1 The Human Performance Laboratory, Department of Kinesiology and Health Education, University of Texas at Austin, Austin, Texas 78712; 2 University of Castilla-la Mancha at Toledo, Toledo 45071, Spain; and 3 Pennington Biomedical Research Center, Baton Rouge, Louisiana 70808
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ABSTRACT |
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We used
-adrenergic receptor stimulation and blockade as a tool to study
substrate metabolism during exercise. Eight moderately trained subjects
cycled for 60 min at 45% of
O2 peak 1) during a control trial (CON); 2) while
epinephrine was intravenously infused at 0.015 µg · kg
1 · min
1
(
-STIM); 3) after ingesting 80 mg of propranolol
(
-BLOCK); and 4) combining
-BLOCK with intravenous
infusion of Intralipid-heparin to restore plasma fatty acid (FFA)
levels (
-BLOCK+LIPID).
-BLOCK suppressed lipolysis (i.e.,
glycerol rate of appearance) and fat oxidation while elevating
carbohydrate oxidation above CON (135 ± 11 vs. 113 ± 10 µmol · kg
1 · min
1;
P < 0.05) primarily by increasing rate of
disappearance (Rd) of glucose (36 ± 2 vs. 22 ± 2 µmol · kg
1 · min
1;
P < 0.05). Plasma FFA restoration (
-BLOCK+LIPID)
attenuated the increase in Rd glucose by more than one-half
(28 ± 3 µmol · kg
1 · min
1;
P < 0.05), suggesting that part of the compensatory
increase in muscle glucose uptake is due to reduced energy from fatty
acids. On the other hand,
-STIM markedly increased glycogen
oxidation and reduced glucose clearance and fat oxidation despite
elevating plasma FFA. Therefore, reduced plasma FFA availability with
-BLOCK increased Rd glucose, whereas
-STIM increased
glycogen oxidation, which reduced fat oxidation and glucose clearance.
In summary, compared with control exercise at 45%
O2 peak (CON), both
-BLOCK and
-STIM reduced fat and increased carbohydrate oxidation, albeit
through different mechanisms.
lipid; glucose; propranolol; epinephrine; stable isotopes
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INTRODUCTION |
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THE ROLE OF
-ADRENERGIC RECEPTOR (
-AR)
activity for balancing carbohydrate and fat metabolism during exercise
is not fully understood.
-AR blockade largely reduces fat
availability and oxidation, which requires a compensatory
increase in carbohydrate oxidation to maintain energy production.
Interestingly, during moderate-intensity exercise, this compensatory
increase in carbohydrate metabolism involves increases in plasma
glucose uptake (2, 3, 32, 38). The mechanisms that govern
the increased plasma glucose utilization with
-AR blockade during
exercise have not been determined.
Administration of nicotinic acid to dogs during exercise reduces plasma
free fatty acid (FFA) concentration and increases the rate of
disappearance (Rd) of glucose, whereas plasma FFA replacement progressively lowers Rd glucose
(5). These animal data suggest that the increases in
Rd glucose are not an action exclusive to -AR
blockade but are possibly a compensatory response to an energy
deficit created by reduced fat availability. A way to determine whether
the reduction in fat availability is responsible for the increase in
plasma glucose utilization is to intravenously infuse an
Intralipid-heparin solution to restore intravascular lipolysis and
plasma FFA during
-AR blockade. If the reduced fat availability is
responsible for the increased Rd glucose during
-AR
blockade, the infusion of Intralipid-heparin should increase fat
oxidation and reverse the increase in Rd glucose.
However, in one report (41), when Intralipid-heparin was
infused into -blocked humans during intense exercise (70%
Wmax
85%
O2 peak), fat
oxidation did not increase.
-AR blockade does not impair plasma
fatty acid uptake (35); thus the lack of increase in fat
oxidation did not seem to be due to insufficient fatty acid
availability. It is possible that high exercise intensity increases
glycogenolysis to a level high enough to impair fatty acid oxidation.
It has recently been reported that an increase in glycolytic flux
associated with high exercise intensity (36) or high
carbohydrate availability (12) may actively impair the
entry of FFA into the mitochondria for oxidation. During
moderate-intensity exercise (i.e., 45%
O2 peak), elevations in Rd
glucose elicited by a preexercise carbohydrate meal (i.e., insulin
mediated) are not reduced by Intralipid-heparin infusion that raises
plasma FFA (21). Likewise, during intense exercise (85%
O2 max), Intralipid-heparin infusion
does not reduce Rd glucose level (34). Despite
these previous reports, we hypothesize that, during moderate-intensity
exercise (45%
O2 peak) with
-AR
blockade, restoring fat availability with Intralipid-heparin infusion
may reduce Rd glucose. We think that this may be the case
because, during moderate-intensity exercise with
-AR blockade, glycolytic flux is not stimulated from high glycogenolysis or hyperinsulinemia.
-AR stimulation by epinephrine infusion in humans during exercise
increases lipolysis and plasma fatty acid concentration. However, this
increase in fat availability does not result in increased fat
oxidation; rather, it is reduced (27). Recent studies
performed at moderate and high exercise intensities (45-85%
O2 max) have shown that carbohydrate
use is reduced in the presence of high FFA from Intralipid-heparin
infusion. In those studies, fat oxidation increased through a reduction
in glycogenolysis (21, 29, 34) without affecting glucose
uptake, which does not support the Randle effect (31). Not
surprisingly, epinephrine infusion that stimulates glycogenolysis
(14, 22) does not allow for fat oxidation to increase
despite elevated plasma FFA concentration. Apparently, the stimulation
of glycogenolysis limits the entry of fatty acids into the mitochondria
for oxidation (12, 36), despite having high circulating
FFA levels.
