Department of Veterinary Clinical Sciences, College of
Veterinary Medicine, The Ohio State University, Columbus, Ohio 43210
This study examined the effects of
preexercise glucose administration, with and without epinephrine
infusion, on carbohydrate metabolism in horses during exercise. Six
horses completed 60 min of treadmill exercise at 55 ± 1% maximum
O2 uptake 1) 1 h after oral administration of
glucose (2 g/kg; G trial); 2) 1 h after oral glucose and with
an intravenous infusion of epinephrine (0.2 µmol · kg
1 · min
1;
GE trial) during exercise, and 3) 1 h after water only (F
trial). Glucose administration (G and GE) caused hyperinsulinemia and hyperglycemia (~8 mM). In GE, plasma epinephrine concentrations were
three- to fourfold higher than in the other trials. Compared with F,
the glucose rate of appearance was ~50% and ~33% higher in G and
GE, respectively, during exercise. The glucose rate of disappearance
was ~100% higher in G than in F, but epinephrine infusion completely
inhibited the increase in glucose uptake associated with glucose
administration. Muscle glycogen utilization was higher in GE [349 ± 44 mmol/kg dry muscle (dm)] than in F (218 ± 28 mmol/kg dm) and G
(201 ± 35 mmol/kg dm). We conclude that 1) preexercise glucose augments utilization of plasma glucose in horses during moderate-intensity exercise but does not alter muscle glycogen usage
and 2) increased circulating epinephrine inhibits the increase in glucose rate of disappearance associated with preexercise glucose administration and increases reliance on muscle glycogen for energy transduction.
 |
INTRODUCTION |
CARBOHYDRATES, IN THE FORM of muscle glycogen and
blood-borne glucose, are important substrates for contracting skeletal
muscle. Among the numerous factors that regulate carbohydrate
metabolism in muscle, there is evidence that adrenergic stimulation can
affect both uptake of glucose by muscle and the rate of intramuscular glycogen utilization. In humans at rest, both basal and
insulin-stimulated glucose clearance are impaired during epinephrine
infusion (3). Furthermore, studies in the rat have shown that skeletal
muscle is the main tissue in which glucose uptake is decreased by
epinephrine (6). The effects of epinephrine on utilization of blood
glucose during exercise are less well studied. However, epinephrine
infusion reduces leg glucose uptake in humans during cycling exercise
(30). Similarly, epinephrine infusion in running dogs inhibits glucose clearance but does not affect hepatic glucose production (28). Moreover, administration of propranolol inhibits the effects of epinephrine on glucose clearance in exercising dogs (28), implicating
-adrenergic mechanisms in this restraint of glucose uptake.
In contrast to the inhibitory effects of
-adrenergic stimulation on
glucose uptake by muscle, studies in animals (28, 53) and humans (9,
30) have demonstrated that epinephrine infusion increases muscle
glycogen utilization during submaximal exercise. Furthermore,
adrenomedullation in rats (52) or
-adrenergic blockade (27) in dogs
reduces muscle glycogen use during exercise. In dogs,
-adrenergic
blockade also reverses the effect of epinephrine infusion on muscle
glycogenolysis (28). It is apparent, therefore, that
-adrenergic
mechanisms can exert opposing effects on muscle glycogenolysis and the
uptake of glucose by muscle.
It is well known that exercise is a potent stimulus for epinephrine
secretion. However, the magnitude of the plasma epinephrine response is
affected by carbohydrate availability. In humans during exercise,
plasma epinephrine concentrations are higher in fasted than in fed
subjects (15, 49). Conversely, short-term (3-4 days) consumption
of a high-carbohydrate diet (29) or ingestion of glucose during
exercise (45) attenuates the exercise-associated increase in plasma
epinephrine. Given the aforementioned effects of
-adrenergic
stimulation on carbohydrate metabolism in muscle, it is possible that
such alterations in the plasma epinephrine response can modify the
relative contributions of blood-borne glucose and intramuscular
glycogen to total carbohydrate oxidation (CHOox) during exercise.
Few studies have examined the role of adrenergic mechanisms in
regulation of carbohydrate metabolism in horses during exercise. However, similar to other species,
-adrenergic blockade accelerates glucose utilization in horses during low- and moderate-intensity exercise. Carbohydrate availability also modifies the sympathoadrenal response to sustained exertion. Gabbard et al. (13) have reported that,
compared with a high-carbohydrate meal (corn), preexercise consumption
of feed low in soluble carbohydrate (alfalfa hay) exacerbated the
plasma catecholamine response to light exercise. Furthermore, the
increase in glucose supply associated with an intravenous infusion of
glucose attenuates the increase in plasma epinephrine in horses during
exercise at 35% of maximum oxygen uptake
(
O2 max) (16). Thus
there is evidence that
-adrenergic stimulation constrains glucose
uptake in horses during exercise. Moreover, on the basis of plasma
epinephrine concentrations, an increase in carbohydrate availability
reduces
-adrenergic stimulation during exercise. Taken together,
these findings suggest that, under conditions of increased carbohydrate
availability, attenuation of the sympathoadrenal response may
contribute to an increase in whole body glucose disposal. To date,
however, this hypothesis has not been tested.
The present studies were, therefore, undertaken to determine the
effects of glucose supply (preexercise oral glucose administration vs.
water placebo) and adrenergic mechanisms (preexercise glucose administration with and without an intravenous infusion of epinephrine during exercise) on tracer-determined whole body glucose uptake in
horses during moderate-intensity exercise. It was
hypothesized that preexercise glucose administration would attenuate
the sympathoadrenal response to exercise and increase utilization of
blood glucose, evidenced by an increase in glucose rate of
disappearance (Rd). Conversely, the increase in
-adrenergic stimulation associated with epinephrine infusion would
reverse the effects of preexercise glucose administration such that
whole body glucose disposal would be similar to that measured in
control (a 24-h fast before exercise) trials. Because
-adrenergic
stimulation can also affect muscle glycogenolysis, a further objective
was to determine the effects of these interventions on net muscle
glycogen utilization.
 |
MATERIALS AND METHODS |
All animal experiments were conducted after approval by the
Institutional Laboratory Animal Care and Use Committee of The Ohio
State University and were performed in compliance with their guidelines
and recommendations.
Horses.
The subjects were six horses (2 Standardbred and 4 Thoroughbred; all
geldings), 3-9 yr old, body mass 408-527 kg (471 ± 25 kg,
mean ± SD). The horses were conditioned and undertaking regular treadmill exercise (5 days/wk) for 2 mo before the study. Four days per
week, horses exercised for 30-45 min at 55% of
O2max, and on the fifth
day a protocol of moderate- (20 min at 55%
O2max) and higher (10 min
at 75%
O2max)
intensity exercise was completed. During the conditioning and
experimental periods, the horses were housed indoors and fed a diet of
timothy grass and alfalfa hay and a pelleted concentrate ration, and
they had access to a salt and mineral block. Between experimental
trials, horses received 3 days of light treadmill exercise (20 min of
trotting at 4-4.5 m/s with the treadmill set at a 4° incline).
