High-altitude acclimation increases the triacylglycerol/fatty
acid cycle at rest and during exercise
Grant B.
McClelland1,3,
Peter W.
Hochachka1,
Shannon P.
Reidy2, and
Jean-Michel
Weber2
1 Department of Zoology, University of British Columbia,
Vancouver, British Columbia V6T 1Z4, 2 Department of Biology,
University of Ottawa, Ottawa, Ontario, Canada K1N 6N5; and
3 Department of Integrative Biology, University of California,
Berkeley, Berkeley, California 94720
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ABSTRACT |
High-altitude acclimation
alters lipid metabolism during exercise, but it is unknown whether this
involves changes in rates of lipolysis or reesterification, which form
the triacylglycerol/fatty acid (TAG/FA) cycle. We combined indirect
calorimetry with [2-3H]glycerol and
[1-14C]palmitate infusions to simultaneously measure
total lipid oxidation, lipolysis, and rate of appearance
(Ra) of nonesterified fatty acids (NEFA) in
high-altitude-acclimated (HA) rats exercising at 60% maximal
O2 uptake (
O2 max). During
exercise, relative total lipid oxidation
(%
O2) equaled sea-level control (SL)
values; however, acclimation greatly stimulated lipolysis (+75%) but
had no effect on Ra NEFA. As a result, TAG/FA cycling
increased (+119%), due solely to an increase in recycling (+144%)
within adipocytes. There was no change in either group in these
variables with the transition from rest to exercise. We conclude that,
in HA, 1) acclimation is a potent stimulator of lipolysis;
2) rats do not modify TAG/FA cycling with the transition to
exercise; and 3) in normoxia, HA and SL derive the same
fraction of their total energy from lipids and carbohydrates.
rats; exercise intensity; lipid metabolism; lipolysis; nonesterified fatty acids; reesterification; carbohydrates; glycerol; oxidative fuel; maximal oxygen uptake
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INTRODUCTION |
THE EFFECTS OF
ENVIRONMENTAL HYPOXIA on metabolic fuel selection during exercise
have attracted much attention, because their study provides
fundamental insights into the complex relationship between oxygen
and substrate supply to living cells. We have recently shown (14,
15) that the relative contribution of lipids and carbohydrates
to energy metabolism is not affected by acclimation to hypoxia, even
though favoring carbohydrates would offer significant savings in
oxygen. Animals running at the same percentage of their aerobic
capacity (%
O2 max) oxidize the
same proportions of lipids and carbohydrates when exercising at
their respective acclimation PO2. Therefore,
high-altitude acclimation does not modify this seemingly robust
relationship between exercise intensity (defined as
%
O2 max) and the relative importance
of the different oxidative fuels (3, 21). However,
acclimation may have several other important effects on fuel selection
that remain to be addressed. For example, it is not known whether the
above conclusion still holds when both experimental groups, acclimated and nonacclimated, exercise under normoxic conditions. Furthermore, acclimation causes intriguing changes in lipid metabolism, even though
relative fat oxidation appears to remain constant. In particular, plasma fatty acid (FA) concentration can be elevated through
acclimation but not as a result of increased release into the
circulation (15), and the mechanism behind this effect is
not understood. It is conceivable that acclimation could modify the
rate of lipolysis [FA mobilization from triacylglycerol (TAG)]
or have an impact on FA reesterification, the only possible fate
other than oxidation. Lipolysis always exceeds oxidative needs;
therefore, a fraction of all the FAs released must be reesterified.
Simultaneous flux through lipolysis and reesterification form the
TAG/FA cycle, a substrate cycle known to play an important role in
adjusting FA supply during exercise (28). Substrate cycles
consist of two opposing reactions, require energy input, but result in
no net generation of product. The role of such cycles in the regulation of fuel utilization has never been investigated in altitude-acclimated individuals. At sea level, the TAG/FA cycle allows lipid metabolism to
adjust to changes in metabolic rate when the demand for FAs is rapidly
modified. The percentage of all FAs released being reesterified drops
from ~70% at rest to ~25% with the onset of moderate exercise,
thereby increasing availability for oxidation (28).
