Resistance to Exercise-Induced Increase in GlucoseUptake During Hyperinsulinemia in Insulin-ResistantSkeletal Muscle of Patients With Type 1 DiabetesPauliina Peltoniemi,

1Hannele Yki-Jarvinen,

2Vesa Oikonen,

1Airi Oksanen,

3Teemu O. Takala,

1

Tapani Ronnemaa,4

Matti Erkinjuntti,5

M. Juhani Knuuti,1

and Pirjo Nuutila1,4

Insulin and exercise have been shown to activate glu-cose transport at least in part via different signalingpathways. However, it is unknown whether insulin re-sistance is associated with a defect in the ability of anacute bout of exercise to enhance muscle glucose up-take in vivo. We compared the abilities of insulin andisometric exercise to stimulate muscle blood flow andglucose uptake in 12 men with type 1 diabetes (age 24 61 years, BMI 23.0 6 0.4 kg/m2) and in 11 age- and weight-matched nondiabetic men (age 25 6 1 years, BMI 22.3 60.6 kg/m2) during euglycemic hyperinsulinemia (1 mU zkg21 z min21 insulin infusion for 150 min). One-leggedexercise was performed at an intensity of 10% of max-imal isometric force for 105 min (range 45–150). Ratesof muscle blood flow, oxygen consumption, and glucoseuptake were quantitated simultaneously in both legsusing [15O]water, [15O]oxygen, [18F]-2-fluoro-2-deoxy-D-glucose, and positron emission tomography. Resting ratesof oxygen consumption were similar during hyperinsulin-emia between the groups (2.4 6 0.3 vs. 2.0 6 0.5 ml zkg21 muscle z min21; normal subjects versus patients withtype 1 diabetes, NS), and exercise increased oxygen con-sumption similarly in both groups (25.3 6 4.3 vs. 20.1 63.0 ml z kg21 muscle z min21, respectively, NS). Rates ofinsulin-stimulated muscle blood flow and the incre-ments in muscle blood flow induced by exercise were alsosimilar in normal subjects (129 6 14 ml z kg21 z min21) andin patients with type 1 diabetes (115 6 12 ml z kg21 zmin21). The patients with type 1 diabetes exhibited resis-tance to both insulin stimulation of glucose uptake (34 66 vs. 76 6 9 mmol z kg21 muscle z min21, P < 0.001) andalso to the exercise-induced increment in glucose up-take (82 6 15 vs. 162 6 29 mmol z kg21 muscle z min21,P < 0.05). We conclude that the ability of exercise to in-crease insulin-stimulated glucose uptake in vivo is bluntedin patients with insulin-resistant type 1 diabetes com-pared with normal subjects. This could be caused by ei-ther separate or common defects in exercise- and insulin-stimulated pathways. Diabetes 50:1371–1377, 2001

Insulin resistance characterizes patients with type 1diabetes (1,2). Contraction increases glucose uptakeindependent of insulin (3,4). For example, contrac-tion results in recruitment of GLUT4 to the cell

membrane, activation of glycogen synthase, and inductionof hexokinase II gene expression independently of insulin(5–7). These data raise the possibility that exercise in-creases glucose uptake normally in skeletal muscle ofinsulin-resistant subjects. This has indeed been shown tobe true in soleus muscles from muscle-specific insulin-receptor knockout (MIRKO) mice (6). Although exerciseand insulin can stimulate glucose uptake independently,the two stimuli also seem to have synergistic effects onglucose uptake. In MIRKO mice, despite extreme insulinresistance, insulin and exercise stimulate glucose uptaketogether more than either stimuli alone (6). This phenom-enon has been attributed to insulin action on nonmusclecells within skeletal muscle or to signaling events down-stream, insulin receptor tyrosine phosphorylation, andphosphatidylinositol (PI) 3-kinase activity. In humans, theability of exercise to increase insulin-stimulated glucoseuptake is higher in trained than in untrained muscle (8).

Most patients with type 1 diabetes are characterized byvarying degree of insulin resistance. Numerous studieshave documented 30–50% average reductions in rates ofinsulin-mediated glucose uptake in type 1 diabetic patients(1,2,9,10). Chronic hyperglycemia, i.e., glucose toxicity, isbelieved to be largely responsible for insulin resistance intype 1 diabetic patients (2). Elevation of plasma glucoseconcentrations for 24 h, to pathophysiologically relevantglucose concentrations (14–20 mmol/l), significantly de-creases insulin-stimulated glucose uptake (9). One possi-ble mechanism underlying the defect in insulin stimulationof glucose uptake has been suggested to be an overactivityof the hexosamine pathway (11,12). Activation of thispathway induces, at least in rats, defects in the insulin-signaling cascade, such as decreases in insulin receptorsubstrate-1 (IRS-1) tyrosine phosphorylation, IRS-1 asso-ciation with PI 3-kinase, and PI 3-kinase activity (13). Itis unknown whether exercise stimulates glucose uptakenormally in insulin-resistant type 1 diabetic patients.

It is not possible to compare the abilities of insulin andexercise to stimulate muscle glucose uptake if the studiesare not performed under identical metabolic conditions. Inthe present study, both resting and exercising legs were

From the 1Turku PET Centre, University of Turku, Turku, Finland; the2Department of Medicine, University of Helsinki, Helsinki, Finland; and theDepartments of 3Rehabilitation and Physical Medicine, 4Medicine, and 5Clin-ical Neurophysiology, University of Turku, Turku, Finland.