Preliminary testing.
For each horse,
O2 max and the
relationship between oxygen consumption
(
O2) and speed were
determined during an incremental exercise test 1 wk before the first
experiment. The incremental exercise test consisted of the horse
running on a high-speed treadmill (Sato) inclined at 4° for 90 s at
4 m/s, after which the treadmill speed was increased by 1 m/s every 90 s until the horse was no longer able to maintain its position on the
treadmill.
O2 was measured
every 10 s during the exercise test.
O2max was defined as the
value at which
O2 reached a
plateau, despite further increases in speed. A plateau was defined as a
change in
O2 of <4
ml · kg
1 · min
1
with an increase in speed. From linear regression analysis (speeds below
O2max), the running
speed that elicited 55% of
O2max was calculated for
each horse.
Experimental design.
The effects of preexercise glucose administration and of epinephrine
infusion on carbohydrate metabolism during moderate-intensity exercise
were examined in a three-way crossover study. All horses undertook 60 min of treadmill exercise at a workload equivalent to 55%
O2max in each of
three experimental conditions: 1) 1 h after administration of a
20% glucose solution (2 g/kg body wt; G trial); 2) 1 h after
administration of a 20% glucose solution and with an intravenous
infusion of epinephrine (0.2 µmol · kg
1 · min
1),
commencing at the onset of exercise (GE trial); and 3) 1 h after administration of water only (F trial). For each horse, the
administered fluid volumes (glucose solution or water) were the same
(~8-9 liters). Trials were separated by 7 days, and the order of
the trials was randomized but balanced among treatments.
Experimental protocol.
The horses were fasted for 24 h before each experiment and were
confined to their stalls during this time period. Body weight was
measured on entry to the laboratory. After aseptic preparation and
local anesthesia of the overlying skin, catheters (14 gauge, 51/4 in.; Angiocath, Becton Dickinson) were inserted into the
right and left jugular veins for isotope infusion and blood collection, respectively. Thereafter, a blood sample was obtained for subsequent determination of background isotopic enrichment. For determination of
glucose kinetics, a primed (18.0 µmol/kg), continuous (0.248 ± 0.007 µmol · kg
1 · min
1)
infusion of [6,6-2H]glucose (99% enriched;
Cambridge Isotopes, Cambridge, MA) in 0.9% saline was then initiated
with the use of a calibrated infusion pump (PHD 2000; Harvard
Apparatus, South Natick, MA). After a 90-min equilibration period,
during which the horses stood in stocks, the glucose or water
treatments were administered by nasogastric gavage. Five minutes before
exercise, a sample of middle gluteal muscle was obtained by
percutaneous biopsy (see Sampling procedures). Thereafter, the
horses were positioned on the treadmill (4° incline), and a
loose-fitting face mask for measurement of respiratory gas exchange was
applied. A thermocouple (model T-180; Physitemp Instruments, Clifton,
NJ) attached to a thermometer (BAT-10, Physitemp Instruments) was
inserted 20-25 cm beyond the anal sphincter for measurement of
temperature within the rectum during exercise. The horses completed a
5-min warm-up (3 m/s treadmill belt speed) followed by 60 min of
running at a speed calculated to elicit 55%
O2max. The rate of
[6,6-2H]glucose infusion was tripled (0.747 ± 0.02 µmol · kg
1 · min
1)
during the warm-up. In the GE trial, an epinephrine solution (12.5 mg
of epinephrine diluted in 500 ml of 0.9% saline for a concentration of
25 µg/ml) was infused (0.2 µmol · kg
1 · min
1)
by a second pump (volumetric infusion pump, model Vet/IV 2.2, Heska),
whereas an equivalent volume of 0.9% saline was administered during
the F and G trials. The epinephrine or saline solutions were infused
via a three-way stopcock to allow simultaneous administration of the
tracer and epinephrine or saline. The epinephrine and saline infusions
were commenced at the onset of exercise (after completion of the
warm-up). During the exercise test, fans mounted 0.5 m in front and to
the sides of the treadmill were used to maintain an air velocity of
3.5-4 m/s over the horse. Ambient conditions were similar for all
trials; values for room temperature and relative humidity during the
experiments were 22.4 ± 0.9°C and 58 ± 5% (means ± SD),
respectively. The horses were reweighed after completion of the
exercise protocol.
Respiratory gas exchange measurements.
O2, carbon dioxide
production (
CO2), and
respiratory exchange ratio (RER) were measured with an open-circuit
calorimeter (Oxymax-XL, Columbus Instruments, Columbus, OH) as
previously described (24). Flow through the system was ~1,500 l/min
STP when the horse was stationary and 9,000 l/min during running. Data for expired O2 (electrochemical
cell, Columbus Instruments) and CO2 (single-beam
nondispersive infrared sensor, Columbus Instruments) concentrations
were measured continuously and were reported at 10-s intervals. The
gas-analysis system was calibrated before the start of each exercise
test by using gas mixtures with O2 and CO2
concentrations that spanned the measurement range. The overall accuracy
of the system was verified repeatedly by the nitrogen dilution
method (10). Discrepancy between simulated
O2 produced by
nitrogen dilution and the value measured by the system was ±3% at
nitrogen flow rates equivalent to a
O2 of 54 l/min
(~140
ml · kg
1 · min
1
for a 385-kg horse). Standard equations were used to calculate
O2 and
CO2 and R
values were calculated by dividing
CO2 by
O2.
Rectal temperature.
Temperature within the rectum (Tre) was measured at rest
before the start of exercise and at 10-min intervals during the
exercise trial. The thermocouple had a response time of ~1°C/s
and was calibrated in a heated water bath with a precision thermometer (Fisher Scientific, Mississauga, Ontario).
Sampling procedures.
Blood samples were obtained at
75,
60,
45,
30, and
15 min before exercise and at 0, 5, 10, 20, 30, 40, 50, and 60 min of exercise (where the "
75" sample was
collected 75 min after the start of isotope infusion and the water or
glucose solutions were administered immediately after collection of the
"
60" sample). Blood samples were divided (6-ml aliquots)
into four different tubes for subsequent analysis. Two aliquots of each
sample were placed in evacuated tubes containing EDTA. These samples
were later analyzed for plasma isotopic enrichment, hematocrit, plasma total protein, nonesterified fatty acid (NEFA), and glycerol
concentrations. A 5-ml aliquot was placed in a tube containing sodium
fluoride-potassium oxalate for subsequent determination of plasma
glucose and lactate concentrations. The final aliquot was placed in a
tube containing no additive and was later analyzed for serum insulin
concentration. Additional blood samples for subsequent measurement of
epinephrine and norepinephrine concentrations were obtained at 0, 10, 20, 40, and 60 min of exercise and were placed in test tubes that held
120 µl of a solution containing 0.24 M EGTA-reduced glutathione. Plasma or serum was obtained by centrifugation (3,000 rpm for 20 min at
4°C) within 30 min of collection and frozen at
20°C (
80°C for hormone and tracer samples) until analysis.
