Structural and metabolic properties of rat muscle exposed to weightlessness aboard Cosmos 1887.

Male Wistar rats were subjected to 12.5 days of weightlessness aboard Cosmos 1887. Histomorphometric and biochemical analyses were investigated in soleus (SOL), plantaris (PL) and extensor digitorum longus (EDL) muscles of flight rats (group F) and compared with data from two groups of terrestrial controls: one group living free in a vivarium (group V) and another subjected to a flight simulation except for the state of weightlessness (group S). Relative to groups V and S, no alteration in the percentage distribution of fibres had occurred in SOL, PL or EDL, after the flight. In SOL muscles from group F animals, cross-sectional areas of all fibre types were reduced to a greater extent (-40%) than capillary to fibre ratio (-24%) leading to a higher capillary density (+33%) than in V and S groups. In PL, type I, IIA and IIB fibre cross-sectional areas were less decreased (-25%). In EDL, only fast-twitch fibre cross-sectional areas showed an average decrease of 30%. Capillary per fibre ratio was reduced by 15% and 28% respectively in PT and EDL muscles from group F rats compared to control groups V and S. Citrate synthase and 3-hydroxyacyl-coenzyme A dehydrogenase activities remained unchanged in SOL, PL and EDL following spaceflight. These findings indicate greater atrophy and functional alterations (capillarity) compared to those observed after 7 days of microgravity on Cosmos 1667.

Cardiac output during exercise in paraplegic subjects.

The purpose of this study was to measure the cardiac output using the CO2 rebreathing method during submaximal and maximal arm cranking exercise in six male paraplegic subjects with a high level of spinal cord injury (HP). They were compared with eight able bodied subjects (AB) who were not trained in arm exercise. Maximal O2 consumption (VO2max) was lower in HP (1.11.min, SD 0.1; 17.5 ml.min-1.kg-1, SD 4) than in AB (2.5 l.min-1, SD 0.6; 36.7 ml.min-1.kg, SD 10.7). Maximal cardiac output was similar in the groups (HP, 14 l.min-1, SD 2.6; AB, 16.8 l.min-1, SD 4). The same result was obtained for maximal heart rate (fc,max) (HP, 175 beats.min-1, SD 18; AB, 187 beats.min-1, SD 16) and the maximal stroke volume (HP, 82 ml, SD 13; AB, 91 ml, SD 27). The slopes of the relationship fc/VO2 were higher in HP than AB (P less than 0.025) but when expressed as a %VO2max there were no differences. The results suggest a major alteration of oxygen transport capacity to active muscle mass in paraplegics due to changes in vasomotor regulation below the level of the lesion.

Effect of almitrine administration on pulmonary arterial pressure in resting and exercising dogs.

In addition to its well-known ventilatory effect, a small rise in pulmonary arterial pressure or pulmonary vascular resistance is occasionally observed with chronic administration of almitrine. In order to test the hypothesis of enhancement of exercise pulmonary vasoconstriction by almitrine, mongrel dogs were studied at rest and during submaximal exercise before and after 4 weeks of chronic ingestion of almitrine (10 mg/kg). It was shown that resting pulmonary arterial pressure (PAP) remained unchanged by almitrine treatment. However, when exercise was superimposed on almitrine medication, PAP was significantly increased throughout the exercise bout. Thus, the rise in PAP during the 20th min of exercise averaged 8.7 +/- 3.4 mm Hg after almitrine treatment while PAP increased by only 1.3 +/- 1.7 mm Hg before medication. The exaggerated exercise-induced PAP response in conjunction with the enhanced secretion of norepinephrine that we observed during almitrine treatment suggests that catecholamine could be involved in the pulmonary haemodynamic adjustments. Furthermore, mixed-venous PO2 (PvO2) both during rest and exercise declined with the prolongation of almitrine ingestion, suggesting that PvO2 might possibly be implicated in the pulmonary haemodynamic response to almitrine, in the same way as it is involved in the hypoxia-induced pulmonary vasoconstriction. These findings demonstrate that almitrine medication, even at a high dose, does not have any deleterious effect on pulmonary vasculature in resting conditions, but prolonged submaximal exercise should be proscribed in patients on a long-term therapy.