Address correspondence and reprint requests to Dr. Pauliina Peltoniemi,Turku PET Centre, University of Turku, P.O. Box 52, FIN-20520 Turku,Finland. E-mail: papelto@utu.fi.

Received for publication 16 June 2000 and accepted in revised form21 February 2001.

AMPK, AMP-activated protein kinase; FDG, 2-fluoro-2-deoxy-D-glucose;FFA, free fatty acid; IRS-1, insulin receptor substrate-1; MIRKO, muscle-specific insulin-receptor knockout; PET, positron emission tomography; PI,phosphatidylinositol; rGU, rate of glucose uptake.

DIABETES, VOL. 50, JUNE 2001 1371

exposed to similar concentrations of insulin and otherhormones and metabolites to determine whether theability of acute exercise to increase glucose uptake is

preserved in insulin-resistant type 1 diabetic patientscompared with nondiabetic subjects.

RESEARCH DESIGN AND METHODS

A total of 12 men with type 1 diabetes with no family history of diabetes orhypertension volunteered for the study (Table 1). Duration of type 1 diabetesaveraged 8 6 2 years. The patients were treated with multiple regimens ofinsulin injection. The mean daily dose was 53 6 4 U. The subjects withdiabetes were normotensive and had no clinical or laboratory evidence ofdisease, other than type 1 diabetes, and had no signs of microvascular ormacrovascular disease, as determined by retinal photographs and autonomicnervous system function tests. The normal men were healthy as judged byhistory, physical examination, and routine laboratory tests and were nottaking any medications. The normal subjects and the patients with type 1diabetes performed regular aerobic exercise an average of 3.4 6 0.8 and 3.1 60.5 h per week, respectively (P 5 0.81). Written informed consent wasobtained after the nature, purpose, and potential risks of the study wereexplained to the subjects. The study was approved by the Joint Commissionof the Ethics of the University of Turku and Turku University Central Hospital.

Before the positron emission tomography (PET) study, maximal oxygenconsumption and maximal isometric contractile force of the quadricepsfemoris muscle were determined as detailed below. The design of this studyis shown in Fig. 1. The PET study was performed after an overnight fast.Alcohol and caffeine were prohibited 1 day before the study, and the subjectswere instructed to avoid strenuous physical activity for 1 day before the study.In the morning of the study, the usual dose of intermediate-acting insulin wasreduced by one-half, and no short-acting insulin was given. Two catheterswere inserted: one in an antecubital vein of the left hand for the infusion ofglucose and insulin and injections of [15O]H2O and [18F]-2-fluoro-2-deoxy-D-glucose (FDG), and the other in the radial artery for blood sampling. Thesubjects were lying supine during the study, resting between 0 and 45 min and

FIG. 1. Study design. Insulin was infused intravenously at a rate of 1 mU z kg21 z min21, and normoglycemia was maintained by glucose infusion.The insert shows the setup for the one-legged exercise, which consisted of intermittent isometric contractions at an intensity that correspondedto 10% of the subject’s maximal isometric force. [15O]H2O (intravenous bolus), [15O]O2 (inhalation), and [18F]FDG (intravenous bolus) wereadministered to quantify rates of blood flow, oxygen consumption, and glucose uptake simultaneously in both legs.

TABLE 1Subject characteristics

Type 1 diabeticpatients

Normalsubjects

n 12 11Age (years) 24 6 1 25 6 1Weight (kg) 74 6 2 71 6 2BMI (kg/m2) 23 6 0.4 22 6 0.6Percentage body fat 15.1 6 1.0 13.9 6 1.2VO2max (ml z kg21 z min21) 45 6 2 50 6 3Maximal isometric force (N)** 618 6 37 583 6 51Work load during PET study

absolute (N) 59 6 3 62 6 7Percentage of maximal work load 9.6 6 0.1 10.4 6 0.1Fasting glucose (mmol/l) 11.1 6 1.2 5.0 6 0.2†Fasting Insulin (mU/l) 4.9 6 1.4 4.5 6 0.6HbA1c (%) 7.0 6 0.3 5.1 6 0.1†Fasting cholesterol (mmol/l) 4.2 6 0.3 4.0 6 0.2Fasting HDL cholesterol (mmol/l) 1.5 6 0.1 1.4 6 0.1Fasting LDL cholesterol (mmol/l) 2.4 6 0.2 2.2 6 0.2Fasting triglycerides (mmol/l) 0.8 6 0.1 1.0 6 0.2

Data are n and means 6 SE. *Maximal isometric force of kneeextensors; †P , 0.001.

