Thermosensitivity of the extrahypothalamic brain stem in conscious goats

In 5 conscious goats, 84 experiments with 881 perfusion periods were performed to explore the brain stem between the rostral medulla and preoptic region for thermo-sensitive structures involved in temperature regulation. The chronically implanted thermodes consisted of 24 or 27 single probes, which were arranged in 8 or 9 rows. The rows of probes were individually perfused with water of 25-46 degrees C to produce discrete temperature stimuli along the brain stem. When the animals were exposed to an air temperature of +4 degrees C, local cooling at various levels of the lower brain stem augmented shivering and increased heat production, which was not regularly followed by a rise in rectal temperature. Ongoing shivering was reduced by local warming of the same sites. In comparison to the effects of hypothalamic thermal stimuli, the magnitude of the lower brain stem responses was reduced. At an air temperature of +30 degrees C local warming of discrete areas of the lower brain stem increased panting and caused a significant rise in respiratory evaporative heat loss. However, panting and shivering were not affected by the same site, and the effective sites of the various animals were not found at corresponding anatomical positions. Thus, thermosensitive sites which are not associated with defined anatomical structures, appear to be dispersed in the lower brain stem of the goat and to interfere with the temperature regulating system.

Air humidity and carotid rete function in thermoregulation of the goat

1. The effects of air humidity on respiratory rate have been studied in conscious goats exposed to an air temperature of + 33 degrees C. Before the experiments the animals had been chronically implanted with hypothalamic thermodes and intravascular heat exchangers to manipulate hypothalamic and general body core temperatures.2. Raising air humidity from 37 to 96% at constant air temperature resulted in a rise of respiratory rate, an immediate increase in hypothalamic temperature and a delayed smaller increase in general body core temperature.3. The rise of respiratory rate was smaller when general body core temperature was clamped at its control level and was absent when hypothalamic and general body core temperatures were clamped at their control levels during the humid air phase.4. It is concluded that the effect of high air humidity on respiratory rate in goats is predominantly the result of a rise in hypothalamic temperature acting on local thermosensitive structures. The carotid rete heat exchanger is thought to provide the thermal link between the evaporating surfaces of the upper respiratory tract and the hypothalamus.5. This function of the carotid rete heat exchanger is restricted to heat stressed animals. In animals subject to central cooling no local effects on hypothalamic temperature could be observed when the temperature of the inspired air was altered from + 33 to - 17 degrees C.

Control of respiratory evaporative heat loss in exercising goats

Investigations were carried out to determine whether a nonthermal input is involved in the control of respiratory evaporative heat loss (REHL) in exercising goats. Two goats were implanted with hypothalamic perfusion thermodes and three goats were implanted with intravascular heat exchangers to clamp hypothalamic temperature and total body core temperature, respectively. At 30 degrees C air temperature REHL was measured while the animals were resting or walking on a treadmill (3 km.h-1, 5 degrees gradient). When the hypothalamic temperature was clamped between 33.0 and 43.0 degrees C the slopes of the responses relating increased REHL to hypothalamic temperature were similar during rest and exercise. However, the threshold hypothalamic temperatures for the increased REHL responses were lower during exercise than at rest, presumably due to higher extrahypothalamic temperatures. When the body core temperature was clamped between 37.0 and 40.4 degrees C the slopes of the responses relating increased REHL to total body core temperature during exercise showed only minor differences compared to those at rest, none of them conclusively indicating nonthermal influences.

Hypothalamic thermosensitivity in conscious Pekin ducks

Conscious Pekin ducks with chronically implanted hypothalamic thermodes were submitted to thermoneutral (Ta 25 degrees C), cold (Ta 5 degrees C), and warm (Ta 33 degrees C) ambient temperatures. Hypothalamic temperature (Thy) was varied in nine steps between 27.9 and 43.5 degrees C in repeated experiments. Cooling of the hypothalamus induced a fall of core temperature (Tc) that was linearly related to Thy and amounted to 1.1--1.3 degrees C at highest cooling intensity. The decrease of Tc was caused by inhibition of metabolic heat production and/or vasodilatation in the skin at cold and thermoneutral Ta and by activation of panting at warm Ta. After the end of cooling a temporary overshoot of heat production occurred, the degree of which depended on the degree of cooling and on Ta, and led to a rapid normalization of Tc. Warming of the hypothalamus induced a slight fall of Tc due to a reduction of metabolic heat production at cold and thermoneutral Ta and to an activation of panting at warm Ta. It is concluded that no specific cold reception and a weak specific warm reception exist in the duck's hypothalamus. A "nonsensory" temperature susceptibility of hypothalamic control functions is responsible for those reactions of thermoregulatory effector activities which do not fall into the categories of adequate thermoregulatory responses to a central thermal stimulus.

