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

DOI: 10.1186/2046-7648-2-1

Keywords: Altitude, Exercise, Human, Opioid, Hypoxia

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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.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.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.In both healthy subjects and patients, dyspnoea and leg fatigue are the main symptoms limiting exercise capacity [1-4]. Dyspnoea is accompanied by activation of cortico-limbic structures implicated in interoceptive awareness and nociceptive sensations, such as pain, and involves the opioid system [5]. In patients with dyspnoea, exogenous opioids can alleviate breathing-related discomfort and improve exercise performance [5-8], while injection of naloxone hydrochloride, a non-specific opioid antagonist that crosses the blood–brain barrier, can decrease performance [5]. Opioids can relieve dyspnoea by altering central processing of efferent and afferent sensory information [5]. Sgherza et al. [9] found that in normoxia, in healthy trained subjects, naloxone compared to placebo decreased incremental exercise performance and suggested that sensation of exertion is under influence of endogenous opioids and may be a limiting fac


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