INSULIN, EXERCISE, AND GLUCOSE UPTAKE IN TYPE 1 DIABETES

1372 DIABETES, VOL. 50, JUNE 2001

performing intermittent isometric exercise with one leg between 45 and 150min, as described in detail below (Fig. 1). At 0 min, an intravenous infusion ofinsulin (1 mU z kg21 z min21) was started. The study for each subject consistedof a 150-min normoglycemic hyperinsulinemic (1 mU z kg21 z min21) period.During hyperinsulinemia, normoglycemia was maintained using variable rateof infusion of 20% glucose. Muscle blood flow (at 60 min) and muscle oxygenconsumption (at 90 min) were measured in both femoral regions simulta-neously, using [15O]H2O infusion and [15O]O2 inhalation techniques, as de-scribed in detail below. Thereafter, at 120 min, a bolus of [18F]FDG wasinjected for quantitation of muscle glucose uptake. Blood samples formeasurement of radioactivity were collected as detailed below. Blood sam-ples for determination of circulating serum free insulin, lactate, and free fattyacid (FFA) concentrations were collected before insulin infusion and at30-min intervals during insulin infusion.Production of PET tracers. For production of 15O (t1/2 5 123 s), alow-energy deuteron accelerator Cyclone 3 was used (Ion Beam Application,Louvain-la-Neuve, Belgium). 15O was produced by the [14N](d,n)[15O] reactionon natural nitrogen gas (14). Radiochemical purity of [15O]O2 was 97%.[15O]H2O was produced using a dialysis technique in a continuously workingwater module (15). Sterility and pyrogeneity tests were performed daily toverify the purity of the product. [18F]FDG (t1/2 5 109 min) was synthesizedwith an automatic apparatus as described by Hamacher et al. (16). At the endof the synthesis, the specific radioactivity was 76 GBq/mmol, and the radio-chemical purity exceeded 98%.Image acquisition and processing. An 8-ring ECAT 931/08-tomograph(Siemens/CTI, Knoxville, TN) was used for image acquisition. The scanner hasan axial resolution of 6.7 mm and an in-plane resolution of 6.5 mm (17). Theimages were obtained from the femoral region. Before the emission scannings,a transmission scan for correction of photon attenuation was performed for 20min with a removable ring source containing 68Ge. All data were corrected fordead time, decay, and measured photon attenuation. For image processing, arecently developed Bayesian iterative reconstruction algorithm using medianroot prior with 150 iterations and the Bayesian coefficient of 0.3 was applied(18).Regions of interest. Regions of interests were drawn in the anteromedialmuscle compartments of both femoral regions in four consecutive cross-sectional slices in both legs, carefully avoiding large blood vessels. Localiza-tion of the muscle compartments was verified by comparing the flow imageswith the transmission image, which provides a topographical distribution oftissue density. The regions of interest outlined in the flow images were copiedto the [15O]O2 and [18F]FDG images to obtain quantitative data from identicalregions.Measurement of muscle blood flow oxygen consumption. For measure-ment of blood flow, 1.2–1.7 GBq of [15O]H2O was injected intravenously anddynamic scanning was performed for 6 min (6 3 5 s, 6 3 15 s, and 8 3 30 sframes). To determine the input function, radioactivity of arterial bloodwithdrawn using a pump was measured using a two-channel detector system(Scanditronix, Uppsala, Sweden) as previously described (19,20). The arterialinput curve was corrected for dispersion and delay, as previously described(20). The autoradiographic method and a 250-s integration time were appliedto calculate blood flow pixel by pixel. This method has been previouslyvalidated against the steady-state method (19) and blood flow measured usingstrain-gauge plethysmography (20).

Muscle oxygen consumption was measured with [15O]O2 using a single-bolus inhalation technique as previously described (21). We recently validatedthis method also for the measurement of oxygen consumption in skeletalmuscle (22). With nostrils closed, subjects inhaled the gas containing 1.32 60.1 GBq of [15O]O2 via a mouthpiece and a short connecting ventilator hose for1 min. PET imaging of the femoral region was thereafter performed for 7 minwith time frames of 6 3 5 s, 6 3 15 s, 6 3 30 s, and 2 3 60 s.Measurement of muscle glucose uptake. For measurement of glucoseuptake, 0.19–0.25 GBq of [18F]FDG was injected intravenously over 2 min, anda dynamic scan for 30 min was started (8 3 15s, 2 3 30s, 2 3 120s, 1 3 180s,and 4 3 300s). Arterial blood samples for measurement of plasma radioactiv-ity were collected, as previously described (23). Plasma and tissue time-activity curves for the anteromedial muscle compartments were analyzedgraphically to quantitate the fractional rate of tracer uptake Ki (24). Theactivity points 5–30 min after injection of [18F]FDG were used to determinethe slope by linear regression. The rate of glucose uptake (rGU) is obtained bymultiplying Ki by the plasma glucose concentration [Glc]p divided by a lumpconstant term (LC): rGU 5 [Glc]p/LC 3 Ki. The lumped constant accounts fordifferences in the transport and phosphorylation of [18F]FDG and glucose. Alumped constant value of 1.2 for skeletal muscle was used (25–27). We havepreviously shown that the relationship between whole-body insulin-stimulatedglucose uptake, measured with the insulin clamp technique, and femoralmuscle glucose uptake, measured independent of the clamp using FDG and

PET, is similar in normal subjects and insulin-resistant patients with type 1diabetes (28). The relationship between whole-body glucose uptake andfemoral muscle glucose uptake as measured with FDG and PET is alsounaffected by marked elevations in FFA concentrations (23). Glucose extrac-tion was calculated by dividing muscle glucose uptake by muscle blood flow.Whole-body glucose uptake. Whole-body glucose uptake was quantitated,independent of PET measurements, using the euglycemic hyperinsulinemicclamp technique (29). During hyperinsulinemia, the rate of glucose infusion,corrected for changes in the glucose pool size, was used as a measure ofwhole-body glucose uptake (30), and normoglycemia was maintained using avariable rate of infusion of 20% glucose based on arterial plasma glucosemeasurements (31). The whole-body glucose uptake was calculated from thesame time period when the measurements of blood flow, oxygen consump-tion, and muscle glucose uptake were performed (60–150 min).Design of isometric knee extension during the PET study and measure-

ment of maximal oxygen uptake.

