Cardiorespiratory and metabolic responses to exercise in HbSC sickle cell patients.

PURPOSE: Relative to healthy control individuals with normal hemoglobin (Hb), patients carrying the double heterozygous form of sickle cell disease (HbSC) display an impaired oxygen transport capacity. The present study was undertaken to determine the influence of the decreased oxygen availability associated with the presence of HbSC on the cardiorespiratory and metabolic responses to endurance exercise. METHODS: Eleven black men affected by the double heterozygous form of the sickle cell disease (HbSC group) and seven healthy subjects with normal Hb (HbAA group) of the same ethnic origin submitted successively to an incremental exercise test to exhaustion on a cycle ergometer for the determination of their maximal tolerated power and to a 20-min endurance exercise. RESULTS: The HbSC had a significantly lower exercise tolerance than the HbAA. During the endurance exercise, they exhibited furthermore significantly lower VO2, VCO2, and minute ventilation V(E) than the HbAA. Despite the fact that the HbSC exercised at a significantly lower mean absolute work rate than the HbAA, except for the ventilatory equivalent for CO2 (V(E)/VCO2), which was higher (P < 0.001) in the HbSC group, the other parameters recorded during the 20-min endurance exercise (heart rate, arterial PaO2, PaCO2, pH, lactate, and VE/VO2, the ventilatory equivalent for O2) and during the subsequent recovery (blood lactate) were similar for both groups. CONCLUSION: The study underscores the importance of considering relative work rate as well as absolute work rate to arrive at a correct interpretation of exercise and recovery data. The results give evidence that the modifications of homeostasis brought into play by exercise were shifted toward distinctly lower absolute work rates in HbSC patients.

Short endurance training improves lactate removal ability in patients with heart transplants.

Eight male patients with heart transplants at least a year after the operation were submitted to a 6-wk endurance training program and explored for their blood lactate kinetics before and after exercise. The tests consisted of a bicycle exercise upgraded by 20 W every 2 min until volitional fatigue. Training induced a significant (P < 0.025) decrease in lactate concentrations from the 40-W to the 120-W exercise step and a significant increase (P < 0.025) in the time into exercise (9.87 +/- 0.87 min vs 7.17 +/- 0.90 min) at which a lactate concentration of 2 mmol.l-1 was reached. Lactate recovery curves were significantly lower (P < 0.036) after training than before training, except at minutes 1, 2, 8, and 60. The fits of a biexponential mathematical model to the lactate recovery curves reveal a significant (P < 0.036) training-induced increase (+71%) in the slow-velocity constant gamma 2v of the model. In view of the functional meaning given to this parameter, namely the ability to remove lactate, it is concluded that training lowers blood lactate concentrations during exercise and recovery in patients with heart transplants at least in part by raising the efficiency with which lactate is removed, and that the ability to remove lactate can be a valuable criterion to evaluate physical fitness.

Lactate exchange and removal abilities in sickle cell trait carriers during and after incremental exercise.

Arterial blood lactate concentrations and pH were measured on seven black male sickle cell trait (SCT) carriers before, during and after incremental exhaustive bicycle exercise (25 W increments per minute) and compared with those of six control individuals of the same ethnic origin having a similar physical fitness level. The object of the experiment was to determine if SCT has an effect on lactate kinetics. At volitional exhaustion which was reached at a comparable overall mean absolute work rate for both groups, oxygen consumption expressed per kilogram body mass was significantly lower for the SCT carriers than for the control volunteers. Lactate concentrations were higher for the SCT carriers after the 150 W exercise step but differences reached statistical significance only at exhaustion. Concentrations were distinctly higher for the SCT group during the following 40 minutes of recovery. While there were no observable differences in blood pH between the SCT and control subjects during the exercise, this variable became significantly lower for the SCT than for the control group 8 minutes after the end of exercise. Lactate recovery curves were fitted by a biexponential time function where the two velocity constants inform on the body's overall ability to exchange and remove lactate. The ability to remove lactate was comparable for the two groups. The present results do not warrant drawing a definite conclusion on impairment of the ability to exchange lactate in the presence of SCT. However, SCT carriers are likely to produce more lactate than control subjects reaching exhaustion at similar mean absolute work rate during exhaustive incremental bicycle exercise.