Epinephrine infusion appears to determine the source of carbohydrate
used for substrate during exercise. The powerful increase in
glycogenolysis with epinephrine infusion reduces plasma glucose clearance in the contracting muscle of rodents (9). To our knowledge the effects of both -AR blockade and stimulation in the
same individuals during exercise have not been reported.
The main purpose of this study was to determine whether the reductions
in fat availability are responsible for the increases in plasma glucose
turnover during -AR blockade. To achieve this goal, we restored
plasma fatty acid levels during
-AR blockade. Second, we increased
-AR stimulation during exercise via intravenous epinephrine infusion
to determine whether the stimulation of glycogenolysis and fat
availability (lipolysis) affects plasma glucose kinetics and substrate
oxidation during moderate intensity exercise in humans.
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METHODS |
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Subjects.
Eight moderately trained men (n = 4) and women
(n = 4) participated in this experiment. Subjects were
healthy and were not taking any medication. Women were premenopausal,
were not taking oral contraceptives, and were tested during the
follicular phase. Subjects' (mean ± SD) age, peak oxygen
consumption (O2 peak), peak heart rate,
body weight, and percent body fat were 26 ± 7 yr, 55 ± 8 ml · kg
1 · min
1, 185 ± 10 beats/min, 65 ± 10 kg, and 17 ± 6%, respectively.
Before participation in the testing, subjects were informed of the
possible risks involved and signed a consent form approved by the
Internal Review Board of the University of Texas at Austin.
Experimental protocol.
On four different occasions, subjects arrived at the laboratory in the
morning, after an overnight fast (12 h). After 60 min of rest, subjects
pedaled the cycle-ergometer (Jaeger-Ergotest) for 60 min at a constant
work rate that elicited 45% of their O2 peak (Fig.
1). In one trial,
-ARs were stimulated by infusion of epinephrine (Adrenalin Chloride Solution, Parke-Davis, NJ) at a constant rate (0.015 µg · kg
1 · min
1) from the
15- to 60-min period of exercise (
-STIM). In another trial,
-AR
blockade was produced by ingesting 80 mg of propranolol 2 h before
exercise (
-BLOCK). In another trial, the
-blockade treatment was
combined with intravenous infusion of a 20% triglyceride emulsion
(Intralipid; Clintec Nutrition, Deerfield, IL) with sodium heparin
(Elkins-Sinn, Cherry Hill, NJ) to restore lipolysis and plasma FFA
concentration (
-BLOCK+LIPID). One hour before exercise, a sodium
heparin bolus (7.1 U/kg) was infused, followed by a constant-rate infusion of Intralipid (0.46 ml · kg
1 · h
1) with sodium
heparin (5.5 U · kg
1 · h
1)
throughout rest and exercise. Finally, the normal responses to 60 min
of 45%
O2 peak exercise were measured
during the control trial (CON). During all four trials,
electrocardiogram tracing was monitored throughout the testing period
to confirm that normal sinus rhythm was maintained. The order of
the trials was randomized, and they were separated by
48 h.
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Isotope infusion.
When subjects arrived at the laboratory, Teflon catheters were inserted
into an antecubital vein in each arm for infusion and blood sampling,
respectively. A heating pad was affixed to the sampling forearm to
obtain arterialized blood. A blood sample (4 ml) was withdrawn for
determination of background isotopic enrichment. Then, a primed,
constant-rate infusion of [2H2]glucose
(prime = 20 µmol/kg; 0.25 µmol · kg1 · min
1),
[2H5]glycerol (prime = 3.7 µmol/kg;
0.25 µmol · kg
1 · min
1;
Isotec, Miamisburg, OH), and [1-13C]palmitate (no prime;
0.04 25 µmol · kg
1 · min
1;
Cambridge Isotope Laboratories, Andover, MA), bound to 5% human albumin (Bayer, Elkhart, NJ), was started using calibrated syringe pumps (Harvard Apparatus, South Natick, MA). Subjects were infused for
1 h before the start of exercise to allow attainment of isotopic equilibrium. The rates of infusion were maintained during the 60 min of
exercise. Before being infused into subjects, each isotope was diluted
in sterile saline, tested for pyrogenicity, and passed through a 0.2- to 0.45-µm syringe filter (Acrodisc, Gelman Scientific, Ann Harbor, MI).
Blood sampling and analysis.
For determination of resting glucose, glycerol, and palmitate kinetics,
blood samples were withdrawn 5 min and immediately before exercise.
During exercise and recovery, blood samples (~14 ml) were collected
every 10 min. After collection, blood samples were divided into four
different, prechilled tubes according to the constituents to be
analyzed. For each tube, plasma was separated by centrifugation (1,000 g for 20 min at 4°C) and immediately frozen at 70°C
until analysis. The blood collected during the
-BLOCK+LIPID trials
was immediately centrifuged and processed to prevent in vitro
lipolysis. Four milliliters of each blood sample were placed into tubes
containing 0.2 ml of EDTA solution (25 mg/ml) and analyzed for isotopic
enrichment of the heptafluorobutyric anhydride derivative of glucose
and glycerol (16) and the methyl ester derivative of
palmitate (18) by means of gas chromatography-mass spectrometry (Hewlett-Packard 5989). Five milliliters of plasma were
placed in tubes containing 0.25 ml of EDTA (25 mg/ml) for determination
of plasma glycerol [fluorometric assay (13)], plasma
free fatty acids [FFA, colorimetric assay (28)], glucose (glucose auto-analyzer, YSI, 23A), and lactate [spectrophotometric assay (17)]. Three milliliters of blood were
mixed in a tube containing 0.3 ml of a solution of reduced gluthathione
(4.5 mg), sodium heparin (50 IU), and 20 µl of 0.24 EGTA for
determination of epinephrine and norepinephrine concentration [HPLC
with electrochemical detection (20)]. The final 2 ml of
each blood sample were placed into a test tube containing 0.2 ml of an
EDTA (24 mg/ml)-aprotinin (0.5 TiU/ml) solution and analyzed for plasma
insulin concentration (radioimmunoassay, Linco Research, St. Charles, MO).
