Modulation of adrenergic receptors and adrenergic functions in cold adapted humans

To specify the role of adrenoceptors in mediating adrenergic functions after adaptation of humans to cold, effect of administration of increasing concentrations of beta1 and beta2 adrenomimetics (Dobutamine, Bricanyl) on resting metabolic rate, heart rate, systolic blood pressure, rectal and skin temperatures of control humans and of cold adapted winter swimmers was studied. Increase in metabolic rate, mediated by beta1 and beta2 adrenomimetics, was attenuated after cold adaptation, indicating downregulation of beta1 and beta2 adrenoceptors. Since cold adapted humans have greater capacity of nonshivering thermogenesis, than that mediated by both beta1 and beta2 adrenoceptors, the role of other subtypes of adrenoceptors in mediating nonshivering thermogenesis is anticipated. Heart rate increased after administration of the beta2 agonist, but was not influenced by the beta1 agonist. The significance of beta2 adrenoceptors in mediating heart rate was depressed after cold adaptation. Data indicate that modifications of activity of beta adrenoceptors play crucial role in mechanisms responsible for adaptation of humans to cold.

Thermal, cardiac and adrenergic responses to repeated local cooling

The aim of this study was to ascertain whether repeated local cooling induces the same or different adaptational responses as repeated whole body cooling. Repeated cooling of the legs (immersion into 12 degrees C water up to the knees for 30 min, 20 times during 4 weeks = local cold adaptation - LCA) attenuated the initial increase in heart rate and blood pressure currently observed in control subjects immersed in cold water up to the knees. After LCA the initial skin temperature decrease tended to be lower, indicating reduced vasoconstriction. Heart rate and systolic blood pressure appeared to be generally lower during rest and during the time course of cooling in LCA humans, when compared to controls. All these changes seem to indicate attenuation of the sympathetic tone. In contrast, the sustained skin temperature in different areas of the body (finger, palm, forearm, thigh, chest) appeared to be generally lower in LCA subjects than in controls (except for temperatures on the forehead). Plasma levels of catecholamines (measured 20 and 40 min after the onset of cooling) were also not influenced by local cold adaptation. Locally cold adapted subjects, when exposed to whole body cold water immersion test, showed no change in the threshold temperature for induction of cold thermogenesis. This indicates that the hypothermic type of cold adaptation, typically occurring after systemic cold adaptation, does not appear after local cold adaptation of the intensity used. It is concluded that in humans the cold adaptation due to repeated local cooling of legs induces different physiological changes than systemic cold adaptation.

Steroids and thermogenesis

Apart from thyroid hormones, as the main hormonal regulators of obligatory thermogenesis, and catecholamines, as major hormonal regulators of facultative thermogenesis, production of heat in homeotherms can also be influenced by steroids. Generally, hormones can influence heat production by regulating the activity of various enzymes of oxidative metabolism, by modulating membrane protein carriers and other membrane or nuclear protein factors. Proton carriers in the inner mitochondrial membrane, known as uncoupling proteins, play the key role in heat dissipation to the detriment of the formation of energy-rich phosphates. In this minireview we have focused on the effects of steroids and thyroid hormones on heat production in brown adipose tissues and in skeletal muscles, with particular respect to their effect on uncoupling protein expression. Apart from hormonal steroids, dehydroepiandrosterone, an important precursor in the metabolic pathway leading to hormonal steroids which possess many, mostly beneficial effects on human health, modulates metabolic pathways which may lead to increased heat production. Recent studies demonstrate that 7-oxo-dehydroepiandrosterone, one of its 7-oxygenated metabolites, is even more effective than dehydroepiandrosterone. Recent findings of various actions of these steroids support the view that they may also participate in modulating thermogenic effects.

