Tolerance of altitude-acclimatized rats to exercise in the cold

The tolerance of altitude-acclimatized (18,000 ft 4 wk) and unacclimatized rats to exercise at 5 degrees was determined. Fewer unacclimatized than acclimatized rats became fatigued during 9 hr of exercise in the cold. Normal body temperatures were maintained in both groups during 9 hr in the cold at rest, but after exercise unacclimatized rats became mildly hypothermic (body temperature 35 degrees) and acclimatized rats severely hypothermic (body temperature 27.9 degrees). Polycythemia (hematocrit 69) was produced during the altitude acclimatization. Altitude-acclimatized rats developed more severe hypoglycemia and lower liver glycogen and serum lactic acid concentrations after exercise than did controls. No pathological changes were found in resting altitude-acclimatized rats, but after exercise in the cold, a higher percentage of acclimatized than unacclimatized rats developed focal myocardial necrosis within 4 days. Reduced exercise tolerance is attributed to severe hypothermia with associated decreased metabolism, polycythemia, hypoglycemia, and a higher incidence of pathological changes in the cardiac and striated muscles.