Thermoregulatory and metabolic responses to repeated bouts of prolonged cycle-ergometer exercise in man

Changes in body temperature, oxygen uptake (VO2), heart rate (HR), sweating rate and plasma osmolarity were examined in 10 human subjects, performing four successive 30 min exercise-bouts of the same intensity (50% VO2 max) separated by 30 min rest periods. In spite of the rest intervals and replacement of body fluid loss there was a progressive increase in VO2. HR, rectal (Tre) and mean body (Tb) temperatures in consecutive exercise bouts. The thermoregulatory efficiency showed an increasing tendency, and a delay in the sweating response at the beginning of each exercise was shortened. It is concluded that a drift in metabolic and temperature responses to exercise, reported throughout a long-term continuous work, occurs also in the euhydrated subjects performing a prolonged intermittent exercise. It is not caused by an impaired thermoregulation during exercise but rather by insufficient restitution of metabolic processes during rest intervals.     

Hypothalamic, rectal, and muscle temperatures in exercising dogs: effect of cooling

To investigate mechanisms that may be involved in the prolongation of exercise performance with body cooling hypothalamic (Thy), rectal (Tre), and exercising muscle (Tm) temperatures, as well as the heart rate, respiratory rate, blood lactic acid concentration ( [LA] ), and plasma osmolality (Osm) were measured in five dogs during exhaustive treadmill exercise at an ambient temperature (Ta) of 22 +/- 1 degree C without cooling (control) and with external cooling by use of ice packs. In both series of experiments, dehydration of animals was prevented. Compared with exercise with noncooling, exercise with cooling resulted in 1) increased exercise duration from 90 +/- 14 to 145 +/- 15 min (62%, P less than 0.05); 2) attenuated increases in Thy, Tre, and Tm; 3) decreased respiratory and heart rates; and 4) lowered LA. Significant negative correlations were found between both Tm and delta Tm attained at 60 min of the run and time of exercise until exhaustion (r = -0.72 and -0.74, respectively; P less than 0.02). This work failed to differentiate clearly changes or equilibrium levels of brain, core, or muscle temperature as separate factors affecting work tolerance. However, the inverse relationship between Tm reached at 60 min of the run (in both experiments) and the total duration of exercise indicates that sustained muscle hyperthermia may largely contribute to limitation of working ability.     

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.     

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, 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.     

Effect of propranolol on thyroxine-induced changes in body temperature and metabolism during exercise in dogs.

Effects of thyroxine on temperature and metabolism during exercise were studied in dogs after beta-adrenergic blockade. Dogs performed 60 min treadmill exercise of moderate intensity 5 and 72 h following thyroxine injected s.c. in a single dose of 0.1 mg/kg b.w. Thyroxine increased significantly the lipolytic response to exercise as well as blood lactate (LA) concentrations and rectal temperature (Tre) during exercise as early as 5h following the hormone administration. The changes became more pronounced 72 h after the injection. At rest Tre, blood FFA and LA levels in the thyroxine-treated dogs did not differ from the control values, and blood glucose was slightly, but significantly higher. Propranolol given intravenously in a dose of 0.25 mg/kg at 30 min of the exercise performed 72 h following thyroxine injection abolished the plasma FFA rise, and inhibited to a certain extent increases in Tre and blood LA concentrations during the next 30 min of exercise.     

Enhanced glucose availability for working muscles reduces exercise hyperthermia in dogs

Body temperature and metabolic responses to 2 h treadmill exercise in dogs given glucose intravenously (25-30 mg.kg-1 X min-1 throughout the run) were compared with those measured in the same animals with elevated plasma FFA concentrations (soya bean oil ingestion + intravenous heparin) and in control experiments (24 h fasting). In comparison with control conditions enhanced glucose availability for the working muscles caused a reduction in the exercise-induced increases in both rectal (by 0.9 +/- 0.11 degree C) and muscle (by 0.9 +/- 0.16 degree C) temperatures, a lower rate of oxygen uptake (by 16%) and an elevated respiratory exchange ratio. A tendency towards enhanced body temperature responses to exercise, accompanied by increases in VO2 and cardiac frequency was noted in dogs with elevated plasma FFA concentrations as compared with the control animals. The estimated amount of heat effectively dissipated from the body, expressed as a fraction of heat load (thermoregulatory efficiency) was significantly higher in dogs infused with glucose (0.962 +/- 0.0035), than in the controls (0.947 +/- 0.0043) and those with elevated plasma FFA concentrations (0.931 +/- 0.0029). It is concluded that the increased contribution of carbohydrates to the energy yield during exercise results in a marked attenuation of hyperthermia, associated with a reduced metabolic rate and improved thermoregulatory efficiency.     

