Plasma beta-endorphin response of thoroughbred horses to maximal exercise

Ten horses underwent a standardised strenuous treadmill exercise test, before, during and after which measurements were made of plasma beta-endorphin and cortisol concentrations, blood lactate, glucose, haemoglobin and pH, the activities of creatine kinase, lactate dehydrogenase and aspartate amino-transferase, and heart rate, oxygen uptake and expired minute volume. The correlations between the exercise-induced response of beta-endorphin and the changes observed in the other physiological measurements were examined. There was a large variation in the beta-endorphin response of the horses to exercise. The increase in beta-endorphin was correlated significantly and inversely with the treadmill velocity at which maximal oxygen uptake was reached. It was also significantly and directly correlated with the heart rate during recovery, the increase in plasma lactate concentration and the change in blood pH, indicating that the exercise-induced increase in beta-endorphin concentration was smaller in horses with a higher aerobic capacity.