Preliminary testing, diet, and training.
O2 peak was determined while subjects
cycled an ergometer (Monark-819, Varberg, Sweden) by use of an
incremental protocol lasting 7-10 min. Two days before the first
experimental trial, subjects performed the experimental exercise
protocol (60 min at 45%
O2 peak) to ensure homogeneity of the
last exercise bout. Subjects refrained from training during the 24 h before the experimental trials. Subjects were asked to exactly replicate the last meal at the same time of day before each of the four trials.
Measurement of gas exchange.
Periodically during exercise, subjects inhaled through a two-way
Daniels valve while inspired air volume was measured with a
Parkinson-Cowan CD4 dry gas meter (Rayfield Equipment, Waitsfield, VT).
The expired gases were continuously sampled from a mixing chamber and
analyzed for oxygen (Applied Electrochemistry, SA3, Ametek, Pittsburgh,
PA) and carbon dioxide (Beckman LB2; Schiller Park, IL). These
instruments were interfaced to a computer for calculation of
O2 and carbon dioxide
production (
CO2).
Calculations.
Plasma glucose, glycerol, and palmitate kinetics were calculated using
the one-pool model non-steady-state equations of Steele (39), modified for use with stable isotopes
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Statistical analysis. SPSS for Windows software was used for statistical analysis. Statistical differences among treatments and over time were identified by using analysis of variance with repeated measures in a complete within-subjects design. Time points were specifically examined for significance by use of contrasts solved by univariate repeated measures. Statistical significance was defined as P < 0.05. The results are presented as means ± SE for eight subjects.
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RESULTS |
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Plasma catecholamine concentrations.
Resting plasma epinephrine concentration was similar among trials
(0.6 ± 0.2 nM). -STIM increased plasma epinephrine
concentration above CON from 0.8 ± 0.1 to 2.2 ± 0.4 nM at
60 min of exercise (P < 0.05).
-BLOCK also
increased epinephrine above CON to 3.0 ± 0.8 nM at 60 min of
exercise (P < 0.05).
-BLOCK+LIPID increased epinephrine levels above CON (2.1 ± 0.9 nM; P < 0.05) but less than with
-BLOCK alone. Plasma norepinephrine was
similar among trials at rest (1.4 ± 0.3 nM).
-STIM did not
affect the normal increase in norepinephrine observed during CON
(2.2 ± 0.2 nM at 60 min). However,
-BLOCK and
-BLOCK+LIPID
increased plasma norepinephrine throughout exercise (3.3 ± 0.5 and 2.9 ± 0.4 nM, respectively, P < 0.05).
Plasma glucose and insulin concentrations.
During the 20- to 60-min period of exercise, plasma glucose
concentration (Fig. 2A)
increased above CON (4.8 ± 0.1 mM) during -STIM (5.4 ± 0.1 mM; P < 0.05). In contrast, plasma glucose
concentration decreased during the first 30 min of exercise with
-BLOCK (4.5 ± 0.1 mM; P < 0.05), returning to
CON values thereafter. Plasma glucose levels were maintained similar to
CON during
-BLOCK+LIPID (4.8 ± 0.2 mM; P < 0.05). After 15 min of exercise, plasma insulin (Fig. 2B)
declined in all trials by ~3.0 ± 0.3 µU/ml. After 30 min of
exercise, plasma insulin increased with
-STIM above CON values
(6.7 ± 0.6 vs. 5.4 ± 0.4 µU/ml, respectively;
P < 0.05). However, during
-BLOCK and
-BLOCK+LIPID, insulin concentrations were reduced below CON
(3.5 ± 0.3 and 3.7 ± 0.5 µU/ml, respectively; P < 0.05).
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Total carbohydrate oxidation.
During the 20- to 55-min period of exercise, carbohydrate
oxidation increased above CON (113 ± 10 µmol · kg1 · min
1) during
-STIM (127 ± 12 µmol · kg
1 · min
1;
P < 0.05) and was even higher with
-BLOCK (135 ± 9 µmol · kg
1 · min
1,
respectively; P < 0.05). However,
-BLOCK+LIPID ameliorated the increase in carbohydrate oxidation
observed with
-BLOCK alone to levels not significantly different
from CON (120 ± 10 µmol · kg
1 · min
1).
Plasma glucose kinetics.
During the 20- to 55-min period of exercise, -STIM did not elevate
Ra glucose above CON (22 ± 2 vs. 22 ± 2 µmol · kg
1 · min
1).
However,
-BLOCK markedly raised Ra glucose above CON
(37 ± 2 vs. 22 ± 2 µmol · kg
1 · min
1;
P < 0.05; Fig.
3A).
-BLOCK+LIPID
attenuated the increase in Ra glucose during
-BLOCK by
approximately one-half, but Ra glucose still remained
higher than CON (28 ± 3 vs. 22 ± 2 µmol · kg
1 · min
1;
P < 0.05). Rd glucose (index of glucose
uptake) was similar to Ra glucose during CON and
-BLOCK+LIPID; thus plasma glucose levels were maintained.
-STIM
tended to reduce Rd glucose below Ra glucose
[20 ± 2 vs. 22 ± 2 µmol · kg
1 · min
1; not
significant (NS)], explaining the mild hyperglycemia (i.e., 5.4 mM).
During the first 15 min of exercise with
-BLOCK, Rd glucose exceeded Ra glucose (28 ± 3 vs. 21 ± 3 µmol · kg
1 · min
1), which
was responsible for the mild hypoglycemia. Plasma glucose clearance
(Fig. 3B) was significantly reduced below CON during
-STIM (4.5 ± 0.3 vs. 3.6 ± 0.3 ml · kg
1 · min
1,
respectively; P < 0.05) and increased above CON during
-BLOCK (8.0 ± 0.5 ml · kg
1 · min
1;
P < 0.05) and
-BLOCK+LIPID (6.0 ± 0.8 ml · kg
1 · min
1;
P < 0.05).
