Effect of PO2 on growth and physiological characteristics of Candida utilis in a multistage tower fermentor

The effect of increasing the partial pressure of oxygen in the aeration gas on growth and physiological activity of the yeast Candida utilis in a multistage tower fermentor was studied. The measurements were made at steady states of continuous culture for single values of dilution rate, temperature, and pH in all stages of the fermentor and with one given ethanol concentration in the growth medium feed. The partial pressure of oxygen in the gas phase was changed in the range from 165 to 310 torr. The results revealed the existence of the upper critical value of the partial oxygen pressure in the gas phase. It was demonstrated that the upper critical value of PO ₂ influences not only the growth rate, biomass yield, and productivity but also the cell physiology resulting in changes of respiration activity and activity of alcohol and aldehyde dehydrogenases.     

Effect of O2 on vesicle formation, acetylene reduction, and O2-uptake kinetics in Frankia sp. HFPCcI3 isolated from Casuarina cunninghamiana

The effect of the partial pressure of oxygen (PO2) on the formation of vesicles, which are thought to be the site of N2 fixation in Frankia, was studied in HFPCcI3, an effective isolate from Casuarina cunninghamiana. Unlike other actinorhizal root nodules, vesicles are not produced by the endophyte in Casuarina nodules. However, in culture under aerobic conditions, large, phase-bright vesicles are formed in HFPCcI3 within 20 h following removal of NH+4 from the culture medium and reach peak numbers within 72 to 96 h. In vivo acetylene reduction activity parallels vesicle formation. Optimum rates of acetylene reduction in short-term assays occurred at 20% O2 (0.2 atm (1 atm = 101.325 kPa] in the gas phase. O2 uptake (respiration) determined polarographically showed diffusion-limited kinetics and remained unsaturated by O2 until 300 microM O2. In contrast, respiration in NH+4-grown cells was saturated by O2 between 8 and 10 microM O2. These results indicate the presence of a diffusion barrier associated with the vesicles. Vesicle development was repressed in cells incubated in N-free media sparged with gas mixtures with PO2 between 0.001 and 0.003 atm. Nitrogenase was induced under these conditions, but acetylene reduction was extremely O2 sensitive. The kinetics of O2 uptake as a function of dissolved O2 concentration in avesicular cells were similar to those in NH+4-grown cells indicating the lack of a diffusion barrier. These results demonstrate that vesicle formation and the development of the O2 protection mechanisms of nitrogenase are regulated by ambient PO2 in HFPCcI3.     

Acclimation of a terrestrial plant to submergence facilitates gas exchange under water

Flooding imposes stress upon terrestrial plants since it severely hampers gas exchange rates between the shoot and the environment. The resulting oxygen deficiency is considered to be the major problem for submerged plants. Oxygen microelectrode studies have, however, shown that aquatic plants maintain relatively high internal oxygen pressures under water, and even may release oxygen via the roots into the sediment, also in dark. Based on these results, we challenge the dogma that oxygen pressures in submerged terrestrial plants immediately drop to levels at which aerobic respiration is impaired. The present study demonstrates that the internal oxygen pressure in the petioles of Rumex palustris plants under water is indeed well above the critical oxygen pressure for aerobic respiration, provided that the air‐saturated water is not completely stagnant. The beneficial effect of shoot acclimation of this terrestrial plant species to submergence for gas exchange capacity is also shown. Shoot acclimation to submergence involved a reduction of the diffusion resistance to gases, which was not only functional by increasing diffusion of oxygen into the plant, but also by increasing influx of CO₂, which enhances underwater photosynthesis.     

NO synthesis, unlike respiration, influences intracellular oxygen tension.

We have developed a new phosphorescent probe, PdTCPPNa(4), whose luminescence properties are affected by local variations of intracellular oxygen tension (PO(2)). Spectrofluorometric measurements on living human umbilical venous endothelial cells loaded with this molecule show that a decrease in extracellular oxygen tension induces a decrease of PO(2), illustrating the phenomenon of oxygen diffusion and validating the use of this probe in living cells. Moreover, KCN- or 2,4-dinitrophenol-induced modifications of respiration do not lead to detectable PO(2) variations, probably because O(2) diffusion is sufficient to allow oxygen supply. On the contrary, activation by acetylcholine or endothelial nitric oxide synthase (eNOS), which produces NO while consuming oxygen, induces a significant decrease in PO(2), whose amplitude is dependent on the acetylcholine dose, i.e., the eNOS activity level. Hence, activated cytosolic enzymes could consume high levels of oxygen which cannot be supplied by diffusion, leading to PO(2) decrease. Other cell physiology mechanisms leading to PO(2) variations can now be studied in living cells with this probe.     

