Diminished in trained
hormonal rats
responses
to exercise
H. GALBO, E. A. RICHTER, J. J. HOLST, AND N. J. CHRISTENSEN Institute of Medical Physiology B, University of Copenhagen; Department of Clinical Chemistry, Bispebjerg Hospital, Copenhagen; and 2nd Clinic of Internal Medicine, Kommunehospitalet, Aarhus, Denmark
GALBO, H., E. A. RICHTER, J. J. HOLST, AND N. J. CHRISDiminished hormonal responses to exercise in trained ruts. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 43(6): 953-958, 1977.- Male rats (120 g) either were subjected to a 12-wk physical training program (T rats) or were sedentary controls (C rats). Subsequently the rats were killed at rest or after a 45- or 90-min forced swim. At rest, T rats had higher liver and muscle glycogen concentrations but lower plasma insulin. During exercise, blood glucose increased 60% in T rats but decreased 20% in C rats. Plasma glucagon and insulin concentrations did not change in T rats but plasma glucagon increased and insulin decreased markedly in C rats. Plasma epinephrine (90 min: range, 0.78-2.96 ng*ml+, (T) vs. 4.42-15.67 (C)) and norepinephrine (90 min: 0.70-2.22(T) vs. 2.50-6.10 (C)) were lower in T than in C rats. Hepatic glycogen decreased substantially and, as with muscle glycogen, the decrease was parallel in T and C rats. The plasma concentrations of free fatty acids were higher but lactate and alanine lower in T than in C rats. In trained rats the hormonal response to exercise is blunted partly due to higher glucose concentrations. In these rats adipose tissue sensitivity to catecholamines is increased, and changes in glucagon and insulin concentrations are not necessary for increased lipolysis and hepatic glycogen depletion during exercise. TENSEN.
related to the rate of decline of blood glucose concentrations (lo), trained rats ought to show diminished hormonal responses to exercise. These, in turn, might explain the diminished rate of hepatic glycogen depletion during exercise that has been found in trained rats (3). In the present study an exercise test was carried out in trained and untrained rats to evaluate the role of blood glucose concentrations for the regulation of the hormonal response to exercise and furthermore the role of hormones for the hepatic glycogen depletion during exercise. METHODS
Fifty-one male Wistar rats weighing 110-130 g were randomly divided into two groups, both of which were provided with food and water ad libitum. One group (26 rats) was subjected to a 12-wk physical training program consisting of swimming in water maintained at 33-34°C. The trained rats (T rats) swam 5 days/wk. The first day the duration of swimming was 30 min. Then it was gradually increased until after 8 wk of training the rats swam continuously 6 h/day (Fig. 1). The rats in the other group (25 rats) were sedentary epinephrine; norepinephrine; metabolism; liver; muscles; glycontrols (C rats). To avoid stress reactions during the cogen;lipid mobilization; fatty acids; glucose final exercise test due to handling and immersion in water, C rats were subjected to 15 min of swimming fop 4 days the week before the test (19). Sixty-four hours after the rats had been trained for DURING PROLONGED EXERCISE the secretion ofglucagon and catecholamines increases while the secretion of the last time (T rats) or accustomed to handling (C insulin decreases (5, 6, 10-13, 16, 19, 22, 28, 29). These rats), they participated in an exercise test. The test was carried out in the morning, and the rats fasted 6 h changes in hormone secretion should favor hepatic in advance. Nine T rats and nine C rats were forced to glucose production, and the administration of glucagon antibodies to exercising rats has in fact been shown to swim1 for 90 min, and nine T rats and eight C rats diminish hepatic glycogen depletion (12). During exer- were forced to swim1 for 45 min in water maintained at cise the decrease in insulin secretion is probably essen- 33-34OC. Eight T rats and eight C rats remained resting tially due to the stimulation by catecholamines of B- for 90 min. At the end of exercise or rest period the animals were immediately anesthetized with ether, and cell-inhibiting receptors (6, 11). The secretion of gluca8 ml blood were drawn by cardiac puncture. Then gon and epinephrine appears to be significantly influsamples of the liver, of the superficial and deep parts of enced by glucose-sensitive receptors since maintenance of euglycemia by glucose infusion diminishes the rates the vastus lateralis muscle, and of the soleus muscle --of secretion of these hormones during exercise (10). l On the day of the exercise test the randomly selected rats in Endurance training of rats has been shown to result in each of the groups of T and C rats were ranked in regard to body a slower decrease of blood glucose concentrations during weight. T rats were loaded with a tail weight of lead amounting to prolonged exercise (3). Thus, according to the concept 4% of body weight, and C rats were loaded with the same weights that the hormonal responses to exercise are closely as T rats with identical rank. Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.081.226.078) on January 18, 2019.
954
GALBO,
were quickly removed and immediately frozen in liquid nitrogen. The heart was removed too and was weighed after it had been opened, rinsed, and blotted. Epinephrine and norepinephrine in the first drawn 100 ~1 of plasma were determined by a previously published radioenzymatic assay (8) modified for measurement of very small amounts. Catecholamine determinations were carried out only in two T and two C rats at rest and in five T and five C rats afier 90 min of exercise. Insulin was determined by a radioimmunoassay relying on charcoal separation (1). The detection limit for the assay system was 0.30 pmol l-l, and the within-assay coefficient of variation (n = 20) was 3% at 46 pmol 1-l. The methods of sampling and analysis of blood and tissue for glucagon, glucose, pyruvate, alanine, lactate, FFA (free fatty acids), glycerol, and glycogen have been reported elsewhere (12, 13). All analyses of a hormone were carried out in a single assay run. Glycogen concentrations measured in samples from six different lobes within the same liver were practically identical (296, 299, 299, 307, 319, 328 mmol 1-l). However, in the present study the sites of tissue sampling were always the same. Statistical evaluation of the data was made by means of correlation analysis and by means of t-test and the nonparametric rank sum test (25). Differences were considered to be significant if P values of less than 0.05 were obtained with both these tests.
RICHTER,
HOLST,
AND
CHRISTENSEN
(0.37 t 0.02%, mean t SD) than in C rats (0.26 t 0.02%). Training significantly decreased plasma insulin (Fig. 2) and blood pyruvate concentrations (Table 3) at rest and increased glycogen concentrations in the liver (35% (14-56%), mean and 95% confidence limits) (Fig. 2) and in the deep part of the vastus lateralis muscle (39% (B-60%)) (Table 2). The other variables did not, Gum
SE /t
mmol
- - - Trained - - --- Untrained
lO-
fats rats
l
8-
l
6-
t 0
I
t
45
90
GLUCAGON
500 -
l
pmol
/I 400 -
3uoc
i
200 r roo-
RESULTS
uL
T rats increased their body weight less than C rats (Fig. 1) but developed higher heart weights (1166 t 14 mg vs. 994 k 17 mg, mean -+ SE). Accordingly, at death the relative heart weight was higher in T rats
I 45
L 0
1NSULlN 70 pmot
/I
50
WEIGHT gram
.
c .
