Exercise Physiology Laboratory, Department of Integrative Biology, University of California, Berkeley, Berkeley, California 94720
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ABSTRACT |
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Related to hepatic autoregulation we
evaluated hypotheses that 1) glucose production would be
altered as a result of a glycerol load, 2) decreased glucose
recycling rate (Rr) would result from increased glycerol uptake, and
3) the absolute rate of gluconeogenesis (GNG) from glycerol
would be positively correlated to glycerol rate of disappearance
(Rd) during a glycerol load. For these purposes, glucose
and glycerol kinetics were determined in eight men during rest and
during 90 min of leg cycle ergometry at 45 and 65% of peak
O2 consumption (O2 peak).
Trials were conducted after an overnight fast, with exercise commencing
12 h after the last meal. Subjects received a continuous infusion
of [6,6-2H2]glucose,
[1-13C]glucose, and
[1,1,2,3,3-2H5]glycerol without (CON) or with
an additional 1,000 mg (rest: 20 mg/min; exercise: 40 mg/min) of
[2-13C]- or unlabeled glycerol added to the infusate
(GLY). Infusion of glycerol dampened glucose Rr, calculated as the
difference between [6,6-2H2]- and
[1-13C]glucose rates of appearance (Ra), at
rest [0.35 ± 0.12 (CON) vs. 0.12 ± 0.10 mg · kg
1 · min
1 (GLY),
P < 0.05] and during exercise at both intensities
[45%: 0.63 ± 0.14 (CON) vs. 0.04 ± 0.12 (GLY); 65%:
0.73 ± 0.14 (CON) vs. 0.04 ± 0.17 mg · kg
1 · min
1 (GLY),
P < 0.05]. Glucose Ra and oxidation were
not affected by glycerol infusion at rest or during exercise.
Throughout rest and both exercise intensities, glycerol Rd
was greater in GLY vs. CON conditions (rest: 0.30 ± 0.04 vs.
0.58 ± 0.04; 45%: 0.57 ± 0.07 vs. 1.19 ± 0.04; 65%:
0.73 ± 0.06 vs. 1.27 ± 0.05 mg · kg
1 · min
1, CON vs.
GLY, respectively). Differences in glycerol Rd
(
Rd) between protocols equaled the unlabeled glycerol
infusion rate and correlated with plasma glycerol concentration
(r = 0.97). We conclude that infusion of a glycerol
load during rest and exercise at 45 and 65% of
O2 peak 1) does not affect
glucose Ra or Rd, 2) blocks glucose
Rr, 3) increases whole body glycerol Rd in a
dose-dependent manner, and 4) results in gluconeogenic rates
from glycerol equivalent to CON glucose recycling rates.
gluconeogenesis; glycerol kinetics; exertion; stable isotopes
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INTRODUCTION |
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UNDER POSTPRANDIAL CONDITIONS, hepatic autoregulation maintains euglycemia by coordinated changes in glycogenolysis and gluconeogenesis (GNG). In resting humans, GNG provides 20-40% of hepatic glucose production (HGP) after an overnight fast (5, 13, 17, 27, 32, 34). The contribution of GNG to HGP increases to 90% after 40-60 h of fasting rest (13, 22, 27). During steady-state exercise, GNG and glycogenolysis increase to match glucose production and utilization. The absolute and relative contributions of GNG to HGP have been reported to increase with exercise duration (1), intensity (5, 9), and training (5).
Investigators have attempted to quantify GNG in humans during rest and exercise by several methods, including splanchnic catheterization (1, 34), labeling the gluconeogenic pathway with tracer CO2 (15, 28), and incorporation of precursor carbon isotope into glucose (5, 17, 18, 20, 30). The most common techniques for the estimation of GNG in vivo are based on precursor-to-product (p/p) relationships. Unfortunately, estimation of GNG by the p/p approach is confounded by several factors, including inaccessibility of the triose phosphate pool (TPP, the "true" gluconeogenic precursor pool) for measurement, and dilution of infused carbon label in the tricarboxylic acid (TCA) cycle (21). In an attempt to account for isotopic dilution, correction factors determined from the recovery of infused tracer acetate (14) have been developed. However, correction factors are species and condition specific and are potentially affected by extrahepatic metabolism of acetate (30).