Effect of thyroxine treatment on metabolic responses to a single insulin injection

Metabolic responses to a single i.v. injection of cristalline insulin (0.2 i.u./kg b.w.) were compared in control and T4-treated dogs both at rest and after prolonged physical exercise. The post-insulin decrease in blood glucose was significantly correlated with the pre-insulin BG concentration. Thus, the insulin-induced fall of BG was greatest in T4-treated dogs at rest, in which significantly higher BG levels were found in comparison with controls, and smallest in the same dogs after exercise, i.e. at the lowest initial BG concentrations. The post-insulin hypoglycaemia caused marked increases in the plasma FFA level in control dogs, both at rest and after physical effort, and in T4-treated dogs at rest. They were accompanied by elevations in the plasma adrenaline levels. In T4-treated dogs given insulin after exercise decreases both in the plasma FFA and A concentrations were found. In the majority of the control and T4-treated dogs insulin injected at rest caused an increase in blood LA levels, being more pronounced in the latter. Insulin injected after physical exercise did not change blood LA level in T4 treated dogs, and it caused its decrease in the control animals. The results of these investigations show that both T4-treatment and physical exercise, performed prior to insulin injection, modify the metabolic response to insulin and post-insulin hypoglycaemia.     

Exercise hyperthermia as a factor limiting physical performance: temperature effect on muscle metabolism

The muscle contents of high-energy phosphates and their derivatives [ATP, ADP, AMP, creatine phosphate (CrP), and creatine], glycogen, some glycolytic intermediates, pyruvate, and lactate were compared in 11 dogs performing prolonged heavy exercise until exhaustion (at ambient temperature 20.0 +/- 1.0 degrees C) without and with trunk cooling using ice packs. Without cooling, dogs were able to run for 57 +/- 8 min, and their rectal (Tre) and muscle (Tm) temperatures increased to 41.8 +/- 0.2 and 43.0 +/- 0.2 degrees C, respectively. Compared with noncooling, duration of exercise with cooling was longer by approximately 45% while Tre and Tm at the time corresponding to the end of exercise without cooling were lower by 1.1 +/- 0.2 and 1.2 +/- 0.2 degrees C, respectively. The muscle contents of high-energy phosphates (ATP + CrP) decreased less, the rate of glycogen depletion was lower, and the increases in the contents of AMP, pyruvate, and lactate as well as in the muscle-to-blood lactate ratio were smaller. The muscle content of lactate was positively correlated with Tm. The data indicate that with higher body temperature equilibrium between high-energy phosphate breakdown and resynthesis was shifted to the lower values of ATP and CrP and glycolysis was accelerated. The results suggest that hyperthermia developing during prolonged muscular work exerts an adverse effect on muscle metabolism that may be relevant to limitation of endurance.     

Physiological effects of dehydration and rehydration with water and acidic or neutral carbohydrate electrolyte solutions

Five healthy young men exercised on an ergocycle for six 25-min periods separated by 5-min rest intervals in a warm dry environment (36 degrees C). After 1 h of exercise without fluid intake, the subjects continued to be dehydrated or were rehydrated either with water (W) or with isosmotic electrolyte carbohydrate solutions, either acidic (AISO) or close to neutrality (NISO). The average amount of the fluid ingested progressively every 10 min (120 ml) at 20 degrees C was calculated so as to compensate for 80% of the whole body water loss due to exercise in the heat. Dehydration associated with hyperosmotic hypovolaemia elicited large increases in heart rate (HR), and in rectal temperature (Tre), while no decrease was found in either whole body or local sweat rates. Rehydration with water significantly reduced the observed disturbances, except for plasma osmolality and Na+ concentration which were significantly lower than normal. With both AISO and NISO there was no plasma volume reduction and osmolality increase. Although a plasma volume expansion was induced by NISO ingestion, the cardiac cost was not improved, as reflected by the absence of a decrease in HR. With NISO, sweating was not enhanced and Tre tended to remain higher. It is concluded that efficient rehydration requires the avoidance of plasma volume expansion at the expense of interstitial and intracellular rehydration. During rehydration by oral ingestion of fluid, the pH of the drink may be an important factor; its effect remains unclear, however.     

Seasonal changes in circadian rhythms of body temperatures in humans living in a dry tropical climate

Seven volunteers (3 females and 4 males; 3 Caucasians and 4 Africans) participated in two 24 h sessions during the cool dry (CD) and the hot dry (HD) seasons of the sahelian tropical climate. Body temperatures were taken on portable cassette recorders for 24 h. Rectal (Tre) and mean skin (Tsk) temperatures decreased in the HD compared to the CD conditions, meeting one of the criteria for adaptation to heat. No ethnic differences in thermal responses were found. Males and females differed in their body temperature rhythms and in their reactions to heat. Body temperatures were higher in females than in males. Males reacted to heat with a decrease in Tre, without change in the Tre-Tsk gradient. Females showed a decrease in both Tre and Tsk, more marked for Tsk, with an increase in the Tre-Tsk gradient. It was concluded that males showed seasonal acclimatization to heat via a decrease in metabolism confirmed by a decrease in plasma levels of thyroid stimulating hormone (TSH) in the HD condition. Females showed a mixed metabolic and thermolytic type of acclimatization, with an absence of variation in plasma TSH levels. In conclusion, the steady rise in temperature between the CD and HD conditions was sufficient to trigger an acclimatization to heat similar in Caucasian and African subjects, although exposure to the external climate differed widely.     

