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Early adaptations in blood substrates, metabolites, and hormones to prolonged exercise training in man . M. J. GREEN,S. JONES,M. BALL-BURNETT, AND I. FRASEW Department sf Kinesiology, University of Waterloo, Waterloo, Ont., Canada N2L 3GP

Received July 20, 1990 GREEN,H. J., JONES,S., BALL-BURNET'F, M., and FRASER, I. 1991. Early adaptations in blood substrates, metabolites, and hormones to prolonged exercise training in man. Can. J. Physiol. Pharmacol. 69: 1222- 1229. This study was designed to investigate the effect of short-term, submaximal training on changes in blood substrates, metaboBites, and hormonal concentrations during prolonged exercise at the-same power output. Cycle training was performed daily by eight male subjects (vo2max = 53.0 f 2.0 mL - kg-' min-', X f SE) for 10- 12 days with each exercise session lasting for 2 h at an average intensity of 59% sf Vo,max. This training protocol resulted in reductions ( p < 0.05) in blood lactate concentration (mM)at 15 min (2.96 f 0.46 vs. 1.43 $ 0.23), 30 rnin (2.92 $- 0.46 vs. 1.70 f 0.22), 60 min (2.96 f 0.53 vs. B .72 f 0.29), and 98 min (2.58 1.3 vs. 1.62 $- 0.23) of exercise. The reduction in blood lactate was also accompanied by lower (p < 0.05) concentrations of both ammonia and uric acid. Similarly, following training lower concentrations ( p c 0.0%)were observed for blood @-hydroxybutyrate(6Qand 90 min) and serum free fatty acids (90 rnin). B l o d glucose (15 and 38 min) and blood glycerol (30 and 6 3 rnin) were higher ( p < 8.85) following training, whereas blood alanine and pymvate were unaffected. For the hormones insulin, glucagon, epinephrine, and norepinephrine, only epinephrine and norepinephrine were altered with training. For both of the catecholamines, the exercise-induced increase was blunted ( p < 0.05) at both 60 and 98 min. As indicated by the changes in blood lactate, ammonia, and uric acid, a depression in glycoiysis and IMP formation is suggested as an early adaptive response to prolonged submaximal exercise training. Key work: exercise, training, blood metabolites, substrates, hormones.



GREEN, H. 9., JONES, S., BALL-BUWNET'F, M., et FRASER, I. 1991. Early adaptations in blood substrates, metabolites, and hormones to prolonged exercise training in man. Can. 9. Physiol. Pharmacol. 69 : 1222- 1229. Cette etude a CtC conCue pour examiner I'effet d'un entrainement submaximal, de courte durke, sur les variations des concentrations d'homones, de m6tabslites et de substrats sanguins durant un exercice prolongt. L'e~traPnementsur bicyclette a CtC effectuC quotidiennement par huit sujets miles (Vo,max + = 53,0 f 2,O mL kg- min-', X f ET), pendant 10- 12 jours; chaque session a slur6 2 h, B une intensit6 moyenne de Vs,max de 59%. Ce protocole d'entrainement a provoquC des r6ductions ( p < Q,05) de la concentration Be lactate sanguin (mM)aprks 15 min (2,96 0,46 vs. 1,73 f 8,23), 38 rnin (2,92 f 0,46 vs. 1,70 f 0,22), 60 min (2,96 f 0,53 vs. B ,72 0,291 et 90 min (2,58 f 1,3 vs. 1,62 f 0,231 d9exercice. La r ~ u c t i o nde lactate sanguin a aussi ttC accompagnCe de plus faibles concentrations ( p < 0,05) d'ammoniaque et d'acide urique. Aprb entrainement, de plus faibles concentrationsde P-hydroxybutyrate sanguin (48 et 90 min) et d'acides gras libres sCriques (90 rnin) ont aussi 6t6 observCes, Les concentrations de glucose sanguin (15 et 30 min) et de glycCrol sanguin (30 et QO min) ont Ct6 plus ClevCes ( p < 0,05) aprks l'entrainernent, alms que celles de pymvate et d'analine sanguins n90nt pas 6t6 rnodifiCes. Pour ce qui des hormones insuline, glucagon, CpinCphrine et norCpinCphrine, seules les deux derni&res ont kt6 alt6rCes avec B'entrainement. Pour ces deux catCcholamines, 19augmentationinduite par 19exercicea 6t6 rauite (p < 0,05) 2t 60 et B 90 rnin. Comme l'ont indiqu6 les variations de lactate sanguin, d'ammoniaque et d9acideurique, une diminution de glycolyse et de formation dq1MBserait possiblement une rCponse adaptatrice rapide h un entrainement submaximal prolongC. MOPSc)& : exereice, entrainement, sang, mttabolites, substrats, hormones. [Traduit par la rCdaction]



