Stimulatory effects of cold exposure and cold acclimation on glucose uptake in rat peripheral tissues ANDRfi L. VALLERAND, FLEURETTE PfiRUSSE, AND LUDWIK J. BUKOWIECKI Lava1 University, Medical School, Department of Physiology, Quebec, Quebec GlK 7P4; and Defence and Civil Institute of Environmental Medicine, Toronto, Ontario M3M 3B9, Canada
VALLERAND,ANDRI% L., FLEURETTE~~~RUSSE, ANDLUDWIK J. BUKOWIECKI. Stimulator-y effects of cold exposure and cold acclimation on glucose uptake in rat peripheral tissues. Am. J. Physiol. 259 (Regulatory Integrative Comp. Physiol. 28): R1043-R1049, 1990.-The effects of cold exposure on the net rates of 2-[3H]deoxy-D-glucose uptake (K;) in rat peripheral tissues were investigated comparatively in warm- and coldacclimated animals to determine whether cold acclimation induces regulatory alterations in glucose metabolism. Acute exposure of warm-acclimated (25°C) rats to cold (48 h at 5°C) markedly increased the Ki values in red and white skeletal muscles (2-5 times), in the heart (8 times), in several white adipose tissue (WAT) depots (4-20 times), and in brown adipose tissue (BAT) (110 times). After cold acclimation (3 wk at 5”C), the Ki values further increased in the heart (15 times) and WAT (up to 29 times) but decreased in BAT (36 times). Remarkably, glucose uptake was still increased in muscles of cold-exposed/cold-acclimated animals (that do not shiver), demonstrating that enhanced glucose uptake may occur in muscles in the absence of shivering thermogenesis (or contractile activity). When cold-acclimated rats were returned to the warm for 18 h, the Ki values of all tissues, except WAT, returned to control levels. Cold exposure synergistically potentiated the stimulation of tissue glucose uptake induced by a maximal effective dose of insulin (0.5 U/kg iv) in warm- as well as in cold-acclimated animals. The data demonstrate that 1) activation of shivering and/or nonshivering thermogeneses by cold exposure results in a rapid increase of glucose uptake in peripheral tissues; 2) cold acclimation enhances the capacity of heart and WAT for cold-induced glucose uptake, but, with the exception of WAT, it does not induce long-term alterations in glucose metabolism; and 3) cold exposure, but not cold acclimation, increases insulin responsiveness (maximum velocity) in all tissues tested. The results also indicate that activation of nonshivering thermogenesis in BAT and muscles of cold-acclimated animals represents the principal phenomenon explaining the improvement by cold exposure of glucose tolerance and insulin action. brown adipose tissue; white adipose heart; 2-deoxy-D-glucose; nonshivering ing thermogenesis
tissue; skeletal thermogenesis;
muscle; shiver-
COLD EXPOSUREMARKEDLY AFFECTS glucose homeostasis in warm-acclimated rats. It stimulates glucose turnover, increases plasma glucose clearance rates, improves glucose tolerance, enhances insulin action, and reverses the diabetogenic effects of high-fat feeding (1, 5, 7, 14, 18, 23, 25-29). Cold exposure exerts these beneficial effects in spite of the fact that it decreasesplasma insulin 0363-6119/90
$1.50 Copyright
levels as well as the insulin response during a glucose tolerance test (23,29). The sparing effect of cold exposure on insulinemia is particularly evident in starved animals living in the cold where plasma insulin levels are barely detectable by radioimmunoassay. In these animals, glucose tolerance is improved, in spite of the fact that glucose infusion hardly affects plasma insulin levels (23). It is likely that cold exposure improves glucose tolerance by activating shivering and/or nonshivering thermogenesis in peripheral tissues. It is known that when a warmacclimated rat is acutely exposed to cold it produces the extra heat necessary for maintaining its body temperature constant mainly via shivering thermogenesis. If that animal is maintained in the cold for several days, it progressively stops shivering because its capacity for producing heat by nonshivering thermogenesis progressively increases (10, 16, 22). Thus rats adapt to cold by increasing their capacity for producing heat via nonshivering thermogenesis. Blood flow studies have demonstrated that the calorigenic response to norepinephrine of brown adipose tissue (BAT) and the skeletal muscles is much higher in cold-acclimated rats than in warmacclimated controls (8). On the other hand, we have previously demonstrated that when warm-acclimated rats are exposed to cold for 48 h, the net rates of glucose uptake, assessedby the 2[3H]deoxy-D-glucose (2-[3H]DG) technique, are markedly enhanced in the skeletal muscles, heart, and white and brown adipose tissues (23, 29). We have suggested that cold exposure stimulates tissue glucose uptake via at least three mechanisms: 1) by stimulating contractile activity in the skeletal muscles, 2) by increasing insulin responsiveness in skeletal muscles and white and brown adipose tissues, and 3) by stimulating glucose uptake via noninsulin-mediated pathways. However, these experiments were performed with warm-acclimated animals that were acutely exposed to cold, i.e., with animals in which shivering thermogenesis is the main heat producing mechanism, and the role of nonshivering thermogenesis in the control of glucose metabolism in cold-acclimated animals still remained to be investigated. On this basis, the effects of cold exposure and warm reexposure on glucose uptake were investigated in peripheral tissues of warm- and cold-acclimated rats. The first objective of the present studies was to determine whether cold exposure would stimulate glucose uptake in the skeletal muscles of cold-acclimated rats, i.e., in the absence of shivering. The skeletal muscles represent a
0 1990 the American
Physiological
Society
R1043
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
R1044
NONSHIVERING
THERMOGENESIS
major site of glucose disposal in rats, and it was important to determine whether muscle possesses the capacity for increasing its glucose utilization during cold exposure, activity. independently of the stimulati .on of contractile The second objective was to assess whether adaptive alterations in glucose metabolism and insulin responsiveness develop in rat peripheral tissues after prolonged cold exposure. Indeed, it is known that the calorigenic response to catecholamines of skeletal muscles and brown adipose tissue (BAT) markedly increases during cold acclimation (8, 24). To this end, the rats were first adapted to cold (5°C) or to warm (25°C) for 3 wk. At the end of the acclimation period, one -half of the warm-acclimated rats were acutely exposed to cold for 48 h and one-half of the co1.d-acclimated rats were reexposed to warm (18 h), the other animal s remaining at their respective acclimation temperatures. The following four groups of rats were formed: warm- accli mated rats exposed to warm, warm-acclimated rats exposed to cold, cold- accl imated rats exposed to cold, and cold-acclimated rats exposed to warm. Such an experimental design allows the differentiation of alterations in glucose metabolism associated with cold exposure per se from those associated with cold acclimation. The plasma disappearance rates of 2-13H]DG (K,) were determined in these four groups in the presence and absence of a maximally effective dose of insulin. The net rates of 2-[“H]DG uptake (Ki) (12, 23, 27) were also determined in different types of skeletal muscles (vastus lateralis, vastus internus, extensor digitorum long-us, and soleus), heart, and white and brown adipose tissues. METHODS
A nimals and experimental design. Male SPWU .e-Daw1eY rats of -100 g were divided into 8 groups of -10 animals each, according to a 2 x 2 x 2 factorial design. Warm-acclimated rats were housed in individual cages located in a controlled room held at 25°C with a standard 12:12 h light-dark cycle for 3 wk. Cold-acclimated rats were treated similarly, but they were housed in a cold room held at 5°C for the same period of time.. After the acclimation period, these two groups were subdivided in two other groups. One-half of the warm-acclimated rats were acutely exposed to cold (48 h at 5”C), whereas onehalf of the cold-acclimated animals were returned to the warm (25°C) for 18 h. The other animals remained at their respective acclimation temperatures. The following groups of animals were formed: 1) warm acclimated/ warm exposed, 2) warm acclimated/cold exposed, 3) cold acclimated/cold exposed, and 4) cold acclimated/warm exposed. On the moment of the 2-[“H]DG tests (see below), one-half of the animals in each of the four groups were injected with a maximally effective dose of insulin (0.5 U/kg iv; Eli Lilly, Indianapolis, IN) and the other one-half with carrier. All eight groups of animals were supplied with fresh tap water and Purina lab chow ad libitum (Ralston Purina, St. Louis, MO). The rats were housed in individual cages and were tested in their own cage to avoid stress due to handling. 2-[“H]DG tests. A few days before the 2-[“H]DG tests, the rats were cannulated via the right external jugular
