phosphorylation on the AMPK site Ser(565) (P = 0.007), suggesting a blunted skeletal muscle lipolysis in obesity. Total forearm glycerol uptake during baseline did not differ significantly between groups, whereas higher net fatty acid uptake across the forearm was observed in the obese (P = 0.064). ISO induced an increase in total glycerol release from skeletal muscle, which was not significantly different between groups. Interestingly, this was accompanied by an increase in HSL Ser(659) phosphorylation in obese subjects during ISO compared with baseline (P = 0.008).

CONCLUSIONS

Obesity is accompanied by impaired fasting glycerol release, lower HSL protein expression, and serine phosphorylation. It remains to be determined whether this is a primary factor or an adaptation to the obese insulin-resistant state.

Catecholamine regulation of local lactate production in vivo in skeletal muscle and adipose tissue: role of -adrenoreceptor subtypes

CONTEXT

The regulation of lactate production in skeletal muscle (SM) and adipose tissue (AT) is not fully elucidated.

OBJECTIVE

Our objective was to investigate the catecholamine-mediated regulation of lactate production and blood flow in SM and AT in healthy, normal-weight subjects by using microdialysis.

METHODS

First, lactate levels in SM and AT were measured during an iv norepinephrine infusion (n = 11). Local blood flow was determined with the 133Xe-clearance technique. Second, muscle lactate was measured during hypoglycemia and endogenous epinephrine stimulation (n = 12). Third, SM was perfused with selective beta(1-3)-adrenoreceptor agonists in situ (n = 8). Local blood flow was measured with the ethanol perfusion technique.

RESULTS

In response to iv norepinephrine, the fractional release of lactate (difference between tissue and arterial lactate) increased by 40% in SM (P = 0.001), whereas remaining unchanged in AT. Blood flow decreased by 40% in SM (P < 0.005) and increased by 50% in AT (P < 0.05). In response to hypoglycemia, epinephrine increased 10-fold, and the fractional release of lactate in SM doubled (P < 0.0001). The blood flow remained unchanged. The beta2-agonist, terbutaline, caused a marked concentration-dependent increase of muscle lactate and blood flow (P < 0.0001). The beta(1)-agonist, dobutamine, induced a discrete increase of muscle lactate (P < 0.0001), and the blood flow remained unchanged. The beta3-agonist, CPG 12177, did not affect muscle lactate or blood flow.

CONCLUSIONS

Catecholamines stimulate lactate production in SM, but not in AT. In SM, the beta2-adrenoreceptor is the most important beta-adrenergic receptor subtype in the regulation of lactate production.

Lactate release from adipose tissue and skeletal muscle in vivo: defective insulin regulation in insulin-resistant obese women

To study the local tissue lactate production in the normal state and its possible disturbances in insulin resistance, rates of lactate release from adipose tissue (AT) and skeletal muscle (SM) were compared postabsorptively and during a hyperinsulinemic euglycemic clamp in 11 healthy nonobese and 11 insulin-resistant obese women. A combination of microdialysis, to measure interstitial lactate, and the 133Xe clearance technique, to determine local blood flow, were used. In the controls, local blood flow increased by 40% in SM (P<0.05) and remained unchanged in AT, whereas the interstitial-plasma difference in lactate doubled in AT (P<0.005) and was unaffected in SM during hyperinsulinemia. In the obese, blood flow and interstitial-plasma difference in lactate remained unchanged in both tissues during hyperinsulinemia. The lactate release (micromol100 g-1min-1) was 1.17+/-0.22 in SM and 0.43+/-0.11 in AT among the controls (P<0.01) and 0.86+/-0.23 in SM and 0.83+/-0.25 in AT among the obese women in the postabsorptive state. During insulin infusion, lactate release in the controls increased to 1.92+/-0.26 in SM (P<0.005) and to 1.14+/-0.22 in AT (P<0.005) but remained unchanged in the obese women. It is concluded that AT and SM are both significant sources of lactate release postabsorptively, and AT is at least as responsive to insulin as SM. The ability to increase lactate release in response to insulin is impaired in AT and SM in insulin-resistant obese women, involving defective insulin regulation of both tissue lactate metabolism and local blood flow.