Endogenous antipyretics: neuropeptides and glucocorticoids

Based on observations that fever is suppressed under several physiological circumstances the existence of endogenous antipyretically active molecules has been postulated. A large number of experimental and some clinical studies provided evidence that the neuropeptides arginine vasopressin (AVP), alpha- and gamma-melanocyte stimulating hormones (alpha-MSH, gamma-MSH) and adrenocorticotropic hormone (ACTH) as well as glucocorticoids are capable to antagonize febrile responses to pyrogens. Endogenous antipyresis is mediated by actions of these molecules within the central nervous system or, at least in some cases, by peripheral effects. Brain sites where endogenous antipyresis is activated include the septal area of the limbic system and the anterior hypothalamus. The precise neuronal mechanisms of how the aforementioned endogenous mediators cause a limitation or even suppression of fever are not known. There is, however, evidence that endogenous antipyretics cancel changes in neuronal activities which have been induced by endogenous pyrogens such as cytokines and prostaglandins. At the level of the hypothalamic controller of thermoregulation antipyretic peptides seem to cause a reversion of the pyrogen-induced upward shift of the threshold body core temperature for activation of metabolic heat production. Such a change in thermoregulatory characteristics is compatible with a limitation of fever in strength and duration.

[Physiologic basis of human adaptation to cold]

Intermittent exposures to cold (15 head out immersions into 14 degrees C water, for 1 h, within 3 weeks) induce physiological changes leading to the restriction of heat loss from the body, due to hypothermia and vasoconstriction. These adaptational changes may save more than 25% of energy during 1 hour cold test. Insulative adaptation due to increased amount of the subcutaneous fat is less prominent. Winter swimmers show similar mechanisms of cold adaptationn as humans adapted by intermittent cold exposures in the laboratory. Additionally, winter swimmers exert a greater capacity of nonshivering thermogenesis than nonadapted controls. Thus, adaptation of humans to cold involves hypothermic, metabolic and insulative mechanisms. Time courses of development of individual mechanisms of adaptation differ. Intermittent local cold exposures (20 immersions of legs into 12 degrees C water, for 45 min, within 4 weeks lower sympathetic tone and induce redistribution of cardiac output, in order to ensure preferential warming of central parts of the body. Basal metabolism and cold induced thermogenesis are increased, but the threshold for induction of cold thermogenesis is not influenced. In contrast to changes induced by repeated whole body immersions, these changes may not ensure greater resistance to the cold stress.

Dynamics of cytokine production in human peripheral blood mononuclear cells stimulated by LPS or infected by Borrelia

To specify the role of individual cytokines in the immune response to pyrogens, isolated and cultivated human peripheral blood mononuclear cells (PBMC) were used for the experiments. Different pyrogens (lipopolysaccharide from Escherichia coli - LPS and live Borrelia afzelii) were applied and the time course of changes in concentrations of different cytokines in the medium was followed using the ELISA method. It was found that nonstimulated human PBMC proliferate under in vitro conditions and produce IL-6, TNF-alpha, IL-10 and finally also IL-1beta. Productions of IL-12 and INF-gamma are not changed. Proliferation of PBMC is potentiated after incubation with LPS or live Borrelia. PBMC stimulated by LPS increase the net production (stimulated minus unstimulated) of IL-1beta and TNF-alpha significantly, while production of IL-6 was smaller. A delayed increase in the production of IL-10 was also observed. Productions of IL-12 and INF-gamma were not influenced. In contrast to LPS, stimulation of PBMC with live Borrelia, increases also the production of IL-12 and IFN-gamma, besides IL-1beta, TNF-alpha, IL-6 and IL-10. Productions of IL-1beta, IL-6 and TNFalpha increased immediately after incubation with both LPS and Borrelia, while productions of IL-12 and INF-gamma begin to increase 8 hours and production of IL-10 12 hours after stimulation. Data indicate that stimulation with different pyrogens may activate the cells of the immune cascade in a different way. Stimulation of BPMC by LPS seems to activate the initial steps of the immune response (macrophages and granulocytes) only, while infection with live Borrelia also stimulates the later phase of the immune response, probably due to effect of initially produced cytokines.