Central thermosensitivity in conscious goats: hypothalamus and spinal cord versus residual inner body

Experiments were performed on conscious goats to confirm the suggestion that in this species the inner body contains more thermosensitive structures than those residing in the hypothalamus and spinal cord. For this purpose goats were chronically implanted with local thermodes and intravascular heat exchangers to allow independent temperature control of the hypothalamus, spinal cord and residual inner body. With the hypothalamus and spinal cord clamped simultaneously at different levels between 32 degrees C and 40 degrees C, residual internal temperature was lowered by subtracting heat via the intravascular heat exchanger. The residual internal temperature at which shivering and increased heat production occured due to heat extraction, was directly related to the value of the combined hypothalamic and spinal cord clamp temperature. The higher hypothalamic and spinal cord clamp temperatures were, the lower residual internal temperature fell before shivering occurred and heat production rose. Plots relating residual internal temperature to hypothalamic and spinal cord temperature at different levels of heat production showed the signal input generated within the residual inner body to be of nearly the same order of magnitude as that from the hypothalamus and spinal cord.

Effects of total body core cooling on heat production of conscious goats

Experiments were performed to investigate the effect of general body core cooling on heat production at various air temperatures between +1 degree C and +56 degrees C in conscious goats. An intravascular heat exchanger (IVHE) was used to alter body core temperature independently of air temperature. Heat loss via the IVHE caused a fall in body core temperature, the extent of which depended on the rate of extraction and air temperature. Irrespective of air temperature the decrease in body core temperature resulted in shivering and an increase in heat production, which eventually balanced the heat loss. During steady state conditions the extra heat production was approximately equal to that lost via the IVHE. The threshold body core temperature at which heat production increased in response to central cooling did not significantly alter with air temperature. However, the slopes of the curves describing this response were smaller at higher than at lower air temperatures, which indicated that central thermosensitivity decreased with increasing air temperature. Irrespective of air temperature the threshold temperatures for shivering were higher and the slopes of the curves were steeper than those previously found with combined cooling of the hypothalamus and spinal cord in the same species which indicated the existence of central thermosensors outside the above two mentioned areas.

Anterior and posterior hypothalamus: effects of independent temperature displacements on heat production in conscious goats

Three goats were chronically implanted with thermodes to alter the temperatures of the anterior and posterior hypothalamus independently of each other. At an air temperature of +14 degrees C the anterior hypothalamus was cooled with different intensities, while the posterior hypothalamus was simultaneously either warmed (39 degrees C) or cooled (29 degrees C). In both conditions cooling anterior hypothalamus increased heat production. However, the increase was smaller, when the posterior hypothalamus was cooled. The inhibiting effect was most pronounced during the first parts of the periods and diminished with time. Nevertheless, in a separate series of experiments, the effects of posterior hypothalamic cooling were found to persist over periods of 3 h. At an air temperature of +3 degrees C the posterior hypothalamus temperature was altered between 28 and 42 degrees C, while anterior hypothalamus temperature was kept close to its control level. Shivering and heat production decreased with cooling and increased with warming of the posterior hypothalamus. The results suggest that those neurons which reside in the posterior hypothalamus and mediate shivering, are sensitive to temperature. Thermosensitivity of these allegedly integrative neurons affects shivering and heat production in a way inverse to the thermosensitivity of the temperature sensing neurons in the anterior hypothalamus.

Intravascular heat exchanger for conscious goats

Polyethylene tubings were chronically implanted into the vascular system of goats and served as heat exchangers to remove heat directly from the body core at a rate equalling several times resting heat production.

