Extracellular hyperosmolality and body temperature during physical exercise in dogs

Abstract
The hypothesis that thermoregularion during exercise can be affected by extracellular fluid hyperosomolality [Osm] without changing the plasma Na+ concentration was tested. 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 [Na+] was increased by 12 meq/l (P < 0.05) after NaCl infusion, and decreased by 9 meq/l (P < 0.05) after NaCl infusion, and decreased by 9 meq/l (P < 0.05) after mannitol infusion. Both infusions increased plasma [Osm] by 15 mosmol/kg (P < 0.05). After both infusions, Tre was essentially constant during 60 min rest. Compared with the noninfusion exercise increase in Tre of 1.3.degree. C, Tre increased by 1.9.degree. C (.DELTA. Tre = 0.5.degree. C, P < 0.05) after both postinfusion exercise experiments. The induction of extracellular hyperosomolality, without elevation of plasma [Na+], can induce excessive increases in Tre during exercise but not at rest.