Introduction Prolonged exercise of moderate intensity results in a timedependent change in the proportion of different substrates used by the working muscle. The utilization of different substrates at different time intervals has been described as biphasic and is characterized by an initial emphasis on carbohydrate, bath muscle glycogen and blood glucose, followed by a progressive reliance on free fatty acids (Felig and Wahren 1975; Holloszy 1990). The increased free fatty acid utilization by the working muscle with time is believed to occur, in large part, as a result of increased plasma free fatty acid concentration secondary to increased ligolysis of triglycerides in extramuscular storage sites (Galbo 1983; Holloszy 1990). The alteration in substrate selection is accompanied by extensive changes in the concentration of several hormones, which appear to be intimately involved in extramuscular substrate mobilization and utilization. Most notable are the hormones of the sympathetic adrenergic system, epinephrine and norepinephrine, which progressively increase during the exercise and two hormones of the pancreas, glucagon and insulin, which increase and decrease, Printed in Canada I BmgrirnC au Canada


respectively (Galbo 1983;Galbo et al. 1979; Hoelzer ct al. 1986). If the exercise regime is repeated on a regular basis over an extended period of time, free fatty acids become a more dominant substrate (Galbo 1983; Holloszy and Coyle 1984). However, the strategy used to promote increased utilization does not appear to depend on hrther elevations in plasma free fatty acid concentration. Plasma free fatty acids have been found to be lower following training than before training tat the same absolute work intensity and, particularly, Hate in prolonged exercise (Koivisto et wl. 198%;Winder et wl. 1979). As compared with the untrained state, these adaptations are accompanied by an attenuation of the sympathetic response (Winder et ak. 19791, a greater glucagon release (Koivisto et al. 1982), and no further change in insulin (Koivisto et aI. 1982; Winder et al. 1979). These hormonal changes in conjunction with an apparently greater muscle triglyceride utilization during exercise have k e n used to support the hypotheses that training results in an increased dependency on free fatty acids derived from intramuscular Bipolysis (Molloszy 1990).

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However, it is becoming increasingly clear that metabolic control and substrate behaviour is far more complex than previously supposed (Galbs and Kjaer 1987; Vranic et al. 1987) and that redundancy seems to be a characteristic (Hselzer et al. 1986). Evidence has been presented that some of these adaptations occur much earlier than previously believed winder et al. 1978, 1979), and there is some suggestion that there is a dissociation between the hormonal behaviour and the fuel utilization during exercise particularly early in the training. The blood catecholamines, as an example, have been observed to decrease within the first weeks of training (Winder et al. 1978), before increases in free fatty acid utilization and altered muscle metabolic behaviour is believed to have occurred. Indeed we have found pronounced reductions in the levels of both epinephrine and norepinephrine after 3 consecutive days of prolonged exercise (Green et al. 1989) with no training induced effect on the concentration of either blood insulin or glucagon. During exercise following this abbreviated training program we were unable to observe any training influence on the level of plasma free fatty acids (Green et al. 1989). These observations, in conjunction with the fact that the respiratory exchange ratio, 8, was unchanged, led to the speculation that the proportion of carbohydrates and fats oxidized during exercise following training may depend on the development of other adaptations not evident during the first few days of training. In this study, we have extended our training model with the object of determining whether or not the concentrations of the blood substrates and metabolites change as previously found for more extended training and whether or not such changes are accompanied by further alterations in the fuel utilization and the hormonal response to exercise. Since there is some suggestion that the initial training hyprvolemia may be lost with extended training at the same intensity and duration (Senay et al. 19761, we were particularly concerned with the behaviour of plasma volume relative to the hormonal changes and specifically the catecholamines.

