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PubMed1994

Ventilatory and metabolic responses to cold and CO-induced hypoxia in awake rats.

Experiments were carried out in awake rats to compare the effects of ambient and CO-induced hypoxia on thermoregulation and ventilatory control. Measurements of metabolic rate (VO2), ventilation (V), shivering (EMG) and colonic temperature (Tc) were made at fixed ambient temperature (Ta) of 25, 15 and 5 degrees C. Animals were exposed to ambient hypoxia (FIO2 of 21, 17, 14, 12 and 10%) or to CO hypoxia (FICO of 0.03% in air). The results show that: (1) Both ambient and CO-induced hypoxia provoked decreases in VO2 and Tc which were more marked at low Ta values; non-shivering thermogenesis was depressed with both types of hypoxia, whereas shivering was depressed only with ambient hypoxia; (2) Ventilatory response to ambient hypoxia was blunted at low Ta values and CO-induced hypoxia did not affect ventilation. It is concluded that: (1) hypoxia affects markedly the control of Tc by altering thermogenesis: inhibition of non-shivering thermogenesis seems to result from a decrease in CaO2 whereas inhibition of shivering seems to result from a decrease in PaO2; (2) during hypoxia, ventilation is controlled by the opposite stimulation from chemoreceptors and inhibition from hypometabolism. However, as revealed by CO-induced hypoxia, another stimulatory factor may also interact with the control of breathing.

Animals↗
PubMed2000

Effects of severity of dystocia on cold tolerance and serum concentrations of glucose and cortisol in neonatal beef calves.

Effects of dystocia on rectal temperature and serum cortisol and glucose concentrations, were studied in neonatal calves exposed to 0 degree C. Primiparous dams were observed continuously during parturition and if Stage II (labor) was not completed within 2 h after appearance of the allantochorion, delivery was completed with obstetrical assistance. Parturitions were scored (CDS) for difficulty and obstetric assistance required: CDS 1, no assistance (n = 8); CDS 2, minor manual assistance (n = 7); CDS 3, use of a mechanical calf puller (n = 5); CDS 4, cesarean section (n = 6). A blood sample, rectal temperature, and body weight were obtained within 30 min after birth. Calves were then fed 38 degrees C pooled colostrum, muzzled to prevent suckling, and placed back with their dam in a heated (22 degrees C) barn. At 4 h of age an indwelling jugular catheter was inserted. At 5 h of age calves were placed in a 0 degree C room for 140 min and blood samples and rectal temperatures were obtained every 10 or 20 min. A shivering score (1 = no shivering; 2 = moderate shivering; 3 = intense shivering) was assigned at each sampling time. Rectal temperatures were higher (P < 0.01) in CDS 1, 2 and 4 calves (39.0, 39.3, and 39.0 +/- .02 degrees C, respectively) than in calves with CDS 3 (38.3 +/- 0.02 degrees C) and were affected by duration of cold exposure (time; P < 0.01). Shivering was not affected by CDS but was affected by time (P < 0.01). Glucose concentrations were higher (P < 0.01) in CDS 3 calves (110.1 +/- 1.6 mg/dL) than in CDS 1, 2, or 4 calves (77.2, 86.4, and 89.0 +/- 1.3 mg/dL, respectively) and changed over time (P < 0.01). Cortisol concentrations were higher in CDS 1 calves (80.0 +/- 1.7 ng/mL) than in CDS 2, 3 or 4 calves (62.7, 58.2, and 57.7 +/- 2.0 ng/mL, respectively) and were affected by time (P < 0.01). We conclude that severe dystocia (CDS 3) resulted in lower calf rectal temperature, reduced serum cortisol, and increased serum glucose which could affect the ability of the calf to withstand cold stress. Minor dystocia did not cause and timely cesarean delivery prevented, the physiological aberrations encountered in severe dystocia.

Adaptation, Physiological↗
PubMed1975

Thermoregulatory changes induced by cholinomimetic substances introduced into the cerebral ventricles of sheep.

