Effects of exercise on plasma concentrations of caffeine and its metabolites in horses.

The effects of exercise on the metabolism of caffeine (CA) were studied 3 h after administration of the drug to race horses which then underwent exercise sets (1000-m gallop). Analysis was made of pharmacokinetics of CA, changes in its plasma concentrations, its metabolites, i.e., theophylline (TP), theobromine (TB) and paraxanthine (PX), and the molar concentration ratios of CA to these metabolites. After exercise, AUC and t1/2 tended to decrease, and the concentration of CA decreased, while the concentrations of TP and TB significantly increased. The TP/CA ratio and TB/CA ratio significantly increased from 6 to 74h and from 25 to 50h after drug administration, respectively. This indicated promotion of metabolism of CA into TP and TB. The effects on PX were minimal.