Extracellular hyperosmolality and body temperature during physical exercise in dogs.

The purpose of this study was to test the hypothesis that thermoregulation during exercise can be affected by extracellular fluid hyperosmolality [Osm] without changing the plasma Na+ concentration. The effects of preexercise venous infusions of hypertonic mannitol and NaCl solutions on rectal temperature (Tre) responses were compared in dogs running at moderate intensity for 60 min on a treadmill. Plasma E1Na+] was increased by 12 meq/l (P less than 0.05) after NaCl infusion, and decreased by 9 meq/l (P less than 0.05) after mannitol infusion. Both infusions increased plasma [Osm] by 15 mosmol/kg (P less than 0.05). After both infusions, Tre was essentially constant during 60 min rest. However, compared with the noninfusion exercise increase in Tre of 1.3 degrees C, Tre increased by 1.9 degrees C (delta delta Tre = 0.5 degrees C, P less than 0.05) after both postinfusion exercise experiments. It was concluded that inducing extracellular hyperosmolality, without elevating plasma [Na+], can induce excessive increases in Tre during exercise but not at rest.