Effect of antidepressants on tolerance to hypoxia and physical exercise

Laboratory of Pharmacology, S. Ordzhonikidze All-Union Pharmaceutical Chemical Research Institute, Moscow. Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 112, No. 8, pp. 156–158, August, 1991.     

Influence of passive lower-body heating on muscle metabolic perturbation and high-intensity exercise tolerance in humans

The purpose of this investigation was to determine the influence of heat stress on the dynamics of muscle metabolic perturbation during high-intensity exercise. Seven healthy males completed single-legged knee-extensor exercise until the limit of tolerance on two separate occasions. In a randomized order the subjects underwent 40 min of lower-body immersion in warm water at 42°C prior to exercise (HOT) or received no prior thermal manipulation (CON). Following the intervention, muscle metabolism was measured at rest and throughout exercise using (31)P-MRS. The tolerable duration of high-intensity exercise was reduced by 36% after passive heating (CON: 474?±?146 vs. HOT: 303?±?76?s; P?=?0.005). Intramuscular pH was lower over the first 60?s of exercise (CON: 7.05?±?0.02 vs. HOT: 7.00?±?0.03; P?=?0.019) in HOT compared to CON. The rate of muscle [PCr] degradation during exercise was greater in the HOT condition (CON: -0.17?±?0.08 vs. HOT: -0.25?±?0.10%?s(-1); P?=?0.006) and pH also tended to change more rapidly in HOT (P?=?0.09). Muscle [PCr] (CON: 26?±?14 vs. HOT: 29?±?10%), [Pi] (CON: 504?±?236 vs. HOT: 486?±?186%) and pH (CON: 6.84?±?0.13 vs. HOT: 6.80?±?0.14; P?>?0.05) were not statistically different at the limit of tolerance (P?>?0.05 for all comparisons). These results suggest that the reduced time-to-exhaustion during high-intensity knee-extensor exercise following lower-body heating might be related, in part, to accelerated rates of change of intramuscular [PCr] and pH towards 'critical' values that limit muscle function.     

Carbohydrate supplementation improves moderate and high-intensity exercise in the heat

The aim of the present study was to clarify the effect of carbohydrate (CHO) supplementation on moderate and high-intensity endurance exercise in the heat. Eight endurance-trained men [maximal oxygen uptake ( VO(2max)) 59.5+/-1.6 ml kg(-1) bw(-1), mean+/-SE] cycled to exhaustion twice at 60% VO(2max) and twice at 73% VO(2max) at an ambient temperature of 35 degrees C. Subjects ingested either a 6.4% maltodextrin solution (CHO) or an artificially flavoured and coloured placebo (PLA). Time to fatigue was significantly greater with CHO in both the 60% and 73% VO(2max) trials (14.5% and 13.5% improvement, respectively). Heart rate and oxygen uptake ( VO(2)) did not differ at any point between PLA and CHO. Hypoglycaemia was not seen in any condition but plasma glucose concentrations tended to be higher at both intensities when CHO was fed. CHO oxidation rates were similar at 60% VO(2max) between CHO and PLA. There were no differences between PLA and CHO in the rate of rise of rectal temperatures ( T(rec)) at either intensity but there was a trend for subjects to fatigue at a high temperature when taking CHO. Ratings of perceived exertion (RPE) tended to be lower throughout both CHO trials; this was significant at 80 min and at fatigue at 60% VO(2max). It is concluded that supplementation with CHO improves exercise performance in the heat at both moderate and high endurance intensities. In the absence of a clear metabolic explanation, a central effect involving an increased tolerance of rising deep body temperature merits further investigation.     

急性重复缺氧对小鼠缺氧耐受性的影响及其机制的初步探讨

小鼠在密闭缺氧的重复作用下,对缺氧的耐受性逐次递增,第2、3、4、5次的耐受时间分别较第1次增加1.8、2.5、3.0和3.6倍;第4次重复缺氧后的动物在更低氧分压下的存活时间较正常对照动物延长10倍;其氰化钾中毒致死时间较正常动物推迟4倍。腹腔注入缺氧耐受小鼠脑匀浆提取液,可显著延长小鼠在低氧分压下的存活时间,与腹腔注入等体积生理盐水和注入等体积正常小鼠脑匀浆提取液组相比较,分别延长1.8和2.1倍。结果提示,急性重复缺氧,可能使组织细胞,特别是脑细胞发生某种可塑的或适应的变化,从而导致动物对缺氧具有非常高的耐受水平;急性重复缺氧小鼠脑中的一种或多种可提取并可通过血脑屏障的水溶性物质可能为该适应变化的一部分。     