AND
MUSCLE
GLUCOSE
UPTAKE
vein with a piece of polyethylene tubing (PE-50, ClayAdams, Parsipanny, NJ) as previously described (29). The animals were allowed to recover from the operation for at least 3 days. Glucose uptake in rat peripheral tissues was estimated using the 2-[“H]DG method (12), exactly as previously described (23, 27). In brief, the rats (weighing -200 g) were injected in the jugular vein with 1 ml of a saline solution containing 250 &i of 2- [“HI DG, 25 &i of [ ‘*C]sucrose (ICN Radiochemicals, Irvine, CA), and 0.25% albumin. Food (but not water) was removed from the cages 6 h before the injections. On the moment of the injections, the animals were conscious and were quietly laying in their own cage in a temperature-controlled room at 25” or 5°C. Immediately after the injection, the cannulas were flushed with 0.4 ml of warm saline, and blood samples (0.2 ml) were taken at the times indicated in Fig. 1. Withdrawn blood was replaced with an equivalent volume of heparinized saline (50 U/ml). Animals were decapitated within a few seconds after the collection of the last sample (20 min after the initial injection). The following tissues were dissected within 5 min: four skeletal muscles (vastus lateralis, vastus internus, extensor digitorum long-us, and soleus), heart, interscapular BAT, and three white adipose tissue depots (subcutaneous, gonadal, and retroperitoneal). All tissues were rapidly frozen, and the 3H-‘4C radioactivity in the plasma samples and solubilized tissue aliquots was counted in a Rackbeta LCS 1215 scintillation counter (LKB- Wallac, Finland). The extracellular volume of each tissue sample was estimated using [ ‘*C]sucrose and served for calculating the intracellular 2- [“H]DG concentration, as previously described (23, 27). Plasma glucose was determined with a Beckman glucose analyzer. Determination of the plasma KP. The plasma concentrations of 2-[“H]DG after the single injection were plotted on a semilogarithmic scale (see Fig. 2), and the elimination rate was calculated from the slope obtained by a linear regression analysis of the data using KP = slope x -2.303. Plasma half-life was obtained by tl12 = 0.693/K,. Areas under the plasma 2-[3H]DG levels were calculated by the trapezoidal rule, and the plasm.a clearante was calculated by clearance = dose/area. Determination of rates of net tissue Ki. The Ki of individual tissues was determined using the following equation K i- -
Cit
x /
KP \
Cpo X (1 - es%‘) where Cit is the intracellular 2- [3H]DG disintegrations per minute (dpm) per milligram of tissue at death, KP is the rate of plasma 2-[“H]DG disappearance described above, CpOis the extrapolated plasma 2-[“H]DG concentration at time 0, and t is the duration of the test (20 min) (23, 27). Statistics. The main effects of cold exposure, cold acclimation, and insulin treatment as well as the possible interactions between these treatments were evaluated by a threeway (2 X 2 X 2) factorial analysis of variance. One-way analysis of variance and Duncan’s post hoc test were used to evaluate significant differences between the
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
NONSHIVERING
THERMOGENESIS
m w o+) H
WARM-ACCLIMATED/WARM-EXPOSED WARM-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
>
H w w H
WARM-ACCLIMATED/WARM-EXPOSED] WARM-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
1
- INSULIN
+ INSULIN
I
I
I
5
10
15
TIME
IN
AND
1 20
MINUTES
FIG. 1. Effects of cold exposure, cold acclimation, and insulin treatment on plasma glucose levels during 2- [3H]deoxy-o-glucose (2- [3H] DG) tests. Warm-acclimated (25°C) and cold-acclimated (5°C) animals were injected at time 0 with 2-[3H]DG and [‘4C]sucrose, in conjunction with insulin or saline, as described in METHODS. The injections were performed in precannulated conscious, undisturbed animals either in warm (25°C) or in cold (5°C). Insulin injection significantly decreased plasma glucose levels that stabilized 5-10 min after injection. Total areas under glucose curves are reported in Table 1.
various groups of rats. Data were transformed in loglo when Bartlett’s test showed significant heterogeneous variance. Results are expressed as means t SE. RESULTS
Effects of cold exposure, cold acclimation, and insulin injection on plasma glucose levels and on kinetics of plasma Z-[“H]DG elimination. The integrated glucose areas under the plasma glucose curves measured during the 20 min of 2-[3H]DG tests were similar between all groups
MUSCLE
GLUCOSE
R1045
UPTAKE
with the exception of cold-adapted/cold-exposed rats, which had slightly higher glucose levels (P < 0.01) than the warm-adapted/warm-exposed controls (Fig. 1, Table 1). Insulin treatment (0.5 U/kg) significantly reduced plasma glucose levels as well as the total integrated area under the plasma glucose curve (Fig. 1, Table 1). The decrease in plasma 2-[“H]DG levels after the intravenous injection of the analogue was linear in all groups when the radioactivity values were plotted on a semilogarithmic scale (Fig. 2). This linearity facilitates the mathematical analysis of the kinetics of 2-13H]DG disappearance from plasma (12) (Table 1). In warm- and coldacclimated animals, cold exposure significantly increased the plasma KP and 2-[3H]DG clearance and decreased 2[ “H]DG tl12 (Table 1). Significantly, these values returned to normal when cold-acclimated animals were reexposed to warm for 18 h. Similarly to cold exposure, insulin treatment increased the KP and the clearance and decreased tliz (Table 1). When data from all eight groups were statistically analyzed, it was found that there was a significant interaction (P < 0.01) between the main effects of cold exposure and insulin treatment. Cold acclimation, however, did not affect per se plasma 2-[3H] DG clearance, the KP, or t1,2. Effects of cold exposure, cold acclimation, and insulin injection on Ki values. Cold exposure increased the Ki in all four skeletal muscles investigated, in the heart, in BAT, and in different white adipose tissue depots of warm- and cold-acclimated rats (Figs. 3-6). Confirming our previous results (23, 27), the effects of cold exposure were particularly evident in BAT of warm-acclimated rats where the Ki values increased from 6.1 t 1.9 to 685 t 78, i.e., by X00 times (Fig. 5). In skeletal muscles, which are quantitatively a major site of glucose uptake, cold exposure maximally increased the Ki by -4OO-500%. In BAT and in the skeletal muscles, the stimulatory effects of cold exposure were more important in warmacclimated than in cold-acclimated rats. The contrary was observed in the heart and white adipose tissue. When the cold-acclimated rats were removed from the cold room and returned to warm for 18 h (cold adapted/warm exposed group), all Ki values returned to levels observed in controls (warm adapted/warm exposed) with the exception of the three white adipose tissue depots (subcutaneous, gonadal, and retroperitoneal). As expected, in-
1. Effects of cold exposure, cold acclimation, and insulin treatment
TABLE
Warm-Warm -Insulin
10
n
K,, x lo-’ min-’ Half-life, min Clearance, ml. min-’ kg-’ Glucose area, mg/dl Total area, x 10s3 dpm+pl
4.79t0.27 14.87kO.78 22.6k1.2
Warm-Cold +Insulin
11 11.39&0.44*
-Insulin
Cold-Cold +Insulin
9 7.98+0.37$
6.33t0.22*
8.83+0.38$
62.9*5.9*
38.2*1.3$
9 11.26&0.32*
6.19t0.16* 82.2+7.0*$
-Insulin
10 7.76kO.411
Cold+Insulin
8 12.87*0.80*t
9.15kO.473: 5.54*0.35* 34.4+1.4$ 79.2&5.3* t
-Insulin
10 5.32k0.38 13.63t0.99 21.8kl.l
Warm +Insulin
10 11.46*0.34* 6.1&0.18* 56.5t2.3*
l
1,613.7+69.1 1,067.8*35,5* 25.97k1.52
9.73t0.64*
1,775.2+54.2-j- 831.3+46.9*$ 16.19+0.66$
7.93*0.74*t
1,991.2+62.6$ 956.Ok38.0" 17.89&0.83t
7.95t0.61*t
1,617.1&48.8 1,007.7+57.0* 28.12k1.66
9.63&0.72*
Plasma 2-[3H]deoxy-~-glucose rate of disappearance (K,), half-life, clearance, glucose area, and total area in warm-acclimated/warm-exposed (warm-warm), warm-acclimated/cold-exposed (warm-cold), cold-acclimated/cold-exposed (cold-cold) and cold-acclimated/warm-exposed (coldwarm) rats that were or were not treated with insulin. Data were calculated as described in RESULTS and were derived from values of Figs. 1 and 2. Experimental conditions were as described in Fig. 1. Significantly different from corresponding data without insulin: * P < 0.01. Significantly different from corresponding warm-acclimated controls: t P < 0.05, $ P -C 0.01.
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
R1046
NONSHIVERING D--Ii o---o 0-0 a-n
WARM-ACCLIMATED/WARM-EXPOSED-) WARM-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
H u +-+ H
WARM-ACCLIMATED/WARM-EXPOSED) WARM-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
THERMOGENESIS
AND
8 3 s?
