The effect of carbon monoxide on respiration

A Novel Possible Role for Met Hemoglobin as Carrier of Hydrogen Sulfide in the Blood

Significance: Along with other gasotransmitters nitric oxide (NO) and carbon monoxide, hydrogen sulfide (H₂S) has recently emerged as an important signaling molecule with a particularly complex metabolism. Endogenous H₂S reacts with multiple cellular targets, including protein ferric heme groups, to elicit physiological responses, such as regulation of local blood flow. Recent Advances: Recent in vitro evidence suggests that H₂S at low physiological concentrations is carried in the blood as bound to the small fraction of oxidized ferric hemoglobin (metHb). A relatively stable metHb-sulfide complex forms when H₂S and purified metHb react in vitro, with an affinity within the in vivo physiological range of sulfide in the blood. Formation and subsequent redox metabolism of metHb-sulfide complex have also been confirmed in isolated intact red blood cells (RBCs) containing enhanced metHb levels. Thus, H₂S may function as an endocrine signaling molecule and elicit responses at sites away from the site of production. In addition, metHb, considered as an inert or pathological hemoglobin derivative, may have a novel potential physiological role in the transport of H₂S in the blood. Critical Issues: The transport of H₂S in the blood mediated by metHb would represent an O₂-independent pH-dependent mechanism for the blood-mediated control of blood flow and as such it is critical to understand the in vivo significance of this transport. Future Directions: Major challenges must be resolved to understand how metHb may carry H₂S in the RBCs, in particular determination of metHb-sulfide levels in the blood and identification of targets in the vasculature.

Carbon Monoxide and Cyanide Poisoning in the Burned Pregnant Patient: An Indication for Hyperbaric Oxygen Therapy

Carbon monoxide (CO) is a small molecule poison released as a product of incomplete combustion. Carbon monoxide binds hemoglobin, reducing oxygen delivery. This effect is exacerbated in the burned pregnant patient by fetal hemoglobin that binds CO 2.5- to 3-fold stronger than maternal hemoglobin. With no signature clinical symptom, diagnosis depends on patient injury history, elevated carboxyhemoglobin levels, and alterations in mental status. The standard of care for treatment of CO intoxication is 100% normobaric oxygen, which decreases the half-life of CO in the bloodstream from 5 hours to 1 hour. Hyperbaric oxygen (HBO2) is a useful adjunct to rapidly reduce the half-life of CO to 20 minutes and the incidence of delayed neurologic sequelae. Because of the slow disassociation of CO from hemoglobin in the fetus, there is a far stronger indication for HBO2 in the burned pregnant patient than in other burn patient populations.Cyanide intoxication is often a comorbid disease with CO in inhalation injury from an enclosed fire, but may be the predominant toxin. It acts synergistically with CO to effectively lower the lethal doses of both cyanide and CO. Diagnosis is best made in the presence of high lactate levels, carboxyhemoglobin concentrations greater than 10%, injury history of smoke inhalation from an enclosed fire, and alterations in consciousness. While treatment with hydroxocobalamin is the standard of care and has the effect of reducing concomitant CO toxicity, data indicate cyanide may also be displaced by HBO2.Carbon monoxide and cyanide poisoning presents potential complications impacting care. This review addresses the mechanism of action, presentation, diagnosis, and treatment of CO and cyanide poisonings in the burned pregnant patient and the use of HBO2 therapy.

Subcellular Energetics and Metabolism: Potential Therapeutic Applications

Part I of this review discussed the similarities between embryogenesis, mammalian adaptions to hypoxia (primarily driven by hypoxia-inducible factor-1 [HIF-1]), ischemia-reperfusion injury (and its relationship with reactive oxygen species), hibernation, diving animals, cancer, and sepsis, and it focused on the common characteristics that allow cells and organisms to survive in these states. Part II of this review describes techniques by which researchers gain insight into subcellular energetics and identify potential future tools for clinicians. In particular, P nuclear magnetic resonance to measure high-energy phosphates, serum lactate measurements, the use of near-infrared spectroscopy to measure the oxidation state of cytochrome aa3, and the ability of the protoporphyrin IX-triplet state lifetime technique to measure mitochondrial oxygen tension are discussed. In addition, this review discusses novel treatment strategies such as hyperbaric oxygen, preconditioning, exercise training, therapeutic gases, as well as inhibitors of HIF-1, HIF prolyl hydroxylase, and peroxisome proliferator-activated receptors.

