Europ. J. appl. Physiol. 34, t83--t90 (1975) 9 by Springer-Verlag 1975
Leg Muscle Metabolism during Exercise in the Heat and Cold* W. J. F i n k , D. L. Costill, a n d P. J. V a n H a n d e l Human Performance Laboratory, Ball State University, Muncie, Indiana 47306 Received January 6, 1975 Abstract. In an effort to assess the effects of environmental heat stress on muscle metabolism during exercise, 6 men performed work in the heat (Tdb = 41 ~ C, RH = 15 %) and cold (Tdb = 9~ C, RH = 55 %). Exercise consisted of three 15-min cycling bouts at 70 to 85 % ~m~x, with 10-min rest between each. Muscle biopsies obtained from the vastus lateral!s before and after each work bout were analyzed for glycogen and triglyceride content. Venous blood samples drawn before and after exercise were assayed for lactate, glucose, free fatty acids, hemoglobin, and hematocrit. Oxygen uptake, heart rates and rectal temperatures were all significantly higher during exercise in the heat. Blood lactate concentration was roughly twice as great during the heat experiments as that measured in the 9~ environment. Muscle glycogen utilization per 60 rain was significantly greater in the heat ( -- 74 m moles/kg-wet muscle) as compared to the cold exercise ( - 42 m molesfkg-wet muscle.) On the average, muscle triglyceride declined 23 % during exercise in the cold and 11% in the heat. The findings of an enhanced glycolysis during exercise in the heat is compatible with earlier studies which demonstrate a decreased availability of oxygen due to a reduction in muscle blood flow. Key words: Muscle Glycogen -- Muscle Triglyceride -- Blood Lactate -- Plasma Volume. H e a t stress a p p e a r s to affect t h e d i s t r i b u t i o n o f blood flow d u r i n g s u b m a x i m a l exercise, b u t causes l i t t l e or no increase in c a r d i a c o u t p u t [5, 6, i0]. T h e q u a n t i t y of b l o o d flow a v a i l a b l e for skin d u r i n g such exercise d e p e n d s on how m u c h b l o o d flow can be r e d i s t r i b u t e d from w o r k i n g muscles a n d i n a c t i v e organs. A l t h o u g h muscle blood flow is g e n e r a l l y d e t e r m i n e d b y local m e t a b o l i c factors during exercise, it has b e e n d e m o n s t r a t e d t h a t i n c r e a s e d s y m p a t h e t i c n e r v e a c t i v i t y m a y even induce v a s o c o n s t r i c t i o n in t h e w o r k i n g muscles [2, 12]. R o w e l l [9] suggests t h a t since visceral v a s o c o n s t r i c t i o n a p p e a r s t o reflect i n c r e a s e d s y m p a t h e t i c v a s o m o t o r outflow d u r i n g exercise in t h e h e a t , i t is possible t h a t w o r k i n g muscle responds to this v a s o m o t o r outflow b y r e d u c i n g its own blood flow. M e a s u r e m e n t s of b l o o d l a c t a t e p r o v i d e i n d i r e c t s u p p o r t for this thesis, a n d suggest t h a t exercise in t h e h e a t m a y e n h a n c e t h e r a t e of glycolysis as a r e s u l t of this s y m p a t h e t i c drive. I n t h e absence of m o r e precise evidence, t h e p r e s e n t i n v e s t i g a t i o n was undert a k e n to assess t h e effects of e n v i r o n m e n t a l t e m p e r a t u r e on t h e r a t e of muscle glycogen utilization, a n d to e x p l a i n one f a c t o r p o t e n t i a l l y responsible for exh a u s t i o n d u r i n g exercise in t h e heat.
Methods The subjects were 6 men, ranging in age from 21 to 39 years. Since these experiments were conducted during the winter months of January to March, none of the men were heat accli* This research was supported by grants from the National Institutes of Health, Grant Number R0t AM 17083-01, and the Ball State University Faculty Research Committee.
