Physiological responses to prolonged physical exercise in dogs

1. Duration of exercise until exhaustion is significantly correlated with body weight of dogs. 2. Rectal temperature (Tre) achieved at the end of the effort depends on the resting value of Tre, but the exercise-induced increases in Tre are unrelated to the initial Tre. 3. The magnitude of exercise-induced decrease in blood glucose (BG) level is positively correlated with the resting BG level and negatively correlated with the elevations of the plasma free fatty-acid (FFA) concentration. 4. A significant positive relationship is found between the exercise-induced increases in the plasma FFA levels and noradrenalinaemia.     

Metabolic and temperature responses to physical exercise in thyroidectomized dogs.

The purpose of the present work was to further elucidate the role of thyroid hormones in the control of body temperature and metabolism during physical exercise. Changes in rectal temperature (Tre), some parameters of exercise-metabolism and in the plasma noradrenaline (NA) levels were examined in eight dogs performing submaximal treadmill exercise to exhaustion before and after thyroidectomy (THY). The metabolic 'responses to adrenaline (A) infusion were also compared in intact and THY dogs. During the exercise performed by THY dogs Tre increases were markedly attenuated, plasma FFA level increases were reduced and the pattern of plasma NA changes was modified in comparison with control runs. The reduced exercise-induced FFA mobilization in THY dogs might be attributed to a lower activation of the adrenergic system in the later stage of exercise and to the weaker lipolytic action of catecholamines. The attenuated Tre increases during exercise performed by THY dogs and the exercise-hyperthermia described previously in dogs treated with thyroid ormones suggest that an optimum level of thyroid hormones is necessary to induce typical changes in body temperature during physical exercise.     

Metabolic, body temperature and hormonal responses to repeated periods of prolonged cycle-ergometer exercise in men.

This study was designed to find out whether rest intervals and prevention of dehydration during prolonged exercise inhibit a drift in metabolic rate, body temperature and hormonal response typically occurring during continuous work. For this purpose in ten healthy men the heart rate (fc), rectal temperature (Tre), oxygen uptake (VO2), as well as blood metabolite and some hormone concentrations were measured during 2-h exercise at approximately 50% maximal oxygen uptake split into four equal parts by 30-min rest intervals during which body water losses were replaced. During each 30-min exercise period there was a rapid change in Tre and fc superimposed on which, these values increased progressively in consecutive exercise periods (slow drift). The VO2 showed similar changes but there were no significant differences in the respiratory exchange ratio, pulmonary ventilation, mechanical efficiency and plasma osmolality between successive periods of exercise. Blood glucose, insulin and C-peptide concentrations decreased in consecutive exercise periods, whereas plasma free fatty acid, glycerol, catecholamine, growth hormone and glucagon concentrations increased. Blood lactate concentrations did not show any regular drift and the plasma cortisol concentration decreased during the first two exercise periods and then increased. In conclusion, in spite of the relatively long rest intervals between the periods of prolonged exercise and the prevention of dehydration several physiological and hormonal variables showed a distinct drift with time. It is suggested that the slow drift in metabolic rate could have been attributable in the main to the increased concentrations of heat liberating hormones.     

Metabolic responses to exercise after fasting

Fasting before exercise increases fat utilization and lowers the rate of muscle glycogen depletion. Since a 24-h fast also depletes liver glycogen, we were interested in blood glucose homeostasis during exercise after fasting. An experiment was conducted with human subjects to determine the effect of fasting on blood metabolite concentrations during exercise. Nine male subjects ran (70% maximum O2 consumption) two counterbalanced trials, once fed and once after a 23-h fast. Plasma glucose was elevated by exercise in the fasted trial but there was no difference between fed and fasted during exercise. Lactate was significantly higher (P less than 0.05) in fasted than fed throughout the exercise bout. Fat mobilization and utilization appeared to be greater in the fasted trial as evidenced by higher plasma concentrations of free fatty acids, glycerol, and beta-hydroxybutyrate as well as lower respiratory exchange ratio in the fasted trial during the first 30 min of exercise. These results demonstrate that in humans blood glucose concentration is maintained at normal levels during exercise after fasting despite the depletion of liver glycogen. Homeostasis is probably maintained as a result of increased gluconeogenesis and decreased utilization of glucose in the muscle as a result of lowered pyruvate dehydrogenase activity.     

