Cold-induced inhibition of thermal panting in shorn sheep 3. Effect of previous habituation to cold

SUMMARY1. Twenty-six shorn Scottish Blackface sheep received nine daily cold shocks (down to −10°C) each lasting 1 hr. This treatment reduced the metabolic response to cold (lower heart rates) and caused passive body cooling—defined as habituation. Eight selected sheep showing maximum habituation and 16 untreated controls then received blocking treatment (re-shorn and cooled for 2 hr at 18°C or 8°C) before being heat-stressed (at 42°C) to elicit panting.2. Thermal panting, delayed as expected by blocking treatment, was further delayed by previous cold habituation. Habituated sheep blocked at 18°C panted 30 min later than blocked controls. In habituated sheep blocked at 8°C, panting was completely inhibited even after 90 min heat stress; their final mean respiration rate was 19/min v. 220/min in blocked controls and 250/min in unblocked controls. Rectal temperatures of habituated sheep were lower than controls during blocking treatment.3. Habituation, by restraining the normal metabolic response to cold, increased the heat debt during blocking treatment and lessened the heat load during subsequent heat stress. This apparently reduced the drive for panting. Habituation may also have favoured a direct inhibitory effect of cold on the respiratory centres.

STUDIES ON HABITUATION TO COLD PAIN

It was possible to produce habituation to cold in a group of human subjects by immersing the left hand in cold water for [Formula: see text] minutes twice a day for 19 days. The right hand did not adapt. Another group of subjects was exposed similarly with the difference that an anxiety test (mental arithmetic test) was always given simultaneously with the cold-water test. In this second group the original blood pressure response, i.e. for the first day, was greater than in the first group because of the cumulative effects of the two tests. After 19 days definite evidence was obtained for adaptation to these two tests administered together. However, when these tests were given separately to the second group, no adaptation was evident; adaptation occurred only to both tests given simultaneously. These results indicate that no adaptation develops to cold per se if the subjects are distracted from cold discomfort. It was also found that adaptation of one hand to cold water not only failed to induce adaptation in the opposite hand but even reinforced responses of the unadapted hand. These findings suggest a participation of the central nervous system in adaptation to cold pain, and tend to minimize the importance of local peripheral changes.

Metabolic and thermal responses to muscular exertion in the cold

The metabolic and thermal responses to muscular exertion in the cold were studied in outdoor and indoor workers. The metabolic rate during exercise was the same in the cold as in a thermally neutral (warm) environment when workload was sufficiently high to prevent a fall in the rectal temperature. At lower levels of work the metabolic rate in the cold was higher than that in the warm. No significant differences in the work metabolism were found between the groups. When the resting subjects were exposed to the cold environment prior to the exercise the skin temperature decreased. The rate of decrease diminished as exercise progressed and at higher load of work the extra heat generated in the body was enough to rewarm the skin, even on the most peripheral parts. The rate of decrease of skin surface cooling during rest in the cold was found to be slower in the outdoor workers than in the indoor workers, and the onset of the rewarming occurred much quicker in the former. These differences in skin temperature between outdoor and indoor workers indicate an acclimation to cold affecting the vasomotor control of the skin circulation. The hypothesis is suggested that habituation to cold leads to a lower set point of the thermosensitive cells in the thermoregulating center, so that vasodilatation impulses are discharged at a lower temperature. Submitted on September 17, 1962

Hand circulation in the cold of Lapps and North Norwegian fishermen

The hand blood flows of Norwegian Lapps and North Norwegian fishermen were compared with those of a group of control subjects to determine whether habituation to cold alters the local vascular response. Blood flow was measured using venous occlusion plethysmographs with local temperatures of 40°, 20°, 10°C and a few degrees above zero. In further experiments using Dewar-flask calorimeters, the rate of heat elimination and rapidity of onset of the cold vasodilatation response on immersing the hand in water at 0°C were also recorded. The subjects were kept warm in order to remove as far as possible the effects of differing degrees of vasoconstrictor fiber activity. The onset of the cold vasodilatation was found to be more rapid in the cold-habituated subjects than in the control subjects. The levels of hand blood flow were, however, the same in all three groups of subjects, implying that habituation to cold does not alter the local vascular response to temperature. It is suggested that the difference reported by other workers in similar studies are probably found only when subjects examined are partially vasoconstricted, and reflect a more general process of adaptation to cold which results in a reduced vasoconstrictor fiber activity on exposure to a given thermal environment. Submitted on August 19, 1959