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Calculated minimal glycogen oxidation and plasma lactate.
During the 20- to 55-min period of exercise, -STIM increased the
calculated minimal glycogen oxidation above CON (108 ± 11 vs.
92 ± 9 µmol · kg
1 · min
1,
respectively; P < 0.05), peaking 10 min after the
onset of the epinephrine infusion (i.e., 25 min of exercise; Fig.
4A).
-BLOCK also increased
the calculated minimal glycogen oxidation above CON, but the increases
were smaller and not statistically significant (99 ± 9 vs.
92 ± 9 µmol · kg
1 · min
1; NS).
During the 20- to 55-min period of exercise,
-BLOCK+LIPID returned
calculated glycogen oxidation to CON values (92 ± 9 vs. 92 ± 9 µmol · kg
1 · min
1,
respectively). However, during the first 25 min of exercise,
-BLOCK+LIPID tended to reduce glycogen oxidation below CON (92 ± 11 vs. 100 ± 10 µmol · kg
1 · min
1; NS)
and significantly reduced it below
-BLOCK levels (107 ± 10 µmol · kg
1 · min
1;
P < 0.05). During the 20- to 60-min period of
exercise, plasma lactate concentrations followed a pattern similar to
the calculated minimal glycogen oxidation. Compared with CON (1.4 ± 0.2 mM),
-STIM increased plasma lactate concentration
(2.2 ± 0.4 mM; P < 0.05), whereas
-BLOCK+LIPID tended to decrease plasma lactate levels (1.2 ± 0.2 mM; NS). Furthermore,
-BLOCK did not change plasma lactate
concentration compared with CON (Fig. 4B).
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Ra glycerol and Rd
FFA.
During the 20- to 55-min period of exercise, -STIM tended to raise
Ra glycerol (Fig.
5A) above CON (7.8 ± 1.2 vs. 7.1 ± 0.8 µmol · kg
1 · min
1; NS).
In contrast,
-BLOCK totally prevented Ra glycerol from increasing above rest, and it remained lower than CON throughout exercise (2.6 ± 0.4 µmol · kg
1 · min
1;
P < 0.05).
-BLOCK+LIPID resulted in higher
Ra glycerol than all the other trials (9.6 ± 1 µmol · kg
1 · min
1; all
P < 0.05) due to the Intralipid-heparin infusion. A
response pattern similar to the one reported for Ra
glycerol was observed for plasma glycerol concentrations.
Rd FFA (Fig. 5B) is an index of fatty acid
availability from plasma for oxidation or reesterification.
-STIM
increased Rd FFA above CON; however, the increases did not
reach significance until the 35- to 45-min period (18 ± 3 vs.
12 ± 1 µmol · kg
1 · min
1;
P < 0.05).
-BLOCK prevented Rd FFA from
increasing above rest, and it was lower than CON throughout exercise
(6 ± 0.8 µmol · kg
1 · min
1;
P < 0.05).
-BLOCK+LIPID increased Rd
FFA above CON to levels equal to
-STIM during the 45- to 55-min
period (17 ± 3 µmol · kg
1 · min
1;
P < 0.05).
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Plasma FFA concentration and FFA
oxidation.
A response pattern similar to Rd FFA was observed for
plasma FFA concentrations (Fig.
6A). During the 20- to 60-min
period of exercise, -STIM raised plasma FFA concentration above CON (0.42 ± 0.1 vs. 0.32 ± 0.06 mM, respectively;
P < 0.05).
-BLOCK suppressed the increase in plasma
FFA concentration (0.15 ± 0.01 mM; P < 0.05),
remaining at resting levels during exercise.
-BLOCK+LIPID elevated
plasma FFA to levels similar to
-STIM during the 40- to 60-min
period of exercise (0.44 ± 0.05 mM; P < 0.05).
Fatty acid oxidation (Fig. 6B) increased throughout exercise
during CON (13.2 ± 3 vs. 19.2 ± 1 µmol · kg
1 · min
1; for 7 vs. 55 min of exercise; P < 0.05).
-STIM
reduced fatty acid oxidation below CON soon after the onset of
epinephrine infusion (8.1 ± 2 vs. 13.8 ± 1 µmol · kg
1 · min
1 at 22 min; P < 0.05) but returned to CON values thereafter
(18.3 ± 2 µmol · kg
1 · min
1 at 60 min of exercise).
-BLOCK greatly reduced fatty acid oxidation throughout exercise.
-BLOCK+LIPID increased fatty acid oxidation at
the onset of exercise above all other trials (16.2 ± 2 µmol · kg
1 · min
1;
P < 0.05 at 7 min of exercise). However, fatty acid
oxidation did not increase with exercise duration during the
constant-rate Intralipid infusion (16.2 ± 2 vs. 16.2 ± 1 µmol · kg
1 · min
1 for 7 vs. 55 min of exercise). Because CON increased fat oxidation throughout
exercise whereas
-BLOCK+LIPID did not, fat oxidation was higher at
60 min of exercise during CON vs.
-BLOCK+LIPID (19.2 ± 1 vs.
16.2 ± 1 µmol · kg
1 · min
1;
P < 0.05).
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Energy expenditure.
Table 1 summarizes the estimated caloric
contribution from fat, blood glucose, and muscle glycogen during the
20- to 55-min period of exercise. The total rate of energy expenditure
was similar during all trials (129 ± 12 cal
· kg1 · min
1). Compared
with CON,
-STIM increased the percentage of energy derived from
muscle glycogen from 37 ± 9 to 59 ± 11% (P < 0.05).