A three-dimensional multiscale model for gas exchange in fruit

Respiration of bulky plant organs such as roots, tubers, stems, seeds, and fruit depends very much on oxygen (O2) availability and often follows a Michaelis-Menten-like response. A multiscale model is presented to calculate gas exchange in plants using the microscale geometry of the tissue, or vice versa, local concentrations in the cells from macroscopic gas concentration profiles. This approach provides a computationally feasible and accurate analysis of cell metabolism in any plant organ during hypoxia and anoxia. The predicted O2 and carbon dioxide (CO2) partial pressure profiles compared very well with experimental data, thereby validating the multiscale model. The important microscale geometrical features are the shape, size, and three-dimensional connectivity of cells and air spaces. It was demonstrated that the gas-exchange properties of the cell wall and cell membrane have little effect on the cellular gas exchange of apple (Malus×domestica) parenchyma tissue. The analysis clearly confirmed that cells are an additional route for CO2 transport, while for O2 the intercellular spaces are the main diffusion route. The simulation results also showed that the local gas concentration gradients were steeper in the cells than in the surrounding air spaces. Therefore, to analyze the cellular metabolism under hypoxic and anoxic conditions, the microscale model is required to calculate the correct intracellular concentrations. Understanding the O2 response of plants and plant organs thus not only requires knowledge of external conditions, dimensions, gas-exchange properties of the tissues, and cellular respiration kinetics but also of microstructure.     

CUTANEOUS GAS EXCHANGE IN VERTEBRATES: DESIGN, PATTERNS, CONTROL AND IMPLICATIONS

1. The exchange of oxygen and carbon dioxide between skin and environment is commonplace in the vertebrates. In many lower vertebrates, the skin is the major or even sole avenue for respiration.
2. As implied by the physical laws governing diffusion of gases, the skin diffusion coefficient, surface area, gas diffusion distance and transcutaneous gas partial pressures may independently or jointly affect cutaneous respiration. In vertebrates, each of these variables has undergone modification that may be related to dependence upon cutaneous gas exchange.
3. Both theoretical models and experimental data suggest that cutaneous gas exchange is limited by the rate of diffusion. However, changes in convection of the respiratory medium and of blood may partially compensate for diffusion limitation, and potentially function in the regulation of cutaneous gas exchange.
4. Typically, the skin is one of several gas exchangers, although many salamanders and some species in other vertebrate groups breathe solely through the skin. The cutaneous contribution to overall gas exchange is often most important in small animals, at cool temperatures, at low levels of activity and in normoxic and normocapnic conditions. Branchial and pulmonary respiration increasingly predominate in other circumstances.
5. Often, the skin figures more prominently in CO₂, excretion than in O₂, uptake.
6. Cutaneous gas exchange emerges in vertebrates as a process perhaps less effective and more constrained than branchial or pulmonary exchange but also less energetically costly. Its utility is indicated by its wide and successful exploitation in vertebrates occupying a diverse array of habitats.     

Respiratory sinus arrhythmia is associated with efficiency of pulmonary gas exchange in healthy humans

Respiratory sinus arrhythmia (RSA) may be associated with improved efficiency of pulmonary gas exchange by matching ventilation to perfusion within each respiratory cycle. Respiration rate, tidal volume, minute ventilation (.VE), exhaled carbon dioxide (.VCO(2)), oxygen consumption (.VO(2)), and heart rate were measured in 10 healthy human volunteers during paced breathing to test the hypothesis that RSA contributes to pulmonary gas exchange efficiency. Cross-spectral analysis of heart rate and respiration was computed to calculate RSA and the coherence and phase between these variables. Pulmonary gas exchange efficiency was measured as the average ventilatory equivalent of CO(2) (.VE/.VCO(2)) and O(2) (.VE/.VO(2)). Across subjects and paced breathing periods, RSA was significantly associated with CO(2) (partial r = -0.53, P = 0.002) and O(2) (partial r = -0.49, P = 0.005) exchange efficiency after controlling for the effects of age, respiration rate, tidal volume, and average heart rate. Phase between heart rate and respiration was significantly associated with CO(2) exchange efficiency (partial r = 0.40, P = 0.03). These results are consistent with previous studies and further support the theory that RSA may improve the efficiency of pulmonary gas exchange.     

