Skeletal muscle changes after endurance training at high altitude A. X. BIGARD,

A. BRUNET,

C. Y. GUEZENNEC,

AND

H. MONOD

Division de Physiologie Me’tabolique et Hormonale, Centre d’Etudes et de Recherches de Mkdecine A&ospatiale, 91228 Br&igny Cedex; and Laborutoire de Physiologie de la Motricitk’, URA 385 Centre Nationale de la Recherche Scientifique, 75634 Paris Cedex 13, France BIGARD, A. X., A. BRUNET, C. Y. GUEZENNEC, AND H. M~NOD. Skeletal muscle changes after endurance training at 1991.-The high altitude. J. Appl. Physiol. 71(6): 2114-2121,

effects of endurance training on the skeletal muscle of rats have been studied at sea level and simulated high altitude (4,000 m). Male Wistar rats were randomly assignedto one of four groups: exercise at sea level, exercise at simulated high altitude, sedentary at sealevel, and sedentary at high altitude (n = 8 in each group). Training consisted of swimming for 1 h/day in water at 36°C for 14 wk. Training and exposure to a high-altitude environment produced a decreasein body weight (P < 0.001).There wasa significant linear correlation between musclemassand body weight in the animals of all groups (r = 0.89, P < 0.001). High-altitude training enhancedthe percentageof type IIa fibers in the extensor digitorum longus muscle (EDL, P < 0.05) and deep portions of the plantaris muscle (dPLA, P < 0.01). High-altitude training alsoincreasedthe percentage of type IIab fibers in fast-twitch muscles.These muscles showedmarked metabolic adaptations: training increased the activity levels of enzymes involved in the citric acid cycle (citrate synthase, CS) and the ,&oxidation of fatty acids (3 hydroxyacyl CoA dehydrogenase,HAD). This increaseoccurred mainly at high altitude (36 and 31% for HAD in EDL and PLA muscles;24 and 31% for CS in EDL and PLA muscles).Training increasedthe activity of enzymesinvolved in glucosephosphorylation (hexokinase). High-altitude training decreased lactate dehydrogenaseactivity. Endurance training performed at high altitude and sealevel increasedthe isozyme l-to-total lactate dehydrogenaseactivity ratio to the sameextent. These findings showed that high-altitude training induces more marked adaptative changes than sea-level training. Neither high-altitude nor sea-leveltraining affected the soleusmuscle. Interpretation of these results should take into account that in terms of mechanical work the absolute work rate of exercises was constant. Consequently, relative work rates (in terms of the percentage of maximal aerobic power) for exercisesperformed at high altitude and at sealevel differed. chronic hypoxia; acclimatization to high altitude; swimming exercise; histochemistry; musclefiber composition

NATURE AND IMPORTANCE of training-induced changes vary with the intensity and the daily duration of the exercise performed (8, 10, 15). Many studies have shown that both animal and human skeletal muscle adapt to endurance training with an increase in respiratory capacity. Nevertheless, few reports suggest that endurance training produces a conversion of type IIb to IIa fibers in human (1, 18, 26, 27) or animal skeletal muscle

THE

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$1.50 Copyright

(12). All studies have shown that these adaptative transitions are obtained through high-intensity endurancetraining programs involving exercises performed at a high work rate and for a long duration; for example, Green et al. (12) showed that, after a 15wk training program, rodents were able to run for 2 h/day at a rate of 27 m/min at 15’. Exposure to a high-altitude environment decreases maximal aerobic work capacity. In addition, acclimatization (to a natural altitude) or acclimation (to a simulated altitude) induces structural and metabolic adaptations, especially in skeletal muscles. It is well known that chronic hypoxia induces a loss of muscle mass and, consequently, a significant decrease in its mean fiber crosssectional area. This is true for both human subjects (5, 16) and animals (29). These structural changes lead to an improvement in the diffusion of oxygen from capillaries to muscle fibers (4). Recent data on human subjects have shown that long-term exposure to high-altitude environments reduces the oxidative capacity of skeletal muscle (5, 16, 17). Thus, prolonged exposure to a high-altitude environment leads to an improvement in the availability of oxygen to skeletal muscle. It also decreases the oxidative capacity of skeletal muscle. Little is known about the effects of high-altitude endurance training on the muscles of animals living in chronic hypoxia environments. Terblanche et al. (32) observed that the increase in muscular respiratory capacity is greater in rats trained at moderate altitude (1,750 m) than in those trained at sea level. In contrast, Terrados et al. (33) reported an equal increased work capacity in human subjects after high-altitude and sea-level training. It is important to note that, in their study, well-trained human subjects exercised only in a hypobaric chamber. They did not live at simulated altitude when exercises were being performed. More recently, Mizuno et al. (24) studied the morphological and histochemical characteristics of the gastrocnemius and triceps brachii muscles of 10 well-trained cross-country skiers examined before and after 2 wk of high-altitude training (2,700 m). Their results suggest that short training periods at high altitude have no effect on aerobic work capacity. Furthermore such training is thought to induce an improvement in short-term exercise performance. Therefore we have no data concerning the effects of high-altitude endurance training on the muscle tissue of

