Acute norepinephrine reuptake inhibition decreases performance in normal and high ambient temperature.

Department of Human Physiology and Sports Medicine, Faculty of Physical Education and Physiotherapy, Vrije Universiteit Brussel, Brussels, Belgium.
Journal of Applied Physiology (Impact Factor: 3.43). 08/2008; 105(1):206-12. DOI: 10.1152/japplphysiol.90509.2008
Source: PubMed

ABSTRACT Combined inhibition of dopamine (DA)/norepinephrine (NE) reuptake improves exercise performance and increases core temperature in the heat. A recent study demonstrated that this effect may primarily be related to increased DA activity. NE reuptake inhibition (NERI), however, has received little attention in humans, certainly in the heat, where central fatigue appears to be a main factor influencing performance. Therefore the present study examines the effect of NERI (reboxetine) on exercise capacity, thermoregulation, and hormonal response in normal and high temperature. Nine healthy well-trained male cyclists participated in this study. Subjects ingested either placebo (Pla; 2 x 8 mg) or reboxetine (Rebox; 2 x 8 mg). Subjects exercised in temperate (18 degrees C) or warm (30 degrees C) conditions and cycled for 60 min at 55% W(max) immediately followed by a time trial (TT; Pla18/Rebox18; Pla30/Rebox30) to measure exercise performance. Acute NERI decreased power output and consequently exercise performance in temperate (P = 0.018) and warm (P = 0.007) conditions. Resting heart rate was significantly elevated by NERI (18 degrees C: P = 0.02; 30 degrees C: P = 0.018). In Rebox18, heart rate was significantly higher than in the Pla18, while in the heat no effect of the drug treatment was reported during exercise. In Rebox30, all hormone concentrations increased during exercise, except for growth hormone (GH), which was significantly lower during exercise. In Rebox18, prolactin (PRL) concentrations were significantly elevated; GH was significantly higher at rest, but significantly lower during exercise. In conclusion, manipulation of the NE system decreases performance and modifies hormone concentrations, thereby indicating a central NE effect of the drug. These findings confirm results from previous studies that predominantly increased DA activity is important in improving performance.

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    ABSTRACT: Acute oral tyrosine administration has been associated with increased constant-load, submaximal exercise capacity in the heat. This study sought to determine whether self-paced exercise performance in the heat is enhanced with the same tyrosine dosage. Following familiarisation, seven male endurance-trained volunteers, unacclimated to exercise in the heat, performed two experimental trials in 30°C (60% relative humidity) in a crossover fashion separated by at least 7 days. Subjects ingested 150 mg·kg body mass tyrosine (TYR) or an isocaloric quantity of whey powder (PLA) in 500 mL of sugar-free flavoured water in a randomised, double-blind fashion. Sixty minutes following drink ingestion subjects cycled for 60 min at 57 ± 4% peak oxygen uptake (O2peak), then performed a simulated cycling time-trial requiring completion of an individualised target work quantity (393.1 ± 39.8 kJ). The ratio of plasma tyrosine plus phenylalanine (tyrosine precursor): amino acids competing for brain uptake (free-tryptophan, leucine, isoleucine, valine, methionine, threonine, lysine) increased 2.5-fold from rest in TYR, and remained elevated throughout exercise (P < 0.001), whereas it declined in PLA from rest to pre-exercise (P = 0.004). Time-trial power output (P = 0.869) and performance (34.8 ± 6.5 min and 35.2 ± 8.3 min in TYR and PLA, respectively; P = 0.4167) were similar between trials. Thermal sensation (P > 0.05), RPE (P > 0.05), core temperature (P = 0.860), skin temperature (P = 0.822) and heart rate (P = 0.314) did not differ between trials. These data indicate that acute tyrosine administration did not influence self-paced endurance exercise performance in the heat. Plasma tyrosine availability is apparently not a key determinant of fatigue processes under these conditions.
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