Regional differences in glucose clearance: effects of insulin and resistance training on arm and leg glucose clearance in older hypertensive individuals

The purpose of this study was to compare insulin's ability to stimulate glucose uptake in the arm and leg in a group of older hypertensive individuals (n = 13, 66 +/- 2 yr old). We also examined the effect of a 4-mo whole body resistance-training (RT) program on arm and leg glucose clearance (GC) during a hyperinsulinemic-euglycemic clamp. During the hyperinsulinemic-euglycemic clamp, GC was assessed by simultaneous measurement of arm and leg blood flow (BF) and assessment of fractional glucose extraction (GE) in blood samples from the brachial artery, brachial vein, and popliteal vein. At baseline, a significant main effect (arm vs. leg) demonstrated greater GC and BF in the arm than in the leg (P = 0.006 for GC and P = 0.012 for BF). Insulin significantly increased GE, BF, and GC in the arm and leg (main effects: P = 0.0001 for GE, P = 0.0001 for BF, and P = 0.0001 for GC) at baseline. However, the effect of insulin was similar in the arm and leg. After RT, a significant main effect (baseline vs. RT) demonstrated greater GE and GC in the leg (P = 0.024 for GE and P = 0.053 for GE), but not in the arm (P = 0.31 for GE and P = 0.14 for GC). No significant main effect (baseline vs. RT) for BF in the arm or leg was observed after RT. In conclusion, the greater GC in the arm than in the leg at baseline is primarily due to enhanced arm BF. Furthermore, whole body RT appears to increase GC in the leg but not in the arm.

Adipocyte differentiation-related protein in human skeletal muscle: relationship to insulin sensitivity

OBJECTIVE

To determine whether adipocyte differentiation-related protein (ADRP), a lipid droplet-associated protein that binds to and sequesters intracellular fatty acids, is 1) expressed in human skeletal muscle and 2) differentially regulated in human skeletal muscle obtained from obese non-diabetic (OND) and obese diabetic (OD) subjects.

RESEARCH METHODS AND PROCEDURES

Ten OND subjects and 15 OD subjects underwent a weight loss or pharmacological intervention program to improve insulin sensitivity. Anthropometric data, hemoglobin A(1C), fasting glucose, lipids, and glucose disposal rate were determined at baseline and at completion of studies. Biopsies of the vastus lateralis muscle (SkM) were obtained in the fasting state from OND and OD subjects. Protein expression was determined by Western blotting.

RESULTS

ADRP was highly expressed in SkM from OND (4.4 +/- 1.54 AU/10 microg, protein, n = 10) and OD (5.02 +/- 1.33 AU/10 microg, n = 12) subjects. OND subjects undergoing weight loss had decreased triglyceride levels and improved insulin action. SkM ADRP content increased with weight loss from 5.14 +/- 2.15 AU/10 microg to 9.92 +/- 1.57 AU/10 microg (p < 0.025). OD subjects were treated with either troglitazone or metformin, together with glyburide, for 3 to 4 months. Both treatments attained similar levels of glycemic control. OD subjects with lower baseline ADRP content (2.85 +/- 1.07 AU/10 microg, n = 6) displayed up-regulation of ADRP expression (to 9.27 +/- 2.76 AU/10 microg, p < 0.025).

DISCUSSION

ADRP is the predominant lipid droplet-associated protein in SkM, and low ADRP expression is up-regulated in circumstances of improved glucose tolerance. Up-regulation of ADRP may act to sequester fatty acids as triglycerides in discrete lipid droplets that could protect muscle from the detrimental effects of fatty acids on insulin action and glucose tolerance.

Major differences in noradrenaline action on lipolysis and blood flow rates in skeletal muscle and adipose tissue in vivo

AIMS/HYPOTHESIS

The regulation of skeletal muscle lipolysis is not fully understood. In the present study, the effects of systemic and local noradrenaline administration on lipolysis and blood flow rates in skeletal muscle and adipose tissue were studied in vivo.

METHODS

First, circulating noradrenaline levels were raised tenfold by a continuous i.v. infusion (n=12). Glycerol levels (an index of lipolysis) were measured in m. gastrocnemius and in abdominal adipose tissue using microdialysis. Local blood flow was determined with the (133)Xe clearance technique and whole-body lipolysis rates assessed with a stable glycerol isotope technique ([(2)H(5)] glycerol). Second, interstitial glycerol levels in m. gastrocnemius, m. vastus and adipose tissue were measured by microdialysis during local perfusion with noradrenaline (10(-8)-10(-6) mol/l) (n=10). Local blood flow was monitored with the ethanol perfusion technique.