Influence of prior exercise on muscle [phosphorylcreatine] and deoxygenation kinetics during high-intensity exercise in men

(31)Phosphate-magnetic resonance spectroscopy and near infrared spectroscopy (NIRS) were used for the simultaneous assessment of changes in quadriceps muscle metabolism and oxygenation during consecutive bouts of high-intensity exercise. Six male subjects completed two 6 min bouts of single-legged knee-extension exercise at 80% of the peak work rate separated by 6 min of rest while positioned inside the bore of a 1.5 T superconducting magnet. The total haemoglobin and oxyhaemoglobin concentrations in the area of the quadriceps muscle interrogated with NIRS were significantly higher in the baseline period prior to the second compared with the first exercise bout, consistent with an enhanced muscle oxygenation. Intramuscular phosphorylcreatine concentration ([PCr]) dynamics were not different over the fundamental region of the response (time constant for bout 1, 51 +/- 15 s versus bout 2, 52 +/- 17 s). However, the [PCr] dynamics over the entire response were faster in the second bout (mean response time for bout 1, 72 +/- 16 s versus bout 2, 57 +/- 8 s; P < 0.05), as a consequence of a greater fall in [PCr] in the fundamental phase and a reduction in the magnitude of the 'slow component' in [PCr] beyond 3 min of exercise (bout 1, 10 +/- 6% versus bout 2, 5 +/- 3%; P < 0.05). These data suggest that the increased muscle O(2) availability afforded by the performance of a prior bout of high-intensity exercise does not significantly alter the kinetics of [PCr] hydrolysis at the onset of a subsequent bout of high-intensity exercise. The greater fall in [PCr] over the fundamental phase of the response following prior high-intensity exercise indicates that residual fatigue acutely reduces muscle efficiency.     

Systemic hypoxia promotes lymphocyte apoptosis induced by oxidative stress during moderate exercise

Blood undergoes oxidative stress during severe hypoxia or intense exercise. Excessive exposure to oxidative stress induces replicative senescence and apoptosis of lymphocytes. This study determines how various exercises with/without hypoxia affect lymphocyte subset mobilization and oxidative stress-induced lymphocyte apoptosis. Eighteen sedentary males randomly engaged in two normoxic exercise bouts [severe exercise (SE) (up to VO_2max) and moderate-intensity exercise (ME) (50%VO_2max) while exposed to 21%O₂], two hypoxic exercise bouts (ME while exposed to 12%O₂ and 15%O₂) and two hypoxic resting conditions (resting while exposed to 12%O₂ and 15%O₂) in a normobaric hypoxia chamber. Under normoxic conditions, SE but not ME (1) increased the percentages of senescent (CD28⁻ and CD57⁺)/activated (CD62L⁻ and CD11a⁺)-form lymphocytes mobilized into the peripheral blood compartment; (2) decreased the levels of surface thiol and intracellular total (t-GSH) and reduced-form glutathione (r-GSH) of lymphocytes in blood; and (3) further enhanced the extents of H₂O₂-induced mitochondria trans-membrane potential diminishing, caspases 3/8/9 activation, poly(ADP-ribose) polymerase cleavage and phosphotidyl serine exposure in blood lymphocytes. However, no significant change occurred in the subset mobilization, antioxidant levels or apoptosis of lymphocytes following exposure to either 12%O₂ or 15%O₂. Although both 12%O₂ and 15%O₂ ME increased the mobilization of senescent/activated-form lymphocytes, only 12%O₂ ME enhanced H₂O₂-induced lymphocyte thiol, t-GSH and r-GSH consumption and apoptotic responses. Therefore, we conclude that the 12%O₂ exposure increases the mobilization of senescent/activated-form lymphocytes into the peripheral blood compartment and simultaneously enhances oxidative stress-induced lymphocyte apoptosis by diminishing cellular antioxidant levels during exercise.     