Measuring and interpreting respiratory critical oxygen pressures in roots

BACKGROUND AND AIMS: Respiratory critical oxygen pressures (COPR) determined from O(2)-depletion rates in media bathing intact or excised roots are unreliable indicators of respiratory O(2)-dependency in O(2)-free media and wetlands. A mathematical model was used to help illustrate this, and more relevant polarographic methods for determining COPR in roots of intact plants are discussed. METHODS: Cortical [O(2)] near the root apex was monitored indirectly (pea seedlings) from radial oxygen losses (ROL) using sleeving Pt electrodes, or directly (maize) using microelectrodes; [O(2)] in the root was controlled by manipulating [O(2)] around the shoots. Mathematical modelling of radial diffusive and respiratory properties of roots used Michaelis-Menten enzyme kinetics. KEY RESULTS: Respiration declined only when the O(2) partial pressure (OPP) in the cortex of root tips fell below 0.5-4.5 kPa, values consistent with depressed respiration near the centre of the stele as confirmed by microelectrode measurements and mathematical modelling. Modelling predictions suggested that the OPP of a significant core at the centre of roots could be below the usual detection limits of O(2)-microelectrodes but still support some aerobic respiration. CONCLUSIONS: In O(2)-free media, as in wetlands, the COPR for roots is likely to be quite low, dependent upon the respiratory demands, dimensions and diffusion characteristics of the stele/stelar meristem and the enzyme kinetics of cytochrome oxidase. Roots of non-wetland plants may not differ greatly in their COPRs from those of wetland species. There is a possibility that trace amounts of O(2) may still be present in stelar 'anaerobic' cores where fermentation is induced at low cortical OPPs.     

Effects of Barometric Pressure and Abnormal Gas Mixtures on Gaseous Exchange by the Avian Embryo

Gas moves through the pores of the egg shell by diffusion inthe gas phase. The gas flux is therefore determined by the productof the effective conductance of the shell and the partial pressuregradient of the gas between the ambient air and the inner sideof the shell. The partial pressure gradient of oxygen is decreasedby a reduction of the oxygen partial pressure in the ambientair. This can be achieved by reducing barometric pressure atnormal ambient oxygen concentration or by reducing ambient oxygenconcentration at standard barometric pressure. Both methodsare reported to decrease oxygen consumption of the embryo butto a different degree. At the same ambient oxygen pressure thereduction is less in eggs exposed to a reduced barometric pressure.In an attempt to explain this difference, chicken embryos aged16–19 days were exposed to various oxygen concentrationsand carbon dioxide production was measured. At subnormal oxygenconcentrations carbon dioxide output diminished as the oxygenconcentration was lowered and the duration of exposure was prolonged.At oxygen concentrations above normal a small but significantincrease in carbon dioxide production was found. Finally theresults are compared with those in the literature on the diverseeffects of a continuous reduction of barometric pressure andambient oxygen concentration. This difference is ascribed tothe fact that a reduction of barometric pressure not only decreasesoxygen partial pressure in the ambient air but also increaseseffective conductance of the egg shell, the latter being inverselyproportional to the barometric pressure.     

Some factors affecting oxygen uptake by red blood cells in the pulmonary capillaries

In this study we investigate the equations governing the transport of oxygen in pulmonary capillaries. We use a mathematical model consisting of a red blood cell completely surrounded by plasma within a cylindrical pulmonary capillary. This model takes account of convection and diffusion of oxygen through plasma, diffusion of oxygen through the red blood cell, and the reaction between oxygen and haemoglobin molecules. The velocity field within the plasma is calculated by solving the slow flow equations. We investigate the effect on the solution of the governing equations of: (i) mixed-venous blood oxygen partial pressure (the initial conditions); (ii) alveolar gas oxygen partial pressure (the boundary conditions); (iii) neglecting the convection term; and (iv) assuming an instantaneous reaction between the oxygen and haemoglobin molecules. It is found that: (a) equilibrium is reached much more rapidly for high values of mixed-venous blood and alveolar gas oxygen partial pressure; (b) the convection term has a negligible effect on the time taken to reach a prescribed degree of equilibrium; and (c) an instantaneous reaction may be assumed. Explanations are given for each of these results.     