Dynamics of cytokine production in human peripheral blood mononuclear cells stimulated by LPS or infected by Borrelia

To specify the role of individual cytokines in the immune response to pyrogens, isolated and cultivated human peripheral blood mononuclear cells (PBMC) were used for the experiments. Different pyrogens (lipopolysaccharide from Escherichia coli - LPS and live Borrelia afzelii) were applied and the time course of changes in concentrations of different cytokines in the medium was followed using the ELISA method. It was found that nonstimulated human PBMC proliferate under in vitro conditions and produce IL-6, TNF-alpha, IL-10 and finally also IL-1 beta. Productions of IL-12 and INF-gamma are not changed. Proliferation of PBMC is potentiated after incubation with LPS or live Borrelia. PBMC stimulated by LPS increase the net production (stimulated minus unstimulated) of IL-1 beta and TNF-alpha significantly, while production of IL-6 was smaller. A delayed increase in the production of IL-10 was also observed. Productions of IL-12 and INF-gamma were not influenced. In contrast to LPS, stimulation of PBMC with live Borrelia, increases also the production of IL-12 and IFN-gamma, besides IL-1 beta, TNF-alpha, IL-6 and IL-10. Productions of IL-1 beta, IL-6 and TNF alpha increased immediately after incubation with both LPS and Borrelia, while productions of IL-12 and INF-gamma begin to increase 8 hours and production of IL-10 12 hours after stimulation. Data indicate that stimulation with different pyrogens may activate the cells of the immune cascade in a different way. Stimulation of BPMC by LPS seems to activate the initial steps of the immune response (macrophages and granulocytes) only, while infection with live Borrelia also stimulates the later phase of the immune response, probably due to effect of initially produced cytokines.

Human physiological responses to immersion into water of different temperatures

To differentiate between the effect of cold and hydrostatic pressure on hormone and cardiovascular functions of man, a group of young men was examined during 1-h head-out immersions in water of different temperatures (32 degrees C, 20 degrees C and 14 degrees C). Immersion in water at 32 degrees C did not change rectal temperature and metabolic rate, but lowered heart rate (by 15%) and systolic and diastolic blood pressures (by 11 %, or 12%, respectively), compared to controls at ambient air temperature. Plasma renin activity, plasma cortisol and aldosterone concentrations were also lowered (by 46%, 34%, and 17%, respectively), while diuresis was increased by 107%. Immersion at 20 degrees C induced a similar decrease in plasma renin activity, heart rate and systolic and diastolic blood pressures as immersion at thermoneutrality, in spite of lowered rectal temperature and an increased metabolic rate by 93%. Plasma cortisol concentrations tended to decrease, while plasma aldosterone concentration was unchanged. Diuresis was increased by 89%. No significant differences in changes in diuresis, plasma renin activity and aldosterone concentration compared to subjects immersed to 32 degrees C were observed. Cold water immersion (14 degrees C) lowered rectal temperature and increased metabolic rate (by 350%), heart rate and systolic and diastolic blood pressure (by 5%, 7%, and 8%, respectively). Plasma noradrenaline and dopamine concentrations were increased by 530% and by 250% respectively, while diuresis increased by 163% (more than at 32 degrees C). Plasma aldosterone concentrations increased by 23%. Plasma renin activity was reduced as during immersion in water at the highest temperature. Cortisol concentrations tended to decrease. Plasma adrenaline concentrations remained unchanged. Changes in plasma renin activity were not related to changes in aldosterone concentrations. Immersion in water of different temperatures did not increase blood concentrations of cortisol. There was no correlation between changes in rectal temperature and changes in hormone production. Our data supported the hypothesis that physiological changes induced by water immersion are mediated by humoral control mechanisms, while responses induced by cold are mainly due to increased activity of the sympathetic nervous system.

Hibernation triggers and cryogens: do they play a role in hibernation?

