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. Keywords:Altitude, Exercise, Human, Opioid, Hypoxia Background 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 dis- comfort 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 dys- pnoea by altering central processing of efferent
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 dis- comfort 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 dys- pnoea 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 factor for maximum aerobic exercise capacity. Acute hypoxia is a potent stressor, especially when combined with an exercise challenge, changing the per- ceived level of exertion [10]. Acute exposure to hypoxia increases ventilation and cardiac output in order to min- imise the reduction in arterial oxygen content and sys- temic mass oxygen transport. Despite these acute adaptations, incremental exercise testing in such condi- tions (e.g. an FiO 2equivalent to an altitude of 5,000 m or higher, hereafter referred to as severe hypoxia) invari- ably results in compromised aerobic exercise capacity. The mechanisms behind this limitation of exercise * Correspondence:bengt.kayser@unige.ch 2 Institute of Movement Sciences and Sports Medicine, Faculty of medicine, University of Geneva, 10, rue du Conseil Général, Genève 4 1205, Switzerland Full list of author information is available at the end of the article © 2013 Koglin and Kayser; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Koglin and KayserExtreme Physiology & Medicine2013,2:1 http://www.extremephysiolmed.com/content/2/1/1
capacity in hypoxia are still poorly understood [11-13]. A puzzling observation is that despite maximum effort in severe hypoxia, cardiac output remains submaximal, suggesting early motor drive withdrawal [11,12]. How- ever, while the locomotor muscles are not driven as hard during large muscle volume effort in such conditions, the contrary is the case for the respiratory muscles. Ventilation is higher for any given level of oxygen con- sumption during exercise in hypoxia, as compared to normoxia, and is accompanied by a concomitant in- crease in the sensation of breathing effort [10]. Since blocking the effect of endogenous opioids on the sensa- tion of exertion can increase symptom intensity and cur- tail performance in normoxia [9], it is possible that such an effect would be exacerbated in hypoxia. We therefore hypothesised that blocking opioid receptors may de- crease aerobic exercise performance more in hypoxia than in normoxia. To test this hypothesis, we compared incremental cycling exercise in conditions of normoxia and hypoxia (11% O 2) after injection of the opioid antagonist naloxone in comparison to a placebo. Our expecta- tions were that under naloxone the sensation of breathing effort and that of cycling would increase and curtail exercise performance more in hypoxia as compared to normoxia. Methods Subjects Thirteen healthy trained men volunteered to participate in the study. Inclusion criteria were an age between 18 and 30 years and >4 h/week of endurance training. Ex- clusion criteria were presence of any relevant chronic or acute disease, having done a maximal capacity training or a race during the last 48 h, symptoms/signs of viral illness in the week preceding the experiments or expos- ure to altitude in the 2 months before the study. The study was approved by the research ethics commission of the Geneva University Hospitals and SwissMedic and com- plied with the principles of the Declaration of Helsinki. All subjects were screened by a physician, were fully informed of the nature and risks of the experiments, knew of their right to withdraw at any time and signed an informed consent form. Exercise protocol After inclusion, the subjects first performed two
