Extracellular hyperosmolality and body temperature during physical exercise in dogs.

Area of Science:

• Exercise Physiology
• Thermoregulation
• Body Fluid Homeostasis

Background:

• Thermoregulation during exercise is crucial for maintaining physiological stability.
• The role of extracellular fluid osmolality in exercise thermoregulation is not fully understood.
• Previous research has focused on plasma sodium concentration's impact on heat balance.

Purpose of the Study:

• To investigate if extracellular fluid hyperosmolality affects thermoregulation during exercise independently of plasma sodium concentration.
• To test the hypothesis that increased extracellular fluid osmolality can alter core body temperature responses during exercise.
• To differentiate the effects of hyperosmolality from hypernatremia on exercise thermoregulation.

Main Methods:

• Utilizing canine models performing moderate-intensity treadmill exercise for 60 minutes.
• Administering pre-exercise intravenous infusions of hypertonic mannitol and sodium chloride solutions.
• Measuring rectal temperature (Tre) and plasma osmolality ([Osm]) before and after infusions and during exercise.

Main Results:

• Both mannitol and NaCl infusions increased plasma osmolality by 15 mosmol/kg.
• NaCl infusion elevated plasma sodium concentration, while mannitol decreased it.
• Rectal temperature increased significantly more during exercise after both infusions compared to control (1.9°C vs. 1.3°C).

Conclusions:

• Extracellular fluid hyperosmolality, independent of plasma sodium levels, can lead to excessive core body temperature increases during exercise.
• This effect on thermoregulation appears specific to exercise conditions, not occurring during rest.
• Maintaining extracellular fluid osmolality is critical for effective thermoregulation during physical exertion.