Recently, Hellerstein and colleagues (12, 26) have developed a technique, termed mass isotopomer distribution analysis (MIDA), to measure fractional GNG without the use of correction factors for determination of GNG in vivo. Unlike the p/p approach, estimation of GNG by MIDA is theoretically not confounded by dilution or isotopic exchange of carbon label in the TCA cycle. Furthermore, MIDA is purported to account for GNG from all, not just 3-carbon gluconeogenic substrates. However, the isotopic glycerol load required for the reliable application of MIDA could elicit a significant increase in blood glycerol concentration, thereby affecting HGP or GNG.
In the past, elevated gluconeogenic precursor supply has been reported to increase both absolute GNG (20) and the relative contributions of the infused substrates to HGP in dogs (31, 36) and humans (18, 38). However, results of splanchnic catheterization (2) and isotopic dilution studies (18, 20, 38) on humans indicate that elevated gluconeogenic precursor supply does not drive HGP. Thus it appears that hepatic autoregulatory mechanisms maintain normal HGP despite elevated gluconeogenic precursor supply in resting humans. However, the effects of increasing GNG precursor supply during exercise, a condition in which HGP is elevated, remain unknown.
The present investigation was performed to evaluate effects of a glycerol load sufficient to perform MIDA on glucose turnover, oxidation, and recycling at rest and during moderate exercise after an overnight fast. Specifically, we evaluated the hypotheses that 1) glucose production would be altered as a result of a glycerol load; 2) decreased glucose recycling would result from increased glycerol uptake; and 3) the absolute rate of GNG from glycerol would be positively correlated to glycerol rate of disappearance (Rd) during a glycerol load. Results indicate tight control of hepatic autoregulation in resting and exercising men.
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METHODS |
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Subjects.
Eight endurance-trained male subjects were recruited from the
University of California, Berkeley campus, by posted notice and
electronic mailing. Trained subjects were used to extend the period of
time that constant metabolic flux rates could be sustained during
exercise. Subjects were considered endurance trained if they had been
competing in the US Cycling Federation or collegiate mountain or road
cycling competitions for >3 yr and had a peak oxygen consumption
(O2 peak) >60
ml · kg
1 · min
1 during leg
cycling exercise. Subjects were nonsmokers, diet and weight stable with
a body fat percentage <10%, had a 1-s forced expiratory volume
70%
of vital capacity, and were injury and disease free as determined by
medical questionnaire and physical examination. The protocol was
approved by the University of California Committee for the Protection
of Human Subjects (CPHS 98-4-83), and subjects gave informed
written consent.
Screening tests.
O2 peak was determined on three
occasions by means of a progressive leg cycle ergometer protocol
(Monark Ergometric 829E), beginning at 100 W and increasing 25 or 50 W
every 3 min until voluntary cessation. Two
O2 peak tests were performed before the
isotope trials to assure a true maximum effort, and blood was collected
from an antecubital vein during the second test for determination of
lactate threshold. A third
O2 peak test
was conducted 1 wk after the last isotope test to ensure that
O2 peak was unchanged over the 6-wk
experimental period. Respiratory gases were continuously collected and
analyzed via an open-circuit, indirect calorimetry system (Ametek S-3A1 O2 and Ametek CD-3A CO2 analyzers), and
respiratory parameters were determined every minute by a real-time,
online PC-based system (5, 9). Three-day dietary records
were collected before the first and fourth isotope trial to assess
dietary habits and monitor individual caloric intake and macronutrient
composition during the 6-wk testing period. Analysis of dietary records
was performed using the Nutritionist III program (N-Squared Computing, Salem, OR). Body composition was determined by skinfold measurements, as previously reported (9).
Experimental design.
After screening, five stable-isotope infusion trials were performed on
each subject (see Tracer protocol). During the 24 h preceding each isotope trial, subjects refrained from exercise and
consumed a standardized diet (3,240 kcal: 66% carbohydrate, 19% fat,
and 14% protein) prepared by laboratory staff. The dietary protocol
included a final snack (566 kcal: 52% carbohydrate, 33% fat, and 15%
protein), consumed 12 h before the onset of exercise. Subjects
reported to the laboratory at 0700 on the morning of the isotope trial,
7.5 h after their last meal. After collection of background
samples, tracer infusion began, and subjects rested for 3.75 h,
followed by 90 min of leg ergometer cycling at either 45 (easy) or 65%
(moderate intensity exercise) O2 peak. Trials were performed in a randomized order, with no fewer than 5 days
between. Subjects were instructed to maintain their initial dietary and
training regimens throughout the 6-wk testing period.