-BLOCK reduced the percent energy from fat from 51 ± 11 to 24 ± 7% (P < 0.05) while significantly
increasing the energy derived from plasma glucose from 12 ± 8 to
20 ± 6%; P < 0.05. Finally, compared with CON,
-BLOCK+LIPID increased the energy derived from plasma glucose from
12 ± 8 to 16 ± 11% (P < 0.05).
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DISCUSSION |
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This study altered -AR activity, through either blockade or
epinephrine stimulation, as a means to perturb substrate metabolism during exercise to systematically study the mechanisms of substrate regulation and energy compensation. The observation that both
-BLOCK
and
-STIM reduced fat oxidation during exercise at 45%
O2 peak suggests that the plasma
epinephrine concentration and
-AR activity that normally occur
(i.e., CON) are optimal for maximizing fat oxidation (27).
The first new finding of this study is that, during exercise at 45%
O2 peak, moderately trained subjects
accelerate plasma Rd glucose to compensate for reduced
lipolysis and fatty acid availability with
-BLOCK, and this appears
to be in response to a deficit in energy from fat oxidation. More than
one-half of the increase in Rd glucose was reversed to CON
levels when plasma fatty acid levels were restored (i.e.,
-BLOCK+LIPID) and fat oxidation was increased. This agrees with the
concept that plasma fatty acid availability and oxidation can regulate
plasma glucose utilization [Randle-like effect (31)], in
this case, when fat availability and oxidation are low due to
-BLOCK. In contrast,
-STIM increased glycogen oxidation, which
greatly reduced fat oxidation despite increased plasma FFA levels,
indicating that carbohydrate metabolism can also regulate fat metabolism.
Previous reports show that -AR blockade increases plasma
glucose turnover in healthy people (2, 32) and in type 1 diabetic patients (3, 38) during submaximal exercise
(~50%
O2 max). However, to our
knowledge, this is the first study in humans to directly test the
hypothesis that it is related to fatty acid availability. Our data
suggest that
-BLOCK increased Rd glucose during exercise
to compensate for the energy deficit created by the reduction in
lipolysis and fatty acid oxidation. Elevating plasma fatty acid levels
during
-BLOCK+LIPID increased fat oxidation while simultaneously
reducing Rd glucose compared with
-BLOCK. However, we
cannot exclude the possibility that the reductions in Rd
glucose with
-BLOCK+LIPID could have been due to a direct effect of
the elevated plasma FFA concentration per se, impairing glucose uptake,
rather than to an energy compensatory effect of elevating fat
oxidation. Hargreaves et al. (19) observed that raising
plasma fatty acids from 0.5 to 1.4 mM with a triglyceride-heparin infusion reduced glucose uptake without increasing FFA uptake or fat
oxidation (nonenergy compensatory effect), prompting the idea that the
high plasma fatty acid levels impaired glucose transport across the
cell membrane. The exercise model of Hargreaves et al. was knee
extension at 80% of work capacity, and plasma FFA concentrations were
twofold higher than presently observed, making conditions quite
different compared with the present experiment. In contrast, elevations
in plasma FFA by Intralipid-heparin have not reduced glucose uptake
during cycling exercise at intensities ranging from 40 to 85%
O2 peak (21, 29, 33). Thus it is unlikely that the presently observed reductions in Rd
glucose during
-BLOCK+LIPID compared with
-BLOCK were due to a
direct effect of the elevated plasma FFA concentration.
Although the cellular mechanisms behind the increase in glucose uptake
with -BLOCK are not clear, the present observations, together with
previous reports of cellular energy disturbance, suggest it to be a
compensation for reduced lipolysis and thus fatty acid availability
(5, 32). Insulin did not stimulate the elevated
Rd glucose, in that the highest Rd glucose
occurred at the lower plasma insulin level (i.e.,
-BLOCK), and
-BLOCK+LIPID reduced Rd glucose whereas plasma insulin
remained unchanged (Fig. 2B). Not only does
-AR blockade
reduce fatty acid availability, but it also has the potential to
attenuate the normal rate of glycogenolysis that might occur under
control conditions due to inhibition of cAMP-induced activation of
phosphorylase (8). However,
-BLOCK did not
significantly alter the calculated glycogen oxidation, probably because
the severe reductions in FFA availability activated other intracellular
stimulators of glycogen phosphorylase (Ca2+,
Pi, ADP, AMP) that counteracted the inhibitory effects of
-BLOCK. Likewise, other investigators have found that
-AR
blockade does not reduce muscle glycogen utilization as assessed by
muscle biopsies during submaximal, prolonged exercise (10, 24,
40). In that
-AR blockade reduces lipolysis as well as cAMP
activation of glycogen phosphorylase, it makes sense that the main
compensatory pathway for maintained substrate flux would be increased
glucose uptake (Table 1).
In support of our hypothesis that Rd glucose increased in
response to disturbed cellular homeostasis, the elevation of plasma fatty acid availability with -BLOCK+LIPID decreased the calculated minimal glycogen oxidation below
-BLOCK levels during the 5- to
25-min period of exercise, suggesting that the disturbance of cellular
homeostasis was lessened (Fig. 4A). However,
-BLOCK+LIPID did not completely return Rd glucose to CON levels, despite
elevating plasma fatty acids and lipolysis above CON levels. During
moderate-intensity exercise (50%
O2 peak), both plasma fatty acid and
intramyocellular triglyceride are oxidized (12). Although
-BLOCK+LIPID increased lipolysis in the intravascular space from the
infused Intralipid-heparin, it likely did not restore the intramuscular
triglyceride lipolysis that was also reduced by
-BLOCK
(10). Thus deficient intramuscular fat availability may
still have limited fat oxidation, thus disturbing cellular homeostasis
and increasing Rd glucose above control. Sufficient plasma
FFA was disposed into cells of the whole body (Rd FFA; Fig.
