Exercise training-induced hypervolemia in the horse.

This study examined whether a long-term increase in blood volume (hypervolemia) is a result of endurance training in horses. The findings reveal that following training, plasma volume in horses increased, and although water intake remained unchanged, urine output decreased. This suggests that the training-induced increased blood volume in horses is primarily managed through kidney control mechanisms influencing water reabsorption.

Objective and Methodology of the Study

• The main aim of this research was to determine if endurance exercise training in horses results in a chronic increase in blood volume, a condition known as hypervolemia.
• Six mature horses that were previously inactive were used for the experiment, which lasted five weeks.
• Five of the horses were trained for 14 days on a treadmill at a constant speed and gradient for varying lengths of time. The sixth horse was trained at a trotting pace in a round pen.

Findings of the Research

• The study found that after training, the horses’ plasma volume, a component of blood volume, increased by approximately 29.1%, suggesting the occurrence of hypervolemia.
• There was no change in daily water intake during the training period. However, the 24-hour urine output decreased by an average of about 24.5%, indicating a change in fluid regulation in the body.
• The resting glomerular filtration rate and the rate of sodium clearance—markers of kidney function—remained unaffected by the training. However, clearance of urea, potassium, and osmotic substances decreased, while the clearance of free water increased post-training.
• Resting concentrations of plasma aldosterone and arginine vasopressin, hormones regulating blood volume and pressure, were not altered by training.
• Plasma potassium concentration significantly decreased after two weeks of training.

Conclusions of the Research

• The data from the research suggests that kidney control mechanisms affecting water reabsorption might contribute to the primary route for the training-induced increase in blood volume seen in horses.
• This is achieved through the re-absorption of urea and other osmotically active substances, not including sodium. This understanding might help in harnessing the health and performance benefits of hypervolemia in athletic horses.