SCLE
q m m m
1
UPTAKE WARM-ACCLIMATED/WARM-EXPOSED WARM-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
175 -
+ INSULIN 1
150 125 -
125
100,000 80,000 60,000
40,000
” -INSULIN
VASTUS
5 F 2 m
GLUCOSE
_ INSULIN
200,000
2 3. 8 2 B t;
MI
175
-
150
-
125
-
+INSULIN
LATERAL
-INSULIN
IS
+INSULIN
VASTUS
INTERNUS
20,000
2 2 6
125 d
10,000 0
CI
Y
I
I
I
I
5
10
15
20
TIME
IN MINUTES
2. Effects of cold exposure, cold acclimation, and insulin treatment on plasma 2-[3H]DG levels. Experimental conditions are described in legend to Fig. 1. Results are plotted on a semilogarithmic scale. A best-fit line for plasma 2-[3H]DG values was made by linear regression analysis. Regression coefficients for 8 groups were 0.99,0.98, 0.99, 0.98, 0.97, 0.92, 0.90, and 0.99 (P < 0.01 for all values) (same sequence as in legend). Complete pharmacokinetic analysis of data is reported in Table 1. Elimination of label from plasma was increased by cold exposure and insulin treatment. It returned to normal when cold-acclimated rats were returned to warm for 18 h.
m; 100 z 22 75
-
50
-
50
FIG.
sulin increased the tissue Ki values in all the tissues investigated. In addition, insulin synergistically potentiated the stimulatory effects of cold exposure in warmacclimated as well as in cold-acclimated animals. The statistical analysis of the data revealed that the interaction between the main effects of insulin treatment and cold exposure on the Ki was significant for the heart (P < 0.05) and highly significant (P c 0.01) for all other tissues. In contrast, there was no significant interaction between the effects of cold acclimation and insulin treatment in all tissues with the exception of white adipose tissue (P < 0.01 for the subcutaneous and gonadal depots and P c 0.05 for the retroperitoneal depot). DISCUSSION
The present data demonstrate that cold exposure markedly increases glucose uptake in skeletal muscles, heart, and white and brown adipose tissues not only in warm-acclimated rats but also in cold-acclimated animals (Figs. 3-6). This observation is particularly important for the skeletal muscles because it clearly shows that glucose uptake may be increased in that tissue in the absence of shivering (or contractile activity). Indeed, several studies with isolated or perfused muscles have demonstrated that contractile activity per se may in-
25
- -INSULIN
+INSULIN
EDL
0
-INSULIN
+INSULIN
SOLEUS
FIG. 3. Rates of net 2-[3H]DG uptake (K;) in skeletal muscles of warm- and cold-acclimated rats. Experimental conditions are described in legend to Fig. 1. Four muscles of different fiber type characteristics were compared. Vastus lateralis and extensor digitorum longus (EDL) are mainly composed of fast-twitch glycolytic fibers, soleus is a slowtwitch oxidative muscle, whereas vastus internus is a mixed type of muscle. Ki values were determined as described in METHODS. Significantly different from corresponding data without insulin: b P c 0.01. Significantly different from corresponding warm-acclimated controls: c P < 0.05, d P < 0.01.
crease glucose utilization in the absence of insulin (19, 20, 30, 31). Although enhanced muscle contractile activity may explain the stimulation of glucose uptake in warm-acclimated animals (25), this mechanism cannot be invoked in cold-acclimated animals because their muscles do not shiver (10, 22). The highest rates of glucose uptake in muscles of cold-acclimated rats were observed in the soleus muscle (slow-twitch oxidative muscle), where the Ki values were increased by 370% in comparison with the warm-acclimated/warm-exposed controls (Fig. 6). In the other types of muscles (fast-twitch glycolytic or mixed type), glucose uptake was increased by only 50-lOO%, but the increase was statistically significant. Several factors may contribute to increase glucose uptake in the skeletal muscles of cold-acclimated rats. First, the present data demonstrate that cold exposure synergistically potentiates the effects of insulin in warmand cold-acclimated rats (Figs. 3-6 I). However, the magnitude of this potentiation appears too small to explain,
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
NONSHIVERING
THERMOGENESIS
I]
WARM-ACCLIMATED/
FA
WARM-ACCLIMATED/COLD-EXPOSED
m
COLD-ACCLIMATED/COLD
m
COLD-ACCLIMATED/
AND
MUSCLE
GLUCOSE
0
WARM-ACCLIMATED/WARM-EXPOSED WARM-ACCLIMATED/COLD-EXPOSED 69 COLD-ACCLIMATED/COLD EXPOSED n COLD-ACCLIMATED/WARM-EXPOSED
WARM-EXPOSED
q
WAT EXPOSED
R1047
UPTAKE
240
240
240
WARM-EXPOSED 200 160
d b
HEART
d b
T
80 -
80
40 -
40
oA% -INSULIN +INSULIN SUBCUTANEOUS
FIG. 6. Rates and retroperitoneal rats. Experimental
+INSULIN
-INSULIN 4. Rates of net 2-[3H]DG rats. Experimental cold-acclimated to Fig. 1. FIG.
uptake (K;) in heart of warmand conditions are described in legend
II
WARM-ACCLIMATED/
m
WARM-ACCLIMATED/COLD-EXPOSED
WARM-EXPOSED
m
COLD-ACCLIMATED/COLD
m
COLD-ACCLIMATED/WARM-EXPOSED
EXPOSED
2500 1 2000 -
I-BAT
- 1500 T z 2 1000 -
500 -
O-INSULIN
+INSULIN
FIG. 5. Rates of net 2-[3H]DG uptake (Ki) in interscapular brown adipose tissue (IBAT) of warm- and cold-acclimated rats. Experimental conditions are described in Fig. 1.
for instance, the 370% increase of glucose uptake observed in the soleus of cold-acclimated rats, especially if one considers that cold exposure decreases plasma insulin levels by -50% (23, 27). Thus additional hypotheses must be invoked to explain the effects of cold exposure on the stimulation of glucose uptake in muscle of coldacclimated animals. One possibility might be an enhanced blood flow through the skeletal muscles that insulin and would enhance the supply of circulating glucose (6, 21). Another one would be that norepinephrine released from svmnathetic nerves directlv activates
0
of net 2-[3H]DG uptake (Ki) in subcutaneous, gonadal, white adipose tissue of warm- and cold-acclimated conditions are described in Fig. 1.
nonshivering thermogenesis in skeletal muscles and consequently stimulates glucose utilization as a fuel for thermogenesis. Until now, there existed little evidence directly supporting this bP0 thesis, but recently it has been demonstrated that cold acclim .atio n markedly increases the capacity of skeletal muscles for responding calorigenically to norepinephrine (24). Thus during cold acclimation, the primary trigger stimulating glucose uptake in the skeletal muscles would shift from an increased muscle contractile activity (shivering thermogenesis) to increased norepinephrine responsiveness (nonshivering thermogenesis). The most important effect of cold exposure on tissue Ki was seen in BAT of warm-acclimated rat where the Ki increased by X00 times (from 6 X 10m3to 0.6) (Fig. 5). In the presence of insulin, the Ki of cold-exposed animals increased above 2, a value that was -two orders of magnitude higher than that observed in other tissues (Figs. 3-6). Although the absolute value of BAT Ki decreased by -35% in cold-acclimated/cold-exposed rats compared with the warm-acclimated/cold-exposed rats, the total capacity of BAT for glucose uptake augmented during cold acclimation because that tissue undergoes a remarkable hyperplasia. Indeed, the total tissue cellularity, protein content, and cytochrome oxidase activity increases by ~600-800% during cold acclimation (3, 4). Of all the tissues investigated, the heart was the only one in which the Ki increased during cold acclimation (Fig. 4). The reason why the heart adapted to the cold stress differently from the skeletal muscles (Fig. 3) is not known. However, it might be pointed out that heart activity rapidly increases when a rat is exposed to cold, and it remains elevated as long as the animal is maintained in the cold. In contrast, shivering (or contractile activity) progressively decreases in skeletal muscles during the development of the cold-acclimated state. It is therefore possible that glucose, in the form of glycogen, represents a substrate that is more important for contractile activity than for thermogenesis. The lack of glycogen in skeletal mu .scles has alway s been associated with the induction of fatigue and a similar situation
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
R1048
NONSHIVERING
THERMOGENESIS
might prevail in the heart where contractile activity remains elevated throughout the acclimation process. There were at least three major differences between brown and white adipose tissue regarding the effects of cold exposure on glucose uptake. First, as mentioned above, the Ki values were -7-10 times higher in BAT than in white adipose tissue. Second, insulin responsiveness increased during cold acclimation in white adipose tissue (Fig. 6), but it decreased in BAT (Fig. 5). Third, this enhanced responsiveness still persisted in white adipose tissue when cold-acclimated rats returned to the warm for 18 h, whereas in BAT (as in the other tissues) there was no difference between the Ki values of coldacclimated/warm-exposed animals and warm-acclimated/warm-exposed controls. Thus, with the exception of white adipose tissue, cold acclimation does not induce permanent alterations in glucose metabolism as they rapidly disappear when cold-acclimated animals are returned to the warm for 18 h. The effects of cold exposure on glucose metabolism are similar to those induced by exercise. Exercise training also reduces the insulin response to an intravenous glucose tolerance test but generally does not improve glucose tolerance. If training stops, the insulin response to a glucose load returns to control values in a few days (11, 13, 30). Thus exercise and cold exposure both increase the apparent sensitivity of peripheral tissues to insulin, but only in a transient manner (4). The (neuro-) hormonal mechanisms by which cold exposure increases glucose utilization in vivo remain to be investigated. In vitro, however, we recently observed that very low concentrations of norepinephrine (lo-“10v7 M) stimulated glucose uptake in isolated rat brown adipocytes in the absence of insulin (17). Physiological concentrations of norepinephrine also decreased the half maximum velocity of the stimulatory effects of insulin by -50 times. Such a remarkable increase of insulin sensitivity induced by norepinephrine might explain, at least partly, the fact that cold exposure reverses the diabetogenic effects of high-fat feeding and that it potentiates the effect of insulin on in vivo glucose uptake (28, 29). In addition, norepinephrine released via activation of the sympathetic nervous system might increase per se glucose uptake via insulin-independent pathways (2,30), at least in brown adipose tissue (17). Finally, it might be pointed out that the sequence of potencies of insulin-stimulated glucose uptake observed in the present experiments (BAT > heart > red muscle > white muscle > white adipose tissue) parallels the tissue abundance of an insulin-regulatable glucose transporter (43 kDa) that was recently identified in insulinsensitive tissue (15). Whether this protein is the only glucose transporter that is recruited in cold-exposed animals still remains to be determined. Indeed, it is possible that there exist in BAT at least two pools of glucose transporters, one pool that might be mobilized by insulin (9) and another one by catecholamines (17). In conclusion, the present study demonstrates that cold exposure markedly increases glucose uptake and insulin responsiveness in skeletal muscle, heart, BAT, and white adipose tissue not only in warm-acclimated rats but also in cold-acclimated animals. It is suggested
AND
MUSCLE
GLUCOSE
UPTAKE
that, in warm-acclimated rats, cold exposure glucose uptake in skeletal muscles principally lating muscle contractile activity, and that in mated animals, augmented glucose uptake mainly triggered by norepinephrine activation shivering thermogenesis.
