Influence of water temperature on heart rate and rectal temperature of swimming rats

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Abstract
The heart rate and rectal temperature of 12 adult male hooded rats were studied during 10-min swims in water of 37, 20, and 42 C. Both the heart rate and the rectal temperature stabilized in water of 37 C, suggesting that swimming in thermoneutral water is a submaximal exercise for rats. In water of 20 C, rectal temperature and heart rate of swimming animals fell exponentially to 28 C and 251 beats/min, respectively. The relationship of heart rate to rectal temperature in these swimming, cooling animals was not different from that reported in the literature for unanesthetized, inactive, hypothermic rats. It appeared that exercise had no effect on the heart rate of rats when the body temperature was dropping. In water of 42 C the rectal temperature rose exponentially to 42.2 C. The heart rate rose to 521 beats/min at the 8th min of swimming and remained stable thereafter. It is suggested that inadequate cardiac output resulting from severe changes in body temperature may be one factor which limits swimming capacity of small animals in hot and cold water.

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