Exercise training-induced hypervolemia in the horse.

The purpose of this study was to determine if a chronic hypervolemia would accompany endurance exercise training in the horse. Six mature previously inactive horses were utilized for this study. During the 5-wk experiment, five of the horses were trained for 14 d on a treadmill ergometer at a constant treadmill speed of 5.6 km X hr-1 and a constant grade of 12.5% for graduated lengths of time. One horse was trained by lunging at a trotting pace in a round pen. Following training, plasma volume increased by 4.7 1 (29.1%, P less than 0.05). Although the rate of daily water intake did not change during the training period, 24-h urine output decreased by an average of 3.5 1 X d-1 (-24.5%, P less than 0.05). Resting glomerular filtration rate and the rate of sodium clearance were not altered by training. However, urea, potassium, and osmotic clearance were decreased by training (P less than 0.05) while free water clearance was increased (P less than 0.05). Resting plasma aldosterone and arginine vasopressin concentrations were not altered by training. Plasma potassium concentration was significantly decreased (P less than 0.05) following the 2 wk of training. These data would appear to suggest that renal control mechanisms affecting water reabsorption via the re-absorption of urea and osmotically active substances other than sodium provide the primary route for the training-induced hypervolemia seen in horses.

Exercise training-induced hypervolemia in greyhounds: role of water intake and renal mechanisms.

The purpose of this study was to determine if the chronic hypervolemia that accompanies endurance exercise training is due only to an increase in the rate of water intake or if there were contributions from renal mechanisms. Four greyhound dogs, previously sedentary for 3 yr, were utilized. During the 28-day experiment each dog was trained on a treadmill ergometer for 14 consecutive days at 65% of its pretraining maximal work intensity. After training, plasma volume increased 472 ml (27.5%, P less than 0.05). The rate of water intake increased 328 ml/day (33%, P less than 0.05), whereas urine output increased 87 ml/day (20.8%, P less than 0.05). The mean resting 24-h values for clearance of sodium increased 0.29 ml/min (90.3%, P less than 0.05), and clearance of potassium decreased 1.51 ml/min (16.1%, NS). Glomerular filtration rate, free water clearance, and osmotic clearance were not significantly altered. These data suggest that the primary mechanism for the exercise training-induced hypervolemia in dogs is a net positive water balance via increased water consumption without significant contribution from an increase in renal water reabsorption.