Effect of dehydration on body-to-brain temperature difference in heat-stressed fowl (Gallus domesticus)

Abstract

Summary. 1. The effect of dehydration on bodyand hypothalamic temperatures was studied in fowls exposed to ambient temperatures from 26 to 42 ~ 2. A mean body-to-brain temperature difference of 0.68 ~ was found in the normally hydrated state. The slope of the regression line relating hypothalamic temperature to body temperature (0.79) was significantly lower than unity (P< 0.001). This indicates an increased brain cooling with increased body temperature.3. Dehydration induced by water deprivation reduced the body-to-brain temperature difference significantly (P<0.001), to 0.35 ~ The slope of the regression line relating hypothalamic temperature to body temperature (0.96) was not significantly different from unity, but was significantly higher than in the normally hydrated state (P< 0.02). This shows a reduced, yet constant degree of brain cooling during dehydration.4. Plasma osmolality and sodium and chloride concentrations, as well as the sodium-to-calcium ratio, were significantly higher in the dehydrated fowls (P<0.001 and P<0.02).5. The present findings suggest a decreased evaporative cooling in the dehydrated fowls resulting in an decreased brain-to-body temperature difference. This might also be related to an elevated hypothalamic thermoregulatory set-point which would cause an inhibition of the evaporative cooling. Together with the activation of the osmoregulatory system, these mechanisms contribute to the maintenance of water balance during dehydration and heat stress.

Body temperature and plasma arginine vasotocin in fowls adapted to high‐ and low‐Nacl diets

“…The differences in the present study in electrolyte concentrations between the HS and LS groups and the associated difference in plasma AVT are qualitatively and quantitatively similar to those found between dehydrated and normallyhydrated fowls 1984;1985;Arnason et al, 1986;Koike et al, 1977).…”

“…This is also in accordance with findings in dehydrated fowls 1984;1985) and with the concept that the hypothalamic thermoregulatory set-point in birds, as well as in mammals, is dependent on the sodium and calcium concentrations or on the sodium-tocalcium ratio in blood and therefore in the tissue fluid bathing the hypothalamic thermosensitive neurons (Denbow and Edens, 1980;1981;Edens, 1976;Jones et al, 1978;Myers and Veale, 1970;Saxena, 1976;Senay, 1979). Baker and Doris (1982) argued against the set-point concept, as no evidence has been provided so far for an experimental alteration of body temperature in thermoneutral environments.…”

“…No attempt was made to test this in the present study. However, a recent study in the dehydrated fowl revealed that a new body-to-brain temperature difference was effectively regulated during exposure to high ambient temperatures (Arad et al, 1984). Senay (1979) has integrated the elevated thermoregulatory set-point, as a consequence of an elevated sodium-to-calcium ratio, into a model summarising the events that take place during hypohydration.…”

Selective Brain Cooling in Mammals and Birds.

“…6, however, and in most other studies [39,46,47,49,50] the slopes were not significantly different from 1.0. Only in hydrated fowl [50] and resting kestrel [48] did SBC significantly increase with rising body temperature; however, the increases were not different from 1.0 when the animals were dehydrated or flying, respectively.…”

“…The difference between both temperatures, i.e., the degree of SBC, was reduced when evaporation was inhibited by bypass-breathing and the occlusion of beak, nares, and eyes [39], and enhanced with forced convective cooling of the evaporating surfaces, either by blowing dry air over the eyes of birds resting [40] or flying [48]. Dehydration had no effect on Japanese quail [49] and reduced the degree of SBC in fowl [50]. Figure 6 shows the relationships between cloacal and brain temperatures in six species whose range of mean body masses was 0.047 to 30 kg [45].…”