Eur J Appl Physiol (1982) 48:201-205 European Journal of

Applied Physiology and Occupational Physiology �9 Springer-Verlag 1982

The Effect of Beta-Adrenergic Receptor Blockade on Intramuscular Glycogen Mobilization During Exercise in the Rat*

Jan Gdrski and Krystyna Pietrzyk

Department of Physiology, Institut of Physiology and Pathology, Medical School, ul. Mickiewicza 2c, 15-230 Bialystok, Poland

Summary. The effect of beta-adrenergic receptor blockade (with propran- olol) on exercise-induced glycogen mobilization in rat skeletal muscles was studied. Treatment with propranolol either fully or partly prevented glycogen mobilization or had no effect at all on the process. The effects of propranolol depended on the intensity and duration of exercise and on the fiber composition of the muscle studied.

Key words: Exercise - Muscle glycogen - Muscle types - Beta-adrenergic blockade - Rat

There are numerous reports concerning the role of the adrenergic system in intramuscular glycogen mobilization during exercise. The results obtained are, however, controversial. It has been shown in man that blockade of the beta-adrenergic receptors has no effect on this process (Harris et al. 1971). In exercising dogs both an increase (Nazar et al. 1971) and a decrease (Issecutz 1978) in muscle glycogen utilization after treatment with propranolol have been described. In rats, propranolol was found either partly to block (Maling et al. 1966) or to have no effect (Gollnick et al. 1970) on exercise-induced glycogenolysis in skeletal muscles. In the same animals, adrenomedullectomy partly inhibited the process (Struck and Tipton 1974). Adrenomedullectomy combined with 6-hydroxydopamine treatment has been shown to have no effect (Sembrowich et al. 1974) and to fully prevent (Galbo et al. 1978, Richter et al. 1980) muscle glycogen breakdown during exercise. In the experiments quoted above different work loads were used (e.g., vigorous running - Sembrowich et al. 1974, and swimming for 75 rain - Galbo et al. 1978). This might suggest that adrenergic involvement in muscle glycogen breakdown during exercise could depend on the work load applied. This assumption has been fully confirmed in the present study, in which the effect of beta-adrenergic receptor blockade on mobilization of glycogen in muscles of rats exercising with different intensity and duration was examined.

* Supported by Polish Academy of Sciences, the project 10.4.2.01.3.2. Offprint requests to: Dr. J. G6rski (address see above)

0301-5548/82/0048/0201/$ 01.00

202 J. G6rski and K. Pietrzyk

Methods

The experiments were carried out on male Wistar rats, 230-250 g of body weight, fed ad libitum on a commercial pellet diet for rodents. The rats undertook either running or swimming exercises. An electrically driven treadmill was used to run the rat. The rats assigned to running exercises were familiarized with the treadmill for 5 mill daily for 6 days preceeding the final experiment, with a speed and slope as in the final run (details in Table 1). The rats assigned to swimming exercise were made to swim in a circular tank, 56 cm in diameter filled with water at a temperature of 34-35 ~ C to a level of 44 cm. Each rat had a load equal to 2% of its body weight attached to its tail. Six rats always swum at the same time. The swimming rats were familiarized with the water, 10 rain daily, for 6 days preceeding the final experiment. Propranolol (Polfa) was administered intraperitoneally in a dose of 6 mg/kg (Gollnick et al. 1970) 20 rain before exercise. Rats were anaesthetized with urethane administered intraperitoneally. Samples of the following skeletal muscles were taken: 1. the superficial layer of the left vastus lateralis (consisting mainly of fast-twitch-glycolytic fibers - "FG" muscle), 2. the deepest layer of the same muscle (consisting mainly of fast-twitch-oxidative-glycolytic fibers - "FOG" muscle), and 3. the left soleus (consisting mainly of slow-twitch-oxidative fibers - "SO" muscle). The muscle samples were taken according to Baldwin et al. (1972) and Pater et al. (1972).

The muscle samples were immediately weighed on a torsion balance, placed in hot 30% KOH and boiled for 20 min. Glycogen was subsequently assayed quantitatively by the method of Carroll et al. (1956). In blood from the abdominal aorta the glucose level was determined by the method of Hultman (1959). The results presented are means + standard deviation. Each result is the mean value of the results obtained in ten rats. The results obtained were evaluated statistically using the Student-t test for unpaired data. Differences with a p value < 0.05 were considered signifi- cant.

