Effect of Acute Administration of Clenbuterol on Athletic Performance in Horses

The aim of the study was to determine the effect of clenbuterol on the anaerobic threshold of horses on a treadmill with increasing physical stress, measuring heart rate (HR) and blood levels of lactate, glucose, and insulin. Twelve Arabian horses were submitted to two physical tests separated by a 10-day interval. Clenbuterol (CL) at 0.8 μg/kg or saline (control—C) was administered intravenously 30 minutes before the test. The treadmill exercise test consisted of an initial warmup followed by a gradually increasing effort. There was no statistical difference in either V₂ or V₄ (velocity at which plasma lactate concentration reached 4 and 2 mmol/L, respectively) between the two experimental groups. For the CL group, V₂₀₀, V₁₈₀, V₁₆₀, and V₁₄₀ (velocity at which the rate heart is 140, 160, 180, and 200 beats/minute, respectively) decreased significantly. At rest as well as times 4, 6, and 10 minutes, insulin levels were higher in the group that received clenbuterol (P < .05). Contrary to what was expected, apparently, there was no improvement in aerobic metabolism in animals when given a therapeutic dose of the bronchodilator. The elevated heart rate observed could have been attributable to the stimulation of cardiac β₁ adrenoceptors and the increased insulin levels to the stimulation of pancreatic β₂ receptors.