Circulatory and EEG changes during resuscitation in experiments

In anaesthetized dogs blood pressure (BP), carotid artery flow (CAF), oxygen tension of arterial blood (paO2) and brain surface (bs-pO2), ECG, frequency analyzed EEG were measured and/or recorded. The changes in these parameters were detected before and during ventricular fibrillation with external and direct heart massage as well as after defibrillation. Comparisons were made between the effect of oxygen and room air ventilation. The ratio of successful resuscitation was 88% with direct heart massage and O2 respiration, 76% with external thoracic massage and O2 ventilation, and 68% with external massage combined with room air respiration. The highest BP and CAF could be achieved with direct heart massage. The results suggest that the minimal requirement for success is to achieve a BP of 5 kPa (40 mmHg) and 13 ml/min CAF. In dogs with large and stiff thorax this minimal pressure and flow could not be attained. During effective massage with room air ventilation brain surface pO2 increases moderately, but with O2-respiration it returns to normal. Small or no changes in bs-pO2 indicates unfavourable prognosis. The start bs-pO2 elevation coincides with the reappearance of the first EEG waves, while pO2 increase on the brain surface precedes the normalization of EEG.     

Adaptation of the "Dynamic Method" for measuring the specific respiration rate in oxygen transfer systems through diffusion membrane

Monitoring the specific respiration rate (Q(O2)) is a valuable tool to evaluate cell growth and physiology. However, for low Q(O2) values the accuracy may depend on the measurement methodology, as it is the case in animal cell culture. The widely used "Dynamic Method" imposes serious difficulties concerning oxygen transfer cancellation, especially through membrane oxygenation. This paper presents an improved procedure to this method, through an automated control of the gas inlet composition that can minimize the residual oxygen transfer driving force during the Q(O2) measurement phase. The improved technique was applied to animal cell cultivation, particularly three recombinant S2 (Drosophila melanogaster) insect cell lines grown in a membrane aeration bioreactor. The average measurements of the proposed method reached 98% of stationary liquid phase balance method, taken as a reference, compared to 21% when the traditional method was used. Furthermore, this methodology does not require knowledge of the volumetric transfer coefficient k(L)a, which may vary during growth.     

Measurement and interpretation of the oxygen isotope composition of carbon dioxide respired by leaves in the dark

We measured the oxygen isotope composition (delta(18)O) of CO(2) respired by Ricinus communis leaves in the dark. Experiments were conducted at low CO(2) partial pressure and at normal atmospheric CO(2) partial pressure. Across both experiments, the delta(18)O of dark-respired CO(2) (delta(R)) ranged from 44 per thousand to 324 per thousand (Vienna Standard Mean Ocean Water scale). This seemingly implausible range of values reflects the large flux of CO(2) that diffuses into leaves, equilibrates with leaf water via the catalytic activity of carbonic anhydrase, then diffuses out of the leaf, leaving the net CO(2) efflux rate unaltered. The impact of this process on delta(R) is modulated by the delta(18)O difference between CO(2) inside the leaf and in the air, and by variation in the CO(2) partial pressure inside the leaf relative to that in the air. We developed theoretical equations to calculate delta(18)O of CO(2) in leaf chloroplasts (delta(c)), the assumed location of carbonic anhydrase activity, during dark respiration. Their application led to sensible estimates of delta(c), suggesting that the theory adequately accounted for the labeling of CO(2) by leaf water in excess of that expected from the net CO(2) efflux. The delta(c) values were strongly correlated with delta(18)O of water at the evaporative sites within leaves. We estimated that approximately 80% of CO(2) in chloroplasts had completely exchanged oxygen atoms with chloroplast water during dark respiration, whereas approximately 100% had exchanged during photosynthesis. Incorporation of the delta(18)O of leaf dark respiration into ecosystem and global scale models of C(18)OO dynamics could affect model outputs and their interpretation.     