Rat soleus muscle ultrastructure after hindlimb suspension.

The aim of the present investigation was to determine, by quantitative electron microscopy, the effects of a 5-wk tail-suspension period on rat soleus muscle ultrastructure. A marked decline (-60%) in muscle mass occurred. The mean fiber cross-sectional area decreased to a greater extent (-75%) than the capillary-to-fiber ratio (-37%), leading to a higher capillary density (+148%) after hypokinesia. The total mitochondrial volume density remained unchanged, whereas the volume density of myofibrils was slightly but significantly reduced (-6%). A shift from subsarcolemmal to interfibrillar mitochondria occurred. Interfibrillar mitochondrial volume density was highest near the fiber border and decreased toward the fiber center. An increase in volume density of satellite cells suggested muscle regenerative events. Soleus atrophy with tail suspension greatly decreases the muscular volume but leaves the ultrastructural composition of muscle fibers relatively unaffected.

Thermoregulation at rest and during exercise in prepubertal boys.

Thermal balance was studied in 11 boys, aged 10-12 years, with various values for maximal oxygen uptake (VO2max), during two standardized sweating tests performed in a climatic chamber in randomized order. One of the tests consisted in a 90-min passive heat exposure [dry bulb temperature (Tdb) 45 degrees C] at rest. The second test was represented by a 60-min ergocycle exercise at 60% of individual VO2max (Tdb 20 degrees C). At rest, rectal temperature increased during heat exposure similar to observations made in adults, but the combined heat transfer coefficient reached higher values, reflecting greater radiative and convective heat gains in the children. Children also exhibited a greater increase in mean skin temperature, and a greater heat dissipation through sweating. Conversely, during the exercise sweating-test, although the increase in rectal temperature did not differ from that of adults for similar levels of exercise, evaporative heat loss was much lower in children, suggesting a greater radiative and convective heat loss due to the relatively greater body surface area. Thermophysiological reactions were not related to VO2max in children, in contrast to adults.

Skeletal muscle adaptation in rats flown on Cosmos 1667.

Seven male Wistar rats were subjected to 7 days of weightlessness on the Soviet biosatellite Cosmos 1667. Muscle histomorphometry and biochemical analyses were performed on the soleus (SOL) and extensor digitorum longus (EDL) of flight rats (group F) and compared with data from three groups of terrestrial controls: one subjected to conditions similar to group F in space except for the state of weightlessness (group S) and the others living free in a vivarium (V1, V2). Relative to group V2 (its age and weight-matched control group), group F showed a greater decrease of muscle mass in SOL (23%) than in EDL (11%). In SOL a decrease in the percentage of type I fibers was counterbalanced by a simultaneous increase in type IIa fibers. The cross-sectional area of type I fiber was reduced by 24%. No statistically significant difference in capillarization and enzymatic activities was observed between the groups. In EDL a reduction in type I fiber distribution and 3-hydroxyacyl-CoA-dehydrogenase activity (27%) occurred after the flight. The small histochemical and biochemical changes reported suggest the interest in studying muscular adaptation during a flight of longer duration.

Skeletal muscle adaptation to physical training and beta-adrenergic blockade in spontaneously hypertensive rats.

The effects of training alone or in combination with long-term, non-selective, beta-adrenergic blockade on histochemical and biochemical properties of fast-twitch [extensor digitorum longus muscle (EDL)] and slow-twitch [soleus muscle (Sol)] muscle were analyzed in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto strain rats (WKY). Fiber type distribution of Sol was drastically modified in SHR with fewer type I fibers and more type IIA fibers. No such histochemical alterations were observed in EDL. While prolonged swimming training remained ineffective in inducing both histochemical and biochemical improvement in WKY, SHR displayed a significant enhancement of capillarization and oxidative capacity in both Sol and EDL. However, in long-term beta-blocks rats training failed to improve significantly the oxidative capacity of SHR muscles, suggesting that beta-adrenoreceptor stimulation is necessary for a fully efficient adaptation of muscular metabolism to physical training.

