Exercise delays the hypoxic thermal response in rats

Departments of 1 Exercise and Nutrition Science, 2 Physiology and Biophysics, 3 Neurology, and 4 Pathology and Anatomical Sciences and The Center for Sleep Disorders Research, Schools of Public Health and Health Professions and Medicine and Biomedical Sciences, University at Buffalo, Buffalo, New York 14214 Submitted 21 January 2003 ; accepted in final form 3 March 2003 Exercise exacerbates acute mountain sickness. In infants and small mammals, hypoxia elicits a decrease in body temperature (T b ) [hypoxic thermal response (HTR)], which may protect against hypoxic tissue damage. We postulated that exercise would counteract the HTR and promote hypoxic tissue damage. T b was measured by telemetry in rats ( n = 28) exercising or sedentary in either normoxia or hypoxia (10% O 2 , 24 h) at 25°C ambient temperature (T a ). After 24 h of normoxia, rats walked at 10 m/min on a treadmill (30 min exercise, 30 min rest) for 6 h followed by 18 h of rest in either hypoxia or normoxia. Exercising normoxic rats increased T b (°C) vs. baseline (39.68 ± 0.99 vs. 38.90 ± 0.95, mean ± SD, P < 0.05) and vs. sedentary normoxic rats (38.0 ± 0.09, P < 0.05). Sedentary hypoxic rats decreased T b (36.15 ± 0.97 vs. 38.0 ± 0.36, P < 0.05) whereas T b was maintained in the exercising hypoxic rats during the initial 6 h of exercise (37.61 ± 0.55 vs. 37.72 ± 1.25, not significant). After exercise, T b in hypoxic rats reached a nadir similar to that in sedentary hypoxic rats (35.05 ± 1.69 vs. 35.03 ± 1.32, respectively). T b reached its nadir significantly later in exercising hypoxic vs. sedentary hypoxic rats (10.51 ± 1.61 vs. 5.36 ± 1.83 h, respectively; P = 0.002). Significantly greater histopathological damage and water contents were observed in brain and lungs in the exercising hypoxic vs. sedentary hypoxic and normoxic rats. Thus exercise early in hypoxia delays but does not prevent the HTR. Counteracting the HTR early in hypoxia by exercise exacerbates brain and lung damage and edema in the absence of ischemia. thermoregulation; hypoxic tissue damage; cerebral edema; pulmonary edema; altitude illness Address for reprint requests and other correspondence: J. A. Krasney, Dept. of Physiology and Biophysics, 124 Sherman Hall, School of Medicine and Biomedical Sciences, Univ. at Buffalo, Buffalo, NY 14214 (E-mail: krasney{at}Buffalo.edu ).