Thermal panting in dehydrated dogs: effects of plasma volume expansion and drinking

Dehydrated mammals reduce thermoregulatory evaporation and regulate deep body temperature (Tb) at elevated levels when they are exposed to heat. These experiments were designed to study the effects of plasma volume (PV) replacement and the effects of drinking H2O or 0.9% NaCl on thermal panting in dehydrated dogs resting at Ta 40-41 degrees C. Five dogs were studied when they were hydrated (control) and in 4 different experiments when they had been dehydrated (D) by removal of drinking water: (1) D, no treatment; (2) D + intravenous dextran infusion; (3) D + drinking H2O; (4) D + drinking NaCl. Compared to controls, dehydrated dogs had lower respiratory frequency (f), higher Tb, lower PV and blood volume (BV) and higher plasma osmolality (pOsmol). Intravenous dextran infusion restored BV to the control level without reducing pOsmol; but f and Tb remained at dehydrated levels after the infusion. When the dogs drank either H2O or NaCl,f increased significantly during the first minute after drinking. pOsmol decreased by 6 min after drinking H2O but did not change after drinking NaCl. BV measured 5 min after the end of drinking H2O was not different from dehydrated BV. It is concluded that (1) Restoration of BV to hydrated levels in dehydrated, heat-exposed dogs with elevated pOsmol does not restore f or Tb to hydrated levels; (2) There is a rapid recovery of thermal panting after drinking which is not dependent on changes in pOsmol or in BV.