5B) that, if oxidized, should have returned fat oxidation to
CON levels. However, it is unlikely that all of the Rd FFA
was disposed into the exercising muscle (1) and completely oxidized (12). It is likely that not enough fatty acids
were delivered to the mitochondria of exercising myocytes to allow for
complete restoration of fat oxidation, thus necessitating the elevation
of Rd glucose above control during
-BLOCK+LIPID. Other
possible explanations for the maintained elevation in Rd glucose above CON during
-BLOCK+LIPID could involve reduced blood flow (15) and oxygen delivery to the exercising muscles.
It has been hypothesized that compromising oxygen delivery during exercise favors the utilization of oxygen-efficient fuels such as
glucose instead of fatty acids (11). Contraction when
hypoxic increases glucose uptake (7), and superimposition
of
-AR blockade further increases glucose uptake (32).
-STIM tended to lower Rd glucose (i.e., 9%; NS),
whereas it significantly reduced plasma glucose clearance by 20 ± 0.8% (P < 0.05; Fig. 3B). Because
-BLOCK increased whereas
-STIM reduced glucose clearance, it is
logical to first consider a common mechanism that might govern both
responses.
-STIM significantly increased muscle glycogen oxidation
and plasma lactate, and it probably increased glucose 6-phosphate in
muscle (9, 22). In contrast,
-BLOCK reduces glucose
6-phosphate levels (6, 8). Therefore, it is possible that
plasma glucose clearance was inversely related to glucose 6-phosphate
levels via the hexokinase activity. On the other hand, plasma
Ra FFA and concentration were increased with
-STIM but
reduced with
-BLOCK. However, it is unlikely that the small increase
in fatty acid level with
-STIM (0.2 mM) was responsible for the
reduced glucose clearance. High plasma FFA levels during
-BLOCK+LIPID were associated with plasma glucose clearance above
control, whereas equally high FFA levels during
-STIM caused plasma
glucose clearance to be significantly reduced below control. In line
with our previous study of graded intravenous epinephrine infusion at
25%
O2 peak, it appears that
-STIM
results in a rapid reduction in fat oxidation that is associated with
increased glycogenolysis despite a high concentration of plasma fatty
acids (27). Interestingly,
-BLOCK also reduced fat
oxidation, but via reduced lipolysis and fatty acid availability, whereas the compensatory increase in carbohydrate oxidation was derived
from increased glucose uptake. Therefore, it appears that control
levels of epinephrine and
-AR activation are optimal for fat
oxidation at 45%
O2 peak.
General theories of substrate regulation during exercise have usually
argued in favor either of the premise that fat metabolism regulates
carbohydrate oxidation [e.g., the Randle effect; (31)] or of the converse, that carbohydrate metabolism regulates fat oxidation (12, 37). Our present observations suggest that either of these theories of regulation can appear dominant, depending upon the nature of the cellular disturbance and the availability of
substrates for compensation. For example, in agreement with the idea
that fat regulates carbohydrate, it was observed that increased
fatty acid availability during -BLOCK+LIPID reduced glucose uptake
compared with
-BLOCK. Furthermore, the restoration of plasma fatty
acid during
-BLOCK+LIPID reduced muscle glycogenolysis below
-BLOCK during the 5- to 25-min period of exercise (Fig. 4A). On the other hand, in agreement with the idea that
carbohydrate metabolism regulates fat oxidation, it was observed that
increases in muscle glycogen oxidation during
-STIM were associated
with reduced fat oxidation. Previous studies have indicated that
increased glycolytic flux directly reduces fat oxidation, possibly by
attenuating the transport of fatty acid across the mitochondrial
membrane (12, 36). Given the myriad of responses, a
hierarchy should be sought on the basis of potency for regulation of
substrate oxidation. Muscle glycogen normally dominates oxidation, even during control exercise at 45%
O2 peak, as observed in the present
study (see Table 1). Furthermore, glycogen oxidation can be readily
increased, as occurred in the present study with
-STIM, or by other
common factors such as increased exercise intensity (33).
As aforementioned, increased glycolytic flux appears to actively reduce
fat oxidation even when plasma free fatty acids are elevated, as
presently observed. Therefore, when glycogen oxidation and plasma FFA
appearance were stimulated via
-STIM in the present study,
carbohydrate oxidation predominated and fat oxidation decreased. In
light of the large ability to increase glycolytic flux in muscle during
exercise, it seems that carbohydrate metabolism has great potential to
dominate fat metabolism. However, when the potential for increased
glycogenolysis and lipolysis was limited, as in the present study
during
-BLOCK, plasma FFA availability regulated both glucose uptake
and reduced glycogen oxidation during the 5- to 25-min period of
-BLOCK+LIPID. Therefore, it seems that fat metabolism regulated
carbohydrate metabolism against the background of restrained
glycogenolysis from
-BLOCK. Other situations where fatty acid
availability regulates carbohydrate oxidation (the Randle effect) are
rest, low-intensity exercise, and elevated glucose uptake induced by
euglycemic insulinemic clamp (25).
Little is known regarding the regulation of intramuscular lipolysis
during exercise and the extent to which it is influenced by muscle
carbohydrate metabolism. -STIM clearly increased Ra and
Rd FFA by ~50%, indicating that lipolysis of adipose
tissue triglyceride was markedly elevated. It is interesting to note, however, that whole body lipolysis was only slightly and
nonsignificantly elevated above control during
-STIM. This could
suggest that lipolysis of intramuscular triglyceride was reduced by
-STIM. We have recently reported that epinephrine stimulation of
lipolysis, assessed by clamping plasma epinephrine concentration via
intravenous infusion, is attenuated as exercise intensity and
glycogenolysis are increased from 25 to 45%
O2 peak (27). The notion put forth by these observations is that the stimulation of muscle glycogenolysis during exercise might reduce lipolytic sensitivity to
epinephrine in muscle during exercise.
