Acute interleukin-6 administration does not impair muscle glucose

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					                        J Physiol (2003), 548.2, pp. 631–638                                                                                               DOI: 10.1113/jphysiol.2002.032938
                        © The Physiological Society 2003                                                                                                         

                        Acute interleukin-6 administration does not impair muscle
                        glucose uptake or whole-body glucose disposal in healthy
Journal of Physiology

                        Adam Steensberg*†, Christian P. Fischer*†, Massimo Sacchetti*, Charlotte Keller*†, Takuya Osada*,
                        Peter Schjerling*, Gerrit van Hall*, Mark A. Febbraio*‡ and Bente Klarlund Pedersen*†
                        *The Copenhagen Muscle Research Centre and †The Department of Infectious Diseases, Rigshospitalet, University of Copenhagen, Denmark, and
                        ‡The School of Medical Sciences, RMIT University, Victoria, Australia

                                             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.
                                             (Received 19 September 2002; accepted after revision 24 January 2003; first published online 14 March 2003)
                                             Corresponding author A. Steensberg: The Copenhagen Muscle Research Centre, Rigshospitalet 7641, Blegdamsvej 9,
                                             DK-2100 Copenhagen, Denmark. Email:

                        Elevated plasma levels of the cytokines tumour necrosis            uptake during graded exercise. Thus, IL-6 appears to be
                        factor-a and interleukin (IL)-6 have been consistently             upregulated in the muscle when the requirement for
                        observed in patients with type 2 diabetes mellitus, and as a       glucose uptake is augmented. Hence, rather than indicate
                        consequence both have been related to insulin resistance           that IL-6 impairs glucose uptake, our recent data suggest
                        and/or impaired glucose disposal (for review see                   that IL-6 levels are augmented during periods of enhanced
                        Hotamisligil, 1999; Febbraio & Pedersen, 2002). Research           glucose uptake, although it must be noted that IL-6 release
                        from our laboratory has focussed on the effect of muscle           occurs after 60 min of exercise, whereas glucose uptake is
                        contraction on the IL-6 response (for review see Pedersen          increased at the onset of muscle contraction (Steensberg et
                        et al. 2001). We have demonstrated that contracting muscle         al. 2001).
                        releases IL-6, and that this release can account for much of
                                                                                           Recently, Wallenius et al. (2002) demonstrated that IL-6-
                        the systemic increase in [IL-6] found in response to
                                                                                           deficient mice developed markedly impaired glucose
                        exercise (Steensberg et al. 2000), even though the brain
                                                                                           disposal, compared with littermate control mice during
                        (Nybo et al. 2002) and the peritendon (Langberg et al.
                                                                                           an intravenous glucose tolerance test. In addition,
                        2002) can release small amounts of IL-6 during exercise. In
                                                                                           Stouthard et al. (1996) demonstrated that IL-6 enhanced
                        addition, IL-6 mRNA is upregulated in contracting muscle
                                                                                           both basal and insulin-stimulated glucose uptake in
                        (Keller et al. 2001; Starkie et al. 2001; Steensberg et al. 2001).
                                                                                           cultured 3T3-L1 adipocytes, while Hardin et al. (2000)
                        Of note, IL-6 mRNA expression, the nuclear transcriptional
                                                                                           observed increased glucose transport in jejunal tissue
                        rate of IL-6, and release of this protein from human skeletal
                                                                                           incubated with IL-6 compared with controls. The effect
                        muscle are enhanced when exercising with low intra-
                                                                                           of acute IL-6 administration on whole-body glucose
                        muscular glycogen stores (Keller et al. 2001; Steensberg et al.
                                                                                           metabolism is unclear. Tsigos et al. (1997) demonstrated
                        2001). In addition, Helge et al. (2002) recently demonstrated
                                                                                           that recombinant human IL-6 (rhIL-6) administration to
                                                             correlated from leg-glucose
                        that IL-6 release was positivelyDownloaded with J Physiol ( by guest on May 7, 2011
                        632                                                   A. Steensberg and others                                            J Physiol 548.2