Compartment Syndrome Resulting from Carbon Monoxide Poisoning

Every year, especially in the cooler Fall and Winter months, hundreds of people die because of carbon monoxide poisoning. This occurs usually as an accident. It is a significant cause of poisoning worldwide. We present a case of compartment syndrome in both lower extremities with accompanying acute renal failure and systemic capillary leakage syndrome because of carbon monoxide poisoning.

Investigation of oxygen vacancies on Pt- or Au-modified CeO2 materials for CO oxidation

Metal (Au or Pt)-modified CeO₂ materials with excellent redox ability for the oxidation of CO to CO₂ were prepared by a redox precipitation method and a certain amount (4.2%) of oxygen vacancies was necessary to improve the CO oxidation on Pt-CeO₂.

Anaerobic carboxydotrophic bacteria in geothermal springs identified using stable isotope probing

Carbon monoxide (CO) is a potential energy and carbon source for thermophilic bacteria in geothermal environments. Geothermal sites ranging in temperature from 45 to 65°C were investigated for the presence and activity of anaerobic CO-oxidizing bacteria. Anaerobic CO oxidation potentials were measured at up to 48.9 μmoles CO g⁻¹ (wet weight) day⁻¹ within five selected sites. Active anaerobic carboxydotrophic bacteria were identified using ¹³CO DNA stable isotope probing (SIP) combined with pyrosequencing of 16S rRNA genes amplified from labeled DNA. Bacterial communities identified in heavy DNA fractions were predominated by Firmicutes, which comprised up to 95% of all sequences in ¹³CO incubations. The predominant bacteria that assimilated ¹³C derived from CO were closely related (>98% 16S rRNA gene sequence identity) to genera of known carboxydotrophs including Thermincola, Desulfotomaculum, Thermolithobacter, and Carboxydocella, although a few species with lower similarity to known bacteria were also found that may represent previously unconfirmed CO-oxidizers. While the distribution was variable, many of the same OTUs were identified across sample sites from different temperature regimes. These results show that bacteria capable of using CO as a carbon source are common in geothermal springs, and that thermophilic carboxydotrophs are probably already quite well known from cultivation studies.

Metagenomic analysis for the microbial consortium of anaerobic CO oxidizers

Metagenomics analysis has been applied to identify the dominant anaerobic microbial consortium of the carbon monoxide (CO) oxidizers in anaerobic sludge. Reads from the hypervariable V6 region in the bacterial 16s rDNA were aligned and finally clustered into operational taxonomic units (OTUs). The OTUs from different stages in anaerobic CO condition were classified. Alphaproteobacteria, clostridia, betaproteobacteria and actinobacteria were the most abundant groups, while alphaproteobacteria, betaproteobacteria and actinobacteria were variable groups. CO consumption and production efficiency of the microbial consortium were studied. Semi-continuous trials showed that these anaerobic CO oxidizers formed a stable microbial community, and the CO conversion rate was at over 84%, with the highest CO consumption activity of 28.9 mmol CO/g VSS●day and methane production activity at 7.6 mmol CH₄/g VSS●day during six cycles.

Confocal and fluorescence lifetime imaging sheds light on the fate of a pyrene-tagged carbon monoxide-releasing Fischer carbene chromium complex

The synthesis of a new pyrene-containing Fischer carbene complex is described. The complex has a broad absorbance spectrum between 300 and 400 nm and, on excitation at 345 nm in CH2Cl2 solution, emission is observed at 395 and 415 nm. Emission is also observed in PBS buffer, but in this case the resulting spectra are much broader. Confocal and fluorescence lifetime imaging indicate that emission occurs on treating HeLa cells with the complex and co-localisation studies demonstrate that this is from the mitochondria and lipid-rich regions of the cell.