Methods fiperimental design Details of the experimental design used to investigate the effects of short-berm training have been reported in the preceding articles (Green et al. 1990, 1991). Briefly, the study involved the performance of 2 h of cycle exercise per day at approximately 59% of peak aerobic power (8/s2max), as established from the Vo2max determined prior to training, for 18- 12 consecutive days. Measurement of vascular volumes and selected blood hematological measurements were performed prior to and approximateIy 48 h after the last training session. In addition, following the initial blood measurements, progressive cycle exercise to exhaustion was performed by the subjects for measurement of Vo2max. These tests were repeated following the study. To evaluate the effects of training during exercise, a prolonged submaximal protocol was used. This protocol involved continuous cycling at the same absolute power output prior to and following training with measurements of gas exchange, cardiac function, and core temperature. These results have been previously reported (Green et ai. 1990). No dietary control was implemented during the training. Rather, the subjects were encouraged to follow a normal balanced diet similar to what they had been consuming prior to the training. In previous work (Green et al. 19891, we have found this approach to be satisfactory since blood glucose homeostasis and resting muscle glycogen concentrations were not adversely affected. Changes in the composition of selected blood components with training, which is the fmus of this paper, was accomplished by periodic


sampling of arterialized venous blood both before the start of exercise and at 30, g0, and !XI min of exercise. For extraction of blood samples, a catheter (Angiocath 21) was inserted into a hand vein approximately 90 min before the start of the exercise and the hand kept warm with a heating pad throughout the experimental period. The initial two blood samples were collected prior to exercise, with the arm extended and supported at a Ievel slightly below the heart, after the subject had been seated stationary in a chair or on the cycle for at least 1%min. The exercise samples were taken with the arm in a similar position while the subject continued cycling. B l s d analysis Following extraction of the blssd, the sample was divided into three parts and prepared for subsequent analysis of blood metabolites, hormones, protein, and electrolytes. In the case of the blood metabolites, a portion of the blood sample was immediately placed in a prechilled tube containing 8.6 M perchloric acid. The samples were neutralized in 1.25 mM KHCO,, centrifuged, and the supernatant was analyzed by fluorometric techniques for the concentrations of glucose, glycerol, lactate, alanine, 8-hydroxybutyrate, and pymvate according to Lowry and Passonneau (1972). For the determination of serum free fatty acids, a second portion of the blood sample was placed in nornheparinized tubes, allowed to clot, and then rimmed and spun (Miles et a!. 1983). Uric acid was determined on serum based on the uricase-catalase colour reaction using kits purchased from Boehringer Mannheirn (Canada) Etd., Dorval, Que. Plasma ammonia concentration was based on the enzymatic conversion of a-ketoglutarate to glutamate using a Sigma kit (Sigma Chemical Co., St. Louis, MO). For this assay, the plasm was separated immediately following withdrawal of the blood, stored on an ice bath temporarily, and analyzed within 15 min. Bsmolarity was analyzed using a Wescor V a p r Pressure Bsmometer (model 5100e). Serum protein and serum albumin were determined by standard techniques. In addition to the above assays, the concentration of the blwd hormones glucagon, insulin, and the catecholamines were also determind. Both insulin and glucagon were analyzed by radioimmunoassay with 12", using diagnostic kits obtained from Cambridge Medical Diagnostics, Inc. and Diagnostic Products Corporation, respectively. For the analysis of glucagon, the b l d sample was collected in a vacutainer containing Na-EDTA and aprotinin, centrihged, and the plasma removed. For insulin, whole blood was placed in plain tubes, allowed to coagulate, centrifuged, and the serum extracted. Samples for both insulin and glucagon were stored until analysis. Analyses of plasma catecholamines, at -80.C epinephrine, and norepinephrine were performed by HPEC techniques using electrochemical detection according to the method of Weicher et al. (1984). With this procedure, 3 mE of whole blood is collected in a tube containing EGTA and glutathione as an antioxidant, the b l d is centrifuged at 2 W g at 2'61, and the plasma is removed. We have found no change in the stability of the preparation for catecholamines when stored at -80°C for several months. To optimize analytical control, samples from a given subject for a given variable were analyzed in duplicate during the same analytical session under conditions as nearly as identical as possible. The values reported represent the average of the duplicate analyses. Statistical treatment To determine the effect of training on the blwd concentration of the various constituents, a two-way analysis of variance for repeated measures was employed. Post hoc analysis for the detection of differences between specific means where a significant main effect was found was performed using the Newman-KeuI procedure. A standard 95% confidence level was employed for all comparisons.