1 Thermoregulatory responses have been recorded from Welsh Mountain sheep exposed to warm, neutral or cold environments while injections of cholinomimetic drugs and/or their antagonists have been given into a lateral cerebral ventricle. 2. Carbachol and physostigmine inhibited panting of animals at high ambient temperature (ta), caused vasoconstriction and initiated shivering at neutral ta, and accentuated shivering at low ta. Rectal temperature (tre) invariably increased. Oxotremorine had apparently identical effects. 3. Nicotine and another ganglionic stimulant, the quaternary methyl derivative of dopamine, had no effects on thermoregulation. 4. Atropine given 10 min before injections of carbachol, physostigmine or oxotremorine completely inhibited their hyperthermic effects, but pretreatment with the ganglion-blocking drug, pempidine, caused no inhibition. The cholinergic synapses that respond to cholinomimetic drugs injected into the lateral cerebral ventricles of sheep are therefore muscarinic and not nicotinic. 5. When atropine was given to sheep exposed to cold, no detectable reduction of shivering occurred and tre decreased only slightly, even with doses of atropine far greater than needed to inhibit shivering induced by physostigmine. This may be because shivering is controlled by neural pathways unaffected by drugs administered intracerebroventricularly or because the cholinergic synapses activated by physostigmine do not carry the input from cold sensors.

Animals↗
PubMed1977

Effect of glucagon on thermogenesis in the pigeon.

The effects of glucagon injection on temperature regulation and some metabolic parameters were studied in the pigeon. Glucagon (100 microng/kg) always inhibited shivering and caused a fall in the oxygen consumption and body temperature of the unanesthetized pigeon at + 6 degrees C. At + 34 degrees C, the same dose of glucagon had no effect on these parameters. At 22 degrees C, glucagon produced an elevation in plasma free fatty acid (FFA) and blood glucose levels. The rise in FFA at 22 degrees C coincided with the suppression of shivering at 6 degrees C. The glucagon-mediated rise in plasma FFA, but not glucose level, was potentiated by cold ambient temperature. Adrenergic blocking agents given prior to glucagon did not abolish its effects. Phentolamine even prolonged the absence and accelerated the suppression of shivering. A dissociation in the mechanisms by which catecholamines and glucagon suppress shivering is suggested. Although mobilizing energy reserves, glucagon does not seem to be calorigenic in the pigeon at this dose. The interpretation of the changes in plasma FFA levels is discussed in relation to fuel consumption during shivering.

Animals↗
PubMed1976

Effect of propranolol on endotoxin-induced pyrogenesis in newborn and adult guinea pigs.

Previous studied have shown that nonshivering thermogenesis (NST) is the prevailing mechanism of fever production in both newborn (during the first 2-3 wk of life) and adult cold-acclimated guinea pigs. This study was undertaken to determine whether this process may be mediated by noradrenergic sympathetics. The temperatures in the interscapular brown fat pad (Tbat) and the colon (Tre), the rate of oxygen consumption (VO2), and shivering activity were measured continuously for 5 h at 27 degrees C ambient temperature in 8- and 16-day-old and adult cold-acclimated guinea pigs following 2 mug/kg iv of Salmonella enteritidis endotoxin, with and without 6.0 mg/kg of propranolol (a beta-adrenergic blocker) injected ip 2 min before the endotoxin. In the older animals, the increase in Tbat normally produced by endotoxin was reduced by propranolol administration, but shivering set in, maintaining both VO2 and Tre at their febrile levels. In the newborn animals, Tbat also was decreased by propranolol, but shivering did not set in, so that VO2 and Tre fell below their febrile values. It is concluded that endotoxin-induced NST is controlled by noradrenergic sympathetics. The failure of NST to be replaced in the present newborn guinea pigs by visible shivering might be related to other observations that the onset of shivering at this age occurs only when Tre is significantly reduced.

Adipose Tissue, Brown↗
PubMed1989

Effects of hypoxia on metabolic rate of conscious adult cats during cold exposure.

Oxygen consumption (VO2) and shivering movements were recorded in adult, conscious cats in a thermoneutral (24-27 degrees C) and in a cold (3-8 degrees C) environment during normoxia, hypoxia, or hyperoxia for 55 min. In the cold environment, VO2 correlated with shivering index (SI) under conditions of normoxia or ambient hypoxia (FIO2 = 0.12). During normoxia, VO2 was 63% higher in the cold than the thermoneutral environment. Ambient hypoxia acutely reduced VO2 in cold and thermoneutral environments, the decrement being greater for the former than the latter. Similarly, the variation in VO2 for unit change in SI was greater in hypoxia than normoxic conditions, suggesting that hypoxia influenced nonshivering as well as shivering components of cold-induced VO2. Hypoxia induced by CO (FICO = 0.002) also reduced VO2 and SI, a result that is consistent with previous results indicating that carotid body chemoreceptors do not mediate the suppression of shivering by ambient hypoxia. Hyperoxia increased VO2 and SI in the cold, and the effects of both hypoxia and hyperoxia in the cold were antagonized by increasing FICO2 to 0.03. The results demonstrate that hypoxia suppresses VO2 in the cold by reducing the intensity of shivering and, probably, by an action on metabolic rate that is unrelated to cold-induced calorigenesis.