RESULTS

With regard to systemic noradrenergic stimulation, no change in fractional release of glycerol (difference between tissue and arterial glycerol) was seen in skeletal muscle. In adipose tissue it transiently increased twofold (p<0.0001), and the rate of appearance of glycerol in plasma showed the same kinetic pattern. Blood flow was reduced by 40% in skeletal muscle (p<0.005) and increased by 50% in adipose tissue (p<0.05). After noradrenaline stimulation in situ, a discrete elevation of skeletal muscle glycerol was registered only at the highest concentration of noradrenaline (10(-6) mol/l) (p<0.05). Adipose tissue glycerol doubled already at the lowest concentration (10(-8) mol/l) (p<0.05). In skeletal muscle a decrease in blood flow was seen at the highest noradrenaline concentrations (p<0.05).

CONCLUSIONS/INTERPRETATION

Lipolysis and blood flow rates are regulated differently in adipose tissue and skeletal muscle. Adipose tissue displays a high, but transient (tachyphylaxia) sensitivity to noradrenaline, leading to stimulation of both lipolysis and blood flow rates. In skeletal muscle, physiological concentrations of noradrenaline decrease blood flow but have no stimulatory effect on lipolysis rates.

Heterogeneity in limb fatty acid kinetics in type 2 diabetes

AIMS/HYPOTHESIS

In order to test the hypothesis that disturbances in skeletal muscle fatty acid metabolism with type 2 diabetes are not equally present in the upper and lower limbs, we studied fatty acid kinetics simultaneously across the arm and leg of type 2 diabetic patients (n=6) and matched control subjects (n=7) for 5 h under baseline conditions and during a 4-h hyperinsulinaemic-euglycaemic clamp.

METHODS

Limb fatty acid kinetics was determined by means of continuous [U-(13)C]palmitate infusion and measurement of arteriovenous differences.

RESULTS

The systemic palmitate rate of appearance was 3.6+/-0.4 and 2.7+/-0.3 micromol.kg lean body mass(-1).min(-1) and decreased during the clamp by 26% (p=0.04) and 43% (p<0.01) in the diabetic patients and in the control subjects respectively. At baseline, palmitate uptake across the arm was similar in the two groups, whereas leg palmitate uptake was lower than in the arm in the diabetic patients. During the clamp, palmitate uptake decreased in the arm (-48%, p=0.02) and the leg (-38%, p=0.04) of the control subjects, whereas it decreased in the arm (-30%, p=0.04) but not in the leg of the diabetic patients. Similarly, during the clamp palmitate release was substantially suppressed in the arm (-47%, p<0.01) and the leg of the control subjects (-45%, p<0.01), but only in the arm of the diabetic patients (-45%, p<0.01).

CONCLUSIONS/INTERPRETATION

The present data indicate that type 2 diabetes is characterised by heterogeneity in the dysregulation of skeletal muscle fatty acid metabolism, with only the leg, but not the arm, showing an impairment of fatty acid kinetics at baseline and during a hyperinsulinaemic-euglycaemic clamp causing a physiological increase in insulin concentration.

Glucose clearance is higher in arm than leg muscle in type 2 diabetes

Insulin-mediated glucose clearance (GC) is diminished in type 2 diabetes. Skeletal muscle has been estimated to account for essentially all of the impairment. Such estimations were based on leg muscle and extrapolated to whole body muscle mass. However, skeletal muscle is not a uniform tissue and insulin resistance may not be evenly distributed. We measured basal and insulin-mediated (1 pmol min-1 kg-1) GC simultaneously in the arm and leg in type 2 diabetes patients (TYPE 2) and controls (CON) (n=6 for both). During the clamp arterio-venous glucose extraction was higher in CON versus TYPE 2 in the arm (6.9+/-1.0 versus 4.7+/-0.8%; mean+/-s.e.m.; P=0.029), but not in the leg (4.2+/-0.8 versus 3.1+/-0.6%). Blood flow was not different between CON and TYPE 2 but was higher (P<0.05) in arm versus leg (CON: 74+/-8 versus 56+/-5; TYPE 2: 87+/-9 versus 43+/-6 ml min-1 kg-1 muscle, respectively). At basal, CON had 84% higher arm GC (P=0.012) and 87% higher leg GC (P=0.016) compared with TYPE 2. During clamp, the difference between CON and TYPE 2 in arm GC was diminished to 54% but maintained at 80% in the leg. In conclusion, this study shows that glucose clearance is higher in arm than leg muscles, regardless of insulin resistance, which may indicate better preserved insulin sensitivity in arm than leg muscle in type 2 diabetes.