Muscle biopsy samples were collected percutaneously from the middle
gluteal muscle via the needle biopsy technique (40). Muscle biopsies
were obtained 5 min before commencement of exercise and within 3 min of
cessation of exercise. Muscle samples were immediately placed in liquid
nitrogen and stored at
80°C until analysis.
In the G and GE trials, urine was collected during the 30-min
postexercise period. Urine volume was measured, and an aliquot was
saved for subsequent measurement of glucose concentrations.
Plasma isotopic enrichment.
Plasma samples (0.7 ml) were deproteinized by adding 1.2 ml of 0.3 N
Zn(SO)4 and 1.2 ml of 0.3 N Ba(OH)3. Each tube
was then vortexed and incubated in the ice bath for 20 min. After
centrifugation (3,000 rpm for 20 min at 4°C), the supernatant was
harvested, and the water was removed from the tubes by vacuum
centrifugation (Savant Instruments, Farmingham, NY). The penta-acetate
derivative of [6,6-2H]glucose was prepared by
adding 100 µl of a 2:1 acetic anhydride and pyridine mixture to the
dried sample. After a 60-min incubation at 60°C, the reaction
mixture was partitioned by sequential addition of 1.5 ml
double-distilled water and 0.4 ml of methylene chloride. After gentle
shaking, the tubes were centrifuged at 2,000 rpm for 10 min. The upper
water phase was discarded, and the remaining methylene chloride phase
was evaporated under N2. Before injection into the gas
chromatograph-mass spectrometer, the samples were dissolved in 50 µl
of ethyl acetate. Standards of known isotopic enrichment were prepared
in an identical fashion and were analyzed with each batch of samples.
Isotopic enrichment was determined by gas chromatography-mass
spectrometry on a Hewlett-Packard 5989A mass spectrometer equipped with
a 30-m × 0.25-mm B-5 capillary column (J & W Scientific, Folsom,
CA) and with the use of 1-µl injections. The initial oven temperature
was set at 110°C and was gradually increased by 35°C/min until
it reached a final temperature of 255°C. The transfer line was set
at 250°C, the source temperature was set at 200°C, and the
quadrupole temperature was set at 115°C. Ions were formed by
electron impact ionization (70 eV), and molecular ions of
mass-to-charge ratios (m/z) of 200 (m + 0), 201 (m + 1), and 202 (m + 2) were selectively
monitored, i.e., m/z 200 and m/z 202 correspond to the
unlabeled and labeled ions, respectively. The tracer-to-tracee ratio
was calculated directly from measured ion abundance ratios and was
equal to R
Ro, where R and Ro represent the measured tracer and tracee ion abundance ratios for enriched and
unenriched (background or preinfusion) samples, respectively. Correction was made for the contribution of singly labeled molecules (m/z 201) to the apparent enrichment at m/z 202 (61).
The intra-assay coefficient of variation was 1.5 ± 0.5%, and the
interassay coefficient of variation was 5.6 ± 2.1%. To control for
between-day variability, all samples for a given horse (F, G, and GE)
were analyzed during the same analytic session.
Plasma biochemical analyses.
Plasma glucose and lactate concentrations were measured by the glucose
oxidase and lactate oxidase methods, respectively, with an automated
analyzer (Yellow Springs Instruments, Yellow Springs, OH). Urine
glucose concentration was also measured by use of the glucose oxidase
autoanalyzer. Plasma NEFA concentration was determined by using a
commercial kit that employs an enzymatic colorimetric method (NEFA test
kit; Wako Chemicals USA, Dallas, TX). Plasma glycerol concentration was
measured by an enzymatic spectrophotometric method [triglycerides
kit 337A (without the triglyceride hydrolysis step); Sigma
Chemical]. Intra- and interassay coefficients of variation for
these biochemical methods were <1.0% and 2.5%, respectively.
Hematocrit was measured by the microhematocrit technique. Plasma total
protein was measured by refractometry (Cambridge Instruments, Buffalo,
NY). All samples were analyzed in duplicate.
Plasma hormone analyses.
Plasma epinephrine and norepinephrine concentrations were determined by
HPLC using electrochemical detection (42). Serum immunoreactive insulin
(IRI) was determined in duplicate by use of a commercially available
RIA (insulin kit, Coat-a-Count Diagnostics, Los Angeles, CA) that has
been validated for horse blood (51). Intra- and interassay coefficients
of variation were 6.0 ± 1.5% and 11.5 ± 2.1%, respectively.
Muscle glycogen and lactate.
Muscle samples were weighed and subsequently freeze-dried. The
freeze-dried samples were reweighed and dissected free of any blood and
connective tissue. For each sample, duplicate pieces of muscle were
analyzed. One portion was extracted according to the general procedures
of Harris et al. (23), and duplicate extracts were analyzed for lactate
content by enzymatic fluorometric methods (41). Muscle glycogen
concentrations (as glucosyl units) were determined on a second
freeze-dried aliquot, which was extracted and analyzed according to the
procedure of Passoneau and Lauderdale (48). Intra-assay (12 replicates
of a single sample) and interassay (6 replicates) coefficients of
variation for the muscle glycogen assay were 2.5% and 4.1%, respectively.
Calculations of glucose kinetics.
Glucose rate of appearance (Ra) and Rd (=tissue
uptake) at rest were calculated by using the steady-state tracer
dilution equation (61)
where
F is the infusion rate of the isotope (in
µmol · kg
1 · min
1);
IEi and IEp are the stable isotopic enrichment
of the infusate and plasma, respectively; and
1 accounts for the
tracer's contribution to the turnover rate of the substrate (61). The
rate of infusion was calculated by multiplying the infusion pump rate
by the concentration of glucose in the infusate. During exercise,
Ra and Rd were calculated according to the
non-steady-state equation developed by Steele (57). This equation is
modified for use with stable isotopes as the amount of tracer infused
is no longer negligible
and
where
Vd is the effective volume of distribution, E is the plasma
isotopic enrichment, Cm is the measured plasma
concentration of the tracee, and dE/dt and
dCm/dt are maximum rates of change in enrichment
and glucose concentration, respectively, as a function of time. By
using this fixed, one-compartment model of Steele, it is assumed that
1) the apparent glucose space is 25% of body weight, and
2) 65% of this space represents the rapidly mixing portion of
the glucose pool. Therefore, the effective Vd for glucose was assumed to be 162 ml/kg. However, our interpretation of the data
presented herein is not affected by the assumed value for Vd because the difference in Ra did not exceed
10% when values were calculated using values for Vd
ranging from 40 to 210 ml/kg.