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

2-months restriction of physical activity of dogs markedly reduced their capacity for prolonged running. The rate of exercise-induced Tre increases was significantly higher in the cage-confined dogs in comparison with controls. At the point of exhaustion blood glucose concentration and muscle glycogen content were similar in the control and cage-confined animals, in spite of the much shorter time of exercise until exhaustion in the latter. The exercise-induced increases in plasma FFA concentration were considerably lower in dogs after prolonged inactivity period in spite of the greater activation of the adrenergic system. It is concluded, that there are several factors which may contribute to the reduction of the ability of cage-confined dogs to perform prolonged physical exercise. The most important seems to be the diminished muscle glycogen content, modifications in exercise metabolism and exercise-induced hyperthermia.     

Influence of caffeine on metabolic responses of men at rest in 28 and 5 degrees C

Cold stress and caffeine ingestion are each reported to increase plasma catecholamines, free fatty acid (FFA) concentrations, and energy metabolism. This study examined the possible interaction of these two metabolic challenges in four double-blind counterbalanced trials. Young adult men (n = 6) ingested caffeine (5 mg/kg) or placebo (dextrose, 5 mg/kg) and rested for 2 h in 28 or 5 degrees C air. Cold stress alone elevated (P less than 0.05) plasma norepinephrine, metabolism (O2 consumption, VO2), and respiratory exchange ratio (RER). Caffeine alone increased (P less than 0.05) plasma epinephrine and FFA but not RER. When the two challenges were combined (caffeine plus 5 degrees C for 2 h) norepinephrine and epinephrine were increased (P less than 0.05) as was FFA. However, VO2, RER, and skin and rectal temperatures were not different from the responses observed at 5 degrees C after placebo ingestion. The data suggest that caffeine selectively increases plasma epinephrine, whereas cold air increases norepinephrine. During the cold exposure, increasing epinephrine and FFA above normal levels did not appear to influence the metabolic or thermal responses to the cold stress. In fact the increase in RER suggested a greater carbohydrate oxidation.     

Effect of free fatty acid mobilization on the electrophoretic mobility of alpha-lipoproteins in the dog

Dogs were given infusions of norepinephrine and subsequent additional infusions of propranolol and nicotinic acid over a 4-hr period. Under different physiological conditions, alpha-lipoproteins of three different electrophoretic mobilities were identified by means of paper electrophoresis; they were designated alpha-lipoproteins X, Y, and Z. During norepinephrine infusion, alpha-lipoprotein Y fell from 45% (of all lipoproteins) to 14%. There was a reciprocal rise in alpha-lipoprotein Z. On the other hand, alpha-lipoprotein X was not significantly changed. There was evidence that alpha-lipoprotein Y was progressively transformed into alpha-lipoprotein Z by increasing plasma FFA concentrations. The percentages of both alpha-lipoproteins Y and Z returned to original values after the dogs were given either nicotinic acid or propranolol. The alterations in the alpha-lipoprotein peaks Y and Z were rapid, being noted within 5 min of change in plasma FFA concentration. However, there appeared to be a threshold of plasma FFA concentration of 1200 micro Eq/liter, below which no changes in alpha-lipoproteins were noted. It was concluded that alpha-lipoprotein Y is rapidly, progressively, but reversibly transformed into alpha-lipoprotein Z by binding to plasma FFA above a threshold level of 1200 micro Eq/liter. However, alpha-lipoprotein X does not appear to be involved in the binding of plasma FFA.     

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.     

Influences of age and gender on human thermoregulatory responses to cold exposures

To delineate age- and gender-related differences in physiological responses to cold exposure, men and women between the ages of 20 and 29 yr and 51 and 72 yr, wearing minimal clothing, were exposed at rest for 2 h to 28, 20, 15, and 10 degrees C room temperatures with 40% relative humidity. During the coldest exposure, the rates of increase in metabolic rate (W X m-2 or ml X kg lean body mass-1 X min-1 were similar for all groups. However, older women (n = 7) may have benefited from a larger (P less than 0.05) early metabolic (M) increase (40% within 15 min) than young men (18%) (n = 10), young women (5%) (n = 10), or older men (5%) (n = 10). A similar rapid M response in older women occurred during the 15 degrees C exposure. During all cold exposures, older women maintained constant rectal temperature (Tre) and young women maintained Tre only during the 20 degrees C exposures, whereas Tre of the men declined during all cold exposures (P less than 0.01). Changes in Tre and mean skin temperature (Ts) during cold exposure were largely related to body fat, although age and surface area/mass modified the changes in men. The data suggest that older men are more susceptible to cold ambients than younger people, since they did not prevent a further decline in their initially relatively low Tre. Despite greater insulation from body fat, the older women maintained a constant Tre at greater metabolic cost than men or younger women.     