Muscle metabolism during prolonged physical exercise in dogs

In order to provide reference data, adenine nucleotide, creatine phosphate, glycogen, glycolytic intermediates and lactate muscle contents were measured in 49 dogs under resting conditions and during prolonged physical exercise of moderate intensity performed until exhaustion. Both the resting and exercise values of the measured variables were remarkably similar to those described in human subjects, except muscle lactate content which achieved higher values during submaximal exercise in dogs than in men.     

Diminished adrenergic response to 2-deoxy-D-glucose after prolonged, exhausting physical exercise in dogs.

Effect of 2-deoxy-D-glucose (2DG) on plasma adrenaline, glucose and free fatty acid concentrations was studied in dogs under control conditions and after prolonged, exhausting physical exercise. The increase in all three variables in response to 2DG was significantly reduced following the exercise. The results suggest diminished responsiveness of adrenal medulla to the glucopenic stimulus after exhausting exercise.     

Influence of hydration state on hormonal responses to exercise in dogs.

The purpose of these studies was to determine how plasma levels of arginine vasopressin (pAVP) are related to workload, plasma osmolality (pOsm), blood volume (BV) and plasma angiotensin II (pAII) in exercising dogs. Measurements were made in dogs running on a treadmill at 7.5 km. hr-1 at slopes of 0, 10% and 20% when they were hydrated ad lib and when they had been deprived of drinking water and also in dogs running on a 20% slope after an IV infusion of hypertonic NaCl. Dehydration increased pOsm by 6.6% and reduced BV by 10% in resting dogs. In dehydrated animals, pAVP, pAII and pOsm were elevated above hydrated levels at rest and during exercise at all three workloads. In hydrated dogs, pOsm rose during exercise at 10% and 20% slopes but pAVP rose above resting levels only at the highest workload and pAII was not affected by exercise. In dehydrated dogs, pOsm and pAVP rose during exercise at 10% and 20% slopes and pAII was elevated at the 20% slope. BV decreased during exercise at the highest workload in both hydrated and dehydrated animals. After hypertonic NaCl, pAVP rose during exercise but pOsm and pAII did not. The results suggest that both osmotic and nonosmotic factors contribute to the release of AVP in exercising dogs and that exercise leads to a leftward shift in the relationship of pAVP to pOSM which could be a result of reduced blood volume.     

Metabolic effects of testosterone during prolonged physical exercise and fasting

Previous studies have shown a decrease in plasma testosterone during prolonged physical exercise and 72 h fasting in rats. To determine whether this hormonal change has an influence upon energy metabolism, two experiments were carried out, in which the plasma levels of testosterone were elevated during prolonged physical exercise and fasting in male wistar rats. The effects of acute and chronic increases in the levels of circulating testosterone were studied, on the one hand after human chorionic gonadotropin (H.C.G.) injection, and on the other by prolonged testosterone perfusion with an osmotic minipump. Blood and tissue sampling were performed to evaluate blood glucose, alanine, and lactate, and tissue glycogen. The results in fed and rest control rats showed no changes in blood parameters under the effect of hypertestosteronemia but there was an increase in muscle glycogen after testosterone perfusion. In 72 h fasted rats both types of hypertestosteronemia were associated with a decrease in blood alanine and lactate ranging from 25% to 35%. Only testosterone perfusion was associated with higher concentrations of muscle glycogen. After 7 h of treadmill running, testosterone perfusion and H.C.G. injection induced a 35% decrease in blood alanine and a slight decrease in blood glucose, with no change in other parameters. Whereas an elevation in the level of testosterone can induce muscle glycogen compensation in the fed resting state, it cannot counteract the exhaustion of muscle glycogen during running.     