The glucose metabolic clearance rate (MCR) was calculated as the
Rd of glucose divided by the average glucose concentration (C) over that period
The glucose kinetics were calculated between two different
blood-collection time points (t1 and
t2). However, these data are presented as
t2. For example, the Ra at t = 20 min during exercise is actually the Ra between 10 and 20 min.
Rates of energy expenditure and whole body substrate oxidation.
Total energy expenditure (TEE) and absolute rates of
CHOox and lipid oxidation were calculated by using the
following equations (12)
|
(1)
|
|
(2)
|
|
(3)
|
where
O2 and
CO2 are in liters per
minute and it was assumed that protein oxidation made negligible
contribution to
O2 and
CO2. The
calculated values were based on respiratory gas exchange values
averaged over 5-min intervals for the first 10 min of exercise and
averaged over 10-min intervals thereafter. CHOox in grams
per minute was converted to micromoles per kilogram per minute by
dividing the molecular weight of glucose (180) and the horse's body
weight. Similarly, rates of fat oxidation were converted to micromoles
per kilogram per minute by dividing by the molecular weight of
palmitate (259) and the horse's body weight.
Studies in dogs (62) and in humans (35) have demonstrated that
increases in glucose uptake by working muscle account for most of the
increment in whole body Rd during exercise. Furthermore, ~90-95% of glucose Rd is oxidized during exercise
(33). Thus glucose Rd provides a reasonable approximation
of blood glucose oxidation. Muscle glycogen (plus lactate) oxidation
was calculated as the difference between total CHOox and
glucose Rd. Net muscle glycogen utilization was also
calculated as the difference of the pre- and postexercise muscle
glycogen concentrations. The relative contributions by plasma glucose
(glucose Rd), other carbohydrate sources (muscle glycogen
and lactate), and lipid to TEE during the 30-60 min period of
exercise were estimated by using standard caloric equivalents.
Statistical analyses.
Unless otherwise stated, data are presented as means ± SE.
The data were analyzed as a three-way crossover design by use of a
two-way repeated-measures ANOVA [repeated measures on treatment (i.e., F, G, or GE) and time factors]. As the data for muscle glycogen did not exhibit homogeneous variances, these data were subject
to logarithmic transformation before ANOVA. Data for net muscle
glycogen utilization and the absolute and relative contributions by
different substrates to TEE during the 30-60 min period of exercise were analyzed by a one-way repeated-measures ANOVA (repeated measures on the treatment factor). Percent data (relative contributions by different substrates) were subject to arcsine transformation before
ANOVA. The null hypothesis was rejected at
= 0.05 for the
main effects (treatment and time) and
= 0.10 for the interaction. Significant differences identified by ANOVA were isolated using the
Student-Newman-Keuls post hoc test. The Sigmastat 2.0 software package
(Jandel Scientific, San Rafael, CA) was used for statistical computations.
 |
RESULTS |
Individual values for
O2max
ranged from 127 to 168 ml · kg
1 · min
1
(mean 141 ± 6.8 ml · kg
1 · min
1).
Mean running speed during the exercise protocol was 5.5 ± 0.2 m/s
(range 5.2-6.0 m/s). The mean
O2 measured during the 60 min
of exercise corresponded to a relative workload of 55 ± 1.0% of
O2max (range 53-57%).
Tre was not different throughout exercise in F compared
with G. However, Tre was significantly lower in GE than in
G and in F between 30 and 60 min of exercise (Table 1). Preexercise body weight was similar
among the three trials (F: 493 ± 15; G: 495 ± 17; GE: 496 ± 15 kg). Mean values for loss of body weight during exercise were higher in
GE (18.8 ± 1.2 kg) than in F (15.5 ± 1.2 kg) and G (15.3 ± 1.3 kg). However, this difference did not reach statistical significance
(P = 0.15; power at
= 0.05:0.21).
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Table 1.
Hematocrit, plasma total protein and lactate concentrations, and rectal
temperature before and during 60 min of exercise at 55 ± 1% of
O2max
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Plasma glucose concentration.
At rest and before administration of glucose or water, plasma glucose
concentrations were similar among the three trials (Fig. 1). During the 60-min period after
administration of the oral glucose load, plasma glucose concentration
increased during both G and GE (8.2 ± 0.4 and 8.3 ± 0.3 mM,
respectively; P < 0.05 vs. F) but remained unchanged in F
(5.1 ± 0.1 mM). During exercise in F, plasma glucose concentration
increased steadily to reach a peak of 10.3 ± 0.5 mM. In G, plasma
glucose decreased from preexercise values during the first 20 min of
exercise but subsequently increased, and, between 20 and 60 min of
exercise, plasma glucose concentration was similar in F and G. During
GE, epinephrine infusion resulted in marked hyperglycemia during
exercise, with peak values of 17.2 ± 0.3 mM. Plasma glucose
concentrations were higher (P < 0.05) throughout exercise in
GE than in F. Similarly, plasma glucose was higher (P < 0.05)
in GE than in G between 20 and 60 min of exercise (Fig. 1).

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Fig. 1.
Plasma glucose concentration during rest and 60 min of exercise at 55%
of maximum oxygen uptake
( O2 max) when fasted
(F), administered glucose 60 min before exercise (G), or administered
glucose 60 min before exercise and infused with epinephrine during
exercise (GE). Values are means ± SE; n = 6. * Mean
glucose concentration during G and GE significantly greater than during
F trial, P < 0.05. # Mean plasma glucose concentration
during GE significantly greater than during F and G, P < 0.05.
|
|
Serum insulin.
As expected, oral glucose administration in G and in GE resulted in a
substantial increase (time × treatment, P < 0.0001) in
serum IRI (Fig. 2). During exercise, serum
IRI decreased in all trials but remained elevated (P < 0.05)
in the G trial compared with F and GE for the first 30 min of exercise.
No differences in serum IRI concentration existed among trials between
40 and 60 min of exercise.

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Fig. 2.
Serum immunoreactive insulin (IRI) concentration during rest and 60 min
of exercise at 55% of
O2 max. Values are
means ± SE; n = 6. * Mean IRI concentration during G and
GE significantly greater than during F trial, P < 0.05. # Mean IRI concentration during G significantly greater than during
F and GE, P < 0.05.
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|
Urine glucose.
Five horses in the G trial urinated during the early postexercise
period, whereas urine was obtained from all six horses in the GE trial.
On average, urine was collected 15-20 min after exercise. The mean
volume of urine collected in G and GE was 1.9 ± 0.1 and 2.1 ± 0.2 liters, respectively. Urine glucose concentration was significantly
higher in GE (47.5 ± 14.5 mM; range 9.5-121.4 mM) than in G
(mean 5.7 ± 2.6 mM; range 2.1-8.5 mM).