Exercise during the PET study. The subjects were lying supine in the PETscanner with the femoral regions in the gantry and the right leg, fixed at a 50°ankle, fastened to a dynamometer (I-KON; Chattanooga Group, Oxfordshire,England) (Fig. 1). Exercise consisted of 2-s isometric knee extension inter-mittent with 2 s of rest during 45–150 min of hyperinsulinemia. Exerciseintensity was set at 10% of maximal isometric force because preliminarystudies showed it to be feasible to maintain this intensity for the entire studyperiod. The subject performed isometric exercise after a sound signal. Theintensity of the exercise was monitored by a light signal, which was green ifthe intensity of the exercise corresponded to 10% of maximal isometric force(Fig. 1). Maximal isometric force of the knee extensors was measured beforethe PET study with a dynamometer (KinCom; Chattex, Chattanooga, TN).Maximal oxygen uptake (VO2max) was determined using an electrically brakedcycle ergometer (Ergoline 800 S; Bunnik, Mijnhardt, Netherlands) with acontinuous incremental protocol. Direct respiratory measurements weremade using an automated system (Medikro 202; Medikro Oy, Kuopio, Fin-land). The VO2max test consisted of a short warm-up, after which the load wasincreased by 30 watts every 2 min until exhaustion. The criteria used toestablish the VO2max were a plateau in VO2 with increasing exercise intensityand respiratory exchange ratio .1.10.Other measurements. Arterial and plasma glucose were determined induplicate by the glucose oxidase method (Analox GM7 Analyzer; AnaloxInstruments, Hammersmith, London, England). Serum free insulin concentra-tions were measured using radioimmunoassay (Pharmacia Insulin RIA kit;Pharmacia Diagnostics AB, Uppsala, Sweden) after precipitation with poly-ethyleneglycol (32). Serum FFAs were measured (33) by a fluorometricmethod and lactate was measured by a spectrophotometric method (34). Bodyfat content was estimated from four skin folds (subscapular, triceps brachii,biceps brachii, and crista iliaca), as measured with caliper (35). Retinalphotography was performed after mydriatic installation with a Canon CR4–45NM fundus camera (Canon, Kanagawa, Japan); one 45% field photograph,including areas of papilla and macula, was taken of each eye. All photographswere analyzed by the same person (T.R). To exclude significant autonomicneuropathy, a series of standardized noninvasive cardiovascular reflex testswere performed on subjects with diabetes (36). Autonomic nerve functiontests measuring mainly the parasympathetic control included a deep breathingtest and an orthostatic test. Diastolic blood pressure response to isometrichand-grip test was used as the measure of sympathetic autonomic nervoussystem control.Statistical methods. All results are expressed as mean 6 SE. The differencesbetween the two groups were compared using Student’s unpaired t test whenappropriate. Concentrations of insulin and metabolites over time between thetwo groups were analyzed using analysis of variance for repeated measures,followed by pairwise comparison using the Tukey’s studentized range test.Spearman’s rank correlation coefficients were calculated when appropriate.Statistical calculations were performed using the SAS statistical programpackage (SAS Institute, Cary, NC). Significance was set at the 0.05 level.

RESULTS

Metabolic characteristics. Absolute and relative workforce of knee extensors and VO2max were similar in bothgroups (Table 1). During hyperinsulinemia, serum freeinsulin concentrations were comparable (53 6 1 vs. 56 64 mU/l, normal subjects versus type 1 diabetic patients,NS), as were plasma glucose concentrations (5.3 6 0.1 vs.5.5 6 0.1 mmol/l, respectively, NS) and plasma lactateconcentrations (0.8 6 0.09 vs. 0.9 6 0.07 mmol/l, respec-tively, NS) during the time period when blood flow and

P. PELTONIEMI AND ASSOCIATES

DIABETES, VOL. 50, JUNE 2001 1373

whole-body and femoral muscle glucose uptake measure-ments were recorded (60–150 min). During the 60- to150-min period, serum FFA concentrations were higher intype 1 diabetic patients than in normal subjects (231 6 23vs. 155 6 12 mmol/l, respectively, P , 0.05).Whole-body glucose uptake. Insulin-stimulated whole-body glucose uptake, expressed per body weight, was 73%higher in the normal subjects (45 6 3 mmol z kg21 bodyweight z min21) than in the type 1 diabetic patients (26 64 mmol z kg21 body weight z min21, P , 0.01). Thecorrelation coefficient between glucose uptake in femoralmuscles and whole body was 0.83 in the normal subjects(P , 0.001) and 0.75 (P , 0.01) in the type 1 diabeticpatients.Femoral muscle blood flow. Resting rates of muscleblood flow were similar in both groups (37 6 6 vs. 29 6 6ml z kg21 muscle z min21, normal subjects versus type 1diabetic patients, NS). During exercise and insulin stimu-lation, muscle blood flow increased fivefold in both groups(165 6 18 vs. 145 6 15 ml z kg21 muscle z min21, normalsubjects versus type 1 diabetic patients, NS betweengroups). The increment induced by exercise in blood flowwas similar in both groups (129 6 14 vs. 115 6 12 ml z kg21

muscle z min21, respectively, NS between groups) (Fig. 2).Femoral muscle oxygen consumption. Resting rates ofoxygen consumption were comparable during hyperinsu-linemia (2.4 6 0.3 vs. 2.0 6 0.5 ml z kg21 muscle z min21,normal subjects versus patients with type 1 diabetes, NS)and during combined exercise and insulin stimulation(Fig. 2). The exercise-induced increments in muscle oxy-

