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Effect of an acute β-adrenergic blockade on the blood glucose response during lactate minimum test
Effect of an Acute 13-adrenergic Blockade on the Blood Glucose Response during Lactate Minimum Test Pedro Balikian J~nior 1, Cassiano Merussi Neiva 1 & Benedito Sergio Denadai 2 lUNAERP, Ribeir~o Preto, S~o Paulo, Brasil. 2Human Performance Laboratory - IB - UNESP, Rio Claro, S~o Paulo, Brasil.
Balikian, P.B., Neiva, C.M., & Denadai, B.S. (2001). Effect of an acute [~-adrenergic blockade on the blood glucose response during lactate minimum test. Journal of Science and Medicine in Sport 4 (3): 257-265. The aim of this study was to determine the relationship between blood lactate and glucose dining an incremental test after exercise induced lactic acidosis, under normal and acute [3-adrenergic blockade. Eight fit males (cyclists or triathletes) performed a protocol to determine the intensity corresponding to the individual equilibrium point between lactate entry and removal from the blood (incremental test after exercise induced lactic acidosis), determined from the blood lactate (Lacmin) and glucose (Glucmin) response. This protocol was performed twice in a double-blind randomized order by ingesting either propranolol (80 rag) or a placebo (dextrose), 120 rain prior to the test. The blood lactate and glucose concentration obtained 7 minutes after anaerobic exercise (Wingate test) was significantly lower (p<0.01) with the acute [~-adrenergic blockade (9.1_+1.5 raM; 3.9_+0.1 raM), respectively than in the placebo condition (12.4_+1.8 mM; 5.0_+0.1 mM). There was no difference (p>0.05) between the exercise intensity determined by Lacmin (212.1_+17.4 W) and Glucmin (218.2_+22.1 W) during exercise performed without acute ~-adrenergic blockade. The exercise intensity at Lacmin was lowered (p<0.05) from 212.1 -+17.4 to 181.0-+15.6 W and heart rate at Lacmin was reduced (p<0.01) from 161.2_+8.4 to 129.3_+6.2 beats min -1 as a result of the blockade. It was not possible to determine the exercise intensity corresponding to Glucmin with [3-adrenergic blockade, since the blood glucose concentration presented a continuous decrease duling the incremental test. We concluded that the similar pattern response of blood lactate and glucose during an incremental test after exercise induced lactic acidosis, is not present duling ~-adrenergic blockade suggesting that, at least in part, this behavior depends upon adrenergic stimulation.
Introduction D e s p i t e t h e l a c k of a g r e e m e n t a m o n g r e s e a r c h e r s a b o u t t h e u n d e r l y i n g m e c h a n i s m s a n d t e r m i n o l o g y ( W a s s e r m a n et al., 1973; Brooks, 1985; G a e s s e r & Poole, 1986), t h e blood l a c t a t e r e s p o n s e d u r i n g i n c r e m e n t a l exercise h a s b e e n widely u s e d in s p o r t a n d m e d i c a l r e s e a r c h . Practical a p p l i c a t i o n s of t h e b l o o d l a c t a t e r e s p o n s e i n c l u d e t h e m e a s u r e m e n t of a e r o b i c c a p a c i t y t r a i n i n g effects (Kohrt et al., 1989; W e l t m a n et al., 1992) a n d p r e s c r i p t i o n of a p p r o p r i a t e exercise i n t e n s i t y ( K i n d e r m a n n et al., 1979; Billat, 1996). M a n y s t u d i e s h a v e p r o p o s e d p r o t o c o l s to d e t e r m i n e t h e exercise i n t e n s i t y c o r r e s p o n d i n g to t h e m a x i m a l l a c t a t e s t e a d y s t a t e (MLSS), i.e., t h e h i g h e s t c o n s t a n t w o r k l o a d t h a t c a n b e p e r f o r m e d w i t h e q u i l i b r i u m b e t w e e n l a c t a t e e n t r y a n d r e m o v a l from t h e b l o o d ( S t e g m a n n et al., 1981; H e c k et al, 1985).
257
Effect of an Acute 13-adrenergi(: blockade...
Tegtbur et al. (1993) have proposed an interesting protocol (incremental test after exercise induced lactic acidosis) to identify the intensity corresponding to the individual equilibrium point between lactate entry and removal from the blood (Lacmin). The authors found that MLSS could be obtained for all subjects during long term exercise at Lacmin intensity. Moreover, differently from others protocols (Maassen & Busse, 1989; Berry et al., 1991) the intensity corresponding to the Lacmin is not influenced by muscle glycogen stores (Tegtbur et al., 1993) or glucose or caffeine ingestion (Campbell et al., 1998). Recently, Sim6es et al. (1999) reported that it was possible to predict the MISS intensity from the plasma glucose response during a Lacmin test in endurance runners. Moreover, Campbell et al. (1998) have demonstrated that the relationship between blood lactate and glucose during Lacmin test is not altered by glucose or caffeine ingestion. These findings suggest the possibility of using the plasma glucose response to estimate the Lacmin intensity. However, the factors and mechanisms responsible for this association are not completely understood. A possible mechanism is the catecholamine response during exercise, since epinephrine and norepinephrine have a critical role in the body's adjustment to stressful activities. In muscle, through ~-adrenergic receptors, epinephrine initiates a cascade of events that stimulates the degradation of glycogen and an increase in lactate production (Podolin et al., 1991). Moreover, many studies have demonstrated that circulating epinephrine is an important controller of both hepatic glucose production and peripheral glucose uptake during exercise (Kjaer, 1992). Studies that analyze the integrated blood lactate and glucose responses during exercise are important, so beyond the evident theoretical aspects that can be investigated, there is the possibility to validate indirect methods. This will allow its use for a greater n u m b e r of professionals interested to identify the MISS, without specific equipment for blood lactate determination. Therefore, the main purpose of this study was to determine the relationship between blood lactate and glucose during an incremental test after exercise induced lactic acidosis, under normal and acute ~-adrenergic blockade.