Plasma catecholamines.
Preexercise epinephrine and norepinephrine concentrations did not
differ among trials (Fig. 3, A and
B). Epinephrine infusion (GE) resulted in plasma concentrations
three- to fourfold higher (P < 0.0001) than those in F and G
(Fig. 3A). On the other hand, plasma norepinephrine
concentrations were lower (P < 0.001) in GE than in F and G
throughout exercise (Fig. 3B). After 60 min of exercise, plasma
epinephrine was significantly lower in G than in F (Fig. 3A).

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Fig. 3.
Plasma epinephrine (A) and norepinephrine (B)
concentrations before and during 60 min of exercise at 55% of
O2 max. Values are
means ± SE; n = 6. * Values for GE significantly different
from F and G, P < 0.001. # G significantly lower than F,
P < 0.05.
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Hematocrit, plasma total protein, NEFA, glycerol, and lactate.
Administration of glucose or water was not associated with changes in
hematocrit and plasma total protein concentration (t =
75 min vs. t = 0 min in Table 1). In all trials,
hematocrit and plasma total protein increased significantly early
during exercise, but no differences were detected among trials.
Resting plasma NEFA concentrations were between 0.45 and 0.68 mM (Fig.
4B). Glucose administration (G and
GE) resulted in significant decreases in plasma NEFA. During the F
trial, plasma NEFA concentration decreased during the first 5 min of
exercise and thereafter gradually increased to reach a peak of 0.97 ± 0.10 mM after 60 min of exercise (Fig. 4B). Plasma NEFA
concentrations also increased gradually during exercise in G. However,
plasma NEFA was significantly lower in G than in F throughout exercise. In contrast, epinephrine infusion (GE) resulted in plasma NEFA concentrations that were similar to those measured in F, but
significantly higher compared with G. In all trials, plasma glycerol
concentrations increased during exercise (Fig. 4A). However,
throughout exercise plasma glycerol was higher (P < 0.05) in
GE than in G and F. Plasma glycerol concentrations were lower
(P < 0.05) in G than in F between 20 and 60 min of exercise.

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Fig. 4.
Plasma glycerol (A) and nonesterified fatty acid (NEFA;
B) concentrations during rest and 60 min of exercise at 55% of
O2 max. Values are mean ± SE; n = 6. a Values for F and GE
significantly greater than those for G, P < 0.05. b Values for GE significantly greater than those for F,
P < 0.05. c Values for F significantly greater
than those for G and GE, P < 0.05.
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Epinephrine infusion greatly intensified (P < 0.0001, time × treatment effect) the increase in plasma lactate concentration early in exercise (Table 1). In the G and F trials, plasma lactate concentrations rose gradually, reaching a peak at the end of exercise of 7.0 ± 0.9 and 6.1 ± 0.7 mM, respectively. In GE, however, plasma lactate increased rapidly from 0.7 ± 0.1 mM at rest to 6.4 ± 0.7 mM
after 10 min of exercise, with values of ~8-9 mM for the
remainder of exercise. Plasma lactate concentration was significantly
higher in GE than in F and G between 5 and 60 min of exercise.
Gas exchange and whole body CHOox and fat oxidation.
There was a small but significant (P < 0.001) increase in
O2 during exercise, but
O2 was similar among the
three trials (Table 2). There was a
significant (P < 0.0001) treatment × time interaction
for RER. In GE, RER was greater than 1.0 for the first 15 min of
exercise and was significantly higher compared with G and F during this
time period. RER was higher (P < 0.05) in GE than in F for
the remainder of exercise. RER was also higher in G than in F at most
time points during the exercise trial. However, RER was not different
between 20 and 60 min of exercise in GE compared with G (Table 2).
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Table 2.
Oxygen uptake, respiratory exchange ratio, and calculated rates of
carbohydrate and fat oxidation during 60 min of exercise at 55 ± 1% of O2max
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Whole body rates of CHOox and fat oxidation are presented
in Table 2. As RER was greater than 1.0 during the first 15 min of
exercise in GE, estimates of whole body substrate oxidation rates were
restricted to the 20- to 60-min period. Total CHOox decreased during exercise in F but remained stable in G. Total CHOox was higher in G and GE than in F between 20 and 60 min of exercise (Tables 2 and 3). Conversely, fat oxidation was
suppressed (P < 0.05, all time points) in G and GE compared
with F. After 60 min of exercise, fat oxidation rates were 48 ± 2, 34 ± 2, and 29 ± 4 µmol · kg
1 · min
1
for F, G, and GE, respectively (Table 2).
Glucose kinetics.
Before oral administration of glucose, plasma isotopic enrichment (Fig.
5), glucose Ra (Fig.
6A), and glucose Rd
(Fig. 6B) were similar among the three trials. In F, an
isotopic steady state was maintained throughout the preexercise period,
but isotopic enrichment decreased during exercise despite the threefold
increase in tracer infusion rate. During both G and GE, isotopic
enrichment decreased after oral glucose administration and was lower
(P < 0.05) throughout exercise compared with that in F (Fig.
5).

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Fig. 5.
Plasma [6,6-2H]glucose enrichment during rest
and 60 min of exercise at 55% of
O2 max. Values are
means ± SE; n = 6. * Values for F significantly greater
than those for G and GE, P < 0.05.
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Fig. 6.
Glucose rate of appearance (Ra; A) and
disappearance (Rd; B) during rest and 60 min of
exercise at 55% of
O2 max. Values are
means ± SE; n = 6. a Values for G
significantly greater than those for F, P < 0.05. b Values for G significantly greater than those for GE,
P < 0.05. c Values for GE significantly
greater than those for F, P < 0.05.
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At rest after oral glucose administration, the mean glucose
Ra and Rd during both G and GE were two- to
threefold greater than during F (P < 0.05, Fig. 6, A
and B). Throughout exercise, the glucose Ra during
G remained ~70-80% higher (P < 0.05) than during F. Whereas glucose Ra reached a plateau of ~42-44
µmol · kg
1 · min
1
between 30 and 60 min of exercise in F, the Ra increased
throughout exercise in G to a peak of 76.4 ± 2.1 µmol · kg
1 · min
1
at 60 min. Glucose Ra during GE was intermediate to that
during G (P < 0.05 between 20 and 60 min) and F (P < 0.05 between 30 and 50 min; Fig. 6A).
During exercise, there was a significant time × trial interaction
(P < 0.0001) for glucose Rd. Whereas glucose
Rd increased with the onset of exercise in all trials, the
increase was much larger in G (Fig. 6B). In F, glucose
Rd increased gradually and reached a plateau of
~32-34
µmol · kg
1 · min
1
during the last 20 min of exercise. After the early sharp increase, glucose Rd decreased between 10 and 30 min of exercise in G
but thereafter remained steady at ~55
µmol · kg
1 · min
1
(P < 0.05 vs. F at all time points during exercise).