gen consumption were also comparable (25.3 6 4.3 vs.20.1 6 3.0 ml z kg21 muscle z min21, respectively). Theincrement induced by exercise in muscle blood flow wassignificantly correlated with the increment in oxygenconsumption in the normal subjects (r 5 0.77, P , 0.01)and in the patients with type 1 diabetes (r 5 0.66, P , 0.05)(Fig. 3).Femoral muscle glucose uptake. During hyperinsulin-emia, glucose uptake in resting femoral muscle was 124%higher in the normal subjects (76 6 9 mmol z kg21 musclez min21) than in the patients with type 1 diabetes (34 6 6mmol z kg21 muscle z min21, P , 0.001) (Figs. 2 and 4).Exercise superimposed upon hyperinsulinemia increasedrates of muscle glucose uptake significantly in both groups(to 238 6 34 vs. 116 6 20 mmol z kg21 muscle z min21,normal subjects versus patients with type 1 diabetes, P ,0.01). The increment induced by exercise was, however,49% lower in the type 1 diabetic patients (82 6 15 mmol zkg21 muscle z min21) than in the normal subjects (162 6 29mmol z kg21 muscle z min21, P , 0.05).Glucose extraction. The fraction of glucose extracted byresting muscle during hyperinsulinemia was significantlyhigher in the normal subjects (0.54 6 0.06) than in thepatients with type 1 diabetes (0.34 6 0.08, P , 0.05).Exercise decreased glucose extraction significantly and by;40% in both groups (20.19 6 0.06 vs. 20.14 6 0.05 innormal subjects versus patients with type 1 diabetes, NSbetween groups). In the leg exposed to both insulin andexercise, fractional extraction was significantly higher in

FIG. 2. Cross-sectional PET images of blood flow (top), oxygen consumption (middle), and glucose uptake (bottom) in the femoral region of anormal subject and a patient with type 1 diabetes during euglycemic-hyperinsulinemic conditions. The resting leg (Rest) is shown on the left, andthe exercising leg is shown on the right (Exercise).

INSULIN, EXERCISE, AND GLUCOSE UPTAKE IN TYPE 1 DIABETES

1374 DIABETES, VOL. 50, JUNE 2001

the normal subjects (0.35 6 0.04) than in the patients withtype 1 diabetes (0.20 6 0.04, P , 0.05).

DISCUSSION

In the present study, we compared the abilities of acuteisometric exercise and insulin to stimulate glucose uptakein patients with type 1 diabetes and in normal subjects.The effects of exercise and insulin were studied underidentical metabolic conditions, i.e., where both legs wereexposed to similar concentrations of insulin and otherhormones and metabolites. We found a clear defect in theability of isometric exercise to increase skeletal muscleglucose uptake in the type 1 diabetic patients, who alsohad a defect in insulin-stimulated glucose uptake. Thesedata imply that type 1 diabetic patients have defects ineither exercise- and insulin-stimulated signaling pathwaysor in a pathway that the two stimuli share in skeletalmuscle.

Use of PET combined with radioactive tracers forquantitation of skeletal muscle perfusion and metabolismoffers some advantages over other methods. First, theseparameters can be measured directly in skeletal muscle,

thus avoiding artifacts induced by interindividual varia-tion in subcutaneous fat (37). Also, any confounding ef-fects caused by intra-arterial catheters can be avoided.[18F]FDG was used for measurement of femoral muscleglucose uptake (23), applying a lumped constant of 1.2,which was recently validated for human skeletal muscle(25–27). The lumped constant during exercise has notbeen determined in human studies; therefore, the calcu-lated glucose uptake rates during exercise may be quanti-tatively inaccurate. However, because normoglycemictype 1 diabetic patients are normally insulin-sensitive(2,10) and insulin action involves both stimulation ofglucose transport and phosphorylation, it is unlikely thatthe affinities of glucose transporters or hexokinases forglucose and 2-deoxy-glucose differ between normal sub-jects and type 1 diabetic patients. In previous studies inwhich glucose uptake has been measured across thewhole leg during moderate- (38) to high-intensity (39)bicycle ergometer exercise and supraphysiological orphysiological insulin concentrations, rates of glucose up-take have averaged 230 (39) and 281 (38) mmol z kg21

muscle z min21. In the present study, glucose uptakeaveraged 238 mmol z kg21 muscle z min21.

Skeletal muscle oxygen consumption was quantitatedusing a bolus inhalation of 15O-labeled oxygen, as previ-ously described and validated in humans (22). Measure-ment of oxygen consumption was considered importantbecause fuel use can only be reliably compared betweentwo groups if rates of oxygen consumption are similar. Inthis study, VO2max was similar between the groups. This isin keeping with previous studies in which patients withuncomplicated type 1 diabetes had a normal aerobicexercise capacity (40). We chose the intensity of isometricexercise based on individual muscular strengths, ratherthan on VO2max, because use of the latter would lead todifferent work loads if groups differed with respect toVO2max. Because maximal isometric contraction forceswere similar between the groups, both absolute and rela-tive work loads and rates of oxygen consumption werematched between the groups.

Several studies have recently demonstrated that humanskeletal muscle contains both intramyocellular and ex-tramyocellular fat (41,42). Possibly, impaired insulin ac-tion on extramyocellular fat within muscle or on lipolysisin adipose tissue could diminish the ability of exercise tostimulate glucose uptake. In the present study, the findingof slightly but significantly higher FFA concentrationsduring hyperinsulinemia in the type 1 diabetic patientsthan in the normal subjects is consistent with this possi-bility. Even if FFAs were not the cause of the impairedexercise-induced glucose uptake, and considering thatplasma lactate concentrations were similar between thegroups, FFA was the most likely fuel that compensated forthe decrease in glucose uptake in the type 1 diabeticpatients. It is also possible that the higher FFA concentra-tions might have caused the lower rate of glucose uptake,either via impairing insulin signaling (43,44) or via sub-strate competition (23).