Metabolic and hormonal response to physical exercise during beta 1-selective and non-selective beta-blockade

The effects of a beta 1-selective (metoprolol, 150 mg per day) and a non-selective beta-blocking agent (propranolol, 120 mg per day) on metabolic and hormonal responses to physical exercise (a 30 min bicycle ergometer test) were investigated against placebo in seven healthy male volunteers with a double blind cross-over design. The blood glucose level remained unchanged during placebo, it tended to increase during metoprolol, whereas it decreased during propranolol. Both metoprolol and propranolol counteracted the exercise-induced increase in plasma free fatty acids and caused a slight decrease in muscle glycogenolysis. The increase in blood lactate concentration during exercise was not influenced by beta-blockade. The secretion of glucagon and cortisol was not modified significantly by beta-blockade, whereas the growth hormone response to exercise was promoted equally by both beta-blocking agents. It has been assumed previously that, during treatment with beta-blocking agents, diminished hepatic gluconeogenesis, caused by the lack of lactate or free fatty acids, may result in a decline in blood glucose levels. The present results indicate that an inhibition of beta 2-mediated hepatic glycogenolysis by propranolol may also influence blood glucose homeostasis during exercise.     

Metabolic adaptation to prolonged exercise.

A study was undertaken to evaluate and to examine the role of substrate supply in 50 healthy subjects after long distance events, such as 10 km, 25 km, and marathon races. The metabolic, variables of carbohydrate metabolism were greatest in 10-km runners, with the highest increase in glucose, lactate, and pyruvate, while in marathon runners only moderate changes were observed. Marathon competitors gave the greatest decrease in insulin concentration whereas glucagon and cortisol showed a contrary tendency. As for lipid concentrations, the most remarkable point was that after the marathon competition the best runners had the highest increase in free fatty acids; the longer the race, the higher were the beta-hydroxybutyrate and acetoacetate levels after the competition. It is important to emphasize that the limiting factor up to 90 min duration is the competitor's ability to deplete the stores of glycogen. Beyond 90 min (or 25 km) the decrease in insulin, the rise in cortisol and the higher concentration of ketnne bodies found indicate a change in metabnlic response.     

Metabolic and endocrine responses to prolonged exercise in rats under beta 2-adrenergic blockade

The respective roles of allosteric regulators and catecholamines in the control of muscle glycogen breakdown during exercise remain a matter of controversy. This study was designed to reassess the role of the sympathoadrenal system during prolonged exercise in rats. Animals were studied at rest or after treadmill exercise (28 m.min-1; 8% slope) to exhaustion in a control situation or following administration of a specific beta 2-adrenergic receptor antagonist (ICI 118,551, 1 mg.kg-1, i.v.). Running times to exhaustion were 54 and 36 min in control and treated rats, respectively. For the purpose of comparison, another group of control rats was studied after a 36-min exercise bout. The reduction in endurance in treated rats was associated with an impairment in glycogen utilization, as measured by muscle glycogen stores, in soleus muscle but not in superficial vastus lateralis or gastrocnemius lateralis muscles. Utilization of liver glycogen stores was similar in the two groups of animals, but plasma glucose (7 vs. 13 mM) and lactate (4 vs. 7 mM) levels were significantly lower in rats under beta-blockade than in control rats run for 36 min. Plasma free fatty acid and glycerol concentrations were not significantly different between groups. On the other hand, plasma epinephrine concentration was significantly higher in treated rats (13 vs. 5 mM), which might reflect a compensatory increase in adrenal activity. These results suggest that glycogen breakdown during prolonged exercise is under the control of the sympathoadrenal system in predominantly slow-twitch but not in predominantly fast-twitch muscles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic responses to catecholamines in dogs injected with a single dose of triiodothyronine.