[Recent data on respiratory control during muscular exercise in man]. / Données récentes sur le contrôle ventilatoire à l'exercice musculaire chez l'homme.

During the last two decades, numerous investigations have been conducted to determine the degree of involvement and mechanism of action of various factors responsible for ventilatory adaptation to exercise in human. The neuromechanical ventilatory system plays a role, exercise ventilation representing a compromise between the necessity of maintaining an adequate level of chemical arterial stimulus and of avoiding mechanical overload and, consequently, respiratory fatigue. The role of arterial chemoreceptors is essential and that of muscular chemoreceptors likely. The limb mechanoreceptors also play a role essentially via small nerve fibers on the other hand. To date, there is no experimental evidence of ventilatory reflexes elicited by pulmonary chemoreceptors. Further, the action of cardiopulmonary baroreceptors was proved only in extraphysiological conditions in animals. It is also possible that a feed-forward mechanism originating in the central nervous system plays an important role. In conclusion, the time course of pulmonary ventilation during exercise could be explained by the action of humoral and neurogenic stimuli.

Effects of gluconeogenic precursor flux alterations on glycogen resynthesis after prolonged exercise.

The importance of gluconeogenic substrates (i.e., lactate, glycerol, and alanine) in the glycogen resynthesis observed in fasting rats after exhausting submaximal exercise [R.D. Fell et al. Am. J. Physiol. 238 (Regulatory Integrative Comp. Physiol. 7): R328-R332, 1980] was examined in muscles and liver in response to pharmacological alterations of gluconeogenic precursor flux. The minor role of lactate for glycogen resynthesis after prolonged submaximal exercise was confirmed by the insignificant accumulation of lactate neither in muscles nor in plasma. When the rate of lipolysis is reduced either by beta-blockade or by nicotinic acid injection, the replenishment of muscle glycogen persisted, suggesting that glycerol released by triglycerides hydrolysis did not play an important role in glycogen resynthesis. On the other hand, when pyruvate oxidation is enhanced by dichloroacetate (DCA), thus reducing plasma levels of lactate and alanine, glycogen resynthesis was completely blocked in liver and partly in some but not all muscles. This failure in total inhibition of glycogen resynthesis associated with the significant reduction of the plasma alanine level could be attributed to the possible stimulation of gluconeogenesis from alanine by DCA (R.A. Harris and D.W. Crabb. Arch. Biochem. Biophys. 189: 364-371, 1978). The results could point out alanine as the major gluconeogenic substrate during recovery from exhaustive exercise in fasting conditions.

Effect of spontaneous recovery or retraining after hindlimb suspension on aerobic capacity.

The purpose of this study was to compare the effects of spontaneous recovery or recovery by treadmill training (180 min/day, 5 days/wk, 30 m/min for 8 wk) on maximal O2 uptake (VO2max), histochemical and biochemical muscular properties (soleus), of rats subsequent to 5 wk of hindlimb suspension. Spontaneous recovery reversed the 15% reduction in VO2max, whereas training posthypokinesia induced a 20% increase over control values. In the spontaneous recovery group, both citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities, decreased by hypokinesia (-40%), increased but remained 20% below the control level. In the training posthypokinesia group, an increase of these activities over control occurred (+50 and +20%, respectively). Recovery or training led to a 100% type I distribution in soleus muscle and to a recovery of all fibers' cross-sectional areas. In the spontaneous recovery group, capillaries per fiber, decreased by 46%, returned to the normal range. In the training posthypokinesia group, training induced an increase in capillaries per fiber above their control values (+23%). These results point to the plasticity of the muscle and indicate the necessity of a posthypokinesia training program for recovery of the total oxidative enzyme capacity.

