Body cooling attenuates the decrease in maximal oxygen uptake associated with cardiovascular drift during heat stress

Previous research suggests cardiovascular drift (CV drift) is associated with decreased maximal oxygen uptake [Formula: see text] during heat stress, but more research manipulating CV drift with subsequent measurement of [Formula: see text] is needed to assess whether this relationship is causal. To assess causation, [Formula: see text] was measured during the same time interval that CV drift occurred (between 15 and 45 min of submaximal exercise under different conditions of body cooling intended to manipulate CV drift). Ten men completed a control graded exercise test (GXT) in 22 degrees C to measure [Formula: see text] then on separate occasions they cycled in 35 degrees C at 60% [Formula: see text] for 15 min (15 max), 45 min with no cooling (NC), and 45 min with fan airflow (FAN) beginning at approximately 18 min into exercise, and each bout was immediately followed by a GXT to measure [Formula: see text] In NC, [Formula: see text] decreased 18%, heart rate (HR) increased 16%, and stroke volume (SV) fell 12% (P < 0.05) from min 15 to min 45. In FAN, [Formula: see text] fell less (5.7%, P < 0.05) , HR rose less (4%, P < 0.05) and SV decreased less (3%, P < 0.05) from 15 to 45 min. The fall in [Formula: see text] associated with CV drift during exercise in a hot environment is attenuated with body cooling via fan airflow. The findings support the notion that a causal link exists between CV drift that occurs during prolonged exercise in a hot environment and a decrease in [Formula: see text].     

Methamphetamine-induced toxicity: An updated review on issues related to hyperthermia

Reports of methamphetamine-related emergency room visits suggest that elevated body temperature is a universal presenting symptom, with lethal overdoses generally associated with extreme hyperthermia. This review summarizes the available information on methamphetamine toxicity as it pertains to elevations in body temperature. First, a brief overview of thermoregulatory mechanisms is presented. Next, central and peripheral targets that have been considered for potential involvement in methamphetamine hyperthermia are discussed. Finally, future areas of investigation are proposed, as further studies are needed to provide greater insight into the mechanisms that mediate the alterations in body temperature elicited by methamphetamine.     

Alcohol and calories: A matter of balance

Many epidemiological studies show that alcohol-derived calories added to food intake of men and women in amounts of 0–25% of total energy do not appreciably alter the average daily intake of other macronutrients (carbohydrate, fat, and protein). With such lack of evidence for caloric compensation, alcohol and its calories seem to make little contribution to metabolic energy, body weight, or body composition (as indicated by the body mass index, BMI). In fact, a major study by Colditz et al. (Am. J. Clin. Nutr. 54:49–55; 1991) reported a clear inverse relationship between alcohol intake and BMI for women! Research on alcohol metabolism has left unresolved some apparent contradictions regarding the effect of alcohol on caloric control, appetite and satiation, and body mass and composition. To resolve those apparent contradictions, the National Institute on Alcohol Abuse and Alcoholism cosponsored with the National Institute of Diabetes and Digestive and Kidney Diseases and the Beltsville Human Nutrition Research Center of the U.S. Department of Agriculture and all-day workshop titled “Alcohol and Calories: A Matter of Balance” on January 27, 1993. The workshop included sessions on calorimetry and body mass maintenance, alcohol metabolism, thermoregulation, and an overview of energy balance. This report provides summaries of the four discussion sessions at the workshop.     

Thermally-induced activities of the mesencephalic reticulospinal and rubrospinal neurons in the rat

Unit activities of 226 midbrain reticulospinal (mRfS) and non-mRfS neurons and 238 rubrospinal (RbS) and non-RbS neurons were investigated during changes in temperatures of midbrain (Tmb), preoptic and anterior hypothalamus (Thyp) and skin (Ts) in the urethane-anesthetized rat. Responsiveness to Tmb, Thyp and Ts were found in 43.5%, 41.6% and 51.5% of neurons of midbrain reticular formation (mRf), and in 35.2%, 32.7% and 17.6% of neurons of red nucleus (Rb). Higher incidence of responsiveness to remote temperatures was found among Tmb responsive neurons than Tmb unresponsive neurons in both mRf and Rb. The mRf contains significantly greater numbers of neurons having such multiple thermal responsiveness and also of neurons which were activated by falls in temperatures (cold-responsive neurons) than the Rb. These characteristics were more conspicuously seen among mRfS neurons, showing a high degree of convergence of cold signals from different sites of body. On the other hand, RbS neurons did not differ from non-RbS neurons regarding thermal characteristics and showed no particular combinations of responsiveness to temperatures of different sites. Microinjection of procaine and glutamate into the mRf just dorsolateral to the Rb, but not into the Rb, decreased and increased cold-induced increase in EMG activity and shivering without changes in cardiovascular and respiratory parameters and pilomotor activity. The results suggest that mRfS neurons are involved in the control of thermoregulatory muscle tone and shivering.     