Influence of moderate cold exposure on blood lactate during incremental exercise

Summary This study examined the effect of exposure of the whole body to moderate cold on blood lactate produced during incremental exercise. Nine subjects were tested in a climatic chamber, the room temperature being controlled either at 30°C or at 10°C. The protocol consisted of exercise increasing in intensity in 35 W increments every 3 min until exhaustion. Oxygen consumption (VO₂) was measured during the last minute of each exercise intensity. Blood samples were collected at rest and at exhaustion for the measurement of blood glucose, free fatty acid (FFA), noradrenaline (NA) and adrenaline (A) concentrations and, during the last 15 s of each exercise intensity, for the determination of blood lactate concentration [la^–]_b. TheVO₂ was identical under both environments. At 10°C, as compared to 30°C, the lactate anaerobic threshold (Th_{an, la} ^–) occurred at an exercise intensity 15 W higher and [Th_{an, la} ^–]_b was lower for submaximal intensities above the Th_{an, la} ^– Regardless of ambient temperature, glycaemia, A and NA concentrations were higher at exhaustion while FFA was unchanged. At exhaustion the NA concentration was greater at 10°C [15.60 (SEM 3.15) nmol·l^–1] than at 30°C [8.64 (SEM 2.37) nmol·l^–1]. We concluded that exposure to moderate cold influences the blood lactate produced during incremental exercise. These results suggested that vasoconstriction was partly responsible for the lower [la^–]_b observed for submaximal high intensities during severe cold exposure.     

Muscle contractile characteristics: relationship to high-intensity exercise

We investigated the relationship between muscle contractile characteristics, collected using percutaneous electrical stimulation, and high-intensity exercise performance. Seventeen participants performed a muscle performance test for the calculation of rate of torque development (RTD), rate of relaxation (RR_1/2), rate of fatigue and fatigue resistance. On a second visit the participants completed a Wingate cycle ergometer test with peak power, mean power, fatigue index and fatigue rate calculated. The muscle fatigue index related significantly to the WAnT fatigue index and fatigue rate (p < 0.01). The change in rate of torque development (%ΔRTD) was also related significantly to the fatigue rate (W/s) during the WAnT. Subjects displaying the greatest reduction in RTD had the greatest fatigue rate during the WAnT and greater fatigue during the electrical stimulation protocol. There were no significant relationships between peak (r 0.36; p > 0.01) or mean power (r −0.11, p > 0.01) with any of the muscle performance measures. These findings demonstrate that muscle contractile characteristics, elicited during standardised in vivo electrical stimulation, relate to performance during a Wingate anaerobic test. They suggest that muscle contraction characteristics play an important role in high-intensity exercise performance and indicate that electrical stimulation protocols can be a useful additional tool to explore muscle contraction characteristics in relation to exercise performance and trainability.     

Effects of hyperthermia on the metabolic responses to repeated high-intensity exercise

In this study, we investigated the metabolic and performance responses to hyperthermia during high-intensity exercise. Seven males completed two 30-s cycle sprints (SpI and SpII) at an environmental temperature of 20.6 (0.3) °C [mean (SD)] with 4 min recovery between sprints. A hot or control treatment preceded the sprint exercise. For the hot trial, subjects were immersed up to the neck in hot water [43°C for 16.0 (3.2) min] prior to entering an environmental chamber [44.2 (0.8)°C for 30.7 (7.1) min]. For the control trial, subjects were seated in an empty bath (15 min) and thereafter in a normal environment [20.2 (0.6)°C for 29.0 (1.9) min]. Subjects core temperature prior to exercise was 38.1 (0.3)°C in the hot trial and 37.1 (0.3)°C in the control trial. Mean power output (MPO) was significantly higher in the hot condition for SpI [683 (130) W hot vs 646 (119) W control (P<0.025)]. Peak power output (PPO) tended to be higher in the hot trial compared with the control trial for SpI [1057 (260) W hot vs 990 (245) W control (P=0.03, NS)]. These differences in power output were a consequence of a faster pedal cadence in the hot trial (P<0.025). There were no differences in sprint performance in SpII in the hot trial compared to the control trial; however, MPO was significantly reduced from SpI to SpII in the hot condition but not in the control condition (P<0.025). Plasma ammonia was higher in the hot trial at 2 min post-SpI [169 (65) mol l^-1 hot vs 70 (26) mol l^-1 control (P<0.01)], immediately and at 2 min post-SpII [231 (76) mol l^-1 hot vs 147 (72) mol l^-1 control (P<0.01)]. Blood lactate was higher in the hot trial compared with the control trial at 5 min post-SpII (P<0.025). The results of this study suggest that an elevation in core body temperature by 1°C can improve performance during an initial bout of high-intensity cycle exercise but has no further beneficial effect on subsequent power production following a 4-min recovery period.     