Conversely, percent reesterification increases to >90% after the end
of prolonged exercise to prevent the overshoot in plasma FA
concentration from reaching toxic levels (13, 28). It is
not clear whether acclimation will influence TAG/FA cycling or whether
this cycle will play a different role than in individuals exercising at
sea level in normoxia. Decreasing cycling would save energy, and
ultimately oxygen, but increased cycling may be the preferred response
if, for instance, high-altitude acclimation triggers a decreased demand
for circulatory FA with no change or an increase in lipolysis. Finally,
two pathways are available for TAG/FA cycling: intracellular cycling
(where the FAs remain in the cell in which they were released through
lipolysis) and extracellular cycling (where reesterification occurs
only after FA transit through the circulation). Interestingly, both
pathways can be modified independently. In humans, for instance, burn
trauma mainly affects intracellular cycling (27), whereas
cold stress preferentially targets extracellular cycling
(25). It is not clear which pathway(s) will be influenced
by hypoxia acclimation if any.
Here, we use indirect calorimetry simultaneously with continuous tracer
infusions in high-altitude-acclimated (HA) and sea-level control (SL)
rats at rest and exercising at 60%
O2 max to address the following
questions. 1) Does exercise intensity (defined as
%
O2 max) also determine the relative
importance of lipids and carbohydrates when SL and HA individuals are
both exercised under normoxic conditions? 2) Does
high-altitude acclimation have an effect on lipolytic rate in resting
or running individuals? 3) Is TAG/FA cycling affected by
acclimation, and, if so, what pathway(s) is/are implicated at rest and
during exercise?
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METHODS |
Animals.
All procedures used in this study were approved by the animal ethics
committees of the Universities of British Columbia and Ottawa.
Twenty-four female Wistar rats (Charles River, Canada, St Hyacinthe,
QC, Canada) were randomly assigned to two groups, one kept under
normoxic, sea-level conditions (SL) and the other under hypobaric
hypoxia equivalent to a high altitude of 4,300 m (HA). Each group had
free access to food (Rodent Lab Diet, PMI Nutrition International, St.
Louis, MO: protein, 28%; fat, 12%; carbohydrates (CHO), 60%), water,
and a water supplement (Transgel, Charles River, St Hyacinthe, QC,
Canada). The HA rats were acclimated to hypoxia by progressive
decrease in pressure over a 10-day period, as described previously
(14), by use of an on-line vacuum system (Animal Care and
Veterinary Services, University of Ottawa). Once the final
pressure of 450 mmHg was reached, it was not allowed to vary by more
than ±10%, and the animals were acclimated to these conditions for 8 wk before experiments.
Exercise protocols.
Aerobic capacity (
O2 max) was estimated
from previous measurements on untrained rats. Because
O2 max is not significantly different
between untrained SL and HA rats [unlike male Wistars,
(1)] exercising in normoxia (SL = 82 ± 2, HA = 80 ± 3 ml · kg
1 · min
1,
FIO2 = 0.2094; G. B. McClelland and
P. W. Hochachka, unpublished data), both groups were run at the
same average speed of 12.33 ± 0.13 m/min. Oxygen consumption
(
O2) was monitored continuously, starting at a speed of 12.5 m/min. If
O2
failed to stabilize after 30 min of exercise, the speed was decreased
by 0.5 m/min steps until a plateau was reached. Running speed was never
decreased below 11 m/min.
Glycerol and nonesterified FA kinetics.
When acclimation was completed, the animals had jugular and carotid
artery catheters surgically implanted under halothane anesthesia, as
described before (14). They were given
buprenorphine (0.01 mg/kg) before and for 48 h after
surgery. All rats were closely monitored and allowed to recover for
2-4 days after surgery. HA rats spent the first 24 h of
recovery in normoxia before returning to the hypobaric chambers until
the exercise trials 24-48 h later. On the experimental day,
extensions were added to the catheters, and the animals were placed on
a Plexiglas-enclosed treadmill (Columbus Instruments, Columbus, OH).