                        healthy volunteers increased circulating plasma glucose in           remained supine during the entire experiment. They were only
                        a dose-dependent manner. Although these authors could                permitted to consume water during the experiment. After 10 min,
                        not determine whether rhIL-6 resulted in increased                   the femoral artery and vein of both legs were cannulated as
                        glucose production or decreased glucose disposal, they               described previously (Steensberg et al. 2001). Thereafter, one
                                                                                             catheter was placed in a forearm vein for infusion of the stable
                        speculated that IL-6 might have produced peripheral
Journal of Physiology

                                                                                             isotopes. One femoral arterial catheter was used for the infusion of
                        resistance to insulin action. This hypothesis is plausible           IL-6 (or saline). Hence, one leg received the IL-6 infusion dose,
                        given the fact that acute rhIL-6 infusion increases circulating      and the other the systemic IL-6 concentration. The other arterial
                        free fatty acids (FFA; Lyngsø et al. 2002), which according          and two femoral venous lines were used for blood sampling. At
                        to the classic glucose–fatty acid cycle can lead to impaired         each sampling time point the femoral arterial blood flow in
                        glucose disposal (Randle et al. 1963). In contrast, Stouthard        both legs was measured with ultrasound Doppler flowmetry, as
                        et al. (1995) studied patients with metastatic renal cell cancer     described previously (Rådegran, 1997). Multiple measures were
                                                                                             made at each sampling point and the coefficient of variation for
                        receiving rhIL-6 infusion, and observed an increase in               this technique has been determined to be ~ 6 % (Rådegran, 1997).
                        glucose appearance and whole-body glucose disposal using             This allowed us to calculate any net changes across the legs by
                        the isotopic tracer dilution method. Importantly, however,           multiplying the a–fv differences with the blood flow using Fick’s
                        in this study (Stouthard et al. 1995) all subjects experienced       principle.
                        clinical symptoms, such as fever, and consequently increased         At ~08.00 h the infusion of the stable isotopes commenced.
                        their whole-body oxygen consumption (◊J). In addition,               Two hours later, when steady state was achieved, the IL-6 (or
                        circulating levels of hormones such as glucagon, adrenaline          saline) infusion commenced. Muscle biopsy samples were obtained
                        (epinephrine) and noradrenaline (norepinephrine) were                after application of a local anaesthetic (lidocaine (lignocaine)
                        elevated. Therefore, the authors could not determine                 20 mg ml_1) from the vastus lateralis of both limbs using the
                        whether the enhanced glucose disposal was a direct effect of         percutaneous needle biopsy technique with suction, 30 min
                        IL-6, whether skeletal muscle was the site of the enhanced           before the commencement of the infusion, 30 and 90 min into the
                                                                                             infusion, at the cessation of the infusion, and 180 min and 24 h
                        disposal, or whether the response was characteristic of
                                                                                             following the infusion.
                        healthy humans.
                                                                                             IL-6 infusates
                        Hence, the aim of the present study was to investigate the           The two different concentrations of rhIL-6 (Sandoz, Basle,
                        role of acute IL-6 administration on whole-body glucose              Switzerland) were infused in doses lower than those reported to be
                        disposal and leg-glucose uptake in healthy humans by                 safe in other studies (Stouthard et al. 1995). The IL-6 doses were
                        using the isotopic tracer dilution method and arterial–              chosen on the basis of pilot experiments. We aimed to reach