Epinephrine infusion almost completely inhibited the augmentation in
glucose Rd associated with preexercise glucose
administration. With the exception of the 5-min time point, glucose
Rd in GE was not different from that in F. Glucose MCR
demonstrated a similar pattern (Fig. 7).
MCR was approximately twofold higher (P < 0.05) in G than in
F and GE throughout exercise, whereas there were no differences between
F and GE for MCR.

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Fig. 7.
Glucose metabolic clearance rate (MCR) during 60 min of exercise at
55% of O2 max. Values
are means ± SE; n = 6. * Values for G significantly
greater than those for F and GE, P < 0.05.
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Muscle glycogen and lactate.
Preexercise muscle glycogen concentration was similar among the three
trials. However, postexercise muscle glycogen concentration was lower
(P < 0.04) in GE than in F and G (Fig.
8A). Therefore, net muscle glycogen
utilization was higher (P < 0.03) in GE (349 ± 44 mmol/kg
dry muscle) compared with F (218 ± 28 mmol/kg dry muscle) and G (201 ± 35 mmol/kg dry muscle). There was no difference in net muscle
glycogen utilization in F compared with G. Similarly, the
exercise-associated increase in muscle lactate concentration was
similar in the F and G trials (Fig. 8B). However, end-exercise muscle lactate was higher (P < 0.05) in GE than in F and G.

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Fig. 8.
Muscle glycogen (A) and lactate (B) concentrations
before (Pre-Ex) and after (Post-Ex) 60 min of exercise at 55% of
O2 max. Values are
means ± SE; n = 6. dw, Dry weight. * Significant
difference compared with Pre-Ex values, P < 0.05. # Post-Ex values significantly different in GE compared with F and
G, P < 0.05.
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The calculated rates of muscle glycogen oxidation (total
CHOox
glucose Rd) during the final 30 min of exercise corroborated the muscle biopsy data (Table
3). Muscle glycogen oxidation was significantly greater in GE than in F (all time points) and G (30- to
40-min and 40- to 50-min periods).
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Table 3.
Total carbohydrate oxidation, rate of glucose disappearance, and muscle
glycogen (and lactate) oxidation during exercise at 55 ± 1% of
O2max
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Overall pattern of substrate utilization.
Figure 9 depicts the estimated relative
caloric contribution from fat, muscle glycogen (plus lactate), and
blood glucose during the final 30-min period of exercise. The total
rate of energy expenditure was similar among the three trials (342 ± 18, 346 ± 21, and 350 ± 22 cal · kg
1 · min
1
for F, G, and GE, respectively). In F, the relative energy expenditure from fat, blood glucose, and muscle glycogen was 32 ± 2, 5.7 ± 0.5, and 62.3 ± 2%, respectively (Fig. 9). The contribution by fat to
total energy was ~33% lower in G (22.5 ± 2%) and GE (20 ± 3%)
compared with F. Thus the contribution by all sources of carbohydrate
(blood glucose, muscle glycogen, lactate) to energy use was
significantly higher in G (77 ± 3%) and GE (80 ± 2%) compared with F (68 ± 2%). In GE there was a shift to greater use of muscle glycogen (74.6 ± 3%), whereas the contribution by blood glucose (5.5 ± 1%) was similar to that during F. In contrast, the relative contribution of blood glucose during G (12.5 ± 2%) was significantly greater than in F and GE, whereas the contribution from muscle glycogen
(65 ± 2%) was lower (P < 0.05) compared with GE but not different from F (Fig. 9).

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Fig. 9.
Relative caloric contributions from other carbohydrates (CHO) (muscle
glycogen plus lactate), lipid, and blood glucose in 6 horses during the
30- to 60-min period of exercise. * Values significantly different
between G and GE vs. F, P < 0.05. ** Contribution from
blood glucose greater in G than in F and GE, P < 0.05. # Contribution from other carbohydrates greater in GE than in F and
G, P < 0.05.
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DISCUSSION |
The present study examined the interactive effects of increased blood
glucose availability (preexercise glucose administration) and increased
circulating epinephrine (preexercise glucose administration and
infusion of epinephrine during exercise) on carbohydrate metabolism in
horses during moderate-intensity exercise. The main findings were
1) a more than twofold increase in whole body glucose uptake during exercise after oral glucose administration compared with exercise after a 24-h fast; 2) almost complete inhibition of
the increase in glucose Rd associated with preexercise
glucose administration produced by an infusion of epinephrine;
3) an increase in net intramuscular glycogen use and lactate
accumulation when epinephrine was infused during exercise; and
4) augmentation of CHOox in both trials preceded by
oral administration of glucose.
Glucose kinetics.
As expected, preexercise administration of glucose resulted in marked
hyperglycemia (Fig. 1) and hyperinsulinemia (Fig. 2). The increase in
plasma glucose concentration was accompanied by increments in glucose
Ra and Rd. Similarly, during exercise, glucose Ra was significantly higher in the trials preceded by
glucose administration compared with the fasting condition. In humans, ingestion of glucose 30-50 min before exercise results in an
increase in glucose Ra during exercise compared with
placebo trials (1, 44). However, preexercise glucose ingestion markedly
suppresses hepatic glucose production during exercise, and the increase
in glucose Ra reflects ongoing intestinal uptake of glucose
(44). In the present study, we did not measure the contribution of
gut-derived Ra (intestinal uptake) to the total glucose
Ra. Nonetheless, it is probable that continued uptake of
glucose from the intestinal tract contributed to the higher glucose
Ra in the trials preceded by oral glucose administration.
However, systemic glucose availability during exercise was lower in GE
than in G (Fig. 6A). The reason for this difference cannot be
determined from our data.
Consistent with findings in humans (44), the hyperglycemia and
hyperinsulinemia associated with preexercise glucose administration was
accompanied by a large increase in glucose Rd early in
exercise (G trial; Fig. 6B). As the increment in glucose
Rd in G was not matched by a similar increase in glucose
Ra, plasma glucose concentration decreased during the first
20 min of exercise (Fig. 1). In marked contrast, epinephrine infusion
(GE) inhibited the increase in glucose uptake associated with
preexercise glucose administration, and plasma glucose concentrations
increased progressively throughout exercise. Because values for glucose
Rd are derived from estimates of glucose Ra,
part of the attenuation in glucose uptake during GE can be attributed
to the reduction in glucose Ra compared with the G trial.
