C. N. Olievier

Sex-related Differences in the Influence of Morphine on Ventilatory Control in Humans

Background Opiate agonists have different analgesic effects in male and female patients. The authors describe the influence of sex on the respiratory pharmacology of the micro‐receptor agonist morphine. Methods The study was placebo‐controlled, double‐blind, and randomized. Steady‐state ventilatory responses to carbon dioxide and responses to a step into hypoxia (duration, 3 min; oxygen saturation, [approximately] 82%; end‐tidal carbon dioxide tension, 45 mmHg) were obtained before and during intravenous morphine or placebo administration (bolus dose of 100 micro gram/kg, followed by a continuous infusion of 30 micro gram [center dot] kg sup ‐1 [center dot] h sup ‐1) in 12 men and 12 women. Results In women, morphine reduced the slope of the ventilatory response to carbon dioxide from 1.8 +/‐ 0.9 to 1.3 +/‐ 0.7 l [center dot] min sup ‐1 [center dot] mmHg sup ‐1 (mean +/‐ SD; P < 0.05), whereas in men there was no significant effect (control = 2.0 +/‐ 0.4 vs. morphine = 1.8 +/‐ 0.4 l [center dot] min sup ‐1 [center dot] mmHg sup ‐1). Morphine had no effect on the apneic threshold in women (control = 33.8 +/‐ 3.8 vs. morphine = 35.3 +/‐ 5.3 mmHg), but caused an increase in men from 34.5 +/‐ 2.3 to 38.3 +/‐ 3 mmHg, P < 0.05). Morphine decreased hypoxic sensitivity in women from 1.0 +/‐ 0.5 l [center dot] min sup ‐1 [center dot] % sup ‐1 to 0.5 +/‐ 0.4 l [center dot] min sup ‐1 [center dot] % sup ‐1 (P < 0.05) but did not cause a decrease in men (control = 1.0 +/‐ 0.5 l [center dot] min sup ‐1 [center dot] % sup ‐1 vs. morphine = 0.9 +/‐ 0.5 l [center dot] min sup ‐1 [center dot] % sup ‐1). Weight, lean body mass, body surface area, and calculated fat mass differed between the sexes, but their inclusion in the analysis as a covariate revealed no influence on the differences between men and women in morphine‐induced changes. Conclusions In both sexes, morphine affects ventilatory control. However, we observed quantitative and qualitative differences between men and women in the way morphine affected the ventilatory responses to carbon dioxide and oxygen. Possible mechanisms for the observed sex differences in the respiratory pharmacology of morphine are discussed.

Anesthetic potency and influence of morphine and sevoflurane on respiration in μ-opioid receptor knockout mice

Background The involvement of the &mgr;-opioid receptor (&mgr;OR) system in the control of breathing, anesthetic potency, and morphine- and anesthesia-induced respiratory depression was investigated in mice lacking the &mgr;OR. Methods Experiments were performed in mice lacking exon 2 of the &mgr;OR gene (&mgr;OR−/−) and their wild-type littermates (&mgr;OR+/+). The influence of saline, morphine, naloxone, and sevoflurane on respiration was measured using a whole body plethysmographic method during air breathing and elevations in inspired carbon dioxide concentration. The influence of morphine and naloxone on anesthetic potency of sevoflurane was determined by tail clamp test. Results Relative to wild-type mice, &mgr;OR-deficient mice displayed approximately 15% higher resting breathing frequencies resulting in greater resting ventilation levels. The slope of the ventilation–carbon dioxide response did not differ between genotypes. In &mgr;OR+/+ but not &mgr;OR−/− mice, a reduction in resting ventilation and slope, relative to placebo, was observed after 100 mg/kg morphine. Naloxone increased resting ventilation and slope in both genotypes. Sevoflurane at 1% inspired concentration induced similar reductions in resting ventilation and slope in the two genotypes. Anesthetic potency was 20% lower in mutant relevant to wild-type mice. Naloxone and morphine caused an increase and decrease, respectively, in anesthetic potency in &mgr;OR+/+ mice only. Conclusions The data indicate the importance of the endogenous opioid system in the physiology of the control of breathing with only a minor role for the &mgr;OR. The &mgr;OR gene is the molecular site of action of the respiratory effects of morphine. Anesthetic potency is modulated by the endogenous &mgr;-opioid system but not by the &kgr;- and &dgr;-opioid systems.

