Effects of dehydration on thermoregulation in cats exposed to high ambient temperatures.

Measurements of evaporative water loss (EWL), metabolic rate (MR), body core temperature (Tc), and respiratory rate (RR) were made in nine hydrated cats exposed to several high ambient temperatures. Measurements were made in animals hydrated ad libitum and during progressive dehydration. At 43 and 40 degrees C, no significant change in EWL occurred during dehydration. At 38 and 35 degrees C, EWL in animals dehydrated for 4 days was reduced to about 50% of hydrated levels. Reductions in RR correlated highly with reduced EWL, suggesting that control of evaporation in dehydrated cats is dependent largely on control of RR. Significant elevations in Tc occurred in dehydrated animals at 43, 40, and 38 degrees C, and an insignificant upward trend was observed at 35 degrees C. Multiple linear regression analysis of data for EWL, Tc, and level of dehydration indicates that dehydration reduces the evaporative response to elevations in Tc and suggests that a change in the sensitivity of the thermoregulatory system is occurring. A similar analysis for MR, Tc, and level of dehydration shows that dehydration results in a lower MR at a given Tc. Changes in MR, corrected for body temperature, during dehydration are principally related at altered ventilatory energy expenditure. A model has been formed for estimating the metabolic cost of ventilation in the cat.