A Catecholamine Precursor Does Not Influence Exercise Performance in Warm Conditions

Purpose Acute doses of Sinemet (R) (l-DOPA combined with carbidopa) previously failed to influence prolonged exercise performance in a temperate environment, but it is not known whether acute doses of l-DOPA timed to reach maximum plasma concentrations (C-max) during exercise will improve prolonged cycling performance in warm conditions (30.2 degrees C +/- 0.2 degrees C, 50% +/- 1%). Methods Ten physically active men (age, 26 +/- 4 yr; height, 1.76 +/- 0.08 m; body mass, 76.3 +/- 10.6 kg; VO2peak, 57 +/- 8 mL center dot kg(-1)center dot min(-1)) were recruited for this study. Participants cycled for 1 h at 60% VO2peak followed by a 30-min exercise test, during which they were instructed to complete as much work as possible. Heart rate, skin and core temperatures, as well as RPE and thermal stress were recorded throughout the exercise, and blood samples were collected at rest, at 15-min intervals during the first hour of exercise, and at the end of the exercise test. Finger tapping tests at the beginning and end of the exercise were conducted to examine fine motor control. Results There was no significant difference in the work done on the placebo (314 +/- 43 kJ) and l-DOPA trials (326 +/- 48 kJ, P = 0.276). Prolactin concentrations were increased at the end of the exercise in all trials (P < 0.001), but this response was attenuated at the end of the exercise for the l-DOPA trial (11.4 +/- 5.5 ng center dot mL(-1)) and placebo trials (20.8 +/- 3.3 ng center dot mL(-1), P = 0.003). No differences between trials were found for any other measure. Conclusions The results suggest that increasing central catecholamine availability inhibits the normal prolactin response to exercise in the heat but does not alter performance, thermoregulation, or sympathetic outflow.