Previous studies have documented insulin resistance intype 1 diabetes to be fully reversible if glycemic control isnormalized (2,10,45,46). The improvement is observedeven in the absence of an increase in insulin requirements,

FIG. 3. The relationship between the exercise-induced increment inmuscle oxygen consumption and blood flow during hyperinsulinemia inthe normal subjects (r 5 0.77, P < 0.01) (A) and in the patients withtype 1 diabetes (r 5 0.66, P < 0.05) (B).

P. PELTONIEMI AND ASSOCIATES

DIABETES, VOL. 50, JUNE 2001 1375

and insulin resistance can be induced within 24 h bychronic hyperglycemia (2,9). One molecular mechanismthat might underlie glucose-induced insulin resistance isoveractivation of the hexosamine pathway (11,12). Activa-tion of this pathway induces, at least in rats, defects in theinsulin-signaling cascade, such as decreases in IRS-1 ty-rosine phosphorylation, IRS-1 association with PI 3-kinase,and PI 3-kinase activity (13). These defects are similar tothose characterizing obese subjects and patients with type2 diabetes (47), although the cause of the alterations maydiffer (13,47). Regarding exercise-stimulated signaling path-ways, an AMP-activated protein kinase (AMPK) has recent-ly been proposed to be one potential regulator of GLUT4translocation in response to muscle contraction (48–51).AMPK is activated by an increase in the AMP-to-ATP ratioand by a decrease in phosphocreatine. Activation of AMPKwith nucleoside 5-aminoimidazole-4-carboxyamide-1-b-D-ribofuranoside has been shown to increase skeletalmuscle glucose uptake, but this increase is insensitive towortmannin, a known PI-3 kinase inhibitor (49,52). It iscurrently unknown whether type 1 diabetic patients havedefects in the PI 3-kinase–independent pathway stimu-lating glucose uptake. If not, the mechanism explainingthe resistance of skeletal muscle to exercise-induced in-creases in glucose uptake could be located in insulin-signaling pathways that are common to both stimuli (6).The present data do not exclude the possibility that theability of exercise alone to stimulate glucose uptake intype 1 diabetic patients is intact. This possibility cannot,however, be tested in the absence of insulin in humans.

In conclusion, these in vivo data demonstrate that theability of exercise to increase glucose uptake is blunted inpatients with insulin-resistant type 1 diabetes comparedwith normal subjects. Whether this is because of separateor common defects in exercise- and insulin-stimulatedpathways leading to decreased glucose uptake cannot beresolved based on the present data.

ACKNOWLEDGMENTS

This work was supported by grants from the Academy ofFinland (P.N., H.Y.-J.), Novo Nordisk Foundation (P.N.),

the Turku University Foundation (P.P), the Yrjo JahnssonFoundation (P.P), the Aarne Koskelo Foundation, and theJuho Vainio Foundation (P.P).

We thank the personnel at the Turku PET Center forskillful technical assistance.

REFERENCES

1. DeFronzo RA, Hendler R, Simonson D: Insulin resistance is a prominentfeature of insulin-dependent diabetes. Diabetes 31:795–801, 1982

2. Yki-Jarvinen H, Koivisto VA: Natural course of insulin resistance in type Idiabetes. N Engl J Med 315:224–230, 1986

3. Ploug T, Galbo H, Richter EA: Increased muscle glucose uptake duringcontractions: no need for insulin. Am J Physiol 247:E726–E731, 1984

4. Nesher R, Karl IE, Kipnis DM: Dissociation of effects of insulin andcontraction on glucose transport in rat epitrochlearis muscle. Am J

Physiol 249:C226–C232, 19855. Hayashi T, Wojtaszewski JF, Goodyear LJ: Exercise regulation of glucose

transport in skeletal muscle. Am J Physiol 273:E1039–E1051, 19976. Wojtaszewski JF, Higaki Y, Hirshman MF, Michael MD, Dufresne SD, Kahn

CR, Goodyear LJ: Exercise modulates postreceptor insulin signaling andglucose transport in muscle-specific insulin receptor knockout mice.J Clin Invest 104:1257–1264, 1999

7. Koval JA, DeFronzo RA, O’Doherty RM, Printz R, Ardehali H, Granner DK,Mandarino LJ: Regulation of hexokinase II activity and expression inhuman muscle by moderate exercise. Am J Physiol 274:E304–E308, 1998

8. Dela F, Mikines KJ, von Linstow M, Secher NH, Galbo H: Effect of trainingon insulin-mediated glucose uptake in human muscle. Am J Physiol

263:E1134–E1143, 19929. Vuorinen-Markkola H, Koivisto VA, Yki-Jarvinen H: Mechanisms of hyper-

glycemia-induced insulin resistance in whole body and skeletal muscle oftype I diabetic patients. Diabetes 41:571–580, 1992

10. Yki-Jarvinen H, Koivisto VA: Insulin sensitivity in newly diagnosed type 1diabetics after ketoacidosis and after three months of insulin therapy.J Clin Endocrinol Metab 59:371–378, 1984

11. McClain DA, Crook ED: Hexosamines and insulin resistance. Diabetes

45:1003–1009, 199612. Yki-Jarvinen H, Daniels MC, Virkamaki A, Makimattila S, DeFronzo RA,

McClain D: Increased glutamine:fructose-6-phosphate amidotransferaseactivity in skeletal muscle of patients with NIDDM. Diabetes 45:302–307,1996