-1
\ \
--
- - Trained
------I
rats
Untsaincd
I
90
.
30
rats 10 0 500 LIVERGLYCOGEN mmol
I kg 300
I-tours of daily swimming I I 45
I 0 Duration
2
4 Duration
of
1 90
swimming
min
2. Concentrations at rest and during exercise of glucose in blood, of glucagon and insulin in plasma, and of glycogen in the liver, in trained and untrained rats (7 pm01 1-l of insulin equals 1 PU nP; glycogen concentrations were expressed as millimoles of glucose ner kilogram of wet tissue). FIG.
6 8 10 of training weeks
l
mu. 1. Growth curves schedule is inserted.
of trained
and
untrained
rats.
Training
l
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DIMINISHED
HORMONAL
RESPONSES
TO
EXERCISE
AFTER
at rest, differ significantly in T and C rats (Fig. 2, Tables l-3). The hormonal responses to exercise were markedly diminished by training (Fig. 2, Table 1). During exercise, glucagon and insulin concentrations in plasma did not change significantly in T rats, whereas in C rats, glucagon concentrations increased to five times the concentrations at rest while insulin concentrations decreased markedly (Fig. 2). Furthermore, during exercise, epinephrine as well as norepinephrine concentrations in plasma were considerably higher in C rats than in T rats (Table 1). During exercise, liver as well as muscle glycogen concentrations decreased in parallel in T and C rats (Fig. 2, Table 2). In the trained animals, blood glucose concentrations throughout exercise were significantly higher than at rest (Fig. 2). In the untrained animals, glucose concentrations did not increase significantly
955
TRAINING
3. Concentrations in serum (FFA and glycerol), blood (lactate and pyruvate), and plasma (alanine) of substrates in trained and untrained rats at rest and immediately after 45 min and 90 min of swimming TABLE
1
Glycerol, mmolll
0,790 40.209
0.303 20.083
Trained
Rats
Untrained
Rats
Rest Epinephrine
0.46 and 0.68
0.18 and 0.52
(2) Norepinephrine
(21
0.72 and 1.86
0.30 and 0.84
(2)
(3 Exert
Epinephrine
ise
1.53 + 0*40*? 6) 1.46 2 0.30* (5)
Norepinephrine Values are either (Exercise). Number * Values in trained t Any value obtained obtained at rest.
8.73 2 2.43*-f (5) 4.04 -+ 0.61*? (5)
single observations (Rest) or means t SE of observations is shown in parentheses. and untrained rats differ significantly. during exercise is higher than the values
TABLE 2. Muscle glycogen concentrations (mmol glucose/kg wet tissue) in trained and untrained ruts at rest and immediately after 45 and 90 min of swimming Superficial
Rest 45-Min
swim
90-Min
swim
Rest
Vastus
39 t 3 (8) 32 k 2*-f (9) 23 k 2*$ (9)
33 k 2
(8) 45-Min
swim
26 2 2V
90-Min
swim
21 5 3*$ (9)
(81
Deep Vastus
Soleus
Trained rats 32 + 2t (8) 21 + 2*:t (9) 16 t 2*t (9)
16 + 2* (9 12 ?I 1* (9)
Untrained rats 23 iz 2t
18 + 1
(8) 13 iz 2*‘t
20 2 1
(8)
(8) 16 k 2
(8)
(8)
9 2 1*t (9)
14 -4 1* (9)
Values are means -+ SE. Number of observations is shown in parentheses, * Values are significantly different from resting values. t Values at rest or during exercise differ significantly between trained and untrained rats. $ Values after 90 min of swimming differ significantly from values after 45 min of swimming.
Lactate, mmolll
Trained Rest
(8) 45Min
swim
goMin swim 3
ruts 2.06 20.14
(8)
(8)
0.850 -+o.o57t (9) 1.461 +O.l20*t$ (9)
0.505 *0.044*$ (9)
0,380 kO.020
2.37
0.410 20.086
0.236 20.035
(9)
+0.28t (9)
3.06 kO.43 19)
Untrained
Rest
(8) TABLE 1. Plasma concentrations of epinephrine and norepinephrine (nglml) in trained and untrained rats at rest and immediately ufter 90 min swimming
1
FFA, meqll
45-Min
swim
90-Min
swim
0.409 k0.063-l (7) 0.593 -+0.073t (9)
(8) 0.452 *0.044* (7) 0.429 +0.031* (9)
I
Pyruvate, pmolll
117 t8t
(8)
I
Alanine, pmolll
491 213
(81
120 St (9) 117 214 (9)
239 +13*'t (9) 255 226*t (9)
141 +7t
460 +21 (7) 894 *6O*t (71 613 2807 (9)
ra
2.49 20.23 (8) 9.04 c2.41v
(8) 6.07 *1.51* (9)
(8) 158 e16t (7) 152 213 (9)
Values are means 2 SE. Number of observations is shown in parentheses. * Values are significantly different from resting values. t Values at rest or during exercise differ significantly between trained and untrained rats. $ Values after 90 min of swimming differ significantly from values after 45 min of swimming.
