Gerrit van Hall

Production of interleukin-6 in contracting human skeletal muscles can account for the exercise-induced increase in plasma interleukin-6.

1 Plasma interleukin (IL)‐6 concentration is increased with exercise and it has been demonstrated that contracting muscles can produce IL‐ The question addressed in the present study was whether the IL‐6 production by contracting skeletal muscle is of such a magnitude that it can account for the IL‐6 accumulating in the blood. 2 This was studied in six healthy males, who performed one‐legged dynamic knee extensor exercise for 5 h at 25 W, which represented 40% of peak power output (Wmax). Arterial‐femoral venous (a‐fv) differences over the exercising and the resting leg were obtained before and every hour during the exercise. Leg blood flow was measured in parallel by the ultrasound Doppler technique. IL‐6 was measured by enzyme‐linked immunosorbent assay (ELISA). 3 Arterial plasma concentrations for IL‐6 increased 19‐fold compared to rest. The a‐fv difference for IL‐6 over the exercising leg followed the same pattern as did the net IL‐6 release. Over the resting leg, there was no significant a‐fv difference or net IL‐6 release. The work was produced by 2.5 kg of active muscle, which means that during the last 2 h of exercise, the median IL‐6 production was 6.8 ng min−1 (kg active muscle)−1 (range, 3.96‐9.69 ng min−1 kg−1). 4 The net IL‐6 release from the muscle over the last 2 h of exercise was 17‐fold higher than the elevation in arterial IL‐6 concentration and at 5 h of exercise the net release during 1 min was half of the IL‐6 content in the plasma. This indicates a very high turnover of IL‐6 during muscular exercise. We suggest that IL‐6 produced by skeletal contracting muscle contributes to the maintenance of glucose homeostasis during prolonged exercise.

Interleukin‐6 production in contracting human skeletal muscle is influenced by pre‐exercise muscle glycogen content

1 Prolonged exercise results in a progressive decline in glycogen content and a concomitant increase in the release of the cytokine interleukin‐6 (IL‐6) from contracting muscle. This study tests the hypothesis that the exercise‐induced IL‐6 release from contracting muscle is linked to the intramuscular glycogen availability. 2 Seven men performed 5 h of a two‐legged knee‐extensor exercise, with one leg with normal, and one leg with reduced, muscle glycogen content. Muscle biopsies were obtained before (pre‐ex), immediately after (end‐ex) and 3 h into recovery (3 h rec) from exercise in both legs. In addition, catheters were placed in one femoral artery and both femoral veins and blood was sampled from these catheters prior to exercise and at 1 h intervals during exercise and into recovery. 3 Pre‐exercise glycogen content was lower in the glycogen‐depleted leg compared with the control leg. Intramuscular IL‐6 mRNA levels increased with exercise in both legs, but this increase was augmented in the leg having the lowest glycogen content at end‐ex. The arterial plasma concentration of IL‐6 increased from 0.6 ± 0.1 ng l−1 pre‐ex to 21.7 ± 5.6 ng l−1 end‐ex. The depleted leg had already released IL‐6 after 1 h (4.38 ± 2.80 ng min−1 (P < 0.05)), whereas no significant release was observed in the control leg (0.36 ± 0.14 ng min−1). A significant net IL‐6 release was not observed until 2 h in the control leg. 4 This study demonstrates that glycogen availability is associated with alterations in the rate of IL‐6 production and release in contracting skeletal muscle.

Blood lactate is an important energy source for the human brain

Lactate is a potential energy source for the brain. The aim of this study was to establish whether systemic lactate is a brain energy source. We measured in vivo cerebral lactate kinetics and oxidation rates in 6 healthy individuals at rest with and without 90 mins of intravenous lactate infusion (36 μmol per kg bw per min), and during 30mins of cycling exercise at 75% of maximal oxygen uptake while the lactate infusion continued to establish arterial lactate concentrations of 0.89 ± 0.08, 3.9 ± 0.3, and 6.9 ± 1.3 mmol/L, respectively. At rest, cerebral lactate utilization changed from a net lactate release of 0.06 ± 0.01 to an uptake of 0.16 ± 0.07 mmol/min during lactate infusion, with a concomitant decrease in the net glucose uptake. During exercise, the net cerebral lactate uptake was further increased to 0.28 ± 0.16 mmol/min. Most 13C-label from cerebral [1-13C]lactate uptake was released as 13CO2 with 100% ± 24%, 86% ± 15%, and 87% ± 30% at rest with and without lactate infusion and during exercise, respectively. The contribution of systemic lactate to cerebral energy expenditure was 8% ± 2%, 19% ± 4%, and 27% ± 4% for the respective conditions. In conclusion, systemic lactate is taken up and oxidized by the human brain and is an important substrate for the brain both under basal and hyperlactatemic conditions.