increases by stimucold-accliwould be of non-
The assistance of Ingrid Schmegner was highly appreciated. These studies were supported in part by grants from the Canadian Diabetes Association and the Medical Research Council of Canada. Address for reprint requests: L. J. Bukowiecki, Laboratory of Endocrinology, Dept. of Physiology, Medical School, Quebec, Quebec GlK 7P4, Canada. Received
27 December
1989; accepted
in final
form
20 June
1990.
REFERENCES 1. BUDOHOSKI, L., R. A. CHALLISS, F. J. LOZEMAN, B. MCMANUS, AND E. A. NEWSHOLME. Increased insulin sensitivity in soleus muscle from cold-exposed rats: reversal by an adenosine-receptor agonist. FEBS Lett. 175: 402-406, 1984. 2. BUKOWIECKI, L. J. Energy balance and diabetes. The effects of cold exposure, exercise training, and diet composition on glucose tolerance and glucose metabolism in rat peripheral tissues. Can. J. Physiol. Pharmacol. 67: 382-393, 1989. 3. BUKOWIECKI, L., A. J. COLLET, N. FOLLEA, G. GUAY, AND L. JAHJAH. Brown adipose tissue hyperplasia: a fundamental mechanism of adaptation to cold and hyperphagia. Am. J. Physiol. 242 (Endocrinol. Metab. 5): E359, 1982. L. J., N. FOLLI~A, A. PARADIS, AND P. J. COLLET. 4. BUKOWIECKI, Stereospecific stimulation of brown adipose tissue respiration by catecholamines via beta-adrenergic receptors. Am. J. Physiol. 238 (Endocrinol. Metab. 1): E552-E563, 1980. 5. CUNNINGHAM, J. J., M. A. GULINO, P. A. MEARA, AND H. H. BODE. Enhanced hepatic insulin sensitivity and peripheral glucose uptake in cold-acclimated rats. Endocrinology 117: 1585-1589, 1985. 6. DEFRONZO, R. A., E. FERRANNINI, Y. SATO, P. FELIG, AND J. WAHREN. Synergistic interaction between exercise and insulin on peripheral glucose uptake. J. CZin. Invest. 68: 1468-1474, 1981. 7. DEPOCAS, F., AND R. MASIRONI. Glucose as fuel for thermogenesis. Am. J. Physiol. 199: 1051-1058, 1960. 8. FOSTER, D. O., AND M. L. FRYDMAN. Nonshivering thermogenesis in the rat. II. Measurements of blood flow with microspheres point to brown adipose tissue as the dominant site of the calorigenesis induced by noradrenaline. Can. J. Physiol. PharmacoZ. 56: 110-122, 1978. 9. GRECO, P. R., J. F. ASSIMACOPOULOS, AND B. JEANRENAUD. Insulin modifies the properties of glucose transporters in rat brown adipose tissue. Biochem. J. 247: 63-68, 1987. 10. HART, J. S., 0. HBROUX, AND F. DEPOCAS. Cold acclimation and the electromyogram of unanaesthetized rats. J. Appl. Physiol. 9: 404-408,1956. J. O., S. H. CONSTABLE, AND D. A. YOUNG. Activation 11. HOLLOSZY, of glucose transport in muscle by exercise. Diabetes Metab. Reu. 1: 409-423,1986. 12. HOM, F. G., C. J. GOODNER, AND M. A. BERRIE. A 2-[“Hldeoxyglucose method for comparing rates of glucose metabolism and insulin responses among rat tissues in vivo. Validation of the model and the absence of an insulin effect on brain. Diabetes 33: 141-152, 1984. 13. HORTON, E. S. Exercise and physical training: effects on insulin sensitivity and glucose metabolism. Diabetes Metab. Reu. 2: l-17, 1986. 14. HOWLAND, R. J., AND K. D. BOND. Cold acclimation increases insulin sensitivity of brown adipocytes. Horm. Metab. Res. 19: 503504,1987. 15. JAMES, D. E., M. STRUBE, AND M. MUECKLER. Molecular cloning and characterization of an insulin-regulatable glucose transporter. Nature Lond. 338: 83-87, 1989. 16. JANSKY, L. Nonshivering thermogenesis and its thermoregulatory significance. Biol. Rev. 48: 85-132, 1973. 17. MARETTE, A., AND L. J. BUKOWIECKI. Stimulation of glucose
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
NONSHIVERING
18.
19.
20.
21.
22.
23.
24.
THERMOGENESIS
transport by insulin and norepinephrine in isolated rat brown adipocytes. Am. J. Physiol. 257 (Cell Physiol. 26): C714-C721,1989. MINAIRE, Y., J. FORICHON, G. DALLEVET, AND M. J. JOMAIN. Combined effects of cold and somatostatin on glucose kinetics in dogs. Eur. J. Appl. Physiol. Occup. Physiol. 46: 249-259, 1981. NESHER, R., I. E. KARL, AND D. M. KIPNIS. Dissociation of effects of insulin and contraction on glucose transport in rat epitrochlearis muscle. Am. J. Physiol. 249 (Cell Physiol. 18): C226-C232, 1985. PLOUG, T., H. GALBO, AND E. A. RICHTER. Increased muscle glucose uptake during contractions: no need for insulin. Am. J. Physiol. 247 (Endocrinol. Metab. 10): E726-E731, 1984. SCHULTZ, T. A., S. B. LEWIS, D. K. WEBSTIE, J. D. WALLIN, AND J. E. GEHRICH. Glucose delivery: a modulator of glucose uptake in contracting skeletal muscle. Am. J. Physiol. 233 (Endocrinol. Metab. Gastrointest. Physiol. 2): E514-E518, 1977. SELLERS, E. A., J. W. SCOTT, AND N. THOMAS. Electrical activity of skeletal muscle of normal and acclimated rat exposed to the cold. Am. J. Physiol. 177: 372-376, 1954. SHIBATA, H., F. PERUSSE, A. L. VALLERAND, AND L. J. BUKOWIECKI. Cold exposure reverses the inhibitory effects of fasting on peripheral glucose uptake in rats. Am. J. Physiol. 257 (Regulatory Integrative Comp. Physiol. 26): R96-RlOl, 1989. SHIOTA, M., AND S. MASUMI. Effect of norepinephrine on consumption of oxygen in perfused skeletal muscle from cold-exposed rats. Am. J. Physiol. 254 (Endocrinol. Metab. 17): E482-E489,1988.
AND
MUSCLE
GLUCOSE
UPTAKE
R1049
25. SMITH, 0. K. L., AND S. B. DAVIDSON. Shivering thermogenesis and glucose uptake by muscles of normal and diabetic rats. Am. J. Physiol. 242 (Regulatory Integrative Camp. Physiol. 11): R109R115,1982. 26. SMITH, S. A., P. YOUNG, AND M. A. CAWTHORNE. Quantification in vivo of the effects of insulin on glucose utilization in individual tissues of warmand cold-acclimated rats. Biochem. J. 237: 789795, 1986. 27. VALLERAND, A. L., J. LUPIEN, AND L. J. BUKOWIECKI. Interactions of cold exposure and starvation on glucose tolerance and insulin response. Am. J. Physiol. 245 (Endocrinol. Metab. 8): E575-E581, 1983. 28. VALLERAND, A. L., J. LUPIEN, AND L. J. BUKOWIECKI. Cold exposure reverses the diabetogenic effects of high-fat feeding. Diabetes 35: 329-334, 1986. 29. VALLERAND, A. L., F. P~RUSSE, AND L. J. BUKOWIECKI. Cold exposure potentiates the effect of insulin on in vivo glucose uptake. Am. J. Physiol. 253 (Endocrinol. Metab. 16): E179-E186, 1987. 30. VRANIC, M., D. WASSERMAN, AND L. J. BUKOWIECKI. Metabolic implications of exercise and physical fitness in physiology and diabetes. In: Diabetes Me&us, edited by H. Rifkin and D. Porte, Jr. New York: Elsevier, 1990, p. 198-219. 31. WALLBERG, H. H., S. H. CONSTABLE, D. A. YOUNG, AND J. 0. HOLLOSZY. Glucose transport into rat skeletal muscle: interaction between exercise and insulin. J. Appl. Physiol. 65: 909-913, 1988.