184
W.J. Fink et at.
matized. However, all participants were physically active, running 4 to 10 miles daily, 4 to 6 days per week. During the experiments the men wore only shoes, shorts, and socks. The experiments were conducted in a chamber maintained at either 4t ~ C with a relative humidity (RH) of t5 % (heat), or at 9~ C with a RH of 55 % (cold). Exercise consisted of three 15-rain cycling bouts at 70 to 85 % of the subject's aerobic capacity, with a 10-rain period between each bout. During this rest interval the men were weighed and required to ingest sufficient water to maintain body weight at the pre-exereise level. As a result, mean postexercise body weights were 0.2 and 0.1% less than the pre-exercise weights in the heat and cold experiments, respectively. Needle biopsies were obtained from the vastus lateralis before and immediately after each of the 15-min exercise bouts. Each sample was cleaned of all visible fat, connective tissue and blood, and divided into two parts. After being weighed, one piece of the muscle sample was frozen in liquid nitrogen and later analyzed for glycogen according to the methods of Lowry [7]. The second piece of muscle was weighed and analyzed for triglyceride content [3]. Muscle weights were corrected for water evaporation by repeated timed weighing as previously described [3]. Venous blood samples were obtained from a forearm vein without stasis before and immediately after each exercise bout. These samples were assayed for lactate [8], free fatty acids [4], glucose [11 ], hemoglobin, and hematocrit. Oxygen consumption was measured via a Douglas bag method utilizing Beckman oxygen (E-2) and carbon dioxide (LB-1) gas analyzers. Gas samples were collected during the 5th, 10th, and 15th rain of each exercise bout. Heart rates and rectal temperatures were monitored continuously during all exercise sessions. Since a comparison of these environmental conditions necessitates a precise regulation of the workload, preliminary studies were conducted to confirm that the environmental temperature had no effect on the pedal force required to perform a given work task. This was accomplished by placing load cells on the pedals and performing a given workload in both the heat and cold. Comparisons of mean differences were tested for significance using a t-test for paired observations.
Results Despite the uniform workloads (mean = 138 watts) during each exercise bout, there was a significant increase ( P < 0.05) in total b o d y oxygen consumption and heart rates during exercise in the heat (Fig. i). Core temperatures showed similar increases during the first 80 to 35 rain in the h e a t and cold (Fig. l). During the final exercise bout in the heat, rectal t e m p e r a t u r e ( T R ) continued to increase. At the same interval in the 9 ~ environment f/'a remained unchanged. Subjectively all of the subjects experienced great difficulty in completing the exercise task in the heat, b u t had little trouble in the cooler environment. Three of the m e n d e m o n s t r a t e d s y m p t o m s of heat syncope upon completing the final exercise b o u t in t h e 4 t ~ C environment. Mean :t: SE values for serum free f a t t y acids (FFA), triglyceride (STG) and glucose are presented in Table t. Although F F A showed a significant ( P < 0.05) increase as a result of exercise, there was no difference between the two experimental conditions. Serum glucose and STG, on the other hand, showed inverse patterns in the heat and cold. T h a t is, in the 4~~ e n v i r o n m e n t serum glucose increased ( P < 0.05) and STG t e n d e d to decrease during exercise. I n the cold, however, serum glucose showed a small decline while STG increased significantly
(P < 0.05). Based on changes in hemoglobin and hematoerit, plasma volume was calculated [i] to have decreased roughly 9% during the first 15-min exercise period, a p p a r e n t l y as a result of a transcapillary fluid flux. As illustrated in Fig. 2, this m o v e m e n t of plasma water was independent of the environmental temperature or repeated bouts of exercise.