Tracer protocol. All trials were conducted at the same time of day. On the morning of the isotope trials, a catheter was inserted into a dorsal hand vein that was subsequently warmed for collection of arterialized blood. A second catheter was placed into the antecubital vein of the contralateral arm for continuous infusion of the isotope and glycerol solutions. After collection of background blood and breath samples, [6,6-2H2]glucose, [1-13C]glucose, and [1,1,2,3,3-2H5]glycerol were continuously infused (Baxter Travenol 6200 infusion pump) without (CON) or with (GLY) the addition of 13C-labeled or unlabeled glycerol (J. T. Baker, Phillipsburg, NJ). Glucose and [1,1,2,3,3-2H5]glycerol isotopes were infused at 4.0 and 0.4 mg/min, respectively, at rest. To maintain isotopic enrichments of blood metabolites, tracer infusion rates were doubled during exercise. The tracer infusion rates employed have been previously demonstrated by our laboratory (9, 10) to maintain stable plasma isotopic enrichments for the measurement of substrate kinetics throughout rest and the two exercise intensities studied. In GLY trials, [2-13C]- or unlabeled glycerol was infused at the rate of 20 mg/min during rest and was increased to 40 mg/min during exercise to replicate conditions required for MIDA. Isotopes (Cambridge Isotope Laboratories, Woburn, MA) and unlabeled glycerol were diluted in 0.9% sterile saline, pyrogenicity and sterility tested (University of California, San Francisco, School of Pharmacy), and, on the day of the experiment, passed through a 0.2-µm Millipore filter (Nalgen, Rochester, NY) before infusion.
Sampling and analyses. Blood was sampled at minutes 0, 180, 195, 210, and 225 of the 3.75-h rest period and at minutes 30, 45, 60, 75 and 90 of exercise. Samples were immediately chilled on ice and centrifuged at 3,000 g for 18 min, and the supernatant was collected and frozen until analysis. Blood samples for determination of glucose and glycerol isotope enrichments and glucose and lactate concentrations were collected in 8% perchloric acid. Samples for free fatty acids (FFA) and glycerol concentrations were transferred to vacutainers containing EDTA, thoroughly mixed, and chilled on ice before centrifugation. Glucose and lactate concentrations were measured enzymatically in duplicate or triplicate using hexokinase (Sigma Chemical, St. Louis, MO) and lactate dehydrogenase (11), respectively. Plasma FFA and glycerol concentrations were determined using commercially available kits (NEFA-C, WAKO, Richmond, VA, and GPO-Trinder, Sigma). Hematocrit was measured at each sampling point, and subjects were instructed to drink tap water to ensure that metabolite and hormone concentrations were not affected by changes in plasma volume.
Isotopic enrichment analyses. Glucose and glycerol isotopic enrichments were measured using gas chromatography-mass spectrometry (GC-MS; GC model 5890, series II, and MS model 5989A, Hewlett-Packard, Palo Alto, CA) analyses of the glucose pentaacetate and glycerol triacetate derivatives. Before GC-MS, analysis samples were neutralized with 2 N KOH, transferred to cation (AG 50W-X8, 50- to 100-mesh H+ resin) and anion (AG 1-X8, 100- to 200-mesh formate resin) exchange columns, and eluted with deionized water. Samples were then lyophilized, resuspended in methanol, and transferred to a 2-ml microreaction vial. One hundred microliters of an acetic anhydride-pyridine solution (2:1) were added to each vial and heated for 10 min at 60°C. Samples were subsequently dried under nitrogen, resuspended in 200 µl of ethyl acetate, and transferred to GC-MS vials for analysis. For GC-MS analyses of glucose and glycerol isotopomers, injector temperature was set at 200°C and initial oven temperatures were set at 110°C for glucose and 80°C for glycerol. Oven temperature was increased 35°C/min until final temperatures of 255 and 225°C were reached for glucose and glycerol, respectively. Helium was used as the carrier gas for all analyses with a 35-to-1 ml/min splitless injection ratio; transfer line temperature was set at 250°C, source temperature at 200°C, and quadruple temperature at 116°C, and positive chemical ionization was performed.
At each of the 10 sampling times, duplicate 10-ml aliquots of expired air were collected in 10-ml evacuated containers for the determination of 13CO2 isotopic enrichment. Breath samples were analyzed in duplicate by use of isotope ratio mass spectrometry (IRMS) by Metabolic Solutions (Acton, Nashua, NH).Calculations.
