Control and sensation of breathing during cycling exercise in hypoxia under naloxone: a randomised controlled crossover trial

Abstract

Background: Opioid receptors are possibly involved in the perception of exertion and the ventilatory response to exercise. We compared incremental cycling exercise in conditions of normoxia and hypoxia (11% O2) after injection of the opioid receptor antagonist naloxone (30 mg i.v.) or placebo. Naloxone was expected to increase sensation of breathing and cycling and to curtail exercise performance more in hypoxia.

Methods: Ten healthy subjects (29 ± 2 years, 183 ± 6 cm, 75 ± 7 kg, mean ± SD) cycled in normoxia and hypoxia until voluntary exhaustion, receiving naloxone or placebo in a balanced double-blind crossover design.

Results: Hypoxia decreased peak power output by 37%-39% with placebo and naloxone (P < 0.001, no effect of naloxone). Switching to normoxia at exhaustion in hypoxia allowed continuing up to 97%-100% of power developed in normoxia with placebo and naloxone (P < 0.001, no effect of naloxone). Perceived exertion increased in hypoxia, dropped upon switching to normoxia and increased again towards exhaustion, no effect of naloxone. SpO2 (earlobe oximetry) was lower in hypoxia, dropping to 64%-68% with naloxone and placebo. The ventilatory response to exercise in normoxia and hypoxia was not changed by naloxone.

Conclusions: It follows that in healthy subjects the ventilatory response and the perception of exertion in hypoxia as compared to normoxia do not involve the endogenous opioid system, and the latter does not play a role in limiting maximum exercise capacity in hypoxia.

Figure 1

The top panel shows the overall RPE vs. power output. At exhaustion from exercise in hypoxia, the subjects were switched to room air and strongly encouraged to continue pedalling until reaching secondary exhaustion. The arrows indicate the measurements after the switch; for reasons of legibility, the arrows are not shown on the other graphs. The middle panel shows the RPE breathing vs. power output. The bottom panel shows the RPE legs vs. power output. Error bars were omitted for clarity. The coloured symbols and lines represent the same conditions for all figures.

Figure 2

The top panel shows the total ventilation (V'E) vs. power output. The middle panel shows the end-tidal CO₂ tension (P_ETCO₂) vs. power output. The bottom panel shows the respiratory frequency vs. power output.

Figure 3

The top panel shows normalised integrated vastus lateralis surface electromyogram (iEMG) vs. power output. The middle panel presents the arterialized blood lactate concentration [La] vs. power output. The bottom panel presents the arterial oxygen saturation (earlobe oximetry) vs. power output.

Figure 4

The top panel shows RPE breathing vs. total ventilation. The bottom panel shows RPE legs vs. iEMG.