                                                                                             plasma levels of IL-6 characteristic of intense prolonged exercise
                        femoral venous (a–fv) differences across the legs. In order
                                                                                             or low-grade inflammation during LoIL-6. In this trial, the rate of
                        to determine whether any effect of IL-6 was direct, or               rhIL-6 infusion was 30 mg h_1, using saline as a vehicle. Due to the
                        secondary to changes in energy turnover and/or regulatory            low bioavailability of IL-6 administered in saline, we could not use
                        hormones, we chose to infuse both a low and a high dose of           this method during HiIL-6. Hence, further pilot experiments
                        rhIL-6. The low dose elicited plasma concentrations of               were conducted using human albumin as a vehicle. These pilot
                        IL-6 comparable with the highest concentrations observed             experiments revealed that a dose of 15 mg h_1 was required during
                        during prolonged strenuous exercise (Pedersen et al.                 this trial. Saline was infused during control infusion (Con).
                        2001). We hypothesized that IL-6 infusion would increase             Blood analysis
                        glucose uptake by the muscle.                                        Blood samples for IL-6 were measured by high-sensitivity enzyme-
                                                                                             linked immunosorbent assay, as described previously (Steensberg
                                                                                             et al. 2001). Plasma insulin (Insulin RIA 100, Amersham
                        METHODS                                                              Pharmacia Biotech, Uppsala, Sweden), glucagon (Linco Research,
                        Subjects                                                             St Charles, USA) and cortisol (Diagnostic Products Corporation,
                        Eighteen healthy, active but not specifically trained males were     Los Angeles, USA) were analysed by radioimmunoassay, and
                        recruited in the study. Each was assigned to one of three groups;    plasma adrenaline and noradrenaline by high-performance liquid
                        control (Con: age 23 ± 1 years; body mass 78 ± 3 kg; height          chromatography. These analyses are described in detail elsewhere
                        183 ± 4 cm; body mass index (BMI) 23.4 ± 0.5 kg m_2), low            (Blomstrand & Saltin, 1999; Steensberg et al. 2002). Plasma
                        rhIL-6 infusion (LoIL-6: age 24 ± 1 years; body mass 80 ± 2 kg;      glucose was determined using an automatic analyser (Cobas Fara,
                        height 184 ± 4 cm; BMI 23.5 ± 0.8 kg m_2) and high IL-6 infusion     Roche, France). Furthermore, an aliquot of blood was mixed with
                        (HiIL-6: age 26 ± 1 years; body mass 77 ± 2 kg; height 178 ± 2 cm;   lithium heparin and spun in a centrifuge (2200 g for 15 min at
                        BMI 24.3 ± 0.7 kg m_2). The study was approved by the Ethical        4 °C). The resultant plasma was stored for measurement of
                        Committee of Copenhagen and Frederiksberg Communities,               [6,6-2H2] glucose enrichment, as described previously (Steensberg
                        Denmark, and performed according to the Declaration of               et al. 2002).
                        Helsinki. Subjects were informed about the possible risks and        Muscle analysis
                        discomfort involved before giving their written consent to           Muscle biopsy samples were divided into two pieces upon
                        participate.                                                         sampling, and then frozen in liquid nitrogen for subsequent
                        Protocol                                                             analysis. One portion was freeze-dried, dissected free from visible
                        Subjects reported to the laboratory at 07.00 h after an overnight    blood and connective tissue, extracted and analysed for glycogen.
                        fast. They voided, changed into appropriate hospital attire and      The second portion of the muscle biopsy sample was extracted for
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                        J Physiol 548.2                                        IL-6 and glucose metabolism                                                     633