Results The blood metabolites glucose, lactate, 0-hydroxybutyrate, glycerol, and free fatty acids were all significantly ( p < 0.05) affected by both the exercise and the training (Table 1 , Fig. 1). Blood alanine concentration, on the other hand, was only



TABLE1 . The effects s f exercise and training on the concentration of blood metabolites

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Time (mim)

Glucose Pretrainiang Posttraining Lactate Pretraining Posttraining Pymvate Pretraining Posttraining 8-Hydrsxybutyrnte Pretraining Posttraining Alanime Pretraining Posttraining NOTE:Values are X f SE (rnM); n = 8. A significant ( p < 0.05) interaction effect was shown only for glucose, lactate. and P-hydroxybmtyrate. "Signifacantly different from pretraining ( p < 0.05). "~ignificantl~ different from -60 mia ( p < 0.05). 'Significantly different from 0 min ( p < 0.05). J%ignifieanalydifferent from 15 mia ( p < 0.05). 'Significantly different from 30 min ( p < 0.05).

affected by the exercise, while changes in blood pyruvate were not evident with either exercise or training (Table I). For blood glucose, exercise resulted in a lower concentration over rest, with the timing and the magnitude of the change dependent on the training state. Prior to training, reductions in glucose were noted early, by 15 min of exercise, whereas folBowing training, glucose was only found to be reduced at 90 min of exercise. Training resulted in higher glucose concentrations at both 15 and 30 min of exercise. Exercise resulted in a persistent elevation of lactate both pre- and post-training. However, with training, the increase was not as marked. As with blood alanine, increases in blood @-hydroxybutyrateconcentration only occurred late in the exercise, by 90 min in the case of 0-hydroxybutyrate and by 60 rnin in the case of danine. Only in the case of @-kaydroxybutyratewas an effect of training detected, with lower concentrations observed at both 60 and 90 rnin following training. In the case of blood glycerol and serum free fatty acids (Fig. 11, exercise resulted in an increase in concentration, the magnitude of the increase depending on the training state. In the case of glycerol, a progressive increase in concentration was noted following 25 min of exercise and specifically at 30 and 60 min. For comparison, the increase in serum free fatty acids was more delayed and was not evident until 60 min of exercise. At 90 min the serum free fatty acid concentration was lower following training. For blood glycerol, higher concentrations were observed at 15, 30, and 60 min of exercise following training than prior to training. Changes in the concentration of plasma ammonia and serum uric acid were also examined and these appear in Fig. 2. Both of these measures were significantly influenced by exercise and training and in both cases, the increase noted during exercise was reduced following training. With training, lower concentrations were found for both ammonia and uric acid at 30, 60, and 90 min of exercise. The effects of exercise and training on serum protein, serum albumin, and osrnolality are presented in Table 2. Both serum protein and serum albumin were found to be elevated at each

