Homeothermy: Does it impede the response to cellular injury?

Dielectric properties of hypothermic rat artery

The temperature and frequency dependencies of the dielectric parameters for the rat artery are used to analyse effects of hypothermia on this tissue. Measurements were performed over the frequency range 500 Hz to 100 kHz and at temperatures from 19 to 60°C. The artery samples contained about 12% water by mass at room temperature at a relative humidity of 70%. The frequency dependencies of the loss tangent for the control, mild hypothermic and moderate hypothermic artery exhibit two peaks at 2 kHz and 35 kHz in the α-dispersion region. The results were discussed in terms of the distribution of relaxation frequencies and the activation energy for the conduction and polarization mechanisms particularly in the elastin-water and collagen-water systems. The knowledge about dielectric behavior of the hypothermic rat artery in vitro is important due to clinical application of local and systemic hypothermia.

Pharmaceutical metabolic down-regulation by protein synthesis inhibition in a conscious rat model

Pharmaceutically induced metabolic down-regulation may be a useful therapeutic adjunct when tissue oxygen supply is restricted. We hypothesized that protein synthesis inhibition in a non-hibernating species should lower oxygen demand, resulting in aerobic metabolic rate depression at the whole animal level. We compared metabolic responses and measures of systemic oxygenation of conscious catheterized rats given either protein synthesis inhibition (PSI) agents or carrier controls (normal saline and DMSO). Core temperature was measured by implanted transmitters, and VO2 was determined in an open flow-through metabolic chamber at 25 degrees C. Mean arterial pressure MAP and heart rate HR were determined from arterial pressure transducer tracings; arterial blood gases and lactate were sampled every 15 min. PSI rats exhibited an immediate transient decline in VO2, followed by a secondary decline to new resting levels; VO2 for the first hour was significantly lower than that for rats receiving DMSO vehicle. Unlike controls, PSI rats showed an overall 3.5 degrees C decline in core temperature, coupled with increased arterial lactate. There were no differences in MAP and HR of PSI rats compared to controls. Although hypothermic response to toxic agents typical of rodents cannot be ruled out completely, the mild hypothermia and reduced VO2 exhibited by PSI rats may be partially attributed to the action of protein synthesis agents.

Moderate hypoxia does not affect the zone of thermal comfort in humans

The zone of thermal comfort was determined during normoxia and hypoxia in 15 healthy normothermic young subjects. Subjects dressed only in shorts/shorts and bikini top donned a water-perfused suit and assumed a supine position on a bench. The ambient temperature was maintained at a mean (SD) of 25.7 (0.3) degrees C. The thermal comfort zone was determined by increasing the temperature of the water perfusing the suit from cool to warm. During the heating process, subjects were instructed to report when their perception of the thermal stimulus provided by the suit changed from unpleasant to pleasant, and again from pleasant to unpleasant. The boundaries of the thermal comfort zone were assumed to be the temperatures of the water perfusing the suit at the time the subjects reported a change in the affective component of their thermal perception. In normoxia, subjects inspired room air and in hypoxia a gas mixture containing 10% O(2) in N(2). Tympanic temperature was similar in the normoxia and hypoxia conditions (P>0.05). The average (SD) lower and upper limits of the thermal comfort zone were 30.5 (1.5) and 34.7 (3.3) degrees C, respectively, during normoxia, and 30.5 (1.7) and 35.1 (3.4) degrees C, respectively, during hypoxia. No significant differences were observed between the normoxia and hypoxia conditions (P>0.05). Also, no gender-related differences were observed in the characteristics of the thermal comfort zone. The results of the present study indicate that acute hypoxic exposure simulated in the present study does not affect the zone of thermal comfort in humans.

Cooling the newborn after asphyxia - physiological and experimental background and its clinical use

Many years of experimental work on hypoxic-ischaemic injury have supported the hypothesis that cooling the body and brain after the primary injury offers permanent neuroprotection. Clinically, the question of how late cooling can start after the insult and still have a protective effect is important and not fully investigated. Pilot studies in human adults initiated cooling after 10-18 h (trauma, stroke), however animal data suggest cooling is not effective if started later than 6 h. There might be a threshold for 'cooling dose' - by depth or duration - to achieve permanent protection. Hypothermia must be administered with understanding of the extensive physiological effects. Different enzymes have different sensitivity to changes in temperature, hence some effects may be beneficial and some deleterious. Hypothermia and cardiovascular responses and coagulation needs careful monitoring.

Behavioral thermoregulatory response to maitotoxin in mice

Many types of marine algal toxins induce marked hypothermic responses in mice. However, it is not known if the thermoregulatory response to these toxins results from dysfunction in the control of core temperature (Tc) or is a coordinated response to lower Tc as occurs with a variety of xenobiotic insults. Female CD-1 mice were administered purified maitotoxin (338 ng/kg; IP) and placed in a temperature gradient for 5 h that permitted the selection of ambient temperatures (Ta) ranging between 15 and 37 degrees C. Tc was monitored simultaneously by radiotelemetric probes that were surgically implanted into the abdominal cavity at least one week before maitotoxin injection. Maitotoxin led to a rapid reduction in Tc from 37 to 34 degrees C within 30 min after injection. There was a simultaneous 4 degrees C reduction in Ta selected by mice within 15 min after injection. Selected Ta recovered rapidly, increased above baseline for approximately one hour, then remained near baseline levels for the remainder of the test period in the gradient. Tc remained approximately 1 to 2 degrees C below control levels throughout the test period. In the temperature gradient, mice can select Ta's warm enough to offset the hypothermic effects of maitotoxin. That cooler Ta's are selected initially after maitotoxin injection suggest that the central neural control of body temperature is affected by the toxin. We postulate that the hypothermic response may represent an adaptive response to enhance survival following exposure to polyether toxins.

Rapid brain cooling in diving ducks

Hypothermia may limit asphyxic damages to the brain, and many small homeotherms have been shown to use anapyrexic strategies when exposed to asphyxic conditions. Larger homeotherms do not seem to use the same strategy, but could save oxygen and prevent hypoxic brain damage by employing selective brain cooling (SBC) in connection with asphyxia. To test the hypothesis that selective brain cooling may take place in connection with asphyxia, we have recorded brain [hypothalamic (THyp)] and body [colonic (TC)] temperatures and heart rates in four Pekin ducks during 5-min simulated (head submersion) diving in cold water (10 degrees C). Diving resulted in a drop in THyp (3.1 +/- 1.4 degrees C) that continued into the recovery period (P < 0.001). Restricting heat loss from the buccal cavity and eyes during diving compromised brain cooling in an additive manner. TC was not influenced by diving. Control cooling of the head with crushed ice during a 5-min period of undisturbed breathing had no effect on THyp. Warm water (35 degrees C) markedly reduced brain cooling, and dive capacity was reduced by approximately 14% (P < 0.05) compared with diving in water at 10 degrees C. The data suggest that SBC is used in ducks during diving, and we propose that this mechanism may enable the bird to save oxygen for prolonged aerobic submergence and to protect the brain from asphyxic damages.