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Articles in PresS, published online ahead of print August 23, 2002
J Appl Physiol, 10.1152/jap.00521.2002
Submitted on June 13, 2002
Accepted on August 19, 2002
1 Joan and Joel Smilow Cardiac prevention and Rehabilitation Center, The Rusk Institute of Rehabilitation Medicine and Department of Medicine, Division of Pulmonary and Critical Care Medicine, New York University School of Medicine, New York, New York, USA; Physical Education and Exercise Science, Brooklyn College of CUNY, New York, New York, USA
2 Joan and Joel Smilow Cardiac prevention and Rehabilitation Center, The Rusk Institute of Rehabilitation Medicine and Department of Medicine, Division of Pulmonary and Critical Care Medicine, New York University School of Medicine, New York, New York, USA
3 Physical Education and Exercise Science, Brooklyn College of CUNY, New York, New York, USA
* To whom correspondence should be addressed. E-mail: francois.haas{at}med.nyu.edu.
Purpose: To assess the effects of naloxone, an opioid antagonist, on exercise capacity. Method: Thirteen men and 5 women (mean age = 30.1, range = 21-35 years) performed 25 watts/min incremental cycle ergometer tests to exhaustion on three days: 1) a familiarization trial, 2) following an intravenous bolus of 30 mg of naloxone or following a placebo. The order of the last 2 days was determined by a double-blind cross-over design. Minute ventilation (VE), O2 consumption (VO2), CO2 production (V CO2), heart rate (HR) were monitored. Perceived exertion rating (0-10 scale) and venous blood samples for lactate (La-) analysis were obtained each minute. Lactate threshold (LT) and ventilatory threshold (VT) were derived from lactate and gas exchange data, respectively. Blood pressure was obtained before exercise, 5 minutes after infusion, at maximum exercise, and 5 minutes postexercise. Results: There were no significant differences observed between the control and placebo trials. Comparing naloxone to placebo data or control data , however, demonstrated a decrease in: exercise time (96% of placebo, P<0.01), total cumulative work (96% of placebo, P<0.002), VO2peak (94% of placebo, P<0.02), and HR (96% of placebo P<0.01). No differences were found in LT, VT, or blood pressure. In contrast, at the final common work load where load, HR, and VE were the same, perceived exertion was higher (8.1± 0.5 vs 7.1 ± 0.4) following naloxone than placebo infusion (P<0.01). The threshold for effort perception amplification occurred at ~60 ± 4% of the placebo VO2peak. Conclusion: Since subjects stopped exercise at levels below their measured maximum total cumulative work, VO2peak, and HR following naloxone, these data indicate that peak work capacity was limited by the individual's perceived exertion--which can be attenuated by endogenous opioids--rather than by physiological limits.
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