Effect of carbon monoxide donor CORM-2 on vitamin D3 metabolism

BACKGROUND/AIMS

Carbon monoxide (CO) interferes with cytochrome-dependent cellular functions and acts as gaseous transmitter. CO is released from CO-releasing molecules (CORM) including tricarbonyl-dichlororuthenium (II) dimer (CORM-2), molecules considered for the treatment of several disorders including vascular dysfunction, inflammation, tissue ischemia and organ rejection. Cytochrome P450-sensitive function include formation of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) by renal 25-hydroxyvitamin D3 1-alpha-hydroxylase (Cyp27b1). The enzyme is regulated by PTH, FGF23 and klotho. 1,25(OH)2D3 regulates Ca(2+) and phosphate transport as well as klotho expression. The present study explored, whether CORM-2 influences 1,25(OH)2D3 formation and klotho expression.

METHODS

Mice were treated with intravenous CORM-2 (20 mg/kg body weight). Plasma 1,25(OH)2D3 and FGF23 concentrations were determined by ELISA, phosphate, calcium and creatinine concentrations by colorimetric methods, transcript levels by quantitative RT-PCR and protein expression by western blotting. Fgf23 mRNA transcript levels were further determined in rat osteosarcoma UMR106 cells without or with prior treatment for 24 hours with 20 µM CORM-2.

RESULTS

CORM-2 injection within 24 hours significantly increased FGF23 plasma levels and decreased 1,25(OH)2D3 plasma levels, renal Cyp27b1 gene expression as well as renal klotho protein abundance and transcript levels. Moreover, treatment of UMR106 cells with CORM-2 significantly increased Fgf23 transcript levels.

CONCLUSION

CO-releasing molecule CORM-2 enhances FGF23 expression and release and decreases klotho expression and 1,25(OH)2D3 synthesis. © 2013 S. Karger AG, Basel.

Carbon monoxide poisoning

Carbon monoxide (CO) poisoning is the leading cause of death as a result of unintentional poisoning in the United States. CO toxicity is the result of a combination of tissue hypoxia-ischemia secondary to carboxyhemoglobin formation and direct CO-mediated damage at a cellular level. Presenting symptoms are mostly nonspecific and depend on the duration of exposure and levels of CO. Diagnosis is made by prompt measurement of carboxyhemoglobin levels. Treatment consists of the patient's removal from the source of exposure and the immediate administration of 100% supplemental oxygen in addition to aggressive supportive measures. The use of hyperbaric oxygen is controversial.

Carbon monoxide-sensitive apoptotic death of erythrocytes

Carbon monoxide (CO) intoxication severely interferes with the oxygen-transporting function of haemoglobin. Beyond that, CO participates in the regulation of apoptosis. CO could be generated from CO-releasing molecules (CORM), such as the tricarbonyl-dichlororuthenium (II) dimer (CORM-2), which is presently considered for the treatment of vascular dysfunction, inflammation, tissue ischaemia and organ rejection. CORM-2 is at least partially effective by modifying gene expression and mitochondrial potential. Erythrocytes lack nuclei and mitochondria but may undergo suicidal cell death or eryptosis, characterized by cell shrinkage and phospholipid scrambling of the cell membrane. Eryptosis is triggered by the increase in cytosolic Ca²⁺ activity ([Ca²⁺](i)). The present study explored whether CORM-2 influences eryptosis. To this end, [Ca²⁺](i) was estimated from Fluo-3-fluorescence, cell volume from forward scatter, phospholipid scrambling from annexin-V-binding and haemolysis from haemoglobin release. CO-binding haemoglobin (COHb) was estimated utilizing a blood gas analyser. As a result, exposure of erythrocytes for 24 hr to CORM-2 (≥5 μM) significantly increased COHb, [Ca²⁺](i) , forward scatter, annexin-V-binding and haemolysis. Annexin-V-binding was significantly blunted by 100% oxygen and was virtually abolished in the nominal absence of Ca²⁺. In conclusion, CORM-2 stimulates cell membrane scrambling of erythrocytes, an effect largely due to Ca²⁺ entry and partially reversed by O₂.