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
VALLERAND,ANDRI% L., FLEURETTE~~~RUSSE, ANDLUDWIK J. BUKOWIECKI. Stimulator-y effects of cold exposure and cold acclimation on glucose uptake in rat peripheral tissues. Am. J. Physiol. 259 (Regulatory Integrative Comp. Physiol. 28): R1043-R1049, 1990.-The effects of cold exposure on the net rates of 2-[3H]deoxy-D-glucose uptake (K;) in rat peripheral tissues were investigated comparatively in warm- and coldacclimated animals to determine whether cold acclimation induces regulatory alterations in glucose metabolism. Acute exposure of warm-acclimated (25°C) rats to cold (48 h at 5°C) markedly increased the Ki values in red and white skeletal muscles (2-5 times), in the heart (8 times), in several white adipose tissue (WAT) depots (4-20 times), and in brown adipose tissue (BAT) (110 times). After cold acclimation (3 wk at 5”C), the Ki values further increased in the heart (15 times) and WAT (up to 29 times) but decreased in BAT (36 times). Remarkably, glucose uptake was still increased in muscles of cold-exposed/cold-acclimated animals (that do not shiver), demonstrating that enhanced glucose uptake may occur in muscles in the absence of shivering thermogenesis (or contractile activity). When cold-acclimated rats were returned to the warm for 18 h, the Ki values of all tissues, except WAT, returned to control levels. Cold exposure synergistically potentiated the stimulation of tissue glucose uptake induced by a maximal effective dose of insulin (0.5 U/kg iv) in warm- as well as in cold-acclimated animals. The data demonstrate that 1) activation of shivering and/or nonshivering thermogeneses by cold exposure results in a rapid increase of glucose uptake in peripheral tissues; 2) cold acclimation enhances the capacity of heart and WAT for cold-induced glucose uptake, but, with the exception of WAT, it does not induce long-term alterations in glucose metabolism; and 3) cold exposure, but not cold acclimation, increases insulin responsiveness (maximum velocity) in all tissues tested. The results also indicate that activation of nonshivering thermogenesis in BAT and muscles of cold-acclimated animals represents the principal phenomenon explaining the improvement by cold exposure of glucose tolerance and insulin action. brown adipose tissue; white adipose heart; 2-deoxy-D-glucose; nonshivering ing thermogenesis
tissue; skeletal thermogenesis;
muscle; shiver-
COLD EXPOSUREMARKEDLY AFFECTS glucose homeostasis in warm-acclimated rats. It stimulates glucose turnover, increases plasma glucose clearance rates, improves glucose tolerance, enhances insulin action, and reverses the diabetogenic effects of high-fat feeding (1, 5, 7, 14, 18, 23, 25-29). Cold exposure exerts these beneficial effects in spite of the fact that it decreasesplasma insulin 0363-6119/90
$1.50 Copyright
levels as well as the insulin response during a glucose tolerance test (23,29). The sparing effect of cold exposure on insulinemia is particularly evident in starved animals living in the cold where plasma insulin levels are barely detectable by radioimmunoassay. In these animals, glucose tolerance is improved, in spite of the fact that glucose infusion hardly affects plasma insulin levels (23). It is likely that cold exposure improves glucose tolerance by activating shivering and/or nonshivering thermogenesis in peripheral tissues. It is known that when a warmacclimated rat is acutely exposed to cold it produces the extra heat necessary for maintaining its body temperature constant mainly via shivering thermogenesis. If that animal is maintained in the cold for several days, it progressively stops shivering because its capacity for producing heat by nonshivering thermogenesis progressively increases (10, 16, 22). Thus rats adapt to cold by increasing their capacity for producing heat via nonshivering thermogenesis. Blood flow studies have demonstrated that the calorigenic response to norepinephrine of brown adipose tissue (BAT) and the skeletal muscles is much higher in cold-acclimated rats than in warmacclimated controls (8). On the other hand, we have previously demonstrated that when warm-acclimated rats are exposed to cold for 48 h, the net rates of glucose uptake, assessedby the 2[3H]deoxy-D-glucose (2-[3H]DG) technique, are markedly enhanced in the skeletal muscles, heart, and white and brown adipose tissues (23, 29). We have suggested that cold exposure stimulates tissue glucose uptake via at least three mechanisms: 1) by stimulating contractile activity in the skeletal muscles, 2) by increasing insulin responsiveness in skeletal muscles and white and brown adipose tissues, and 3) by stimulating glucose uptake via noninsulin-mediated pathways. However, these experiments were performed with warm-acclimated animals that were acutely exposed to cold, i.e., with animals in which shivering thermogenesis is the main heat producing mechanism, and the role of nonshivering thermogenesis in the control of glucose metabolism in cold-acclimated animals still remained to be investigated. On this basis, the effects of cold exposure and warm reexposure on glucose uptake were investigated in peripheral tissues of warm- and cold-acclimated rats. The first objective of the present studies was to determine whether cold exposure would stimulate glucose uptake in the skeletal muscles of cold-acclimated rats, i.e., in the absence of shivering. The skeletal muscles represent a
0 1990 the American
Physiological
Society
R1043
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
R1044
NONSHIVERING
THERMOGENESIS
major site of glucose disposal in rats, and it was important to determine whether muscle possesses the capacity for increasing its glucose utilization during cold exposure, activity. independently of the stimulati .on of contractile The second objective was to assess whether adaptive alterations in glucose metabolism and insulin responsiveness develop in rat peripheral tissues after prolonged cold exposure. Indeed, it is known that the calorigenic response to catecholamines of skeletal muscles and brown adipose tissue (BAT) markedly increases during cold acclimation (8, 24). To this end, the rats were first adapted to cold (5°C) or to warm (25°C) for 3 wk. At the end of the acclimation period, one -half of the warm-acclimated rats were acutely exposed to cold for 48 h and one-half of the co1.d-acclimated rats were reexposed to warm (18 h), the other animal s remaining at their respective acclimation temperatures. The following four groups of rats were formed: warm- accli mated rats exposed to warm, warm-acclimated rats exposed to cold, cold- accl imated rats exposed to cold, and cold-acclimated rats exposed to warm. Such an experimental design allows the differentiation of alterations in glucose metabolism associated with cold exposure per se from those associated with cold acclimation. The plasma disappearance rates of 2-13H]DG (K,) were determined in these four groups in the presence and absence of a maximally effective dose of insulin. The net rates of 2-[“H]DG uptake (Ki) (12, 23, 27) were also determined in different types of skeletal muscles (vastus lateralis, vastus internus, extensor digitorum long-us, and soleus), heart, and white and brown adipose tissues. METHODS
A nimals and experimental design. Male SPWU .e-Daw1eY rats of -100 g were divided into 8 groups of -10 animals each, according to a 2 x 2 x 2 factorial design. Warm-acclimated rats were housed in individual cages located in a controlled room held at 25°C with a standard 12:12 h light-dark cycle for 3 wk. Cold-acclimated rats were treated similarly, but they were housed in a cold room held at 5°C for the same period of time.. After the acclimation period, these two groups were subdivided in two other groups. One-half of the warm-acclimated rats were acutely exposed to cold (48 h at 5”C), whereas onehalf of the cold-acclimated animals were returned to the warm (25°C) for 18 h. The other animals remained at their respective acclimation temperatures. The following groups of animals were formed: 1) warm acclimated/ warm exposed, 2) warm acclimated/cold exposed, 3) cold acclimated/cold exposed, and 4) cold acclimated/warm exposed. On the moment of the 2-[“H]DG tests (see below), one-half of the animals in each of the four groups were injected with a maximally effective dose of insulin (0.5 U/kg iv; Eli Lilly, Indianapolis, IN) and the other one-half with carrier. All eight groups of animals were supplied with fresh tap water and Purina lab chow ad libitum (Ralston Purina, St. Louis, MO). The rats were housed in individual cages and were tested in their own cage to avoid stress due to handling. 2-[“H]DG tests. A few days before the 2-[“H]DG tests, the rats were cannulated via the right external jugular
AND
MUSCLE
GLUCOSE
UPTAKE
vein with a piece of polyethylene tubing (PE-50, ClayAdams, Parsipanny, NJ) as previously described (29). The animals were allowed to recover from the operation for at least 3 days. Glucose uptake in rat peripheral tissues was estimated using the 2-[“H]DG method (12), exactly as previously described (23, 27). In brief, the rats (weighing -200 g) were injected in the jugular vein with 1 ml of a saline solution containing 250 &i of 2- [“HI DG, 25 &i of [ ‘*C]sucrose (ICN Radiochemicals, Irvine, CA), and 0.25% albumin. Food (but not water) was removed from the cages 6 h before the injections. On the moment of the injections, the animals were conscious and were quietly laying in their own cage in a temperature-controlled room at 25” or 5°C. Immediately after the injection, the cannulas were flushed with 0.4 ml of warm saline, and blood samples (0.2 ml) were taken at the times indicated in Fig. 1. Withdrawn blood was replaced with an equivalent volume of heparinized saline (50 U/ml). Animals were decapitated within a few seconds after the collection of the last sample (20 min after the initial injection). The following tissues were dissected within 5 min: four skeletal muscles (vastus lateralis, vastus internus, extensor digitorum long-us, and soleus), heart, interscapular BAT, and three white adipose tissue depots (subcutaneous, gonadal, and retroperitoneal). All tissues were rapidly frozen, and the 3H-‘4C radioactivity in the plasma samples and solubilized tissue aliquots was counted in a Rackbeta LCS 1215 scintillation counter (LKB- Wallac, Finland). The extracellular volume of each tissue sample was estimated using [ ‘*C]sucrose and served for calculating the intracellular 2- [“H]DG concentration, as previously described (23, 27). Plasma glucose was determined with a Beckman glucose analyzer. Determination of the plasma KP. The plasma concentrations of 2-[“H]DG after the single injection were plotted on a semilogarithmic scale (see Fig. 2), and the elimination rate was calculated from the slope obtained by a linear regression analysis of the data using KP = slope x -2.303. Plasma half-life was obtained by tl12 = 0.693/K,. Areas under the plasma 2-[3H]DG levels were calculated by the trapezoidal rule, and the plasm.a clearante was calculated by clearance = dose/area. Determination of rates of net tissue Ki. The Ki of individual tissues was determined using the following equation K i- -
Cit
x /
KP \
Cpo X (1 - es%‘) where Cit is the intracellular 2- [3H]DG disintegrations per minute (dpm) per milligram of tissue at death, KP is the rate of plasma 2-[“H]DG disappearance described above, CpOis the extrapolated plasma 2-[“H]DG concentration at time 0, and t is the duration of the test (20 min) (23, 27). Statistics. The main effects of cold exposure, cold acclimation, and insulin treatment as well as the possible interactions between these treatments were evaluated by a threeway (2 X 2 X 2) factorial analysis of variance. One-way analysis of variance and Duncan’s post hoc test were used to evaluate significant differences between the
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
NONSHIVERING
THERMOGENESIS
m w o+) H
WARM-ACCLIMATED/WARM-EXPOSED WARM-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
>
H w w H
WARM-ACCLIMATED/WARM-EXPOSED] WARM-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
1
- INSULIN
+ INSULIN
I
I
I
5
10
15
TIME
IN
AND
1 20
MINUTES
FIG. 1. Effects of cold exposure, cold acclimation, and insulin treatment on plasma glucose levels during 2- [3H]deoxy-o-glucose (2- [3H] DG) tests. Warm-acclimated (25°C) and cold-acclimated (5°C) animals were injected at time 0 with 2-[3H]DG and [‘4C]sucrose, in conjunction with insulin or saline, as described in METHODS. The injections were performed in precannulated conscious, undisturbed animals either in warm (25°C) or in cold (5°C). Insulin injection significantly decreased plasma glucose levels that stabilized 5-10 min after injection. Total areas under glucose curves are reported in Table 1.