Methods and Procedures SubjeCts Eight healthy, physically active male volunteers participated in the study: age (SD) = 21.1 (1.7)yr.; height = 171.4 (8.5) cm; weight =64.7 (9.5) Kg; and VO2max = 62.3 (5.8) ml.kg-l.min -1. Six were competitive triathletes, and two were competitive cyclists. No one was taking regular medication or had any remarkable medical history. The nature of the study was explained to each subject before written informed consent was obtained.
Experimental protocol The participants were instructed to arrive at the laboratory in a rested and fully hydrated state, at least 3 h post-prandial and to avoid strenuous exercise in the 48 h preceding a test session. For each subject, the tests took place at the same time of the day to avoid the effects of diurnal biological variation. Subjects reported to the laboratory three times, at intervals of 2-3 days between the sessions. At the first visit, in order to determine the VO2max, a maximal exercise test was performed on an electrically braked cycle ergometer (Lifestyle 5500). The initial workload was 50 W and the intensity was increased by 25 W every 1 rain until voluntary exhaustion. Respiratory gas exchange data were 258
Effect of an Acute 13-adrenergic blockade...
determined continuously throughout the exercise tests (Vista CPX, Vacumed). In the two subsequent sessions, the intensity corresponding to the Lacmin was determined in a double-blind randomized order with the subject ingesting either propranolol (80 rag) or a placebo (dextrose) 120 m i n prior to the test. In order to determine the intensity corresponding to the kacmin, the subjects were submitted initially to the Wingate test (Bar-Or, 1987), for lactic acidosis induction. The classical variables obtained from the Wingate test were not measured. At 8 min of recovery after the Wingate test, a s u b s e q u e n t incremental exercise test was performed. The initial workload was 100 W and the intensity was increased by 25 W every 3 min until exhaustion. After 7 rain of recovery and at the end of each stage, 25 pl of blood was collected from the ear lobe, into microcentrifuge tubes containing 50 pl NaF (1%), for lactate m e a s u r e m e n t (YSL 2700 STAT). Simultaneously, 5 ml of blood was sampled at 7 min of recovery and during the last 30 seconds of each workload from a n antecubital vein via an indwelling catheter for glucose m e a s u r e m e n t (glucose oxidase - Cobas Mira Plus- Roche). The lowest blood lactate and glucose concentration during the incremental test and its respective intensity were calculated using a spline function, and were considered Lacmin and Glucmin, respectively (Tegtbur et al., 1993; Sim6es et al., 1999) (Figures 1 and 2). Heart rate was m e a s u r e d continuously during the incremental test (Vantage XL, Polar Electro Oy, Oulu, Finland). S t a t i s t i c a l analysis
Results are reported as m e a n s +SD. propranolol were made using Student intensity at Lacmin and Glucmin was analysis and a Pearson product m o m e n t set at P_<0.05.
Comparisons between placebo and t-test. Relationship between exercise accomplished via a linear regression correlation. Statistical significance was
Glucmin
12
6
10
E
5
8
,7
6
32
4
2:= (3 1
O
.J
2
A
0
i
Lacmin i
i
~
i
~
i
i
[
i
-
0
100 125 150 175 200 225 250 275 300
Intensity (Watts) ].,
Lac~te
= Glucose I
Figure 1: Determination of intensities correspondingto the lower lactate (Lacmin)and glucose (Glucmin) concentration during the incremental test (single subject) without beta-adrenergicblockade.A = blood lactate and glucose 7 minutes after anaerobic exercise (Wingate test).
259
Effect of an Acute ~-adrenergic blockade...
12 10
5
8
E
3~
6
o 2 _=
4 2 0
Lacmin
1
i
i
i
i
~
i
i
100
125
150
175
200
225
250
0
Intensity (Watts)
I'
•
Lactate
•
Glucose I
Figure 2: Determination of intensity corresponding to the lower lactate (Lacmin) concentration during the incremental test (single subject) with beta-adrenergic blockade. A = blood lactate and glucose 7 minutes after anaerobicexercise (Wingate test).
Placebo Lactate(mM) Glucose(mM) LacmJn(Watts) Glucim(Watts) HR-Lacmin(bqm) HR-Glucmin(bqm)
12.4 5.0 212.1 218.2 16~.2 166.2
_+ _+ _+ _+ + _+
1.8 0.1 17.4 22.1 8.4 9.2
~-adrenergic blockade
Difference (%)
9.1 -+ 1.5" 3.9 -+ 0.1" 181.0 _+ 15.6"
-26.7 -21.9 -14.0
_+ 7.6 _+ 3.5 _+ 9.7
129.3 _+
-19.3
_+ 3.4
6.2*
Values are means _+SD. *p
Table 1:
Blood lactate and glucose 7 minutes after anaerobic exercise and the intensity (Watts) and heart rate (HR) corresponding to lower blood lactate (Lacmin) and glucose (Glucmin) concentration during an incremental test, with and without B-adrenergicblockade (n=8).
ReSults The blood lactate and glucose concentration obtained 7 minutes after the Wingate test were significantly lower with acute ~-adrenergic blockade (Table 1). The exercise intensity and heart rate corresponding to the Lacmin were significantly lower with acute ~-adrenergic blockade (Table 1). There was no difference between the exercise intensity determined by Lacmin and Glucmin during the protocol performed without acute ~-adrenergic blockade. It was not possible to determine the exercise intensity corresponding to Glucmin with~-adrenergic blockade, since the blood glucose concentration presented a continuous decrease during the incremental test, as showed for one subject in Fig. 2. All others subjects displayed a similar pattern of blood glucose response. There was a significant correlation (r=0.93; P<0.01) between the exercise intensity determined by Lacmin and Glucmin in tests without ~-adrenergic blockade (Figure 3). However, there was no significant correlation (r=0.31; P>0.05) 260
Effect of an Acute ~-adrenergic blockade...