In summary, reducing energy availability from fatty acids when muscle
glycogenolytic sensitivity is attenuated with -BLOCK permits a large
increase in plasma glucose utilization (Rd glucose) to
compensate for the substrate deficit. In this particular situation, fatty acid availability controls glucose uptake (Randle-like effect;
-BLOCK+LIPID). However, when muscle glycogenolysis is stimulated by
epinephrine infusion (
-STIM) and plasma fatty acid concentration is
also high, carbohydrate metabolism dominated fat oxidation. It appears
that carbohydrate oxidation can be robustly increased during exercise
via either glycogenolysis or glucose uptake when fat oxidation is
reduced by disturbing the normal stimulation of
-adrenergic
receptors during exercise.
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ACKNOWLEDGEMENTS |
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We greatly appreciate the technical support of Drs. Jeffrey Horowitz, Pamela Price, and Asker Jeukendrup. We also appreciate the assistance from Melissa Domenick and the participants of this study.
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FOOTNOTES |
---|
This study was supported by a National Institutes of Health grant subcontract from Dr. R. R. Wolfe to E. F. Coyle.
Address for reprint requests and other correspondence: E.F. Coyle, The Univ. of Texas at Austin, Rm. 222, Bellmont Hall, Austin, TX 78712 (coyle e mail.utexas.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 18 August 2000; accepted in final form 26 January 2001.
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REFERENCES |
---|
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---|
1.
Ahlborg, G,
and
Juhlin-Dannfelt A.
Effect of -receptor blockade on splanchnic and muscle metabolism during prolonged exercise in men.
J Appl Physiol
76:
1037-1042,
1994
2.
Ahlborg, G,
Felig P,
Hagenfeldt L,
Hendler R,
and
Wahren J.
Substrate turnover during prolonged exercise in man: splanchnic and leg metabolism of glucose, free fatty acids, and amino acids.
J Clin Invest
53:
1080-1090,
1974[ISI][Medline].
3.
Benn, JJ,
Brown PM,
Beckwith LJ,
Farebrother M,
and
Sonksen PH.
Glucose turnover in type I diabetic subjects during exercise. Effect of selective and nonselective beta-blockade and insulin withdrawal.
Diabetes Care
15:
1721-1726,
1992[Abstract].
4.
Bergeron, R,
Kjaer M,
Simonsen L,
Bülow J,
and
Galbo H.
Glucose production during exercise in humans: a-hv balance and isotopic-tracer measurements compared.
J Appl Physiol
87:
111-115,
1999
5.
Bracy, DP,
Zinker BA,
Jacobs JC,
Lacy DC,
and
Wasserman DH.
Carbohydrate metabolism during exercise: influence of circulating fat availability.
J Appl Physiol
79:
506-513,
1995
6.
Broberg, S,
Katz A,
and
Sahlin K.
Propranolol enhances adenine nucleotide degradation in human muscle during exercise.
J Appl Physiol
65:
2478-2483,
1988
7.
Cartee, GD,
Douen AG,
Ramlal T,
Klip A,
and
Holloszy JO.
Stimulation of glucose transport in skeletal muscle by hypoxia.
J Appl Physiol
70:
1593-1600,
1991
8.
Chasiotis, DR,
Brandt R,
Harris RC,
and
Hultman E.
Effects of -blockade on glycogen metabolism in human subjects during exercise.
Am J Physiol Endocrinol Metab
245:
E166-E170,
1983
9.
Chiasson, J-L,
Shikama H,
Chu DTW,
and
Exton JH.
Inhibitory effect of epinephrine on insulin-stimulated glucose uptake by rat skeletal muscle.
J Clin Invest
68:
706-713,
1981[ISI][Medline].
10.
Cleroux, J,
Van Nguyen P,
Taylor AW,
and
Leenen FHH
Effects of 1- vs.
1+
2-blockade on exercise endurance and muscle metabolism in humans.
J Appl Physiol
66:
548-554,
1989
11.
Cooper, DM,
Wasserman DH,
Vranic M,
and
Wasserman K.
Glucose turnover in response to exercise during high-and low-FIO2 breathing in man.
Am J Physiol Endocrinol Metab
251:
E209-E214,
1986
12.
Coyle, EF,
Jeukendrup AE,
Wagenmakers AJM,
and
Saris WHM
Fatty acid oxidation is directly regulated by carbohydrate metabolism during exercise.
Am J Physiol Endocrinol Metab
273:
E268-E275,
1997
13.
Eggstein, M,
and
Kuhlmann E.
Triglycerides and glycerol determination after alkaline hydrolysis.
In: Methods of Enzymatic Analysis, edited by Bergmeyer HU. New York: Academic, 1974, p. 1825-1831.
14.
Febbraio, MA,
Lambert DL,
Starkie RL,
Proietto J,
and
Hargreaves M.
Effect of epinephrine on muscle glycogenolysis during exercise in trained men.
J Appl Physiol
84:
465-470,
1998
15.
Frisk-Holmberg, M,
Juhlin-Dannfelt A,
and
Astrom H.
Haemodynamic and metabolic responses to prolonged exercise after chronic 1-adrenoceptor blockade in hypertensive man.
Clin Physiol
5:
231-242,
1985[ISI][Medline].
16.
Gilker, CD,
Pesola GR,
and
Matthews DE.
A mass spectrometric method for measuring glycerol levels and enrichments in plasma using 13C and 2H stable isotopic tracers.
Anal Biochem
205:
172-178,
1992[ISI][Medline].
17.
Gutman, I,
and
Wahlefeld W.
L-(+)-Lactate determination with lactate dehydrogenase and NAD.
In: Methods of Enzymatic Analysis, edited by Bergmeyer HU. New York: Academic, 1974, p. 1464-1468.
18.
