Horses undergoing supramaximal fixed-speed treadmill exercise after administration of furosemide experience less severe acidosis and slower onset of fatigue.

Pre-exercise furosemide induces alkalosis and significantly extends time to fatigue in exercising Thoroughbred horses.

This randomized crossover study investigated the effects of pre-exercise furosemide administration on acid-base balance, ion homeostasis, and exercise performance in six fit Thoroughbred horses. Animals received either 0.5 mg/kg IV furosemide or placebo four hours before galloping to fatigue on a treadmill at 115% of maximal oxygen consumption (VO2max). Furosemide induced a preexercise hypochloremic metabolic alkalosis, evidenced by significantly lower chloride concentrations and higher pH, strong ion difference (SID), and bicarbonate concentrations across all time points from preexercise through one minute postexercise. Mixed venous PCO2 was lower at fatigue and postexercise in furosemide-treated horses, suggesting improved CO2 transport efficiency. Notably, VO2max and peak CO2 production were not significantly different between groups, indicating that furosemide did not enhance aerobic capacity per se. However, time to fatigue was significantly longer following furosemide administration (mean difference: 15.4 seconds; 95% CI, 6.6–24.3 seconds), representing approximately a 2% improvement in performance. The furosemide-associated alkalosis appeared to provide a greater buffering reserve against exercise-induced acidosis, thereby delaying fatigue onset. The approximately 2% body weight loss from diuresis may also have contributed to performance improvements. These findings suggest that furosemide's ergogenic effects in horses are mediated primarily through acid-base modulation rather than alterations in aerobic metabolism, with clinical implications for understanding performance-enhancing mechanisms and regulatory considerations in equine competition.

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