Selective brain cooling reduces respiratory water loss during heat stress

Terrestrial mammals developed several mechanisms to reduce water loss to counteract water shortage. One avenue of water loss is the evaporative heat loss by sweating and panting, which increases with body temperature. Sweating and panting are activated by temperature signals of the body, whereby the brain is the most important site in generating temperature signals. Goats, like other artiodactyls, can cool their brains selectively below the temperature of the trunk core. The aim of the present study is to determine whether an inhibition of the selective brain cooling (SBC) mechanism will increase substantially the respiratory evaporative water loss during heat stress due to the higher brain temperature. The inhibition of SBC was performed by increasing brain temperature experimentally at the same rate as trunk temperature by means of extracorporeal heat exchangers. These experiments without SBC resulted in higher respiratory evaporative water loss compared to experiments with normal SBC. Eighteen experiments in two conscious goats had shown that at a trunk temperature of 40 degrees C the respiratory water loss was reduced on average by 29 g/hr (0.7 l/day) due to the effect of SBC. This amount of water corresponds to about one third of the general water requirements. In conclusion, SBC substantially reduces the water loss in goats during heat stress and consequently improves survival chances during water shortage.