Animals↗
PubMed1991

Effects of hypoxia and cold acclimation on thermoregulation in the rat.

The effects of hypoxia (inspired O2 fraction = 0.12) on thermoregulation and on the different sources of thermogenesis were studied in rats before and after periods of 1-4 wk of cold acclimation. Measurements of metabolic rate (VO2) and body temperature (Tb) were made at 5-min intervals, and shivering activity was recorded continuously in groups of rats subjected to three protocols. In protocol 1, rats were exposed to normoxia to an ambient temperature (Ta) of 5 degrees C for 2 h. In protocol 2, at Ta of 5 degrees C, rats were exposed for 30 min to normoxia, then for 45 min to hypoxia, and finally for 30 min to normoxia. In protocol 3, in the non-cold-acclimated (NCA) rats, Ta was decreased from 30 to 5 degrees C in steps of 5 degrees C and of 30-min duration while in cold-acclimated (CA) rats at 5 degrees C for 4-wk, Ta was increased from 5 to 30 degrees C in steps of 5 degrees C and of 30-min duration. Recordings were made in normoxia and in hypoxia on different days in the same animals. The results showed that 1) in NCA rats, cold exposure in normoxia induced increases in VO2 and shivering that were proportional to the decrease in Ta; 2) in CA rats in normoxia, for a given Ta, VO2 and Tb were higher than in NCA rats, whereas shivering was generally lower; and 3) in both NCA and CA rats, hypoxia induced a transient decrease in shivering and a sustained decrease in nonshivering thermogenesis associated with a marked decrease in Tb that was about the same in NCA and CA rats. We speculate that hypoxia acts on Tb control to produce a general inhibition of thermogenesis. Nonshivering thermogenesis is markedly sensitive to hypoxia, especially demonstrable in CA rats; a recovery or even an increase in shivering can compensate for the decrease in nonshivering thermogenesis.

Acclimatization↗
PubMed1992

A second postcooling afterdrop: more evidence for a convective mechanism.

An attempt was made to demonstrate the importance of increased perfusion of cold tissue in core temperature afterdrop. Five male subjects were cooled twice in water (8 degrees C) for 53-80 min. They were then rewarmed by one of two methods (shivering thermogenesis or treadmill exercise) for another 40-65 min, after which they entered a warm bath (40 degrees C). Esophageal temperature (Tes) as well as thigh and calf muscle temperatures at three depths (1.5, 3.0, and 4.5 cm) were measured. Cold water immersion was terminated at Tes varying between 33.0 and 34.5 degrees C. For each subject this temperature was similar in both trials. The initial core temperature afterdrop was 58% greater during exercise (mean +/- SE, 0.65 +/- 0.10 degrees C) than shivering (0.41 +/- 0.06 degrees C) (P < 0.005). Within the first 5 min after subjects entered the warm bath the initial rate of rewarming (previously established during shivering or exercise, approximately 0.07 degrees C/min) decreased. The attenuation was 0.088 +/- 0.03 degrees C/min (P < 0.025) after shivering and 0.062 +/- 0.022 degrees C/min (P < 0.025) after exercise. In 4 of 10 trials (2 after shivering and 2 after exercise) a second afterdrop occurred during this period. We suggest that increased perfusion of cold tissue is one probable mechanism responsible for attenuation or reversal of the initial rewarming rate. These results have important implications for treatment of hypothermia victims, even when treatment commences long after removal from cold water.

Adult↗
PubMed1996

Eucapnic hypoxia lowers human cold thermoregulatory response thresholds and accelerates core cooling.

