Effect of naloxone on perceived exertion and exercise capacity during maximal cycle ergometry

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Abstract
We assessed the effects of naloxone, an opioid antagonist, on exercise capacity in 13 men and 5 women (mean age = 30.1 yr, range = 21–35 yr) during a 25 W/min incremental cycle ergometer test to exhaustion on different days during familiarization trial and then after 30 mg (iv bolus) of naloxone or placebo (Pl) in a double-blind, crossover design. Minute ventilation (V˙e), O2 consumption (V˙o 2), CO2 production, and heart rate (HR) were monitored. Perceived exertion rating (0-10 scale) and venous samples for lactate were obtained each minute. Lactate and ventilatory thresholds were derived from lactate and gas-exchange data. Blood pressure was obtained before exercise, 5 min postinfusion, at maximum exercise, and 5 min postexercise. There were no control-Pl differences. The naloxone trial demonstrated decreased exercise time (96% Pl; P < 0.01), total cumulative work (96% Pl; P < 0.002), peakV˙o 2 (94% Pl; P < 0.02), and HR (96% Pl; P < 0.01). Other variables were unchanged. HR and V˙e were the same at the final common workload, but perceived exertion was higher (8.1 ± 0.5 vs. 7.1 ± 0.5) after naloxone than Pl ( P < 0.01). The threshold for effort perception amplification occurred at ∼60 ± 4% of Pl peakV˙o 2. Thus we conclude that peak work capacity was limited by perceived exertion, which can be attenuated by endogenous opioids rather than by physiological limits.