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Exercise Physiology Laboratory, Department of Integrative Biology, University of California, Berkeley, Berkeley, California 94720
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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