The involvement of the μ-opioid receptor in ketamine-induced respiratory depression and antinociception

N-methyl-d-aspartate receptor antagonism probably accounts for most of ketamine’s anesthetic effects; its analgesic properties are mediated partly via N-methyl-d-aspartate and partly via opioid receptors. We assessed the involvement of the &mgr;-opioid receptor in S(+) ketamine-induced respiratory depression and antinociception by performing dose-response curves in exon 2 &mgr;-opioid receptor knockout mice (MOR−/−) and their wild-type littermates (WT). The ventilatory response to increases in inspired CO2 was measured with whole body plethysmography. Two antinociceptive assays were used: the tail-immersion test and the hotplate test. S(+) ketamine (0, 10, 100, and 200 mg/kg intraperitoneally) caused a dose-dependent respiratory depression in both genotypes, with greater depression observed in WT relative to MOR−/− mice. At 200 mg/kg, S(+) ketamine reduced the slope of the hypercapnic ventilatory response by 93% ± 15% and 49% ± 6% in WT and MOR−/− mice, respectively (P < 0.001). In both genotypes, S(+) ketamine produced a dose-dependent increase in latencies in the hotplate test, with latencies in MOR−/− mice smaller compared with those in WT animals (P < 0.05). In contrast to WT mice, MOR−/− mice displayed no ketamine-induced antinociception in the tail-immersion test. These results indicate that at supraspinal sites S(+) ketamine interacts with the &mgr;-opioid system. This interaction contributes significantly to S(+) ketamine-induced respiratory depression and supraspinal antinociception.

influences of Morphine on the Ventilatory Response to Isocapnic Hypoxia

Background: The ventilatory response to hypoxia is composed of the stimulatory activity from peripheral chemoreceptors and a depressant effect from within the central nervous system. Morphine induces respiratory depression by affecting the peripheral and central carbon dioxide chemoreflex loops. There are only few reports on its effect on the hypoxic response. Thus the authors assessed the effect of morphine on the isocapnic ventilatory response to hypoxia in eight cats anesthetized with alpha‐chloralose‐urethan and on the ventilatory carbon dioxide sensitivities of the central and peripheral chemoreflex loops. Methods: The steady‐state ventilatory responses to six levels of end‐tidal oxygen tension (PO2) ranging from 375 to 45 mmHg were measured at constant end‐tidal carbon dioxide tension (PET CO2, 41 mmHg) before and after intravenous administration of morphine hydrochloride (0.15 mg/kg). Each oxygen response was fitted to an exponential function characterized by the hypoxic sensitivity and a shape parameter. The hypercapnic ventilatory responses, determined before and after administration of morphine hydrochloride, were separated into a slow central and a fast peripheral component characterized by a carbon dioxide sensitivity and a single offset B (apneic threshold). Results: At constant PET CO2, morphine decreased ventilation during hyperoxia from 1,260 +/‐ 140 ml/min to 530 +/‐ 110 ml/min (P < 0.01). The hypoxic sensitivity and shape parameter did not differ from control. The ventilatory response to carbon dioxide was displaced to higher PET CO2 levels, and the apneic threshold increased by 6 mmHg (P < 0.01). The central and peripheral carbon dioxide sensitivities decreased by about 30% (P < 0.01). Their ratio (peripheral carbon dioxide sensitivity:central carbon dioxide sensitivity) did not differ for the treatments (control = 0.165 +/‐ 0.105; morphine = 0.161 +/‐ 0.084). Conclusions: Morphine depresses ventilation at hyperoxia but does not depress the steady‐state increase in ventilation due to hypoxia. The authors speculate that morphine reduces the central depressant effect of hypoxia and the peripheral carbon dioxide sensitivity at hyperoxia.