0 1991 the American

Physiological

Society

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SKELETAL

MUSCLE

EFFECTS

acclimatized subjects. The purpose of this report was to determine the biochemical and histochemical changes that occur in the muscle cell after endurance training, exposure to a simulated hypobaric altitude (4,000 m), and the combination of both factors METHODS

male albino Wistar rats weighing d75assigned to one of two groups: a high-altitude group (HA, n = 16) and a sea-level group (SL ,n = 16). They were housed four per cage with a 12:12-h dark-light cycle and maintained on a standard diet with water ad libitum. A hypobaric chamber was used to simulate a high-altitude environment. During the first 2 days, pressure was maintained at 635 Torr (1,500 m). After 3 days, it was gradually reduced to 463 Torr (4,000 m). In each group (SL or HA), the animals were assigned to either a sedentary or a trained section. Thus the 32 rats were divided into four groups, each containing 8 animals. The four groups included the sea-level sedentary group (SL-S), the sea-level trained group (SL-T), the high-altitude sedentary group (HA-S), and the high-altitude trained group (HA-T). The endurance-training program consisted of a swimm.ing exercise. Endurance -trained animals swam in cylindrical tanks 65 cm high and 50 cm diam. Three tanks were used for each trained group. The water was strictly regulated at 36OC. On the 1st day, the rats swam for 20 min. The swimming period was progressively increased to 1 h/day. The swimming program was carried out for 5 days/wk for 12 consecutive weeks. The SL-T group’s exercise sessions were carried out at sea level; the HA-T group’s exercise sessions were performed in a large hypobaric chamber. Each day, before being moved to the hypobaric chamber, the rats were returned to sea-level atmospheric pressure. Then they were moved to the hypobaric chamber, where they trained at simulated high altitude. The retention period in a normal atmospheric environment did not exceed 30-45 min. During this period, food and water dispensers were refilled and animal cages were cleaned. The four groups of animals were killed 24 h after the last training session. They were anesthetized with pentobarbital sodium (50 mg/kg body wt ip). Soleus (SOL), extensor digitorum longus (EDL), and plantaris (PLA) muscles from the left hindlimb were dissected and quickly frozen in liquid nitrogen for biochemical assays. The same muscles from the right hindlimb were excised and cleaned of adipose and connective tissue. Then they were weighed and frozen in isopentane cooled to freezing point by liquid nitrogen. All samples were stored at -80°C until histochemical and biochemical analysis. One of the rats in the HA-T group drowned while swimming during the last week of training. Histochemical methods. Serial transverse sections (la20 pm) were cut in a cryostat at -2OOC. After being preincubated in alkaline and acid buffers, fibers were stained for myofibrillar adenosinetriphosphatase and classified according to the Brooke and Kaiser method (6). The thre e main fiber types- I (slow oxidative), IIa Thirty-two