                        total RNA and analysed for GLUT 4 mRNA by Northern blot                   was used. If such analysis revealed significant differences, a
                        analyses. These procedures are described in detail elsewhere              Newman-Keuls post hoc test was used to locate the specific
                        (Steensberg et al. 2002).                                                 differences. The net leg-glucose uptake was analysed by a Friedman
                                                                                                  non-parametric test, because a normal distribution could not be
                        Physiological variables
                                                                                                  obtained. P < 0.05 was accepted as significant.
                        Heart rate (HR) and blood pressure (BP) were measured every
Journal of Physiology

                        60 min using electrocardiography and sphygmomanometry,
                        respectively. Temperature (T) was also measured at this time              RESULTS
                        point via a tympanic probe. Expired pulmonary gases were
                        collected and analysed for ◊J on-line using a Medgraphics CPX/D           There were no significant differences between the trials for
                        metabolic cart (St Paul, MN, USA).                                        HR, T or BP (data not shown). Due to technical difficulties
                                                                                                  we were unable to sample expired pulmonary gases during
                        All data are presented as means ± S.E.M. (n = 6). Log plasma              HiIL-6. There were no differences in ◊J when comparing
                        [IL-6], GLUT 4 mRNA and glucagon were normally distributed;               Con with LoIL-6 (data not shown). The subjects did not
                        therefore the log-transformed values were used for the statistical        report any discomfort during Con or LoIL-6. During
                        analyses. To analyse changes over time and between groups, a              HiIL-6 all subjects experienced approximately 30 min of
                        two-way repeated-measures analysis of variance (RM-ANOVA)                 shivering and discomfort 30–60 min after the IL-6 infusion

                                          Figure 1. Effect of infusion of high and low doses of recombinant human interleukin-6
                                          (rhIL-6) on arterial and venous plasma [IL-6] and net differences in [IL-6]
                                          Arterial plasma [IL-6 ](A), venous plasma [IL-6] from the infusion leg (INFLEG) (B) and net [IL-6]
                                          differences in the systemic leg (SYSLEG) (C) during and after 3 h infusion of saline, a low or a high dose of
                                          rhIL-6 (Con, LoIL-6 and HiIL-6, respectively). Data are presented as means ± S.E.M. (n = 6). # Significantly
                                          different from Con (P < 0.05); $ Significantly different from LoIL-6 (P < 0.05).
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                        634                                                      A. Steensberg and others                                                J Physiol 548.2
Journal of Physiology

                        was initiated. This effect was transient and none of the                  effluent from the infusion leg (Fig. 1B). In contrast, IL-6
                        subjects reported any severe side effects.                                concen-tration in the venous blood from the infusion
                                                                                                  leg was elevated (P < 0.05) compared with pre-infusion
                        The arterial plasma [IL-6] is shown in Fig. 1A. During
                                                                                                  concentrations in LoIL-6. This effect was markedly
                        LoIL-6 and HiIL-6, the mean arterial plasma [IL-6]s were
                                                                                                  augmented during HiIL-6 (P < 0.05). There was a net
                        143 and 319 pg ml_1, respectively. Saline infusion did
                                                                                                  disappearance of IL-6 from the plasma across the leg
                        not increase the concentration of IL-6 in the venous
                                                                                                  receiving the systemic IL-6 dose in both LoIL-6 and HiIL-6
                                                                                                  (P < 0.05) (Fig. 1C).
                                                                                                  There was a gradual decline (P < 0.05) in the rate of
                                                                                                  glucose appearance (Ra) and a concomitant fall (P < 0.05)

                        Figure 2. Effect of infusion of high and low doses of
                        rhIL-6 on the rate of appearance (Ra) and disappearance                  Figure 3. Effect of infusion of high and low doses of
                        (Rd) of 6,6-2H glucose, and on the metabolic clearance                   rhIL-6 on arterial glucose concentration and net leg-
                        rate (MCR)                                                               glucose uptake in the infusion leg
                        [6, 6-2H] glucose Ra (A), Rd (B) and MCR (C) before (Pre), during        Arterial glucose concentration (A) and net leg-glucose uptake in
                        and after 3 h infusion of saline, a low or a high dose of rhIL-6. Data   the infusion leg (B) before, during and after 3 h infusion of saline, a
                        are presented as means ± S.E.M. (n = 6). § Time effect for Con,          low or a high dose of rhIL-6. Data are presented as means ± S.E.M.
                        LoIL-6 and HiIL-6 (P < 0.05).                                            (n = 6).
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                        J Physiol 548.2                                         IL-6 and glucose metabolism                                           635
Journal of Physiology

                             Figure 4. Effects of infusion of high and low doses
                             of rhIL-6 on muscle glycogen content
                             Muscle glycogen content in the infusion leg before, during
                             and after 3 h infusion of either saline, a low or a high dose of
                             rhIL-6. Data are presented as means +S.E.M. (n = 6).