Hypercapnia induces c-fos expression in neurons of retrotrapezoid nucleus in cats

To investigate which neurons in the medulla oblongata produced the nuclear protein FOS during stimulation of respiration by hypercapnia, we subjected six anaesthetized cats to 10% CO2 in air for one hour. Four animals inhaled room air. Coronal sections from the medulla oblongata were processed for FOS immunohistochemistry. Only the retrotrapezoid nucleus (RTN) of the animals exposed to CO2 contained a large population of labelled neurons. This indicates that RTN neurons are strongly activated during hypercapnia.

Antioxidants prevent depression of the acute hypoxic ventilatory response by subanaesthetic halothane in men

We studied the effect of the antioxidants (AOX) ascorbic acid (2 g, I.V.) and α‐tocopherol (200 mg, P.O.) on the depressant effect of subanaesthetic doses of halothane (0.11 % end‐tidal concentration) on the acute isocapnic hypoxic ventilatory response (AHR), i.e. the ventilatory response upon inhalation of a hypoxic gas mixture for 3 min (leading to a haemoglobin saturation of 82 ± 1.8 %) in healthy male volunteers. In the first set of protocols, two groups of eight subjects each underwent a control hypoxic study, a halothane hypoxic study and finally a halothane hypoxic study after pretreatment with AOX (study 1) or placebo (study 2). Halothane reduced the AHR by more than 50 %, from 0.79 ± 0.31 to 0.36 ± 0.14 l min−1 %−1 in study 1 and from 0.79 ± 0.40 to 0.36 ± 0.19 l min−1 %−1 in study 2, P < 0.01 for both. Pretreatment with AOX prevented this depressant effect of halothane in the subjects of study 1 (AHR returning to 0.77 ± 0.32 l min−1 %−1, n.s. from control), whereas placebo (study 2) had no effect (AHR remaining depressed at 0.36 ± 0.27 l min−1 %−1, P < 0.01 from control). In a second set of protocols, two separate groups of eight subjects each underwent a control hypoxic study, a sham halothane hypoxic study and finally a sham halothane hypoxic study after pretreatment with AOX (study 3) or placebo (study 4). In studies 3 and 4, sham halothane did not modify the control hypoxic response, nor did AOX (study 3) or placebo (study 4). The 95 % confidence intervals for the ratio of hypoxic sensitivities, (AOX + halothane) : halothane in study 1 and (AOX ‐ sham halothane) : sham halothane in study 3, were [1.7, 2.6] and [1.0, 1.2], respectively. Because the antioxidants prevented the reduction of the acute hypoxic response by halothane, we suggest that this depressant effect may be caused by reactive species produced by a reductive metabolism of halothane during hypoxia or that a change in redox state of carotid body cells by the antioxidants prevented or changed the binding of halothane to its effect site. Our findings may also suggest that reactive species have an inhibiting effect on the acute hypoxic ventilatory response.

