The steady-state transport of oxygen through hemoglobin solutions

The steady-state transport of oxygen through hemoglobin solutions was studied to identify the mechanism of the diffusion augmentation observed at low oxygen tensions. A novel technique employing a platinum-silver oxygen electrode was developed to measure the effective diffusion coefficient of oxygen in steady-state transport. The measurements were made over a wider range of hemoglobin and oxygen concentrations than previously reported. Values of the Brownian motion diffusion coefficient of oxygen in hemoglobin solution were obtained as well as measurements of facilitated transport at low oxygen tensions. Transport rates up to ten times greater than ordinary diffusion rates were found. Predictions of oxygen flux were made assuming that the oxyhemoglobin transport coefficient was equal to the Brownian motion diffusivity which was measured in a separate set of experiments. The close correlation between prediction and experiment indicates that the diffusion of oxyhemoglobin is the mechanism by which steady-state oxygen transport is facilitated.     

Phase fluorometric sterilizable optical oxygen sensor

We report here on a low-cost, optical oxygen sensor as an attractive alternative to the widely used amperometric Clark-type oxygen electrode for measuring dissolved oxygen tensions in cell cultures and bioreactor. Our sensor is based on the defferential quenching of the fluorescence lifetime of chromophore in response to the partial pressure of oxygen. This is measured as a phase shift in fluorescence emission from the chromophore due to oxygen quenching when excited by an intensity modulated beam of light. In this article we demonstrate the advantages of lifetime-based optical methods over both intensity based optical methods and amperometric electrodes. Our sensor is particularly suitable for measuring dissolved oxygen in bioreactors. It is autoclavable, is free of maintenance requirements, and solvents the problems of long-term stability, calibration drifts, and reliable measurement of low oxygen tensions in dense microbial cultures that limit the utility of Clark-type elcectordes. (c) 1994 John Wiley & Sons, Inc.     

Oxygen consumption rate of tissue measured by a micropolarographic method

A new method for measuring the oxygen consumption rate of a sheet of homogeneous tissue is described. The method measures, by a Clark-type oxygen electrode without a membrane, the time for the tissue to consume all its dissolved oxygen. The electrode is applied to one surface of the tissue sheet and the other surface is sealed from the atmosphere by a cover slip. The consumption is calculated from an estimate of the oxygen dissolved in the tissue at the moment it is covered and the time for the oxygen tension at one surface to fall to zero. The data also yield the oxygen diffusion coefficient in the oxygen-consuming tissue.     

Measurement of low levels of oxygen and their effect on respiration in cell suspensions maintained in an open system

The presence of low levels of oxygen may have profound effects on the cytotoxic activity of radiation, radiosensitizers, and bioreductive alkylating agents. As others have shown, low oxygen tensions may significantly alter rates of cellular and chemical oxygen consumption. When experiments are performed at very low oxygen concentrations, the opposing effects of oxygen leakage into and cellular/chemical oxygen consumption from the system can lead to unpredictable results. Use of a newly designed, highly sensitive Clark-type oxygen sensor has permitted accurate and reproducible measurement of low levels of oxygen. Cellular depletion of oxygen at various cell densities has been monitored for a series of oxygen tensions in solution and the corresponding respiration rates have been calculated. Although oxygen depletion was found to be quite significant at low oxygen tensions, not all oxygen present could be removed by cellular respiration. Respiration rate decreased as oxygen tension decreased and approached zero at low oxygen tensions. This result was independent of cell density. A model is presented to account for the observed effect of oxygen tension on cellular oxygen utilization.     

A Mathematical Model of the Controlled Plant of the Réspiratory System

Ability to predict the dynamic response of oxygen, carbon dioxide tensions, and pH in blood and tissues to abrupt changes in ventilation is important in the mathematical modeling of the respiratory system. In this study, the controlled plant (the amount and distribution of O₂ and CO₂) of the respiratory system is modeled. Although the body tissues are divided into a finite number of “compartments” (three tissue groups), in contrast to earlier models, the blood and tissue gas tensions within each compartment are considered to be continuously distributed in time and in one spatial coordinate. The mass conservation equations for oxygen and carbon dioxide involved in the blood-tissue gas exchange are described by a set of partial differential equations which take into account convection of O₂ and CO₂ caused by the flow of blood as well as diffusion due to local tension gradients. Nonlinear algebraic equations for the dissociation curves, which take into account the Haldane and Bohr effects in blood, are used to obtain the relationships between concentrations and partial pressures. Time-variable delays caused by the arterial and venous transport of the respiratory gases are also included. The model so constructed successfully reproduced actual O₂ and CO₂ tensions in arterial blood, and in muscle venous and mixed venous blood when ventilation was abruptly changed.     