Heat stress does not modify lactate exchange and removal abilities during recovery from short exercise.

Arterial and femoral venous lactate concentrations were measured before, during, and after short intermittent exercise (55-118% of maximal O2 consumption) in thermoneutral (N, 25 degrees C, 10.5 Torr) and hot (H, 45 degrees C, 17.5 Torr) conditions. The thermal load induced significantly higher heart rate and rectal temperature in H relative to N. All the arterial lactate (La) recovery curves were fitted to an equation containing two exponential time functions of the form La(t) = La(0) + A1a(1 - e-gamma 1at) + A2a(1 - e-gamma 2at) where the velocity constants gamma 1a and gamma 2a are the body's overall ability to exchange and remove lactate after exercise, respectively, and t is time. There was no significant difference in these constants, regardless of thermal conditions. The arterial lactate concentration at the end of exercise, the peak lactate concentration during recovery, the amplitudes A1a and A2a of the biexponential function, and the arteriofemoral venous lactate concentration difference during recovery were not significantly different in H relative to N. However, measured and computed arterial lactate concentrations during recovery, especially at the end of the tests, were higher in H (P < 0.04). The more elevated lactate concentrations in H at rest at the end of recovery denote a higher basal lactate production, and they were not due to muscle hypoxia.

Lactate exchange and removal abilities in sickle cell patients and in untrained and trained healthy humans.

Arterial blood lactate concentrations obtained on seven black males with hemoglobin sickle cell disease (SC) before, during, and after graded bicycle exercise up to exhaustion were compared with those of seven untrained (HU) and seven trained (HT) healthy males of the same ethnic origin. Lactate recovery curves were fitted by a biexponential time function consisting of a rapidly increasing and a slowly decreasing component. Higher work rates were reached by the HU and HT than by the SC group. Blood lactate rose distinctly over the corresponding preexercise resting values after the 25-, 50-, and 100-W exercise steps for the SC, HU, and HT groups, respectively. The arterial oxygen content was significantly lower for the SC than for the HU group at rest and at the end of exercise. The velocity constants of the slowly decreasing component of the lactate recovery curves were similar for the SC, HU, and HT groups despite the fact that they cycled up to different absolute work rates. The velocity constant of the rapidly increasing component was significantly higher for the HT. In terms of the functional meaning given to these constants and in view of their inverse relationship with absolute work rate (Freund et al. J. Appl. Physiol. 61: 932-939, 1986), these results indicate that, relative to the HU, the HT and the SC display improved and impaired abilities, respectively, to exchange and to remove lactate.

[The effect of supramaximal exercise on the recovery kinetics of lactate]. / Influence de l'exercice supramaximal sur la cinétique de récupération du lactate.

Blood lactate recovery curves from muscular exercise can be described by a sum of two exponential terms consisting in a rapidly increasing and a slowly decreasing component. A two-compartment model consisting of the previously working muscles and the remainder of the lactate space furnishes the simplest but nevertheless realistic explanation of this evolution pattern. The velocity constants of the rapidly increasing (gamma 1) and the slowly decreasing (gamma 2) components inform respectively on the abilities of the body to exchange and remove lactate. However the blood lactate recovery curves observed in three untrained subjects after 3-min 107-115% VO2max cycling display a transitory plateauing of the lactate concentration between the increasing and decreasing phases of the curves. This levelling off of the concentrations at their highest value was not observed in an endurance trained subject after a 3-min 107% VO2max exercise. Despite the plateauing and consequent less good fit than to the usual evolution curves, the recovery curves could still be accurately fitted by the biexponential time function. After supramaximal exercise the abilities to exchange and to remove lactate are severely impaired. These impairments in the functional properties of the organism are very likely associated with and/or linked to physico-chemical modifications which have been observed as much at the cellular level as in the body fluids during and after supramaximal exercise.