Carbon monoxide targeting mitochondria

Mitochondria present two key roles on cellular functioning: (i) cell metabolism, being the main cellular source of energy and (ii) modulation of cell death, by mitochondrial membrane permeabilization. Carbon monoxide (CO) is an endogenously produced gaseoustransmitter, which presents several biological functions and is involved in maintaining cell homeostasis and cytoprotection. Herein, mitochondrion is approached as the main cellular target of carbon monoxide (CO). In this paper, two main perspectives concerning CO modulation of mitochondrial functioning are evaluated. First, the role of CO on cellular metabolism, in particular oxidative phosphorylation, is discussed, namely, on: cytochrome c oxidase activity, mitochondrial respiration, oxygen consumption, mitochondrial biogenesis, and general cellular energetic status. Second, the mitochondrial pathways involved in cell death inhibition by CO are assessed, in particular the control of mitochondrial membrane permeabilization.

An unusual case of suicidal carbon monoxide poisoning

The authors present a case of suicide by carbon monoxide poisoning in a confined space with interesting elements related to the method and the circumstances in which it took place. Carbon monoxide poisoning is a very common cause of death in accidental and suicidal cases, but the method used in this case, consisting of burning charcoal in a small office bathroom, presents some particularly interesting aspects. The circumstances, moreover, appear very interesting because the person who committed suicide was an employee of the Public Administration and he had received notification regarding his involvement in a legal prosecution.

Carbon monoxide-dependent energy metabolism in anaerobic bacteria and archaea

Despite its toxicity for the majority of living matter on our planet, numerous microorganisms, both aerobic and anaerobic, can use carbon monoxide (CO) as a source of carbon and/or energy for growth. The capacity to employ carboxidotrophic energy metabolism anaerobically is found in phylogenetically diverse members of the Bacteria and the Archaea. The oxidation of CO is coupled to numerous respiratory processes, such as desulfurication, hydrogenogenesis, acetogenesis, and methanogenesis. Although as diverse as the organisms capable of it, any CO-dependent energy metabolism known depends on the presence of carbon monoxide dehydrogenase. This review summarizes recent insights into the CO-dependent physiology of anaerobic microorganisms with a focus on methanogenic archaea. Carboxidotrophic growth of Methanosarcina acetivorans, thought to strictly rely on the process of methanogenesis, also involves formation of methylated thiols, formate, and even acetogenesis, and, thus, exemplifies how the beneficial redox properties of CO can be exploited in unexpected ways by anaerobic microorganisms.

Population pharmacokinetic analysis of carboxyhaemoglobin concentrations in adult cigarette smokers

AIMS

To develop a population-based model to describe and predict the pharmacokinetics of carboxyhaemoglobin (COHb) in adult smokers.

METHODS

Data from smokers of different conventional cigarettes (CC) in three open-label, randomized studies were analysed using NONMEM (version V, Level 1.1). COHb concentrations were determined at baseline for two cigarettes [Federal Trade Commission (FTC) tar 11 mg; CC1, or FTC tar 6 mg; CC2]. On day 1, subjects were randomized to continue smoking their original cigarettes, switch to a different cigarette (FTC tar 1 mg; CC3), or stop smoking. COHb concentrations were measured at baseline and on days 3 and 8 after randomization. Each cigarette was treated as a unit dose assuming a linear relationship between the number of cigarettes smoked and measured COHb percent saturation. Model building used standard methods. Model performance was evaluated using nonparametric bootstrapping and predictive checks.

RESULTS

The data were described by a two-compartment model with zero-order input and first-order elimination with endogenous COHb. Model parameters included elimination rate constant (k(10)), central volume of distribution (Vc/F), rate constants between central and peripheral compartments (k(12) and k(21)), baseline COHb concentrations (c0), and relative fraction of carbon monoxide absorbed (F1). The median (range) COHb half-lives were 1.6 h (0.680-2.76) and 30.9 h (7.13-367) (alpha and beta phases, respectively). F1 increased with increasing cigarette tar content and age, whereas k(12) increased with ideal body weight.

