Dehydration increases the magnitude of selective brain cooling independently of core temperature in sheep

Fuller, Andrea; Meyer, Leith C. R.; Mitchell, Duncan; Maloney, Shane K.

doi:10.1152/ajpregu.00074.2007

Cited by 32 publications

(50 citation statements)

References 34 publications

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“…As a result of the concession to fluid and circulatory homeostasis (diminished thermoregulatory drives), however, body temperature should be regulated at a higher level, as is seen in the present study (higher T es and T sk during Ex2 in NFR, Fig. 1, A and B), and in resting sheep (20), exercising dogs (3), and exercising goats (6).…”

Section: Discussionmentioning

confidence: 49%

Effect of hypohydration on hyperthermic hyperpnea and cutaneous vasodilation during exercise in men

Fujii

Honda²,

Hayashi³

et al. 2008

Journal of Applied Physiology

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Nishiyasu T. Effect of hypohydration on hyperthermic hyperpnea and cutaneous vasodilation during exercise in men. J Appl Physiol 105: 1509-1518. First published September 11, 2008 doi:10.1152/japplphysiol.01206.2007.-We tested the hypothesis that, in humans, hypohydration attenuates hyperthermic hyperpnea during exercise in the heat. On two separate occasions, thirteen male subjects performed a fluid replacement (FR) and a no-fluid replacement (NFR) trial in random order. The subjects performed two bouts of cycle exercise (Ex1 and Ex2, 30 -60 min) at 50% peak oxygen uptake (V O2 peak) in 35°C separated by a 70-to 80-min rest period, during which they drank water containing 25 mosmol/l sodium in the FR trial but not the NFR trial. The drinking in the FR trial nearly restored the body fluid to the euhydrated condition, so that the body fluid status differed between the trials before Ex2 (the difference in plasma osmolality before Ex2 was 9.4 mosmol/kgH 2O; plasma volume was 7.6%, and body weight was 2.5%). The slopes of the linear relationships between ventilatory variables (minute ventilation, ventilatory equivalents for oxygen uptake and carbon dioxide output, tidal volume, respiratory frequency, and end-tidal CO 2 pressure) and esophageal temperature (Tes) did not significantly differ between Ex1 and Ex2, or between the FR and NFR trials. On the other hand, during Ex2 in the NFR trial, the Tes threshold for the onset of increased forearm vascular conductance (FVC) was higher, and the slope and peak values of the relationship between FVC and Tes were lower than during Ex1 in the NFR trial and during Ex2 in the FR trial. These findings suggest that hypohydration does not affect the hyperthermic hyperpnea during exercise, although it markedly attenuates the cutaneous vasodilatory response. thermoregulation; respiration; ventilation; skin circulation IN MANY SPECIES of mammals and birds, an elevation in body temperature stimulates ventilation and increases evaporative heat loss for thermoregulation (the so-called panting response) (42). In 1905, Haldane (25) was the first to report that hyperthermia also increases ventilation in humans, and the recent review by White (53) suggested that the hyperthermic hyperventilatory response provides countercurrent cooling of blood perfusing the brain (selective brain cooling). However, the mechanism and the physiological significance of this response in humans are not fully understood.Hayashi et al. (26) and Nybo and Nielsen (37) recently reported that during prolonged submaximal exercise [50 -57% peak oxygen uptake (V O 2 peak )] in the heat in humans, minute ventilation (V E) increases linearly with increasing esophageal temperature (T es ), but this ventilatory response appears independent of skin temperature (T sk ) and the rate of increase in core temperature (26). Such prolonged exercise in the heat usually leads to profuse sweating, which can lead to hypohydration. Indeed, body weight was reduced by 1.5% in Hayashi's study (unpublished data), and by 0.7% in Nybo and Ni...

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Section: Discussionmentioning

confidence: 49%

Effect of hypohydration on hyperthermic hyperpnea and cutaneous vasodilation during exercise in men

Fujii

Honda²,

Hayashi³

et al. 2008

Journal of Applied Physiology

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“…The Journal of Experimental Biology 215 (22) (Fuller et al, 2007) and Bedouin goats Jessen et al, 1998). Compared with euhydration, dehydrated goats and sheep increased selective brain cooling with no alteration in the threshold body temperature at which selective brain cooling was implemented.…”

Section: Discussionmentioning

confidence: 99%

Selective brain cooling in Arabian oryx (Oryx leucoryx): a physiological mechanism for coping with aridity?