High triacylglycerol turnover rate in human skeletal muscle

In the present study we investigated the relationship between plasma fatty acids (FA) and intramuscular triacylglycerol (IMTAG) kinetics of healthy volunteers. With this aim [U-(13)C]-palmitate was infused for 10 h and FA kinetics determined across the leg. In addition, the rate of FA incorporation into IMTAG in vastus lateralis muscle was determined during two consecutive 4-h periods (2-6 h and 6-10 h). Fifty to sixty per cent of the FA taken up from the circulation were esterified into IMTAG, whereas 32 and 42% were oxidized between 2-6 and 6-10 h, respectively. IMTAG fractional synthesis rate was 3.4 +/- 0.8% h(-1) and did not change between the two 4- h periods, despite an increase in arterial FA concentration (34%, P < 0.01). IMTAG concentration was also unchanged, implying that the IMTAG fractional synthesis rate was balanced by an equal rate of breakdown. FA oxidation increased over time, which could be due to the observed decline in plasma insulin concentration (-74%, P < 0.01). In conclusion, a substantial fraction of the fatty acids entering skeletal muscle in post-absorptive healthy individuals is esterified into IMTAG, due to its high turnover rate (29 h pool(-1)). An increase in FA level, as a consequence of short-term fasting, does not seem to increase IMTAG synthesis rate and pool size.

Microdialysis methods for measuring human metabolism

PURPOSE OF REVIEW

To discuss the advantages and limitations of the microdialysis technique as a diagnostic and research tool using recent findings on human metabolism.

RECENT FINDINGS

Results from several studies have supported the potential of microdialysis as a diagnostic tool for metabolic monitoring in difficult accessible tissues (brain, liver, intestine). However, despite promising results, no clear diagnostic measures have yet emerged. Several studies combining microdialysis with stable isotope tracers have shown that this approach has great potential for studying human metabolism non-invasively in specific tissue beds in a more dynamic way.

SUMMARY

Bearing in mind the limitations and assumptions, the microdialysis technique is a useful tool in investigations of human metabolism. Its main advantages are that it can be used safely with low-grade invasiveness in humans, and thereby allows continuous sampling over prolonged periods of time from specific tissues without taking any biopsies. At present, microdialysis would seem to be useful as a diagnostic tool when integrated in the total clinical, physiological and pharmacological evaluation. Within human metabolic research, microdialysis has been and will be a very useful technique.

Impaired beta-adrenergically mediated lipolysis in skeletal muscle of obese subjects

AIMS/HYPOTHESIS

The aim of this study was to investigate whether the beta2-adrenergically mediated increase in interstitial glycerol concentrations (used as an indicator of local lipolysis) was impaired in the skeletal muscle (the gastrocnemius muscle) of obese subjects compared with in that of lean subjects, and whether any differences in muscle lipolysis were related to differences in intramyocellular muscle triglyceride content.

METHODS

A microdialysis experiment was performed in the gastrocnemius muscle of eight lean and eight obese men (body fat 22.1+/-1.6% vs 32.7+/-1.6% respectively). After determining baseline extracellular glycerol concentrations, the probe was perfused with increasing concentrations of the beta2-agonist, salbutamol (doses of 1, 10 and 100 micromol/l for 45-min periods). Local blood flow was determined using the ethanol dilution technique. Intramyocellular lipid content was determined using 1H-magnetic resonance spectroscopy.

RESULTS

Compared with that in lean subjects, the beta2-adrenergically mediated increase in glycerol concentrations (absolute and percentage change) was blunted in obese subjects (at 100 micromol/l of salbutamol, percentage change 12.0+/-12% vs 48+/-12%, p<0.05). The decrease in ethanol out:in ratio was less pronounced in the obese individuals ( p<0.05), indicating a diminished increase in local muscle blood flow. Intramyocellular lipid content was comparable in both groups.

CONCLUSIONS/INTERPRETATION

The capacity to increase skeletal interstitial glycerol concentrations during direct beta2-adrenergic stimulation is impaired in obese subjects with normal intramyocellular concentrations, suggesting that this may be an early event in the process of triglyceride accumulation.