Aerobic microbial growth at low oxygen concentrations

Sterilizable membrane probes were used to study the relation between oxygen concentration and respiration rate in Candida utilis growing on acetate. When the organism was grown in a continuous fermentor at various dissolved oxygen concentrations (0.23 x 10(-6) to 32 x 10(-6)m), with time allowed for full adaptation to each oxygen concentration, the relationship between oxygen concentration and growth rate simulated Michaelis-Menten behavior, giving an apparent K(m) for oxygen of 1.3 x 10(-6)m. When respiration rate was measured at various oxygen concentrations without allowing time for adaptation, it was found that the respiration rate was directly proportional to O(2) concentration at low O(2) concentrations, and independent of O(2) concentration at high O(2) concentrations. Transition from one type of behavior to the other was fairly abrupt. The respiration rate in the presence of excess oxygen depended on the O(2) concentration at which the cells were grown, but the rate at low O(2) concentrations did not. There was evidence that, at low oxygen concentrations, oxygen diffusion through the cell substance limits respiration rate, at least in part.     

Electron paramagnetic resonance oximetry as a quantitative method to measure cellular respiration: a consideration of oxygen diffusion interference

Electron paramagnetic resonance (EPR) oximetry is being widely used to measure the oxygen consumption of cells, mitochondria, and submitochondrial particles. However, further improvement of this technique, in terms of data analysis, is required to use it as a quantitative tool. Here, we present a new approach for quantitative analysis of cellular respiration using EPR oximetry. The course of oxygen consumption by cells in suspension has been observed to have three distinct zones: pO(2)-independent respiration at higher pO(2) ranges, pO(2)-dependent respiration at low pO(2) ranges, and a static equilibrium with no change in pO(2) at very low pO(2) values. The approach here enables one to comprehensively analyze all of the three zones together-where the progression of O(2) diffusion zones around each cell, their overlap within time, and their potential impact on the measured pO(2) data are considered. The obtained results agree with previously established methods such as high-resolution respirometry measurements. Additionally, it is also demonstrated how the diffusion limitations can depend on cell density and consumption rate. In conclusion, the new approach establishes a more accurate and meaningful model to evaluate the EPR oximetry data on cellular respiration to quantify related parameters using EPR oximetry.     

Different pathways of the oxygen supply in the sapwood of young<Emphasis Type="Italic"> Olea europaea</Emphasis> trees

Gaseous transport through lenticels is widely accepted to be the main pathway for oxygen supply to the parenchymatous tissues of the wood. Circumstantial evidence exists that the oxygen required for respiration by these living cells can be obtained from the transpiration stream. However, there has been no functional confirmation of this role. To address this problem and to quantify the contribution of the different pathways to the oxygen supply of the sapwood, we have developed a three-electrode miniaturized oxygen-selective sensor to be implanted into the sapwood for long-term determination of the oxygen concentration. In spring, during the active growing season, the oxygen concentration of the sapwood of young olive (Olea europaea L.) trees changed from 80-90 micromol O(2) l(-1) around midday to 20-30 micromol O(2) l(-1) in the night. These concentrations correspond to a deficit of oxygen for the sapwood between 65-70% and 88-90% of an aqueous solution saturated with air. In the daylight hours, almost all the oxygen present in the sapwood was delivered by the transpiration stream, driven by the soil-plant-atmosphere water-potential gradient. During the night the diffusion of oxygen via the sap-filled lumina of the tracheids and vessels (xylary diffusion in the aqueous phase) accounted for about 87% of all the oxygen present, whereas only the remaining 13% was assessed as supplied by radial diffusion in the aqueous or gaseous phase.     

Photosynthetic consequences of phenotypic plasticity in response to submergence: Rumex palustris as a case study

Survival and growth of terrestrial plants is negatively affected by complete submergence. This is mainly the result of hampered gas exchange between plants and their environment, since gas diffusion is severely reduced in water compared with air, resulting in O2 deficits which limit aerobic respiration. The continuation of photosynthesis could probably alleviate submergence-stress in terrestrial plants, but its potential under water will be limited as the availability of CO2 is hampered. Several submerged terrestrial plant species, however, express plastic responses of the shoot which may reduce gas diffusion resistance and enhance benefits from underwater photosynthesis. In particular, the plasticity of the flooding-tolerant terrestrial species Rumex palustris turned out to be remarkable, making it a model species suitable for the study of these responses. During submergence, the morphology and anatomy of newly developed leaves changed: 'aquatic' leaves were thinner and had thinner cuticles. As a consequence, internal O2 concentrations and underwater CO2 assimilation rates were higher at the prevailing low CO2 concentrations in water. Compared with heterophyllous amphibious plant species, underwater photosynthesis rates of terrestrial plants may be very limited, but the effects of underwater photosynthesis on underwater survival are impressive. A combination of recently published data allowed quantification of the magnitude of the acclimation response in this species. Gas diffusion resistance in terrestrial leaves underwater was about 15,000 times higher than in air. Strikingly, acclimation to submergence reduced this factor to 400, indicating that acclimated leaves of R. palustris had an approximately 40 times lower gas diffusion resistance than non-acclimated ones.     