A survey of the literary evidence on cryogens and hibernation induction triggers is given and the results of experiments on the effect of hypothalamic or i.v. injections of opioids and plasma from hibernating European hamsters on body temperature control of rabbits are presented. Pharmacological doses of a delta opioid--DADLE (25 or 50 micrograms), when injected into the anterior hypothalamus, induce a small and short-lasting hypothermic effect in cold exposed rabbits, due to the downward shift of the temperature threshold for shivering. Lower doses (5 micrograms) are without effect, similarly as i.v. administrations (500 micrograms/kg) of this substance. Intrahypothalamic injections of met-enkephalin (0.1-1 microgram) induce a slight hyperthermia due to the shift of all thermoregulatory effectors to higher body temperatures. Intrahypothalamic injections of plasma from hibernating European hamsters do not influence the body temperature control in rabbits.

Change in sympathetic activity, cardiovascular functions and plasma hormone concentrations due to cold water immersion in men

The purpose of this study was to determine whether or not repeated short-term cold water immersions can induce a change in the activity of the sympathetic nervous system and, consequently, in cardiovascular functions in healthy young athletes. Changes in some plasma hormone concentrations were also followed. A single cold water immersion (head-out, at 14 degrees C, for 1 h) increased sympathetic nervous system activity, as evidenced by a four-fold increase (P < 0.05) in plasma noradrenaline concentration. Plasma adrenaline and dopamine concentrations were not increased significantly. Plasma renin-angiotensin activity was reduced by half (P < 0.05) during immersion but plasma aldosterone concentration was unchanged. Stimulation of the sympathetic nervous system during immersion did not induce significant changes in heart rate, but induced peripheral vasoconstriction (as judged from a decrease in skin temperature) and a small increase (by 10%) in systolic and diastolic blood pressures. No clear change in reactivity of the sympathetic nervous system was observed due to repeated cold water immersions (three times a week, for 6 weeks). Neither the plasma renin-angiotensin activity, aldosterone concentration nor cardiovascular parameters were significantly influenced by repeated cold water immersions. A lowered diastolic pressure and an increase in peripheral vasoconstriction were observed after cold acclimation, however. Evidently, the repeated cold stimuli were not sufficient to induce significant adaptational changes in sympathetic activity and hormone production.

Changes in thermal homeostasis in humans due to repeated cold water immersions

The purpose of this study was to monitor changes in body and skin temperatures, heat production, subjective shivering, cold sensation and body fat content in humans after intermittent cold water immersion. Repeated exposures of young sportsmen to cold water (head out, 14 degrees C, 1 h, 3 times per week for 4-6 weeks) induced changes in regulation of thermal homeostasis. "Cold acclimated" subjects exhibited an hypothermic type of adaptation. Central and peripheral body temperatures at rest and during cold immersion were lowered. The metabolic response to cold was delayed and subjective shivering was attenuated. The observed hypothermia was due to the shift of the threshold for induction of cold thermogenesis to lower body temperatures. "Cold acclimated" subjects also showed a lowered cold sensation. Because of the observed physiological changes, about 20% of the total heat production was saved during one cold water immersion of "cold acclimated" subjects. Maximal aerobic and anaerobic performances were not altered. No change in the thermosensitivity of the body temperature controller, as assessed from the unchanged slope of the relation between the deep body temperature and total heat production, was observed. Changes in cold sensation and regulation of cold thermogenesis were noticed first after four cold water immersions and persisted for at least 2 weeks after termination of the adaptation procedure. A trend towards a small increase in the body fat content was also observed. This finding, as well as the increased vasoconstriction, evidenced by the lowered skin temperature, indicate that slight changes in body insulation may also occur after "cold acclimation" in humans.