Comparative lactate kinetics after short and prolonged submaximal exercise.

Arterial blood lactate concentrations were determined in two groups of eleven males before, during and after near 2 W.kg body mass-1 bicycle exercise. One group of subjects cycled for 3 min, whereas the second group exercised for 60 min. All the lactate curves during recovery could be fitted to a bi-exponential time function consisting of a rapidly increasing and a slowly decreasing component. This typical evolution pattern indicates that the two-compartment model which has been proposed to represent the movements of lactate after short exercise applies also to recovery from prolonged exercise. Lengthening exercise duration decreased (respectively 10% and 28%) the value of both velocity constants of the fits to the lactate recovery curves, with the difference (28%) being statistically significant for the velocity constant describing the slowly decreasing part of the curves. This result indicates that extending exercise from 3 to 60 min impairs the ability to remove lactate after the exercise.

Lactate removal ability and graded exercise in humans.

Venous lactate concentrations of nine athletes were recorded every 5 s before, during, and after graded exercise beginning at a work rate of 0 W with an increase of 50 W every 4th min. The continuous model proposed by Hughson et al. (J. Appl. Physiol. 62: 1975-1981, 1987) was well fitted with the individual blood lactate concentration vs. work rate curves obtained during exercise. Time courses of lactate concentrations during recovery were accurately described by a sum of two exponential functions. Significant direct linear relationships were found between the velocity constant (gamma 2 nu) of the slowly decreasing exponential term of the recovery curves and the times into the exercise when a lactate concentration of 2.5 mmol/l was reached. There was a significant inverse correlation between gamma 2 nu and the rate of lactate increase during the last step of the exercise. In terms of the functional meaning given to gamma 2 nu, these relationships indicate that the shift to higher work rates of the increase of the blood lactate concentration during graded exercise in fit or trained athletes, when compared with less fit or untrained ones, is associated with a higher ability to remove lactate during the recovery. The results suggest that the lactate removal ability plays an important role in the evolution pattern of blood lactate concentrations during graded exercise.

Blood lactate during constant-load exercise at aerobic and anaerobic thresholds.

Venous blood lactate concentrations [1ab] were measured every 30 s in five athletes performing prolonged exercise at three constant intensities: the aerobic threshold (Thaer), the anaerobic threshold (Than) and at a work rate (IWR) intermediate between Thaer and Than. Measurements of oxygen consumption (VO2) and heart rate (HR) were made every min. Most of the subjects maintained constant intensity exercise for 45 min at Thaer and IWR, but at Than none could exercise for more than 30 min. Relationships between variations in [1ab] and concomitant changes in VO2 or HR were not statistically significant. Depending on the exercise intensity (Thaer, IWR, or Than) several different patterns of change in [1ab] have been identified. Subjects did not necessarily show the same pattern at comparable exercise intensities. Averaging [1ab] as a function of relative exercise intensity masked spatial and temporal characteristics of individual curves so that a common pattern could not be discerned at any of the three exercise levels studied. The differences among the subjects are better described on individual [1ab] curves when sampling has been made at time intervals sufficiently small to resolve individual characteristics.

Comparison of arterial and venous blood lactate kinetics after short exercise.

Seventeen healthy male volunteers participated in this study designed to compare arterial with both arterialized venous and venous lactate kinetics after short exercise. Blood samples drawn before, during, and after bicycle exercise were analyzed continuously for lactate. A mathematical function incorporating two exponential terms was fitted to the arterial, arterialized venous, and venous lactate recovery curves, and the parameters of the mathematical function were compared using a linear regression. All parameters measured on or fitted to the arterialized venous curves correlated well with the respective arterial data (correlation coefficient R = 0.82 to 0.99, P less than 0.001). Among the parameters obtained from the fit to the venous curves, only those describing lactate removal correlated closely with the arterial results. It is concluded that for lactate kinetic studies during recovery following short-term muscular exercise, the information obtained from arterialized venous blood is comparable to arterial blood, whereas the use of venous blood, from the sampling site in this study, appears suitable for determining only the parameters for lactate disappearance. These conclusions are illustrated by the comparison between arterial, arterialized venous, and venous parameters as a function of the work rate of the previously performed exercise.

