Inhibition of thermal panting by intracarotid infusion of hypertonic saline in dogs.

Six dogs were surgically prepared with a right carotid loop and with the left common carotid artery ligated low in the neck. Respiratory frequency (f) and body temperature (Tb) were measured while the animals were at rest in a warm chamber (36-40 degrees C) in four separate experiments: 1) right intracarotid infusion of hypertonic NaCl; 2) right intracarotid infusion of isotonic NaCl; 3) intravenous infusion of hypertonic NaCl; and 4) no infusions. In a fifth experiment, right and left jugular and cephalic venous blood samples were collected before, during, and after the hypertonic intracarotid infusions. A 10-min infusion of hypertonic NaCl into the right common carotid artery elevated right jugular plasma osmolality by 33 mosmol/kg H2O, left jugular plasma osmolality by 26 mosmol/kg H2O, and cephalic plasma osmolality by 8 mosmol/kg H2O. A significant reversible drop in f and a rise in Tb occurred during hypertonic intracarotid infusions. Neither intracarotid infusion of isotonic NaCl nor intravenous infusion of hypertonic NaCl affected f or Tb. In experiments with no infusion, f and Tb did not change. It is concluded that brain receptors sensitive to extracellular solute concentration are able to influence the rate of thermoregulatory panting. This may underlie, in part, the reduced evaporation and elevated body temperatures that occur in dehydrated mammals exposed to heat.