ENERGY-COST AND THERMOREGULATION OF UNRESTRAINED RATS DURING EXERCISEIN THE COLD

The effect of cold ambient temperature on the energy cost and thermore gulation during exercise was studied in 18 male Sprague-Dawley rats ac customed to running for 2 weeks. For measuring T-b and ensuring an unr estrained exercise performance, the rats were implanted with intraperi toneal telemetric radio transmitters. The rats ran at three submaximal exercise intensities (10, 15 and 20 m . min(-1)) at different T-a val ues (22 degrees, 0 degrees, -10 degrees and -20 degrees C) for 30 min. VO2 and T-b were continuously measured. During exercise at 0 degrees C a 100% substitution of thermoregulatory heat production by exercise thermogenesis was observed. At T-a values below 0 degrees C the physic al strain of exercise increased considerably: VO2 increased 34-61% at -10 degrees C and 51% at -20 degrees C compared to the corresponding e xercise intensity at T-a 22 degrees C. A partial substitution of therm oregulatory heat production by exercise thermogenesis was observed in the cold. At -10 degrees C the cost for thermoregulation during exerci se was 28% and at -20 degrees C 31% lower than at rest at the correspo nding T-a values. The rat could not maintain normal T-b during exercis e below 0 degrees C. The T-b dropped 1.5-1.8 degrees C at -10 degrees C and 2.5 degrees C at -20 degrees C. It is concluded that exercising below 0 degrees C at submaximal levels (below 50% of VO2max) partially substitutes for thermoregulatory costs but is thermally unfavorable f or the small-sized rat.