Tracing Acid-Base Variables in Exercising Horses: Effects of Pre-Loading Oral Electrolytes

SIMPLE SUMMARY: Exercise results in changes in blood acid-base status that are proportional to the duration and intensity of the activity. Prolonged activity and transport are associated with substantial losses of water and electrolyte (ions) from the body, mostly through sweating. Because these losses can be large it is recommended that horses drink electrolyte solutions designed to replace lost water and ions. In this study horses were given either 1, 3 or 8 L of water or an oral electrolyte supplement and effects on acid-base state were measured. It was found that small volumes (1 and 3 L) had minimal or no effect. The 8 L of solution was designed to fully replace electrolyte and water losses. The large volume (8 L) electrolyte supplement, compared to water, abolished the mild alkalosis (raised pH) that occurred with long-lasting submaximal exercise. ABSTRACT: Oral electrolyte supplementation may influence acid-base state during exercise due to the intestinal absorption of administered water and electrolytes used to mitigating sweat losses. This study examined the effect of pre-exercise electrolyte supplementation (3 and 8 L) on plasma acid-base variables at rest, during moderate intensity exercise and during recovery. It was hypothesized that electrolyte supplementation will result in improved acid-base state compared to the alkalosis typical of prolonged exercise. In randomized crossover fashion, four horses were administered 3 L or 8 L of a hypotonic electrolyte solution (PNW) intended to replace sweat losses, or water alone (CON), 1 h before treadmill exercise to fatigue (at 35% of peak VO(2)) or for 45 min at 50% peak VO(2). Blood was sampled at 10-min intervals before, during and after exercise, and analyzed for dependent and independent acid-base variables. Effects of 3 L of supplementation at low exercise intensities were minimal. In the 8 L trials, plasma [H(+)] decreased (p < 0.05) during exercise and early recovery in CON but not PNW. Plasma TCO(2) decreased (p < 0.05) by 30 min after PNW reaching a nadir of 28.0 ± 1.5 mmol/L during the early exercise period (p = 0.018). Plasma pCO(2) and strong ion difference [SID] were the primary contributors to changes in [H(+)] and [TCO(2)], respectively. Pre-exercise PNW of 8 L intended to fully replenish sweat loses maintained [H(+)], decreased [TCO(2)] and mitigated the mild alkalosis during moderate intensity exercise.