Leg Nuscle )Iet abolism during Exercise
in the
Heat and Cold
t85
E
ul -1
2>
170
w 160 t.,": 150
t.r.}--t I.}"+t
140 ta# ~:
130
120
EXERCISE
]~EXERCISE>I
0~0~0 ~0
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Leg Muscle Metabolism during Exercise in the Heat and Cold* W. J. F i n k , D. L. Costill, a n d P. J. V a n H a n d e l Human Performance Laboratory, Ball State University, Muncie, Indiana 47306 Received January 6, 1975 Abstract. In an effort to assess the effects of environmental heat stress on muscle metabolism during exercise, 6 men performed work in the heat (Tdb = 41 ~ C, RH = 15 %) and cold (Tdb = 9~ C, RH = 55 %). Exercise consisted of three 15-min cycling bouts at 70 to 85 % ~m~x, with 10-min rest between each. Muscle biopsies obtained from the vastus lateral!s before and after each work bout were analyzed for glycogen and triglyceride content. Venous blood samples drawn before and after exercise were assayed for lactate, glucose, free fatty acids, hemoglobin, and hematocrit. Oxygen uptake, heart rates and rectal temperatures were all significantly higher during exercise in the heat. Blood lactate concentration was roughly twice as great during the heat experiments as that measured in the 9~ environment. Muscle glycogen utilization per 60 rain was significantly greater in the heat ( -- 74 m moles/kg-wet muscle) as compared to the cold exercise ( - 42 m molesfkg-wet muscle.) On the average, muscle triglyceride declined 23 % during exercise in the cold and 11% in the heat. The findings of an enhanced glycolysis during exercise in the heat is compatible with earlier studies which demonstrate a decreased availability of oxygen due to a reduction in muscle blood flow. Key words: Muscle Glycogen -- Muscle Triglyceride -- Blood Lactate -- Plasma Volume. H e a t stress a p p e a r s to affect t h e d i s t r i b u t i o n o f blood flow d u r i n g s u b m a x i m a l exercise, b u t causes l i t t l e or no increase in c a r d i a c o u t p u t [5, 6, i0]. T h e q u a n t i t y of b l o o d flow a v a i l a b l e for skin d u r i n g such exercise d e p e n d s on how m u c h b l o o d flow can be r e d i s t r i b u t e d from w o r k i n g muscles a n d i n a c t i v e organs. A l t h o u g h muscle blood flow is g e n e r a l l y d e t e r m i n e d b y local m e t a b o l i c factors during exercise, it has b e e n d e m o n s t r a t e d t h a t i n c r e a s e d s y m p a t h e t i c n e r v e a c t i v i t y m a y even induce v a s o c o n s t r i c t i o n in t h e w o r k i n g muscles [2, 12]. R o w e l l [9] suggests t h a t since visceral v a s o c o n s t r i c t i o n a p p e a r s t o reflect i n c r e a s e d s y m p a t h e t i c v a s o m o t o r outflow d u r i n g exercise in t h e h e a t , i t is possible t h a t w o r k i n g muscle responds to this v a s o m o t o r outflow b y r e d u c i n g its own blood flow. M e a s u r e m e n t s of b l o o d l a c t a t e p r o v i d e i n d i r e c t s u p p o r t for this thesis, a n d suggest t h a t exercise in t h e h e a t m a y e n h a n c e t h e r a t e of glycolysis as a r e s u l t of this s y m p a t h e t i c drive. I n t h e absence of m o r e precise evidence, t h e p r e s e n t i n v e s t i g a t i o n was undert a k e n to assess t h e effects of e n v i r o n m e n t a l t e m p e r a t u r e on t h e r a t e of muscle glycogen utilization, a n d to e x p l a i n one f a c t o r p o t e n t i a l l y responsible for exh a u s t i o n d u r i n g exercise in t h e heat.
Methods The subjects were 6 men, ranging in age from 21 to 39 years. Since these experiments were conducted during the winter months of January to March, none of the men were heat accli* This research was supported by grants from the National Institutes of Health, Grant Number R0t AM 17083-01, and the Ball State University Faculty Research Committee.