but decreased during the last 45 min of exercise. During the last 45 min of exercise, liver glycogen concentrations in these rats reached very low levels and the rate of decline of liver glycogen concentrations tended to diminish (P < OJ) (Fig. 2). In T (P < 0.05) as well as in C (P < 0,l) rats, the rate of decline of glycogen concentrations in the deep vastus muscle diminished in the last 45 min of exercise (Table 2). Glycerol concentrations in serum increased during exercise in T as well as in C rats, whereas FFA increased significantly only in T rats (Table 3). Blood pyruvate concentrations did not change during exercise and thus remained higher in C than in T rats (Table 3). During exercise plasma alanine and blood lactate concentrations increased in C rats but declined and were unchanged, respectively, in T rats (Table 3). Glucagon correlated with glucose when glucose concentrations in C rats had decreased (90 min: r = -0.75, P < 0.05), whereas insulin correlated with glucose when glucose concentrations in T rats had increased (45 min: r = 0.81, P < 0.01; 90 min: r = 0.88, P < 0.01). Glucagon correlated with plasma alanine concentrations after 45 min of exercise in C rats (r = 0.77, P < 0.05), but neither glucagon nor insulin correlated significantly with FFA, glycerol, lactate, or pyruvate. During exercise, glucagon correlated with epinephrine (r = 0.88, P < 0.05, n = 5) as well as with norepinephrine concentrations (r = 0.86, P < 0.1, n = 5) in C rats and with epinephrine concentrations in T rats (r = 0.93, P < 0.05, n = 5)* In C rats, norepinephrine concentrations correlated with insulin concentrations (r = -0.85, P < 0.1, n = 5), and norepinephrine (r = -0.83, P < 0.1) as
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956 well as epinephrine (r = -0.81, P < 0.1) correlated with glucose concentrations. After 90 min of swimming, in C rats the glycogen concentrations in the superficial vastus muscle correlated positively with blood glucose concentrations (r = 0.86, P < O.Ol), whereas glycogen concentrations in each of the investigated muscle groups correlated negatively (P < 0.05 or < 0.1) with glycerol concentrations in serum. Lactate correlated with pyruvate (90 min: r = 0.85, P < 0.01) and alanine (90 min: r = 0.78, P < 0.05) in T rats, and with alanine (45 min: r = 0.80, P < 0.05; 90 min: r = 0.86, P < 0.01) in C rats, DISCUSSION
In the present study trained rats in contrast to untrained rats maintained high blood glucose concentrations throughout exercise (Fig. 2). Simultaneously the hormonal responses to exercise were markedly diminished in trained rats (Fig. 2, Table 1). The training-induced reduction of plasma epinephrine concentrations was more pronounced than the reduction of norepinephrine concentrations (Table 1). This observation agrees with the previous finding of a selective enhancement of the epinephrine response to exercise provoked by a decrease in plasma glucose concentrations (10). The lower norepinephrine concentrations during exercise after physical training are most likely due to a diminished sympathetic drive accounted for by traininginduced changes in the muscles involved (9, 24, 27). Ordinarily the catecholamine concentrations in plasma increase with increasing work load. In the present study, however, differences in metabolic rate between trained and untrained exercising rats could hardly explain the lower plasma catecholamine concentrations in the trained rats. Thus, although a relatively higher rats more fat content probably made the untrained buoyant (21), the trained and untrained rats were Furthermore, from loaded with the same tail weights. observation of the rats during the exercise test it appeared that trained rats swam more vigorously than untrained rats. Finally, at identical work loads glycogen concentrations in skeletal muscle and liver might be expected to decrease more slowly in trained animals (3, 17, 23, 24). In the present experiments, however, even if the presumably larger liver and muscle mass in the heavier, untrained animals (21) are considered, the rate of depletion of glycogen was not lower in trained rats, indicating that during exercise these animals in fact had a higher metabolic rate in absolute terms than the untra i&d anima 1s but probably genera ted a bout the same percentage of max imum metabolic rate (23) . In the present experiments, plasma glucagon and insulin concentrations changed markedly during exercise in untrained rats but not at all in trained rats (Fig. 2). It might be argued that this difference was due to an altered reactivity in the heavier and possibly obese untrained rats, rather than to an effect of training (14). However, the untrained rats were hardly obese since they did not put on weight to excess (21) and at rest had plasma insulin and glucose concentrations similar to those previously found in smaller rats (12).
GALBO,
RICHTER,
HOLST,
AND
CHRISTENSEN
Furthermore, in rats swimming with a similar load but with a lower body weight (250 g) than that of the trained rats in the present study (Fig. l), glucagon concentrations have previously been shown to increase and insulin concentrations to decrease (12, 13). The differences i .n plasma gluca .gon and insulin concentrations between trained and untrained rats during exercise most likely reflected differences in secretion rates, and not differences in clearance due to differences trained rats in blood flow between and untrained through the liver and ki dney, the organs which predominantly eliminate insulin and glucagon. This is so since the reduction during exercise of splanchnic and renal blood flow depends on the relative work load (9), which, as pointed out, probably did not differ much in trained and untrained rats, and since physical training changed the pl .asma concen tra .tions of glucagon and insulin in opposi .te directions CFi .g. 2). In untrained rats, during the first 45 min of swimming, insulin concentrations declined markedly in the face of unchanged blood glucose concentrations (Fig. 2). This is in accordance with previous findings (12) and with the concept that insulin secretion is inhibited by adrenergic activity during prolonged exercise (6). The lack of decrease in plasma insulin levels during exercise after training (Fig. 2) is most likely bound up with a diminished inhibition of the pancreatic B cells in consequence of the smaller sympathetic discharge in trained rats (Table 1). Also, in trained exercising rats, insulin secretion must have been inhibited since in these rats glucose concentrations increased without a concomitant increase in insulin concentrations (Fig. 2). Hypoglycemia is the single most potent A cell stimulus, and low insulin concentrations too may enhance the lack o f glucaglucagon secret ion (28). Accordingly, gon response to exercise in trained rats (Fig. 2) may be mainly a consequence of the higher glu case and insulin concentrations in these rats compared with un trained rats. However, glucagon secretion has been reported to be depressed by FFA and to be promoted by catecholamines, lactate, and alanine (18, 19, 28). So the higher plasma concentrations of FFA (Table 3) and the lower concentrations of catecholamines (Table l), lactate, and alanine (Table 3) in trained than in untrained rats may partly explain the different glucagon response to exercise in these two groups of rats. That decreases in glucose and insulin concentrations in plasma cannot completely account for the glucagon response to prolonged exercise in rats appears from the fmding of unchanged glucagon concentra tions during the last 45 min of exercise in untrained rats despite declining glucose and insulin concentrations (Fig. 2) During this period the secretion of glucagon w as ha rdi .y maximum since higher glucagon concentrations in exercising rats of the same strain have been reported fro m our laboraincrease in gl ucagon tory (13). The lack of further secretion was possibly due to the changes in FFA, lactate, and alanine concentrations (Table 3) that took place at the same time as glucose concentrations decreased. The influence of training on the catecholamine secretion during exercise is a subject of controversy. After