Antidiuretic responses to thermal stimulation of hypothalamus and spinal cord in the conscious goat

At various ambient temperatures the effects of hypothalamus temperature and spinal cord temperature on urine formation and heat production were studied in conscious goats with chronically implanted thermodes. At neutral air temperature cooling hypothalamus or spinal cord induced a fall in urine volume and a rise in urine osmolality. This antidiuretic response was concurrent with a rise in heat production. Simultaneous occurrence of antidiuresis and increased heat production was also found after cessation of hypothalamic warming. At hot ambient temperature cooling hypothalamus affected neither urine formation nor heat production. Since hypothalamic cooling and spinal cord cooling produce identical effects on kidney function it is concluded that this response is linked to the complex cold defence activity as a whole. The predominent change of free water clearance is tentatively interpreted as caused by an increased ADH concentration in the blood during the cold defence activity.

Interaction of air temperature and core temperatures in thermoregulation of the goat

1. The interaction between air temperature, hypothalamus temperature, and spinal cord temperature in driving heat production and respiratory evaporative heat loss has been studied in conscious goats with chronically implanted thermodes. 2. Thermoregulatory heat production could be described as being approximately proportional to the sum of two linear drives determined by hypothalamus temperature and spinal cord temperature. This was found also for respiratory evaporative heat loss except that it was not influenced by spinal cord cooling. 3. Thermoregulatory heat production could be further described as being approximately proportional to a product of linear drives determined by hypothalamus and spinal cord temperature on one hand and air temperature on the other. Respiratory evaporative heat loss was approximately proportional to the sum of drives determined by spinal cord, hypothalamus and air temperatures. 4. Sensitivity to central cooling was found to undergo long-lasting but temporary changes which interfered with the immediate effects of air temperature on thermoregulation. 5. Central threshold temperatures for heat production and respiratory evaporative heat loss were found to be differently affected by air temperature. This indicates that integrating mechanisms for heat production and respiratory evaporative heat loss are to some extent functionally independent.

Two-dimensional determination of thermosensitive sites within the goat's hypothalamus

In two conscious goats with chronically implanted multithermodes the distribution of thermosensitive sites within the anterior hypothalamus was determined. Changes in heat production at Ta=+5 degrees C and changes in respiratory evaporative heat loss at Ta=+33 degrees C in response to discrete temperature stimuli were measured and the magnitude of the responses was correlated with the histologically assessed position of the probes transmitting the stimuli. In both animals the array of probes was centered close to the center of the thermosensitive area. The density of thermosensitive structures increased toward the center of the area covered by the probes. The most sensitive points were situated close to either side of the midline in those frontal planes which contained the nuclei supraoptici and paraventriculares. No difference was found between cold- and warm-sensitive sites.

Continuous measurement of arterial PO2, PCO2 and pH during autotransplantation of canine hearts

Arterial PO2, PCO2 and pH were measured continuously with electrodes placed in a flow cuvette. Comparison between continuous readings and results of single sample analyses showed practically no discrepancy, and the drifts of the electrodes during time of measurements were negligible. During hypothermic perfusion with a Rygg-Kyvsgård bubble oxygenator, autotransplantation of canine hearts was performed. PO2, PCO2 and pH were measured continuously to check the performance of the heart-lung machine, and to evaluate the therapeutic and diagnostic significance of these blood gas values during cardiac surgery. At the start of perfusion, a steep fall in all three parameters was observed. The average fall in PO2 was 313 mmHg; PCO2 fell by 15 mmHg and pH BY 0.15. PCO2 rapidly returned to normal values, while pH and PO2 increased slowly during the perfusion period. PO2 reached its highest value at the lowest temperature and fell during rewarming. PCO was regulated by the carbon dioxide concentration in the heart-lung machine. pH did not return to normal levels within the time of perfusion. In the transistional period from perfusion, PCO2 increased and pH fell. Alterations in the distribution and direction of blood flow and a low systemic blood pressure are possible explanations of the initial fall in PO2 and the post-perfusion changes in PCO2 and pH. The variations in pH and PCO2 at the start of perfusion were caused by an acid priming fluid with low CO2 content. The post-perfusion changes indicated an unstable circulation, but imminent myocardial failure could not alone be diagnosed by continuous measurement.