Both catheters were fed out of this respirometer, and the jugular
catheter was connected to a calibrated syringe pump (Harvard Apparatus,
South Natick, MA) for tracer infusion. A resting infusion of
[2-3H]glycerol (0.5-1.0 Ci/mmol) and
[1-14C]palmitate (50-60 mCi/mmol) was performed at
0.6 ml/h for 60 min in the resting state. The infusion rate was then
increased to 1.0 ml/h for 60 min of exercise and 2 min of postexercise
recovery. The infusate was prepared in sterile physiological saline
containing 1.1 µM delipidated albumin. Resting infusion rates of
[2-3H]glycerol were 1,408,593 ± 18,523 and
1,200,164 ± 177,694 dpm · kg
1 · min
1 for SL
and HA, respectively. [1-14C]palmitate infusion rates
were 1,599,987 ± 20,713 and 1,673,479 ± 58,613 dpm · kg
1 · min
1. Infusion
rates were increased for exercise and recovery (+67%) to minimize
changes in specific activity (12). Arterial blood samples
(250-300 µl each) were drawn after 50 min of resting infusion, after 15, 30, 45, and 60 min of exercise, and 2 min after the end of
exercise. The samples were centrifuged immediately, and the plasma was
stored in glass tubes at
20°C. Throughout the experiments,
simultaneous measurements of
O2 and
carbon dioxide production (
CO2) were
made using an EcoOxymax system (Columbus Instruments), as detailed
elsewhere (6). Gas exchange data were collected every
30 s for the final 15 min of the resting infusion, during
exercise, and in recovery. At the end of each experiment, the animals
were allowed to recover for
2 days (HA kept under hypobaric hypoxia),
before the gas exchange measurements were repeated but in the absence
of isotope infusion.
Sample analysis.
Plasma samples were prepared as published previously (2).
Briefly, 100-150 µl of plasma were placed in 25 ml of
chloroform-methanol (Folch 2:1 vol/vol), shaken, filtered (Whatman no.
1), and rinsed with Folch (5 ml, 2:1 vol/vol). Distilled water (7.5 ml,
or 0.25 × final volume) was added to each tube, and the layers
were separated by centrifugation (833 g for 10 min).
Chloroform (10 ml) was added to the aqueous phase, and methanol-water
(7.5 ml; 40:30 vol/vol) was added to the organic phase. Phases were
separated after centrifugation and dried on a rotating evaporator
(Büchi RE 121 Rotovapor; 65-70°C; 130-150 rpm). Ten
milliliters of 99% ethanol were added to the aqueous phase to aid
evaporation. The aqueous phase was redissolved in ethanol-water (1:1
vol/vol), and total 3H activity was counted in the
equivalent of 5 µl of plasma. To determine percent total
3H activity in glycerol, the equivalent of 15 µl of
plasma was spotted on a silica gel plate (60 F254, Merck)
for thin-layer chromatography. The plate was developed with
Folch (40:24 vol/vol), and sections (1 × 1 cm) were scraped into
separate vials before ethanol-water (3 ml; 1:1 vol/vol) and 8 ml of ACS
II scintillation fluid (Packard) were added for counting.
Glycerol concentration ([glycerol]) was determined
spectrophotometrically (26). The organic phase was
redissolved in hexane-isopropanol (3:2 vol/vol), and total
14C activity was determined. The equivalent of 100 µl of
plasma was methylated to measure the concentrations of individual
nonesterified FA ([NEFA]) by gas chromatography (2, 13).
Calculations.
Rates of lipid and CHO oxidation were calculated from
O2 and
CO2 with the equations of Frayn
(7), after correcting for nitrogen excretion by use of an
average value of 0.1042 mg/min from 24-h-fasted rats
(17). The rate of appearance (Ra) of palmitate was calculated with the steady-state equation of Steele
(23). Ra NEFA was calculated as Ra
palmitate divided by the fractional contribution of palmitate to total
NEFA. Ra glycerol was calculated using the steady-state
(rest and exercise) or the non-steady-state equations (recovery) of
Steele (23), assuming a volume of distribution of 150 ml/kg (5). Total, intra- and extracellular TAG/FA cycling were calculated as in Wolfe et al. (28). Results were
analyzed using t-test and one- or two-way analyses of
variance (ANOVA). When tests for normality failed, a Mann-Whitney
ranked-sum test or a Kruskal-Wallis ANOVA was used. Pairwise multiple
comparisons were made with the Student-Newman-Keuls or Dunnett's
methods. All percentages were arcsine square root transformed.
Bonferroni-adjusted levels of significance were used to take into
account the number of tests performed. All the values presented are
means ± SE.
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RESULTS |
Respirometry.
O2,
CO2,
and respiratory exchange ratios (RER) of SL and HA rats running in
normoxia at 60%
O2 max appear in Fig.
1. Values are averages of all exercise
trials, pooled from experiments with and without tracer infusions.