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DIMINISHED
HORMONAL
RESPONSES
TO
EXERCISE
AFTER
physical training the urinary excretion rates of epinephrine and norepinephrine connected with exercise have previously been found to be increased (26), decreased (20), and to be unaltered (7) compared to those of untrained subjects. However urinary excretion rates of these catecholamines probably only poorly reflect changes in their secretion rates. Apart from dificulties inherent in the sampling procedure, this is so because only a minor part of the secreted catecholamines is recovered in the urine and because the recovery furthermore depends upon kidney function (flow) and the extent of catecholamine removal elsewhere in the body. That catecholamine excretion data should be interpreted with caution appears from the previous finding that catecholamine excretion was less during maximum exercise than at rest (26). In the rat the plasma catecholamine response to exhausting exercise has been found not to change after training (22). Recently, however, physical training has been reported to decrease the turnover of norepinephrine in rat hearts during exercise (20) and to decrease the plasma catecholamine as well as the insulin response to prolonged exercise in man (16). Correspondingly, when well trained racing cyclists were compared with untrained subjects at work loads representing the same percentage of their maximum capacity, the changes in catecholamine and insulin concentrations during exercise were smaller in the racing cyclists (5). Also in agreement with the present study, during exercise the racing cyclists had higher blood concentrations of glucose and FFA and lower concentrations of lactate, pyruvate, and alanine than the untrained subjects. However, in the cited study (5), glucagon concentrations never rose during exercise but a slight fall was observed only in trained subjects. The capacity to oxidize fatty acids increases in trained muscles (15). Hereby an increased rate of FFA oxidation is made possible at a given metabolic rate and plasma FFA concentration (15). This, together with the higher FFA concentrations in trained compared with untrained rats in the present study (Table 3), suggests a higher turnover of FFA in the trained rats. Considering the blunted hormonal response to exercise in trained rats (Fig. 2, Table 1) it appears that the pancreatic hormones do not account for the increased lipolysis during exercise in trained rats. However, although diminished, the catecholamine response to exercise in these rats may explain the increased lipolysis, because training has been shown to increase the potential of adipose tissue cells to release FFA in response to epinephrine stimulation (2). The negative correlations in the present study between serum glycerol and muscle glycogen concentrations indicate the existence of a negative feedback between the state of function of the working muscles and adipose tissue lipolysis. The increased sensitivity of lipolysis to catecholamines in trained rats (2) allows a h’rg h er lipolysis at a certain muscular glycogen and plasma catecholamine level in these rats compared with untrained rats (Table 1-3) and thereby improves the utilization of the fat depots during exercise. Estimated from the liver glycogen content of 7.5-h fasted rats (Fig. 2) and from knowledge of the liver
957
TRAINING
glycogen content of fed rats (12), the rate of hepatic glycogen depletion was very small in the resting rats in the present study. During exercise the decrease in hepatic glycogen content (Fig. 2) corresponded to a release of energy (5.8 kcal kg-l of body weight h-l), larger than the generally assumed overall metabolic rate in resting rats (about 3.4 kcal kg-l h-l). According to these considerations the rate of hepatic glycogen depletion increased markedly in response to exercise. In the trained rats the increased rate of hepatic glycogen depletion during exercise occurred in the absence of changes in plasma glucagon and insulin levels (Fig. 2). Accordingly such changes are not always essential for increases in hepatic glucose production during exercise, a fact which is in keeping with the previous finding of a normal increase in glucose production in exercising, depancreatized, insulin-infused dogs (29). The existence in trained rats of a markedly increased hepatic glycogen breakdown at the same time as blood glucose concentrations were markedly higher than at rest2 (Fig. 2) is more reasonable than it would be in man. This is so since the rat, due to a large liver (4.3% of body weight vs. 2.1% in man), has a relatively larger hepatic glycogen store, and furthermore has a relatively smaller, obligatory glycolytic brain (0.6% of body weight vs. 1.9% in man) and lower muscle glycogen concentrations (17, 23, 24) than man. In conclusion the present study has demonstrated that in rats, as in man (4), physical training decreases plasma insulin concentrations at rest (Fig. 2). We have furthermore confirmed the finding (3) that in the rat physical training markedly increases glycogen concentrations in the liver and to a smaller extent in the muscles. The major finding, however, is that the improved ability after physical training to avoid a decline in blood glucose concentration is accompanied by a marked diminution of the hormonal responses to exercise, a fact that supports the concept that the hormonal response to exercise is influenced by glucose-sensitive receptors (10). And finally, in the trained rat the sensitivity of adipose tissue to catecholamines probably is increased, and changes in the plasma concentrations of glucagon and insulin are not necessary for an increase in the rates of lipolysis and hepatic glycogen breakdown during exercise. l
l
l
Lisbeth Kall, Vibeke Ulrik, and Rikke GrBnholt performed skilled technical assistance. The investigation was supported by grants from the NOVO Research Foundation, Idraettens Forskningsrad, Forskningsfonden for Storkobenh&vn, Faeroerne og Grsnland, and the J. Weimann Foundation. Received
for publication
21 April
1977,
2 Blood for glucose measurements was drawn within 2 min after the end of exercise, Even if the highly unlikely assumptions are made (30) that hepatic glucose production proceeds unchanged and muscular glucose metabolism ceases immediately at the end of exercise, the estimated increase of blood glucose concentrations during these 2 min is smaller than the observed difference between glucose concentrations in resting and exercised animals. Neither can this difference be explained by hepatic glycogenolysis caused by ether anesthesia since such an effect probably was more pronounced in resting rats having higher hepatic glycogen concentrations than exercised rats.
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GALBO,
958
RICHTER,
HOLST,
AND
CHRISTENSEN
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28. 29.
30.