Characterization of adenosine-A1 receptor-mediated antilipolysis in rats by tissue microdialysis, 1H-spectroscopy, and glucose clamp studies

Increased supply of fatty acids to muscle and liver is causally involved in the insulin resistance syndrome. Using a tissue microdialysis technique in Wistar and Zucker fatty (ZF) rats, we determined tissue glycerol levels as a marker of lipolysis in gastrocnemius muscle (gMT), subcutaneous adipose (SAT), and visceral adipose tissue (VAT) as well as the reduction of plasma free fatty acids, glycerol, and triglycerides caused by the antilipolysis-specific adenosine-A1 receptor agonist (ARA). In Wistar and ZF rats, ARA significantly lowered dialysate glycerol levels in SAT, VAT, and gMT. Whereas in SAT and VAT the decrease in dialysate glycerol indicated adipocytic antilipolysis, this decrease in gMT was not caused by a direct effect of ARA on intramyocellular lipolysis, as demonstrated by the lack of inhibition of the protein kinase A activity ratio in gMT. In addition, no differences of the fed-starved-refed dynamics of intramyocellular triglyceride levels compared with untreated controls were measured by in vivo (1)H-spectroscopy, excluding any adenylate cyclase-independent antilipolysis in muscle. Treatment with ARA resulted in pronounced reductions of plasma free fatty acids, glycerol, and triglycerides. Furthermore, in ZF rats, ARA treatment caused an immediate improvement of peripheral insulin sensitivity measured by the euglycemic-hyperinsulinemic glucose clamp technique.

Depot-specific differences in perilipin mRNA but not protein expression in obesity

BACKGROUND

Perilipin (PLIN) is a lipid droplet-associated protein, which regulates lipolysis in rodent adipocytes by modulating the access of hormone-sensitive lipase to the lipid surface. Little is known regarding the expression and function of PLIN in man. Subcutaneous (s.c.) and omental fat tissue display marked differences in both basal and stimulated lipolysis.

OBJECTIVE AND SUBJECTS

In order to assess whether this can be caused by differences in PLIN expression, we investigated the expression of PLIN mRNA and protein in s.c. and omental fat tissue samples from 16 obese women (n = 8) and men (n = 8).

METHODS

For mRNA detection a quantitative real-time PCR was used. Protein was analysed by Western blot.

RESULTS

PLIN mRNA was significantly reduced in omental versus s.c. tissue (P = 0.022). In contrast, although PLIN protein levels were highly variable between the two depots when related to the reference protein beta-actin, no significant difference in PLIN protein expression was observed between the fat depots (P = 0.61). There were no gender-specific differences in PLIN mRNA or protein.

CONCLUSION

Our results suggest that post-transcriptional mechanisms may influence PLIN protein expression and that PLIN expression per se is probably not a major factor in determining the lipolytic differences between the s.c. and omental fat depots.

Lipolysis in skeletal muscle is decreased in high-fat-fed rats

The intracellular triglyceride content in skeletal muscle is increased in insulin-resistant states such as obesity or high-fat feeding. It has been hypothesized that increased fatty acid oxidation resulting from increased lipolysis of intramyocellular triglycerides may be responsible for this insulin resistance. This study was undertaken to examine whether insulin resistance is associated with increased lipolysis in skeletal muscle in rats fed a high-fat diet. Sprague-Dawley rats were fed a high-fat diet for 5 weeks. Lipolysis in skeletal muscle and adipose tissue was determined by measuring the interstitial glycerol concentrations using a microdialysis method in basal and hyperinsulinemic-euglycemic clamp conditions. In the basal state, plasma free fatty acid (FFA) levels were higher in high-fat-fed rats than in low fat-fed rats (P <.05). In contrast, plasma glycerol levels (P <.001) and interstitial glycerol concentrations of skeletal muscle (P <.05) and adipose tissue (P <.01) were lower in high fat-fed rats than in low fat-fed rats. Plasma (P <.05) and interstitial glycerol concentrations (P <.05 for skeletal muscle, P <.01 for adipose tissue) during the hyperinsulinemic euglycemic clamps were also lower in the high-fat diet group. These results do not support the idea that increased fatty acid oxidation resulting from increased lipolysis of intramyocellular triglycerides is responsible for the insulin resistance in high fat-fed rats.