of the exercise time points examined and in no case was the increase progressive. Similarly for both variables, the training effect that was found was not specific to exercise alone but was manifested in lower concentrations during both rest and exercise. In the case of blood osmoldity, training did not alter the increase that occurred with exercise. Exercise and training also resulted in pronounced changes in the blood concentrations of both norepinephrine and epinephrine (Fig. 3). For both catecholamines, the exercise-induced increase was lower following training at both 60 and 90 min. Both catecholamines also exhibited a similar time dependent change early in the exercise period, namely, in the case of exercise prior to training, a progressive increase during the first 60 miaa of exercise. Following training, norepinephrine plateaued at 30 min and did not increase further over the next 60 min of exercise. In the case of epinephrine, increases were progressive for the first 60 rnin of exercise as in pretraining; however, the magnitude of the increase was considerably reduced. Insulin concentration was affected by exercise only. Significant reductions in insulin concentrations were found at each exercise time point (Fig. 4). The changes in glucagon concentration either for exercise or training were not sigaaificant (Fig. 4). Based on the results of this study, it is apparent that a 10to 12-day period of prolonged cycle training results in extensive changes in the blood concentration of several metabolites and hormones. Particularly significant in this regard was the pronounced reduction in blood lactate concentration that occurred following training, a finding that has been dscumented in humans on several previous occasions (Henriksson 1974; Hurley eo al. 1984, 1986; Winder et ab. 2978, 1979). As demonstrated in previous studies using young (Hickson et al. 1981) and middle-aged subjects (Rogers et al. 19881, the lower blood lactate response is a relatively early phenomenon occurring within 1 week following the onset of heavy training. Since we have previously found (Green et a&.1989), using the

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FIG.1. The effect of exercise and training om the concentra>ons of blood glycerol and serum free fatty acids (FFA).Values are X f SE (mM),n = 8. PRE, pretrainimg; POST, postlraining. "Significantly different from PRE ( p < 0.05. bSignificantly different from -60 min ( p < 0.0%).'Significantly different from 0 min ( p < 0.8%). dSignificantly different from 15 min ( p < 8.05). 'Significantly different from TO min ( p < 0.05).fsignificantly different from 60 min ( p < 0.05).

same testing protocol as employed in the present study, that the lower lactate response was not evident following 3 days at training, it appears that some adaptation occurring between 3 and 10- 12 days of training is responsible for the depressed lactate concentration. Although there is some controversy (Donovan and Brooks 1983), a reduction in exercise blood lactate concentration following training has generally been interpreted to mean that production is reduced as a consequence of a depression in glycolysis (Hurley et al. 1984; Katz and Shlin 1988). It has also been proposed (Henriksson 1977; Holloszy and Coyle 1984), and to some extent confirmed experimentally (Hurley et a&.1986), that the reduction in glycolysis is accompanied by a shift in substrate utilization, namely towards noncarbohydrate sources and in particular the oxidation of free fatty acids. The shift in metabolism and substrate utilization apparently occurs without any change in oxygen consumption during submaximal exercise at the same absolute work intensity prior to and following training (Henriksson 1977; Winder eta&.1979). This interpretation gains its primary support from

FIG. 2 . The effect of exercise and training on the concentrations of blood ammonia and uric acid. Values are X f SE (n = 8). PRE, pretraining; POST, posttraining. "Significantly different fmm PRE ( p < 0.0%).

studies in both man (Henriksson 1977) and rats (Hurley et ak. 1986) where training has been conducted for a relatively long period of time (8 - 12 weeks) and where substantial changes in the aerobic oxidative potential of the muscle have been documented. At issue is whether or not a similar interpretation is justified in this study given the early nature of the response and the lack of a significant elevation in the oxidative potential of the trained muscle (Green et al. 1991). It would appear that a shift towards a more emphasized free fatty acid metabolism occurred during exercise following training in this study as suggested by the Bower R values that were found (Green et (PI. 1990). R values at 30,60, and 90 min of the exercise prior to training were 0.94 f 0.01, 0.93 0.02, and 0.89 -b 0.2, respectively. Following training, the values at comparable time periods were 0.87 f 0.02, 0.86 -b 0.02, and 0.85 0.01. Since arterialized plasm free fatty acid concentrations were not elevated during exercise following training in this study as well as others (Koivisto et a&. 1982; Winder et al. 1979) and since arterial free fatty acid concentration is directly related to free fatty acid utilization by the working muscle (Carlson and Bernow 1961), it would appear that the trained muscle is capable of enhanced extraction or that an increase in muscle triglyceride utilization results (Holloszy and Coyle 1984). The recent finding that exercise reduces muscle triglycerides to a greater extent following training than before training (Hurley et ak. 8986) lends support to the latter mechanism, namely the greater dependency on