Immune system of cold-exposed and cold-adapted humans

The aim of this study was to investigate whether or not the human immune system can be activated by a noninfectious stimulus, thereby improving the physiological status of the individual. The effect of a single cold water immersion (14 degrees C for 1 h) on the immune system of athletic young men, monitored immediately after immersion, was minimal. With the continuation of the cold water immersions (three times a week for a duration of 6 weeks) a small, but significant, increase in the proportions of monocytes, lymphocytes with expressed IL2 receptors (CD25) and in plasma tumour necrosis factor alpha content was induced. An increase in the plasma concentrations of some acute phase proteins, such as haptoglobin and haemopexin, was also observed. After 6 weeks of repeated immersions a trend towards an increase in the plasma concentrations of IL6 and the amount of total T lymphocytes (CD3), T helper cells (CD4), T suppressor cells (CD8), activated T and B lymphocytes (HLA-DR) and a decrease in the plasma concentration of alpha 1-antitrypsin was observed. Concentrations of IL1 beta, neopterin, C-reactive protein, orosomucoid, ceruloplasmin, macroglobulin, immunoglobulins (IgG, IgM, IgA) and C3, C4 components of the complement, as well as the total number of erythrocytes, leucocytes, granulocytes and neutrophils showed no significant changes after the repeated cold water immersions. It was concluded that the stress-inducing noninfectious stimuli, such as repeated cold water immersions, which increased metabolic rate due to shivering the elevated blood concentrations of catecholamines, activated the immune system to a slight extent. The biological significance of the changes observed remains to be elucidated.

Production of systemic and hypothalamic cytokines during the early phase of endotoxin fever

Changes in concentrations of cytokines in plasma and in hypothalamic push-pull perfusates of guinea pigs were measured within the 1st hour after intramuscular injections of bacterial lipopolysaccharide (LPS; Escherichia coli, 20 micrograms/kg) or solvent (0.9% saline). In control animals injected with solvent, interleukin (IL)-1 and tumor necrosis factor alpha (TNF-alpha) were not detectable in plasma. Only IL-6 was present in picogram quantities. Within 45 min after injection of LPS, the concentrations of IL-1, TNF-alpha, and IL-6 increased in the plasma: by several orders of magnitude for TNF-alpha and about tenfold for IL-G. Picogram amounts of biologically active IL-1 were detected in plasma after injection of LPS. No steady state levels of systemic cytokines were reached during the experimental period. In hypothalamic perfusates of animals injected with the solvent, no IL-1 was detectable. TNF-alpha could be detected at higher concentrations than IL-6. IL-6 was detectable at tenfold lower concentrations than in the plasma. In animals injected with LPS, the hypothalamic concentration of IL-6 started to increase during the period 15-30 min and the concentrations of TNF-alpha during the period 30-45 min after LPS injection. The concentrations of IL-6 increased by 300-400% and did not exceed picogram values. No progressive increase of hypothalamic levels of these cytokines was observed during the time course of the experiment. The method used did not detect any changes in the amount of biologically active IL-1 in hypothalamic perfusates of LPS-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

[Changes in body fluids and minerals after cold adaptation]

To characterize fluid and ion shifts during 5-weeks cold adaptation, 6 nonadapted volunteers underwent cold acclimation programme (CAP), consisted of 1 h head-out immersion in water 14 +/- 1 degrees C 3 times a week. Blood samples were analyzed before/after the immersion and in the first and in the last week of CAP. Urine was collected for 10 h before, during, and after immersion for 4 h. Plasma volume (PV) decreased during first immersion (-18%) and less after CAP (-12). Blood volume reduction was 8.5% before and 5.2% after the CAP. Mean corpuscular volume was not changed either after the cold exposition or after the cold adaptation. The concentration of serum proteins increased by 12.1% after first immersion and by 8.1% after the CAP. The changes in serum concentration of Na+, K+ and Cl- before and after the CAP were not significant. Urine flow increased by 102% after first immersion, and by 165% after CAP. Urinary excretion of Na+ increased by 167% and 283%, excretion of K+ by 222% and 362% during first immersion and after CAP, respectively. Serum concentration of aldosterone increased nonsignificantly (+30%) during immersion before CAP and it did not changed after CAP. After the cold adaptation we observed the reduction of PV decrease, and increased diuresis with higher excretions of cations.