Hachey, DL,
Patterson BW,
Reeds PJ,
and
Elsas LJ.
Isotopic determination of organic keto acid pentafluorobenzyl esters in biological fluids by negative chemical ionization gas chromatography/mass spectrometry.
Anal Chem
63:
919-923,
1991[ISI][Medline].
19.
Hargreaves, M,
Kiens B,
and
Richter EA.
Effect of increased plasma free fatty acid concentrations on muscle metabolism in exercising men.
J Appl Physiol
70:
194-201,
1991
20.
Hjemdahl, P.
Catecholamine measurements by high-performance liquid chromatography.
Am J Physiol Endocrinol Metab
247:
E13-E20,
1984
21.
Horowitz, JF,
Mora-Rodriguez R,
Byerley LO,
and
Coyle EF.
Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise.
Am J Physiol Endocrinol Metab
273:
E768-E775,
1997
22.
Jansson, E,
Hjemdahl P,
and
Kaijser L.
Epinephrine-induced changes in muscle carbohydrate metabolism during exercise in male subjects.
J Appl Physiol
60:
1466-1470,
1986
23.
Jeukendrup, AE,
Wagenmakers AJ,
Stegen JH,
Gigsen AP,
Brouns F,
and
Saris WH.
Carbohydrate ingestion can completely suppress endogenous glucose production during exercise.
Am J Physiol Endocrinol Metab
276:
E672-E683,
1999
24.
Kaiser, P,
Tesch PA,
Thorsson A,
Karlsson J,
and
Kaijser L.
Skeletal muscle glycolysis during submaximal exercise following acute -adrenergic blockade in man.
Acta Physiol Scand
123:
285-291,
1985[ISI][Medline].
25.
Kelley, DE,
Mokan M,
Simoneau JA,
and
Mandarino LJ.
Interaction between glucose and free fatty acid metabolism in human skeletal muscle.
J Clin Invest
92:
91-98,
1993[ISI][Medline].
26.
Kjaer, M,
Kiens B,
Hargreaves M,
and
Richter EA.
Influence of active muscle mass on glucose homeostasis during exercise in humans.
J Appl Physiol
70:
552-557,
1991.
27.
Mora-Rodriguez, R,
and
Coyle EF.
Effects of plasma epinephrine on fat metabolism during exercise: interactions with exercise intensity.
Am J Physiol Endocrinol Metab
278:
E669-E676,
2000
28.
Novak, M.
Colorimetric ultramicro method for determination of free fatty acids.
J Lipid Res
6:
431-433,
1965
29.
Odland, LM,
Heigenhauser GJF,
Wong D,
Hollidge-Horvat MG,
and
Spriet LL.
Effects of increased fat availability on fat-carbohydrate interaction during prolonged exercise in men.
Am J Physiol Regulatory Integrative Comp Physiol
274:
R894-R902,
1998
30.
Peronnet, F,
and
Massicotte D.
Table of nonprotein respiratory quotient: an update.
Can J Sport Sci
16:
23-29,
1991[ISI][Medline].
31.
Randle PJ, Hales CN, Garland PB, and Newsholme EA. The
glucose-fatty acid cycle. Its role in insulin sensitivity and the
metabolic disturbances of diabetes mellitus. Lancet I:
785-789, 1963.
32.
Roberts, AC,
Reeves JT,
Butterfield GE,
Mazzeo RS,
Sutton JR,
Wolfel EE,
and
Brooks GA.
Altitude and -blockade augment glucose utilization during submaximal exercise.
J Appl Physiol
80:
605-615,
1996
33.
Romijn, JA,
Coyle EF,
Sidossis LS,
Gastadelli A,
Horowitz JF,
Endert E,
and
Wolfe RR.
Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration.
Am J Physiol Endocrinol Metab
265:
E380-E391,
1993
34.
Romijn, JA,
Coyle EF,
Sidossis LS,
Zhang X-J,
and
Wolfe RR.
Relationship between fatty acid delivery and fatty acid oxidation during strenuous exercise.
J Appl Physiol
79:
1939-1945,
1995
35.
Schrauwen, P,
and
van Baak MA.
The effect of beta-adrenergic blockade on non-esterified fatty acid uptake of exercising skeletal muscle during arm cranking.
Int J Sports Med
16:
439-444,
1995[ISI][Medline].
36.
Sidossis, LS,
Gastaldelli A,
Klein S,
and
Wolfe RR.
Regulation of plasma fatty acid oxidation during low- and high-intensity exercise.
Am J Physiol Endocrinol Metab
272:
E1065-E1070,
1997
37.
Sidossis, LS,
and
Wolfe RR.
Glucose and insulin-induced inhibition of fatty acid oxidation: the glucose-fatty acid cycle reversed.
Am J Physiol Endocrinol Metab
270:
E733-E738,
1996
38.
Simonson, DC,
Koivisto V,
Sherwin RS,
Ferrannini E,
Hendler R,
Juhlin-Dannfelt A,
and
DeFronzo RA.
Adrenergic blockade alters glucose kinetics during exercise in insulin-dependent diabetics.
J Clin Invest
73:
1648-1658,
1984[ISI][Medline].
39.
Steele, R.
Influences of glucose loading and injected insulin on hepatic glucose output.
Ann NY Acad Sci
82:
420-430,
1959[ISI].
40.
Van Baak, MA,
de Haan A,
Saris WHM,
van Kordelaar E,
Kuipers H,
and
van der Vusse GJ.
-Adrenoceptor blockade and skeletal muscle energy metabolism during endurance exercise.
J Appl Physiol
78:
307-313,
1995
41.
Van Baak, MA,
Mooij JMV,
and
Wijnen JAG
Effect of increased plasma non-esterified fatty acid concentrations on endurance performance during beta-adrenoceptor blockade.
Int J Sports Med
14:
2-8,
1993[ISI][Medline].