180 g were randomly

OF TRAINING

AT ALTITUDE

2115

(fast oxidative glycolytic) t and IIb (fast glycolytic)-were studied. In addition, several groups of intermediate (IM) fibers were considered, depending on the staining characteristics they assumed (Fig. 1, A and B). After being preincubated at pH 4.35 (5 min) and 10.3 (9 min), IM fibers and type I and IIa fibers were identified in the SOL muscle. Type Ib IM fibers showed intermediate staining characteristics after alkaline preincubation. On the other hand, type IIc fibers showed intermediate staining characteristics after acid preincubation (26). Moreover, cross sections of PLA and EDL muscles were preincubated at pH 4.55 (5 min), so that type IIa, IIb, and IIab fibers could be identified. Fiber type IIab was intermediate to fiber types showing staining characteristics ranging between those of types IIa and IIb (Fig. 1C) (18,30,31). In these fast-twitch muscles, fibers ranging between types I and II were classified only as type IIc fibers. This classification was made in accordance with the staining characteristics they assumed at pH 4.35. The distribution of muscle fiber types was determined by using a number of fields. These fields were equally distributed over the biopsy, with a minimum of 1,000 fibers/sample. In addition, deep and superficial portions of PLA muscle were separated for histochemical studies. Biochemical methods. All samples were placed in tubes and lyophilized. Blood, fat, and connective tissue were removed from muscles in an air-conditioned room maintained at 20°C with 25% relative humidity. The samples were weighed in the room and homogenized at a dilution level of 1:400. Homogenization was carried out at the local temperature (between 0 and -4°C) in a 0.3 M phosphate buffer (pH 7.7) containing 0.05% bovine serum albumin. NAD/NADH enzymatic fluorometric assays, performed at 25*C with a Gilson spectra/glofluorometer, were used to determine the enzyme activities of this homogenate. The assays were conducted according to the method described by Lowry and Passonneau (22). They were expressed in micromoles of substrate utilized per minute per gram of muscle protein, Muscle protein was determined according to the method described by Lowry et al. (23): bovine serum albumin served as the standard, and a Beckman spectrophotometer was used. The activities of several enzymes were assayed: 3 hydroxyacyl CoA dehydrogenase (HAD, EC 1.1.1.35) was used to estimate the P-oxidation of fatty acids, citrate synthase (CS, EC 4.1.3.7) was used to measure citric acid cycle activity, hexokinase (HK, EC 2.7.1.1) was used to test glucose phosphorylation capacity, and lactate dehydrogenase (LDH, EC 1.1.1.27) was used to determine lactateproduction capacity. LDH isozyme 1 was tested using homogenates thermoinactivated at 65*C and a-ketobutyrate as a substrate, according to the method described by Karlsson et al. (20). Statistics. A multifactor analysis of variance was used to analyze the global effects of exposure to high altitude and endurance training on the following studied vari-

ables: body and skeletal muscle weights, fiber-type distribution, and muscle enzyme activities. In addition, comparisons between groups were made with a Student’s t test for all variables. P < 0.05 was selected to indicate the level of statistical significance.

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OF

TRAINING

AT

ALTITUDE

FIG. 1. Serial cross sections of rat soleus (A and B) and extensor digitorum longus (C) muscles were processed for histochemical demonstration of myosin ATPase activity at preincubation pH values of 4.35 (A), 10.3 (B), and 4.55 (C). I, type I; Ib, type Ib; IIc, type 11~; IIa, type IIa; IIab, type IIab; IIb, type IIb. Scale bar, 20 pm.

RESULTS

Body and muscle weights. Endurance training and exposure to high altitude had significant global effects on body weight. By the end of the study, those animals that had performed training exercises or had been exposed to a high-altitude environment had reduced body weights (P < 0.001). Figure 2 shows that growth rate was reduced by these two factors. Chronic hypoxia reduced the growth rate of both HA-S and HA-T animals during the first 3 wk. Thereafter, all the animals seemed to gain weight at a similar rate. Training at sea level had a significant effect on the body weights of SL-T animals, but the difference between HA-S and HA-T groups failed to be significant (Table 1). A high-altitude environment significantly reduced the food intake of both sedentary rats and those trained on the swimming program (P < 0.001). Training, on the other hand, was without any effect (Table 1). Neither training nor exposure to high altitude had any effect on muscle mass-to-body weight ratio (muscle mass was estimated by taking the sum of EDL, PLA, and SOL muscle weights). Thus, for the animals in all groups, a significant correlation was found between the sum of the mass of the three muscles and total body weight (r = 0.89, P < 0.001).