300
~" 250. y = 46.7 + 0.77x
.E E
.J
r = 0.93 S E E = 7.85
2oo
p < O.01(N = 8)
150 150
200
250
300
Glucmin (watts)
Figure 3: Relationship between exercise intensity at lowest blood lactate (Lacmin) and glucose (Glucmin) concentration, without ~-adrenergic blockade. Dashed line is line of indentity, solid line is regression equation.
"E 3:
~
220
~
Q. i
,_= 170 E
.~-'~-
f = 0.31 SEE = 19.23
I~
p > 0.05 (N = 81
i 120 120
170 Lacmin
220 - propranolol
(watts)
Figure 4: Relationship between exercise intensity at lowest blood lactate (Lacmin) concentration, with and without ~-adrenergic blockade. Dashed line is line of indentity, solid line is regression equation.
between the exercise intensity corresponding to the Lacmin with and without acute ~-adrenergic blockade (Figure 4). DiSCuSSiOn This investigation was designed to determine the relationship between lactate and glucose during an incremental test after exercise induced lactic acidosis, under normal and acute ~-adrenergic blockade. The results of this study indicate two significant findings. Firstly, Glucmin determined from the blood glucose response can predict the intensity corresponding to Lacmin during cycling exercise in normal conditions. Secondly, this association is modified by acute ~-adrenergic blockade. Karlsson et al. (1983) analyzed the effect of acute unselective (propranolol) and ~l-selective (atenolol) ~-adrenoceptor blockade on the Wingate test. Peak and average power of the Wingate test showed a significant impairment after both, unselective and ~]-adrenoceptor blockade (4%-6%). However, peak blood lactate 261
Effect of an Acute 13-adrenergicblockade...
was only reduced after propranolol. Similarly, although the p e a k and average power were not m e a s u r e d in the present study, a 26% reduction of p e a k blood lactate was observed after the Wingate test with acute ~t-adrenergic blockade. Glycogenolysis in skeletal muscle is considered to be mediated via ~2-adrenergic receptor stimulation (Arnold & Selberis, 1968). Therefore, in accordance with the data obtained by Karlsson et al. (1983), the lower blood lactate concentration after the Wingate test in the propranolol trial, is not due only to decreased work performed, but a reduced muscle glycolytic rate due [t-adrenergic blockade. However, some studies have provided conflicting results a b o u t the effects of [ladrenergic blockade on muscle glycogenolysis (Juhlin-Dannfelt et al., 1982) and lactate concentration after intense exercise (Broberg et al, 1988). These discrepancies could be due to differences in the completeness of ~-adrenergic blockade achieved and the exercise intensity performed in the different studies (Hargreaves & Richter, 1988). Tegtbur et al. (1993) found no effect of low muscle glycogen stores on the Lacmin determined in ten male endurance trained runners. Although the pretest blood lactate concentration was lower with low muscle glycogen stores (10.6 mM) t h a n in normal condition (14.1 raM), the running speed corresponding to the Lacmin were not significantly different between the conditions. The authors demonstrated that the running speed corresponding to the Lacmin is independent of the respective blood lactate before the incremental test. Therefore, the lower blood lactate concentration observed before the incremental test in our study, probably had no influence on the lower exercise intensity corresponding to the Lacmin with acute [~-adrenergic blockade (Table 1). Campbell et al. (1998) did not observe effect of oral ingestion of glucose (75 g) 30 minutes before the exercise, with consequent increase of glucemia, over the intensity corresponding to the Glucmin. In our study, the [~-adrenergic blockade determined a lower glucemia, although it is still within the lower limit of normality, after the high intensity exercise. Moreover, with the ~-adrenergic blockade it was not possible the identification of Glucmin, because of the constant decrease of glucemia during the incremental exercise. This decrease probably occurred not j u s t by the lower glucemia before this m o m e n t of protocol, b u t because of the action of the [~-adrenergic blockade during the incremental exercise discussed as follows. Thus, the possible isolated effect that the lower glucemia, patC.icnlarly from a real hypoglycemia, can have u p o n the intensity of Glucmin h a s still to be investigated. In this study, the blood samples for lactate and glucose determination were collected from different sites (ear lobe and antecubital vein, respectively), considering the different methods (equipment and blood volume) used for the determination of theses substrates. Some studies have demonstrated that the site of blood collection could influence blood lactate m e a s u r e m e n t s and indices derived from t h e m w.These studies have verified that differences between venous and arterialysed blood lactate were evident when fixed concentrations are used (4 mM) (Foxdal et al., 1991), b u t not w h e n blood lactate curve is analyzed (lactate threshold - infiexion point) (Robergs et al., 1990). Campbell et al. (1998) and Sim6es et al. (1999) m ~ g use of the same sample of blood to analyze the blood lactate and glucemia (YSL 2300 STAT) found similar results to our study, in relation to the association between the Lacmin and Glucmin without [3-adrenerglc bl6ckade. However, at the present moment, there are no studies that have 262
Effect of an Acute 13-adrenergic blockade...