AND P. T. RICKETTS. Multiple hormonal responses to prolonged exercise in relation to physical training, J. Appl. Physiol. 33: 607-610, 1972. KARLSSON, J., L.-O. NORDESJ~, AND B. SALTIN. Muscle glycogen utilization during exercise after physical training. Acta Phystil. Stand. 90: 210-217, 1974. LUYCKX, A. Se’cre’tion de l’lnsuline et du Glucagon, Etude Clinique et Expe’rimentale. Paris: Masson, 1974. LUYCKX, A. S., A. DRESSE, A. CESSION-FOSSI~N, AND P. J. LEFEBVRE, Catecholamines and exercise-induced glucagon and fatty acid mobilization in the rat. Am. J. Physiol, 229: 376-383, 1975. &TMAN, I., N. 0, SJ~STRAND, AND G. SWEDIN. Cardiac noradrenaline turnover and urinary catecholamine excretion in trained and untrained rats during rest and exercise. Acta Physiol. Stand. 86: 299-308, 1972, PITTS, G, C., AND L. S. BULL. Exercise, dietary obesity, and growth in the rat. Am. J. Physiol. 232: R38-R44, 1977. RAVEN, P. B., T. J. CONNERS, AND E, EVONUK. Effects of exercise on plasma lactic dehydrogenase isozymes and catecholamines. J. Apple Physiol. 29: 374-377, 1970. SALTIN, B., AND J. KARLSSON. Muscle glycogen utilization during work of different intensities. In: Muscle Metabolism during Exercise, edited by B. Pernow and B. Saltin. New York-London: Plenum, 1971, p+ 289-299. SALTIN, B., K. NAZAR, D. L. COSTILL, E. STEIN, E. JANSSON, B. ES&N, AND P, D. GOLLNICK. The nature of the training response; peripheral and central adaptations to one-legged exercise. Acta Physiok Stand. 96: 289-305, 1976. SNEDECOR, G. W., AND W. G. COCHRAN. Statistical Methods. Ames, Iowa: Iowa State Univ. Press, 1965, p. 45, 117, 160. TAYLOR, A. W., J. H. SCHOEMAN, A. R. ESFANDIARY, AND J. C+ RUSSEL. Effect of exercise on urinary catecholamine excretion in active and sedentary subjects. Rev. Can, BioZ, 30: 97-105, 1971. TRAP-JENSEN, CT,, N. J. CHRISTENSEN, J. P. CLAUSEN, B. RASMUSSEN, AND K. KLAUSEN. Arterial noradrenaline and circulatory adjustment to strenuous exercise with trained and nontrained muscle groups. In: Physical Fitness (Proceedings of a Satellite Symposium of the XXV International Congress of Physiological Sciences). Prague: Univ. Karlova Press, 1973, p* 414418. UNGER, R, II,, AND L. ORCI. Physiology and pathophysiology of glucagon. Physiol. Rev. 56: 778-826, 1976, VRANIC, M., R. KAWAMORI, S. PEK, N. KOVACEVIC, AND G. A. WRENSHALL. The essentiality of insulin and the role of glucagon in regulating glucose utilization and production during strenuous exercise in dogs. J* Clin. Invest. 57: 245-255, 1976, WAHREN, J., P. FELIG, R, HENDLER, AND G. AHLBORG. Glucose and amino acid metabolism during recovery after exercise. J. Appl. Physiol. 34: 838-845, 1973.
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hormonal rats
responses
to exercise
H. GALBO, E. A. RICHTER, J. J. HOLST, AND N. J. CHRISTENSEN Institute of Medical Physiology B, University of Copenhagen; Department of Clinical Chemistry, Bispebjerg Hospital, Copenhagen; and 2nd Clinic of Internal Medicine, Kommunehospitalet, Aarhus, Denmark
GALBO, H., E. A. RICHTER, J. J. HOLST, AND N. J. CHRISDiminished hormonal responses to exercise in trained ruts. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 43(6): 953-958, 1977.- Male rats (120 g) either were subjected to a 12-wk physical training program (T rats) or were sedentary controls (C rats). Subsequently the rats were killed at rest or after a 45- or 90-min forced swim. At rest, T rats had higher liver and muscle glycogen concentrations but lower plasma insulin. During exercise, blood glucose increased 60% in T rats but decreased 20% in C rats. Plasma glucagon and insulin concentrations did not change in T rats but plasma glucagon increased and insulin decreased markedly in C rats. Plasma epinephrine (90 min: range, 0.78-2.96 ng*ml+, (T) vs. 4.42-15.67 (C)) and norepinephrine (90 min: 0.70-2.22(T) vs. 2.50-6.10 (C)) were lower in T than in C rats. Hepatic glycogen decreased substantially and, as with muscle glycogen, the decrease was parallel in T and C rats. The plasma concentrations of free fatty acids were higher but lactate and alanine lower in T than in C rats. In trained rats the hormonal response to exercise is blunted partly due to higher glucose concentrations. In these rats adipose tissue sensitivity to catecholamines is increased, and changes in glucagon and insulin concentrations are not necessary for increased lipolysis and hepatic glycogen depletion during exercise. TENSEN.
related to the rate of decline of blood glucose concentrations (lo), trained rats ought to show diminished hormonal responses to exercise. These, in turn, might explain the diminished rate of hepatic glycogen depletion during exercise that has been found in trained rats (3). In the present study an exercise test was carried out in trained and untrained rats to evaluate the role of blood glucose concentrations for the regulation of the hormonal response to exercise and furthermore the role of hormones for the hepatic glycogen depletion during exercise. METHODS
Fifty-one male Wistar rats weighing 110-130 g were randomly divided into two groups, both of which were provided with food and water ad libitum. One group (26 rats) was subjected to a 12-wk physical training program consisting of swimming in water maintained at 33-34°C. The trained rats (T rats) swam 5 days/wk. The first day the duration of swimming was 30 min. Then it was gradually increased until after 8 wk of training the rats swam continuously 6 h/day (Fig. 1). The rats in the other group (25 rats) were sedentary epinephrine; norepinephrine; metabolism; liver; muscles; glycontrols (C rats). To avoid stress reactions during the cogen;lipid mobilization; fatty acids; glucose final exercise test due to handling and immersion in water, C rats were subjected to 15 min of swimming fop 4 days the week before the test (19). Sixty-four hours after the rats had been trained for DURING PROLONGED EXERCISE the secretion ofglucagon and catecholamines increases while the secretion of the last time (T rats) or accustomed to handling (C insulin decreases (5, 6, 10-13, 16, 19, 22, 28, 29). These rats), they participated in an exercise test. The test was carried out in the morning, and the rats fasted 6 h changes in hormone secretion should favor hepatic in advance. Nine T rats and nine C rats were forced to glucose production, and the administration of glucagon antibodies to exercising rats has in fact been shown to swim1 for 90 min, and nine T rats and eight C rats diminish hepatic glycogen depletion (12). During exer- were forced to swim1 for 45 min in water maintained at cise the decrease in insulin secretion is probably essen- 33-34OC. Eight T rats and eight C rats remained resting tially due to the stimulation by catecholamines of B- for 90 min. At the end of exercise or rest period the animals were immediately anesthetized with ether, and cell-inhibiting receptors (6, 11). The secretion of gluca8 ml blood were drawn by cardiac puncture. Then gon and epinephrine appears to be significantly influsamples of the liver, of the superficial and deep parts of enced by glucose-sensitive receptors since maintenance of euglycemia by glucose infusion diminishes the rates the vastus lateralis muscle, and of the soleus muscle --of secretion of these hormones during exercise (10). l On the day of the exercise test the randomly selected rats in Endurance training of rats has been shown to result in each of the groups of T and C rats were ranked in regard to body a slower decrease of blood glucose concentrations during weight. T rats were loaded with a tail weight of lead amounting to prolonged exercise (3). Thus, according to the concept 4% of body weight, and C rats were loaded with the same weights that the hormonal responses to exercise are closely as T rats with identical rank. Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.081.226.078) on January 18, 2019.