Fiber-type distribution. Neither training nor exposure to a high-altitude environment affected fiber-type distribution in the SOL muscle. No significant variation in the percentages of type I, IIa, IIb, or IM (Ib and 11~) fibers appeared in this muscle (Fig. 3). EDL and deep portions of PLA (dPLA) muscles 600

,

500

f

SL-s

a SL-T

T

n

HA-S

0

HA-T

12

13

100

o! 12

FIG.

trained altitude

3

4

5

6

7

8

9

10

11

14

weeks

2. Changes in body weight in rats maintained sedentary or at sea level @L-S and SL-T, respectively) or at simulated high (HA-S and HA-T, respectively).

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SKELETAL

MUSCLE

EFFECTS

1. Body weight, relative skeletal muscle ??mss, and daily food intake TABLE

SL-S

8

SL-T

8

HA-S

8

HAT-T

7

Overall

555.5 291 482.4” +163 426.4t t76 408.7-t _+136

1.915 to.01 1.902 20.01 1.911 -to.01 1.952 to.01

30.51 to.38

P < 0.001 P < 0.001

NS NS

NS P < 0.001

29.16 to.39 22.31-t kO.39 23. lot to.44

effect

Training Altitude

Values are means + SE; it, no. of rats. SL-S, sea-level sedentary; SL-T, sea-level trained; HA-S, high-altitude sedentary; HA-T, high-altitude trained. * Significantly different from corresponding sedentary groups, P < 0.01. t Significantly different from corresponding sea-level groups, P < 0.01.

showed similar modifications (Fig. 3). At high altitude, the percentage of type IIa fibers was enhanced by endurance training (19.10 t 0.60 vs. 23.04 & 1.74%, P < 0.05 for EDL; 19.32 t 1.35 vs. 25.71 t 1.56%, P < 0.01 for dPLA). This increase occurred at the expense of the percentage of type IIb fibers (71.37 t 0.75 vs. 67.57 t 1.69%, P < 0.05 for EDL; 56.34 $- 1.22 vs. 51.92 t 1.67%, P < 0.05 for dPLA). In high-altitude acclimatized rats, the percentage of type IIab fibers in these muscles was greater than in sea-level rats. This was especially the case for sedentary animals (1.44 t 0.37 vs. 5.71 t 0.47%, P < 0.01 for EDL; 2.38 t 0.62 vs. 6.23 + 0.72%, P < 0.01 for dPLA). Despite the lack of a statistically significant global effect, the population of type IIab fibers in sea-level animals was enhanced by endurance training (1.44 + 0.37 vs. 5.71 t 0.47%, P < 0.01 in EDL; 2.38 t 0.62 vs. 4.77 t 0.66%, P < 0.01 in dPLA). There were fewer changes in the percentage of type IIc fibers. A global increasing effect of high altitude was observed only in the EDL muscle (0.17 $- 0.04 vs. 0.57 t 0.13%, P < 0.01 in sedentary rats; 0.14 t 0.05 vs. 0.53 t 0.16%, P < 0.01 in trained rats). No changes were observed in the percentage of the distribution of fiber types in the superficial portion of the PLA muscle. Enzyme activities. In the SOL muscle, HAD (i.e., the key enzyme involved in fatty acid oxidation) and CS (i.e., representative of the citric acid cycle) activities were unaffected by training (Table 2). On the other hand, in HAT animals HAD activity was less than in SL-T (-18%, P < 0.001). Otherwise, exposure to high altitude had an increasing overall effect on HK activity. This was especially the case for HA-S animals: the difference between SL-S and HA-S equaled +39% (P < 0.05). No statistical difference in HK activity was observed between the different groups of trained rats. The isozyme l-to-total LDH activity ratio was increased by endurance training (P < 0.05), mainly at sea level. The difference between SL-S and SL-T groups was greater than between HA-S and HA-T groups (+75% between SL-S and SL-T groups, (P < 0.01; +18% between HA-S and HA-T groups, no statistical difference).