Hypoxia lowers the basic thermoregulatory responses of animals and humans. In cold-exposed animals, hypoxia increases core temperature (Tco) cooling rate and suppresses shivering thermogenesis. In humans, the experimental effects of hypoxia on thermoregulation are equivocal. Also, the effect of hypoxia has not been separated from that of hypocapnia consequent to hypoxic hyperventilation. To determine the isolated effects of hypoxia on warm and cold thermoregulatory responses and core cooling during mild cold stress, we examined the Tco thresholds for sweating, vasoconstriction, and shivering as well as the core cooling rates of eight subjects immersed in 28 degrees C water under eucapnic conditions. On 2 separate days, subjects exercised on an underwater cycle ergometer to elevate Tco above the sweating threshold. They then rested and cooled until they shivered vigorously. Subjects inspired humidified room air during the control trial. For the eucapnic hypoxia trial, they inspired 12% O2-balance N2 with CO2 added to maintain eucapnia. Eucapnic hypoxia lowered the Tco thresholds for vasoconstriction and shivering by 0.14 and 0.19 degrees C, respectively, and increased core cooling rate by 33% (1.83 vs. 1.38 degrees C/h). These results demonstrate that eucapnic hypoxia enhances the core cooling rate in humans during mild cold stress. This may be attributed in part to a delay in the onset of vasoconstriction and shivering as well as increased respiratory heat loss during hypoxic hyperventilation.

Adult↗
PubMed2004

Chronic treatment with antipsychotics enhances intraoperative core hypothermia.

UNLABELLED: Antipsychotics can induce hypothermia, but intraoperative temperature regulation in schizophrenic patients taking antipsychotics remains unclear. We investigated intraoperative temperature regulation and postoperative shivering in chronic schizophrenic patients receiving antipsychotics. We studied 30 schizophrenic patients and 30 control patients who underwent orthopedic surgery. Tympanic membrane temperatures (35.7 degrees C +/- 0.5 degrees C, 35.6 degrees C +/- 0.5 degrees C, 35.6 degrees C +/- 0.4 degrees C, 35.5 degrees C +/- 0.4 degrees C, 35.4 degrees C +/- 0.5 degrees C, and 35.4 degrees C +/- 0.3 degrees C) 15, 30, 45, 60, 75, and 90 min, respectively, after induction in schizophrenic patients were significantly (P < 0.001) lower than those (36.5 degrees C +/- 0.5 degrees C, 36.4 degrees C +/- 0.5 degrees C, 36.3 degrees C +/- 0.4 degrees C, 36.2 degrees C +/- 0.5 degrees C, 36.2 degrees C +/- 0.4 degrees C, and 36.1 degrees C +/- 0.4 degrees C) in control patients. Mean skin temperatures (31.1 degrees C +/- 0.4 degrees C [P = 0.008], 31.1 degrees C +/- 0.3 degrees C [P = 0.007], and 31.1 degrees C +/- 0.2 degrees C [P = 0.006]) 60, 75, and 90 min, respectively, after induction in schizophrenic patients were significantly lower than those (31.5 degrees C +/- 0.3 degrees C, 31.5 degrees C +/- 0.3 degrees C, and 31.5 degrees C +/- 0.3 degrees C) in control patients. Four of 30 schizophrenic patients and 7 of 30 control patients developed postanesthesia shivering. There were no significant differences within 1 h after tracheal extubation in tympanic membrane temperatures between patients who shivered and those who did not shiver. In conclusion, chronic schizophrenic patients were more hypothermic during anesthesia. The incidence of postanesthesia shivering was not significantly increased. IMPLICATIONS: Antipsychotics inhibit autonomic thermoregulation. This is caused by decreased heat production, increased heat loss, and impaired central action at the hypothalamus. Thus, schizophrenic patients receiving antipsychotics may have impaired intraoperative temperature regulation.

Adult↗
PubMed2005

Intraoperative clonidine administration to neurosurgical patients.

The goals of this two-part study were to determine the dose of clonidine to prevent postoperative shivering after mild hypothermia and to evaluate the effect of clonidine on recovery from anesthesia in patients undergoing surgery for intracranial lesions. We enrolled 48 patients undergoing elective supratentorial neurosurgical procedures into one of two studies. In study 1 (n=14) we determined the ED50 of clonidine to prevent postoperative shivering after mild hypothermia (35 degrees C) using Dixon's up-and-down method. Clonidine dose for the first study patient was 3 microg/kg. The dose was then adjusted in 1-microg/kg increments for the following patients. Shivering was assessed for 1 h postoperatively. Study 2 (n=34) was a prospective, randomized, double-blind, placebo controlled study to evaluate the effect of 3 microg/kg clonidine on recovery from anesthesia. At the beginning of dural closure, patients randomly received a 15-min infusion of either clonidine or normal saline. Recovery variables were studied for 2 h after the end of anesthesia. The ED50 of clonidine to prevent shivering was 1.1 +/- 1.5 microg/kg in neurosurgical patients whose target core temperature was 35 degrees C at the end of surgery. Compared with saline, 3 microg/kg of clonidine administered to neurosurgical patients 1 h before the end of anesthesia did not delay emergence from anesthesia nor did it have clinically significant sedative or hemodynamic effects. Our results imply that clonidine may be used in neurosurgical patients to prevent postoperative shivering after mild hypothermia.

