Apr 13, 1992 - Six men were studied during 40 min of cycling exercise to examine the ... muscle glycogen on glucose Liptake in the early stages of exercise in.
Experimental Physiology (1992), 77, 641-644 Printed in Great Britain
MUSCLE GLYCOGEN AND GLUCOSE UPTAKE DURING EXERCISE IN HUMANS M. HARGREAVES, I. MEREDITH* AND G. L. JENNINGS* Department of Physiology, The University of Melbourne, Parkville 3052 and * Alfred and Baker Medical Unit, Alfred Hospital and Baker Medical Research Institute, Prahran 3181, Victoria, Australia (MANUSCRIPT RECEIVED 13 APRIL 1992, ACCEPTED 8 MAY 1992) SUMMARY
Six men were studied during 40 min of cycling exercise to examine the relationship between leg glucose uptake and muscle glycogen concentration. Exercise resulted in significant increases in leg glucose uptake, while muscle glycogen and arterial blood glucose concentrations declined. Arterial plasma insulin levels did not change significantly. There was a significant inverse relationship between muscle glycogen concentration and glucose uptake during exercise which suggests a possible regulatory influence of muscle glycogen on glucose Liptake in the early stages of exercise in humans. INTRODUCTION
During exercise, glucose uptake by contracting skeletal muscle increases in proportion to exercise intensity and duration (Katz, Broberg, Sahlin & Wahren, 1986). This is the result of increased glucose transport across the sarcolemma and activation of the enzymes responsible for glucose disposal. Studies in the rat have suggested that muscle glucose uptake during exercise may be dependent, in part, on a reduction in muscle glycogen (James, Kraegen & Chisholm, 1985). Furthermore, elevation of muscle glycogen levels reduces the contraction-induced glucose uptake in perfused rat muscle (Richter & Galbo, 1986; Hespel & Richter, 1990). The present study was undertaken to examine the possible relationship between muscle glycogen concentration and glucose uptake during exercise in humans. METHODS
Experiments were performed on six male subjects aged 27± 1 years (mean+S.E.M.), with a mean body weight of 71-7±1-4 kg and a mean maximal pLulmonary oxygen uptake (1'O2,max), measured during incremental upright cycling exercise to volitional fatigue, of 3-93±0-18 1 min-1. Subjects were studied during 40 min cycling on an electrically-braked cycle ergometer at a workload estimated to require 50 % V02,max determined from the linear relationship between oxygen uptake and workload obtained during the V02,max test. The study was approved by the Alfred Hospital Human Ethics Review Committee. One subject performed exercise in the upright position; for technical reasons the remaining five subjects exercised in a supine position. The exercise workload in these subjects was derived from their upright V02,max test but was approximately 50 % of the maximal workload attained during an incremental supine cycling test to fatigue. Subjects were studied in the morning after an overnight fast and having abstained from strenuous exercise, tobacco, alcohol and caffeine for at least 24 h. Catheters were positioned in the brachial artery and femoral vein under local anaesthesia (1 % xylocaine, Astra Pharmaceuticals) and were kept patent with heparinized saline (1 U (10 ml saline)-'). Following 30 min of rest and after 15 and 40
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M. HARGREAVES, I. MEREDITH AND G. L. JENNINGS
min of exercise, arterial and venous blood samples were obtained simultaneously, and subsequently analysed for glucose (YSI 23 AM glucose analyser, Yellowsprings Instruments Co., Yellowsprings, OH, USA), arterial plasma insulin (Incstar radioimmunoassay kit), haemoglobin (cyanmethaemoglobin method) and oxygen saturation (OSM 2, Radiometer, Copenhagen). Oxygen content was calculated assuming 1.34 ml (g haemoglobin)-l. Expired gas samples were collected in Douglas bags during blood sampling and subsequently analysed for oxygen (Servomex) and carbon dioxide (Godart) content and volume using a Tissot spirometer. Pulmonary oxygen uptake (°'02) was calculated using standard equations. Immediately after blood samples were taken, muscle samples were obtained from vastus lateralis using a percutaneous needle-biopsy technique. These samples were quickly frozen in liquid N2 and later analysed for glycogen using an enzymatic, fluorometric method (Passonneau & Lauderdale, 1974). Heart rate (HR) was monitored continuously by electrocardiography, and leg blood flow (LBF) estimated according to Jorfeldt & Wahren (1971) from pulmonary oxygen uptake and the brachial artery-femoral vein oxygen difference (A-V 02). The data from the three sampling times were compared using analysis of variance for repeated measures. Significant differences, at the 005 level, were located using the Student-Newman-Keuls post hoc test. All data are reported as means+S.E.M. RESULTS