Exercise
O2,
CO2, and RER values were not
significantly different between experiments when tracer infusion was or
was not performed, for both HA and SL rats (P > 0.05).
There were no significant differences in
O2,
CO2,
or RER between the two groups of rats at any individual time point
before, during, and after exercise (P > 0.05; Fig. 1).
At the beginning of exercise, mean RER values were 0.858 ± 0.012 and 0.853 ± 0.011, whereas at the end of exercise they were 0.797 ± 0.011 and 0.821 ± 0.006 in SL and HA, respectively
(Fig. 1). Likewise, the rates of CHO oxidation were not significantly different between SL and HA (P > 0.05). CHO oxidation
went from 757 ± 61 and 728 ± 56 µM
O2 · kg
1 · min
1
at 15 min of exercise to 432 ± 87 and 531 ± 36 at the end
of exercise in SL and HA, respectively (Fig.
2A). The percent contribution of CHO to
O2 was also not different
between SL and HA (Fig. 2B; P > 0.05). At
the beginning of exercise, CHO oxidation contributed to 46 ± 3 and 45 ± 3% of
O2 in SL and HA,
respectively, and it decreased to 36 ± 2% (HA) and 28 ± 4% (SL) by the end of exercise. At the beginning of exercise, absolute
rates of lipid oxidation were not different between the two groups
(796 ± 60 vs. 704 ± 34 µM
O2 · kg
1 · min
1
for SL and HA, respectively; P > 0.05). These rates
increased during activity to reach 938 ± 27 (SL) and 827 ± 38 µM
O2 · kg
1 · min
1
(HA) by the end of exercise (Fig.
3A). Expressed as relative contributions to total metabolic rate, they were 48 ± 3 and
46 ± 3% of
O2 (SL vs. HA)
initially but increased to 63 ± 4 and 56 ± 2% (SL vs. HA)
by the end of exercise (Fig. 3B).

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Fig. 1.
Oxygen consumption
( O2; A), carbon dioxide
production ( CO2;
B), and respiratory exchange ratio [(RER) = O2/ CO2;
C] in high-altitude-acclimated (HA,
) and sea-level control (SL, ) rats
during exercise at 60% of their estimated maximal O2
uptake ( O2 max); n = 13 for SL rats, and n = 7 for HA rats. REC, recovery.
Values presented are means ± SE.
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Fig. 2.
Total carbohydrate (CHO) oxidation (µmol
O2 · kg 1 · min 1;
A) and expressed relative to total
O2 (%; B) in rats running at
60% O2 max. Symbols are the same as
in Fig. 1.
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Fig. 3.
Total lipid oxidation (µmol
O2 · kg 1 · min 1;
A) and expressed relative to total
O2 (%; B) in rats running at
60% of their estimated O2 max.
Symbols are the same as in Fig. 1.
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Glycerol and NEFA kinetics.
Table 1 shows average resting and
exercise values for NEFA and glycerol metabolism. Although resting
[NEFA] (P = 0.34) and [glycerol] (P = 0.07) were not different, during exercise, HA animals had a lower
mean NEFA concentration (P = 0.002) and a higher mean
[glycerol] (P < 0.001) than the SL controls. Mean Ra NEFA was the same in the two groups (47 ± 3 vs.
48 ± 3 µM · kg
1 · min
1,
P = 0.096), but lipolytic rate was significantly higher
in HA than in SL (Ra glycerol of 56 ± 5 vs. 32 ± 2 µM · kg
1 · min
1;
P < 0.001).
Temporal changes in glycerol concentration, specific activity, and
Ra are shown in Fig. 4.
Palmitate concentration ([palmitate]) and Ra palmitate
were not different between SL and HA (Fig.
5, A and C;
P > 0.05). On the other hand, [NEFA] was lower in HA than in SL after 45 and 60 min of exercise (Fig.
6A; P = 0.012) and during recovery (P = 0.02). Ra NEFA was
not significantly different between the two groups at any time (Fig.
6C; P > 0.05).

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Fig. 4.
Plasma concentration (mM; A), specific
activity (dpm/mM; B), and rate of appearance
(Ra) of glycerol
(µmol · kg 1 · min 1;
C) before (REST), during, and after (REC, recovery) 60 min
of exercise at 60% of estimated
O2 max under normoxia. Rats were SL
( , n = 7) or HA ( ,
n = 8). *Significantly different from SL
(P < 0.05).