High-Density Lipoprotein Modulates Glucose Metabolism in Patients With Type 2 Diabetes Mellitus

Background— Low plasma high-density lipoprotein (HDL) is associated with elevated cardiovascular risk and aspects of the metabolic syndrome. We hypothesized that HDL modulates glucose metabolism via elevation of plasma insulin and through activation of the key metabolic regulatory enzyme, AMP-activated protein kinase, in skeletal muscle. Methods and Results— Thirteen patients with type 2 diabetes mellitus received both intravenous reconstituted HDL (rHDL: 80 mg/kg over 4 hours) and placebo on separate days in a double-blind, placebo-controlled crossover study. A greater fall in plasma glucose from baseline occurred during rHDL than during placebo (at 4 hours rHDL=−2.6±0.4; placebo=−2.1±0.3mmol/L; P=0.018). rHDL increased plasma insulin (at 4 hours rHDL=3.4±10.0; placebo= −19.2±7.4 pmol/L; P=0.034) and also the homeostasis model assessment &bgr;-cell function index (at 4 hours rHDL=18.9±5.9; placebo=8.6±4.4%; P=0.025). Acetyl-CoA carboxylase &bgr; phosphorylation in skeletal muscle biopsies was increased by 1.7±0.3-fold after rHDL, indicating activation of the AMP-activated protein kinase pathway. Both HDL and apolipoprotein AI increased glucose uptake (by 177±12% and 144±18%, respectively; P<0.05 for both) in primary human skeletal muscle cell cultures established from patients with type 2 diabetes mellitus (n=5). The mechanism is demonstrated to include stimulation of the ATP-binding cassette transporter A1 with subsequent activation of the calcium/calmodulin-dependent protein kinase kinase and the AMP-activated protein kinase pathway. Conclusions— rHDL reduced plasma glucose in patients with type 2 diabetes mellitus by increasing plasma insulin and activating AMP-activated protein kinase in skeletal muscle. These findings suggest a role for HDL-raising therapies beyond atherosclerosis to address type 2 diabetes mellitus.

A 2-wk reduction of ambulatory activity attenuates peripheral insulin sensitivity

US adults take between approximately 2,000 and approximately 12,000 steps per day, a wide range of ambulatory activity that at the low range could increase risk for developing chronic metabolic diseases. Dramatic reductions in physical activity induce insulin resistance; however, it is uncertain if and how low ambulatory activity would influence peripheral insulin sensitivity. We aimed to explore if healthy, nonexercising subjects who went from a normal to a low level of ambulatory activity for 2 wk would display metabolic alterations including reduced peripheral insulin sensitivity. To do this, ten healthy young men decreased their daily activity level from a mean of 10,501+/-808 to 1,344+/-33 steps/day for 2 wk. Hyperinsulinemic-euglycemic clamps with stable isotopes and muscle biopsies, maximal oxygen consumption (VO2 max) tests, and blood samples were performed pre- and postintervention. A reduced number of daily steps induced a significant reduction of 17% in the glucose infusion rate (GIR) during the clamp. This reduction was due to a decline in peripheral insulin sensitivity with no effect on hepatic endogenous glucose production. The insulin-stimulated ratio of pAktthr308/total Akt decreased after step reduction, with a post hoc analysis revealing the most pronounced effect after 4 h of insulin infusion. In addition, the 2-wk period induced a 7% decline in VO2 max (ml/min; cardiovascular fitness). Lean mass of legs, but not arms and trunk, decreased concurrently. Taken together, one possible biological cause for the public health problem of Type 2 diabetes has been identified. Reduced ambulatory activity for 2 wk in healthy, nonexercising young men significantly reduced peripheral insulin sensitivity, cardiovascular fitness, and lean leg mass.