13. Patti ME, Virkamaki A, Landaker EJ, Kahn CR, Yki-Jarvinen H: Activationof the hexosamine pathway by glucosamine in vivo induces insulinresistance of early postreceptor insulin signaling events in skeletal muscle.Diabetes 48:1562–1571, 1999

14. Strijckmans K, Vandecasteele C, Sambre J: Production and quality controlof 15O2 and C15O2 for medical use. Int J Appl Radiat Isot 36:279–283,1985

15. Crouzel C, Clarc J, Brihaye C, Långstrom B, Lemaire C, Meyer G, Nebeling

FIG. 4. Rates of skeletal muscle glucose up-take during hyperinsulinemia (Insulin) andhyperinsulinemia and exercise (Insulin 1 ex-ercise) of normal subjects (M) and patientswith type 1 diabetes (f) (mean 6 SE). o,exercise-induced increments in glucose up-take. *P < 0.05; **P < 0.01.

INSULIN, EXERCISE, AND GLUCOSE UPTAKE IN TYPE 1 DIABETES

1376 DIABETES, VOL. 50, JUNE 2001

B, Stone-Nelander S: Radiochemistry automation for PET. In Radiophar-

maceuticals for Positron Emission Tomography. Stocklin G, Pike V, Eds.Dordrecht, the Netherlands, Kluwer Academic Publisher Group, 1993, p.45–90

16. Hamacher K, Coenen HH, Stocklin G: Efficient stereospecific synthesis ofno-carrier-added 2-[18F]-fluoro-2-deoxy-D-glucose using aminopolyethersupported nucleophilic substitution. J Nucl Med 27:235–238, 1986

17. Spinks TJ, Jones T, Gilardi MC, Heather JD: Physical performance of thelatest generation of commercial positron scanner. IEEE Trans Nucl Sci

35:721–725, 198818. Alenius S, Ruotsalainen U: Bayesian image reconstruction for emission

tomography based on median root prior. Eur J Nucl Med 24:258–265, 199719. Ruotsalainen U, Raitakari M, Nuutila P, Oikonen V, Sipila H, Teras M,

Knuuti MJ, Bloomfield PM, Iida H: Quantitative blood flow measurement ofskeletal muscle using oxygen-15-water and PET. J Nucl Med 38:314–319,1997

20. Nuutila P, Raitakari M, Laine H, Kirvela O, Takala T, Utriainen T,Makimattila S, Pitkanen OP, Ruotsalainen U, Iida H, Knuuti J, Yki-JarvinenH: Role of blood flow in regulating insulin-stimulated glucose uptake inhumans: studies using bradykinin, [15O]water, and [18F]fluoro-deoxy-glu-cose and positron emission tomography. J Clin Invest 97:1741–1747, 1996

21. Meyer E, Tyler JL, Thompson CJ, Redies C, Diksic M, Hakim AM:Estimation of cerebral oxygen utilization rate by single-bolus 15O2 inhala-tion and dynamic positron emission tomography. J Cereb Blood Flow

Metab 7:403–414, 198722. Nuutila P, Peltoniemi P, Oikonen V, Larmola L, Kemppainen J, Takala T,

Sipila H, Oksanen A, Ruotsalainen U, Bolli GB, Yki-Jarvinen H: Enhancedstimulation of glucose uptake by insulin increases exercise-stimulatedglucose uptake in skeletal muscle in humans: studies using [15O]-oxygen,[15O]-water, [18F]-fluoro-deoxy-glucose and positron emission tomography.Diabetes 49:1084–1091, 2000

23. Nuutila P, Koivisto VA, Knuuti J, Ruotsalainen U, Teras M, Haaparanta M,Bergman J, Solin O, Voipio-Pulkki LM, Wegelius U: Glucose-free fatty acidcycle operates in human heart and skeletal muscle in vivo. J Clin Invest

89:1767–1774, 199224. Patlak CS, Blasberg RG: Graphical evaluation of blood-to-brain transfer

constants from multiple-time uptake data: generalizations. J Cereb Blood

Flow Metab 5:584–590, 198525. Kelley DE, Williams KV, Price JC, Goodpaster B: Determination of the

lumped constant for [18F]fluorodeoxyglucose in human skeletal muscle.J Nucl Med 40:1798–1804, 1999

26. Utriainen T, Lovisatti S, Makimattila S, Bertoldo A, Weintraub S, DeFronzoRA, Cobelli C, Yki-Jarvinen H: Direct measurement of the lumped constantfor 2-deoxy-[1]-[14C]-glucose in vivo in human skeletal muscle. Am J

Physiol 279:E228–E233, 200027. Peltoniemi P, Lonnroth P, Laine H, Oikonen V, Groop L, Strindberg L,

Knuuti J, Nuutila P: Determination of the lumped constant for [18F]-fluoro-deoxy-glucose in skeletal muscle of obese and nonobese humans. Am J

Physiol 279:E1122–E1130, 200028. Nuutila P, Knuuti J, Ruotsalainen U, Koivisto VA, Eronen E, Teras M,

Bergman J, Haaparanta M, Voipio-Pulkki LM, Viikari J: Insulin resistance islocalized to skeletal but not heart muscle in type 1 diabetes. Am J Physiol

264:E756–E762, 199329. DeFronzo RA, Tobin JD, Andres R: Glucose clamp technique: a method for

quantifying insulin secretion and resistance. Am J Physiol 237:E214–E223,1979

30. Yki-Jarvinen H, DeFronzo RA, Koivisto VA: Normalization of insulinsensitivity in type I diabetic subjects by physical training during insulinpump therapy. Diabetes Care 7:520–527, 1984

31. Kadish AH, Hall DA: A new method for the continuous monitoring of bloodglucose by measurement of dissolved oxygen. Clin Chem 11:869–875, 1965

32. Desbuquois B, Aurbach GD: Use of polyethylene glycol to separate freeand antibody-bound peptide hormones in radioimmunoassays. J Clin

Endocrinol Metab 33:732–738, 197133. Miles J, Glasscock R, Aikens J, Gerich J, Haymond M: A microfluorometric

method for the determination of free fatty acids in plasma. J Lipid Res

24:96–99, 198334. Lowry OH, Passonneau JV: A Flexible System of Enzymatic Analysis.