Effect of exercise duration on lactate kinetics after short muscular exercise.

Arterial blood lactate concentrations were measured in six normal males before, during and after 3- and 6-min bicycle exercises performed at three different work rates. The lactate recovery curves were fitted to a bi-exponential time function consisting of a rapidly increasing and a slowly decreasing component, which supplied an accurate representation of the changes in lactate concentration. Variations in the parameters of this mathematical model have been studied as a function of the duration of exercise and of the work rate, showing a clear dependence on exercise duration such that increasing exercise length decreases the velocity constants of the fitted curves. In terms of the functional meaning which can be given to these constants, this result indicates that extending exercise duration from 3 to 6 min reduces the ability of the whole body to exchange and remove lactate. This effect did not qualitatively modify the one already described, which is due to increased work rates, but it shifted the ability to exchange and remove lactate towards lower values. The main conclusion of the study is that lactate kinetic data vary as a function of time during exercise. This inference must be accounted for in the interpretation of lactate data obtained during muscular exercise.

Prolonged submaximal exercise and L-carnitine in humans.

Changes in the main physiological parameters and circulating indicators of carbohydrate, protein, lipid (and ketone body) metabolism were measured in ten exercising subjects before L-carnitine (L-carn) loading, after 4 weeks of daily loading with 2 g L-carn, and 6-8 weeks after terminating L-carn administration. Measurements were made on venous blood samples collected during each experiment at fixed time intervals over an initial rest of 45 min, 60 min bicycle exercise performed near 50% VO2max and 120 min recovery. Free and total plasma carnitine levels reached a plateau corresponding to an average rise of 25% for both fractions, 9-10 days after the beginning of the L-carn diet. These levels returned to their initial values 6-8 weeks after cessation of the supply. Generally L-carn supplementation did not significantly modify the physiological parameters and circulating metabolites. No distinct increase of the relative participation of endogenous lipids in the fuel supply of prolonged submaximal exercise was observed. In normal human subjects the increased demand for fatty acid oxidation resulting from exercise seems to be adequately supported by endogenous levels of carnitine.

Work rate-dependent lactate kinetics after exercise in humans.

Arterial blood lactate concentrations were measured on 19 subjects before, during, and after a 3-min bicycle exercise at several work rates, and the concentrations during the recovery phases were fitted to a biexponential time function consisting of a rapidly increasing and a slowly decreasing component. Highly significant correlations with the work rate of the exercise preceding the recovery were found for all the parameters of the fitted equation. The two velocity constants show inverse linear relationships, whereas the other parameters vary according to a definite power function. A functional meaning has been given to the two velocity constants, namely the ability of the tissues to exchange and to remove lactate. For the group of subjects studied, after exercises at work rates below about 3.5 W/kg, the tissue's ability to utilize, and possibly to exchange lactate, increases over values generally reported for resting conditions, whereas after exercises at higher work rates the inverse occurs. Lactate kinetics during recovery appear to be the result of two underlying processes, one enhancing the ability of the tissues to exchange and remove lactate and the other restraining it.

Responses of anterior pituitary hormones to heat exposure.

To determine anterior pituitary response to mild hyperthermia, plasma GH, ACTH, PRL and TSH were measured every 10 min in five subjects exposed to hot air (120 min, 50 C, 20 mbar) in a climatic chamber. Mean rectal temperatures increases of 0,8 C and mean sweat losses of 760 g were observed. GH levels increased from 4.4 +/- 0.4 ng/ml to 22.0 +/- 7.0 ng/ml (mean +/- SE) and reached maximum 80 min after the beginning of heat exposure and then declined before the end of the heat stress. These peak levels varied widely with the individual, whereas no significant changes in ACTH, PRL and TSH levels were observed during the exposure period. These results suggest that the mechanisms regulating GH secretion are more sensitive to an acute heat exposure than those of the other pituitary hormones studied.