of the Geneva University Hospitals and SwissMedic and com- plied with the principles of the Declaration of Helsinki. All subjects were screened by a physician, were fully informed of the nature and risks of the experiments, knew of their right to withdraw at any time and signed an informed consent form. Exercise protocol After inclusion, the subjects first performed two habitu- ation experiments, identical to the real experiments ex- cept for the injection, to get used to the equipment and to experience cycling in acute hypoxia. They were instructed not to do any heavy training on the days pre- ceding the experiments and to refrain from caffeine in the 4 h preceding a test. To control for circadian rhythm, for a given subject, all experiments were per- formed at similar times of the day. After the habituation tests, the subjects came to the lab on four different occa- sions at least 24 h apart to perform incremental cycling exercise until voluntary exhaustion, twice in normoxia and twice in acute normobaric hypoxia (FIO 2= 10.65% O 2, in Geneva the equivalent of an altitude of approxi- mately 5,000 m). The subjects cycled at 80 rpm on a mechanically braked ergometer (Monark 282E, Varberg, Sweden). After 3 min of resting baseline measurements, the subjects would warm up at 40 watt for 3 min. In order to obtain similar durations of exercise times be- tween subjects and between conditions of normoxia and hypoxia, the duration of steps was adapted between con- ditions and stages. After warm-up, the subjects incre- mented by 3-min steps of 40 up to 200 watt in normoxia and then by 20 watt up to voluntary exhaustion, while in hypoxia, they incremented by 40 up to 120 watt and then by 20 watt up to voluntary exhaustion. When reaching exhaustion in hypoxia, the subjects were switched to room air (‘normoxia switch’) and strongly encouraged to continue cycling while the load was increased by 20 watt every 90 s until reaching secondary voluntary exhaustion. Intervention In each condition, they did this once after immediate pre-exercise intravenous injection of naloxone (30 mg
by 20 watt up to voluntary exhaustion. When reaching exhaustion in hypoxia, the subjects were switched to room air (‘normoxia switch’) and strongly encouraged to continue cycling while the load was increased by 20 watt every 90 s until reaching secondary voluntary exhaustion. Intervention In each condition, they did this once after immediate pre-exercise intravenous injection of naloxone (30 mg naloxone HCl in 30 ml saline) and once after placebo (30 ml saline). The conditions normoxia and hypoxia were in a randomised order. Naloxone and placebo were administered in a balanced double-blind crossover de- sign. The research support section of the Geneva Univer- sity Hospitals pharmacy prepared the vials, the numbering scheme and the randomization envelopes and released the randomization key after data analysis was completed. The subjects started exercising within 5 min after injection and reached exhaustion within 30 min, approximately one half oftheserumhalf-lifeofnaloxonereportedinhumans[14]. Normoxia and hypoxia Normobaric hypoxia was obtained by mixing N 2into ambient air under control of FIO 2(Altitrainer, SMTec, Nyon, Switzerland). The gas-mixing system was attached via a piece of large-bore low-resistance tubing to the in- spiratory valve of a low-resistance three-way valve (Hans Rudolph 2700, Shawnee, KS, USA) mounted in series with a turbine flow measurement set-up (Vmax 29c, Sensormedics, Loma Linda, CA, USA) attached to a tightly fitted face mask (Hans Rudolph). The subjects al- ways breathed through the same set-up, also in nor- moxia. The gas-mixing device was set to room air for the normoxia experiments and to a simulated altitude of 5,000 m for the hypoxia experiments. Koglin and KayserExtreme Physiology & Medicine2013,2:1 Page 2 of 10 http://www.extremephysiolmed.com/content/2/1/1