Adrenergic alpha-Agonists↗
PubMed1991

Fever in young lambs: temperature, metabolic and cardiorespiratory responses to a small dose of bacterial pyrogen.

Experiments were done on ten lambs ranging in age from 15 to 25 days to define the temperature, metabolic and cardiorespiratory responses to intravenous administration of a small dose of bacterial pyrogen (SAE). Administration of SAE but not normal saline produced a short-lived fever of about 0.7 degrees C. The increase in body-core temperature was preceded by a surge in total body oxygen consumption and the onset of shivering which was influenced by behavioral state (ie, shivering was inhibited during active sleep). The increase in total body oxygen consumption was initially met by an increase in total body oxygen extraction and then by an increase in systemic oxygen delivery. Systemic arterial blood pressure did not change significantly during the febrile response; however, pulmonic arterial blood pressure increased significantly. Thus, our experiments provide new data on oxygen supply and demand during the development of fever and that shivering thermogenesis is inhibited in active sleep following the administration of bacterial pyrogen in young lambs. The influence of active sleep on the overall febrile response, and whether or not there is a shift from shivering thermogenesis to non-shivering thermogenesis remains to be determined.

Animals↗
PubMed1977

The role of catecholamine in the effects of trauma on thermoregulation, studied in rats treated with 6-hydroxydopamine.

Rats in which central noradrenergic terminals have been damaged by injecting 6-hydroxydopamine (6-OHDA) into the cerebral ventricular system have been used to study the possible role of noradrenaline (NA) in the effect of trauma on thermoregulation. In non-injured rats the 6-OHDA treatment did not affect the threshold environmental temperature (Ta) for the onset of shivering or the slope of the regression lines relating intensity of shivering to Ta. It raised the core temperature (Tc) threshold for the opening of the A-V anastomoses in the tail. Whereas in normal rats ischaemic limb injury depresses the threshold Ta for the onset of shivering without altering the slope of the regression line, in 6-OHDA-treated rats no depression of the threshold Ta was observed during limb ischaemia but the slope of the regression line was lower. The Tc threshold for the opening of the A-V anastomoses in the tail was the same as in the 6-OHDA-treated controls. The injection of NA into a lateral cerebral ventricle of a normal rat in a 5 degrees environment inhibited shivering and lowered Tc. The 6-OHDA treatment did not induce hypersensitivity to the injection of NA by this route. It is thought that NA is concerned in the central inhibition of shivering during limb ischaemia.

Animals↗
PubMed2006

Total intravenous anaesthesia with propofol and remifentanil for elective neurosurgical procedures: an audit of early postoperative complications.

BACKGROUND AND OBJECTIVES: This was a prospective audit to assess the incidence and characteristics of early postoperative complications in the recovery room in extubated patients after elective neurosurgical procedures using propofol and remifentanil-based total intravenous anaesthesia. METHODS: Vital signs (temperature, conscious level, respiratory rate, oxygen saturation, pulse and blood pressure) and postoperative complications (shivering, nausea, vomiting and cardiorespiratory) were analysed in 145 adult patients over a 1-yr period. RESULTS: The overall shivering, postoperative nausea and vomiting and postoperative hypertension (systolic blood pressure more than 25% of the preoperative value) incidences were 30.3%, 16.6% and 35.2%, respectively. Fifty-one percent of the patients had at least one of the above complications. The complication rates were found to be widely different among various types of neurosurgery. The surgical procedures were divided into five groups: supratentorial craniotomy, posterior fossa craniotomy, intracranial vascular procedures, transphenoidal hypophysectomy and extracranial procedures. Median extubation time was similar in all groups and patients were fully conscious with no hypoxia in the recovery room. The intracranial vascular group had the highest shivering and postoperative nausea and vomiting rates (58.8% and 29.4%, respectively). In the supratentorial craniotomy group, 46% of the patients had hypertension. The overall complication rate (presence of any complications) was highest in the supratentfial craniotomy (55.4%), posterior fossa craniotomy (75%) and intracranial vascular (76.5%) groups. Shivering and overall complication rate was significantly related to the anaesthetic time (P<or=0.001 and 0.02, respectively). CONCLUSIONS: Despite the potential advantages of total intravenous anaesthesia in titratability, rapid return of consciousness and reduced respiratory complications, making it suitable for planned extubation at the end of neurosurgery, the postoperative complications of shivering, postoperative nausea and vomiting and hypertension were still high.