Glucose and glycerol rates of appearance (Ra),
Rd, metabolic clearance rate (MCR), glucose Rr, and the
fractional incorporation of carbon label into glucose from labeled
3-carbon precursor (GNG) were calculated using equations defined by
Steele and modified for use with stable isotopes, as previously
described (9). Metabolite kinetics were calculated as
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(1) |
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(2) |
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(3) |
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(4) |
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(5) |
Statistical analyses. Data are presented as means ± SE. Representative values for metabolite concentration and substrate kinetics were obtained by averaging values from the final 30 min of rest and exercise. When no significant differences were observed between CON and GLY protocols, data were pooled. Significance of mean differences among infusion protocols and exercise intensities was determined with two-factorial ANOVA with repeated measures. Significance differences among groups and changes over time were determined using repeated-measures factorial ANOVA and post hoc analysis. Post hoc comparisons were made with Fisher's protected least significant difference test. Pearson product-moment correlations were conducted as indicated. Statistical significance was set at P = 0.05.
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RESULTS |
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Subject characteristics, dietary records, and physiological
responses to exercise.
Anthropometric data on subjects are reported in Table
1. During the course of the 6-wk study,
body composition and
O2 peak were unchanged
(Table 1). As determined from 72-h dietary records, macronutrient and
energy contents of individual diets were consistent during the
experimental period (week 1: 61.1 ± 2.7%
carbohydrate, 24.2 ± 2.3% fat, 15.1 ± 0.7% protein,
3,349.9 ± 282.14 kcal vs. week 6: 57.8 ± 2.4%
carbohydrate, 25.4 ± 3.3% fat, 16.8 ± 1.4% protein,
3,126.0 ± 182.81 kcal). Infusion of glycerol did not affect
O2, heart rate, or RER during rest or
exercise (Table 2). In the transition
from rest to exercise,
O2 and heart rate increased in an intensity-dependent manner (Table 2). RER was also
increased at 65%
O2 peak compared with
rest. The cycle ergometer workloads required for the specific exercise
intensities are also reported in Table 2.
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Metabolite concentrations.
Plasma glycerol concentrations were significantly elevated at
rest and during both exercise intensities in GLY compared with CON
trials (Fig. 1A). In all four
trials, glycerol concentration increased in the transition from rest to
exercise in an intensity-dependent manner. Resting plasma FFA
concentrations were not different between GLY and CON trials (0.28 ± 0.04 vs. 0.27 ± 0.03 mM, GLY vs. CON). Similarly, FFA
concentrations increased (P < 0.05) in the transition from rest to exercise in CON and GLY trials (45%
O2 peak: 0.39 ± 0.05 vs.
0.41 ± 0.08; 65%
O2 peak:
0.37 ± 0.05 vs. 0.43 ± 0.06, GLY vs. CON). Furthermore,
infusion of glycerol had no effect on glucose concentrations during
rest or exercise (Fig. 1B). However, during exercise at 65%
O2 peak, glucose concentration decreased over time in 12-h-fasted men despite the infusion of glycerol. During exercise at 65%
O2 peak, lactate concentrations were
increased compared with both rest and 45%
O2 peak. Blood lactate concentrations
were unaffected by glycerol infusion during rest or exercise (Fig.
1C).
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Isotopic enrichments.
Infusion of unlabeled glycerol significantly reduced
[1,1,2,3,3-2H5]glycerol enrichments in GLY
compared with CON (Fig. 2A).
However, stable isotopic enrichments were obtained during the final 30 min of rest and exercise under both protocols. Similarly,
[6,6-2H2]glucose, and
[1-13C]glucose enrichments, shown in Fig. 2, B
and C, respectively, were stable throughout rest and the
final 30 min of exercise in all trials. No differences in
[6,6-2H2]glucose enrichments were observed
between protocols (Fig. 2B). However,
[1-13C]glucose enrichments were lower during
45% O2 peak GLY compared with CON
trials (Fig. 2C). In addition,
[6,6-2H2]- and [1-13C]glucose
enrichments demonstrated an intensity effect during exercise (Fig. 2,
B and C).
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Glycerol kinetics.