various groups of rats. Data were transformed in loglo when Bartlett’s test showed significant heterogeneous variance. Results are expressed as means t SE. RESULTS
Effects of cold exposure, cold acclimation, and insulin injection on plasma glucose levels and on kinetics of plasma Z-[“H]DG elimination. The integrated glucose areas under the plasma glucose curves measured during the 20 min of 2-[3H]DG tests were similar between all groups
MUSCLE
GLUCOSE
R1045
UPTAKE
with the exception of cold-adapted/cold-exposed rats, which had slightly higher glucose levels (P < 0.01) than the warm-adapted/warm-exposed controls (Fig. 1, Table 1). Insulin treatment (0.5 U/kg) significantly reduced plasma glucose levels as well as the total integrated area under the plasma glucose curve (Fig. 1, Table 1). The decrease in plasma 2-[“H]DG levels after the intravenous injection of the analogue was linear in all groups when the radioactivity values were plotted on a semilogarithmic scale (Fig. 2). This linearity facilitates the mathematical analysis of the kinetics of 2-13H]DG disappearance from plasma (12) (Table 1). In warm- and coldacclimated animals, cold exposure significantly increased the plasma KP and 2-[3H]DG clearance and decreased 2[ “H]DG tl12 (Table 1). Significantly, these values returned to normal when cold-acclimated animals were reexposed to warm for 18 h. Similarly to cold exposure, insulin treatment increased the KP and the clearance and decreased tliz (Table 1). When data from all eight groups were statistically analyzed, it was found that there was a significant interaction (P < 0.01) between the main effects of cold exposure and insulin treatment. Cold acclimation, however, did not affect per se plasma 2-[3H] DG clearance, the KP, or t1,2. Effects of cold exposure, cold acclimation, and insulin injection on Ki values. Cold exposure increased the Ki in all four skeletal muscles investigated, in the heart, in BAT, and in different white adipose tissue depots of warm- and cold-acclimated rats (Figs. 3-6). Confirming our previous results (23, 27), the effects of cold exposure were particularly evident in BAT of warm-acclimated rats where the Ki values increased from 6.1 t 1.9 to 685 t 78, i.e., by X00 times (Fig. 5). In skeletal muscles, which are quantitatively a major site of glucose uptake, cold exposure maximally increased the Ki by -4OO-500%. In BAT and in the skeletal muscles, the stimulatory effects of cold exposure were more important in warmacclimated than in cold-acclimated rats. The contrary was observed in the heart and white adipose tissue. When the cold-acclimated rats were removed from the cold room and returned to warm for 18 h (cold adapted/warm exposed group), all Ki values returned to levels observed in controls (warm adapted/warm exposed) with the exception of the three white adipose tissue depots (subcutaneous, gonadal, and retroperitoneal). As expected, in-
1. Effects of cold exposure, cold acclimation, and insulin treatment
TABLE
Warm-Warm -Insulin
10
n
K,, x lo-’ min-’ Half-life, min Clearance, ml. min-’ kg-’ Glucose area, mg/dl Total area, x 10s3 dpm+pl
4.79t0.27 14.87kO.78 22.6k1.2
Warm-Cold +Insulin
11 11.39&0.44*
-Insulin
Cold-Cold +Insulin
9 7.98+0.37$
6.33t0.22*
8.83+0.38$
62.9*5.9*
38.2*1.3$
9 11.26&0.32*
6.19t0.16* 82.2+7.0*$
-Insulin
10 7.76kO.411
Cold+Insulin
8 12.87*0.80*t
9.15kO.473: 5.54*0.35* 34.4+1.4$ 79.2&5.3* t
-Insulin
10 5.32k0.38 13.63t0.99 21.8kl.l
Warm +Insulin
10 11.46*0.34* 6.1&0.18* 56.5t2.3*
l
1,613.7+69.1 1,067.8*35,5* 25.97k1.52
9.73t0.64*
1,775.2+54.2-j- 831.3+46.9*$ 16.19+0.66$
7.93*0.74*t
1,991.2+62.6$ 956.Ok38.0" 17.89&0.83t
7.95t0.61*t
1,617.1&48.8 1,007.7+57.0* 28.12k1.66
9.63&0.72*
Plasma 2-[3H]deoxy-~-glucose rate of disappearance (K,), half-life, clearance, glucose area, and total area in warm-acclimated/warm-exposed (warm-warm), warm-acclimated/cold-exposed (warm-cold), cold-acclimated/cold-exposed (cold-cold) and cold-acclimated/warm-exposed (coldwarm) rats that were or were not treated with insulin. Data were calculated as described in RESULTS and were derived from values of Figs. 1 and 2. Experimental conditions were as described in Fig. 1. Significantly different from corresponding data without insulin: * P < 0.01. Significantly different from corresponding warm-acclimated controls: t P < 0.05, $ P -C 0.01.
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
R1046
NONSHIVERING D--Ii o---o 0-0 a-n
WARM-ACCLIMATED/WARM-EXPOSED-) WARM-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
H u +-+ H
WARM-ACCLIMATED/WARM-EXPOSED) WARM-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
THERMOGENESIS
AND
8 3 s?
SCLE
q m m m
1
UPTAKE WARM-ACCLIMATED/WARM-EXPOSED WARM-ACCLIMATED/COLD-EXPOSED COLD-ACCLIMATED/COLD EXPOSED COLD-ACCLIMATED/WARM-EXPOSED
175 -
+ INSULIN 1
150 125 -
125
100,000 80,000 60,000
40,000
” -INSULIN
VASTUS
5 F 2 m
GLUCOSE
_ INSULIN
200,000
2 3. 8 2 B t;
MI
175
-
150
-
125
-
+INSULIN
LATERAL
-INSULIN
IS
+INSULIN
VASTUS
INTERNUS
20,000
2 2 6
125 d
10,000 0
CI
Y
I
I
I
I
5
10
15
20
TIME
IN MINUTES
2. Effects of cold exposure, cold acclimation, and insulin treatment on plasma 2-[3H]DG levels. Experimental conditions are described in legend to Fig. 1. Results are plotted on a semilogarithmic scale. A best-fit line for plasma 2-[3H]DG values was made by linear regression analysis. Regression coefficients for 8 groups were 0.99,0.98, 0.99, 0.98, 0.97, 0.92, 0.90, and 0.99 (P < 0.01 for all values) (same sequence as in legend). Complete pharmacokinetic analysis of data is reported in Table 1. Elimination of label from plasma was increased by cold exposure and insulin treatment. It returned to normal when cold-acclimated rats were returned to warm for 18 h.
m; 100 z 22 75
-
50
-
50
FIG.