CONCLUSION

A robust model was developed to predict COHb concentrations in adult smokers and to determine optimum COHb sampling times in future studies.

Delivery of drugs and macromolecules to mitochondria

Mitochondria is where the bulk of the cell's ATP is produced. Mutations occur to genes coding for members of the complexes involved in energy production. Some are a result of damages to nuclear coded genes and others to mitochondrial coded genes. This review describes approaches to bring small molecules, proteins and RNA/DNA into mitochondria. The purpose is to repair damaged genes as well as to interrupt mitochondrial function including energy production, oxygen radical formation and the apoptotic pathway.

Lymphocyte cytochrome c oxidase, cyclic GMP and cholinergic muscarinic receptors as peripheral indicators of carbon monoxide neurotoxicity after acute and repeated exposure in the rat

Changes in cerebral cytochrome oxidase (COX) activity, nitric oxide (NO)-cyclic GMP (cGMP) pathway and cholinergic muscarinic receptors (MRs) have been reported in rodents acutely exposed to carbon monoxide (CO). These endpoints measurable in lymphocytes may serve as peripheral markers of CO neurotoxicity. The early and delayed effects of repeated and acute in vivo CO inhalation were investigated on COX activity, cGMP formation and MR binding in rat brain and lymphocytes to assess whether each endpoint was similarly affected both centrally and peripherally. Male Wistar rats either inhaled 500 ppm CO, 6 h/day, 5 days/week, 4 weeks (repeated exposure) or 2,400 ppm, 1 h (single exposure). Neither treatment altered brain or lymphocyte COX activity 1 and 7 days post-treatment. Also ineffective were repeated and acute CO treatments towards (3)H-quinuclidinyl benzilate (QNB) binding to MRs in cerebral cortex, hippocampus, striatum, cerebellum (respective controls, mean+/-S.D.: 171 +/- 45, 245 +/- 53, 263 +/- 14 and 77 +/- 7 fmol/mg protein) and lymphocytes (24 +/- 10 fmol/million cells) at the same time points. In lymphocytes control cGMP levels averaged 1.98 +/- 0.99 pmol/mg protein under basal conditions, and 3.94 +/- 0.55 pmol/mg protein after NO-stimulation. One day after chronic treatment cessation, the CO-treated group displayed about a 50% decrease in both basal and NO-stimulated cGMP values, which persisted up to 7 days after, compared to air-exposed rats. Acutely, CO caused a delayed enhancement (+140%) of NO-induced activation of soluble guanylate cyclase. The finding that the NO-cGMP pathway is a target for the delayed effects of CO in peripheral blood cells is in accordance with our data in brain [Hernández-Viadel, M., Castoldi, A.F., Coccini, T., Manzo, L., Erceg, S., Felipo, V., 2004. In vivo exposure to carbon monoxide causes delayed impairment of activation of soluble guanylate cyclase by nitric oxide in rat brain cortex and cerebellum. Journal of Neurochemistry 89, 1,157-1,165], and supports the use of this peripheral endpoint as a biomarker of CO central effects.

A multicompartment model of carboxyhemoglobin and carboxymyoglobin responses to inhalation of carbon monoxide

We have developed a model that predicts the distribution of carbon monoxide (CO) in the body resulting from acute inhalation exposures to CO. The model includes a lung compartment, arterial and venous blood compartments, and muscle and nonmuscle soft tissues with both vascular and nonvascular subcompartments. In the model, CO is allowed to diffuse between the vascular and nonvascular subcompartments of the tissues and to combine with myoglobin in the nonvascular subcompartment of muscle tissue. The oxyhemoglobin dissociation curve is represented by a modified Hill equation whose parameters are functions of the carboxyhemoglobin (HbCO) level. Values for skeletal muscle mass and cardiac output are calculated from prediction formulas based on age, weight, and height of individual subjects. We demonstrate that the model fits data from CO rebreathing studies when diffusion of CO into the muscle compartment is considered. The model also fits responses of HbCO to single or multiple exposures to CO lasting for a few minutes each. In addition, the model reproduces reported differences between arterial and venous HbCO levels and replicates predictions from the Coburn-Forster-Kane equation for CO exposures of a 1- to 83-h duration. In contrast to approaches based on the Coburn-Forster-Kane equation, the present model predicts uptake and distribution of CO in both vascular and tissue compartments during inhalation of either constant or variable levels of CO.