Material and measurements Gas exchange and breathing parameters were measured breath-by-breath with a metabolic cart (Vmax 29c, Sensormedics). Prior to each experiment, the system was calibrated with a 3-L syringe and gas mixtures of known composition. Heart rate was measured by telemetry with a thoracic belt (Polar, Tampere, Finland). Arterial blood haemoglobin saturation (SpO 2) was measured on an ear- lobe with a pulse oximeter (Ohmeda, Helsinki, Finland) connected to the metabolic cart. Arterialized blood from a hyperaemic earlobe (Trafuril Cream, Ciba-Geigy, Basel, Switzerland) was used to measure lactate (Accutrend, Roche, West Sussex, UK). To quantify locomotor muscle activation, a surface electromyogram (EMG) was obtained from the right vastus lateralis muscle. After cleaning with ether and light abrading of the skin, two electrodes (Kendall H59P, Mansfield, OH, USA) were applied directly next to each other on the distal part of the muscle. In addition to marking the skin with indelible ink, we used transparent foil to mark the sites of the electrodes together with skin marks such as moles and scars to reposition the electrodes on the same sites between sessions. Inter-electrode resist- ance was measured and considered acceptable if <3 kΩ.A reference electrode was placed over a bony area near the knee. The signal was amplified, filtered with a Butterworth band pass between 10 and 200 Hz (BMA-830, CWE, Ardmore, OK, USA), digitised at 1,000 Hz with an AD- board (NI-Daqcard, National Instruments, Austin, TX, USA) and stored on a computer. The data were analysed post hocwith custom routines in Matlab (Matlab, Natick, MA, USA) to obtain, for each single contraction, the inte- grated rectified EMG (iEMG) and, after a fast-Fourier transformation, the median (i.e. centroid, CPF) and mean power (MPF) frequencies, as described before [15]. iEMG was normalised with the signal obtained at 80 watt. Perception of exertion The subjects were asked to rate the rate of perceived ex- ertion (RPE) on a 0–10-point CR-10 Borg ratio scale [2]. At the end of each exercise level, the subjects rated their perception of exertion separately for their legs (How hard is it to cycle?), breathing (How hard is it
with the signal obtained at 80 watt. Perception of exertion The subjects were asked to rate the rate of perceived ex- ertion (RPE) on a 0–10-point CR-10 Borg ratio scale [2]. At the end of each exercise level, the subjects rated their perception of exertion separately for their legs (How hard is it to cycle?), breathing (How hard is it to breathe?) and overall (How hard is the overall effort?). The anchors were 0 for no exertion at all and 10 for the maximum imaginable. When prompted, the subject would point to the scale and nod when the experimenter called the correct corresponding number out loud. Analysis and statistics For each subject and each condition, the data were aver- aged over the last 30 s of each workload with the excep- tion of the maximum when a mean over 15 s was used. The data were analysed with SPSS version 18 (IBM, Chicago, IL, USA). Repeated measures ANOVA was used to test for within-group effects across time. Following sig- nificant main effects, planned pairwise comparisons were made using Holm's sequential Bonferroni procedure. Results are expressed as mean ± SD. Statistical signifi- cance was set atP<0.05. Results Of the 13 recruited subjects, 1 dropped out after the first habituation test (no reason given), 2 subjects were excluded for a vagal reaction to hypoxia at rest, and 10 completed the study (age 29 ± 2 years (mean ± SD), height 183 ± 6 cm, weight 75 ± 7 kg, maximum aerobic capacity (V'O 2max) 50 ± 8 ml/kg/min). The injection of naloxone was well tolerated. Table 1 shows the results observed at exhaustion in the different conditions. Hypoxia decreased power output by 39% in the pla- cebo condition (P< 0.001) and by 37% in the naloxone condition (P< 0.001, no significant difference between conditions). Switching to normoxia at exhaustion in hyp- oxia allowed continuing up to 97% of power developed in normoxic control under placebo (P< 0.001) and to al- most 100% under naloxone (P< 0.001, no significant dif- ference between conditions). V'O 2peak was 3.77 ± 0.80 L/min (placebo) and 3.85