954
GALBO,
were quickly removed and immediately frozen in liquid nitrogen. The heart was removed too and was weighed after it had been opened, rinsed, and blotted. Epinephrine and norepinephrine in the first drawn 100 ~1 of plasma were determined by a previously published radioenzymatic assay (8) modified for measurement of very small amounts. Catecholamine determinations were carried out only in two T and two C rats at rest and in five T and five C rats afier 90 min of exercise. Insulin was determined by a radioimmunoassay relying on charcoal separation (1). The detection limit for the assay system was 0.30 pmol l-l, and the within-assay coefficient of variation (n = 20) was 3% at 46 pmol 1-l. The methods of sampling and analysis of blood and tissue for glucagon, glucose, pyruvate, alanine, lactate, FFA (free fatty acids), glycerol, and glycogen have been reported elsewhere (12, 13). All analyses of a hormone were carried out in a single assay run. Glycogen concentrations measured in samples from six different lobes within the same liver were practically identical (296, 299, 299, 307, 319, 328 mmol 1-l). However, in the present study the sites of tissue sampling were always the same. Statistical evaluation of the data was made by means of correlation analysis and by means of t-test and the nonparametric rank sum test (25). Differences were considered to be significant if P values of less than 0.05 were obtained with both these tests.
RICHTER,
HOLST,
AND
CHRISTENSEN
(0.37 t 0.02%, mean t SD) than in C rats (0.26 t 0.02%). Training significantly decreased plasma insulin (Fig. 2) and blood pyruvate concentrations (Table 3) at rest and increased glycogen concentrations in the liver (35% (14-56%), mean and 95% confidence limits) (Fig. 2) and in the deep part of the vastus lateralis muscle (39% (B-60%)) (Table 2). The other variables did not, Gum
SE /t
mmol
- - - Trained - - --- Untrained
lO-
fats rats
l
8-
l
6-
t 0
I
t
45
90
GLUCAGON
500 -
l
pmol
/I 400 -
3uoc
i
200 r roo-
RESULTS
uL
T rats increased their body weight less than C rats (Fig. 1) but developed higher heart weights (1166 t 14 mg vs. 994 k 17 mg, mean -+ SE). Accordingly, at death the relative heart weight was higher in T rats
I 45
L 0
1NSULlN 70 pmot
/I
50
WEIGHT gram
.
c .
-1
\ \
--
- - Trained
------I
rats
Untsaincd
I
90
.
30
rats 10 0 500 LIVERGLYCOGEN mmol
I kg 300
I-tours of daily swimming I I 45
I 0 Duration
2
4 Duration
of
1 90
swimming
min
2. Concentrations at rest and during exercise of glucose in blood, of glucagon and insulin in plasma, and of glycogen in the liver, in trained and untrained rats (7 pm01 1-l of insulin equals 1 PU nP; glycogen concentrations were expressed as millimoles of glucose ner kilogram of wet tissue). FIG.
6 8 10 of training weeks
l
mu. 1. Growth curves schedule is inserted.
of trained
and
untrained
rats.
Training
l
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DIMINISHED
HORMONAL
RESPONSES
TO
EXERCISE
AFTER
at rest, differ significantly in T and C rats (Fig. 2, Tables l-3). The hormonal responses to exercise were markedly diminished by training (Fig. 2, Table 1). During exercise, glucagon and insulin concentrations in plasma did not change significantly in T rats, whereas in C rats, glucagon concentrations increased to five times the concentrations at rest while insulin concentrations decreased markedly (Fig. 2). Furthermore, during exercise, epinephrine as well as norepinephrine concentrations in plasma were considerably higher in C rats than in T rats (Table 1). During exercise, liver as well as muscle glycogen concentrations decreased in parallel in T and C rats (Fig. 2, Table 2). In the trained animals, blood glucose concentrations throughout exercise were significantly higher than at rest (Fig. 2). In the untrained animals, glucose concentrations did not increase significantly
955
TRAINING
3. Concentrations in serum (FFA and glycerol), blood (lactate and pyruvate), and plasma (alanine) of substrates in trained and untrained rats at rest and immediately after 45 min and 90 min of swimming TABLE
1
Glycerol, mmolll
0,790 40.209
0.303 20.083
Trained
Rats
Untrained
Rats
Rest Epinephrine
0.46 and 0.68
0.18 and 0.52
(2) Norepinephrine
(21
0.72 and 1.86
0.30 and 0.84
(2)
(3 Exert
Epinephrine
ise
1.53 + 0*40*? 6) 1.46 2 0.30* (5)
Norepinephrine Values are either (Exercise). Number * Values in trained t Any value obtained obtained at rest.
8.73 2 2.43*-f (5) 4.04 -+ 0.61*? (5)
single observations (Rest) or means t SE of observations is shown in parentheses. and untrained rats differ significantly. during exercise is higher than the values
TABLE 2. Muscle glycogen concentrations (mmol glucose/kg wet tissue) in trained and untrained ruts at rest and immediately after 45 and 90 min of swimming Superficial
Rest 45-Min
swim
90-Min
swim
Rest
Vastus
39 t 3 (8) 32 k 2*-f (9) 23 k 2*$ (9)
33 k 2
(8) 45-Min
swim
26 2 2V
90-Min
swim
21 5 3*$ (9)
(81
Deep Vastus
Soleus
Trained rats 32 + 2t (8) 21 + 2*:t (9) 16 t 2*t (9)
16 + 2* (9 12 ?I 1* (9)
Untrained rats 23 iz 2t
18 + 1
(8) 13 iz 2*‘t
20 2 1
(8)
(8) 16 k 2
(8)
(8)
9 2 1*t (9)
14 -4 1* (9)
Values are means -+ SE. Number of observations is shown in parentheses, * Values are significantly different from resting values. t Values at rest or during exercise differ significantly between trained and untrained rats. $ Values after 90 min of swimming differ significantly from values after 45 min of swimming.