                        in whole-body rate of glucose disappearance (Rd) in all                 The plasma hormone levels are reported in Fig. 6. Plasma
                        trials, without any significant differences between the                 insulin decreased (P < 0.05) over time, but there were no
                        groups (Fig. 2A and B). As a consequence of the similar fall            differences when comparing groups; nor was there a
                        in both Ra and Rd for glucose in all trials, neither the                group w time interaction (Fig. 6A). Plasma glucagon levels
                        metabolic clearance rate (MCR) of glucose (Fig. 2C) nor                 were similar when comparing the three groups and this
                        the arterial glucose concentration (Fig. 3A) was affected by            hormone was not affected by either time or treatment
                        time or treatment.                                                      (Fig. 6B). There were no differences in plasma cortisol
                                                                                                concentrations when comparing the three groups before
                        Since there were no differences between the infusion leg and
                                                                                                infusion (Fig. 6C). Plasma cortisol concentrations did not
                        the systemic leg during any trial for net leg-glucose uptake,
                                                                                                change during Con, but increased (P < 0.05) during both
                        muscle glycogen or GLUT 4 mRNA, the data are presented
                                                                                                HiIL-6 and LoIL-6. While concentrations of plasma
                        for the infusion leg only. Leg blood flow was not different
                                                                                                cortisol declined after 2 h of infusion in LoIL-6, they
                        when comparing the three trials (Table 1). In addition, net
                                                                                                remained elevated in HiIL-6 (P < 0.05) at 3 h of infusion.
                        leg-glucose uptake, being the product of leg blood flow and
                                                                                                During both IL-6 trials, plasma cortisol levels returned
                        femoral arterial–venous glucose concentration, was not
                                                                                                to pre-infusion values after 3 h of recovery. Plasma
                        different when comparing the trials (Fig. 3B). The muscle
                                                                                                noradrenaline was not affected by time or treatment when
                        glycogen content averaged 370 ± 47 mmol glycosyl U (kg
                                                                                                comparing trials (Fig. 7A). In contrast, plasma adrenaline
                        dry weight)_1 (n = 18) prior to infusion, and the content did
                                                                                                markedly increased (P < 0.05) at the onset of infusion in
                        not differ between groups or over time (Fig. 4). The three
                                                                                                HiIL-6 such that values after 60 min were greater
                        groups did not differ in the expression of GLUT 4 mRNA at
                                                                                                (P < 0.05) compared with LoIL-6 and Con (Fig. 7B). It is
                        any time point; however, the day after the infusion, GLUT 4
                                                                                                of note that LoIL-6 did not affect plasma adrenaline
                        mRNA was lower (P < 0.05) in all groups (Fig. 5) compared
                        with pre-infusion.

                             Figure 5. Effects of infusion of high and low
                             doses of rhIL-6 on muscle GLUT 4 mRNA
                             Muscle GLUT 4 mRNA/28S rRNA before, during
                             and after 3 h infusion of saline, a low or a high dose
                             of rhIL-6. Data are presented as geometric
                             means ± S.E.M. (n = 6). # Significantly different
                             from pre-infusion for Con, LoIL-6 and HiIL-6
                             (P < 0.05).

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                        636                                                   A. Steensberg and others                                              J Physiol 548.2

                        DISCUSSION                                                             us (Pedersen et al. 2001) and others (Gleeson, 2000) to
                                                                                               hypothesize that IL-6 was released by skeletal muscle when
                        Contrary to our hypothesis, the results from this study                intramuscular glycogen content was low in order to
                        demonstrate, on the one hand, that rhIL-6 infusion does                provide a signal to the liver to increase endogenous glucose
                        not increase whole-body glucose disposal or muscle glucose             production (EGP). Despite the fact that EGP occurs
Journal of Physiology