Carbon monoxide specifically inhibits cytochrome c oxidase of human mitochondrial respiratory chain

Carbon monoxide (CO) toxicity is the result of a combination of tissue hypoxia and direct CO-mediated damage at a cellular level, since not all the signs and symptoms presented can be explained only by the formation of carboxyhaemoglobin. Mitochondria, specially the electron transport chain, seem to be the target for CO at a subcellular level. However, the direct effect of CO in individual complexes of the human mitochondrial respiratory chain has not been completely elucidated. We here studied the in vitro effect of CO on individual complexes of the mitochondrial respiratory chain of human mitochondria. We obtained muscle tissue from 10 healthy people who underwent orthopaedic surgery for hip replacement. Isolated mitochondria were incubated for 5 min. under CO concentrations of 50, 100 and 500 ppm. Afterwards, enzymatic activities of individual complexes of the mitochondrial respiratory chain were assessed in vitro and compared with those obtained in basal (synthetic air without CO) conditions. Cytochrome c oxidase (complex IV of the mitochondrial respiratory chain) activity showed a decrease from 836+/-439 nmol/min./mg of mitochondrial protein after air incubation to 670+/-401, 483+/-182, and 379+/-131 nmol/min./mg after 50, 100 and 500 ppm of CO incubation, respectively (20%, 42% and 55% decrease in cytochrome c oxidase activity). This gradual decrease in cytochrome c oxidase was found to be statistically significant (P<0.001). Other complex activities showed no any significant variation. Carbon monoxide is toxic for mitochondria in man, altering the mitochondrial respiratory chain at the cytochrome c oxidase level. This inhibition in cytochrome c oxidase may play a role in the development of the symptoms observed in acute CO poisoning, and in some diseases related to smoking.

Suppression of the induction of long-term depression by carbon monoxide in rat cerebellar slices

Carbon monoxide (CO) suppresses brain functions at doses lower than that suppressing oxygen (O(2)) supply to the brain, and the cerebellum is one of the sites most susceptible to the neurotoxic effects of CO. We investigated the effects of CO on the induction of cerebellar long-term depression (LTD) in the synapses between parallel fibres (PFs) and Purkinje cells. CO, at concentrations between 8 nM and 5 microM, exhibited almost no effect on synaptic responses in Purkinje cells, O(2) consumption and NO release from PFs in rat cerebellar slices. However, the induction of LTD was significantly suppressed by CO at concentrations between 40 and 200 nM. The suppressive effect of 40 nM CO was antagonized by 10 microM NOR3, an NO donor. In contrast, CO exhibited no clear effect on the induction of LTD at concentrations between 1 and 5 microM. The induction of LTD, suppressed by 10 microM N(G)-nitro-L-arginine, an inhibitor of NO synthase, was not restored by 5 microM CO. CO is not only a neurotoxic substance but also a candidate for an intercellular messenger. delta-Aminolevulinate (30 microM), a substance facilitating endogenous CO production, suppressed the induction of LTD, and the effect of delta-aminolevulinate was antagonized by 10 microM NOR3. These findings suggest that CO may have a suppressive effect on the induction of cerebellar LTD at nanomolar concentrations, probably via its effects on NO/cGMP signalling.