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Fig. 5.
Plasma concentration (µmol/l; A), specific
activity (dpm/mM; B), and Ra palmitate
(µmol · kg 1 · min 1;
C) before (REST), during, and after (REC) 60 min of exercise
at 60% of estimated O2 max under
normoxia. Rats were SL (n = 7) or HA (n = 8). Symbols are the same as in Fig. 4.
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Fig. 6.
Plasma concentration (µmol/l; A) and
Ra of total nonesterified fatty acids (Ra NEFA;
µmol · kg 1 · min 1;
B) before (REST), during, and after (REC) 60 min of exercise
at 60% of estimated O2 max under
normoxia. Rats were SL (n = 7) or HA (n = 8). Symbols are the same as in Fig. 4. *Significantly different from
SL (P < 0.05).
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TAG/FA cycling.
In the resting state, mean total TAG/FA cycling was increased in HA but
not to a point significantly different from SL (P = 0.23), whereas the percentage of FA recycled was the same in the two
groups (67 ± 12 and 65 ± 6% for HA and SL, respectively; P > 0.05). During exercise, total cycling in HA
(138 ± 15 vs. 63 ± 6 in SL; P < 0.001) and
percent recycling (74 ± 1 vs. 67 ± 3% in SL;
P < 0.001) were both higher (Fig.
7). These differences were entirely due
to an acclimation-induced increase in intracellular TAG/FA cycling
(from 50 ± 8 to 122 ± 15 µM · kg
1 · min
1 in SL vs.
HA; P = 0.001), because extracellular cycling was not different between the two groups (P > 0.05). Intra-
and extracellular cycling over time are shown in Fig.
8. None of the parameters related to
TAG/FA cycling (i.e., absolute total cycling, %recycled, intra-, and
extracellular cycling) showed a significant change between rest and
exercise in either group (Table 1; P > 0.05).

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Fig. 7.
Rate of total fatty acids (FA) recycled
(µmol · kg 1 · min 1;
A) and percentage of FA recycled back into triacylglycerol
(TAG; B) before (REST), during, and after (REC) 60 min of
exercise at 60% of estimated O2 max
under normoxia. Rats were SL (n = 5) or HA
(n = 8). Symbols are the same as in Fig. 4.
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Fig. 8.
Rate of intracellular FA recycling
(µmol · kg 1 · min 1;
A) within adipocytes and rate of extracellular FA recycling
(B) before (REST), during, and after (REC) 60 min of
exercise at 60% of estimated O2 max
under normoxia. Rats were SL (n = 5) or HA
(n = 8). Symbols arethe same as in Fig. 4.
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DISCUSSION |
This study supports current models of fuel selection showing that
mammals exercising at the same percent
O2 max derive the same fraction of
their total energy from lipids and carbohydrates, (3, 21).
Animals whose aerobic capacity is decreased in hypoxia, even after
prolonged exposure (HA), follow the same general pattern of fuel
oxidation as SL controls (14, 15), even when HA rats are
running under normoxic conditions (Figs. 2 and 3). We also show that
acclimation to hypoxia strongly stimulates lipolysis (+75%) and the
TAG/FA substrate cycle (+119%), whereas Ra NEFA remains
unaffected. The increase in absolute (µmol FA
reesterified · kg
1 · min
1)
and relative cycling (%total FA released that are reesterified back to
TAG) is entirely due to a sharp rise in intracellular cycling (+144%),
because the extracellular component of the cycle is insensitive to
altitude acclimation. However, the TAG/FA cycle is not involved in
regulating FA availability in running rats, because its flux does not
change significantly at the beginning of exercise or in early recovery,
as it does in humans (28).
Oxidative fuel selection.
During exercise, rates of lipid oxidation were lower (although not
significantly different) after acclimation, but they were decreased in
proportion to metabolic rate. This was reflected in the relative
contributions of lipids and carbohydrates that were the same for SL and
HA (Figs. 2 and 3). Therefore, the principle that relative exercise
intensity (%
O2 max) determines the
mixture of fuels oxidized appears to be very robust, because it has now
been observed under many circumstances where aerobic capacity is
modified experimentally. The same relationship between fuel selection
and %
O2 max appears before (SL
controls) as well as after acclimation to hypoxia (HA), and this
conclusion holds true when HA animals run in normoxia (Figs. 2 and 3)
or in hypoxia (14, 15). Moreover, it applies to all
mammalian species measured to date, spanning a surprisingly wide range
of body sizes and aerobic capacities (rats, goats, dogs, and humans; see Ref. 21). So far, the only clear exceptions are for
individuals fed a very high fat diet over a long period of time who
oxidize a significantly higher percentage of lipids than controls fed a
normal diet (9, 21) and for women (8).
