Thermoregulation under conditions of impaired body fluid/osmotic balance in mammals

The state of body hydration has a marked effect on thermoregulation during heat stress. Heat stressed hypohydrated mammals reduce evaporative cooling and metabolic rate, and allow their body temperature to rise. This outcome is not a failure in thermoregulation, but readjustment, leading to improved physical avenues for heat dissipation and water conservation. The modulation of the thermoregulatory system is mostly central. This is exhibited by decreased sensitivity of the thermoregulatory integration center and by an upward shift of temperature thresholds for heat dissipation activities. New evidence suggests alterations in effector organs as well. In the euhydrated body, hyperosmolarity and hypovolemia, the consequences of hypohydration, via activation of osmoreceptors and low pressure baroreceptors, induce the same thermoregulatory responses as hypohydration does. Single unit studies on hypothalamic slices provide a neuronal basis for osmoregulation-thermoregulation interaction. The possible role of arginine vasopressin (AVP) as a central modulator in this interaction is discussed.