GABAA receptor modulation of temperature sensitive neurons in the diagonal band of Broca in vitro

Several regions of the brain, including the horizontal limb of the diagonal band of Broca (HDB), contain neurons that are responsive to changes in local temperature. These neurons are hypothesized to participate in thermoregulation and sleep–wake control. The HDB contains a large number of γ-aminobutyric acid (GABA) terminals, and it has many neurons that utilize GABA as a neurotransmitter. Therefore, in this study we characterized the in vitro effects of the GABA_A receptor agonist muscimol (0.5, 0.25, 0.1 and 0.0625 μM doses) and the GABA_A receptor antagonist bicuculline (3.0 and 1.0 μM doses) on the firing rate and thermosensitivity of HDB neurons. Of the 51 neurons recorded in a submerged slice chamber, 53% were warm sensitive, 45% were temperature insensitive and 2% were cold sensitive. All neurons exposed to bath applied muscimol exhibited reductions in both firing rate and thermosensitivity. Muscimol induced reductions were maintained for at least 20 min after washout. Neurons exposed to bicuculline had no change in firing rate or thermosensitivity. However, after bicuculline washout there were reductions in both firing rate and thermosensitivity. These findings support the hypothesis that GABA_A receptor induced inhibition of HDB thermosensitive neurons can modulate both thermoregulation and sleep–wake control.     

Postmenopausal hot flushes: A disorder of thermoregulation

The changes in cutaneous and body temperature and cutaneous conductance during hot flushes in eight postmenopausal women were studied. The vasomotor changes occurred approx. 45 sec after the patients experienced the initial subjective symptoms of the attacks. The rise in skin conductance appeared to be a more reliable index of the flushing episode than did the change in skin temperature.On the basis of the changes recorded it is suggested that the hot flush syndrome may represent a specific thermoregulatory disorder rather than being due to a non-specific central autonomic discharge. The episodes may be triggered by a neuroendocrine imbalance following the disruption of ovarian function and fall in estrogen production.In assessing the frequency and severity of hot flushes, and the effects of treatment, objective measurements of skin and core temperature and skin conductance should replace subjective criteria.     

Effect of GABA-transminase inhibition on rectal temperature in mice.

gamma-Acetylenic GABA (amino-4-hex-5-ynoic acid) and gamma-vinyl GABA (amino-4-hex-5-enoic acid), two catalytic irreversible inhibitors of GABA-transaminase, produce marked sustained elevations in brain GABA concentrations. Associated with these biochemical changes is a decrease in the rectal temperature of mice. This hypothermia can be reversed by increasing ambient temperature. The results suggest GABA plays an important role in mammalian central thermoregulation.     

Simulation of passive thermal behavior of a cooling biological system: Entry into hibernation

Summary A mathematical model is developed which describes the dynamic generation and transfer of heat in the marmot at normothermic to hibernating body temperatures. Since the animal approximates a ball as it enters hibernation, the form of the model was a sphere divided into three concentric layers-central core, muscle and skin. Each layer was assumed homogeneous in composition but distributed with respect to temperature. Both conductive and convective heat exchange were cosidered—conductive heat exchange occuring radially through the layers and convectivevia the blood flow. Separate relationships between temperature and metabolism were used for heat generation in the different layers. The non-linear partial differential equations describing heat exchange between layers were solved numerically on the IBM 360/50 computer. The temperatures computed from the model were compared with experimental temperatures of marmots entering hibernation. Using nominal values of the estimated parameters, the agreement between model and experimental temperatures was fair. To help improve the model, the principal parameters were varied to determine sensitivities. Changes in metabolic rates and blood flow had only small effects, but the model was quite sensitive to changes in the outer surface heat transfer coefficient and shape factor. A smalla posteriori adjustment was made to the heat transfer coefficient which gave an excellent fit between computed and experimental dynamic temperature behavior.Supported by NASA grant No. NGR 26-004-0025.     

The role of protein synthesis in the hypothalamic mechanism mediating pyrogen fever

The effects of inhibition of protein synthesis by anisomycin on the pathogenesis of fever and normal thermoregulatory processes were investigated in the conscious and unrestrained cat. Subcutaneous administraton of 5.0–25.0 mg/kg of anisomycin prevented the fever normally evoked by an intravenous infusion of either 1.0 ml (10⁸ organisms) of a 1:10 dilution of S. typhosa or 1.0–5.0 ml (3.5×10⁵−2.1×10⁷ cells/ml) of endogenous pyrogen. In addition, systemic pre-treatment with anisomycin delayed and/or blocked the fever typically elicited by a direct micro-injection into the anterior hypothalamic, preoptic area (AH/POA) at AP 12.5–16.0 of 1.0 μl of the endotoxin. Anisomycin did not alter the hyperthermic response to an anterior hypothalamic injection of either 1.0–7.0 μg/1.0 μl of serotonin (5-HT) or 100.0 ng/1.0 μl of prostaglandin (PGE). Inhibition of protein synthesis, furthermore, did not prevent the fall in body temperature usually produced by an intrahypothalamic micro-injection of 2.33–14.0 μg/1.0 μl of either norepinephrine (NE) or dopamine (DA). The thermoregulatory capacity of the cat was unaffected by the administration of comparable doses of anisomycin, i.e., the animal was able to maintain normal body temperature (±0.5°C) when exposed to an ambient temperature of either 10°C or 34°C. These results strongly suggest that the synthesis of new protein within the region of the AH/POA is a functional requisite for the development of a pyrogen-induced fever.     

Prostaglandin E1 fever in the crayfish Cambarus bartoni

Groups of 10 crayfish were injected with prostaglandin E₁, at 1 of 3 pharmacological doses (0.5, 0.1 or 0.05 mg), into the haemocoel, and individually allowed to thermoregulate in electronic shuttleboxes for 24 hr. The mean preferred temperature of each group was then compared with their mean preferred temperature for 24 hr prior to injection, and with the mean preferred temperature of 10 crayfish injected with pyrogen-free saline. Dosage-dependent increases in preferred temperature were observed in the crayfish injected with PGE₁, ranging from 1°C at the lowest dosage to 3.4°C at the highest dosage, above the normal thermal preferendum for this species of 22.1°C.