Patients with heart transplants exhibit impaired exercise capacity and complain of abnormal leg fatigue (25). Furthermore, relative to normal age-matched control subjects, their maximal oxygen consumption is severely reduced (5). The underlying mechanisms of this unfit status are not yet well understood, although peripheral muscular limitation has been highlighted(12,17,19). Several protocols including longterm (13) and short-term(14,15) training programs have been proposed to reverse this peripheral muscular limitation to exercise. In previous works performed with patients with heart transplants(15,18), it has been shown that cardiorespiratory, metabolic, hormonal, muscular, and ultrastructural parameters are improved by a short-term training program. Although a debate still persists over whether absolute or relative work rate should be used as a normalizing independent variable for the interpretation of lactate kinetics data(22), several authors have found evidence that exercise lactate-related parameters such as the “lactate threshold” or the“onset of blood lactate accumulation” supply more accurate and reliable information on the training status of the subjects than maximal O2 consumption (for references see Tanaka, 31). In addition, Freund et al. (7) and Oyono-Enguéllé et al. (21) have mentioned that parameters derived from the fits of a biexponential mathematical model to blood lactate recovery curves could bear pertinent dynamical information on the physical fitness level of the subjects.
The present study was designed to examine whether changes in lactate kinetics parameters can serve as a reliable marker of the metabolic adaptations induced by exercise training in patients with heart transplants. Therefore, we have postulated that improvements in the physical condition of the patients in response to endurance training should be associated with concomitant changes in the efficiency with which lactate is removed during recovery from muscular exercise. Another aim was to foster the debate on the experimental conditions required for such an investigation, and specifically on the relationship between lactate data and absolute or relative work rate.
Eight male patients with heart transplants (HTR), at least 1 yr after the operation, participated in the study. They were all sedentary before and after transplantation and not enrolled in any regular sport or retraining program. Their individual descriptive characteristics are reported inTable 1. All the subjects were informed of the nature and the purpose of the experiments and of the potential risks involved before they gave their written consent. The patients were treated according to a standard immunosuppressive protocol with Cyclosporine (mean dose 325 mg·d-1), Prednisone (0.2 mg·kg-1·d-1) and Azathioprin (0.5-1 mg·kg-1·d-1). Most of them received vasodilator agents (calcium channel antagonist) for moderate hypertension. All were out of rejection, as shown by repeated endomyocardial biopsies. The protocol employed was sanctioned by the local Ethics Committee in view of the reported beneficial effects of a short training program. The experiments took place in the afternoon. The subjects reported to the laboratory 2 h after a light standard lunch. Experiments started at about 2:00 p.m. An initial rest of 1 h was allowed.
The protocol consisted successively of a pre-training, a training, and a post-training period.
Pre-training period. A first graded exercise test (E1) was performed in the upright position using a bicycle ergometer (MEDIFIT 1000 S). It began at 20 W and the work rate was incremented by 20 W every 2 min until volitional exhaustion. That is, despite verbal encouragement, the subjects were no longer able to maintain the requested pedaling rate of 50 rpm. Oxygen consumption (˙VO2) at volitional exhaustion and maximal work rate that could be sustained for 2 min were termed, respectively, peak˙VO2 and maximal tolerated power (MTP). Ventilatory threshold (VT) was determined according to Beaver et al. (2). At least 3 d after E1, the subjects performed a second graded exercise (E2) up to MTP followed by a 60-min recovery (R2). Blood was sampled during E2 and R2. For the first 5 min of R2, subjects rested on the bicycle. Thereafter, they were rapidly moved to an armchair for the remainder of the recovery.
Training period. The training program consisted of three sessions per week for 6 wk of a modified interval training, the Square Wave Endurance Exercise Test (SWEET), proposed by Gimenez et al. (9) and adapted by Lonsdorfer et al. (15). This particular type of exercise consists of nine successive crenels of 5 min duration. During each crenel, a 4-min period of moderate work, termed “base” level, was followed by a 1-min period of very heavy work, termed “peak” level. Base and peak levels had been set, respectively, at individual VT and 90% MTP. Heart rate was monitored continuously during each training session by means of a Sport Tester apparatus. The individual work rates were adjusted in order to keep the exercise heart rate during training between 137 and 148 beats·min-1, which is within the training-sensitive zone for our group in view of their mean age.
Post-training period. To estimate changes in MTP and in peak˙VO2, all the subjects again performed a graded exercise test with a 20-W increment every 2 min up to volitional exhaustion (E3). At least 3 d later, they were submitted to a second incremental exercise (E4) followed by a 60-min recovery phase (R4). During E4 and R4 blood was again sampled. Seven subjects (S1-S7) were asked to stop cycling at their pre-training MTP, while one subject (S8) continued exercising up to his post-training MTP.
