Caloric value of cardiac response to hot environments

Single Comb White Leghorn hens were placed in a controlled temperature chamber, initially at 26 C and subsequently raised to 35 C. Recordings of the surface and rectal temperatures, respiratory rate, heart rate, and oxygen consumption were made at the 26 C basal temperature and compared with those made at the 35 C ambient temperature. Sequential doses of saline and atropine were injected following the thermoregulatory response of birds to 35 C. The thermal challenge resulted in an average increase of 53 breaths per minute in the respiratory rate during the pre-injection period at 35 C and rose steadily to the end of the thermal challenge. The saline elicited no effects on any parameter measured; however, the atropine resulted in a mean increase of 62 beats per minute in the heart rate. There was no difference in the pre-atropine heart rate at 35 C and the basal heart rate at 26 C. Following the atropine induced increase in heart rate there was a 13% increase in energy expenditure. Thermal-bradycardia in hot environments may represent a first line of defense against hyperthermia by reducing the added heat load.