investigated the impact of sampling site on the Lacmin, Glucmin and the relationship between these indices, what do not allow us to confirm definitely what is the influence that the site of blood collection can have upon the relation between the Lacmin and Glucmin. The results of this study support the possibility that the blood glucose response can be used to predict the Lacmin intensity, consistent with previous fmdings (Campbell et al., 1998; Sim6es et al., 1999). Sim6es et al. (1999) verified in welltrained endurance runners, that the velocity determined by Glucmin, was coincident with the velocity corresponding to MLSS, determined by two different incremental tests performed on the track: individual anaerobic threshold (Stegmann et al., 1981), and lactate minimum test (Tegtbur et al., 1993). This phenomenon could, to some extent, be explained by the catecholamines influence on the metabolism, particularly during the strenuous exercise of a graded exercise test. In relation to lactate metabolism, epinephrine binds to ~-adrenergic receptors on the skeletal muscle membrane, initiating a cascade of events leading to glycogen breakdown (via activation of phosphorylase a), which ultimately increases lactate production. This influence is confirmed by many studies performed in humans. Mazzeo and Marshall (1989) manipulated the inflection point in blood lactate during progressive incremental exercise (TLA)via training specificity and found that plasma epinephrine demonstrated an inflection point (TEPI) that was identical to that of TEA across all subjects and exercise tasks. Podolin et al. (1991) have demonstrated that the relationship between TLA and TEPI is not influenced by muscular glycogen stores. In relation to glucose metabolism, some studies have found that epinephrine is directly involved in regulation of both hepatic glucose production (Ra) and peripheral glucose uptake (Rd) during exercise. Ra increases exponentially with workload in parallel with the rise in plasma epinephrine (Kjaer, 1992). During exercise performed in hypoxia, the increase of the catecholamines and Ra is higher compared to normoxia, whereas the responses of pancreatic hormones were similar between the two conditions. Rd also seems to be under the influence of catecholamines during exercise (Kjaer, 1992). Kjaer et al. (1991) found that during combined arm and leg exercise, the catecholamine response as well as Ra rose more markedly compared with the period of leg exercise only. However, the increase of glucose delivery only resulted in a small increase in Rd. The authors suggested that this phenomenon could be determined by the high concentration of epinephi~ne in plasma, as epinephrine has been shown to reduce muscular glucose uptake during exercise (Issekutz, 1985; Hoelzer et al., 1986). Therefore, the catecholamines, particularly epinephrine may play an important role in the mechanism underlying the association between blood lactate and glucose response during incremental exercise. In the present study, the blood glucose response was modified by acute ~adrenergic blockade, making it impossible to identify the inflection in the blood glucose curve. There are no studies in the literature which have analyzed the ~adrenergic blockade effect on the blood glucose response using a similar protocol, which makes it difficult to compare our results to others. However, Galbo et al. (1976) reported a more rapid decline of blood glucose during submaximal exercise after [~-adrenergic blockade. Moreover, the authors found an increase of the carbohydrate combustion rate and less muscle glycogen utilization after ~263
Effect of an Acute ~-adrenergic blockade...
adrenergic blockade. Galbo et al. (1976) suggested that during [~-adrenergic blockade, a smaller inhibition of hexokinase by glucose-6-phosphate derived from glycogen possibly accounted for the larger glucose uptake. Hoelzer et al. (1986) have verified that the hypoglycemia observed during exercise with [3-adrenergic blockade, is not determined by a lower hepatic glucose production. Even though adrenergic stimulation could be important for hepatic glucose production, the fall in insulin levels associated with exercise is not affected by pharmacological ]3-blockade (Hoelzer et al., 1986). Moreover, Hoelzer et al. (1986) also verified an increase of muscular glucose uptake after adrenergic blockade, indicating the importance of catecholamines in preventing hypoglycemia during exercise. This may be due to the lack of a catecholamine action in skeletal muscle (Issekutz, 1985) or indirectly due to lower free fatty acid availability, since lipolysis is depressed under ~-adrenergic blockade. Thus, the increase in muscular glucose uptake may explain the constant decline in blood glucose observed during our protocol with [~-adrenergic blockade. In conclusion, the similar pattern response of blood lactate and glucose duiing an incremental test after exercise induced lactic acidosis, is not present during ~adrenergic blockade suggesting that, at least in part, this behavior depends upon adrenergic stimulation.
References Arnold, A. & Selberis, W.H. (1968). Activities of catecholamines on the rat muscle glycogenolytic (~2) receptor. Experentia 24:1010-11. Bar-Or, O. (1987). The Wingate anaerobic test: An update on methodology, reliability and validity. Sports Medicine 50: 273-82. Berry, M.J., Stoneman, J.V., Weyrich, A.S. & Burney, B. (1991). Dissociation of the ventilatory and lactate threshold following caffeine ingestion. Medicine mid Science in Sports and Exercise 23: 463-9, Bfllat, L.V. (1996). Use of blood lactate measurements for prediction of exercise performance and control of training. Sports Medicine 22: 157-75. Broberg, S., Katz, A. & Sahlin, K. (1988). Propranolol enhances adenine nucleotide degradation in human muscle during exercise. J o u r n a l Applied Physiology 65:2478-83. Brooks, G.A. (1985). Anaerobic threshold: review of the concept and directions for future research. Medicine and Science in Sports and Exercise 17: 22-31. Campbell, C.S.G., SimSes, H. & Denadai, B.S. (1998). Influence of glucose and caffeine administration on identification of maximal lactate steady state. Medicine and Scienee in Sports and Exercise 30: $327. Donovan, C.M., & Brooks, G.A. (1983). Endurance training affects lactate clearance, not lactate production. American Journal Physiology 244: 463-70. Foxdal, P., Sjodin, A., Ostman, B. & Sjodin, B. (1991). The effect of different blood sampling sites and analyses on the relationship between exercise intensity and 4.0 mmol.1-1 blood lactate concentration. European Journal Applied Physiology 63:52-4. Gaesser, G., & Poole, D.C. (1986). Lactate and ventilatory threshold: disparity in time course of adaptation to training. Journal Applied Physiology 61: 999-1004. Gaesser, G., & Rich, R.G. (1985). Influence of caffeine on blood lactate response during incremental exercise. International Journal of Sports Medicine 6: 207-11. Galbo, H., Hoist, J.0., Christensen, N.J., & Hilsted, J. (1976). Glucagon and plasma catecholamine during beta-blockade in exercising man. Journal Applied Physiology 40: 855-63. Hargreaves, M. & Richter, E.A. (1988). Regulation of skeletal muscle glycogenolysis during exercise. Canadian Journal Sport Science. 