Changes in fuel selection with increasing work intensity may reflect
the progressive recruitment of different populations of muscle fibers.
Therefore, future work examining correlations between fiber
composition, fiber type recruitment, and fuel selection may provide
important new insights on this issue. The disproportionately high
reliance on lipids shown after feeding a high-fat diet may be explained by changes in muscle composition, fiber recruitment, or both.
Lipolysis and NEFA kinetics.
This study is the first to investigate the effects of high-altitude
acclimation on glycerol kinetics. It shows that plasma [glycerol] and
lipolytic rate (Ra glycerol) are both significantly increased by HA acclimation (Table 1; Fig. 4), whereas plasma [NEFA]
and Ra NEFA are not (Table 1; Figs. 5 and 6). Reasons for
this response are unclear, because the demand for FA by working muscles
is not higher in HA than in SL during exercise. Catecholamines, norepinephrine in particular, are well-known stimulators of lipolysis (22), and it is possible that their plasma concentration
is increased after prolonged exposure to hypoxia. No such change was
observed in humans after relatively acute exposure to hypoxia of
5
days (18), but individuals acclimated at 4,300 m for a 3-wk period had higher plasma norepinephrine levels than normoxic controls during running (20). Therefore, the higher
lipolytic rate observed here in the HA group may have been caused by
elevated plasma norepinephrine levels compared with SL. Unlike most
other mammals measured to date (26, 28), rats did not show
any significant change in lipolytic rate (Ra glycerol) or
Ra NEFA between rest, exercise, and recovery (Figs.
4-6). Interestingly, the rate of total lipid oxidation increases
in both groups with the onset of exercise, suggesting an increase in
1) extraction rates of circulating NEFA at the muscle,
2) the percentage of NEFA taken up by the muscle that was
oxidized, or 3) the contribution of muscle TAG to overall energy expenditure. This latter scenario seems the most likely, on the
basis of the low Ra NEFA and higher postexercise muscle TAG
depletions seen in trained HA rats running under hypoxic conditions (15). However, we cannot exclude the possibility that an
increase in Ra glycerol takes place in this species, for
example at lower exercise intensity or when work is prolonged for more
than 60 min.
TAG/FA cycling.
Our results show that the TAG/FA substrate cycle is very active in rats
at rest and during moderate exercise (Figs. 7 and 8). Most of the
reesterification takes place directly within the cells where the FA are
originally released by lipolysis (77-89% of total cycling is
intracellular; Table 1), similar to recent data on fasted rats at rest
[57% intracellular (10)]. The extracellular reesterification pathway is a minor component of the TAG/FA cycle (only
11-23% of total reesterification takes place after FA transit through the circulation), and absolute flux through this pathway does
not change with acclimation to hypoxia (Table 1). The most dramatic
effect of high-altitude acclimation on lipid metabolism is a 119%
increase in intracellular TAG/FA cycling (Table 1; Figs. 7 and 8), and
this response is directly linked to the stimulation of lipolysis (Fig.
4), because Ra NEFA remains unchanged (Figs. 5 and 6). This
may not be unique to rats, since a human acclimation study found that
resting [glycerol] was higher, whereas [NEFA] did not change,
suggesting an increase in recycling as well (29).
Regulation of TAG/FA cycling.
Regulation of the TAG/FA cycle is poorly understood but is probably
complex, involving many factors. Some of the potential regulators
include hormones, substrate concentrations, adipose tissue
blood flow, and the interactions among them. None of these factors has
been studied in relation to substrate cycling after altitude
acclimation. At SL, esterification of FA to TAG in adipose tissue is
stimulated by insulin and the production of glycerol 3-phosphate via
glycolysis, but insulin can also inhibit hormone-sensitive lipase.