                        uptake. On the other hand, it is important to note that                rapidly at the onset of exercise, prior to any release of IL-6
                        these results clearly demonstrate that elevated systemic               from the skeletal muscle (Steensberg et al. 2001), this
                        IL-6 does not impair muscle glucose uptake or whole-body               hypothesis was supported by previous studies in which it
                        glucose disposal. Hence, even though circulating IL-6 is               was found that rhIL-6 infusion increased either circulating
                        elevated in patients with type 2 diabetes mellitus, and has            glucose (Tsigos et al. 1997) or isotopic tracer-determined
                        been implicated in insulin resistance and/or impaired                  EGP (Stouthard et al. 1995). However, it must be noted
                        glucose disposal (Tsigos et al. 1997), our data suggest that           that during these previous studies, the glucoregulatory
                        acutely elevating IL-6 does not impair muscle glucose                  hormones glucagon, adrenaline and noradrenaline were
                        uptake or whole-body glucose disposal in healthy humans.               altered to some extent by rhIL-6 infusion. In contrast, the
                        We have shown previously that the release of IL-6 during               present study found that rhIL-6 infusion during LoIL-6
                        prolonged exercise is related to intramuscular glycogen                did not affect the circulating concentration of any of these
                        content (Keller et al. 2001; Steensberg et al. 2001). This led         hormones. The results from the present study demonstrate,
                                                                                               therefore, that elevating IL-6 increases neither EGP (Fig. 2A)
                                                                                               nor arterial glucose concentration (Fig. 3A) in healthy
                                                                                               In the present study we hypothesized that IL-6 infusion
                                                                                               would increase glucose uptake by the muscle. This
                                                                                               hypothesis was based on previous studies that have
                                                                                               examined the effect of IL-6 on specific cells in vitro.
                                                                                               Stouthard et al. (1996) demonstrated that IL-6 increased

                        Figure 6. Effects of infusion of high and low doses of
                        rhIL-6 on plasma levels of insulin, glucagon and cortisol
                        Plasma insulin (A), glucagon (B) and cortisol (C) before, during         Figure 7. Effects of infusion of high and low doses of
                        and after 3 h infusion of saline, a low or a high dose of rhIL-6. Data   rhIL-6 on plasma levels of noradrenaline and adrenaline
                        are presented as means ± S.E.M. (n = 6). § Time effect for Con,          Plasma levels of noradrenaline (A) and adrenaline (B) before,
                        LoIL-6 and HiIL-6 (P < 0.05). * Significantly different from Pre         during and after 3 h infusion of saline, a low or a high dose of
                        (P < 0.05). # Significantly different from Con (P < 0.05).               rhIL-6. Data are presented as means ± S.E.M. (n = 6).
                        $ Significantly different from LoIL-6 (P < 0.05).                        # Significantly different from Con (P < 0.05).
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                        J Physiol 548.2                                  IL-6 and glucose metabolism                                                  637

                        both basal and insulin-stimulated glucose uptake in                with a concomitant increase in glucose uptake (Steensberg
                        cultured 3T3-L1 adipocytes. These authors concluded that           et al. 2001), while IL-6 release during graded exercise was
                        IL-6 acted by increasing intrinsic glucose transporter             positively correlated with leg-glucose uptake (Helge et al.
                        activity. In addition, Hardin et al. (2000) recently observed      2002). In order to test our hypothesis, we chose to infuse
                        increased glucose transport in jejunal tissue incubated            rhIL-6 into the leg. Although we did not observe an
Journal of Physiology