Perception of effort during high-intensity exercise at low,moderate and high wet bulb globe temperatures

The purpose of this study was to determine the effect of low, moderate and high wet bulb globe temperatures (T _wbg) on cardiovascular variables and ratings of perceived exertion (RPE) during moderately prolonged, high-intensity exercise. Six subjects [four men and two women; mean (SD) age, 22.0 (1.2) years; maximum oxygen consumption ({ie519-1}), 51.0 (8.4) ml · kg^–1 · min^–1] completed 30 min of exercise (80% {ie519-2}) on a cycle ergometer at low [14.7 (2.1)°C], moderate [21.0 (1.5)° C], and high [27.4 (2.3)° C]T _wbg. Two additional subjects completed 20 min of exercise in the high temperature condition, but completed 30 min in the moderate and lowT _wbg. Heart rate (f _c), blood pressure, blood lactate (La), mean skin temperature ( _sk), , and RPE were measured at 10, 20 and 30 min. Results showed thatf _c, rate pressure product, RPE, pulmonary ventilation and ventilatory equivalent for oxygen increased (P < 0.05) across time for all conditions, while decreased across time. _sk andf _c were significantly greater across time in the high condition [35.9 (0.65)° C; 176 (12.6) beats · min^–1] compared to the moderate [34.6 (1.5)° C; 170 (17.2) beats · min^–1] and the low condition [31.7 (1.5)° C; 164 (17.1) beats-min^–1]. However, there were no differences throughout exercise in RPE [high,.16.2 (2.0); moderate, 16.4 (2.2); low, 16.3 (1.9)] and across the conditions. These data suggest that RPE is closely related to metabolic intensity but is not a valid indicator of cardiovascular strain during exercise in highT _wbg conditions.     

Carbohydrate consumption prior to repeated bouts of high-intensity exercise

Summary Rapid depletion of muscle glycogen occurs during activities greater than 100% of maximal oxygen uptake. While carbohydrate ingestion prior to an endurance event has been shown to be beneficial, the effects of carbohyrate ingestion on repeated bouts of high-intensity exercise are not known. Therefore, the purpose of this study was to determine if carbohydrate ingestion prior to repeated bouts of high-intensity, short-duration exercise would improve performance. Ten well-trained male cyclists performed two experimental rides, one 15 min after consumption of 5.0 ml·kg^–1 body weight of a 19.7% carbohydrate drink and one following a placebo. The experimental ride consisted of four 1.6 km timed performance rides separated by 4.8 km steadystate rides at 80% of maximal oxygen uptake (between the last two performance rides the steady-state rides were 1.6 km at 80% and 1.6 km at 90%). Blood glucose levels were significantly increased following both the ingestion of the carbohydrate beverage and the performance of the exercise bout. Total exercise time following ingestion of the experimental drink [mean (SD); 25.6 (3.3) min] was not different from that following ingestion of the placebo [25.2 (3.3) min]. Similarly, the sum of all four timed performance rides following ingestion of the experimental drink [6.8 (0.9) min] was not different from that following ingestion of the placebo [6.6 (0.9) min]. In the present study, carbohydrate ingestion 15 min prior to exercise increased blood glucose levels, although performance time was not affected.     

Effects of low and high levels of moderate hypoxia on anaerobic energy release during supramaximal cycle exercise

The purpose of this study was to investigate whether hypoxia can alter anaerobic energy release during supramaximal exercise. Seven male subjects performed 12 submaximal cycling tests to establish the relationship between workload and O₂ demand. The subjects also performed 40 s Wingate tests (WT) under normoxia (room air), two levels of moderate hypoxia of 16.4% O₂ and 12.7% O₂. We measured the power output and oxygen uptake (VO₂) during each test and estimated the O₂ demand, O₂ deficit and percentage of anaerobic energy release (%AnAER). These data were analyzed for each 20 s interval. At all intervals, there were no differences in Pmean·body mass (BM)⁻¹, O₂ demand·BM⁻¹ or O₂ deficit·BM⁻¹ among the three O₂ conditions. However, under hypoxia of 12.7%, VO₂·BM⁻¹ was significantly decreased and %AnAER was significantly increased in the late phase (20–40 s) of the WT, compared to normoxia (P<0.05). There were no such significant differences between normoxia and hypoxia of 16.4%. Thus, the present results show that the degree of hypoxia affects the magnitude of the hypoxia-induced increase in anaerobic energy release in the late phase of the WT and suggest that certain degrees of hypoxia induce significant increases in the amount of anaerobic energy released, compared to normoxia.     