37% in the naloxone condition (P< 0.001, no significant difference between conditions). Switching to normoxia at exhaustion in hyp- oxia allowed continuing up to 97% of power developed in normoxic control under placebo (P< 0.001) and to al- most 100% under naloxone (P< 0.001, no significant dif- ference between conditions). V'O 2peak was 3.77 ± 0.80 L/min (placebo) and 3.85 ± 0.76 L/min (naloxone) in normoxia, reached 2.39 ± 0.4 L/min in hypoxia (both placebo and naloxone) and increased again to 94% (placebo) and 95% (naloxone) of normoxia values after the normoxia switch at exhaustion from hypoxia (no significant difference between condi- tions). Time to exhaustion was similar in normoxia and hypoxia (22 ± 3 min) and reached a total of 27 ± 4 min for hypoxia when adding the additional exercise time after the normoxia switch (no effects of naloxone). Figure 1 shows the overall levels of perceived exertion and those pertaining to breathing and leg effort separ- ately. In normoxia, perceived exertion increased in a curvilinear way, and there was no effect of naloxone. The rate of increase was greater in hypoxia, dropped upon switching to acute normoxia at exhaustion, and increased again towards exhaustion (no effect of naloxone). The top panel of Figure 2 shows the increase in mi- nute ventilation (V' E) with exercise intensity and its more pronounced increase in hypoxia. After the nor- moxia switch, ventilation dropped but not completely to the normoxic level. There was no effect of naloxone. The second panel of Figure 2 shows the evolution of P ETCO2. In conditions of normoxia, the typical pattern of a slight increase followed by a drop beyond the venti- latory threshold was observed. Conversely, hypoxia im- mediately induced hyperventilation and reduced P ETCO2 values, which were not restored upon the normoxia switch. There was no effect of naloxone. The third panel Koglin and KayserExtreme Physiology & Medicine2013,2:1 Page 3 of 10 http://www.extremephysiolmed.com/content/2/1/1
of Figure 2 shows the evolution of respiratory frequency, which followed a similar pattern as that of ventilation and tidal volume (not shown), without any effect of naloxone. iEMG was higher during hypoxia (Figure 3). No differ- ences were observed in CPF or MPF (not shown). At ex- haustion in hypoxia, after the hypoxia switch, iEMG increased and reached higher values at exhaustion com- pared to normoxia. There were no effects of naloxone. Blood lactate levels (Figure 3) increased in a typical curvilinear manner both in normoxia and hypoxia, with an early onset of the exponential increase in hypoxia. There was no effect of naloxone. SpO 2showed a slight drop at higher intensities in con- ditions of normoxia, whereas it dropped right from the start of exercise in hypoxia. Upon the normoxia switch, it normalised rapidly. There was no effect of naloxone. SpO 2reached lower values in hypoxia, dropping to 68% with placebo and 64% with naloxone (no significant dif- ference), and increased after the normoxia switch. Heart rate response to exercise showed the typical linear in- crease with a steeper slope in hypoxia, a drop at the nor- moxia switch and similar maximum heart rates at exhaustion (no effects of naloxone; data not shown). Figure 4 shows the relationship between ventilation and RPE breathing, and iEMG and RPE legs. Both rela- tionships were slightly displaced to the left in hypoxia compared to normoxia, but there was no effect of naloxone. Discussion Contrary to our expectations naloxone had no effect on any of the investigated variables, neither in normoxia nor in hypoxia. It follows that, at least in trained healthy young male subjects, during incremental exercise tests, in normoxia and normobaric hypoxia with an FIO 2of 10.65 (equivalent to approximately 5,000 m), endogen- ous opioid receptors are not involved in the ventilatory and heart rate responses to exercise nor in the sensation of overall levels of perceived exertion or those pertaining to breathing or cycling effort specifically. Naloxone dosage Could it be that there was insufficient blockade of opioid receptors? This seems unlikely. The‘normal’dose for clin- ical use of naloxone