Tracer-measured glycerol Ra was significantly increased at
rest and at both exercise intensities in GLY compared with CON trials
(Fig. 3A). However,
differences in glycerol Ra between protocols were abolished
when the infusion rate of unlabeled glycerol was subtracted from total
glycerol Ra (Fig. 3A). Compared with rest,
glycerol Ra increased during exercise in an
intensity-dependent manner under both GLY and CON conditions. Glycerol
Rd was increased in GLY compared with CON at rest and
during both exercise intensities (Fig. 3B). The absolute
difference in glycerol Rd (Rd) between protocols during the final 30 min of rest and exercise was equal to the
minute infusion rate (If) of unlabeled glycerol in the GLY
trials (Fig. 3C).
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Glucose kinetics.
No effect of glycerol infusion was observed on
[6,6-2H2]glucose-measured Ra and
Rd at rest and during exercise (Fig.
4, A and B).
Compared with rest, glucose Ra increased two- and threefold (P < 0.05) during exercise at 45 and 65%
O2 peak, respectively (Fig.
4A). Mean glucose Ra and Rd were
similar in resting men (Fig. 4B). Glucose Ra and
Rd both demonstrated a significant intensity effect during
exercise. The exercise intensity effect was 17% greater on glucose
Rd compared with Ra, resulting in lower blood glucose concentrations during exercise at 65%
O2 peak. Because no effect of glycerol
infusion was observed on glucose concentration and Rd,
glucose metabolic clearance rate (MCR) was not different between
protocols and was scaled to exercise intensity (Fig.
4C).
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GLCox and RER. GLCox, determined from expired 13CO2 enrichment, was not affected by glycerol infusion at rest or exercise (Table 2). During exercise, GLCox increased in a time- and intensity-dependent manner. In addition, the contribution of glucose oxidation to the TEE increased from rest to exercise in an intensity-dependent manner. Because glucose Ra, Rd, and oxidation were highly correlated in exercising men, the increase in GLCox during exercise reflected changes in glucose Ra and Rd. The oxidation of other carbohydrate-derived fuels (glycogen and lactate), determined from indirect calorimetry, increased with exercise intensity and was unaffected by the infusion of glycerol.
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DISCUSSION |
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The present investigation is the first to examine the
effects of an intravenous glycerol load sufficient to perform MIDA on glucose kinetics in men during sustained exercise. Our results indicate
that glucose turnover (Ra and Rd),
GLCox, and MCR were not affected by glycerol infusion in
resting men. Moreover, glucose Ra and Rd
increased in an intensity-dependent manner during light and moderate
exercise independently of glycerol infusion. Glucose production was
maintained during rest and both exercise intensities despite a twofold
increase in glycerol concentration and removal. During infusion of
glycerol, GNG from glycerol was correlated to glycerol Rd
and was equal to glucose Rr during CON trials. However, provision of
exogenous glycerol did not prevent a fall in blood glucose
concentration during prolonged moderate (65% O2 peak) exercise. Our data extend the
scope of studies on hepatic autoregulatory control (33) to
include the regulation of glucose production in the presence of
increased gluconeogenic substrate supply during exercise.
Glycerol kinetics.
Tracer-measured glycerol Ras were higher under GLY than
under CON conditions, but when corrected for exogenous glycerol
infusion, no differences in endogenous glycerol Ras were
observed between protocols (Fig. 3A). The glycerol
Ra results are interpreted to mean that whole body
lipolysis was not affected by glycerol infusion. The lack of effect of
increased glycerol concentrations on lipolysis is consistent with our
observation that plasma insulin concentrations were not affected by
exogenous glycerol infusion (J. K. Trimmer, J. M. Schwarz,
G. A. Casazza, M. A. Horning, N. Rodriguez, and G. A. Brooks, unpublished observations). Glycerol Ra was
greater during both exercise intensities than it was during rest. In
addition, glycerol Ra was significantly increased during
the final 15 min of exercise at 65 compared with 45%
O2 peak. In this respect, our results
are consistent with those of others showing an exercise intensity
effect on glycerol Ra (10, 29). Glycerol Ra also increased over time during exercise at 65%
O2 peak, a result that is in agreement
with results of previous studies on men engaged in moderate- to
hard-intensity exercise (10, 29). The glycerol
Ra values that we determined tended to be higher than
values previously reported from our laboratory (10). The
distinction likely reflects the difference in dietary controls and
duration of exercise. Previously our subjects had been studied 4-6
h postprandially. However, in the present investigation, subjects were
studied after an overnight fast, with exercise commencing 12 h
after the last meal. Baba et al. (3) have also reported that glycerol Ra increases with time since the last meal.
