Heat acclimation improves regulation of plasma volume and plasma Na+ content during exercise in horses

This study determined the plasma volume (PV) and ion responses to heat accl imation and exercise in six trained Thoroughbred horses during 21 days of e xposure to heat and humidity (33 degrees C, 83% relative humidity) for 4 h/ day. During the 2nd h on days 0, 3, 7, 14, and 21, horses performed a stand ardized treadmill test, running at 50% of peak O-2 uptake until pulmonary a rtery temperature reached 41.5 degrees C. Heat acclimation resulted in an i ncrease in PV from 21.3 +/- 1.1 liters on day 0 to 24.3 +/- 1.0 liters on d ay 14, returning to 22.6 +/- 0.9 liters on day 21. The corresponding total plasma protein contents were 1,273 +/- 53, 1,455 +/- 81, and 1,377 +/- 57 g , respectively, and increases in total plasma Na+ plus Cl- content were 5,1 45 +/- 126, 5,749 +/- 146, and 5,394 +/- 114 mmol, respectively. Thus chang es in PV were accompanied by direct changes in plasma protein and osmolyte contents. With exercise on day 0, PV decreased by 7.1 +/- 0.7% at 5 min of exercise and remained decreased (-6.7 +/- 1.3%) at 5 min of recovery. By da y 21, PV decreased significantly less than on day 0 (by 5.2 +/- 0.9% at 5 m in of exercise), was decreased by only 2.0 +/- 1.6% at 5 min of recovery, a nd was fully restored at 15 min of recovery. Plasma Na+ concentration incre ased 3 meq/l during the first 5 min of exercise and was normalized by 5 min of recovery on day 0 and by end exercise on day 21. It is concluded that i mproved ability to regulate PV during exercise in response to heat acclimat ization is associated with an increased PV and an improved conservation of Na+.