FOSTER, C.B.; COGGER, E.A.; GREENE, H.M.; WICKLER, S.J.; California Polytechnic University, Pomona; California Polytechnic University, Pomona; California Polytechnic University, Pomona; California Polytechnic University, Pomona: Fluid Balance and Electrolyte Changes in Horses During Rest and Exercise at High Altitude (3800m)

Acute exposure to high altitude (HA) in humans produces shifts in fluid including a decrease in total body water (TBW), plasma volume (PV), Na+, and K+. Such changes have not been documented in horses. Both exercise and altitude evoke specific physiological responses; this study compared the physiological response of the horse to exercise, HA, and their interaction. These values were compared to LA values. Fluid balance and electrolyte changes were measured in horses exposed to HA (3800 m). Six horses were measured at low altitude (LA: 225 m) and during 14 days at HA. TBW and PV were measured at rest; serum osmolality (Osm), concentrations of aldosterone (ALD), total protein (TP), and serum K+, Na+, Cl-, were measured at rest and after submaximal exercise. Rest: Initial exposure to HA at rest resulted in an 8% decrease in PV, but TBW increased by 7%. Both TBW and PV normalized by day 14 at HA. Decreases in PV may indicate water loss typical of HA exposure. However, increased TBW implies compensatory water intake, but an inability to access water in the face of decreased PV. Initial HA exposure produced decreases in Osm (5%), K+ (14%), ALD (65%), and an increase in Cl-(4%), and TP (11%). By day 4 at altitude, all variables stabilized and returned to or near baseline values except which K+ returned to baseline on day 13. Exercise: at both LA and HA, exercise stimulated increases in TP (6%), ALD (61%), and K+ (11%); but no change in Na+, Cl-, or Osm. The patterns and magnitudes of these increases and decreases were not affected by altitude–except TP at HA day 2 (when TP at rest was higher than exercise). Supported by an Agricultural Research Initiative grant, SJW & HMG.