Short days induce changes in specific melatonin binding in hamster median eminence and anterior pituitary

Autoradiography of 125I-melatonin binding to hamster brain sections revealed a competitive 125I-melatonin binding in median eminence only. Saturation studies on crude membrane fraction revealed high-affinity melatonin binding sites in median eminence (Kd = 59 pM) and anterior pituitary (Kd = 97 pM). In the hamsters maintained on LD 14:10, the concentration of the binding sites was 10.4 and 6.0 fmol/mg protein in median eminence and anterior pituitary respectively; long-term exposure to LD 8:16 decreased the concentration to less than a half.

The role of dopaminergic pathways in thermoregulation in the rabbit

Dopamine, when injected into the anterior hypothalamus of the rabbit, induced a slight hyperthermia due to an upward shift of the threshold central temperature for induction of cold thermogenesis, panting and vasodilation. A slightly reduced thermosensitivity of the controller regulating vasodilation may also contribute to the hyperthermic effect of dopamine. Intrahypothalamic injections of the dopamine agonist, apomorphine, induced a similar effect to that of dopamine, with the exception that the thermosensitivity of the controller regulating vasodilation was not changed. Intraperitoneal injections of a dopamine antagonist, haloperidol, induced a marked hypothermia, due to a downward shift of the threshold central temperature for induction of cold thermogenesis, panting and vasodilation. A slightly reduced thermosensitivity of the controller regulating vasodilation was also observed. Intrahypothalamic injections of haloperidol did not induce an antagonist effect to dopamine, but rather tended to induce hyperthermia. Thermoregulatory responses, occurring after administration of dopamine or apomorphine, partially resembled those seen after administration of neurotensin or prostaglandins.

Mode of ACTH antipyretic action

The method of intestinal cooling was used to analyze the effect of centrally administered ACTH in microgram quantities on hypothalamic centers regulating activity of thermoregulatory outputs (cold thermogenesis--CT, peripheral vasomotor tone--PVMT, respiratory evaporative heat loss--REHL). ACTH, when injected into the supraoptic area of the anterior hypothalamus of normal rabbits, had no significant effect on body temperature control. Intrahypothalamic administration of ACTH during the early phase of the fever, induced by intravenous injection of exogenous pyrogen, evoked dissociation of temperature thresholds for cold and warm defence, shifting the threshold for induction of cold thermogenesis to lower central temperatures. The thermosensitivity of centers controlling cold thermogenesis was lowered and the maximal values of cold thermogenesis were depressed to about 30% of those in control rabbits. Central administration of ACTH in the late phase of the fever (120 min after IV injection of endotoxin) induced a smaller effect than in the early phase of the fever--the downward shift of the temperature threshold for cold thermogenesis was less evident and the thermosensitivity of the controller remained unchanged. The changes in activity of thermoregulatory centers that occurred after ACTH in febrile rabbits correspond to those observed in the late phase of the fever in ACTH-untreated rabbits. It is suggested therefore, that the presumed increase in ACTH production during fever might represent a negative feed-back mechanism contributing to the termination of the febrile state.

Thermoregulation of the rabbit during the late phase of endotoxin fever

In the late phase of the fever occurring 120 or more min after i.v. injection of endotoxin (1 microgram/kg) to female rabbits, marked shifts of thresholds for respiratory evaporative heat loss and for peripheral vasodilatation to higher body core temperatures were observed. In contrast, the threshold body core temperature for cold thermogenesis was shifted downwards. As a result, the interthreshold zone was widened. Within the body temperature range of 37.4 to 39.9 degrees C neither heat production or heat loss mechanisms were operant and the body temperature was determined mainly by passive heat transfer between the body and the environment. Outside this zone, the sensitivities of the heat and cold defence activities to changes in body core temperature appeared to be unchanged.