Anesthesia, Intravenous↗
PubMed1994

The effects of halothane, isoflurane, and enflurane on thermoregulatory responses in the neuraxis of cats.

BACKGROUND: Normal thermoregulatory function is believed to be modulated by thermosensitive neurons in the preoptic region of the anterior hypothalamus and other sites within the central nervous system including the spinal cord. Previous evidence has demonstrated modulation of segmental spinal cord thermoregulatory mechanisms from more rostral central nervous system sites. The ability of the volatile anesthetics to disrupt normal thermoregulatory function and produce shivering-like activity during emergence is well documented. The purpose of the current investigation was to examine the action purpose of the current investigation was to examine the action of the volatile anesthetics halothane, isoflurane, and enflurane on thermoregulatory responses produced at the preoptic region and spinal cord. METHODS: Cats were chronically instrumented with bilateral cannulas allowing selective heating and cooling of the preoptic region. Electrodes were implanted in hindlimb and forelimb muscles for electromyographic (EMG) analysis. Animals underwent selective heating and cooling of the preoptic region in the awake state, during volatile agent anesthesia and during emergence. In a separate series of animals, pontine-transected cats with epidural thermodes and a thermocouple underwent alternate heating and cooling of the spinal cord. Heating and alternate heating and cooling of the spinal cord. Heating and cooling was performed in the nonanesthetized state, at graded concentrations of halothane, and during emergence. In all animals deep core peritoneal temperature, epidural spinal cord temperature, forelimb and hindlimb EMG activity were continuously recorded and digitally processed. EMG responses in both experiments were quantitated and analyzed for power spectral density. RESULTS: In the chronically prepared animals, heating and cooling of the preoptic region in the conscious state resulted in appropriate thermoregulatory responses, including shivering-like activity and increased EMG power with preoptic region cooling. Halothane, isoflurane, and enflurane each abolished these thermoregulatory responses. During emergence from anesthesia, however, the typical spontaneous increases in EMG power observed at normothermia were significantly attenuated by heating of the preoptic region and augmented by cooling of the preoptic region. In the acutely prepared animals, cooling of the spinal cord produced graded increases in EMG activity. Increased concentrations of halothane dose-dependently diminished this response to cooling of the spinal cord. During emergence, cooling of the spinal cord resulted in a shivering response similar to those observed during control conditions. CONCLUSIONS: The ability of preoptic region heating and cooling to modulate postanesthetic shivering implies that while thermoregulatory pathways remain intact, volatile anesthetics produce an imprecision in the control of thermoregulatory responses at the level of the anterior hypothalamus. Attenuation of shivering-like responses generated at spinal cord levels in pontine-transected cats implies a significant blunting action of thermoregulatory response mechanisms at the level of the spinal cord or lower brain stem.

Anesthetics↗
PubMed2002

Autonomic nervous system responses during sedative infusions of dexmedetomidine.