Structural and functional responses to prolonged hindlimb suspension in rat muscle.

The purpose of this study was to investigate alterations in structural and functional properties in the soleus (SOL) and extensor digitorum longus (EDL) muscles of rats after 1, 2, and 5 wk of tail suspension. Maximal O2 uptake was 19% lower after 5 wk suspension. Loss of muscle mass was greater in SOL (63%) than in EDL (22%) muscle. A reduction of type I distribution was accompanied by an increase of intermediate fiber subgroups (int I in SOL, int II in EDL). The cross-sectional area of all three fiber types was reduced by hypokinesia. The decrease in capillaries per fiber in SOL was greater than the decrease in citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities after 5 wk. No alteration in lactate dehydrogenase activity was noted. In EDL, no changes in fiber area, capillarization, and enzymatic activities occurred. Energy charge remained unchanged (0.91) whatever the muscle. These results suggest that type I fibers showed an earlier and greater susceptibility than type II fibers to suspension which is also accompanied by a decreased aerobic capacity.

Aerobic performance capacity in paraplegic subjects.

To determine adaptation to prolonged exercise in paraplegics, maximal O2 uptake (VO2max) and lactate threshold (LT) were evaluated during an arm cranking exercise in nine patients (P) and nine able-bodied (AB) subjects. Mean VO2max averaged 25.1 and 31.6 ml X min-1 X kg-1 in P and AB groups respectively. VO2max in P was found to be directly related to the level of spinal injury: the higher the lesion the lower the uptake. Lactate threshold expressed as a percentage of VO2max was higher in P (59%) than in AB (43%), and close to that observed in arm-trained athletes. Since training has less effect on VO2max in paraplegics than in able-bodied subjects, attributable to a deficiency in the circulatory adaptation of paraplegics to exercise, the observed differences between AB and P in lactate threshold and submaximal exercise indicate that the possible effect of training in paraplegics is located at the level of intracellular chemistry, with a diminution in glycogenolysis (higher LT) and a higher rate of lipid utilization (lower RQ).

Effects of hypoxia on catecholamine and cardiorespiratory responses in exercising dogs.

The sympathoadrenal contribution to cardiorespiratory response elicited by hypoxia and/or exercise was assessed in the dog. The increased plasma norepinephrine (NE) and dopamine (DA) levels which follow hypoxia (fraction of inspired O2 equals 0.12) while epinephrine (E) remained unchanged ruled out the possibility of a primacy of the adrenal medulla in the response to hypoxia. In contrast to the lack of effect of hypoxic exposure, the adrenal medulla was substantially stimulated during exercise. The exercise-induced sympathoadrenal response remained unchanged during hypoxia as compared to normoxia when expressed as function of relative work intensity. Nevertheless at a given oxygen uptake, all plasma catecholamines were increased by hypoxia. These modifications in hormonal milieu failed, however, to alter the cardiac responses to exercise but were associated with a change in breathing pattern.

Free dopamine in dog plasma: lack of relationship with sympathoadrenal activity.

To investigate the relationship between dopamine (DA) released into the bloodstream and sympathoadrenal activity, levels of free DA, norepinephrine (NE), and epinephrine (E) in plasma were recorded in four dogs subjected to three tests: treadmill exercise at two work levels [55 and 75% maximal O2 uptake; 15 min], normobaric hypoxia (12% O2; 1 h), combined exercise and hypoxia. Normoxic exercise induced slight nonsignificant decreases in the arterial partial pressure of O2 (PaO2), increases in NE [median values and ranges during submaximal work vs. rest: 1086 (457-1,637) vs. 360 (221-646) pg/ml; P less than 0.01] and E [277 (151-461) vs. 166 (95-257) pg/ml; P less than 0.05], but it failed to alter the DA level. Hypoxia elicited large decreases in PaO2 [hypoxia vs. normoxia: 42.8 (40.3-50.0) vs. 97.6 (83.2-117.6) Torr; P less than 0.01], increases in DA [230 (105-352) vs. 150 (85-229) pg/ml; P less than 0.01] and NE [383 (219-1,165) vs. 358 (210-784) pg/ml; P less than 0.05], but it failed to alter the E level. Combined exercise and hypoxia further increased NE levels but did not alter the DA response to hypoxia alone. The data indicate that free DA in plasma may vary independently of the sympathoadrenal activity.