TABLE 2. The effects of exercise and training on the concentration of serum protein, seaham albumin, and osrnolality

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Time (min)

Serum protein (g/1WmL) Pretraining Posttraining Semm albumin (g/HW mL) Pretraining Posttraining Osmolality (mmol/kg) Pretraining Posttraining

7.05f0.12 6.84f0.16

7.34f0.88 7.15f0.15

5.20f0.05 4.94fO.05

5.35f0.87 5.48f0.04 5.13+_0.06 5.36f0.04

285.9f 1.8 286.3k3.4

285.6k2.8 286.5k3.5

7.40-+-0.12 7.68f0.09 7.38+8.14 7.51f 0.16 5.55k0.04 5.38f0.03 294.4k2.2 293.9f3.3

7.73f0.09 7.46k0.12

7.59k0.17 7.47k0.11

5.53k0.05 5.44f0.05

5.49f0.05 5.41k0.08

294.4f2.8 292.1k4.0

290.4k3.3 292.9f3.6

NOTE:Values are X + SB, (n = 8). Main effects for training ( p < 0.05) and time ( p < 0.05) were found for both serum protein and serum albumin. For osmolality only a main effect for rime was found ( p < 0.05). No significant ( p < 0.05) interaction effects were found.



Fne. 4. The effect of exercise training o n the concentrations of plasma insulin and gluwgon. Values are X SE (ao = 8). PRE, pretraining; POST, posttraining. Only a significant ( p < 0.05)main effect for time was found for insulin. No other effects were significant.


FIG.3, The effect of exercise and training on the concentrations of ~ o catecholamines, d norepinephrine and epinephrine. Values are X f SE (n = 8). PWE, pretraining; POST,posttraining. Significantly different from PWE ( p < 0.05). bSignificantly different fmm -60 min ( p < 0.05). CSignificantly different from 0 min ( p < 0.05).dSignificantly different from 30 min ( p < 0.05). 'Significantly different from 60 min ( p < 0.05).

free fatty acids made available from endogenous sources (Hslloszy 1990). It is possible that the 10- to 12-day training period used in this study also resulted in increased muscle triglyceride utilization. If such was the case, it would appear

h a t increases in 0-oxidative and aerobic oxidative potential were not prerequisites (Green et a&.1991). It should be emphasized that the reduction in the exercise R values noted following training may not be a true indication of changes in substrate utilization, but they could reflect a reduction in excess C 0 2 mediated by a lower lactate and lower acidosis (Green eb a!. 1990). However, such an interpretation does not seem plausible given the relatively low blood lactate concentrations observed during exercise prior to and following train-