Results

T h e resul ts of the g lycogen m e a s u r e m e n t s a re p r e s e n t e d in T a b l e 1. In the con t ro l ra ts , each exerc i se r e su l t ed in s ignif icant r educ t i on of g lycogen

levels in the musc les e x a m i n e d excep t for F G musc le of ra ts swimming for 75 min .

T h e effects of p r o p r a n o l o l were d ivers i f ied and d e p e n d e d b o t h on the type of exerc i se and at the t ype of musc le examined . In rats runn ing at a s p e e d of 12 m/min for 30 min on 0 ~ incl ine , p r o p r a n o l o l c o m p l e t e l y p r e v e n t e d mobi l i - z a t i on of g lycogen in the F G and F O G muscles and pa r t l y in the SO muscle . In ra ts runn ing as a b o v e bu t wi th a t r eadmi l l inc l ined at 15 ~ p r o p r a n o l o l c o m p l e t e l y b l o c k e d g lycogen mob i l i z a t i on on ly in the F G musc le and pa r t ly in the F O G and SO muscles . P r o p r a n o l o l had no effect on g lycogen level in any musc le of the ra ts runn ing till exhaus t i on at a s p e e d of 12 m/min . In ra ts runn ing ti l l exhaus t i on at a s p e e d of 30 m/min , p r o p r a n o l o l had no effect on g lycogen level in the F G musc le and pa r t l y p r e v e n t e d its mob i l i za t i on in the F O G and SO muscles . In ra ts swimming for 75 rain, p r o p r a n o l o l c o m p l e t e l y b l o c k e d g lycogen mob i l i z a t i on in b o t h the F O G and the SO muscles .

Blood Glucose Level

The b l o o d glucose level was s ignif icant ly r e d u c e d on ly in the rats runn ing till exhaus t i on at a s p e e d of 12 m/ra in ( res t ing level - 6.24 + 0.89, con t ro l g roup -

Adrenergic System and Muscle Glycogen During Exercise 203

Table 1. The effect of propranolol on exercise-induced intramuscular glycogen level. C - control, exercising rats; P - propranolol-treated exercising rats. Glycogen - ~mol of glucose/g of tissue

Vastus Vastus Soleus superficial deepest (SO muscle) (FG muscle) (FOG muscle)

Resting control 31.7 + 2.9 27.8 ___ 3.3 29.1 + 5.6

Running 30 rain 12 m/min C 27.3 + 2.5 c 19.7 _+ 3.2 c 12.5 + 2.9 0 ~ incline P 34.2 + 4.1 z 25.5 _+ 3.5 x 22.6 + 2.2 z' b

Running 30 rain 12 m/min C 25.5 + 4.7 c 10.4 + 2.0 ~ 11.0 + 2.5 15 o incline P 33.6 + 6.3 x 17.5 + 2.8 z,c 21.2 + 3.6 z,b

Running till exhaustion 12 m/rain C 23.6 + 5.3 c 16.4 + 3.7 c 14.0 + 2.5 0 ~ incline d P 26.8 + 5.0 a 14.1 + 6.4 c 14.3 + 3.1 c

Running till exhaustion 30 m/min C 12.8 + 7.6 c 6.1 + 1.5 c 7.3 + 1.7 c

5 ~ incline P 14.5 + 6.9 ~ 10.2 + 1.9 z, c 16.2 + 2.4 z, ~

Swimming C 31.6 + 4.5 16.0 + 2.3 z,r 16.0 + 3.8 ~ 75 min P 35.1 + 6.3 29.6 + 4.8 27.7 + 4.5 z,c

Resting control vs. the post-exercise levels: a p < 0.02; b p < 0 .01 ; c p < 0.001 P VS. C: x p < 0.01; z p < 0.001 Time of running till exhaustion was: speed 12 m/rain C - 239 _+ 32 rain; P - 186 + 44 rain (p < 0.01 vs. the control); speed 30 m/min C - 19.1 + 8.1 rain; P - 14.5 + 4.9 rain (p > 0.05 vs. the control). Exhaustion was defined as a point in which the rats were unable of further running d In this group of rats two additional subgroups were examined: In the first, the determinations were

carried out in control rats, each of which run for 186 min, i.e., the time equal to the average time of running till exhaustion in the propranolol-treated rats of this group. In the second group the rats were injected with propranolol (6 mg/kg) twice: 20 rain before the exercise and 90 rain after it started. The results obtained in both subgroups were similar to the respective postexercise data presented in the table