                        with IL-6 compared with controls. The authors concluded            enhanced leg-glucose uptake, our method cannot
                        that IL-6 was able to increase the absorption of glucose in the    categorically rule out the possibility that IL-6, produced
                        gut, thereby increasing the plasma glucose levels. Recently it     within the muscle, stimulates glucose uptake. Infused
                        was shown that IL-6-knock-out mice developed impaired              rhIL-6 could act in an alternative way to that which is
                        glucose disposal during an intravenous glucose tolerance           produced endogenously by the muscle, if the muscle-
                        test (Wallenius et al. 2002). Stouthard et al. (1995) also         derived IL-6 is either a different isoform or if IL-6 becomes
                        demonstrated that infusion of recombinant human IL-6               active within the muscle only in a phosphorylated or
                        (rhIL-6) into renal cancer patients increased whole-body           glycosylated state. However, this seems unlikely because
                        glucose disposal and subsequent oxidation compared with            we have shown recently that infusion of rhIL-6 into the
                        a control trial. Although one could argue that this was due        femoral artery of humans activates the stress protein
                        to elevated glucagon increasing EGP, thereby resulting in          HSP72 (Febbraio et al. 2002). These recent data demonstrate
                        an augmented whole-body glucose disposal, this appears             that the form of IL-6 infused in the present study is
                        unlikely since the metabolic clearance rate of glucose was         biologically active in human skeletal muscle.
                        higher in the rhIL-6 infusion trial. Hence, in this previous
                                                                                           Recently Lyngsø et al. (2002) demonstrated that acute
                        study (Stouthard et al. 1995) the relative hyperglycaemia
                                                                                           rhIL-6 infusion markedly increased circulating FFA
                        was not responsible for the augmented glucose disposal. It
                                                                                           concentration. According to the classic glucose–fatty acid
                        must be noted that in this previous study, the rhIL-6 was
                                                                                           cycle of Randle and co-workers (1963), such an increase
                        infused at a rate even greater than the rate of HiIL-6
                                                                                           would result in transient insulin resistance. Indeed, it has
                        infusion in the present study. Furthermore, the subjects
                                                                                           been demonstrated that acute increases in plasma FFA
                        were cancer patients, and had increased energy expenditure
                                                                                           decrease insulin-stimulated glucose uptake in rats (Kim et
                        during the IL-6 infusion, which in itself can increase glucose
                                                                                           al. 1996). However, our results demonstrate clearly that
                        utilization. It is possible, therefore, that the different
                                                                                           acute rhIL-6 infusion does not result in transient insulin
                        results in glucose disposal when comparing the previous
                        with the present results are due to these methodological
                        differences.                                                       Since we hypothesized that IL-6 would increase glucose
                                                                                           uptake, we expected glycogen content to be higher
                        In the present study, we observed a small, but nonetheless
                                                                                           following IL-6 infusion as the increase in glucose uptake
                        significant decline in both glucose Ra and Rd throughout
                                                                                           would result in enhanced glycogen storage. Despite the
                        the experiment (Fig. 2). It should be noted that subjects
                                                                                           markedly elevated levels of IL-6 in the infusion leg during
                        arrived in the laboratory after an overnight fast and,
                                                                                           HiIL-6 and LoIL-6, intramuscular glycogen content was
                        therefore, this result is probably due to a progressive
                                                                                           not affected. The fact that muscle glycogenolysis was not
                        decline in hepatic glycogen concentration, in the absence
                                                                                           increased in HiIL-6 may, on the surface, appear anomalous
                        of any carbohydrate ingestion throughout the experimental
                                                                                           with respect to the marked increase in adrenaline, since
                        period. In addition, GLUT 4 mRNA was lower after 24 h in
                                                                                           adrenaline is know to increase muscle glycogenolysis
                        all trials when compared with pre-infusion. We have no
                                                                                           (Jansson et al. 1986). It must be noted, however, that
                        definitive data to determine the reason for such an
                                                                                           adrenaline appears to exert its glycogenolytic effect in active
                        observation. However, apart from travelling to the
                                                                                           rather that resting skeletal muscle. During contraction,
                        laboratory on the morning of the experiments, subjects
                                                                                           adrenaline converts glycogen phosphorylase (Phos), the
                        were bed rested for 20–24 h, which is known to decrease
                                                                                           enzyme responsible for glycogenolysis from the inactive b
                        GLUT 4 expression (Tabata et al. 1999). It is also possible
                                                                                           form to the active a form, and the concomitant increase in
                        that slight muscle damage associated with the muscle
                                                                                           inorganic phosphate (Pi) provides substrate to keep Phos
                        biopsy procedure decreased GLUT 4 mRNA after 24 h,
                                                                                           in the a form. However, at rest there is insufficient Pi to
                        since muscle damage also decreases GLUT 4 protein
                                                                                           keep Phos in the active form and, even in the presence of
                        expression (Asp et al. 1996). This latter suggestion appears
                                                                                           adrenaline, it rapidly converts back to the inactive form
                        less likely because the repeated biopsy procedure has no
                                                                                           (Ren & Hultman, 1989). Hence, the fact that glycogenolysis
                        effect on HSP72 mRNA (Febbraio et al. 2002), which is a
                                                                                           was not increased in HiIL-6 is consistent with this
                        very sensitive index of cellular stress and trauma.
                        Our hypothesis that IL-6 increases glucose uptake was
                                                                                           Recently, Metzger et al. (2001) found that mice bearing
                        based on our previous observations that IL-6 expression
                                                                                           IL-6-secreting tumours had reduced body fat. In addition,
                        and release in human muscle during contraction coincided
                                                                                           Wallenius et al. (2002) demonstrated that IL-6-deficient
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                        638                                                   A. Steensberg and others                                               J Physiol 548.2