Muscle-derived interleukin-6: Lipolytic, anti-inflammatory and immune regulatory effects

Interleukin-6 (IL-6) is produced locally in working skeletal muscle and can account for the exercise-induced increase in plasma IL-6. The transcription rate for IL-6 in muscle nuclei isolated from muscle biopsies during exercise is very high and is enhanced further when muscle glycogen content is low. Furthermore, cultured human primary muscle cells can increase IL-6 mRNA when incubated with the calcium ionophore ionomycin and it is likely that myocytes produce IL-6 in response to muscle contraction. The biological roles of muscle-derived IL-6 have been investigated in studies in which human recombinant IL-6 was infused in healthy volunteers to mimic closely the IL-6 concentrations observed during prolonged exercise. Using stable isotopes, we have demonstrated that physiological concentrations of IL-6 induce lipolysis. Although we have yet to determine the precise biological action of muscle-derived IL-6, our data support the hypothesis that the role of IL-6 released from contracting muscle during exercise is to act in a hormone-like manner to mobilize extracellular substrates and/or augment substrate delivery during exercise. In addition, IL-6 inhibits low-level TNF-α production, and IL-6 produced during exercise probably inhibits TNF-α-induced insulin resistance in peripheral tissues. Hence, IL-6 produced by skeletal muscle during contraction may play an important role in the beneficial health effects of exercise

Reduced glycogen availability is associated with an elevation in HSP72 in contracting human skeletal muscle

To test the hypothesis that a decrease in intramuscular glycogen availability may stimulate heat shock protein expression, seven men depleted one leg of muscle glycogen the day before performing 4–5 h of exhaustive, two‐legged knee extensor exercise at 40 % of leg peak power output. Subjects then rested for a further 3 h. Muscle biopsies were obtained from the depleted and control leg before, immediately after and 3 h into recovery from exercise. These samples were analysed for muscle glycogen, and HSP72 gene and protein expression. In addition, catheters were placed in one femoral artery and both femoral veins and blood was sampled from these catheters prior to exercise and at 1 h intervals during exercise and into recovery for the measurement of arterial‐venous differences in serum HSP72. Plasma creatine kinase (CK) was also measured from arterial blood samples. Pre‐exercise muscle glycogen content was 40 % lower in the depleted compared with the control leg and this difference was maintained throughout the experiment (P < 0.05; main treatment effect). Neither HSP72 gene nor protein expression was different pre‐exercise. However, both HSP72 gene and protein increased (P < 0.05) post‐exercise in the depleted leg, but not in the control leg. Exercise did not increase plasma CK concentrations and we were unable to detect HSP72 in the serum of any samples. These results demonstrate that while acute, concentric exercise is capable of increasing HSP72 in human skeletal muscle, it does so only when glycogen is reduced to relatively low levels. Hence, our data suggest that HSP72 protein expression is related to glycogen availability. In addition, because CK did not increase and we found no evidence of HSP72 in the venous effluent, our data suggest that skeletal muscle is impermeable to HSP72.

Acute interleukin‐6 administration does not impair muscle glucose uptake or whole‐body glucose disposal in healthy humans

The cytokine interleukin (IL)‐6 has recently been linked with type 2 diabetes mellitus and has been suggested to affect glucose metabolism. To determine whether acute IL‐6 administration affects whole‐body glucose kinetics or muscle glucose uptake, 18 healthy young men were assigned to one of three groups receiving a high dose of recombinant human IL‐6 (HiIL‐6; n= 6), a low dose of IL‐6 (LoIL‐6; n= 6) or saline (Con; n= 6) infused into one femoral artery for 3 h. The stable isotope [6,6‐2H2] glucose was infused into a forearm vein throughout the 3 h infusion period and for a further 3 h after the cessation of infusion (recovery) to determine endogenous glucose production and whole‐body glucose disposal. Infusion with HiIL‐6 and LoIL‐6 resulted in a marked (P < 0.05) increase in systemic IL‐6 concentration throughout the 3 h of infusion (mean arterial plasma [IL‐6]s of 319 and 143 pg ml−1 for HiIL‐6 and LoIL‐6, respectively), followed by a rapid decline (P < 0.05) during the recovery period. Subjects experienced clinical symptoms such as shivering and discomfort during HiIL‐6 administration, but were asymptomatic during LoIL‐6 administration. In addition, only HiIL‐6 elevated (P < 0.05) plasma adrenaline (epinephrine). IL‐6 infusion, irrespective of dose, did not result in any changes to endogenous glucose production, whole‐body glucose disposal or leg‐ glucose uptake. These data demonstrate that acute IL‐6 administration does not impair whole‐body glucose disposal, net leg‐glucose uptake, or increase endogenous glucose production at rest in healthy young humans.