Lactate, mmolll
Trained Rest
(8) 45Min
swim
goMin swim 3
ruts 2.06 20.14
(8)
(8)
0.850 -+o.o57t (9) 1.461 +O.l20*t$ (9)
0.505 *0.044*$ (9)
0,380 kO.020
2.37
0.410 20.086
0.236 20.035
(9)
+0.28t (9)
3.06 kO.43 19)
Untrained
Rest
(8) TABLE 1. Plasma concentrations of epinephrine and norepinephrine (nglml) in trained and untrained rats at rest and immediately ufter 90 min swimming
1
FFA, meqll
45-Min
swim
90-Min
swim
0.409 k0.063-l (7) 0.593 -+0.073t (9)
(8) 0.452 *0.044* (7) 0.429 +0.031* (9)
I
Pyruvate, pmolll
117 t8t
(8)
I
Alanine, pmolll
491 213
(81
120 St (9) 117 214 (9)
239 +13*'t (9) 255 226*t (9)
141 +7t
460 +21 (7) 894 *6O*t (71 613 2807 (9)
ra
2.49 20.23 (8) 9.04 c2.41v
(8) 6.07 *1.51* (9)
(8) 158 e16t (7) 152 213 (9)
Values are means 2 SE. Number of observations is shown in parentheses. * Values are significantly different from resting values. t Values at rest or during exercise differ significantly between trained and untrained rats. $ Values after 90 min of swimming differ significantly from values after 45 min of swimming.
but decreased during the last 45 min of exercise. During the last 45 min of exercise, liver glycogen concentrations in these rats reached very low levels and the rate of decline of liver glycogen concentrations tended to diminish (P < OJ) (Fig. 2). In T (P < 0.05) as well as in C (P < 0,l) rats, the rate of decline of glycogen concentrations in the deep vastus muscle diminished in the last 45 min of exercise (Table 2). Glycerol concentrations in serum increased during exercise in T as well as in C rats, whereas FFA increased significantly only in T rats (Table 3). Blood pyruvate concentrations did not change during exercise and thus remained higher in C than in T rats (Table 3). During exercise plasma alanine and blood lactate concentrations increased in C rats but declined and were unchanged, respectively, in T rats (Table 3). Glucagon correlated with glucose when glucose concentrations in C rats had decreased (90 min: r = -0.75, P < 0.05), whereas insulin correlated with glucose when glucose concentrations in T rats had increased (45 min: r = 0.81, P < 0.01; 90 min: r = 0.88, P < 0.01). Glucagon correlated with plasma alanine concentrations after 45 min of exercise in C rats (r = 0.77, P < 0.05), but neither glucagon nor insulin correlated significantly with FFA, glycerol, lactate, or pyruvate. During exercise, glucagon correlated with epinephrine (r = 0.88, P < 0.05, n = 5) as well as with norepinephrine concentrations (r = 0.86, P < 0.1, n = 5) in C rats and with epinephrine concentrations in T rats (r = 0.93, P < 0.05, n = 5)* In C rats, norepinephrine concentrations correlated with insulin concentrations (r = -0.85, P < 0.1, n = 5), and norepinephrine (r = -0.83, P < 0.1) as
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956 well as epinephrine (r = -0.81, P < 0.1) correlated with glucose concentrations. After 90 min of swimming, in C rats the glycogen concentrations in the superficial vastus muscle correlated positively with blood glucose concentrations (r = 0.86, P < O.Ol), whereas glycogen concentrations in each of the investigated muscle groups correlated negatively (P < 0.05 or < 0.1) with glycerol concentrations in serum. Lactate correlated with pyruvate (90 min: r = 0.85, P < 0.01) and alanine (90 min: r = 0.78, P < 0.05) in T rats, and with alanine (45 min: r = 0.80, P < 0.05; 90 min: r = 0.86, P < 0.01) in C rats, DISCUSSION
In the present study trained rats in contrast to untrained rats maintained high blood glucose concentrations throughout exercise (Fig. 2). Simultaneously the hormonal responses to exercise were markedly diminished in trained rats (Fig. 2, Table 1). The training-induced reduction of plasma epinephrine concentrations was more pronounced than the reduction of norepinephrine concentrations (Table 1). This observation agrees with the previous finding of a selective enhancement of the epinephrine response to exercise provoked by a decrease in plasma glucose concentrations (10). The lower norepinephrine concentrations during exercise after physical training are most likely due to a diminished sympathetic drive accounted for by traininginduced changes in the muscles involved (9, 24, 27). Ordinarily the catecholamine concentrations in plasma increase with increasing work load. In the present study, however, differences in metabolic rate between trained and untrained exercising rats could hardly explain the lower plasma catecholamine concentrations in the trained rats. Thus, although a relatively higher rats more fat content probably made the untrained buoyant (21), the trained and untrained rats were Furthermore, from loaded with the same tail weights. observation of the rats during the exercise test it appeared that trained rats swam more vigorously than untrained rats. Finally, at identical work loads glycogen concentrations in skeletal muscle and liver might be expected to decrease more slowly in trained animals (3, 17, 23, 24). In the present experiments, however, even if the presumably larger liver and muscle mass in the heavier, untrained animals (21) are considered, the rate of depletion of glycogen was not lower in trained rats, indicating that during exercise these animals in fact had a higher metabolic rate in absolute terms than the untra i&d anima 1s but probably genera ted a bout the same percentage of max imum metabolic rate (23) . In the present experiments, plasma glucagon and insulin concentrations changed markedly during exercise in untrained rats but not at all in trained rats (Fig. 2). It might be argued that this difference was due to an altered reactivity in the heavier and possibly obese untrained rats, rather than to an effect of training (14). However, the untrained rats were hardly obese since they did not put on weight to excess (21) and at rest had plasma insulin and glucose concentrations similar to those previously found in smaller rats (12).