13:197-203. Heck, H., Mader, A., Hess, G., Mucke, S., Muller, R., & Hollmarm, W. (1985). Justification of the 4mmol/1 lactate threshold. International Journal of Sports Medicine 6:117-130. Hoelzer, D.R., Dalsky, G.P., Clutter, W.E., Shah, S.D., Holloszy, J.O., & Cryer, P.E. (1986). ~ Glucoregulation during exercise: hypoglycemia is prevented by redundant glucoregulatory systems sympathocromafin activation, and changes in islet hormone secretion. Journal
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Effect of an Acute I~-adrenergic blockade... CHnical Investigation 7 7 : 2 1 2 - 2 1 . Issekutz, B. (1985). Effect of epinephi-ine on carbohydrate metabolism in exercising in dogs. Metabolism 3 4 : 4 57-64. Juhlin-Dannfelt AC, Terblanche SE, Fell RD, Young J C & Holloszy JO. (1982). Effects of betaadrenergic receptor blockade on glycogenolysis during exercise. J o u r n a l Applied Physiology 53:549-54. Karlsson, J., Kjessel, T. &Kaiser, P. (1983). Alpine skiing a n d acute beta-blockade. International Journal o f Sports Medicine 4:190-3 Kjaer, M. (1992). Regulation of hormonal and metabolic responses during exercise in humans. Exercise Sport Science Reviews 20: 161-84. Kjaer, M., Kiens, B., Hargreaves, M. & Richter, E.A. (1991). Influence of active muscle mass on glucose homeostasis during exercise in man. J o u r n a l Applied Physiology 71: 552-7. Kindermann, W., Simon, G. & Keul, J. (1979). The significance of the aerobic-anaerobic transition for the determination of work load intensities during endurance training. European Journal Applied Physiology 42: 25-34. Kohrt, W.M., O'Connor, J.S. & Skinner, J.S. (1989}. Longitudinal assessment of responses by triathletes to swimming, cycling, and running. Medicine and Science in Sports and Exercise 21: 569-75. Maassen, N., & Busse, M.W. (1989). The relationship between lactic acid and work load: a measure for endurance capacity or a n indicator of carbohydrate deficiency?. European Journal Applied Physiology 58: 728-37. Podolin, D.A., Munger, P.A., & Mazzeo, R.S. (1991). Plasma catecholamines and lactate response during graded exercise with varied glycogen conditions. J o u r n a l Applied Physiology 71: 1427-33. Robergs, R.A., Chwalbinska-Moneta, J., Mitchell, J.B., Pascoe, D.D., Houmard, J. & Costill D.L. (1990). Blood lactate threshold differences between arterialized a n d venous blood. International Journal Sports Medicine 11:446-51. Sim~)es, H.G., Campbell, C.S.G, Kokubun, E., Denadai, B.S., & Baldissera, V. (1999). Blood glucose responses in h u m a n s mirror lactate responses for individual anaerobic threshold and for lactate minimum in track tests. European Journal Applied Physiology 80: 34-40. Stegmann, H., Kindermann, W., & Schnabel, A. (1981). Lactate kinetics and individual anaerobic threshold. International Journal of Sports Medicine 2: 160-5. Tegtbur, U., Busse, M.W., & Braumann, K.M. (1993). Estimation of a n individual equilibrium between lactate production and catabolism during exercise. Medicine and Science in Sports and Exercise 25: 620-627. Wasserman, K., Whipp, B.J., Koyal, S.N., & Beaver, W.L. (1973). Anaerobic threshold and respiratory gas exchange during exercise. J o u r n a l Applied Physiology 35: 236-45. Weltman, A., Seip, R.L., Snead, D., Weltman, J.Y., Haskvitz, E.M., Evans, W.S., Veldhuis, J.D., & Rogol, A.D. (1992). Exercise training at and above the lactate threshold in previously untrained women. International Journal of Sports Medicine 13: 257-263.
265
Balikian, P.B., Neiva, C.M., & Denadai, B.S. (2001). Effect of an acute [~-adrenergic blockade on the blood glucose response during lactate minimum test. Journal of Science and Medicine in Sport 4 (3): 257-265. The aim of this study was to determine the relationship between blood lactate and glucose dining an incremental test after exercise induced lactic acidosis, under normal and acute [3-adrenergic blockade. Eight fit males (cyclists or triathletes) performed a protocol to determine the intensity corresponding to the individual equilibrium point between lactate entry and removal from the blood (incremental test after exercise induced lactic acidosis), determined from the blood lactate (Lacmin) and glucose (Glucmin) response. This protocol was performed twice in a double-blind randomized order by ingesting either propranolol (80 rag) or a placebo (dextrose), 120 rain prior to the test. The blood lactate and glucose concentration obtained 7 minutes after anaerobic exercise (Wingate test) was significantly lower (p<0.01) with the acute [~-adrenergic blockade (9.1_+1.5 raM; 3.9_+0.1 raM), respectively than in the placebo condition (12.4_+1.8 mM; 5.0_+0.1 mM). There was no difference (p>0.05) between the exercise intensity determined by Lacmin (212.1_+17.4 W) and Glucmin (218.2_+22.1 W) during exercise performed without acute ~-adrenergic blockade. The exercise intensity at Lacmin was lowered (p<0.05) from 212.1 -+17.4 to 181.0-+15.6 W and heart rate at Lacmin was reduced (p<0.01) from 161.2_+8.4 to 129.3_+6.2 beats min -1 as a result of the blockade. It was not possible to determine the exercise intensity corresponding to Glucmin with [3-adrenergic blockade, since the blood glucose concentration presented a continuous decrease duling the incremental test. We concluded that the similar pattern response of blood lactate and glucose during an incremental test after exercise induced lactic acidosis, is not present duling ~-adrenergic blockade suggesting that, at least in part, this behavior depends upon adrenergic stimulation.