BACKGROUND: The purpose of this study was to determine the effects of dexmedetomidine on systemic and cardiac autonomic reflex responses during rest and during thermal stress. METHODS: Volunteers received either placebo or low- or high-dose dexmedetomidine (target plasma concentrations 0.3 or 0.6 ng/ml, respectively) infusions in a prospectively randomized, double-blinded crossover study design. After 1 h, baroreflex sensitivity was assessed, and then core body temperature was raised to the sweating threshold and then lowered to the shivering threshold. Plasma catecholamines and blood pressure were measured, and cardiac autonomic responses were assessed by analysis of heart rate variability. RESULTS: Compared with placebo, plasma norepinephrine concentrations, blood pressure, heart rate, and some heart rate variability measures were lower after 1-h infusion of dexmedetomidine, but baroreflex responses did not differ significantly. Dexmedetomidine blunted the systemic and cardiac sympathetic effects of sweating observed during placebo infusion but had no effect on parasympathetic measures. Increases in blood pressure, and systemic catecholamines due to shivering were observed during placebo and dexmedetomidine, but these responses were less with dexmedetomidine. During shivering, dexmedetomidine infusion was associated with higher low-frequency and high-frequency heart rate variability power but lower heart rate compared with the sweating threshold and with the control period, suggesting nonreciprocal cardiac autonomic responses. CONCLUSIONS: Infusion of dexmedetomidine results in compensated reductions in systemic sympathetic tone without changes in baroreflex sensitivity. Dexmedetomidine blunts heart rate and the systemic sympathetic activation due to sweating, but it is less effective in blunting cardiac sympathetic responses to shivering. During dexmedetomidine infusion, cardiac sympathetic and parasympathetic tone may have nonreciprocal changes during shivering.

Adult↗
PubMed1988

Acylgalactosylceramides in developing dysmyelinating mutant mice.

Acylgalactosylceramides (AGC) from forebrains of normal and dysmyelinating (quaking and shiverer) mice were purified by Florisil column chromatography and preparative TLC. These procedures resolved the AGC on the basis of their Rf values into two main fractions which comigrate with their homologs from rat forebrains. In control animals, AGC were detectable in mouse forebrains from the eighth postnatal day and reached maximal values within 20 days. The same developmental pattern was obtained in dysmyelinating shiverer mice but the AGC content was reduced to approximately 30% of control values. In quaking mutants, the AGC were hardly detected. They were also present in sciatic nerve of normal mice and to a lesser extent in trembler mice. Gas chromatography-mass spectrometry analysis of both ester- and amide-linked fatty acids isolated from AGC of normal and shiverer mice shows that the shiverer mutant AGC display a chemical structure similar to that of normal AGC. AGC constituents of control myelin are reduced by approximately 70% in shiverer myelin, indicating that these molecules can be considered as early markers of oligodendrocyte differentiation. The early arrest of myelinogenesis in the quaking animals and the near absence of AGC are in good agreement with this proposal. Moreover, the reduced amount of AGC in the trembler PNS indicates that AGC could also be early markers for differentiation of the Schwann cell.

Animals↗
PubMed1996

Effect of hypoglycemia on thermoregulatory responses.

The effects of hypoglycemia on sweating, skin blood perfusion, and shivering responses were investigated in 10 healthy male volunteers. They exercised on an underwater cycle ergometer while immersed to the neck in 28 degrees C water for 20 min at 50% of their maximal work rate. The exercise-induced elevation in esophageal temperature (T(es)) initiated the sweating response (Esw) and increased skin blood perfusion (SkBP) as measured at the forehead. In the 99-min postexercise immersion period, the values of T es relative to resting level (delta T(es)) at which Esw abated, SkBP reached preexercise values, and shivering commenced were defined as the delta T(es) thresholds for cessation of sweating, passive vasodilation, and onset of shivering, respectively. Two trials were conducted 1 wk apart. The subject was hypoglycemic in one trial and euglycemic in the other (plasma glucose was maintained at 2.8 and 5 mM, respectively) with the use of the hyperinsulinemic (insulin infusion rate = 60 mU.m-2.min-1) glucose-clamp technique. Oxygen uptake, Esw, T(es), mean skin temperature, heat flux from the skin, and SkBP were recorded at minute intervals. Although heat flux and SkBP attained significantly higher end-exercise levels during euglycemia, the responses were similar during the postexercise cooling period. Hypoglycemia did not affect the Esw response during the exercise and cooling periods. Whereas the exercise delta T(es) response was unaffected by hypoglycemia, the decrease in T(es) was greater (P < or = 0.005) during the hypoglycemic than during the euglycemic condition. Hypoglycemia did not alter the delta T(es) threshold for cessation of sweating and passive vasodilation but reduced (P < or = 0.001) the delta T(es) threshold for onset of shivering (from -0.09 +/- 0.07 degrees C in the euglycemic condition to -0.65 +/- 0.12 degrees C in the hypoglycemic condition). The present results indicate that hypoglycemia (2.8 mM) does not affect the delta T(es) threshold for cessation of thermoregulatory sweating or the threshold for passive vasodilation during recovery from exercise-induced moderate heat stress but that it decreases the core temperature threshold for shivering during cold exposure.

Adult↗