Effect of bombesin on thermoregulation of the rabbit

Injections of bombesin (BOM, 125, 250 and 500 ng) into the preoptic/anterior hypothalamus caused dose-related decreases of threshold temperatures for metabolic cold defence, cutaneous vasomotor tone and respiratory rate, combined with a reduced sensitivity of these thermoregulatory effectors in response to core temperature changes induced at thermoneutral or warm ambient conditions. Intracisternal (i.c.) injections of BOM (250 ng) produced qualitatively identical thermoregulatory effector changes in response to core temperature changes. Injections of BOM into the posterior hypothalamus did not affect body temperature control. Increased locomotor behavior, licking and grooming was elicited, however, from all injection sites. The results explain the prevailing hypothermic effect of BOM as the consequence of the concerted decrease in threshold temperatures and "gains" of all autonomic thermoregulatory effectors and suggest the activation of warm inputs, relative to cold inputs, at the hypothalamic level as the underlying mechanism. Direct or indirect inhibition of the intrinsic hypothalamic system involving thyrotropin-releasing hormone (TRH) and consequent deactivation of central noradrenergic pathways known to generate the entire autonomic pattern of cold defence might be involved in the neuro-humoral changes resulting in hypothermic effects of centrally applicated BOM.

Effect of external factors on gonadal activity and body mass of male golden hamsters (Mesocricetus auratus)

Effects of photoperiod, temperature and food and water availability on body and testicular mass of golden hamsters were investigated. Lack of water or food inhibited body growth. Short photoperiod (LD 8:16) enhanced body growth in males fed 'poor diet' but it had no effect in animals on 'high quality diet'. Cold exposure diminished body growth in animals with access to water but not in those deprived of water. It is suggested that lowered motor activity is mainly responsible for body mass gain of animals kept under the short photoperiod. All stressful factors (water deprivation, 'poor diet' or cold) induced involution of testes. The time course of testicular involution was similar to that induced by short photoperiod. Testicular involution was, after several weeks, followed by spontaneous recrudescence. Combination of stressful external cues speeded up testicular involution. Since the testicular involution induced by water deprivation, "poor diet" and cold also occurred in animals kept in constant light or under long photoperiod (which are known to produce functional pinealectomy) it is concluded that the inhibitory effects of stressful external factors on testes are not mediated by the pineal. The decrease in testosterone production induced by external cues precedes changes in testicular mass. In individuals kept in the cold, testosterone production remains low, in spite of spontaneous recrudescence of the testes. No simple relationship between testosterone concentration in the blood and testicular mass was observed. The mass of the interscapular brown adipose tissue (ISBAT) changes strikingly during the hibernation cycle. Immediately after exposure to cold it decreases and stays at the low level for 40 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperthermic effect of neurotensin in the rabbit

Neurotensin injected into the anterior hypothalamus of the rabbit induces a consistent upward shift of the threshold core temperatures for inducing cold thermogenesis, peripheral vasomotor tone and respiratory evaporative heat loss, while leaving the efficiency and the intensity of these thermoregulatory outputs unchanged. Neurotensin injections into the PH are without effect on body temperature control. The effect of neurotensin is interpreted as a selective inhibition of activity of warm sensors in the anterior hypothalamus. The possible mediating role of dopaminergic pathways in the neurotensin action is discussed.

Thyroid hormones in rats during long-term cold exposure and hypometabolic effect of reverse triiodothyronine on adrenaline induced thermogenesis

One-day cold exposure to 5 degrees C induced temporal increase of reverse-triiodothyronine (rT3), triiodothyronine (T3) and thyroxine (T4) concentrations in the blood of rats (by 151%, 91% and 107%, respectively). The levels of all thyroid hormones remain elevated for at least 10 days of cold exposure. The injection of rT3 (330 micrograms kg-1 s.c.) inhibited the resting and adrenaline stimulated metabolism 1-3 days after application independently from the thyroid status of the animals. Since the hypothermic effect of rT3 occurred in thyroidectomized rats, it was concluded that rT3 acted primarily on peripheral, tissues, rather than on the thyroid.