Introduction D e s p i t e t h e l a c k of a g r e e m e n t a m o n g r e s e a r c h e r s a b o u t t h e u n d e r l y i n g m e c h a n i s m s a n d t e r m i n o l o g y ( W a s s e r m a n et al., 1973; Brooks, 1985; G a e s s e r & Poole, 1986), t h e blood l a c t a t e r e s p o n s e d u r i n g i n c r e m e n t a l exercise h a s b e e n widely u s e d in s p o r t a n d m e d i c a l r e s e a r c h . Practical a p p l i c a t i o n s of t h e b l o o d l a c t a t e r e s p o n s e i n c l u d e t h e m e a s u r e m e n t of a e r o b i c c a p a c i t y t r a i n i n g effects (Kohrt et al., 1989; W e l t m a n et al., 1992) a n d p r e s c r i p t i o n of a p p r o p r i a t e exercise i n t e n s i t y ( K i n d e r m a n n et al., 1979; Billat, 1996). M a n y s t u d i e s h a v e p r o p o s e d p r o t o c o l s to d e t e r m i n e t h e exercise i n t e n s i t y c o r r e s p o n d i n g to t h e m a x i m a l l a c t a t e s t e a d y s t a t e (MLSS), i.e., t h e h i g h e s t c o n s t a n t w o r k l o a d t h a t c a n b e p e r f o r m e d w i t h e q u i l i b r i u m b e t w e e n l a c t a t e e n t r y a n d r e m o v a l from t h e b l o o d ( S t e g m a n n et al., 1981; H e c k et al, 1985).
257
Effect of an Acute 13-adrenergi(: blockade...
Tegtbur et al. (1993) have proposed an interesting protocol (incremental test after exercise induced lactic acidosis) to identify the intensity corresponding to the individual equilibrium point between lactate entry and removal from the blood (Lacmin). The authors found that MLSS could be obtained for all subjects during long term exercise at Lacmin intensity. Moreover, differently from others protocols (Maassen & Busse, 1989; Berry et al., 1991) the intensity corresponding to the Lacmin is not influenced by muscle glycogen stores (Tegtbur et al., 1993) or glucose or caffeine ingestion (Campbell et al., 1998). Recently, Sim6es et al. (1999) reported that it was possible to predict the MISS intensity from the plasma glucose response during a Lacmin test in endurance runners. Moreover, Campbell et al. (1998) have demonstrated that the relationship between blood lactate and glucose during Lacmin test is not altered by glucose or caffeine ingestion. These findings suggest the possibility of using the plasma glucose response to estimate the Lacmin intensity. However, the factors and mechanisms responsible for this association are not completely understood. A possible mechanism is the catecholamine response during exercise, since epinephrine and norepinephrine have a critical role in the body's adjustment to stressful activities. In muscle, through ~-adrenergic receptors, epinephrine initiates a cascade of events that stimulates the degradation of glycogen and an increase in lactate production (Podolin et al., 1991). Moreover, many studies have demonstrated that circulating epinephrine is an important controller of both hepatic glucose production and peripheral glucose uptake during exercise (Kjaer, 1992). Studies that analyze the integrated blood lactate and glucose responses during exercise are important, so beyond the evident theoretical aspects that can be investigated, there is the possibility to validate indirect methods. This will allow its use for a greater n u m b e r of professionals interested to identify the MISS, without specific equipment for blood lactate determination. Therefore, the main purpose of this study was to determine the relationship between blood lactate and glucose during an incremental test after exercise induced lactic acidosis, under normal and acute ~-adrenergic blockade.
Methods and Procedures SubjeCts Eight healthy, physically active male volunteers participated in the study: age (SD) = 21.1 (1.7)yr.; height = 171.4 (8.5) cm; weight =64.7 (9.5) Kg; and VO2max = 62.3 (5.8) ml.kg-l.min -1. Six were competitive triathletes, and two were competitive cyclists. No one was taking regular medication or had any remarkable medical history. The nature of the study was explained to each subject before written informed consent was obtained.
Experimental protocol The participants were instructed to arrive at the laboratory in a rested and fully hydrated state, at least 3 h post-prandial and to avoid strenuous exercise in the 48 h preceding a test session. For each subject, the tests took place at the same time of the day to avoid the effects of diurnal biological variation. Subjects reported to the laboratory three times, at intervals of 2-3 days between the sessions. At the first visit, in order to determine the VO2max, a maximal exercise test was performed on an electrically braked cycle ergometer (Lifestyle 5500). The initial workload was 50 W and the intensity was increased by 25 W every 1 rain until voluntary exhaustion. Respiratory gas exchange data were 258
Effect of an Acute 13-adrenergic blockade...
determined continuously throughout the exercise tests (Vista CPX, Vacumed). In the two subsequent sessions, the intensity corresponding to the Lacmin was determined in a double-blind randomized order with the subject ingesting either propranolol (80 rag) or a placebo (dextrose) 120 m i n prior to the test. In order to determine the intensity corresponding to the kacmin, the subjects were submitted initially to the Wingate test (Bar-Or, 1987), for lactic acidosis induction. The classical variables obtained from the Wingate test were not measured. At 8 min of recovery after the Wingate test, a s u b s e q u e n t incremental exercise test was performed. The initial workload was 100 W and the intensity was increased by 25 W every 3 min until exhaustion. After 7 rain of recovery and at the end of each stage, 25 pl of blood was collected from the ear lobe, into microcentrifuge tubes containing 50 pl NaF (1%), for lactate m e a s u r e m e n t (YSL 2700 STAT). Simultaneously, 5 ml of blood was sampled at 7 min of recovery and during the last 30 seconds of each workload from a n antecubital vein via an indwelling catheter for glucose m e a s u r e m e n t (glucose oxidase - Cobas Mira Plus- Roche). The lowest blood lactate and glucose concentration during the incremental test and its respective intensity were calculated using a spline function, and were considered Lacmin and Glucmin, respectively (Tegtbur et al., 1993; Sim6es et al., 1999) (Figures 1 and 2). Heart rate was m e a s u r e d continuously during the incremental test (Vantage XL, Polar Electro Oy, Oulu, Finland). S t a t i s t i c a l analysis
Results are reported as m e a n s +SD. propranolol were made using Student intensity at Lacmin and Glucmin was analysis and a Pearson product m o m e n t set at P_<0.05.
Comparisons between placebo and t-test. Relationship between exercise accomplished via a linear regression correlation. Statistical significance was
Glucmin
12
6
10
E
5
8
,7
6
32
4
2:= (3 1
O
.J
2
A
0
i
Lacmin i
i
~
i
~
i
i
[
i
-
0
100 125 150 175 200 225 250 275 300
Intensity (Watts) ].,
Lac~te
= Glucose I
Figure 1: Determination of intensities correspondingto the lower lactate (Lacmin)and glucose (Glucmin) concentration during the incremental test (single subject) without beta-adrenergicblockade.A = blood lactate and glucose 7 minutes after anaerobic exercise (Wingate test).