Mitochondrial cytochrome c oxidase inhibition during acute carbon monoxide poisoning

Clinical symptoms of acute carbon monoxide (CO) poisoning are mainly related to the capability of haemoglobin to bind CO. However, the persistence of some clinical alterations after carboxyhaemoglobin normalization suggests that other heme containing proteins, like cytochrome c oxidase, could play a role in its pathogenesis. We studied mitochondrial enzyme activities of lymphocytes from three patients suffering from acute CO poisoning. HbCO levels were 11.6%. 19.6% and 22.3% in the acute phase, 2.3%, 2.4% and 1.5% on day 3 after admission, and 1.2%, 3.3% and 1.1% on day 12. Complex II, III and glycerol-3-phosphate dehydrogenase activities remained normal along the study, while cytochrome c oxidase (complex IV) activity showed a 76% inhibition compared to controls during acute poisoning (P < 0.01) and 48% at day 3 (P < 0.05). The activity was normal already on day 12 after the complete disappearance of symptomatology. Our results suggest that mitochondrial cytochrome c oxidase is also a target site in human acute CO poisoning, and its extended and generalized inhibition could explain the persistence of different symptoms after the normalization of HbCO levels.

O2 consumption in the in-vitro fetal side human placenta

The relative dependence of placental O2 consumption (MO2) on O2 supply (SupO2) parameters, and the fraction of aerobic O2 utilization, was studied by in-vitro perfusion of the fetal side of term human placental lobes. Placental MO2 was a function of the combined effect of SupO2 parameters (PO2, perfusate flow rate, perfusate effective O2 solubility) and not of each separately. At SupO2 greater than 800 microliters/(min.lobe), MO2 saturated at 260 microliters/(min.lobe). No accumulation of O2 debt could be detected even after 2 h anoxia. Adding CN- or CO to perfusate did not abolish MO2, and a residual MO2 of 31-37% of control MO2 was measured. In its MO2, the placenta is a typical conformer tissue. This conformity enables conservation of O2 transfer to the embryo even when placental SupO2 is radically reduced. Only about 60-70% of placental MO2 is utilized in the aerobic respiratory pathway, while about 30-40% of the oxygen is used in other pathways.

The ventilatory response of rodents to changes in arterial oxygen content

A strong correlation between the arterial oxygen partial pressure (PaO2) values for the threshold of the hypoxic ventilatory response (HVR) and the shoulder of the oxyhemoglobin equilibrium curve (OEC) is retained in heterothermic rodents as body temperature changes despite changes in hemoglobin-oxygen affinity. It has been suggested that this may reflect either temperature-induced changes in the response characteristics of arterial chemoreceptors or an ability to sense changes in arterial O2 content (CaO2). This study examined the extent to which changing CaO2 independent of PaO2 with carbon monoxide could contribute to the HVR in heterothermic (golden-mantled ground squirrels) and non-heterothermic rodents (rats). The HVR of isocapnic, anaesthetized rodents was assessed during both hypoxic hypoxia, which alters PaO2 and CaO2 simultaneously, and carbon monoxide hypoxia, which alters CaO2 independent of PaO2. While both species exhibited ventilatory responses to hypoxic hypoxia and carbon monoxide hypoxia, the HVR of the squirrel was consistently stronger than that of the rat. Reductions in CaO2 independent of PaO2 could still produce 60% of the full HVR seen with hypoxic hypoxia in both species. Simultaneous changes in PaO2, however, were necessary to produce the full response. While it seems likely that the results can be explained by the changes in tissue PO2 which would occur at receptor sites under the various conditions, such an explanation is not totally supported by other studies.

Comparison of carbon monoxide and nitrogen induced effects on synaptic transmission in the rat hippocampal slice

A comparison has been made of the effects of carbon monoxide (CO) or nitrogen (N2) exposure on synaptic transmission in the hippocampal slice. CA1 field potentials, evoked by Schaffer collateral stimulation, were unaffected by superfusion of slices with artificial cerebral spinal fluid (ACSF) equilibrated with either 15% CO or 15% N2 for 120 min. However, superfusion with hypoxic ACSF equilibrated with either 85% CO or 85% N2 caused a rapid depression of synaptic transmission. Reperfusion with control ACSF following 30 min hypoxia led to recovery of evoked responses and a slight hyperexcitability. In the hippocampal slice, synaptic transmission, as assessed by input/output curves, was not different during or following hypoxia induced by exposure to CO or N2. In the short term, CO is not toxic.