A percutaneous indwelling catheter was placed in the cephalic or basilic vein under local anesthesia. Blood was sampled before E2 and E4 after the initial rest of about 1 h, at the end of each work rate increment during E2 and E4, and at minutes 1, 2, 3, 4, 5, 8, 10, 15, 20, 30, 40, 50, and 60 of R2 and R4. For S7, due to a technical incident, blood could not be sampled during one of the recoveries.
During the graded exercises E1 to E4, HR (beats·min-1) was supplied every minute from a continuously monitored ECG (SHILLER CARDIOVIT AT 6) as a safety and check procedure. Peak ˙VO2(ml·min-1 STPD) was determined using a breath-by-breath open spirometric system (MEDISOFT) delivering average values every 15 s.
Blood was collected in dry syringes and whole blood lactate was analyzed by means of a lactate analyzer (Lactate Analyzor Analox GM-7). Measurements were made within 5 min of sampling.
Mathematical and Statistical Procedures
The time into E2 and E4 after which a lactate concentration of 2 mmol·l-1 (t2mmol) was reached has been selected to characterize the pattern of blood lactate evolution for each subject during exercise. The value was interpolated from the blood lactate versus time curve. Although no particular meaning is given to this concentration, it has been chosen because it was reached by all the subjects during E2 and E4, and also because t2mmol represents a simple way to quantify the shift to the right of the blood lactate versus work rate curve in response to the training regimen.
The pre- and post-training individual venous blood lactate recovery curves were fitted to the biexponential time function equation 1
using an iterative nonlinear regression technique. In this equation Lv(0) and Lv(t) (mmol·l-1) are, respectively, the venous lactate concentration at the end of exercise and at any time t into the recovery. A1v and A2v (mmol·l-1) are the amplitudes, andγ1v and γ2v (min-1), the velocity constants of the fitted exponential functions.
Signification of the parameters. A1a, A2a, La(0) obtained from the fits of equation 1 to arterial lactate recovery curves, also termed concentration parameters, are required to describe amplitudes of arterial lactate variations during recovery. As for the velocity constants γ1a and γ2a, they respectively supply direct information on the ability to exchange and to remove lactate during recovery, and indirect information on the same abilities during the previously performed exercise (8). Concerning the analogously defined parameters obtained from venous blood (as in the present study), only A2v and γ2v have been shown to be closely correlated with their arterial counterparts. The differences between Lv(0), A1v, and γ1v and corresponding La(0), A1a, andγ1a depend mainly on the anatomical and functional properties of the subcompartment at the exit of which venous blood has been sampled(20). Therefore, reliable information on lactate removal, but not for lactate exchange, processes can be obtained from venous blood(20).
Differences in MTP before and after training, peak ˙VO2, t2mmol, lactate concentrations during and after E2 and E4, and in the parameters of the fits of equation 1 to venous lactate curves during R2 and R4, were sought for by means of the Wilcoxon ranked test. Statistical significance was set at P < 0.05. InTables 1 and 2 significant differences between the means are indicated with an asterisk.
Subjects experienced no significant change in body weight during the investigation period. Their mean pre- and post-training values for this parameter were 75.1 ± 5.2 and 76.2 ± 5.3 kg, respectively.
As shown in Table 1, the training protocol employed here resulted in a significant 17% increase (P < 0.015) in MTP from 127.5 ± 6.5 W to 148.8 ± 5.8 W, and in a 19% increase in absolute peak ˙VO2 from 1730 ± 90 to 2060 ± 130 ml·min-1. Normalized to body weight, peak ˙VO2 rose from 23.0 ± 1.5 to 27.0 ± 2.7 ml·min-1·kg-1 (+17%).
During E2 and E4, lactate concentrations rose significantly (P< 0.036) over the resting values from the 40-W to the 120-W exercise step(Fig. 1). As shown in the same figure, training shifted the lactate versus work rate curve to the right. The time into E2 and E4 after which a lactate concentration of 2 mmol·l-1 was reached increased significantly (P < 0.025) from 430 ± 52 s (E2) to 592 ± 54 s (E4). Differences in lactate concentrations between E2 and E4 reached statistical significance (P < 0.025) from the 40-W to the 120-W exercise step. Within this interval, post-training lactate concentrations were 23-30% lower than the pre-training ones.