                        mice become markedly glucose intolerant and obese, while              Metzger S, Hassin T, Barash V, Pappo O & Chajek-Shaul T (2001).
                        treatment of these mice with IL-6 partially attenuated                  Reduced body fat and increased hepatic lipid synthesis in mice
                        these perturbations. They suggested that IL-6 could be                  bearing interleukin-6-secreting tumor. Am J Physiol 281, E957–965.
                                                                                              Nybo L, Nielsen B, Pedersen BK, Møller K & Secher NH (2002).
                        used as a therapeutic drug in the treatment of obesity-
                                                                                                Interleukin-6 release from human brain during exercise. J Physiol
                        related illnesses. It was noteable that, in the present study,
Journal of Physiology

                                                                                                542, 991–995.
                        rhIL-6 infusion during LoIL-6 did not result in adverse               Pedersen BK, Steensberg A & Schjerling P (2001). Muscle-derived
                        side effects and there were no clinical symptoms reported               interleukin-6: possible biological effects. J Physiol 536, 329–337.
                        by the subjects or changes in sympathetic or pancreatic               Rådegran G (1997). Ultrasound Doppler estimates of femoral artery
                        hormones. In addition, rhIL-6 infusion did not increase                 blood flow during dynamic knee extensor exercise in humans. J Appl
                        EGP or arterial glucose concentration, demonstrating that               Physiol 83, 1383–1388.
                                                                                              Randle PJ, Garland PB, Hales CN & Newsholme EA (1963). The
                        therapeutic IL-6 treatment in obese and/or insulin-resistant
                                                                                                glucose fatty-acid cycle, its role in insulin sensitivity and the
                        patients would not result in hyperglycaemia, thereby                    metabolic disturbances of diabetes mellitus. Lancet 1, 785–789.
                        reducing the likelihood of the adverse complications                  Ren J-M & Hultman E (1989). Regulation of glycogenolysis in human
                        associated with glucose toxicity.                                       skeletal muscle. J Appl Physiol 67, 2243–2248.
                                                                                              Starkie RL, Arkinstall MJ, Koukoulas I, Hawley JA & Febbraio MA
                        In conclusion, we have shown that acute IL-6 administration             (2001). Carbohydrate ingestion attenuates the increase in plasma
                        does not impair whole-body glucose disposal or net leg-                 interleukin-6, but not skeletal muscle interleukin-6 mRNA, during
                        glucose uptake, or increase endogenous glucose production               exercise in humans. J Physiol 533, 585–591.
                        during rest. Hence, a transient increase in systemic                  Steensberg A, Febbraio MA, Osada T, Van Hall G, Saltin B & Pedersen
                        concentration of IL-6 does not result in impaired glucose               BK (2001). Low glycogen content increases interleukin-6 production
                        homeostasis.                                                            in contracting human skeletal muscle. J Physiol 537, 633–639.
                                                                                              Steensberg A, Keller C, van Hall G, Osada T, Schjerling P, Pedersen BK,
                                                                                                Saltin B & Febbraio MA (2002). Muscle glycogen content and
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                        Keller C, Steensberg A, Pilegaard H, Pedersen BK & Neufer D (2001).      The authors wish to thank the subjects who participated in this
                          Transcriptional activation of the IL-6 gene in human contracting       study for their extraordinary effort. In addition, the technical
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