259
Effect of an Acute ~-adrenergic blockade...
12 10
5
8
E
3~
6
o 2 _=
4 2 0
Lacmin
1
i
i
i
i
~
i
i
100
125
150
175
200
225
250
0
Intensity (Watts)
I'
•
Lactate
•
Glucose I
Figure 2: Determination of intensity corresponding to the lower lactate (Lacmin) concentration during the incremental test (single subject) with beta-adrenergic blockade. A = blood lactate and glucose 7 minutes after anaerobicexercise (Wingate test).
Placebo Lactate(mM) Glucose(mM) LacmJn(Watts) Glucim(Watts) HR-Lacmin(bqm) HR-Glucmin(bqm)
12.4 5.0 212.1 218.2 16~.2 166.2
_+ _+ _+ _+ + _+
1.8 0.1 17.4 22.1 8.4 9.2
~-adrenergic blockade
Difference (%)
9.1 -+ 1.5" 3.9 -+ 0.1" 181.0 _+ 15.6"
-26.7 -21.9 -14.0
_+ 7.6 _+ 3.5 _+ 9.7
129.3 _+
-19.3
_+ 3.4
6.2*
Values are means _+SD. *p
Table 1:
Blood lactate and glucose 7 minutes after anaerobic exercise and the intensity (Watts) and heart rate (HR) corresponding to lower blood lactate (Lacmin) and glucose (Glucmin) concentration during an incremental test, with and without B-adrenergicblockade (n=8).
ReSults The blood lactate and glucose concentration obtained 7 minutes after the Wingate test were significantly lower with acute ~-adrenergic blockade (Table 1). The exercise intensity and heart rate corresponding to the Lacmin were significantly lower with acute ~-adrenergic blockade (Table 1). There was no difference between the exercise intensity determined by Lacmin and Glucmin during the protocol performed without acute ~-adrenergic blockade. It was not possible to determine the exercise intensity corresponding to Glucmin with~-adrenergic blockade, since the blood glucose concentration presented a continuous decrease during the incremental test, as showed for one subject in Fig. 2. All others subjects displayed a similar pattern of blood glucose response. There was a significant correlation (r=0.93; P<0.01) between the exercise intensity determined by Lacmin and Glucmin in tests without ~-adrenergic blockade (Figure 3). However, there was no significant correlation (r=0.31; P>0.05) 260
Effect of an Acute ~-adrenergic blockade...
300
~" 250. y = 46.7 + 0.77x
.E E
.J
r = 0.93 S E E = 7.85
2oo
p < O.01(N = 8)
150 150
200
250
300
Glucmin (watts)
Figure 3: Relationship between exercise intensity at lowest blood lactate (Lacmin) and glucose (Glucmin) concentration, without ~-adrenergic blockade. Dashed line is line of indentity, solid line is regression equation.
"E 3:
~
220
~
Q. i
,_= 170 E
.~-'~-
f = 0.31 SEE = 19.23
I~
p > 0.05 (N = 81
i 120 120
170 Lacmin
220 - propranolol
(watts)
Figure 4: Relationship between exercise intensity at lowest blood lactate (Lacmin) concentration, with and without ~-adrenergic blockade. Dashed line is line of indentity, solid line is regression equation.
between the exercise intensity corresponding to the Lacmin with and without acute ~-adrenergic blockade (Figure 4). DiSCuSSiOn This investigation was designed to determine the relationship between lactate and glucose during an incremental test after exercise induced lactic acidosis, under normal and acute ~-adrenergic blockade. The results of this study indicate two significant findings. Firstly, Glucmin determined from the blood glucose response can predict the intensity corresponding to Lacmin during cycling exercise in normal conditions. Secondly, this association is modified by acute ~-adrenergic blockade. Karlsson et al. (1983) analyzed the effect of acute unselective (propranolol) and ~l-selective (atenolol) ~-adrenoceptor blockade on the Wingate test. Peak and average power of the Wingate test showed a significant impairment after both, unselective and ~]-adrenoceptor blockade (4%-6%). However, peak blood lactate 261
Effect of an Acute 13-adrenergicblockade...
