Effects of exercise and training on plasma and blood volumes in Thoroughbreds and endurance horses

Exercise-and training-induced changes in plasma (PV) and blood volumes (BV) have been well documented in racehorses but not in endurance horses. We aimed to determine and compare the effects of training and exercise on BV and PV in Thoroughbred racehorses and endurance horses. Six endurance horses underwent a 4-month training program under saddle and 11 Thoroughbreds completed a racehorse-appropriate interval training program for 3-4 months on a racetrack. PV and BV were measured before and after training at rest and post-exercise using the indocyanine green dilution technique. For the Thoroughbreds, exercise consisted of a 1000 m breeze on a racetrack. The endurance horses completed rides of 10, 20 and 30 miles during training and 50 miles when fully trained. Data were analysed using the Wilcoxon signed rank test, Mann-Whitney test and paired t test, with significance set at . Although absolute PV and BV were greater in Thoroughbreds, mass-specific PV and BV were mostly greater in endurance horses. Untrained endurance horses had a greater mass-specific PV and BV at rest (PV: 65 ± 4.1 vs 54 ± 5.7 ml/kg, ; BV: 101 ± 8.0 vs 83 ± 8.8 ml/kg; ) and a greater PV (60 ± 3.8 vs 44 ± 4.8 ml/kg; ) but not BV (108 ± 2.0 vs 112 ± 14.3 ml/kg) after exercise. Training induced an increase in resting PV and BV to 73 ± 4.3 and 116 ± 14.1 ml/kg, respectively, in endurance horses (), and an increase in PV and BV after 30 miles endurance exercise (PV: 60 ± 3.8 vs 80 ± 15.8 ml/kg; ; BV: 108 ± 2.0 vs 139 ± 26.5 ml/kg; ). Resting and post-exercise PV and BV increased following training in the Thoroughbreds: resting PV = 57 ± 5.1 ml/kg (), resting BV = 89 ± 8.3 ml/kg (); post-exercise PV = 49 ± 4.8 ml/kg (), post-exercise BV = 112 ± 14.3 ml/kg (untrained) vs 124 ± 18.2 ml/kg (trained), ). PV increased in response to exercise in endurance horses but decreased in Thoroughbreds. Endurance horses had smaller absolute PV and BV but greater mass-specific resting PV and BV than Thoroughbreds.