sulin increased the tissue Ki values in all the tissues investigated. In addition, insulin synergistically potentiated the stimulatory effects of cold exposure in warmacclimated as well as in cold-acclimated animals. The statistical analysis of the data revealed that the interaction between the main effects of insulin treatment and cold exposure on the Ki was significant for the heart (P < 0.05) and highly significant (P c 0.01) for all other tissues. In contrast, there was no significant interaction between the effects of cold acclimation and insulin treatment in all tissues with the exception of white adipose tissue (P < 0.01 for the subcutaneous and gonadal depots and P c 0.05 for the retroperitoneal depot). DISCUSSION
The present data demonstrate that cold exposure markedly increases glucose uptake in skeletal muscles, heart, and white and brown adipose tissues not only in warm-acclimated rats but also in cold-acclimated animals (Figs. 3-6). This observation is particularly important for the skeletal muscles because it clearly shows that glucose uptake may be increased in that tissue in the absence of shivering (or contractile activity). Indeed, several studies with isolated or perfused muscles have demonstrated that contractile activity per se may in-
25
- -INSULIN
+INSULIN
EDL
0
-INSULIN
+INSULIN
SOLEUS
FIG. 3. Rates of net 2-[3H]DG uptake (K;) in skeletal muscles of warm- and cold-acclimated rats. Experimental conditions are described in legend to Fig. 1. Four muscles of different fiber type characteristics were compared. Vastus lateralis and extensor digitorum longus (EDL) are mainly composed of fast-twitch glycolytic fibers, soleus is a slowtwitch oxidative muscle, whereas vastus internus is a mixed type of muscle. Ki values were determined as described in METHODS. Significantly different from corresponding data without insulin: b P c 0.01. Significantly different from corresponding warm-acclimated controls: c P < 0.05, d P < 0.01.
crease glucose utilization in the absence of insulin (19, 20, 30, 31). Although enhanced muscle contractile activity may explain the stimulation of glucose uptake in warm-acclimated animals (25), this mechanism cannot be invoked in cold-acclimated animals because their muscles do not shiver (10, 22). The highest rates of glucose uptake in muscles of cold-acclimated rats were observed in the soleus muscle (slow-twitch oxidative muscle), where the Ki values were increased by 370% in comparison with the warm-acclimated/warm-exposed controls (Fig. 6). In the other types of muscles (fast-twitch glycolytic or mixed type), glucose uptake was increased by only 50-lOO%, but the increase was statistically significant. Several factors may contribute to increase glucose uptake in the skeletal muscles of cold-acclimated rats. First, the present data demonstrate that cold exposure synergistically potentiates the effects of insulin in warmand cold-acclimated rats (Figs. 3-6 I). However, the magnitude of this potentiation appears too small to explain,
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
NONSHIVERING
THERMOGENESIS
I]
WARM-ACCLIMATED/
FA
WARM-ACCLIMATED/COLD-EXPOSED
m
COLD-ACCLIMATED/COLD
m
COLD-ACCLIMATED/
AND
MUSCLE
GLUCOSE
0
WARM-ACCLIMATED/WARM-EXPOSED WARM-ACCLIMATED/COLD-EXPOSED 69 COLD-ACCLIMATED/COLD EXPOSED n COLD-ACCLIMATED/WARM-EXPOSED
WARM-EXPOSED
q
WAT EXPOSED
R1047
UPTAKE
240
240
240
WARM-EXPOSED 200 160
d b
HEART
d b
T
80 -
80
40 -
40
oA% -INSULIN +INSULIN SUBCUTANEOUS
FIG. 6. Rates and retroperitoneal rats. Experimental
+INSULIN
-INSULIN 4. Rates of net 2-[3H]DG rats. Experimental cold-acclimated to Fig. 1. FIG.
uptake (K;) in heart of warmand conditions are described in legend
II
WARM-ACCLIMATED/
m
WARM-ACCLIMATED/COLD-EXPOSED
WARM-EXPOSED
m
COLD-ACCLIMATED/COLD
m
COLD-ACCLIMATED/WARM-EXPOSED
EXPOSED
2500 1 2000 -
I-BAT
- 1500 T z 2 1000 -
500 -
O-INSULIN
+INSULIN
FIG. 5. Rates of net 2-[3H]DG uptake (Ki) in interscapular brown adipose tissue (IBAT) of warm- and cold-acclimated rats. Experimental conditions are described in Fig. 1.
for instance, the 370% increase of glucose uptake observed in the soleus of cold-acclimated rats, especially if one considers that cold exposure decreases plasma insulin levels by -50% (23, 27). Thus additional hypotheses must be invoked to explain the effects of cold exposure on the stimulation of glucose uptake in muscle of coldacclimated animals. One possibility might be an enhanced blood flow through the skeletal muscles that insulin and would enhance the supply of circulating glucose (6, 21). Another one would be that norepinephrine released from svmnathetic nerves directlv activates
0
of net 2-[3H]DG uptake (Ki) in subcutaneous, gonadal, white adipose tissue of warm- and cold-acclimated conditions are described in Fig. 1.
nonshivering thermogenesis in skeletal muscles and consequently stimulates glucose utilization as a fuel for thermogenesis. Until now, there existed little evidence directly supporting this bP0 thesis, but recently it has been demonstrated that cold acclim .atio n markedly increases the capacity of skeletal muscles for responding calorigenically to norepinephrine (24). Thus during cold acclimation, the primary trigger stimulating glucose uptake in the skeletal muscles would shift from an increased muscle contractile activity (shivering thermogenesis) to increased norepinephrine responsiveness (nonshivering thermogenesis). The most important effect of cold exposure on tissue Ki was seen in BAT of warm-acclimated rat where the Ki increased by X00 times (from 6 X 10m3to 0.6) (Fig. 5). In the presence of insulin, the Ki of cold-exposed animals increased above 2, a value that was -two orders of magnitude higher than that observed in other tissues (Figs. 3-6). Although the absolute value of BAT Ki decreased by -35% in cold-acclimated/cold-exposed rats compared with the warm-acclimated/cold-exposed rats, the total capacity of BAT for glucose uptake augmented during cold acclimation because that tissue undergoes a remarkable hyperplasia. Indeed, the total tissue cellularity, protein content, and cytochrome oxidase activity increases by ~600-800% during cold acclimation (3, 4). Of all the tissues investigated, the heart was the only one in which the Ki increased during cold acclimation (Fig. 4). The reason why the heart adapted to the cold stress differently from the skeletal muscles (Fig. 3) is not known. However, it might be pointed out that heart activity rapidly increases when a rat is exposed to cold, and it remains elevated as long as the animal is maintained in the cold. In contrast, shivering (or contractile activity) progressively decreases in skeletal muscles during the development of the cold-acclimated state. It is therefore possible that glucose, in the form of glycogen, represents a substrate that is more important for contractile activity than for thermogenesis. The lack of glycogen in skeletal mu .scles has alway s been associated with the induction of fatigue and a similar situation
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
R1048
NONSHIVERING
THERMOGENESIS
might prevail in the heart where contractile activity remains elevated throughout the acclimation process. There were at least three major differences between brown and white adipose tissue regarding the effects of cold exposure on glucose uptake. First, as mentioned above, the Ki values were -7-10 times higher in BAT than in white adipose tissue. Second, insulin responsiveness increased during cold acclimation in white adipose tissue (Fig. 6), but it decreased in BAT (Fig. 5). Third, this enhanced responsiveness still persisted in white adipose tissue when cold-acclimated rats returned to the warm for 18 h, whereas in BAT (as in the other tissues) there was no difference between the Ki values of coldacclimated/warm-exposed animals and warm-acclimated/warm-exposed controls. Thus, with the exception of white adipose tissue, cold acclimation does not induce permanent alterations in glucose metabolism as they rapidly disappear when cold-acclimated animals are returned to the warm for 18 h. The effects of cold exposure on glucose metabolism are similar to those induced by exercise. Exercise training also reduces the insulin response to an intravenous glucose tolerance test but generally does not improve glucose tolerance. If training stops, the insulin response to a glucose load returns to control values in a few days (11, 13, 30). Thus exercise and cold exposure both increase the apparent sensitivity of peripheral tissues to insulin, but only in a transient manner (4). The (neuro-) hormonal mechanisms by which cold exposure increases glucose utilization in vivo remain to be investigated. In vitro, however, we recently observed that very low concentrations of norepinephrine (lo-“10v7 M) stimulated glucose uptake in isolated rat brown adipocytes in the absence of insulin (17). Physiological concentrations of norepinephrine also decreased the half maximum velocity of the stimulatory effects of insulin by -50 times. Such a remarkable increase of insulin sensitivity induced by norepinephrine might explain, at least partly, the fact that cold exposure reverses the diabetogenic effects of high-fat feeding and that it potentiates the effect of insulin on in vivo glucose uptake (28, 29). In addition, norepinephrine released via activation of the sympathetic nervous system might increase per se glucose uptake via insulin-independent pathways (2,30), at least in brown adipose tissue (17). Finally, it might be pointed out that the sequence of potencies of insulin-stimulated glucose uptake observed in the present experiments (BAT > heart > red muscle > white muscle > white adipose tissue) parallels the tissue abundance of an insulin-regulatable glucose transporter (43 kDa) that was recently identified in insulinsensitive tissue (15). Whether this protein is the only glucose transporter that is recruited in cold-exposed animals still remains to be determined. Indeed, it is possible that there exist in BAT at least two pools of glucose transporters, one pool that might be mobilized by insulin (9) and another one by catecholamines (17). In conclusion, the present study demonstrates that cold exposure markedly increases glucose uptake and insulin responsiveness in skeletal muscle, heart, BAT, and white adipose tissue not only in warm-acclimated rats but also in cold-acclimated animals. It is suggested
AND
MUSCLE
GLUCOSE
UPTAKE
that, in warm-acclimated rats, cold exposure glucose uptake in skeletal muscles principally lating muscle contractile activity, and that in mated animals, augmented glucose uptake mainly triggered by norepinephrine activation shivering thermogenesis.
increases by stimucold-accliwould be of non-
The assistance of Ingrid Schmegner was highly appreciated. These studies were supported in part by grants from the Canadian Diabetes Association and the Medical Research Council of Canada. Address for reprint requests: L. J. Bukowiecki, Laboratory of Endocrinology, Dept. of Physiology, Medical School, Quebec, Quebec GlK 7P4, Canada. Received
27 December
1989; accepted
in final
form
20 June
1990.