Glucose kinetics. Glucose Ra was unaffected by the infusion of glycerol and increased in an intensity-dependent manner during exercise (Fig. 4A). As well, we observed a secondary rise in glucose Ra (Fig. 4A) after 60 min of exercise after a 12-h fast. Glucose Rd was not affected by glycerol infusion during rest or exercise. Glucose turnover rates observed under CON and GLY conditions were similar to values previously reported for trained men by our laboratory (4, 9) and others (6, 24). Similarly, others have observed a lack of effect of increased gluconeogenic substrate availability on glucose turnover in resting postabsorptive dogs (36) and humans (2, 18, 20, 38). The substrates studied include lactate (2, 20), alanine (18, 38), and glycerol (18, 36). We now report that increasing gluconeogenic precursor availability by glycerol infusion does not increase HGP in exercising men.
Total oxidation rates of glucose and other carbohydrates determined by indirect calorimetry were not different between infusion protocols, despite elevated glycerol concentrations in GLY trials. In this regard, our results are in agreement with the findings of Miller et al. (25), who reported that RER was unchanged after an oral glycerol load that elicited a 100-fold increase in plasma glycerol concentrations. Moreover, because absolute and relative glucose oxidation rates were unaffected by glycerol infusion, we conclude that elevation of blood glycerol does not spare blood glucose or redirect glucose disposal during rest or exercise.Glucose recycling and GNG from glycerol.
Glucose recycling rates observed during the control trials indicate
that flux through phosphoenolpyruvate carboxykinase
(PEPCK) accounted for ~18% of total glucose appearance during rest.
Although we recognize that glucose Rr underestimates total GNG, the
recycling rates that we observed are similar to values previously
reported by our laboratory for resting men (9). As well,
the recycling rates that we observed in CON trials are similar to
gluconeogenic rates estimated from the secondary labeling of glucose
carbon after [13C]lactate infusions (5, 17).
Assuming the conversion of glycerol to glucose accounts for ~6% of
glucose Ra in overnight-fasted men (3), we
estimate that GNG accounted for 24% of the net glucose production at
rest in CON experiments, a value similar to results obtained using MIDA
(Refs. 13 and 32 and J. K. Trimmer,
J. M. Schwarz, G. A. Casazza, M. A. Horning, N. Rodriguez, and G. A. Brooks, unpublished observations) and
[13C]bicarbonate (Ref. 7 and J. K. Trimmer, G. A. Casazza, M. A. Horning, and G. A. Brooks,
unpublished observations). In CON trials, glucose recycling accounted
for 16 and 12% of glucose Ra during exercise at 45 and
65% O2 peak, respectively. However,
the absolute glucose recycling rates were increased during both
exercise intensities compared with rest. Thus the glucose recycling
results are interpreted to mean that the absolute, but not relative,
gluconeogenic carbon flux through pyruvate increased during exercise in
CON trials.
Hepatic autoregulation. Although our results are consistent with the idea that high concentrations of a gluconeogenic precursor such as glycerol can influence its relative contribution to GNG, we do not believe that a gluconeogenic precursor load can increase the absolute gluconeogenic rate; i.e., increases in the contribution of any precursor are likely compensated for by decreased contributions of other precursors. This view influences the way in which we interpret the previous contributions of others. In the present investigation, infusion of glycerol elicited a 66% reduction in glucose recycling at rest and a nearly complete cessation of glucose recycling during light and moderate exercise. These results are consistent with those of Wapnir and Stiel (35), who reported that high levels of dihydroxyacetone phosphate reduced PEPCK activity in rat liver preparations. An abundance of glycerol could make it the predominant gluconeogenic precursor by directly influencing the TPP and by indirectly inhibiting PEPCK by causing a backup of gluconeogenic intermediates.
With regard to the effects of exogenous glycerol on GNG and glycerol disposal sites, the decline in glucose Rr, and the positive correlation between glycerol ![]() |
ACKNOWLEDGEMENTS |
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We thank the subjects for their participation in and compliance with the dietary and exercise protocols.
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FOOTNOTES |
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This work was supported by National Institutes of Health Grants AR-42906 and DK-19577.
Address for reprint requests and other correspondence: G. A. Brooks, Exercise Physiology Laboratory, Dept. of Integrative Biology, 5101 Valley Life Sciences Bldg., Univ. of California, Berkeley, Berkeley, CA 94720-3140 (E-mail: gbrooks{at}socrates.berkeley.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 13 March 2000; accepted in final form 15 January 2001.
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