Changes in blood plasma osmolality and states of mania

Manic patients were studied systematically over a period of months. Fluctuations in their blood plasma osmolality were observed and these correlated significantly and inversely with undulations in their mania, rating scores. Successful treatment of patients with lithium carbonate was correlated with increasing levels of their plasma osmolality and with a significant remission of mania symptomatology. Animal studies have shown that adaptations to altered blood plasma osmolalities are accompanied by profound metabolic changes in tissues of the central nervous system. A similar relationship is likely to exist in man. We suggest, therefore, that an altered plasma osmolality is linked to changes in manic symptomatology, through osmotically regulated metabolic changes in the central nervous system.     

Influence of menstrual cycle on shivering, skin blood flow, and sweating responses measured at night

In 10 women, external cold and heat exposures were performed both in the middle of luteal phase (L) and in the early follicular phase (F) of the menstrual cycle. Serum progesterone concentrations in L and F averaged 46.0 and 0.9 nmol X l-1, respectively. The experiments took place between 3:00 and 4:30 A.M., when the L-F core temperature difference is maximal. At neutral ambient temperature, esophageal (Tes), tympanic (Tty), rectal (Tre), and mean skin (Tsk) temperatures averaged 0.59 degrees C higher in L than in F. The thresholds for shivering, chest sweating, and cutaneous vasodilation (heat clearance technique) at the thumb and forearm were increased in L by an average of 0.47 degrees C, related to mean body temperature [Tb(es) = 0.87Tes + 0.13 Tsk] and to Tes, Tty, Tre, or Tsk. The above-threshold chest sweat rate and cutaneous heat clearances at the thumb and forearm were also enhanced in L, when related to Tb(es) or time. The metabolic rate, arm blood flow, and heart rate at thermoneutral conditions were increased in L by 5.0%, 1.1 ml X 100 ml-1 X min-1, and 4.6 beats X min-1, respectively. The concomitant increase in threshold temperatures for all autonomic thermoregulatory responses in L supports the concept of a resetting of the set point underlying the basal body temperature elevation in L. The effects of the increased threshold temperatures are counteracted by enhanced heat loss responses.     

Thermoregulatory model for immersion of humans in cold water

The mathematical models of thermoregulation of Stolwijk and Hardy, and Montgomery were used to develop a model suitable for the simulation of human physiological responses to cold-water immersion. Data were obtained from experiments where 13 healthy male volunteers were totally immersed under resting and nude conditions for 1 h in water temperatures of 20 and 28 degrees C. At these temperatures, the mean measured rectal temperature (Tre) fell by approximately 0.9 and 0.5 degrees C, respectively, yet mean measured metabolic rate (M) rose by approximately 275 and 90 W for the low body fat group (n = 7) and 195 and 45 W for the moderate body fat group (n = 6). To predict the observed Tre and M values, the present model 1) included thermal inputs for shivering from the skin independent of their inclusion with the central temperature to account for the observed initial rapid rise in M, 2) determined a thermally neutral body temperature profile such that the measured and predicted initial values of Tre and M were matched, 3) confined the initial shivering to the trunk region to avoid an overly large predicted initial rate of rectal cooling, and 4) calculated the steady-state convective heat loss by assuming a zero heat storage in the skin compartment to circumvent the acute sensitivity to the small skin-water temperature difference when using conventional methods. The last three modifications are unique to thermoregulatory modeling.     

Reason for the discrepancy between blood plasma osmolality and its Na+ concentration during cold stress in rats

The body temperature decrease to 24 degrees C or intraperitoneal injection of adrenaline (0.3 mg per 100 g body mass) to rats entailed the increased plasma osmolality and reduction in it of Na+ concentration. All plasma samples were ultracentrifuged. Osmolality was reduced and sodium concentration was increased in the internatant of the same samples where the maximal alterations were recorded. Experiments with the blood plasma of the lamprey which is rich under natural conditions in nonesterified fatty acids demonstrated analogous changes in osmolality and Na+ concentration in the plasma and plasma internatant. No significant changes were discovered in breast milk in which the percentage of high density lipoproteins is minimal. The authors assume that high density lipids are involved in the regulation of blood osmolality, with these lipids changing the solvent water activity and thereby lowering or raising blood plasma osmolality.