The potential of exercise-induced changes in peripheral amino acids to alter blood prolactin levels through a serotonergic system modification was investigated in 8 male athletes. In two trials, subjects (N = 8) exercised on a cycle ergometer for 5 hr. The intensity of exercise corresponded to 55% VO(2)max (T-55 or 75% VO(2)max (T-75), respectively. In each trial, each subject received a 25-g energy bar (111 kcal) every 60 min, as well as 300 ml of a 6% carbohydrate solution (90 kcal) every 30 min of exercise duration. Plasma glucose and insulin declined (p less than or equal to .05) in both trials during exercise. Ammonia was augmented (p less than or equal to .05) above the baseline concentration after 120 min in both trials. During the last 2 hr of exercise, plasma free fatty acids were higher (p less than or equal to .05) in T-75 than in T-55). During this time, the plasma free TRP/BCAA ratio was also augmented (p less than or equal to .05) in T-75, while no change was induced in T-55. Plasma prolactin did not change in T-55, while an increase (p less than or equal to .05) was found in T-75. The findings may further support the hypothesis that during endurance exercise changes in peripheral amino acid concentration may influence prolactin response via serotonergic system modification.