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EDL and PLA muscles showed similar metabolic adaptations (Table 2). Training increased oxidative enzyme activities in EDL and PLA muscles (HAD and CS, P < 0.01 for both). These training-induced metabolic adaptations were mainly observed in high-altitude acclimatized rats. HAD increased an average of 36% in the EDL muscle and 31% in the PLA muscle. CS increased an average of 24% in EDL and 31% in PLA. The swimming training program also increased HK in the EDL muscle (P < 0.05) and the PLA muscle (P < 0.005). This increase occurred especially in the SL-T group (21.5%, P < 0.05; +39.8%, P < 0.001). However, the differences in the fast-twitch muscles of HA-S and HA-T groups were not significant. The effect of endurance training was an overall reduction in total LDH activity. This reduction was more marked in the fast-twitch muscles of animals in the high-al .titude acclimatized group. Moreover, endur ante training at sea level and at high altitude increased isozyme l-to-total LDH activity ratio. These metabolic changes occurred to a greater extent in the PLA muscles of acclimatized rats than in those living at sea level (+167% in HA animals and +104% in SL animals). They were observed to have a similar magnitude in EDL (+95% in HA animals and +89% in SL animals). DISCUSSION

Chronic hypobaric hypoxia and endurance training influence growth rate in male animals (Fig. 2). It is well known that chronic exercise is a factor involved in growth rate reduction (8, 11, 12,14). On the other hand, several studies have emphasized a decrease in the body weight of laboratory animals that have experienced long periods of exposure to a high-altitude environment (7, 29). In our study, chronic hypoxia and endurance training did not have cumul .ative effects on body growth rate. On the contrary, there seemed to be less of a loss in body weight in animals in the HA-T group than in those in the HA-S group. Loss in body weight after long-term exposure to a high-altitude environment can be attributed to a decrease in energy intake, an increase in energy requirements, fat or carbohydrate malabsorption, or dehydration. A recent study (25) has shown that anorexia alone cannot explain the degree of body weight loss that occurs in human subjects who have become acclimatized to high altitude. Thus it is likely that other factors responsible for a reduction in energy intake play a role in the loss of body weight at high altitude. However 1,these factors have not been studied here. On the other hand, the skeletal muscle mass estimated by taking the sum of the three muscles studied is well correlated to total body weight. This result confirms the fact that, in animals, neither exposure to high altitude nor endurance training induces any relative change (to body weight) in skeletal muscle mass (9, 11, 14, 29). The intensity of the adaptative responses to endurance training depended on the muscle type. The metabolic adaptations of the SOL muscle, comprising 87% of slow-twitch oxidative type I fibers, differed from those observed in EDL and PLA muscles, which have high proportions of fast-twitch fibers (types IIa and IIb). Neither exposure to high altitude

nor swimming training

in-

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OF

TRAINING

AT

ALTITUDE EDL

80%

80%

60%

60%

0% SL-T

HA-S

HA-T

SL-T

dPLA

80%

60%

60%

m

SL-T

type

t

~~{

HA-S

type

HA-T

HA-S

HA-T

sPLA

80%

SL-S

HA-S

If,,

HA-T

m

type

SL-s

Ilc

II

SL-T

type

lla

type

lab

-1

type

Ilb

3. Fiber composition in soleus, extensor digitorum longus (EDL), and deep and superficial portions of plantaris (dPLA and sPLA, respectively) muscles of SL-S, SL-T, HA-S, and HA-T. Significantly different from corresponding sedentary groups: *‘P < 0.05; **:P < 0.01. Significantly different from corresponding sea-level groups: $P < 0.05; $$P < 0.01. FIG.

fluenced CS activity in the SOL muscle. On the other hand, the capacity of P-oxidation of fatty acids decreased when training was performed at high altitude. This surprising result was difficult to explain. However, we were able to observe that during the swimming exercises the mechanical work performed by the muscles differed from that performed during running exercises. The lack of metabolic changes in SOL muscles after training signaled the loss of its antigravity function during swimming exercises. Indeed, the absence of the SOL support function during swimming may explain the fact that this muscle was not recruited. Armstrong and Laughlin (2) have clearly shown that, during swimming, blood flow is significantly lower in SOL whereas it is elevated in PLA. These data strongly suggest that SOL muscles are not recruited during swimming. Chronic hypoxia induced an increase in type IIab fibers in EDL and dPLA muscles. Enhancement of this type of IM fiber, which represented a transitional state between the two major fast-twitch fiber types, showed that chronic hypoxia slightly alters fiber-type distribution (18, 30, 31). The role of prolonged exposure to high