New York, Academic Press, 1972.35. Durnin JV, Rahaman MM: The assessment of the amount of fat in the

human body from measurements of skinfold thickness. Br J Nutr 21:681–689, 1967

36. Piha SJ: Cardiovascular autonomic reflex tests: normal responses andage-related reference values. Clin Physiol 11:277–290, 1991

37. Utriainen T, Malmstrom R, Makimattila S, Yki-Jarvinen H: Methodologicalaspects, dose-response characteristics and causes of interindividual vari-ation in insulin stimulation of limb blood flow in normal subjects.Diabetologia 38:555–564, 1995

38. DeFronzo RA, Ferrannini E, Sato Y, Felig P, Wahren J: Synergisticinteraction between exercise and insulin on peripheral glucose uptake.J Clin Invest 68:1468–1474, 1981

39. Dela F, Mikines KJ, Sonne B, Galbo H: Effect of training on interactionbetween insulin and exercise in human muscle. J Appl Physiol 76:2386–2393, 1994

40. Nugent AM, Steele IC, al-Modaris F, Vallely S, Moore A, Campbell NP, BellPM, Buchanan KD, Trimble ER, Nicholls DP: Exercise responses inpatients with IDDM. Diabetes Care 20:1814–1821, 1997

41. Brechtel K, Machann J, Jacob S, Strempfer A, Schick F, Haring HU,Claussen CD: In-vivo 1H-MR spectroscopy: the determination of the intra-and extramyocellular lipid content depending on the insulin effect in thedirect offspring of type-2 diabetics. Rofo Fortschr Geb Rontgenstr Neuen

Bildgeb Verfahr 171:113–120, 199942. Krssak M, Falk PK, Dresner A, DiPietro L, Vogel SM, Rothman DL, Roden

M, Shulman GI: Intramyocellular lipid concentrations are correlated withinsulin sensitivity in humans: a 1H NMR spectroscopy study. Diabetologia

42:113–116, 1999 [Published errata appear in Diabetologia 42:386, 1999,and 42:1269, 1999.]

43. Griffin ME, Marcucci MJ, Cline GW, Bell K, Barucci N, Lee D, Goodyear LJ,Kraegen EW, White MF, Shulman GI: Free fatty acid–induced insulin re-sistance is associated with activation of protein kinase C u and alterationsin the insulin signaling cascade. Diabetes 48:1270–1274, 1999

44. Thompson AL, Lim-Fraser MY, Kraegen EW, Cooney GJ: Effects ofindividual fatty acids on glucose uptake and glycogen synthesis in soleusmuscle in vitro. Am J Physiol Endocrinol Metab 279:E577–E584, 2000

45. Yki-Jarvinen H, Koivisto VA: Continuous subcutaneous insulin infusiontherapy decreases insulin resistance in type 1 diabetes. J Clin Endocrinol

Metab 58:659–666, 198446. Revers RR, Kolterman OG, Scarlett JA, Gray RS, Olefsky JM: Lack of in

vivo insulin resistance in controlled insulin-dependent, type I diabeticpatients. J Clin Endocrinol Metab 58:353–358, 1984

47. Cusi K, Maezono K, Osman A, Pendergrass M, Patti ME, Pratipanawatr T,DeFronzo RA, Kahn CR, Mandarino LJ: Insulin resistance differentiallyaffects the PI 3-kinase- and MAP kinase-mediated signaling in humanmuscle. J Clin Invest 105:311–320, 2000

48. Russell RR, Bergeron R, Shulman GI, Young LH: Translocation of myocar-dial GLUT-4 and increased glucose uptake through activation of AMPK byAICAR. Am J Physiol 277:H643–H649, 1999

49. Hayashi T, Hirshman MF, Kurth EJ, Winder WW, Goodyear LJ: Evidencefor 59 AMP-activated protein kinase mediation of the effect of musclecontraction on glucose transport. Diabetes 47:1369–1373, 1998

50. Hutber CA, Hardie DG, Winder WW: Electrical stimulation inactivatesmuscle acetyl-CoA carboxylase and increases AMP-activated protein ki-nase. Am J Physiol 272:E262–E266, 1997

51. Vavvas D, Apazidis A, Saha AK, Gamble J, Patel A, Kemp BE, Witters LA,Ruderman NB: Contraction-induced changes in acetyl-CoA carboxylaseand 59-AMP-activated kinase in skeletal muscle. J Biol Chem 272:13255–13261, 1997

52. Merrill GF, Kurth EJ, Hardie DG, Winder WW: AICA riboside increasesAMP-activated protein kinase, fatty acid oxidation, and glucose uptake inrat muscle. Am J Physiol 273:E1107–E1112, 1997

P. PELTONIEMI AND ASSOCIATES

DIABETES, VOL. 50, JUNE 2001 1377