However, even though glucose Ra was ~40% higher in GE
than in F between 30 and 60 min of exercise, glucose Rd and MCR in GE were not different from F at most time points during exercise
(Fig. 6B). Taken together, these results indicate that increases in circulating epinephrine exert a potent inhibitory effect
on glucose disposal in horses during moderate-intensity exercise, even
when glucose supply is increased by administration of glucose before exercise.
Several mechanisms may account for the effects of epinephrine infusion
on glucose Rd and MCR, including direct effects on glucose
transport into muscle (18) and indirect effects on glucose uptake
because of differences in circulating insulin (59) and NEFA
concentrations (22) and alterations in the rate of intramuscular glycogenolysis (34). Plasma insulin was lower in GE than in G during
the first 30 min of exercise, even though preexercise concentrations
were similar (Fig. 2). The more rapid decrease in plasma insulin
concentrations during exercise in GE likely reflected intensified
-adrenergic-mediated inhibition of insulin secretion associated with
the increase in circulating epinephrine (14). Muscle contractions and
insulin have been shown to have synergistic effects on glucose uptake
by muscle in dogs (62) and humans (59) during exercise. Therefore, the
higher plasma insulin concentration in G compared with F and GE may
have contributed to the increase in glucose Rd and MCR in
the G trial. However, plasma insulin concentrations were similar among
trials during the last 30 min of exercise, indicating that other
mechanisms contributed to the between-trial differences in glucose disposal.
Lower plasma NEFA concentrations in G also may have contributed to the
higher glucose uptake in this trial compared with GE and F. Preexercise
glucose administration resulted in a significant decrease in plasma
NEFA, and, although plasma NEFA increased during exercise in all
trials, concentrations were significantly lower in G than in F and GE
(Fig. 4B). In humans, preexercise carbohydrate ingestion
inhibits lipolysis during low- to moderate-intensity exercise and
thereby suppresses the increase in plasma NEFA concentration (8, 25).
Similarly, in horses, preexercise consumption of a carbohydrate meal
(corn grain) attenuates the increase in fatty acid concentration during
exercise (36, 58). On the other hand,
-adrenergic mechanisms
stimulate lipolysis during exercise (2), probably accounting for the
higher plasma NEFA in GE than in G throughout exercise (Fig.
4B). In dogs, suppression of circulating NEFA concentrations by
infusion of nicotinic acid, an inhibitor of lipolysis, results in
increased whole body glucose disposal during exercise (5). Conversely,
increased plasma NEFA has been demonstrated to attenuate glucose uptake
by muscle in humans during leg exercise (22), although other
investigators have not found an effect of increased plasma NEFA on
glucose uptake (54). Nonetheless, it is possible that the lower
circulating NEFA during exercise in G compared with GE and F may, in
part, explain the increase in glucose uptake during the G trial.
The lower glucose Rd and MCR during exercise in GE also may
have reflected more direct effects of epinephrine on glucose transport into muscle. In vitro studies have demonstrated that
epinephrine inhibits glucose uptake in rat skeletal muscle (18, 31).
The mechanism of this inhibition of glucose uptake into muscle by epinephrine has not been elucidated, although it is possible that
-adrenergic stimulation decreases the intrinsic activity of GLUT-4 in muscle, perhaps by phosphorylation of these transporter proteins. However, Lee et al. (39) showed that, although epinephrine causes phosphorylation of GLUT-4 in skeletal muscle, this had no effect on
glucose transport. Alternatively, the lower glucose uptake may be
related to the inhibitory effects of epinephrine on glucose phosphorylation in muscle. Studies of resting (31) and contracting (17)
muscle have demonstrated accelerated glycogenolysis and accumulation of
intracellular glucose and glucose-6-phosphate during epinephrine
stimulation. Because glucose-6-phosphate is an inhibitor of hexokinase
(34), this accumulation could reduce glucose phosphorylation, thereby
reducing glucose uptake into muscle. We did not measure
glucose-6-phosphate concentrations in muscle biopsies from the horses
in this study. However, given the higher rate of muscle glycogenolysis
in the GE trial, it is possible that accumulation of
glucose-6-phosphate and inhibition of hexokinase contributed to the
lower glucose uptake during exercise with epinephrine infusion.
The inhibitory effect of epinephrine on glucose disposal measured in
the horses in the present study is consistent with data from dog and
humans. Infusion of epinephrine (0.5 µg · kg
1 · min
1)
into running dogs decreased whole body glucose uptake and resulted in
marked hyperglycemia (28). Furthermore,
-blockade with propranolol abolished the effects of epinephrine on glucose uptake, implicating
-adrenergic mechanisms in the effect of epinephrine on glucose disposal (28). Intra-arterial infusion of epinephrine (0.04 µg · kg
1 · min
1)
into one leg decreased its glucose uptake by ~60% compared with uptake of blood-borne glucose by the contralateral leg during two-legged exercise (30). More recently, Mora-Rodriguez and Coyle (46)
demonstrated that an infusion of epinephrine that results in only
modest increases in plasma epinephrine concentrations (control: 0.6 ± 0.1 nM; epinephrine infusion: 1.9 ± 0.2 nM) also suppresses glucose
clearance in men during exercise at 25% of peak
O2.
Muscle glycogen utilization.
A second objective of this study was to determine the effect of
preexercise glucose administration, with and without epinephrine infusion during exercise, on intramuscular glycogen utilization. In
humans, the hyperglycemia and hyperinsulinemia resulting from glucose
ingestion 30 to 60 min before exercise has been associated with an
increase in muscle glycogen utilization in some (7, 21), but not all
(11, 20), studies. The increase in muscle glycogen usage under these
circumstances has been attributed to a reduction in the supply of free
fatty acids in contracting skeletal muscle, thereby increasing the
reliance on muscle glycogen as a fuel source (4). There also is
conflicting evidence regarding the effect of preexercise carbohydrate
ingestion on muscle glycogenolysis in horses during exercise. Lawrence
and colleagues (37) reported that, compared with exercise after an
overnight fast, glycogen utilization was higher when horses consumed 1 kg of corn grain (a high-carbohydrate meal) 1, 3, or 5 h before
exercise. However, other studies from the same laboratory utilizing
similar preexercise feeding regimens did not find an increase in
intramuscular glycogenolysis when a carbohydrate meal was ingested
before exercise (36, 38). Similarly, in the present study, we did not
detect an effect of preexercise glucose administration (G trial) on net
muscle glycogen degradation during exercise (Fig. 8A).
On the other hand, net muscle glycogen utilization was ~65% higher
when epinephrine was infused during exercise. There have been no
previous reports of the effects of epinephrine infusion on muscle
metabolism in horses during exercise. However, an increase in
circulating epinephrine has been demonstrated to increase skeletal muscle glycogenolysis in rats (53), dogs (28), and humans (9, 17, 30)
during submaximal exercise. Epinephrine activates phosphorylase kinase
in muscle, thereby promoting conversion of glycogen phosphorylase from
the inactive b form to the more active a form (19).