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ing. These %owblood lactate concentrations would be expected to have a minimal effect on blood pH levels. The effect of training on the blood concentrations s f glycerol and 0-hydroxybutyrate during exercise has been shown to parallel the response in arterial free fatty acids (Hurley et a&.1986). Such was the case in this study with 8-hydroxybutyrate, which was Bower late in the exercise following training than before training. However, in the case of blood glycerol, the 10- 12 days of training resulted in a higher concentration during the middle stages of the exercise (38 and 60 min) than before training. Since blood glycerol has been suggested to be a better indicator of adipose tissue lipolysis than blood free fatty acids (Hetenyi et al. 1983), our results suggests a possible training induced enhancement in free fatty acid release. If such were the case, training would have to increase arterial free fatty acid extraction since the concentration during this period was unaffected by training. It is of interest that in a shorter training model, 3 days using the same exercise protocol, training was without effect on the concentrations of either serum free fatty acids or B-hydroxybutyrate during the same duration and intensity of the exercise. Similar to the more extended training studies, we have found that the reduction in blood lactate concentration during exercise following training was also accompanied by a reduced muscle lactate concentration (Green et al. 1991). Training also resulted in a pronounced blunting of the increase in the blood concentrations of both ammonia and uric acid. The increase in blood ammonia has previously been shown to parallel the increase in blood lactate particularly during short-term dynamic cycle exercise (Buono et al. 1984; Katz et al. 1986). This relationship has been interpreted to mean that the increase in glycolysis and lactate production in working muscle is also accompanied by an increase in the production of IMP and ammonia via the AMP deaminase reaction (Meyer st al. 1980). As emphasized by Graham et al. (1987), the proposal that lactate and ammonia production are mechanistically linked may not be entirely valid since they have found a progressive drift in blood ammonia concentration with prolonged exercise in contrast with lactate, which plateaued early in the exercise. Our results are similar to Graham et al. (1987). As concluded by Graham e&a [ . (1987), other potential sources of blood ammonia production could conceivably be involved notably amino acid metabolism. Interestingly, blood alanine concentration during exercise was not affected by the shortterm training, a reduction that might be expected if the glucosealanine cycle was involved in the production of ammonia. An enhanced clearance rate of ammonia during exercise following training might also be a significant factor in explaining the lower ammonia in our study and the results of a similar study (Denis el al. 1989). The reduction in blood uric acid that occurred with training, however, is also consistent with a reduction in IMP and ammonia formtion since IMP can be progressively degraded to hypxanthine, xanthine, and uric acid in muscle. Recently, it has been demonstrated that xanthine oxidase, the enzyme used to degrade xanthine to uric acid can exist in this more active oxidase form during heavy exercise, suggesting that the muscle is capable sf producing substantial amounts of uric acid (Hellsten et al. 1988). It has previously been shown by us (Green and Fraser 11988) and others (Sutton ct al. 1980) that blood uric acid is increased with heavy exercise. The fact that b l d uric acid was elevated during exercise prior to training in this study but not in a previous study using prolonged exer-


cise (Green and Fraser 1988) may be explained by the persistently higher, albeit moderate, Bevels of blood lactate found in this study. It is generally accepted that prolonged exercise of moderate intensity results in a reduction in blood glucose concentration (Galbo 1983; Galbo et a1. 1979; Winder et al. 1979) and that training results in an improved ability to defend blood glucose homeostasis (Winder et al. 1979). Although this is not always a consistent finding with training (Hurley et al. 1984; Winder et al. 1979), the discrepant results probably being explained by differences in the intensity and duration of the exercise challenge, an increased blood glucose level was confirmed with the short-term training used in this study. However, although persistently higher glucose concentrations were noted at each of the sampling times, significance was only found at 15 and 30 min of exercise. Since this is too early in the exercise to suggest depletion of liver glycogen stores (Hultman and Nilsson 1989), it would appear that training resulted in a better control between the release of glucose by the liver and uptake and utilization by the working muscle. Galbo and Kjaer (1987) have suggested that the increase in blood glucose observed during this early period of exercise following training results from a more rapid increase in glucose production. Changes in specific hormonal concentrations following training may be central to elaborating a tentative schema to explain the alterations in substrate utilization and muscle metabolic behaviour. In this study, as in a previous study using a 3-day model of prolonged training (Green et a / . 1989), we have found a substantial reduction in the exercise-induced increase in both blood epinephrine and norepinephrine. Similar to the earlier more abbreviated training study, the apparent reduction in the catecholamines was unaccompanied by a training effect on either blood insulin or glucagon concentration. Alterations in catecholamine concentrations have been previously documented as one of the early adaptive responses to prolonged training (Winder et al. 1978, 1979). The mechanism for the reduction remains uncertain, although the substantial training induced elevation in plasma volume that occurred in this study and in the previous study appear to be of some significance. In this study, the training-induced increase in plasma volume amounted to 365 mL or 11.8 % (Green et al. 1990). Acute expansion of plasma volume has been previously shown to depress the blood catecholamine response (Leutscher st al. 1973). Alterations in blood catecholamines could potentially alter the mobilization of both glucose from the liver and free fatty acids from the adipose tissue sites. Among several factors known to affect free fatty acid mobilization during exercise, Bulow (1987) has identified sympathoadrenal activity and the level of circulating insulin to be of major importance. Decreases in the catecholamine response could serve to reduce lipolysis and adipose tissue blood flow, both known to suppress mobilization. Similarly, the catecholamines have long been implicated in liver gluconeogenesis and liver glycogenolysis (Galbo 1983; Galbo et al. 1979; Galbo and Hggaer 1987; Moelzer et al. 1986) and, in fact, epinephrine concentrations appear to associate inversely with blood glucose concentration (Galbo et al. 1979). However, recent evidence (Vranic et al, 1987) would suggest that the glucagon to insulin ratio is of primary importance in regulation of hepatic glucose release. The function of epinephrine has been implicated peripherally, namely to control the flux of glucose into the working muscle by altering muscle glycogenolysis. It is possible that a depression in blood