altitude in fiber-type interconversion has already been surmised by Sillau and Banchero (29). They recorded an enhancement of type IIb fibers in gastrocnemius and tibialis anterior muscles in sedentary rats living at simulated altitude (5,100 m) for 45 days. If subtype IIab fibers represent a real transitory state between type IIa and IIb fibers, then the increase in type IIab fibers in EDL and dPLA muscles may be explained in the following way. According to the findings of Sillau and Banchero (29), the increase in type IIab fibers in the HA-S group, with respect to the SL-S group, may have reflected a tendency toward the enhancement of fast-glycolytic fibers (transition from IIa to IIab and IIb fiber subtypes). On the other hand, exposure to high altitude induced only moderate metabolic changes in glycolytic pathways. The citing of the increase in the activities of LDH and HK is therefore used as an indirect argument to conjecture the enhancement of glycolytic capacity in the phasic muscles (EDL and PLA). Swimming exercise is less strenuous than running. Therefore the metabolic adaptations it produces are less complete and less pronounced. When swimming was

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SKELETAL MUSCLE EFFECTS OF TRAINING TABLE 2. Effects

of endurance

training

and altitude

CS

SL-s SL-T

HA-S HA-T Overall effect Training Altitude SL-s SL-T

HA-S HA-T Overall effect Training Altitude SL-s SL-T

HA-S HA-T Overall effect Training Altitude

63.91 t4.04 69.14 k2.67 65.43 t2.07 64.83 t1.02

NS NS 61.10 24.14 68.75 t3.94 60.28 t2.52 75.01* t3.34 P < 0.01 NS 55.74 t4.93 69.57§ t2.79 59.56 t2.29 78.02*$ t2.38 P < 0.001 NS

2119

AT ALTITUDE

on metabolic adaptations

of SOL, EDL, and PLA muscles

HAD

HK

SOL 48.35 k1.48 48.30 k2.92 43.61 d.98 39.45% t2.03

6.90

985

to.48 7.71 20.29 9.64-t k1.03 7.72 to.19

t59 904 t26 1,015 +55 923 t35

NS

NS

P < 0.025

P < 0.05

EDL 24.45 t1.87 26.36 t1.40 20.51 to.94 27.89" t1.36 P < 0.01

NS

PLA 26.61 k1.35 28.28 t1.51 23.35 k1.36 30.59" t-2.03 P < 0.01 NS

5.75 to.39 6.99s to.41 6.41 to.42 7.63 kO.58 P < 0.05 NS 5.07 to.39 7.09* Iko.29 7.03$ to.37 7.24 to.36 P < 0.005 P < 0.005

LDH

NS NS 3,016 2102 2,764 t98 3,323 -t75t 2,604" a31 P < 0.001 NS 2,779 t134 2,283" t85 3,266$ t117 2,227* 271 P < 0.001 P < 0.05

LDHl/Total

4.71 to.4 8.28* t1.3 7.74 t1.5 9.17 to.8 P < 0.05 NS 2.75 kO.5 5.22* kO.6 3.27 20.6 6.39* to.9 P < 0.001 NS 3.07 to.4 6.27* to.7 2.6 kO.7 6.96* a.4 P < 0.001 NS

Values are means t SE in units/g protein for citrate synthase (CS), hydroxyacyl CoA dehydrogenase (HAD), hexokinase (HK), and lactate dehydrogenase (LDH); values for isozyme l-to-total LDH activity ratio (LDHl/total) are X 100; n, no. of rats. SOL, soleus; EDL, extensor digitorum longus; PLA, plantaris. See Table 1 footnote for explanation of group designations. Significantly different from corresponding sedentary groups: * P < 0.01; 8 P < 0.05. Significantly different from sea-level groups: t P C 0.05; $ P < 0.01.