After activation, there is progressive reversal of phosphorylase back
to the b form, and epinephrine is thought to be less important
for stimulation of glycogenolysis with increasing duration of exercise.
Given the markedly higher values for RER in GE compared with F and G
during the first 20 min of exercise (Table 2), it is possible that the
effect of epinephrine on muscle glycogenolysis was greatest early in
exercise. However, the estimates of muscle glycogen oxidation,
calculated from the difference between whole body CHOox and
glucose Rd, also indicated higher glycogen usage in GE than
in F and G during the last 30 min of the exercise protocol (Table 3).
The exercise-associated increase in muscle lactate concentration was
also enhanced by epinephrine infusion (Fig. 8B). Furthermore, plasma lactate concentration was two- to fourfold higher in GE than in
F and G, particularly early in exercise (Table 1). These findings
suggest that lactic acid production increased when epinephrine was
infused during exercise, and they provide further evidence of the
stimulatory effect of epinephrine on muscle glycogenolysis and
glycolysis. Our findings are in agreement with studies in humans
showing that epinephrine infusion increases lactate accumulation in
muscle during submaximal exercise (9) and results in greater lactate
release from working tissues (30). Furthermore, blood lactate
concentrations in humans during exercise are higher with epinephrine
infusion compared with control trials (9, 60).
Overall pattern of substrate utilization.
The results of the present study indicate that preexercise glucose
administration in horses results in increased CHOox during exercise (Table 2 and Fig. 9). During the final 30 min of exercise, CHOox was ~15-17% higher in G than in F (Table 3),
and, overall, the contribution by carbohydrate to TEE was ~10%
higher in G. During the same time period, rates of fat oxidation were
~30-35% lower in G than in F (Table 2). Previous studies in
humans during moderate-intensity exercise also have demonstrated
increased CHOox, with a concomitant reduction in fat
oxidation, when carbohydrate (glucose or fructose) is ingested before
exercise (8, 25). In humans, the reduction in fat oxidation is due to
inhibition of lipolysis (25) and an increase in CHOox in
muscle (8). Horowitz et al. (25) reported that a 10-30 µU/ml
increase in plasma insulin concentration before exercise completely
abolished the exercise-associated increase in lipolysis. Furthermore,
this low rate of lipolysis limited the availability of free fatty acids during exercise (25). Similarly, in the present study it is possible
that the higher insulin concentrations after glucose administration
reduced whole body lipolysis, evidenced by the lower plasma NEFA and
glycerol concentrations during exercise in G (Fig. 4).
Fat oxidation was also lower in GE compared with F (Table 2). However,
this decrease in fat utilization did not appear to be due to reduced
lipolysis and free fatty acid availability as plasma concentrations of
NEFA and glycerol were similar in the GE and F trials (Fig. 4). On the
other hand, the epinephrine-induced increases in glycogenolysis and
glycolysis may have reduced fat oxidation in muscle. In humans, it has
recently been reported that an increase in glycolytic flux associated
with elevated carbohydrate availability directly reduced muscle fatty
acid oxidation by limiting entry of long-chain fatty acids into
mitochondria (8). Under these circumstances, it is proposed that there
is an increase in the concentration of malonyl-CoA in muscle, with
inhibition of carnitine palmitoyltransferase I, the enzyme that
regulates the entry of long-chain fatty acids into mitochondria (8,
26).
The augmentation in CHOox in the G trial was explained by
an increased utilization of blood-borne glucose, as muscle glycogen utilization did not differ between G and F (Fig. 8A). The
relative contribution of blood glucose to TEE was more than twofold
higher in G than in F (Fig. 9). Interestingly, plasma glucose
concentration was similar in these two trials between 20 and 60 min of
exercise, indicating that factors other than glucose availability were
most important for this increase in glucose uptake. These factors could include insulin-mediated GLUT-4 translocation, lower circulating epinephrine and NEFA concentrations, and activation of oxidative enzymes within contracting muscle (32). Our estimates of the relative
contribution of blood glucose to energy expenditure did not account for
the loss of glucose in urine. However, in the G trial, estimated
urinary glucose excretion represented <0.5% of the total glucose
Rd. On the other hand, the marked hyperglycemia during
exercise in GE resulted in substantial loss (range: ~20-115 mM) of
glucose in urine. If it is assumed that most of this glucose was
excreted during exercise, up to 4-5% of the total glucose Rd in GE was due to renal excretion. Therefore, the actual
contribution of blood glucose to energy use was probably lower in GE
than in F, further emphasizing the potent inhibitory effects of
epinephrine on glucose uptake.
Another finding of the present study was the significant attenuation of
the exercise-induced increase in Tre when epinephrine was
infused. Between 30 and 60 min of exercise, Tre was
~0.7-1.0°C lower in GE than in F and G (Table 1).
Furthermore, mean whole body fluid loss was higher in GE compared with
the other trials, although this difference did not reach statistical
significance. It is likely that this effect of epinephrine on
Tre was due to an increase in evaporative heat loss during
exercise. In horses at rest, epinephrine infusion induces a marked
sweating response, an effect mediated by
-adrenoceptors (56).
Conversely,
-adrenergic blockade reduces sweating in horses during
high-intensity exercise (50) and augments the increase in core
temperature in ponies during submaximal exercise (55). This effect of
epinephrine in horses is in marked contrast to findings in humans.
Mora-Rodriguez et al. (47) have demonstrated that an infusion of
epinephrine that elevated plasma concentrations approximately sixfold
compared with the control condition resulted in an ~0.4°C
increase in esophageal temperature. This impairment to thermoregulation
with epinephrine infusion was due to cutaneous vasoconstriction,
evidenced by significant reductions in forearm and skin vascular
conductance (47).
The lower plasma norepinephrine during exercise in GE (Fig. 3B)
was indicative of decreased sympathetic adrenergic outflow, given that
plasma norepinephrine is mainly derived from the spillover of
norepinephrine by sympathetic nerves (14). Given the high circulating
epinephrine in GE (Fig. 3B), one possible explanation for the
attenuated plasma norepinephrine response is stimulation of
2-adrenoceptors in presynaptic terminals, with a
reduction in norepinephrine release and spillover into circulation.
Alternatively, the marked hyperglycemia in GE (Fig. 1) may have
contributed to the diminished plasma norepinephrine concentrations. In
humans, sustained hyperglycemia (by glucose infusion) attenuates the
plasma norepinephrine response to exercise (43, 47). The mechanism of
this response has not been elucidated.
In summary, this study has demonstrated that preexercise administration
of glucose augments CHOox and utilization of blood-borne glucose in horses during moderate-intensity exercise but does not alter
muscle glycogen usage. Conversely, an infusion of epinephrine during
exercise almost completely inhibits the increase in glucose Rd associated with preexercise glucose administration and
increases