Hyperglycemia is thought to increase reesterification by insulin action
and to a greater extent than it decreases lipolysis, so that TAG/FA
cycling increases (16). Insulin is increased during
acclimation (22), but it is not known whether [glucose] was higher in HA rats in the experiments presented here. However, [glucose] did not change during exercise before or after acclimation in trained rats (14). Interestingly, in isolated perfused
adipose tissue [lactate] enhances reesterification without affecting
glycerol release (4). Low [lactate] seen in most
acclimation studies and equivalent [lactate] in SL and HA rats
(14) point to other mechanisms as potential regulators of
TAG/FA cycling. As mentioned before, lipolysis is stimulated via a
-adrenergic mechanism normally turned on with exercise by
epinephrine or sympathetic stimulation. Catecholamines also affect
adipose tissue blood flow (11), via an
-adrenergic
mechanism, and may be limiting to the release of FA into the
circulation by decreasing the availability of albumin binding sites. As
well, high plasma [NEFA] are thought to feed back on adipose tissue
and increase the rate of TAG/FA cycling, lipolysis being unaffected.
[NEFA] is the same or lower in HA than in SL (Fig. 6), suggesting
that this is not the mechanism at work here. Leptin also plays an
important role in the control of TAG homeostasis in adipocytes and
nonadipocytes (19, 24). Very recent work shows that leptin
administration stimulates TAG/FA cycling in rabbits (S. P. Reidy
and J.-M. Weber, unpublished observations). Therefore, it is
conceivable that the activation of TAG/FA cycling observed after
acclimation to hypoxia is mediated by an increase in circulating leptin
concentration. The question of how hormones, or other factors in
general, are affected by altitude acclimation and the influence of
exercise intensity remains to be fully investigated.
Cost of TAG/FA cycling.
Substrate cycles require energy input and the cost of the TAG/FA cycle
has been estimated at 144 kcal/mol of triacylglycerol recycled. In
men, the cost is fairly low at 1.2% of total energy expenditure at rest, but this value drops during exercise to 0.5% due
to the increase in metabolism and decrease in percent FA recycled (28). Even though the cost is low, decreasing this cycle
may be beneficial at altitude to save energy under hypoxic conditions. As we have shown, acclimation increases the rate of the TAG/FA cycling
with no change in resting
O2 (Fig. 1;
Table 1). Therefore, the cost of this cycle was higher in HA at 7.4%
of total energy expenditure compared with 2.9% at SL. During exercise,
FA recycling remains higher in HA rats, but metabolic rate increases in
both groups. This results in the cost's dropping to 4.1 and 1.8% of metabolic rate in HA and SL, respectively. The differences between SL
and HA rats and the difference between rats and humans remain to be
explained. Moreover, increased recycling in humans is accompanied by an
increase in metabolic rate. This was not seen here in these rats; in
fact, there was a decrease in the average metabolic rate during
exercise despite an over twofold increase in the TAG/FA cycle. HA rats
must presumably decrease the rate of other, yet-unknown, cellular
processes to avoid hypermetabolism.
Conclusions and implications.
This is the first study to measure the TAG/FA cycle after high-altitude
acclimation and the first to measure this cycle and the rate of
lipolysis in running rats. High-altitude acclimation alone greatly
stimulates lipolysis. The FA released must be reesterified back to TAG
within the adipocyte, because Ra NEFA is unaffected by
acclimation. This high rate of cycling may be necessary to protect
against dangerously high intracellular FA concentrations. However, the TAG/FA cycle is not used by rats to increase the availability of FA at the onset of exercise when the demand for fatty acids increases. The regulation of this response is currently unknown and warrants further investigation. Finally, the general model
that relative exercise intensity
(%
O2 max) controls substrate oxidation
rates is supported by this study on HA rats running under normoxia.
Evidence for this model as a generalized condition found in all mammals
under all conditions is growing. Indeed, it may be the exceptions to
this rule that offer the most fertile paths for future research.
 |
ACKNOWLEDGEMENTS |
G. McClelland was supported by a Natural Sciences and Engineering
Research Council of Canada (NSERC) postgraduate scholarship and a
University of British Columbia Graduate Fellowship. This study was
funded through NSERC operating grants to J.-M. Weber and P. W. Hochachka.
 |
FOOTNOTES |
Address for reprint requests and other correspondence: G. McClelland, Dept. of Integrative Biology, Univ. of California,
Berkeley, 3060 Valley Life Sciences Bldg. #3140, Berkeley, CA,
94720-3140 (E-mail: grantm{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 27 November 2000; accepted in final form 9 April 2001.
 |
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