During R2 and R4, lactate concentrations first increased, then reached a maximum, and thereafter decreased with time. For the subjects who exercised up to the same absolute work rate in the pre- and post-training sessions (except S7), lactate concentrations were decidedly lower in the post-training sessions, the differences being statistically significant (P < 0.036) at minutes 0, 3, 4, 5, 10, 15, 20, 30 and 40 of the recovery(Fig. 2). But as shown in Figure 3 for S8, who exercised during the post-training session up to his post-training MTP, the lactate concentrations were similar in the two experimental conditions, with no clear difference in the lactate curves between R2 and R4.
Equation 1 could be fitted to the blood lactate recovery curves. The coefficient of estimation ranged between 95-99%. The parameters of the fits to the individual curves are reported inTable 2. For S1-S6, the training-induced decrease in Lv(0), A1v, and A2v ranged from 26% to 51%, while the rises inγ1v and γ2v were +41 and +71%, respectively. From a statistical point of view, only the changes in Lv(0), A2v, andγ2v were significant (P < 0.036). Relative to R2, during R4, S8 displayed greater Lv(0), A1v, and γ1v, while values of γ2v were practically the same.
It has been established for a long time that training increases MTP and peak ˙VO2. The changes in MTP and peak ˙VO2 recorded in this study in response to the short training protocol are in agreement with those obtained for healthy sedentary individuals by Ekblom et al.(6), but after 16 wk of endurance training. More specifically, as regards short endurance training in patients with heart transplants, the only study to which one can refer for comparison is that of Keteyian et al. (14). In their study on patients with heart transplants, these authors reported an increase of 26.7% in absolute peak ˙VO2 (˙VO2max), and of 19.8% in weight-normalized peak ˙VO2 after a 10-wk supervised endurance training. Compared to our results (respectively 19% and 17% in absolute and normalized peak˙VO2), the changes in the study by Keteyian et al.(14) appear more important when expressed in terms of absolute peak VO2. A possible reason for these differences could lie in the fact that the patients explored by Keteyian et al. were less fit before training (mean peak ˙VO2 of 1.29 l·min-1 vs 1.73 l·min-1 in our study). Rowell (24) has mentioned that following endurance training, peak ˙VO2 increases less in fit than in unfit subjects. Nevertheless, relative to the changes recorded in terms of peak ˙VO2 normalized to body mass (17.3% vs 19.8%) the efficiency of our short training program can be considered as being comparable to that of other short training protocols.
Lactate exercise data have also been employed to demonstrate the positive effects of endurance training (28). The shift to the right of the exercise lactate versus work rate curve has been extensively used for such purposes. In this study, as indicated by the 38% increase(P < 0.025) in t2mmol, training induced a significant shift of the exercise lactate versus work rate curve to the right(Fig. 1). Since blood lactate depends both on the rates of blood lactate appearance and disappearance, the reduced lactate concentrations in E4 relative to E2 could be due to a lower lactate production and/or to an increased lactate clearance (29). In view of the functional meaning given to γ2v (mentioned in the Methods section), the changes induced in this parameter (Table 2) in response to the SWEET training protocol of the present study lend support for an increased lactate clearance, but do not exclude a lower lactate production following training. According to Brooks (3), the major fate of lactate during exercise and recovery is its oxidation. Thus, the increase in subsarcolemnal mitochondrial volumetric density brought on by the SWEET and reported by Mettauer et al. (18) could have some relationship with the important changes in γ2v obtained in this study. Finally, improvement in lactate removal ability following the short endurance training program could have played a role in the concomitant shift of the lactate curve to the right.
Relative to the other measured and/or fitted lactate parameters for the recovery, the lower (26-51%) concentration parameters Lv(0), A1v, and A2v after training compared to before training indicate that exercise had led to a reduced lactate accumulation in the body. With respect toγ1v after training, a 41% increase was recorded. This result is in line with the study by Simon et al. (27) who reported that the peak blood lactate concentration occurred sooner in trained subjects than in the less trained, and with one by Freund et al.(8) who showed higher γ1a in fit individuals than in unfit. Together, these data could mean that lactate exchange processes are improved by training. Such an interpretation corroborates the data obtained by Pilegaard et al. (23) according to which the lactate transport capacity is higher in athletes than in untrained and less trained subjects. However, because a previous study has found γ1v poorly correlated with γ1a (20), the relationship between these two parameters needs reexamination over a larger population before one can arrive at a definite conclusion about the information delivered by venous blood on lactate exchange processes.
