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

Plain Language Overview

• This study investigated how the drug furosemide affects acid-base balance, ion levels, and exercise performance in horses running at very high intensity on a treadmill.
• The results showed that furosemide reduced the severity of acidosis, altered blood chemistry before exercise, and allowed horses to exercise longer before getting fatigued.

Purpose and Design of the Study

• The main goal was to measure changes in acid-base status and ion balance during intense exercise, following administration of furosemide, and to connect these changes to exercise performance.
• Six well-trained Thoroughbred horses were studied in a randomized crossover design, meaning each horse received both the drug (furosemide) and a placebo on separate occasions.
• Furosemide was given intravenously at 0.5 mg/kg four hours before exercise to allow time for its effects to develop.
• Exercise consisted of galloping on a treadmill at a fixed speed corresponding to 115% of their maximum oxygen consumption (supra-maximal effort) until reaching fatigue.

Measurements Taken

• Blood samples were taken from arterial and mixed venous sites before treatment, immediately before exercise, after a 2-minute warm-up, at fatigue, and one minute after exercise.
• Parameters measured included:
  • pH (acidity/alkalinity)
  • Partial pressure of CO2 (Pco2), corrected for body temperature
  • Total CO2 content
  • Blood ion concentrations: sodium (Na+), potassium (K+), chloride (Cl-), bicarbonate (HCO3-)
  • Lactate concentration (a marker of anaerobic metabolism)
  • Strong Ion Difference (SID), a calculated value that reflects the balance of charged ions and influences acid-base status
  • Oxygen consumption (V˙o2max) and carbon dioxide production (V˙co2) during exercise

Key Findings on Blood Chemistry

• Furosemide caused a pre-exercise hypochloremic alkalosis:
  • Lower chloride levels in both arterial and venous blood
  • Higher blood pH, indicating more alkaline blood
  • Increased bicarbonate (HCO3-) concentration and Strong Ion Difference, both associated with alkalosis
  • A higher percentage of total CO2 was carried as bicarbonate in venous blood
• At fatigue and post-exercise measurements, mixed venous Pco2 was lower after furosemide compared to placebo, reflecting less retention of carbon dioxide.
• Sodium, potassium, and lactate concentrations increased similarly with exercise regardless of treatment.

Effects on Performance

• Maximum oxygen consumption (V˙o2max) and peak carbon dioxide production were not significantly different between treatments, indicating the horses’ aerobic capacity was unchanged.
• Time to fatigue was significantly longer after furosemide administration, averaging about 15 seconds more, representing improved tolerance to high-intensity exercise.
• Furosemide administration also resulted in approximately 2% body weight loss prior to exercise, likely due to its diuretic effect.

Interpretation and Implications

• Furosemide induced a mild systemic alkalosis before exercise, characterized by altered blood ion concentrations and pH elevation.
• This altered acid-base environment may have delayed the onset of muscle acidosis that contributes to fatigue during intense exercise.
• By moderating the degree of acidosis and maintaining a more favorable acid-base balance, furosemide allowed horses to sustain maximal effort longer before fatigue.
• The findings suggest that furosemide’s effects extend beyond its known diuretic action and can improve exercise performance through changes in blood chemistry.
• This information is valuable for understanding how furosemide affects athletic horses and the physiological underpinnings of its performance impact.