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epinephrine levels noted with training couM operate to enhance the utilization of other substrates by lowering glycogenolysis in muscle. Infusion of epinephrine has been previously noted to elevate muscle glycogenolysis in humans (Jansson et a&. 1986), although the significance of this finding has been challenged given the unphysiologic dosages employed (Katz and Sahlin 1988). It should be acknowledged that the reduction in blood catecholamines with training may not occur in isolation. Changes in receptor density and sensitivity may d s o occur and result in a modification of the response to a given catecholamine concentration. One factor that could potentially explain the differences in the argerialized blood concentration of a number of metabolites following training is the dilution that resulted as a consequence of the expanded plasma volume and the potential disproportionate loss of fluid that may have resulted during exercise following training as a result of the hypervolemia. The dilution effect seems to be a viable explanation to account for some of the differences in serum protein and serum albumin, since lower vdues following training perslsted throughout both rest and exercise. However, based on the increases in plasma volume that result with training (+356 mL), the calculated resting protein content was increased by approximately 20 g. The increase in plasma protein with this type of training has been reported earlier (Convertino 1987). With regards to the other b l d constituents, resting concentrations were not different for any measure, a finding that would suggest that concentration rather than content is regulated. Further, the differences that resulted during exercising for a large number of the other metabolites appears not to be affected by training-induced differences in plasma volume loss as well. Although, we neglected to record exercise hematocrits in this study, which would have permitted calculation of volume shifts, hematocrit changes have previously been reported (Convertino 1987). These results demonstrate that the loss of plasma volume is proportionate to the plasma volume gain (Convertino 1987). Conceivably changes in water content of the blood with exercise could also occur independent of plasma volume shifts. Although this factor would not be expected to dter the results of the experiment, blood water content as determined by wet to dry weight ratios should be considered in feature studies. In summary, we have found that short-term training involving 10- 12 consecutive days of prolonged exercise results in a wide range s f alterations in the concentrations of selected blood constituents, For some metabolites, lactate, uric acid, ammonia, these changes are typical of what has been found from more extended training where increases in muscle oxidative potential has occumed and where a reduction in glycolysis appears like the most probable interpretation. If the training program used in this study resulted in an increase in the proportion of free fatty acids used during exercise as suggested by the lower R vdues, a fundamentally different mechanism would have to be involved than proposed for studies where increases in free fatty acid utilization occurred in conjunction with increases in muscle metabolic potential. In this regard, a productive direction for future experiments might be to examine alterations in blood flow and substrate exchange across the principal tissues involved in substrate release and uptake, namely the liver, adipose tissue, and working muscle. It would be important to determine if the same strategy occurs during hypervolemia as during other perturbations designed to reduce arterial oxygen content, namely an alteration in blood flow (Saltin el a&,1986). It is tempting to speculate that alterations

in blood Wow may be one of the earliest, albeit transient, adaptive responses, capable s f eliciting generally the same physiologic advantage as those realized from peripheral changes in the muscle energy enzymatic machinery.

This study was supported by the Natural Science and Engineering Research Council of Canada. B u r n , 9. 1987. Regulation of lipid mobilization in exercise. Can. J. Sport Sci. f2(Suppl. 1): 117s-119s.

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Early adaptations in blood substrates, metabolites, and hormones to prolonged exercise training in man.

This study was designed to investigate the effect of short-term, submaximal training on changes in blood substrates, metabolites, and hormonal concent...
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