3 .79 + 0 .86 , p r o p r a n o l o l - t r e a t e d g r o u p - 4 . 2 1 _+ 0 .43 mmol /1 ; f o r e a c h g r o u p

p < 0 .001 vs. t h e r e s t i n g l eve l ) .

Discussion

T h e r e s u l t s o b t a i n e d c l e a r l y s h o w t h a t b l o c k a d e of b e t a a d r e n e r g i c r e c e p t o r s

e i t h e r fu l ly o r p a r t l y p r e v e n t s m u s c l e g l y c o g e n m o b i l i z a t i o n d u r i n g e x e r c i s e o r

d o e s n o t i n t e r f e r e a t al l w i t h t h e p r o c e s s , d e p e n d i n g o n t h e d u r a t i o n a n d

i n t e n s i t y o f t h e e x e r c i s e . I t s e e m s t o d e p e n d a l so o n t h e t y p e o f m u s c l e . I n t h e

r a t , h o w e v e r , t h e i n v o l v e m e n t o f m u s c l e s c o m p o s e d o f d i f f e r e n t f i b r e t y p e s

d u r i n g e x e r c i s e d e p e n d s o n t h e t y p e of e x e r c i s e ( A r m s t r o n g e t al . 1974; B a l d w i n

e t al . 1973) . O n e m i g h t t h e r e f o r e e x p e c t t h a t d i f f e r e n c e s in t h e e f f e c t i v e n e s s o f

t h e b l o c k a d e d u r i n g t h e s a m e e x e r c i s e c o u l d b e d u e r a t h e r to a d i f f e r e n t d e g r e e

204 J. G6rski and K. Pietrzyk

of engagement of the various muscles during exercise and not to a different role of the adrenergic system in activation of glycogenolysis in the muscles.

The present data therefore seem to elucidate the reason for the discrepancies in the results obtained, at least in the rat, by other authors (see Introduction). When a similar exercise-test was applied in different laboratories (swimming in the present work and in the investigations of Galbo et al. 1978) the same results were obtained despite the different methods used to block the adrenergic system.

The results of the present work indicate that, during exercise, glycogenolysis in skeletal muscles of rats might be controlled either by the adrenergic or by the nonadrenergic (i.e., mainly ionized calcium - Drummond, 1971) mechanism as well by the two mechanisms simultaneously. The factor regulating the involvement of different mechanisms controlling muscle glycogenolysis during exercise remains unknown. Propranolol had no effect on glycogen mobilization in either the muscle of rats running till exhaustion at a speed of 12 m/min or in the FG muscle of rats running till exhaustion at a speed of 30 m/rain. In the first case hypoglycemia developed and thus the muscles had a diminished supply of glucose. In the second case, the blood glucose level was unchanged but because the FG muscle had a low hexokinase activity (Peter et al. 1972; Staudte and Pette 1972) its glucose supply was probably far below the requirements during this type of exercise. It is therefore very likely that a shortage of glucose may be the factor triggering the nonadrenergic mechanism stimulating muscle glycogeno- lysis during exercise. Since the shortage of glucose in these cases was probably severe, the nonadrenergic mechanism was activated so strongly that it abolished the inhibitory effect of blockade of the beta-adrenergic receptors. It might be further speculated that, in those cases in which the beta-blockade prevented glycogen mobilization either fully or partly, the glucose supply to the muscles was correspondingly sufficient or slightly lower than the requirement. Thus in the first case, the nonadrenergic mechanism was not activated, whereas in the second its activation was inconsiderable so that it could be only partly responsible for stimulation of glycogenolysis.

References

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Carroll NY, Longley RW, Roe JH (1956) The determination of glycogen in fiver and muscle by use of antbrone reagent. J Biol Chem 220:583-593

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Accepted September 22, 1981