was only reduced after propranolol. Similarly, although the p e a k and average power were not m e a s u r e d in the present study, a 26% reduction of p e a k blood lactate was observed after the Wingate test with acute ~t-adrenergic blockade. Glycogenolysis in skeletal muscle is considered to be mediated via ~2-adrenergic receptor stimulation (Arnold & Selberis, 1968). Therefore, in accordance with the data obtained by Karlsson et al. (1983), the lower blood lactate concentration after the Wingate test in the propranolol trial, is not due only to decreased work performed, but a reduced muscle glycolytic rate due [t-adrenergic blockade. However, some studies have provided conflicting results a b o u t the effects of [ladrenergic blockade on muscle glycogenolysis (Juhlin-Dannfelt et al., 1982) and lactate concentration after intense exercise (Broberg et al, 1988). These discrepancies could be due to differences in the completeness of ~-adrenergic blockade achieved and the exercise intensity performed in the different studies (Hargreaves & Richter, 1988). Tegtbur et al. (1993) found no effect of low muscle glycogen stores on the Lacmin determined in ten male endurance trained runners. Although the pretest blood lactate concentration was lower with low muscle glycogen stores (10.6 mM) t h a n in normal condition (14.1 raM), the running speed corresponding to the Lacmin were not significantly different between the conditions. The authors demonstrated that the running speed corresponding to the Lacmin is independent of the respective blood lactate before the incremental test. Therefore, the lower blood lactate concentration observed before the incremental test in our study, probably had no influence on the lower exercise intensity corresponding to the Lacmin with acute [~-adrenergic blockade (Table 1). Campbell et al. (1998) did not observe effect of oral ingestion of glucose (75 g) 30 minutes before the exercise, with consequent increase of glucemia, over the intensity corresponding to the Glucmin. In our study, the [~-adrenergic blockade determined a lower glucemia, although it is still within the lower limit of normality, after the high intensity exercise. Moreover, with the ~-adrenergic blockade it was not possible the identification of Glucmin, because of the constant decrease of glucemia during the incremental exercise. This decrease probably occurred not j u s t by the lower glucemia before this m o m e n t of protocol, b u t because of the action of the [~-adrenergic blockade during the incremental exercise discussed as follows. Thus, the possible isolated effect that the lower glucemia, patC.icnlarly from a real hypoglycemia, can have u p o n the intensity of Glucmin h a s still to be investigated. In this study, the blood samples for lactate and glucose determination were collected from different sites (ear lobe and antecubital vein, respectively), considering the different methods (equipment and blood volume) used for the determination of theses substrates. Some studies have demonstrated that the site of blood collection could influence blood lactate m e a s u r e m e n t s and indices derived from t h e m w.These studies have verified that differences between venous and arterialysed blood lactate were evident when fixed concentrations are used (4 mM) (Foxdal et al., 1991), b u t not w h e n blood lactate curve is analyzed (lactate threshold - infiexion point) (Robergs et al., 1990). Campbell et al. (1998) and Sim6es et al. (1999) m ~ g use of the same sample of blood to analyze the blood lactate and glucemia (YSL 2300 STAT) found similar results to our study, in relation to the association between the Lacmin and Glucmin without [3-adrenerglc bl6ckade. However, at the present moment, there are no studies that have 262
Effect of an Acute 13-adrenergic blockade...
investigated the impact of sampling site on the Lacmin, Glucmin and the relationship between these indices, what do not allow us to confirm definitely what is the influence that the site of blood collection can have upon the relation between the Lacmin and Glucmin. The results of this study support the possibility that the blood glucose response can be used to predict the Lacmin intensity, consistent with previous fmdings (Campbell et al., 1998; Sim6es et al., 1999). Sim6es et al. (1999) verified in welltrained endurance runners, that the velocity determined by Glucmin, was coincident with the velocity corresponding to MLSS, determined by two different incremental tests performed on the track: individual anaerobic threshold (Stegmann et al., 1981), and lactate minimum test (Tegtbur et al., 1993). This phenomenon could, to some extent, be explained by the catecholamines influence on the metabolism, particularly during the strenuous exercise of a graded exercise test. In relation to lactate metabolism, epinephrine binds to ~-adrenergic receptors on the skeletal muscle membrane, initiating a cascade of events leading to glycogen breakdown (via activation of phosphorylase a), which ultimately increases lactate production. This influence is confirmed by many studies performed in humans. Mazzeo and Marshall (1989) manipulated the inflection point in blood lactate during progressive incremental exercise (TLA)via training specificity and found that plasma epinephrine demonstrated an inflection point (TEPI) that was identical to that of TEA across all subjects and exercise tasks. Podolin et al. (1991) have demonstrated that the relationship between TLA and TEPI is not influenced by muscular glycogen stores. In relation to glucose metabolism, some studies have found that epinephrine is directly involved in regulation of both hepatic glucose production (Ra) and peripheral glucose uptake (Rd) during exercise. Ra increases exponentially with workload in parallel with the rise in plasma epinephrine (Kjaer, 1992). During exercise performed in hypoxia, the increase of the catecholamines and Ra is higher compared to normoxia, whereas the responses of pancreatic hormones were similar between the two conditions. Rd also seems to be under the influence of catecholamines during exercise (Kjaer, 1992). Kjaer et al. (1991) found that during combined arm and leg exercise, the catecholamine response as well as Ra rose more markedly compared with the period of leg exercise only. However, the increase of glucose delivery only resulted in a small increase in Rd. The authors suggested that this phenomenon could be determined by the high concentration of epinephi~ne in plasma, as epinephrine has been shown to reduce muscular glucose uptake during exercise (Issekutz, 1985; Hoelzer et al., 1986). Therefore, the catecholamines, particularly epinephrine may play an important role in the mechanism underlying the association between blood lactate and glucose response during incremental exercise. In the present study, the blood glucose response was modified by acute ~adrenergic blockade, making it impossible to identify the inflection in the blood glucose curve. There are no studies in the literature which have analyzed the ~adrenergic blockade effect on the blood glucose response using a similar protocol, which makes it difficult to compare our results to others. However, Galbo et al. (1976) reported a more rapid decline of blood glucose during submaximal exercise after [~-adrenergic blockade. Moreover, the authors found an increase of the carbohydrate combustion rate and less muscle glycogen utilization after ~263
Effect of an Acute ~-adrenergic blockade...
adrenergic blockade. Galbo et al. (1976) suggested that during [~-adrenergic blockade, a smaller inhibition of hexokinase by glucose-6-phosphate derived from glycogen possibly accounted for the larger glucose uptake. Hoelzer et al. (1986) have verified that the hypoglycemia observed during exercise with [3-adrenergic blockade, is not determined by a lower hepatic glucose production. Even though adrenergic stimulation could be important for hepatic glucose production, the fall in insulin levels associated with exercise is not affected by pharmacological ]3-blockade (Hoelzer et al., 1986). Moreover, Hoelzer et al. (1986) also verified an increase of muscular glucose uptake after adrenergic blockade, indicating the importance of catecholamines in preventing hypoglycemia during exercise. This may be due to the lack of a catecholamine action in skeletal muscle (Issekutz, 1985) or indirectly due to lower free fatty acid availability, since lipolysis is depressed under ~-adrenergic blockade. Thus, the increase in muscular glucose uptake may explain the constant decline in blood glucose observed during our protocol with [~-adrenergic blockade. In conclusion, the similar pattern response of blood lactate and glucose duiing an incremental test after exercise induced lactic acidosis, is not present during ~adrenergic blockade suggesting that, at least in part, this behavior depends upon adrenergic stimulation.
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