Respiratory depression by Tramadol in the cat: Involvement of opioid receptors

Background Tramadol hydrochloride (tramadol) is a synthetic opioid analgesic with a relatively weak affinity at opioid receptors. At analgesic doses, tramadol seems to cause little or no respiratory depression in humans, although there are some conflicting data. The aim of this study was to examine whether tramadol causes dose-dependent inhibitory effects on the ventilatory carbon dioxide response curve and whether these are reversible or can be prevented by naloxone. Methods Experiments were performed in cats under &agr;-chloralose–urethane anesthesia. The effects of tramadol and naloxone were studied by applying square-wave changes in end-tidal pressure of carbon dioxide (Petco2; 7.5–11 mmHg) and by analyzing the dynamic ventilatory responses using a two-compartment model with a fast peripheral and a slow central component, characterized by a time constant, carbon dioxide sensitivity, time delay, and a single offset (apneic threshold). Results In five animals 1, 2, and 4 mg/kg tramadol (intravenous) increased the apneic threshold (control: 28.3 ± 4.8 mmHg [mean ± SD]; after 4 mg/kg: 36.7 ± 7.1 mmHg;P < 0.05) and decreased the total carbon dioxide sensitivity (control: 109.3 ± 41.3 ml · min−1 · mmHg−1) by 31, 59, and 68%, respectively, caused by proportional equal reductions in sensitivities of the peripheral and central chemoreflex loops. Naloxone (0.1 mg/kg, intravenous) completely reversed these effects. In five other cats, 4 mg/kg tramadol caused an approximately 70% ventilatory depression at a fixed Pet co2 of 45 mmHg that was already achieved after 15 min. A third group of five animals received the same dose of tramadol after pretreatment with naloxone. At a fixed Petco2 of 45 mmHg, naloxone prevented more than 50% of the expected ventilatory depression in these animals. Conclusions Because naloxone completely reversed the inhibiting effects of tramadol on ventilatory control and it prevented more than 50% of the respiratory depression after a single dose of tramadol, the authors conclude that this analgesic causes respiratory depression that is mainly mediated by opioid receptors.

The effect of low-dose acetazolamide on the ventilatory CO2 response curve in the anaesthetized cat

1. The effect of 4 mg kg‐1 acetazolamide (I.V.) on the slope (S) and intercept on the Pa,CO2 axis (B) of the ventilatory CO2 response curve of anaesthetized cats with intact or denervated carotid bodies was studied using the technique of dynamic end‐tidal forcing. 2. This dose did not induce an arterial‐to‐end‐tidal PCO2 (P(a‐ET),CO2) gradient, indicating that erythrocytic carbonic anhydrase was not completely inhibited. Within the first 2 h after administration, this small dose caused only a slight decrease in mean standard bicarbonate of 1.8 and 1.7 mmol l‐1 in intact (n = 7) and denervated animals (n = 7), respectively. Doses of acetazolamide larger than 4 mg kg‐1 (up to 32 mg kg‐1) caused a significant increase in the P(a‐ET),CO2 gradient. 3. In carotid body‐denervated cats, 4 mg kg‐1 acetazolamide caused a decrease in the CO2 sensitivity of the central chemoreflex loop (Sc) from 1.52 +/‐ 0.42 to 0.96 +/‐ 0.32 l min‐1 kPa‐1 (mean +/‐ S.D.) while the intercept on the Pa,CO2 axis (B) decreased from 4.5 +/‐ 0.5 to 4.2 +/‐ 0.7 kPa. 4. In carotid body‐intact animals, 4 mg kg‐1 acetazolamide caused a decrease in the CO2 sensitivity of the peripheral chemoreflex loop (Sp) from 0.28 +/‐ 0.18 to 0.19 +/‐ 0.12 l min‐1 kPa‐1. Se and B decreased from 1.52 +/‐ 0.55 to 0.84 +/‐ 0.21 l min‐1 kPa‐1, and from 4.0 +/‐ 0.5 to 3.0 +/‐ 0.6 kPa, respectively, not significantly different from the changes encountered in the denervated animals. 5. It is argued that the effect of acetazolamide on the CO2 sensitivity of the peripheral chemoreflex loop in intact cats may be caused by a direct effect on the carotid bodies. Both in intact and in denervated animals the effects of the drug on Sc and B may not be due to a direct action on the central nervous system, but rather to an effect on cerebral vessels resulting in an altered relationship between brain blood flow and brain tissue PCO2.