Arrest of the circadian pacemaker driving the pineal melatonin rhythm in hibernating golden hamsters, Mesocricetus auratus

Pineal melatonin rhythm in golden hamsters was abolished during hibernation. After arousal in darkness, pineal melatonin increased rapidly regardless of whether the arousal was induced during the day or at night. Rapid increase of pineal melatonin after arousal was markedly diminished in animals exposed to light. In hamsters aroused at midnight, the melatonin rhythm in constant darkness ran with the reversed phase relative to hamsters aroused at noon. Since after arousal the melatonin rhythm obviously starts anew from the same phase, we conclude that the circadian pacemaker driving the rhythm might be arrested during hibernation at the day-time phase.

Oxygen consumption in rat skeletal muscle at various rates of oxygen delivery

Resting oxygen consumption of the vascularly isolated gracilis anticus muscle of the rat perfused by natural circulation via the femoral artery with diluted or undiluted blood depends on oxygen delivery (the product of flow rate and arterial oxygen concentration - DO2) only when DO2 falls below 16 microliters/min/g.

Pineal melatonin in hibernating and aroused golden hamsters (Mesocricetus auratus)

Daily changes of pineal melatonin content were determined in warm-adapted nonhibernating and cold-adapted hibernating golden hamsters (Mesocricetus auratus). Pineal melatonin in nonhibernating golden hamsters showed marked daily rhythm with the night values about 20 times higher than the daytime ones. In hamsters hibernating for 2 and 3 days the melatonin rhythm was abolished, since no increase of pineal melatonin over basal levels occurred throughout 24 hr period. After arousal from hibernation melatonin increased rapidly regardless whether the hamsters were provoked to arousal during day or night.

Shivering and nonshivering thermogenesis in the bat (Myotis myotis Borkh.) during arousal from hibernation

Nonshivering thermogenesis exists in the bat (Myotis myotis Borkh.) arousing from hibernation at environmental temperatures of 4–6 °C. Nonshivering thermogenesis is essential for the start of the arousal, and it is stimulated by noradrenaline since hexamethonium prevents the increase in metabolism and body temperature. Injection of noradrenaline abolishes this inhibition by hexamethonium by inducing nonshivering thermogenesis. After simultaneous administration of hexamethonium and alderlin no calorigenic effect of noradrenaline occurs. Shivering heat production during arousal appears at body temperatures between 10 and 17 °C predominantly. In normothermic bats the calorigenic effect of noradrenaline was observed, which indicates that nonshivering thermogenesis might also be present in awake animals. During arousal at 25 °C a great increase in intensity of shivering was observed. Elimination of nonshivering thermogenesis by hexamethonium does not prevent the attainment of the homoiothermic level of body temperature, and administration of noradrenaline does not speed up the process of arousal. As is evident from the metabolic capacity of the brown fat, the heat derived from this organ could maximally participate in total metabolism by 25% at the beginning and at the late period of arousal. In the middle range of body temperatures its significance for total metabolism is only about 10–13%.

BODY ORGAN CYTOCHROME OXIDASE ACTIVITY IN COLD- AND WARM-ACCLIMATED RATS

The cytochrome oxidase activity was estimated in homogenates of the whole body and in nine body organs of cold- and warm-acclimated rats. The total body cytochrome oxidase activity expressed in terms of oxygen consumption was similar in cold- and warm-acclimated rats. In cold-acclimated animals the total cytochrome oxidase activity did not differ from maximal steady state metabolism measured in vivo, while in warm-acclimated rats the total cytochrome oxidase activity was almost twice as great as the maximal steady state metabolism. The results indicate that warm-acclimated rats do not utilize the full capacity of the cytochrome system and that cold-acclimation makes full exploitation of the oxidase capacity possible. In cold-acclimated rats the cytochrome oxidase activity of the muscles comprised 57% of the total, the liver 22.5%, and the skin 6%, with smaller roles for other organs. The role of the liver was greater in cold-acclimated than in warm-acclimated rats.