Hyperammonaemia in relation to high-intensity exercise duration in man

Adenine nucleotide (AN) degradation has been shown to occur during intense exercise in man and in the horse, at or close to the point of fatigue. The aim of the study was to compare plasma ammonia concentration ([NH₃]) as a result of intense exercise with plasma [lactate]. Plasma glutamine concentration ([Gln]) was also measured pre- and post-exercise. On separate occasions, nine healthy subjects (two females) exercised on a motorised treadmill for periods of between 30 s and 210 s, at 5.6 m · s ^–1 (0% incline). On one occasion, running at the same speed, two subjects ran at +4% incline whilst one other subject ran at + 7% incline. Blood samples were taken and plasma was analysed for [lactate], [NH₃] and [Gln]. Subjects showed varying degrees of AN degradation as indicated by plasma [NH₃]. A comparison of plasma [NH₃] with that of plasma [lactate] indicated a marked increase in AN degradation, corresponding to a [lactate] of around 14 mmol · l^–1 in plasma. The data further support the hypothesis that there is a critical intramuscular pH below which there is a stimulus to AN degradation during intense exercise, possibly as a result of a substantial reduction in the kinetics of adenosine diphosphate (ADP) rephosphorylation provided by phosphocreatine, resulting in an increase in [ADP].     

Influence of prolonged intermittent high-intensity exercise on knee flexor strength in male and female soccer players

This study investigated the effect of an acute, prolonged, intermittent, high-intensity single-leg pedalling exercise task (PIHIET) on the isokinetic leg strength of the knee flexors in six male and seven female collegiate soccer players. Following determination of single-leg V˙O_2peak, subjects completed a PIHIET designed to simulate the energetics of soccer match play (~90 min in total; ~70% single-leg V˙O_2peak). Pre-, mid- and post-PIHIET gravity-corrected indices of knee flexion peak torque (PT) and range of motion-relativised torque at 15% of knee flexion (RRT_15%; 0% = full knee extension) were assessed at a lever-arm angular velocity of 1.05 rad·s⁻¹ for intervention and control limbs using an isokinetic dynamometer. Repeated measures ANOVAs revealed significant condition (PIHIET, control) × time (pre-, mid-, post-PIHIET) interactions for knee flexion PT (F _[2,22]=26.2; P<0.001) and RRT_15% (F _[2,22]=20.1; P<0.001). Flexion PT and RRT_15% were observed to decrease, pre- to post-intervention, from 92.8 (28.7) N·m to 72.1 (28.0) N·m and from 63.8 (17.5) N·m to 47.9 (18.4) N·m respectively, for the intervention limb alone. These data corresponded to 22.3% and 24.9% mean reductions pre–post intervention in PT and RRT_15%. Exploratory post hoc analysis of the pattern of the relative deterioration (%) of PT and RRT_15%, for the intervention limb alone, revealed a three-way interaction [group (male, female) x parameter (PT, RRT_15%) x assessment phase (pre- to mid-PIHIET, mid- to post-PIHIET)] (F _1,11=5.2; P<0.05). This interaction characterised a greater deterioration of strength performance during the mid- to post-PIHIET assessment phase, at the extremes of range of motion (RRT_15%) for the female group. The greater percentage of mid–post phase strength loss observed in women near the end-range extension may potentially be implicated in the higher incidence of knee injury reported in female soccer players. Electronic Publication     

Blood lactate exchange and removal abilities after relative high-intensity exercise: effects of training in normoxia and hypoxia