The reason(s) why patients with heart transplants displayed impaired exercise capacity before being enrolled in any retraining program(25) remains to be clarified. Nevertheless, two main hypotheses can be considered. One deals with the less active lifestyle before the transplantation (30) and the other with the prescribed immunosuppressive therapy. Glucocorticoids are known to induce muscle atrophy (11). Cyclosporine has been shown to impair mitochondrial respiration both in vitro(10) and in vivo(16). In addition to its hepatotoxicity and nephrotoxicity, Cyclosporine might further shift exercise metabolism toward a higher dependence on carbohydrates, more specifically because of an induced sympathetic hyperreactivity(26). All these factors could be involved in exercise limitation for patients with heart transplants. However, the results of the present study lend support to an important role of the sedentary state in which these patients were forced to stay before transplantation. The immunosuppressive therapy was not stopped during the training program, a fact which could have led to modifications in muscle structure(18) and hence in exercise capacity improvement. Other possibilities are that relative to the study by Mercier et al.(16) where much higher Cyclosporine doses (20 mg·kg-1·d-1) were used, those of the present study(4.33 mg·kg-1·d-1) were probably unable to induce similar impairment in mitochondrial respiration, and/or that training could have overcome the muscular side effects of the immunosuppressive therapy.
In a previous study by Freund et al. (8), a different experimental model was used. Their subjects carried the double heterozygous form (HbSC) of the sickle cell disease and could cycle up to about 120 W, as had been done by the patients with heart transplants of the present study. Theγ2a obtained for patients carrying HbSC ranged from 0.038 to 0.078 min (with a mean value of 0.058) and were 29% higher than those reported here in Table 2. Because γ2a andγ2v have been shown to be close (20), and because also of the dependence of γ2a and γ2v on both work rate (7,20) and physical fitness(7,21), this comparison could indicate first that patients with severe heart failure before transplantation were on the whole less active than patients carrying HbSC; and second, that a long time after transplantation (at least 1 yr) and before any training program, their status(as indicated by γ2v) remained essentially unfit. Individualγ2v values indicate that such an assumption could be true at least for S3-S5 of the present study, who displayed the lowest values. But cross-sectional and longitudinal studies with various diseases (and a precise history in each case) would be necessary to learn the relationship between the disease severity and the γ2 obtained for similar absolute work rates.
Another finding of the present study is illustrated by S8. For the post-training session, exercising up to his post-training MTP resulted in practically no change in the γ2v in view of the inverse relationship between γ2v and work rate (20). This observation points out again the importance of the mode of expression used for lactate data. In a recent paper, Basset et al.(1) stated that the lactate removal ability is the same for untrained and trained subjects after exercise at similar relative work rates. This can be noticed for S8 in the present study. On the other hand, several other authors have suggested employing similar absolute work rates to clearly show changes due to training (4,22). Training enhances lactate removal ability, as shown in Table 2, but increasing work rate reduces this ability. Thus, expressing the data relatively and/or using the same experimental design as Basset et al.(1), and illustrated here by S8, could cancel or minimize improvements in lactate removal processes.
In summary, the ability to remove lactate appeared so sensitive to training in the patients with heart transplants explored in this study that it ought to be seriously considered as a gauge of the effects of an endurance training program. In addition, lactate data differences and/or variations are fairly well accounted for when the work intensity is expressed in terms of absolute work rate.
1. Basset, D. R. Jr., P. N. Merril, F. J. Nagle, J. C. Agre, and R. Sampedro. Rate of decline in blood lactate after cycling exercise in endurance trained and untrained subjects. J. Appl. Physiol.
2. Beaver, W. L., K. Wassermann, and B. J. Whipp. A new method for detecting anaerobic threshold by gas exchange. J. Appl. Physiol.
3. Brooks, G. A. The lactate shuttle during exercise and recovery. Med. Sci. Sports Exerc.
4. Casaburi, R., T. W. Storer, and K. Wassermann. Mediation of reduced ventilatory response to exercise after endurance training.J. Appl. Physiol.
5. Cerretelli, P., B. Grassi, A. Colombini, B. Caru, and C. Marconi. Gas exchange and metabolic transients in heart transplant patients.Respir. Physiol.
6. Ekblom, B., P. O. Astrand, B. Saltin, J. Stenberg, and B. Wallstrom. Effects of training on cardiocirculatory responses to exercise.J. Appl. Physiol.
7. Freund, H., S. Oyono-Enguelle, A. Heitz, J. Marbach, C. Ott, and M. Gartner. Effect of work duration on lactate kinetics after short muscular exercise. Eur. J. Physiol. Occup. Physiol.
8. Freund, H., J. Lonsdorfer, S. Oyono-Enguelle, A. Lonsdorfer, and P. Bogui. Lactate exchange and removal abilities in sickle cell patients and in untrained and trained healthy humans. J. Appl. Physiol.