Whey and casein labeled with l-[1-13C]leucine and muscle protein synthesis: effect of resistance exercise and protein ingestion

Muscle protein turnover following resistance exercise and amino acid availability are relatively well described. By contrast, the beneficial effects of different sources of intact proteins in relation to exercise need further investigation. Our objective was to compare muscle anabolic responses to a single bolus intake of whey or casein after performance of heavy resistance exercise. Young male individuals were randomly assigned to participate in two protein trials (n = 9) or one control trial (n = 8). Infusion of l-[1-(13)C]leucine was carried out, and either whey, casein (0.3 g/kg lean body mass), or a noncaloric control drink was ingested immediately after exercise. l-[1-(13)C]leucine-labeled whey and casein were used while muscle protein synthesis (MPS) was assessed. Blood and muscle tissue samples were collected to measure systemic hormone and amino acid concentrations, tracer enrichments, and myofibrillar protein synthesis. Western blots were used to investigate the Akt signaling pathway. Plasma insulin and branched-chain amino acid concentrations increased to a greater extent after ingestion of whey compared with casein. Myofibrillar protein synthesis was equally increased 1-6 h postexercise after whey and casein intake, both of which were higher compared with control (P < 0.05). Phosphorylation of Akt and p70(S6K) was increased after exercise and protein intake (P < 0.05), but no differences were observed between the types of protein except for total 4E-BP1, which was higher after whey intake than after casein intake (P < 0.05). In conclusion, whey and casein intake immediately after resistance exercise results in an overall equal MPS response despite temporal differences in insulin and amino acid concentrations and 4E-BP1.

IL-6 selectively stimulates fat metabolism in human skeletal muscle

Interleukin (IL)-6 is chronically elevated in type 2 diabetes but also during exercise. However, the exact metabolic role, and hence the physiological significance, has not been elucidated. The objective of this study was to investigate the in vivo effect of recombinant human (rh) IL-6 on human fat and glucose metabolism and signaling of both adipose tissue and skeletal muscle. Eight healthy postabsorptive males were infused with either rhIL-6 or saline for 4 h, eliciting IL-6 levels of ∼40 and ∼1 pg/ml, respectively. Systemic, skeletal muscle, and adipose tissue fat and glucose metabolism was assessed before, during, and 2 h after cessation of the infusion. Glucose metabolism was unaffected by rhIL-6. In contrast, rhIL-6 increased systemic fatty acid oxidation approximately twofold after 60 min, and it remained elevated even 2 h after the infusion. The increase in oxidation was followed by an increase in systemic lipolysis. Adipose tissue lipolysis and fatty acid kinetics were unchanged with rhIL-6 compared with saline infusion. Conversely, rhIL-6 infusion caused an increase in skeletal muscle unidirectional fatty acid and glycerol release, indicative of an increase in lipolysis. The increased lipolysis in muscle could account for the systemic changes. Skeletal muscle signaling increased after 1 h of rhIL-6 infusion, indicated by a fourfold increase in the phosphorylated signal transducer and activator of transcription (STAT) 3-to-STAT3 ratio, whereas no changes in phosphorylated AMP-activated protein kinase or acetyl-CoA carboxylase levels could be observed. Our findings suggest that an acute increase in IL-6 at a normophysiological level selectively stimulates lipolysis in skeletal muscle, whereas adipose tissue is unaffected.