The effects of 4 weeks of endurance training in conditions of normoxia or hypoxia on muscle characteristics and blood lactate responses after a 5-min constant-load exercise (CLE) at 90% of the power corresponding to the maximal oxygen uptake were examined at sea-level in 13 sedentary subjects. Five subjects trained in normobaric hypoxia (HT group, fraction of oxygen in inspired gas = 13.2%), and eight subjects trained in normoxia at the same relative work rates (NT group). The blood lactate recovery curves from the CLE were fitted to a biexponential time function: La<SUB>(</SUB><I><SUB>t</SUB></I><SUB>)</SUB>=La<SUB>(0)</SUB>+A<SUB>1</SUB>(1-e<SUP>-</SUP><SUP>γ</SUP><SUP>1·t</SUP>)+A<SUB>2</SUB>(1-e<SUP>-</SUP><SUP>γ</SUP><SUP>2·t</SUP>), where the velocity constants γ₁ and γ₂ denote the lactate exchange and removal abilities, respectively, A₁ and A₂ are concentration parameters that describe the amplitudes of concentration variations in the space represented by the arterial blood, La_{( t )} is the lactate concentration at time t, and La₍₀₎ is the lactate concentration at the beginning of recovery from CLE. Before training, the two groups displayed the same muscle characteristics, blood lactate kinetics after CLE, and γ₁ and γ₂ values. Training modified their muscle characteristics, blood lactate kinetics and the parameters of the fits in the same direction, and proportions among the HT and the NT subjects. Endurance training increased significantly the capillary density (by 31%), citrate synthase activity (by 48%) and H isozyme proportion of lactate dehydrogenase (by 24%), and γ₁ (by 68%) and γ₂ (by 47%) values. It was concluded that (1) endurance training improves the lactate exchange and removal abilities estimated during recovery from exercises performed at the same relative work rate, and (2) training in normobaric hypoxia results in similar effects on lactate exchange and removal abilities to training in normoxia performed at the same relative work rates. These results, which were obtained non-invasively in vivo in humans during recovery from CLE, are comparable to those obtained in vitro or by invasive methods during exercise and subsequent recovery. Electronic Publication     

Effects of moderate exercise training on plasma volume, baroreceptor sensitivity and orthostatic tolerance in healthy subjects

The effect of physical training on an individual's ability to withstand an orthostatic stress is unclear. This study was undertaken to determine the effects on orthostatic tolerance in healthy volunteers of training at a level appropriate for unfit subjects and cardiorespiratory patients. In 11 asymptomatic, untrained subjects the following assessments were made: plasma volume by Evans Blue dye dilution and blood volume derived from haematocrit; carotid baroreceptor sensitivity from the slope of the regression of change in cardiac interval against pressure applied to a neck chamber; orthostatic tolerance as time to presyncope in a test of head-up tilting combined with lower body suction; exercise test relating heart rate to oxygen consumption. Subjects were then given a training schedule (5BX/XBX, Royal Canadian Air Force) involving 11-12 min of mixed exercises per day until an age/sex related 'target' was reached. Following training all subjects showed evidence of improved fitness, seen as decreases in heart rate at an oxygen uptake (Vo2) of 1.5 1 min-1 and in the elevation of the regression line between heart rate and Vo2. All also had increases in plasma and blood volumes and decreases in baroreceptor sensitivity. Seven of the subjects showed increases in orthostatic tolerance. Improvement in orthostatic tolerance was related to a low initial tolerance, and was associated with increases in plasma volume and decreases in baroreceptor sensitivity. These results show that moderate exercise training increases orthostatic tolerance in subjects who do not already have a high initial tolerance and suggest that training may be of value in the management of untrained patients with attacks of syncope due to orthostatic intolerance.     

Effect of moderate exercise on rat T-cells

Summary The aim of this study was a detailed examination of the effects of moderate exercise on T-cells in adult male Wistar rats. The T-cell populations were compared in sedentary rats (C, n = 5) and in rats trained for 4 weeks on a treadmill (30–60 min·day^–1, 6 days·week^–1, 20–30 m·min^–1) and sacrificed at rest (Trest, n=5). In the T-rest rats, there were higher percentages of CD4+CD8–, CD4– CD8 + and CD4 – CD8 –thymocytes (P<0.05, P<0.05 and P<0.01 respectively) and of CD4–CD8 + splenocytes (P< 0.01), and a lower percentage of CD4–CD8+ cells in the lymph nodes (P<0.01). Compared with T-rest or C rats, trained rats (n = 5) or untrained rats (n = 5) sacrificed immediately after a running session (60 min, 30 m·min^–1) had a higher percentage of mononucleated cells CD4 + CD8 -in the blood (P<0.05 and P<0.01). Lastly, compared with C rats, rats (n=5) sacrificed immediately after their 5th day of training (30–60 min·day^–1) presented a higher total splenocyte population (P<0.05) and greater in vitro production of T-cell growth factor (interleukin 2 + interleucin 4) by splenocytes in response to a mitogen (P<0.01). These results would indicate that moderate endurance training modifies the cellular composition of lymphoid organs, without impairing the in vitro functions of T-cells.