9. Gimenez, M., E. Servera, and W. Salinas. Square Wave Endurance Exercise Test for training and assessment in trained and untrained subjects. Eur. J. Appl. Physiol. Occup. Physiol.
10. Hokanson, J. F., J. G. Mercier, and G. A. Brooks. Cyclosporine A decreases rat skeletal mitochondrial respiration in vitro. Am. J. Resp. Crit. Care Med.
11. Horber, F. F., H. Hoppeler, J. R. Scheideigger, B. E. Grunig, and H. Howald. Impact of physical training on midthing muscle in normal subjects and in patients treated with glucocorticoids. J. Clin. Invest.
12. Kao, A. C., P. Van Tright, G. S. Schaeffer McCall, et al. Central and peripheral limitations to upright exercise in untrained cardiac transplant recipients. Circulation
13. Kavanagh, T., M. H. Yacoub, D. J. Mertens, J. Kennedy, R. B. Campbell, and P. Sawyer. Cardiorespiratory responses to exercise training after orthotopic cardiac transplantation. Circulation
14. Keteyian, S., R. Shepard, J. Ehrmann, et al. Cardiovascular response of heart transplant patients to exercise training.J. Appl. Physiol.
15. Lonsdorfer, J., E. Lampert, B. Mettauer, et al. Physical fitness after cardiac transplantation: a proposal for an endurance training program and its assessment. Sci. Sports
16. Mercier, J. G., J. F. Hokanson, and G. A. Brooks. Effects of Cyclosporine A on skeletal muscle mitochondria and endurance time.Am. J. Resp. Crit. Care Med.
17. Mettauer, B., E. Lampert, J. Lonsdorfer, et al. Cardiorespiratory and neurohormonal responses to incremental maximal exercise in patients with denervated transplanted hearts. Transplant. Proc.
18. Mettauer, B., E. Lampert, H. Hoppeler, et al. Modifications ultrastructurales du muscle vaste externe après un entraînement court en endurance chez des transplantés cardiaques.Arch. Int. Physiol. Bioch. Biophys.
19. Meyer, M., P. Cerretelli, C. Marconi, M. Rieu, and C. H. Cabrol. Cardiorespiratory adjustments to exercise after cardiac transplantation. In: Clinical Aspects of O2 Transport and Tissue Oxygenation
, K. Reinhart and K. Eyrich (Eds.). Berlin: Springer-Verlag, 1989, pp. 477-497.
20. Oyono-Enguelle, S., M. Gartner, J. Marbach, A. Heitz, C. Ott, and H. Freund. Comparison of arterial and venous blood lactate kinetics after short exercise. Int. J. Sports Med.
21. Oyono-Enguelle, S., A. Heitz, J. Marbach, et al. Lactate removal ability and graded exercise. J. Appl. Physiol.
22. Oyono-Enguelle, S., and H. Freund. Ability to remove lactate in endurance-trained and -untrained humans. J. Appl. Physiol.
23. Pilegaard, H., J. Bangsbo, E. A. Richter, and C. Juel. Lactate transport studied in sarcolemmal giant vesicles from human muscle biopsies: relation to training. J. Appl. Physiol.
24. Rowell, L. B. Human cardiovascular adjustments to exercise and thermal stress. Physiol. Rev.
25. Savin, W. M., W. L. Haskell, J. S. Schroeder, and E. B. Stinson. Cardiorespiratory response of cardiac transplant patients to graded symptom-limited exercise. Circulation
26. Scherrer, U., S. Vissing, B. Morgan, et al. Cyclosporine induced sympathetic activation and hypertension after transplantation.N. Engl. J. Med.
27. Simon, J., J. L. Young, D. K. Blood, K. R. Segal, and R. B. Case. Plasma lactate and ventilatory thresholds in trained and untrained cyclists. J. Appl. Physiol.
28. Sjodin, B., and J. Svedenhag. Changes in onset of blood lactate accumulation (OBLA) and muscle enzymes after training at OBLA.Eur. J. Appl. Physiol. Occup. Physiol.
29. Stanley, W. C., E. W. Gertz, J. A. Wisneski, D. L. Morris, R. A. Neese, and G. A. Brooks. Systemic lactate kinetics during graded exercise in man. Am. J. Physiol.
249(Endocrinol. Metab. 12): E595-E602, 1985.
30. Stratton, J. R., J. F. Dunn, S. Adamopoulos, G. J. Kemp, A. S. J. Coats, and B. Rajagopalan. Training partially reverses skeletal muscle metabolic abnormalities during exercise in heart failure. J. Appl. Physiol.
31. Tanaka, K. Lactate related factors as a critical determinant of endurance. Ann. Physiol. Anthrop.