The physiological and metabolic responses before and during exercise are summarized in Table 1. Exercise resulted in significant increases in pulmonary oxygen uptake, heart rate, leg blood flow, oxygen extraction and glucose uptake, while muscle glycogen and arterial blood glucose concentrations declined. The increase in leg glucose uptake after 15 min of exercise was entirely due to an increase in estimated leg blood flow, since the brachial artery-femoral vein glucose concentration difference did not change from that at rest. In contrast, the increase in leg glucose uptake between 15 and 40 min was due to a doubling of glucose extraction without change in estimated leg blood flow (Table 1). Regression analysis yielded a significant inverse relationship between muscle glycogen concentration and leg glucose ulptake (r = -078, P < 0.05). DISCUSSION
While the inverse relationship between muLscle glycogen concentration and glucose uptake observed in the present study does not necessarily imply cause and effect, the strong association is consistent with the sLIggestion that muscle glycogen may play a role in the regulation of glucose uptake during the early stages of exercise. Previously, a direct relationship has been observed between the percentage of glycogen-empty muscle cells (estimated histochemically) and leg glucose uptake during exercise in humans (Gollnick, Pernow, Essen, Jansson & Saltin, 1981), supporting the finding of the present study. Such a regulatory mechanism would serve to limit blood glucose utilization at a time when glycogen is readily available within the muscle cell. In addition, the preferential utilization of glycogen during the early stages of exercise is energetically advantageous since the glycolytic ATP yield per glycogen-derived glucosyl unit is greater than that from glucose (three vs. two). Muscle glycogen could influence glucose uptake via effects on membrane glucose transport or glucose disposal or both. Studies in the perfused rat hindlimb have demonstrated that increases in pre-contraction muscle glycogen levels result in reduced membrane glucose transport, as meastured by the rate of accumulation of 3-0-[14C]-methyl-D-glucose in contracting muscle (Hespel & Richter, 1990). To our
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GLYCOGEN AND GLUCOSE UPTAKE DURING EXERCISE
Table 1. Physiological and metabolic responses before and during 40 min of cycling exercise at approximately 50 % V02,max Exercise
V02 (1 min-1) A-V02 (ml 1-1) LBF (1 min- 1)
Rest
15 min
40 min
0-28±0-02
1-89+0.11*
1.95±0-09*
59±7
0-64±0+08
126±7*
4-88+0.22*
131±5*
4-85±0-18*
HR (beats min'-)
58+4
129+6*
140±7*
Arterial glucose (mmol 1-)
4-7±0-1
4 5±0-1
4.2±0.2*
Glucose uptake (mmol min-1)
0(08±0 01
0 44+0.12*
0.89±0.20*
Muscle glycogen (mmol (kg wet wt)-1)
100.0±5.9
79.4+5.2*
63.5±5-7*
Insulin(gUml-1)
10(6±1 2
93+1 3
7-1±1-2
Values are means±S.E.M. (n = 6). * Significantly different from rest, P < 0-05.
knowledge, no data exist for humans. The effect of muscle glycogen on glucose disposal is likely to be mediated via glucose 6-phosphate (G-6-P). The rapid breakdown of glycogen at the onset of exercise will lead to intracellular accumulation of G-6-P (Katz, Sahlin & Broberg, 1991), resulting in inhibition of hexokinase activity and glucose phosphorylation and utilization. As the rate of muscle glycogen breakdown declines with continued exercise, muscle G-6-P levels fall (Katz et al. 1991) and the inhibitory effect on glucose uptake is diminished. Similar results have been obtained in perfused, contracting rat muscle (Hespel & Richter, 1990). In summary, we have observed an inverse relationship between muscle glycogen concentration and glucose uptake during exercise which suggests a possible regulatory influence of muscle glycogen on glucose uptake in the early stages of exercise in humans. The authors acknowledge the assistance of Liz Dewar, Leonie Johnston and Sue Scealy. This research was supported by Abbott Australasia and the National Health & Medical Research Council of Australia. REFERENCES P. GOLLNICK, D., PERNOW, B., ESSEN, B., JANSSON, E. & SALTIN, B. (1981). Availability of glycogen and plasma FFA for substrate utilization in leg muscle of man during exercise. Clinical
Physiology 1, 27-42.
HESPEL, P. & RICHTER, E. A. (1990). Glucose uptake and transport in contracting, perfused rat muscle with different pre-contraction glycogen concentrations. Journal of Physiology 427, 347-359. JAMES, D. E., KRAEGEN, E. W. & CHISHOLM, D. J. (1985). Muscle glucose metabolism in exercising rats: comparison with insulin stimulation. American Journal of Physiology 248, E575-580.
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JORFELDT, L. & WAHREN, J. (1971). Leg blood flow during exercise in man. Clinical Science 41, 459-473. KATZ, A., BROBERG, S., SAHLIN, K. & WAHREN, J. (1986). Leg glucose uptake during maximal dynamic exercise in humans. American Journal of Physiology 251, E65-70. KATZ, A., SAHLIN, K. & BROBERG, S. (1991). ReguLlation of glucose utilization in human skeletal muscle during moderate dynamic exercise. American Journal of Physiology 260, E411-415. PASSONNEAU, J. V. & LAUDERDALE, V. R. (1974). A comparison of three methods of glycogen measurement in tissues. Analytical Biochemistry 60, 405-412. RICHTER, E. A. & GALBO, H. (1986). High glycogen levels enhance glycogen breakdown in isolated contracting skeletal muLscle. Journal of Applied Physiology 61, 827-831.
