Effects of Caffeine on Performance, Cardiorespiratory Function and Plasma Hormonal Responses during Exhaustive Treadmill Exercise in the Thoroughbred Horse

Abstract

The present study investigated the effects of caffeine on performance, cardiorespiratory function and plasma hormonal responses of exercising Thoroughbreds on a treadmill. Five Thoroughbreds completed an incremental graded exercise (IG) test and two endurance exercise tests at 80% and 105% of the maximal O₂ uptake (VO_2max), starting 1 hr after the intramuscular administration of either physiological saline or 2.5 mg/kg caffeine and continuing until exhaustion. The mean exercise time was not significantly affected by caffeine, but there were caffeine-induced inconsistent effects on the individual exercise times in the three exercise tests, which were related to its stimulant effects through the central nervous system (CNS). Caffeine tended to increase blood lactate (LA) and plasma catecholamines (CA) of adrenaline and noradrenaline as well as the cardiorespiratory variables such as O₂ uptake, heart rate and packed cell volume during exercise. Caffeine significantly increased VO_2max and the maximal levels in LA in the IG test. The maximal levels in plasma CA during the IG test were doubled by caffeine. Caffeine-mediated increases in the plasma CA significantly correlated with those in LA. Plasma ACTH levels reflected less exercise stress under caffeine during the endurance exercise test at 105%VO_2max, in which caffeine increased the exercise times of three Thoroughbreds. Caffeine had no effect on the plasma cortisol responses to exercise. The present study suggests that caffeine improves exercise performance through actions on the CNS with the increase in cardiorespiratory function which is probably related to plasma CA responses during exercise.