Hetem

Strauss

Fick

et al. 2012

Journal of Experimental Biology

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SUMMARYSelective brain cooling is a thermoregulatory effector proposed to conserve body water and, as such, may help artiodactyls cope with aridity. We measured brain and carotid blood temperature, using implanted data loggers, in five Arabian oryx (Oryx leucoryx) in the desert of Saudi Arabia. On average, brain temperature was 0.24±0.05°C lower than carotid blood temperature for four oryx in April. Selective brain cooling was enhanced in our Arabian oryx compared with another species from the same genus (gemsbok Oryx gazella gazella) exposed to similar ambient temperatures but less aridity. Arabian oryx displayed a lower threshold (37.8±0.1°C vs 39.8±0.4°C), a higher frequency (87±6% vs 15±15%) and a higher maximum magnitude (1.2±0.2°C vs 0.5±0.3°C) of selective brain cooling than did gemsbok. The dominant male oryx displayed less selective brain cooling than did any of the other oryx, but selective brain cooling was enhanced in this oryx as conditions became hotter and drier. Enhanced selective brain cooling in Arabian oryx supports the hypothesis that selective brain cooling would bestow survival advantages for artiodactyl species inhabiting hot hyper-arid environments.

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“…In mammals, SBC can be observed when water is not available. Euhydrated mammals (goats and sheep), even when exposed to heat stress rarely present SBC, but dehydrated animals show more frequent and a larger degree of SBC, supporting the idea that such a mechanism has an osmoregulatory drive and might not play a thermoregulatory function in these animals (Jessen et al, 1998;Fuller et al, 2007). In contrast, in Pogona, cooling of the head (and by inference, the brain) happened when water was available (i.e.…”

Section: Discussionmentioning

confidence: 91%

Thermoregulatory consequences of salt loading in the lizard, Pogona vitticeps

Scarpellini

Bícego

Tattersall

2015

Journal of Experimental Biology

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Previous research has demonstrated that dehydration increases the threshold temperature for panting and decreases the thermal preference of lizards. Conversely, it is unknown whether thermoregulatory responses such as shuttling and gaping are similarly influenced. Shuttling, as an active behavioural response, is considered one of the most effective thermoregulatory behaviours, whereas gaping has been proposed to be involved in preventing brain over-heating in lizards. In this study we examined the effect of salt loading, a proxy for increased plasma osmolality, on shuttling and gaping in Pogona vitticeps. Then, we determined the upper and lower escape ambient temperatures (UET a and LET a ), the percentage of time spent gaping, the metabolic rate (V˙O 2 ), the evaporative water loss (EWL) during gaping and non-gaping intervals and the evaporative effectiveness (EWL/V˙O 2 ) of gaping. All experiments were performed under isotonic (154 mmol l −1 ) and hypertonic saline injections (625, 1250 or 2500 mmol l −1 ). Only the highest concentration of hypertonic saline altered the UET a and LET a , but this effect appeared to be the result of diminishing the animal's propensity to move, instead of any direct reduction in thermoregulatory set-points. Nevertheless, the percentage of time spent gaping was proportionally reduced according to the saline concentration; V˙O 2 was also decreased after salt loading. Thermographic images revealed lower head than body surface temperatures during gaping; however this difference was inhibited after salt loading. Our data suggest that EWL/V˙O 2 is raised during gaping, possibly contributing to an increase in heat transfer away from the lizard, and playing a role in head or brain cooling.

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Dehydration increases the magnitude of selective brain cooling independently of core temperature in sheep

Cited by 32 publications

References 34 publications

Effect of hypohydration on hyperthermic hyperpnea and cutaneous vasodilation during exercise in men

Effect of hypohydration on hyperthermic hyperpnea and cutaneous vasodilation during exercise in men

Selective brain cooling in Arabian oryx (Oryx leucoryx): a physiological mechanism for coping with aridity?

Thermoregulatory consequences of salt loading in the lizard, Pogona vitticeps

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