5,000 m), endogen- ous opioid receptors are not involved in the ventilatory and heart rate responses to exercise nor in the sensation of overall levels of perceived exertion or those pertaining to breathing or cycling effort specifically. Naloxone dosage Could it be that there was insufficient blockade of opioid receptors? This seems unlikely. The‘normal’dose for clin- ical use of naloxone is 1–4 mg, largely sufficient for full reversal of the effects of exogenously administered opioids and to trigger withdrawal symptoms [16]. Naloxone hydrochloride is partly actively transported through the blood–brain barrier and reaches higher central nervous system (CNS) concentrations than in the plasma [17]. Positron emission tomography studies showed that with 1 mg naloxone, 50% of opioid receptors in the CNS were blocked [18]. Santiago and Edelman [19] recommended a minimal dose of 0.1 mg/kg for peripheral and central receptor blockade. We used 30 mg, i.e. 0.40 ± 0.04 mg/kg, a dose that is four times in excess, to compare our results tothoseofapreviousstudy[9].Naloxonehasahalf-lifeof about 1 h [14]. Our subjects started exercising within 5 min Table 1 Peak values at exhaustion in normoxia, hypoxia and after the normoxia switch at exhaustion in hypoxia Normoxia Hypoxia Normoxia switch after hypoxia Placebo Naloxone Placebo Naloxone Placebo Naloxone Power (watt) 296 ± 48 292 ± 52 182 ± 36 * 184 ± 34 * 282 ± 51 291 ± 49 Time (min) 22.5 ± 2.6 22.3 ± 2.4 22.0 ± 3.8 22.4 ± 3.7 27.1 ± 4.2 27.6 ± 3.9 HR (/min) 184 ± 5 181 ± 7 169 ± 7 * 168 ± 7 * 173 ± 5 170 ± 6 Lactate (mM) 11.3 ± 3.7 10.5 ± 2.4 10.4 ± 2.9 10.3 ± 3.7 11.3 ± 3.0 10.9 ± 5.1 RPE global (a.u.) 9.7 ± 0.5 9.6 ± 0.5 9.4 ± 0.7 9.7 ± 0.5 9.3 ± 0.9 9.4 ± 0.5 RPE resp (a.u.) 9.5 ± 0.7 9.4 ± 0.9 9.5 ± 0.6 9.5 ± 0.7 9.0 ± 1.1 8.9 ± 0.4 RPE legs (a.u.) 9.9 ± 0.3 9.7 ± 0.7 9.6 ± 0.5 9.8 ± 0.4 10.0 ± 0.0 10.0 ±
(a.u.) 9.7 ± 0.5 9.6 ± 0.5 9.4 ± 0.7 9.7 ± 0.5 9.3 ± 0.9 9.4 ± 0.5 RPE resp (a.u.) 9.5 ± 0.7 9.4 ± 0.9 9.5 ± 0.6 9.5 ± 0.7 9.0 ± 1.1 8.9 ± 0.4 RPE legs (a.u.) 9.9 ± 0.3 9.7 ± 0.7 9.6 ± 0.5 9.8 ± 0.4 10.0 ± 0.0 10.0 ± 0.0 SaO 2(%) 91.7 ± 4.1 93.3 ± 6.4 67.5 ± 8.9 * 63.6 ± 8.8 * 90.55± 95.70± P ETCO2(kPa) 4.44 ± 0.70 4.56 ± 0.48 3.50 ± 0.18 * 3.49 ± 0.26 * 4.48 ± 0.32 4.42 ± 0.40 V'O 2(L/min) 3.77 ± 0.8 3.85 ± 0.76 2.39 ± 0.40 * 2.39 ± 0.40 * 3.53 ± 0.71 3.64 ± 0.63 V'CO 2(L/min) 4.92 ± 1.1 4.86 ± 0.80 3.19 ± 0.53 * 3.30 ± 0.57 * 3.86 ± 0.78 4.05 ± 0.85 V' E(L/min) 150 ± 33 146 ± 21 128 ± 20 * 133 ± 24 * 123 ± 21 132 ± 28 V' A(L/min) 159 ± 43 154 ± 27 133 ± 26 * 137 ± 37 * 126 ± 27 138 ± 37 Vt (L) 2.98 ± 0.39 3.16 ± 0.41 2.89 ± 0.51 2.97 ± 0.72 2.91 ± 0.45 3.09± RR (/min) 50 ± 8 47 ± 7 45 ± 7 44 ± 7 42 ± 6 43 ± 5 MEFR (L/sec) 5.1 ± 1.2 5.0 ± 0.8 4.2 ± 0.7 4.3 ± 0.9 4.1 ± 0.8 4.4 ± 1.0 Naloxone had no effect on any of these parameters in either condition. Power, mechanical power output on cycle ergometer; time, time to exhaustion; HR, heart rate; lactate, arterialized lactate concentration; RPE, rate of perceived exertion (overall, respiratory, legs); SaO 2, earlobe oximetry; PETCO2, end-tidal CO2; V'O2, oxygen consumption; V'CO 2, expired CO2;V'E, minute ventilation; V'A, alveolar ventilation; Vt, tidal volume; RR, respiratory frequency; MEFR, peak expiratory flow. * Significantly different from normoxia. Koglin and KayserExtreme Physiology & Medicine2013,2:1 Page 4 of 10 http://www.extremephysiolmed.com/content/2/1/1
end-tidal CO2; V'O2, oxygen consumption; V'CO 2, expired CO2;V'E, minute ventilation; V'A, alveolar ventilation; Vt, tidal volume; RR, respiratory frequency; MEFR, peak expiratory flow. * Significantly different from normoxia. Koglin and KayserExtreme Physiology & Medicine2013,2:1 Page 4 of 10 http://www.extremephysiolmed.com/content/2/1/1
Description
The study investigates the effects of naloxone on breathing sensation during cycling in hypoxia.