Oxygen uptake, diffusion limitation, and diffusing capacity of the bipectinate gills of the abalone, Haliotis iris (Mollusca: Prosobranchia)

Extant abalone retain an ancestral system of gas exchange consisting of paired bipectinate gills. This paper examines the hypothesis that fundamental inefficiencies of this arrangement led to the extensive radiation observed in prosobranch gas exchange organs. Oxygen uptake at 15 degrees C was examined in the right gill of resting adult blackfoot abalone, Haliotis iris Martyn 1784. Pre- and post-branchial haemolymph and water were sampled and oxygen content, partial pressure (Po2), pH, and haemocyanin content measured; in vivo haemolymph flow rate was determined by an acoustic pulsed-Doppler flowmeter. During a single pass across the gills, mean seawater Po2 fell from 138.7 Torr to 83.4 Torr, while haemolymph Po2 rose from 37.2 Torr to 77.0 Torr raising total O2 content from 0.226 to 0.346 mmol L(-1). Haemolymph flowed through the right gill at a mean rate of 9.6 mL min(-1) and carried 0.151 to 0.355 mmol L(-1) of haemocyanin (mean body mass 421 g). Only 34.7% of the oxygen carried in the arterial haemolymph was taken up by the tissues and less than half of this was contributed by haemocyanin. A diffusion limitation index (Ldiff) of 0.47-0.52, a well-matched ventilation-perfusion ratio (1.2-1.4) and a diffusing capacity (D) of 0.174 micromol O2 kg(-1) Torr(-1) indicate that the gills operate efficiently and are able to meet the oxygen requirements of the resting abalone.     

Zacopride and 8-OH-DPAT reverse opioid-induced respiratory depression and hypoxia but not catatonic immobilization in goats

Neurophysiological studies have shown that serotonergic ligands that bind to 5-HT1A, 5-HT7, and 5-HT4 serotonin receptors in brain stem have beneficial effects on respiratory neurons during opioid-induced respiratory depression. The effect of these ligands on respiratory function and pulmonary performance has not been studied. We therefore examined the effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), an agonist of 5-HT1A and 5-HT7 receptors, and zacopride, an agonist of 5-HT4 receptors, to establish whether these ligands would reverse opioid-induced respiratory depression and hypoxia without affecting the immobilizing properties of the opioid drug etorphine. When etorphine was used to sedate and immobilize goats, it significantly decreased respiratory rate (P = 0.013), percent hemoglobin oxygen saturation (P < 0.0001), and arterial oxygen partial pressure [Pa(O2); F(10,70) = 5.67, P < 0.05] and increased arterial carbon dioxide partial pressure [F(10,70) = 3.87, P < 0.05] and alveolar-arterial oxygen partial pressure gradient [A-a gradients; F(10,70) = 8.23, P < 0.0001]. Zacopride and 8-OH-DPAT, coadministered with etorphine, both attenuated the effects of etorphine; respiration rates did not decrease, and percent hemoglobin oxygen saturation and Pa(O2) remained elevated. Zacopride decreased the hypercapnia, indicating an improvement in ventilation, whereas 8-OH-DPAT did not affect the hypercapnia and, therefore, did not improve ventilation. The main beneficial effect of 8-OH-DPAT was on the pulmonary circulation; it improved oxygen diffusion, indicated by the normal A-a gradients, presumably by improving ventilation perfusion ratios. Neither zacopride nor 8-OH-DPAT reversed etorphine-induced catatonic immobilization. We conclude that serotonergic drugs that act on 5-HT1A, 5-HT7, and 5-HT4 receptors reverse opioid-induced respiratory depression and hypoxia without reversing catatonic immobilization.