Ventilatory transients during exercise: peripheral or central control?

The fast component of the ventilatory changes that occur at the transition phases of exercise was studied in awake dogs trained to run on a treadmill. Two questions were examined: firstly, is the fast ventilatory component modified by changes in venous return to the lungs, such as those consecutive either to increased work loads or to beta adrenergic blockade?, and secondly, is this component altered by central ventilatory depressants? The results showed that at the onset of exercise, there is no correlation between the instantaneous increment in ventilation and the intensity of exercise, but at the end of the treadmill run, the fall in ventilation is closely linked to the power of the work performed. Ventilatory transients observed either at the start or at the end of exercise remain unaffected by administration of a beta-adrenergic blocking agent. But central depressant effects on ventilation caused by narcotic analgesics or hypnotic drugs altered the breathing pattern of the fast component of exercise-induced ventilatory changes. It is concluded that the instantaneous changes in ventilation occurring at the transition phases of exercise are controlled by mechanoreceptor mechanisms, but cerebral control is superimposed on the reflex control in regulating both tidal volume and breathing rate.

Catecholamines and metabolic responses to submaximal exercise in untrained men and women.

The influence of exercise on blood borne substrates in relation to plasma catecholamine (CA) levels has been studied in untrained subjects (eight men and eight women). Subjects pedalled a bicycle ergometer for 20 min at a workload approximating to 80% maximal aerobic power. During exercise women reacted similarly to men except that their weight loss and hematocrit were significantly lower. At the end of the bicycling test, plasma dopamine (DA), norepinephrine (NE) and epinephrine (E) concentrations increased similarly in both groups. There was no significant difference in blood energy substrates between men and women except that the plasma free fatty acid (FFA) level was significantly higher in the female. When fitness levels were similar, the previously reported sex-related difference in response to exercise seemed to disappear. The lack of correlation between blood borne substrates variations and CA changes raised the question whether other hormonal factors combined with CA could play a role in the mobilization of energy substrates during exercise.

[Maximal oxygen uptake in French children in relation to age, sex and physical training]. / La consommation maximale d'oxygène chez le jeune français, sa variation avec l'âge, le sexe et l'entraînement.

1. Maximal oxygen uptake (VO2 max) was determined in 451 boys and girls ranging from 11 to 16 years and divided in two groups. Group I (n = 287) was considered as an untrained reference group. Group II (n = 174) was regularly trained. This group concerned school-children affiliated to a swimming club and trained 5 to 10 h per week. Body growth is nearly the same in the two groups until puberty but, at 15-16 years in the boys, 14-16 years in the girls, height and weight are higher in group II. 2. In group I, between 11 and 16 years, VO2 max increases by 50% in boys against only 25% in girls. For the same age, VO2 max remained smaller in girls, even before puberty although there is no difference in body growth. At 11 years, specific VO2 max averaged 47 ml . min-1 . kg-1 in boys and 40 ml . min-1 . kg-1 in girls and decreases with age only in sedentary girls down to 37 ml . min-1 . kg-1. From the results presented here it appears that maximal oxygen uptake in the average french schoolchild is similar to those of subjects from different countries with same cultural and socio-economical level. 3. In group II, VO2 max is similar in both sexes at 11-12 years old, and then up to 16 years of age increases more rapidly in boys (+100%) than in girls (+50%). Specific VO2 max ranges between 57 and 62 m. min-1 . kg-1 in boy swimmers and averages 53 ml . min-1 . kg-1 in girl swimmers. The influence of physical training on aptitude development is discussed and compared with previously published data.