Thesis (M.A.)--Boston UniversityEvidence of physiological acclimatization of man to cold is scanty and not clear-cut. Certainly no adaptations have been described comparable to the dramatic changes which occur when men are acclimatized to heat. This study is an attempt to demonstrate acclimatization to cold in man, not in terms of indices specific for cold stress, but rather by measurement of the systemic strain placed on the body. Experiments were designed to answer two questions: (a) Does cold exposure result in a systemic strain on the body? and (b) Does continued cold exposure give rise to (unknown) specific physiologic a aptations with a consequent reduced strain? Since stimulation of tne pituitary-adrenal cortical axis seems to be a common denominator in all known stress situations, adrenal cortical activity was used as a measure of the degree of distortion to the homeostasis.
The following experimental design was used: Twelve young soldiers previously brought into good physical condition by mild exercise were exposed to the following successive sets of conditions: four weeks of no cold exposure (baseline period) during which control measurements were made; twelve consecutive days of continuous cold exposure at 60°F.; nine days of no cold exposure; five days of re-exposure to cold (60°F.); five days of no cold exposure. Adrenal cortical activity was assessed by means of circulating eosinophil counts and urinary uric acid: creatinine ratios. During cold exposure periods the men remained continuously in the cold room, leaving it only for meals, bathing and control procedures (a total of 4 1/4 hours daily). They reclined nude except for cotton shorts on army beds from 8:00 A.M. till noon and from 1:15 P.M. till 4:30 P.M. In order to obtain comparable conditions throughout all experimental periods for the assessment of basal levels of adrenal cortical activity, the men lay quietly from 7:00- 8:00A.M. in a constant temperature room maintained at 85°F.
+/- 1°F.; at the end of this hour, urine was collected and blood drawn. These samples also served as controls for the measurement of acute responses after four hours in the cold.
Sixty degrees Fahrenheit was selected as the cold temperature because it eliminated the danger of cold injury, making it unnecessary to rewarm the men periodically or to clothe them. It was deemed an adequate stimulus because men exhibit the usual cold responses at this temperature - shivering, diuresis, hemoconcentration and peripheral vasoconstriction - within 45 minutes to two hours after the start of exposure. Furthermore preliminary experiments showed this temperature to produce marked discomfort which, however, was tolerable for prolonged periods.
The following results were obtained:
1. There was a significant depression of basal eosinophil counts on the fourth and eighth days of the first cold period, with a return to approximately control levels on the eleventh day.
2. A "rebound" of basal eosinophil counts to levels significantly above controls was observed in the nine-day period between cold exposures.
3. Eosinophil counts taken on the fourth day of re-exposure were not significantly different from controls, but were significantly below "rebound" values.
4. No marked differences from normal diurnal variation were observed in the eosinophil counts after four hours of cold exposure at 60°F.
5. No trends were observed in basal uric acid: creatinine ratios.
6. During acute four-hour exposure, uric acid: creatinine ratios were significantly higher than controls. Although no correlation existed between eosinophil and (U-A)/C changes, there was a significant correlation between rate of urine flow and (U-A)/C.
Although both eosinophil counts and uric acid: creatinine ratios are preserved to measure activity related to 11-oxygenated corticosteroids, the results show a qualitative difference between the two indices. The eosinophil data indicate changes in basal activity but no marked response to acute exposure, whereas the (U-A)/C data suggest acute responses but no change in basal levels. This confirms the findings of previous workers of a lack of parallelism between these indices in conditions of mild stress.
The changes in basal eosinophil counts suggest positive findings of acclimatization to cold. The significant decreases on the fourth and eighth days of exposure indicate increased adrenal cortical activity; the return toward pre-exposure levels on the eleventh day suggest a return of activity toward normal. These results are consistent with, but do not prove, the thesis that cold exposure under the conditions described results in a systemic strain on the body, and that the strain is lessened on extended exposure. This suggests that physiological acclimatization to cold occurs. These conclusions are based on assumptions that (a) changes in circulating eosinophil counts reflect changes in adrenal cortical activity and (b) changes in such activity are a measure of the severity of strain placed on the body