GALBO,
RICHTER,
HOLST,
AND
CHRISTENSEN
Furthermore, in rats swimming with a similar load but with a lower body weight (250 g) than that of the trained rats in the present study (Fig. l), glucagon concentrations have previously been shown to increase and insulin concentrations to decrease (12, 13). The differences i .n plasma gluca .gon and insulin concentrations between trained and untrained rats during exercise most likely reflected differences in secretion rates, and not differences in clearance due to differences trained rats in blood flow between and untrained through the liver and ki dney, the organs which predominantly eliminate insulin and glucagon. This is so since the reduction during exercise of splanchnic and renal blood flow depends on the relative work load (9), which, as pointed out, probably did not differ much in trained and untrained rats, and since physical training changed the pl .asma concen tra .tions of glucagon and insulin in opposi .te directions CFi .g. 2). In untrained rats, during the first 45 min of swimming, insulin concentrations declined markedly in the face of unchanged blood glucose concentrations (Fig. 2). This is in accordance with previous findings (12) and with the concept that insulin secretion is inhibited by adrenergic activity during prolonged exercise (6). The lack of decrease in plasma insulin levels during exercise after training (Fig. 2) is most likely bound up with a diminished inhibition of the pancreatic B cells in consequence of the smaller sympathetic discharge in trained rats (Table 1). Also, in trained exercising rats, insulin secretion must have been inhibited since in these rats glucose concentrations increased without a concomitant increase in insulin concentrations (Fig. 2). Hypoglycemia is the single most potent A cell stimulus, and low insulin concentrations too may enhance the lack o f glucaglucagon secret ion (28). Accordingly, gon response to exercise in trained rats (Fig. 2) may be mainly a consequence of the higher glu case and insulin concentrations in these rats compared with un trained rats. However, glucagon secretion has been reported to be depressed by FFA and to be promoted by catecholamines, lactate, and alanine (18, 19, 28). So the higher plasma concentrations of FFA (Table 3) and the lower concentrations of catecholamines (Table l), lactate, and alanine (Table 3) in trained than in untrained rats may partly explain the different glucagon response to exercise in these two groups of rats. That decreases in glucose and insulin concentrations in plasma cannot completely account for the glucagon response to prolonged exercise in rats appears from the fmding of unchanged glucagon concentra tions during the last 45 min of exercise in untrained rats despite declining glucose and insulin concentrations (Fig. 2) During this period the secretion of glucagon w as ha rdi .y maximum since higher glucagon concentrations in exercising rats of the same strain have been reported fro m our laboraincrease in gl ucagon tory (13). The lack of further secretion was possibly due to the changes in FFA, lactate, and alanine concentrations (Table 3) that took place at the same time as glucose concentrations decreased. The influence of training on the catecholamine secretion during exercise is a subject of controversy. After
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DIMINISHED
HORMONAL
RESPONSES
TO
EXERCISE
AFTER
physical training the urinary excretion rates of epinephrine and norepinephrine connected with exercise have previously been found to be increased (26), decreased (20), and to be unaltered (7) compared to those of untrained subjects. However urinary excretion rates of these catecholamines probably only poorly reflect changes in their secretion rates. Apart from dificulties inherent in the sampling procedure, this is so because only a minor part of the secreted catecholamines is recovered in the urine and because the recovery furthermore depends upon kidney function (flow) and the extent of catecholamine removal elsewhere in the body. That catecholamine excretion data should be interpreted with caution appears from the previous finding that catecholamine excretion was less during maximum exercise than at rest (26). In the rat the plasma catecholamine response to exhausting exercise has been found not to change after training (22). Recently, however, physical training has been reported to decrease the turnover of norepinephrine in rat hearts during exercise (20) and to decrease the plasma catecholamine as well as the insulin response to prolonged exercise in man (16). Correspondingly, when well trained racing cyclists were compared with untrained subjects at work loads representing the same percentage of their maximum capacity, the changes in catecholamine and insulin concentrations during exercise were smaller in the racing cyclists (5). Also in agreement with the present study, during exercise the racing cyclists had higher blood concentrations of glucose and FFA and lower concentrations of lactate, pyruvate, and alanine than the untrained subjects. However, in the cited study (5), glucagon concentrations never rose during exercise but a slight fall was observed only in trained subjects. The capacity to oxidize fatty acids increases in trained muscles (15). Hereby an increased rate of FFA oxidation is made possible at a given metabolic rate and plasma FFA concentration (15). This, together with the higher FFA concentrations in trained compared with untrained rats in the present study (Table 3), suggests a higher turnover of FFA in the trained rats. Considering the blunted hormonal response to exercise in trained rats (Fig. 2, Table 1) it appears that the pancreatic hormones do not account for the increased lipolysis during exercise in trained rats. However, although diminished, the catecholamine response to exercise in these rats may explain the increased lipolysis, because training has been shown to increase the potential of adipose tissue cells to release FFA in response to epinephrine stimulation (2). The negative correlations in the present study between serum glycerol and muscle glycogen concentrations indicate the existence of a negative feedback between the state of function of the working muscles and adipose tissue lipolysis. The increased sensitivity of lipolysis to catecholamines in trained rats (2) allows a h’rg h er lipolysis at a certain muscular glycogen and plasma catecholamine level in these rats compared with untrained rats (Table 1-3) and thereby improves the utilization of the fat depots during exercise. Estimated from the liver glycogen content of 7.5-h fasted rats (Fig. 2) and from knowledge of the liver
957
TRAINING
glycogen content of fed rats (12), the rate of hepatic glycogen depletion was very small in the resting rats in the present study. During exercise the decrease in hepatic glycogen content (Fig. 2) corresponded to a release of energy (5.8 kcal kg-l of body weight h-l), larger than the generally assumed overall metabolic rate in resting rats (about 3.4 kcal kg-l h-l). According to these considerations the rate of hepatic glycogen depletion increased markedly in response to exercise. In the trained rats the increased rate of hepatic glycogen depletion during exercise occurred in the absence of changes in plasma glucagon and insulin levels (Fig. 2). Accordingly such changes are not always essential for increases in hepatic glucose production during exercise, a fact which is in keeping with the previous finding of a normal increase in glucose production in exercising, depancreatized, insulin-infused dogs (29). The existence in trained rats of a markedly increased hepatic glycogen breakdown at the same time as blood glucose concentrations were markedly higher than at rest2 (Fig. 2) is more reasonable than it would be in man. This is so since the rat, due to a large liver (4.3% of body weight vs. 2.1% in man), has a relatively larger hepatic glycogen store, and furthermore has a relatively smaller, obligatory glycolytic brain (0.6% of body weight vs. 1.9% in man) and lower muscle glycogen concentrations (17, 23, 24) than man. In conclusion the present study has demonstrated that in rats, as in man (4), physical training decreases plasma insulin concentrations at rest (Fig. 2). We have furthermore confirmed the finding (3) that in the rat physical training markedly increases glycogen concentrations in the liver and to a smaller extent in the muscles. The major finding, however, is that the improved ability after physical training to avoid a decline in blood glucose concentration is accompanied by a marked diminution of the hormonal responses to exercise, a fact that supports the concept that the hormonal response to exercise is influenced by glucose-sensitive receptors (10). And finally, in the trained rat the sensitivity of adipose tissue to catecholamines probably is increased, and changes in the plasma concentrations of glucagon and insulin are not necessary for an increase in the rates of lipolysis and hepatic glycogen breakdown during exercise. l
l
l
Lisbeth Kall, Vibeke Ulrik, and Rikke GrBnholt performed skilled technical assistance. The investigation was supported by grants from the NOVO Research Foundation, Idraettens Forskningsrad, Forskningsfonden for Storkobenh&vn, Faeroerne og Grsnland, and the J. Weimann Foundation. Received
for publication
21 April
1977,
2 Blood for glucose measurements was drawn within 2 min after the end of exercise, Even if the highly unlikely assumptions are made (30) that hepatic glucose production proceeds unchanged and muscular glucose metabolism ceases immediately at the end of exercise, the estimated increase of blood glucose concentrations during these 2 min is smaller than the observed difference between glucose concentrations in resting and exercised animals. Neither can this difference be explained by hepatic glycogenolysis caused by ether anesthesia since such an effect probably was more pronounced in resting rats having higher hepatic glycogen concentrations than exercised rats.
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GALBO,
958
RICHTER,
HOLST,
AND
CHRISTENSEN
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