carried out at a steady state, oxygen uptake averaged three times the basal metabolic rate (3). This energy cost corresponds to 55-65% of the maximum aerobic capacity of a rat (28). The mild character of swimming exercises performed at sea level by rats was ascertained by the small effec t it had on metabolic changes and by its lack of effect on the fiber-type composition of skeletal muscle. The important adaptative changes observed in fasttwitch muscles after chronic exercise performed at high altitude constitute the major results of this study. These swimming-induced modifications arose from histochemical and biochemical changes. When swimming training was performed at high altitude, enhancement of type IIa fibers and oxidative activities in phasic muscles appeared statistically significant. These results show that, unlike training performed at sea level, mild exercise, such as swimming, is able to bring about fiber transition of subtype II fibers in fast-twitch muscles. In addition to the increase in subtype IIa fibers, at the expense of subtype IIb fibers in EDL and dPLA muscles, a significant increase in IIab IM fibers was observed. This fiber type, which is histochemically situated between types IIa and

IIb, exhibits a mixed heavy chain composition of IIa and IIb fibers (31). The increase in the percentage of these fibers, ranging between types IIa and IIb, accounted for the muscle’s plasticity and its adaptability, the latter being determined by histochemical techniques. These data confirm similar results obtained in humans and rodents that had undergone high-intensity training at sea level (1, 12,18, 19). Such results are supported by recent data showing that muscle contractile protein changes occur when there is a persistent decrease in creatine phosphate and ATP levels (21). Thus these data allow us to entertain the suggestion that energy phosphate compound levels might alter muscle contractile protein synthesis. On the other hand, Gutierrez et al. (13) clearly showed that, in an isolated blood-perfused rabbit hindlimb preparation, hypoxia can induce a decrease in creatine phosphate and a significant increase in inorganic phosphate. Additional experiments are required to determine the role of high-energy phosphate compounds in bringing about changes in muscle composition after exercise performed at high altitude. After 12 wk of training at high altitude, CS and HAD

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activities increased in both EDL and PLA muscles. In these muscles, HK activity was unaffected by training in a high-altitude environment. We conjecture that the capacity for oxidizing fatty acids increases as the rate of utilization of glucose as a substrate decreases. The decrease in total LDH activity and the increase in the ratio of isozyme 1 to total LDH activity confirmed the greater enhancement in oxidative capacity observed after training at high altitude. The most interesting finding was that, in animals, endurance training performed at high altitude counteracted the specific effects that long-term exposure to high altitude produces in glycolytic pathways (mainly observed in PLA muscle). The high-altitudetrained animals did not exhibit the same increase in glycolytic capacity observed in sedentary animals living in similar environments. On the contrary, a significant increase in the activity of respiratory enzymes, as much involved in the oxidation of fatty acids as in the citric acid cycle, was observed. There are few existing studies on metabolic adaptations following endurance training at high altitude. Terblanche et al. (32) studied the metabolic effects of endurance training at sea level or at moderate altitude (1,750 m). They showed that an improvement in the endurance level of animals trained at high altitude is linked to the greater increase in muscle CS activity exhibited in high-altitude-trained than in sea-level-trained animals. Although the animals used in this study were trained at moderate altitude, differences in metabolic adaptative changes were recorded. In comparison to untrained animals, the gastrocnemius CS activity in animals trained at moderate altitude increased by 86%. Furthermore, under our conditions, CS activity increased by 24% in EDL and 31% in PLA. The difference in intensity between these training-induced changes is primarily explained by the fact that we used a less intensive training program. In conclusion, the data reported in this paper emphasize the number of adaptative changes observed in the fast-twitch muscles of rats after they have been subjected to endurance training performed at high altitude. The results, on the whole, showed that chronic exercise had greater histochemical and biochemical effects when training was performed at high altitude than when it was performed at sea level. On the other hand, we observed that training cancelled or reversed the specific effects of exposure to high altitude on muscle metabolism. A strict interpretation of these differences is not clear, because the absolute work rate was the same at high altitude as it was at sea level. However, the degree of difference in histochemical and biochemical changes following training at high altitude leads us to wonder about the role of intracellular oxygen pressure. Address for reprint requests: A. X. Bigard, Div. MQtabolique et Hormonale, CERMA, Base d’Essais Brbtigny Cedex, France. Received

5 July

1990; accepted

in final

form

9 July

de Physiologie en Vol, 91228

1991.

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Skeletal muscle changes after endurance training at high altitude.

The effects of endurance training on the skeletal muscle of rats have been studied at sea level and simulated high altitude (4,000 m). Male Wistar rat...
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