Lipolytic and glycogenolytic responses to catecholamine infusions were studied in resting dogs before and 20 h following administration of a single dose (0.1 mg/kg) of triiodothyronine (T3). In the dogs pretreated with T3 much higher increases in the plasma FFA concentration were found both during noradrenaline and adrenaline infusions in comparison with control experiments. Adrenaline-induced increases in blood LA and glucose levels were also significantly higher in T3-pretreated dogs than in controls. The blockade of beta-adrenergic receptors with propranolol prevented the increases in blood FFA and LA concentrations during subsequent adrenaline infusion. Phentolamine -- the alpha-adrenergic blocking agent -- infused to the T3-pretreated dog inhibited the adrenaline-induced rise in blood glucose level. The observed changes in the metabolic responses to catecholamines induced by triiodothyronine pretreatment indicate that at least in the dog this hormone potentiates both the lipolytic and glycogenolytic effects of catecholamines acting on appropriate adrenergic receptors.     

The effect of restriction of physical activity on adaptation to prolonged exercise in dogs.

Eight-week restriction of physical activity markedly reduced the capacity for prolonged running. The rate of exercise-induced Tre increases was also significantly higher in comparison with control experiments. At the point of exhaustion blood glucose concentration and muscle glycogen content were similar in the control and cage-confined animals, however the rate of decrease in blood glucose level and muscle glycogen depletion during exercise were higher in the cage-confined dogs than in the controls.     

Diminished hormonal responses to exercise in trained rats

Male rats (120 g) either were subjected to a 12-wk physical training program (T rats) or were sedentary controls (C rats). Subsequently the rats were killed at rest or after a 45- or 90-min forced swim. At rest, T rats had higher liver and muscle glycogen concentrations but lower plasma insulin. During exercise, blood glucose increased 60% in T rats but decreased 20% in C rats. Plasma glucagon and insulin concentrations did not change in T rats but plasma glucagon increased and insulin decreased markedly in C rats. Plasma epinephrine (90 min: range, 0.78-2.96 ng-ml-1, (T) vs. 4.42-15.67 (C)) and norepinephrine (90 min: 0.70-2.22 (T) vs. 2.50-6.10 (C)) were lower in T than in C rats. Hepatic glycogen decreased substantially and, as with muscle glycogen, the decrease was parallel in T and C rats. The plasma concentrations of free fatty acids were higher but lactate and alanine lower in T than in C rats. In trained rats the hormonal response to exercise is blunted partly due to higher glucose concentrations. In these rats adipose tissue sensitivity to catecholamines is increased, and changes in glucagon and insulin concentrations are not necessary for increased lipolysis and hepatic glycogen depletion during exercise.     

Metabolic and hormonal responses during squash

The metabolic and hormonal response during squash was observed in eight normal men. Significant increases from resting were found for blood glucose, lactate, pyruvate, alanine and glycerol while total ketone bodies and plasma nonesterified fatty acids rose after play stopped. Insulin and C-peptide decreased significantly and catecholamines, ACTH, prolactin and growth hormone increased.     

Respiratory responses to intermittent and prolonged exercise in a hot-dry environment

Ventilatory gas exchange ratio (R), V?O₂, ventilation (V?_E), respiration rate (RR), rectal temperature (T_re), and heart rate (HR) were determined for four acclimatized subjects during intermittent and prolonged exercise on a treadmill at 24° and 45°C (dry) as follows: 1) 8 cycles (10 min. exercise and 5 min. rest), and 2) prolonged exercise lasting for 90 min. While during intermittent and prolonged exercise, V?O₂ and V?_E did not differ in the heat, RR, T_re, HR and the respiratory dead space were higher in the hot ambient environment. After steady-state attainment, exercise R was higher in the initial as compared to the last cycles with higher values in neutral as compared to the hot ambient condition. It was concluded that heat was more effective than time in lowering the R, probably with a greater dependence on fat oxidation in the latter exercise cycles which seemed to be more pronounced in the heat.