Effects of Morphine and Physostigmine on the Ventilatory Response to Carbon Dioxide

BackgroundIt has been reported that physostigmine antagonizes morphine-induced respiratory depression, but it is not known whether this is due to a central chemoreceptor effect, an effect on the peripheral chemoreflcx loop, or both. We therefore assessed the effect of morphine and physostigmine on the normoxic hypercapnic ventilatory response mediated by the central and peripheral chemoreceptors in ten α-chloralose-urethan-anesthetized cats. MethodsThe breath-by-breath ventilatory responses to stepwlse changes in end-tidal CO2 tension were determined before (control), after administration of morphine hydro-chloride (0.15 mg.kg−1) and during intravenous infusion of physostigmine sallcylate (bolus of 0.05 mg. kg−1 followed by 0.025 mg. kg−1. h−1). Each response was separated into a central and a peripheral chemoreflex characterized by CO2 sensitivity (Sc and Sp), time constant, time delay, and apneic threshold (a single off-set B). ResultsMorphine increased B and decreased Sc and Sp (P < 0.01), but not the ratio Sp/Sc. Subsequent infusion of physostigmine decreased B (P < 0.01), without further change of Sp and Sc. Premedication with physostigmine decreased B, Sp and Sc (P < 0.01) vs. control, but not Sp/Sc. Subsequent administration of morphine decreased Sp and Sc further but increased B (P < 0.01), while Sp/Sc remained constant. ConclusionsBecause morphine diminishes the Sc and Sp of the chemoreflex loop to the same extent this depressant effect is presumably due to an action on the respiratory integrating centers rather than on the peripheral and central chemoreceptors as such and is not antagonized by physostigmine. We argue that the increase in B may be due to changes in the amount of acetylcholine available in the brain and can be antagonized by physostigmine.

Low‐dose acetazolamide reduces CO2‐O2 stimulus interaction within the peripheral chemoreceptors in the anaesthetised cat

1 Using the technique of end‐tidal CO2 forcing, we measured the effect of the carbonic anhydrase inhibitor acetazolamide (4 mg kg−1, i.v.) on the CO2 sensitivities of the peripheral and central chemoreflex loops both during hyperoxia and hypoxia in 10 cats anaesthetised with α‐chloralose‐urethane. 2 In the control situation, going from hyperoxia (arterial PO2 (Pa,O2) 47.40 ± 3.62 kPa, mean ±s.d.) into moderate hypoxia (Pa,O2 8.02 ± 0.30 kPa) led to an almost doubling of the peripheral CO2 sensitivity (SP): a rise from 0.09 ± 0.07 to 0.16 ± 0.06 l min−1 kPa−1. After acetazolamide, however, lowering the Pa,O2 from 46.95 ± 5.19 to 8.02 ± 0.66 kPa did not result in a rise in SP, indicating the absence of a CO2‐O2 stimulus interaction. 3 In hypoxia, acetazolamide reduced SP from 0.16 ± 0.06 to 0.07 ± 0.05 l min−1 kPa−1. In hyperoxia, however, the effect on SP was much smaller (an insignificant reduction from 0.09 ± 0.07 to 0.06 ± 0.05 l min−1 kPa−1). 4 Acetazolamide reduced both the hyperoxic and hypoxic sensitivities (SC) of the central chemoreflex loop: from 0.45 ± 0.16 to 0.27 ± 0.13 l min−1 kPa−1 and from 0.40 ± 0.16 to 0.26 ± 0.13 l min−1 kPa−1, respectively. In hyperoxia, the apnoeic threshold B (X‐intercept of the ventilatory CO2 response curve) decreased from 2.91 ± 0.57 to 0.78 ± 1.9 kPa (P= 0.005). In hypoxia, B decreased from 1.59 ± 1.22 to −0.70 ± 2.99 kPa (P= 0.03). 5 Because acetazolamide abolished the CO2‐O2 interaction, i.e. the expected increase in SP when going from hyperoxia into hypoxia, we conclude that the agent has a direct inhibitory effect on the carotid bodies. The exact mechanism by which the agent exerts this effect will remain unclear until more detailed information becomes available on the identity of the carbonic anhydrase iso‐enzymes within the carotid bodies and their precise subcellular distribution.