REFERENCES 1. BUDOHOSKI, L., R. A. CHALLISS, F. J. LOZEMAN, B. MCMANUS, AND E. A. NEWSHOLME. Increased insulin sensitivity in soleus muscle from cold-exposed rats: reversal by an adenosine-receptor agonist. FEBS Lett. 175: 402-406, 1984. 2. BUKOWIECKI, L. J. Energy balance and diabetes. The effects of cold exposure, exercise training, and diet composition on glucose tolerance and glucose metabolism in rat peripheral tissues. Can. J. Physiol. Pharmacol. 67: 382-393, 1989. 3. BUKOWIECKI, L., A. J. COLLET, N. FOLLEA, G. GUAY, AND L. JAHJAH. Brown adipose tissue hyperplasia: a fundamental mechanism of adaptation to cold and hyperphagia. Am. J. Physiol. 242 (Endocrinol. Metab. 5): E359, 1982. L. J., N. FOLLI~A, A. PARADIS, AND P. J. COLLET. 4. BUKOWIECKI, Stereospecific stimulation of brown adipose tissue respiration by catecholamines via beta-adrenergic receptors. Am. J. Physiol. 238 (Endocrinol. Metab. 1): E552-E563, 1980. 5. CUNNINGHAM, J. J., M. A. GULINO, P. A. MEARA, AND H. H. BODE. Enhanced hepatic insulin sensitivity and peripheral glucose uptake in cold-acclimated rats. Endocrinology 117: 1585-1589, 1985. 6. DEFRONZO, R. A., E. FERRANNINI, Y. SATO, P. FELIG, AND J. WAHREN. Synergistic interaction between exercise and insulin on peripheral glucose uptake. J. CZin. Invest. 68: 1468-1474, 1981. 7. DEPOCAS, F., AND R. MASIRONI. Glucose as fuel for thermogenesis. Am. J. Physiol. 199: 1051-1058, 1960. 8. FOSTER, D. O., AND M. L. FRYDMAN. Nonshivering thermogenesis in the rat. II. Measurements of blood flow with microspheres point to brown adipose tissue as the dominant site of the calorigenesis induced by noradrenaline. Can. J. Physiol. PharmacoZ. 56: 110-122, 1978. 9. GRECO, P. R., J. F. ASSIMACOPOULOS, AND B. JEANRENAUD. Insulin modifies the properties of glucose transporters in rat brown adipose tissue. Biochem. J. 247: 63-68, 1987. 10. HART, J. S., 0. HBROUX, AND F. DEPOCAS. Cold acclimation and the electromyogram of unanaesthetized rats. J. Appl. Physiol. 9: 404-408,1956. J. O., S. H. CONSTABLE, AND D. A. YOUNG. Activation 11. HOLLOSZY, of glucose transport in muscle by exercise. Diabetes Metab. Reu. 1: 409-423,1986. 12. HOM, F. G., C. J. GOODNER, AND M. A. BERRIE. A 2-[“Hldeoxyglucose method for comparing rates of glucose metabolism and insulin responses among rat tissues in vivo. Validation of the model and the absence of an insulin effect on brain. Diabetes 33: 141-152, 1984. 13. HORTON, E. S. Exercise and physical training: effects on insulin sensitivity and glucose metabolism. Diabetes Metab. Reu. 2: l-17, 1986. 14. HOWLAND, R. J., AND K. D. BOND. Cold acclimation increases insulin sensitivity of brown adipocytes. Horm. Metab. Res. 19: 503504,1987. 15. JAMES, D. E., M. STRUBE, AND M. MUECKLER. Molecular cloning and characterization of an insulin-regulatable glucose transporter. Nature Lond. 338: 83-87, 1989. 16. JANSKY, L. Nonshivering thermogenesis and its thermoregulatory significance. Biol. Rev. 48: 85-132, 1973. 17. MARETTE, A., AND L. J. BUKOWIECKI. Stimulation of glucose
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
NONSHIVERING
18.
19.
20.
21.
22.
23.
24.
THERMOGENESIS
transport by insulin and norepinephrine in isolated rat brown adipocytes. Am. J. Physiol. 257 (Cell Physiol. 26): C714-C721,1989. MINAIRE, Y., J. FORICHON, G. DALLEVET, AND M. J. JOMAIN. Combined effects of cold and somatostatin on glucose kinetics in dogs. Eur. J. Appl. Physiol. Occup. Physiol. 46: 249-259, 1981. NESHER, R., I. E. KARL, AND D. M. KIPNIS. Dissociation of effects of insulin and contraction on glucose transport in rat epitrochlearis muscle. Am. J. Physiol. 249 (Cell Physiol. 18): C226-C232, 1985. PLOUG, T., H. GALBO, AND E. A. RICHTER. Increased muscle glucose uptake during contractions: no need for insulin. Am. J. Physiol. 247 (Endocrinol. Metab. 10): E726-E731, 1984. SCHULTZ, T. A., S. B. LEWIS, D. K. WEBSTIE, J. D. WALLIN, AND J. E. GEHRICH. Glucose delivery: a modulator of glucose uptake in contracting skeletal muscle. Am. J. Physiol. 233 (Endocrinol. Metab. Gastrointest. Physiol. 2): E514-E518, 1977. SELLERS, E. A., J. W. SCOTT, AND N. THOMAS. Electrical activity of skeletal muscle of normal and acclimated rat exposed to the cold. Am. J. Physiol. 177: 372-376, 1954. SHIBATA, H., F. PERUSSE, A. L. VALLERAND, AND L. J. BUKOWIECKI. Cold exposure reverses the inhibitory effects of fasting on peripheral glucose uptake in rats. Am. J. Physiol. 257 (Regulatory Integrative Comp. Physiol. 26): R96-RlOl, 1989. SHIOTA, M., AND S. MASUMI. Effect of norepinephrine on consumption of oxygen in perfused skeletal muscle from cold-exposed rats. Am. J. Physiol. 254 (Endocrinol. Metab. 17): E482-E489,1988.
AND
MUSCLE
GLUCOSE
UPTAKE
R1049
25. SMITH, 0. K. L., AND S. B. DAVIDSON. Shivering thermogenesis and glucose uptake by muscles of normal and diabetic rats. Am. J. Physiol. 242 (Regulatory Integrative Camp. Physiol. 11): R109R115,1982. 26. SMITH, S. A., P. YOUNG, AND M. A. CAWTHORNE. Quantification in vivo of the effects of insulin on glucose utilization in individual tissues of warmand cold-acclimated rats. Biochem. J. 237: 789795, 1986. 27. VALLERAND, A. L., J. LUPIEN, AND L. J. BUKOWIECKI. Interactions of cold exposure and starvation on glucose tolerance and insulin response. Am. J. Physiol. 245 (Endocrinol. Metab. 8): E575-E581, 1983. 28. VALLERAND, A. L., J. LUPIEN, AND L. J. BUKOWIECKI. Cold exposure reverses the diabetogenic effects of high-fat feeding. Diabetes 35: 329-334, 1986. 29. VALLERAND, A. L., F. P~RUSSE, AND L. J. BUKOWIECKI. Cold exposure potentiates the effect of insulin on in vivo glucose uptake. Am. J. Physiol. 253 (Endocrinol. Metab. 16): E179-E186, 1987. 30. VRANIC, M., D. WASSERMAN, AND L. J. BUKOWIECKI. Metabolic implications of exercise and physical fitness in physiology and diabetes. In: Diabetes Me&us, edited by H. Rifkin and D. Porte, Jr. New York: Elsevier, 1990, p. 198-219. 31. WALLBERG, H. H., S. H. CONSTABLE, D. A. YOUNG, AND J. 0. HOLLOSZY. Glucose transport into rat skeletal muscle: interaction between exercise and insulin. J. Appl. Physiol. 65: 909-913, 1988.
Downloaded from www.physiology.org/journal/ajpregu by ${individualUser.givenNames} ${individualUser.surname} (129.108.009.184) on November 4, 2018. Copyright © 1990 American Physiological Society. All rights reserved.