184
W.J. Fink et at.
matized. However, all participants were physically active, running 4 to 10 miles daily, 4 to 6 days per week. During the experiments the men wore only shoes, shorts, and socks. The experiments were conducted in a chamber maintained at either 4t ~ C with a relative humidity (RH) of t5 % (heat), or at 9~ C with a RH of 55 % (cold). Exercise consisted of three 15-rain cycling bouts at 70 to 85 % of the subject's aerobic capacity, with a 10-rain period between each bout. During this rest interval the men were weighed and required to ingest sufficient water to maintain body weight at the pre-exereise level. As a result, mean postexercise body weights were 0.2 and 0.1% less than the pre-exercise weights in the heat and cold experiments, respectively. Needle biopsies were obtained from the vastus lateralis before and immediately after each of the 15-min exercise bouts. Each sample was cleaned of all visible fat, connective tissue and blood, and divided into two parts. After being weighed, one piece of the muscle sample was frozen in liquid nitrogen and later analyzed for glycogen according to the methods of Lowry [7]. The second piece of muscle was weighed and analyzed for triglyceride content [3]. Muscle weights were corrected for water evaporation by repeated timed weighing as previously described [3]. Venous blood samples were obtained from a forearm vein without stasis before and immediately after each exercise bout. These samples were assayed for lactate [8], free fatty acids [4], glucose [11 ], hemoglobin, and hematocrit. Oxygen consumption was measured via a Douglas bag method utilizing Beckman oxygen (E-2) and carbon dioxide (LB-1) gas analyzers. Gas samples were collected during the 5th, 10th, and 15th rain of each exercise bout. Heart rates and rectal temperatures were monitored continuously during all exercise sessions. Since a comparison of these environmental conditions necessitates a precise regulation of the workload, preliminary studies were conducted to confirm that the environmental temperature had no effect on the pedal force required to perform a given work task. This was accomplished by placing load cells on the pedals and performing a given workload in both the heat and cold. Comparisons of mean differences were tested for significance using a t-test for paired observations.
Results Despite the uniform workloads (mean = 138 watts) during each exercise bout, there was a significant increase ( P < 0.05) in total b o d y oxygen consumption and heart rates during exercise in the heat (Fig. i). Core temperatures showed similar increases during the first 80 to 35 rain in the h e a t and cold (Fig. l). During the final exercise bout in the heat, rectal t e m p e r a t u r e ( T R ) continued to increase. At the same interval in the 9 ~ environment f/'a remained unchanged. Subjectively all of the subjects experienced great difficulty in completing the exercise task in the heat, b u t had little trouble in the cooler environment. Three of the m e n d e m o n s t r a t e d s y m p t o m s of heat syncope upon completing the final exercise b o u t in t h e 4 t ~ C environment. Mean :t: SE values for serum free f a t t y acids (FFA), triglyceride (STG) and glucose are presented in Table t. Although F F A showed a significant ( P < 0.05) increase as a result of exercise, there was no difference between the two experimental conditions. Serum glucose and STG, on the other hand, showed inverse patterns in the heat and cold. T h a t is, in the 4~~ e n v i r o n m e n t serum glucose increased ( P < 0.05) and STG t e n d e d to decrease during exercise. I n the cold, however, serum glucose showed a small decline while STG increased significantly
(P < 0.05). Based on changes in hemoglobin and hematoerit, plasma volume was calculated [i] to have decreased roughly 9% during the first 15-min exercise period, a p p a r e n t l y as a result of a transcapillary fluid flux. As illustrated in Fig. 2, this m o v e m e n t of plasma water was independent of the environmental temperature or repeated bouts of exercise.
Leg Nuscle )Iet abolism during Exercise
in the
Heat and Cold
t85
E
ul -1
2>
170
w 160 t.,": 150
t.r.}--t I.}"+t
140 ta# ~:
130
120
EXERCISE
]~EXERCISE>I
0~0~0 ~0
/o
~.o/~
,--,--.--.
--"