Cross-talk of NO, superoxide and molecular oxygen, a majesty of aerobic life

Because nitric oxide (NO) reacts with various molecules, such as hemeproteins, superoxide and thiols including glutathione (GSH) and cysteine residues in proteins, biological effects and metabolic fate of this gaseous radical are affected by these reactants. Although the lifetime of NO is short particularly under air atmospheric conditions (where the oxygen tension is unphysiologically high), it increases significantly under physiologically low oxygen concentrations. Because oxygen tensions in human body differ from one tissue to another and change depending on their metabolism, biological activity of NO in various tissues might be affected by local oxygen tensions. To elucidate the role of NO and related radicals in the regulation of circulation and energy metabolism, their effects on arterial resistance and energy metabolism in mitochondria, mammalian cells and enteric bacteria were studied under different oxygen tensions. Kinetic analysis revealed that NO-dependent generation of cGMP in resistance arteries and their relaxation were strongly enhanced by lowering oxygen tensions in the medium. NO reversibly suppressed the respiration and ATP synthesis of isolated mitochondria and intact cells particularly under low oxygen tensions. Kinetic analysis revealed that cross-talk between NO and superoxide generated in and around endothelial cells regulates arterial resistance particularly under physiologically low oxygen tensions. NO also inhibited the respiration and ATP synthesis of E. coli particularly under low oxygen tensions. Because concentrations of NO and H⁺ in gastric juice are high, most ingested bacteria are effectively killed in the stomach. However, the inhibitory effects of NO on the respiration and ATP synthesis of H. pylori are extremely small. Kinetic analysis revealed that H. pylori generates the superoxide radical thereby inhibiting the bactericidal action of NO in gastric juice. Based on such observations, critical roles of the cross-talk of NO, superoxide and molecular oxygen in the regulation of energy metabolism and survival of aerobic and microaerophilic organisms are discussed.     

Cross-talk of NO,superoxide and molecular oxygen,a majesty of aerobic life

Because nitric oxide (NO) reacts with various molecules, such as hemeproteins, superoxide and thiols including glutathione (GSH) and cysteine residues in proteins, biological effects and metabolic fate of this gaseous radical are affected by these reactants. Although the lifetime of NO is short particularly under air atmospheric conditions (where the oxygen tension is unphysiologically high), it increases significantly under physiologically low oxygen concentrations. Because oxygen tensions in human body differ from one tissue to another and change depending on their metabolism, biological activity of NO in various tissues might be affected by local oxygen tensions. To elucidate the role of NO and related radicals in the regulation of circulation and energy metabolism, their effects on arterial resistance and energy metabolism in mitochondria, mammalian cells and enteric bacteria were studied under different oxygen tensions. Kinetic analysis revealed that NO-dependent generation of cGMP in resistance arteries and their relaxation were strongly enhanced by lowering oxygen tensions in the medium. NO reversibly suppressed the respiration and ATP synthesis of isolated mitochondria and intact cells particularly under low oxygen tensions. Kinetic analysis revealed that cross-talk between NO and superoxide generated in and around endothelial cells regulates arterial resistance particularly under physiologically low oxygen tensions. NO also inhibited the respiration and ATP synthesis of E. coli particularly under low oxygen tensions. Because concentrations of NO and H⁺ in gastric juice are high, most ingested bacteria are effectively killed in the stomach. However, the inhibitory effects of NO on the respiration and ATP synthesis of H. pylori are extremely small. Kinetic analysis revealed that H. pylori generates the superoxide radical thereby inhibiting the bactericidal action of NO in gastric juice. Based on such observations, critical roles of the cross-talk of NO, superoxide and molecular oxygen in the regulation of energy metabolism and survival of aerobic and microaerophilic organisms are discussed.     

The Acid-Base Status in Cyanotic Congenital Heart Disease

The relation between oxygen consumption, metabolic status and prognosis was studied in two infants with identically low arterial oxygen tensions (20 mm. Hg) due to cyanotic congenital heart disease. The first patient had low oxygen consumption, arterial blood acidosis and increased arterial lactate, and died at the age of 36 hours. The second had normal oxygen consumption, arterial acid-base balance, lactate and pyruvate, and survived. Since arterial oxygen tensions were similar in both, it was concluded that compensatory factors, such as cardiac output, pulmonary and systemic blood flow and increased oxygen saturation at normal pH levels (Bohr effect), are important in maintaining tissue oxidation and preventing anaerobiosis and lactate production. The importance of a knowledge of acid-base status in the immediate prognosis of cyanotic congenital heart disease is stressed. The treatment of acidosis by buffer therapy may be an important palliative, improving cardiac output and tissue oxidation, and should be undertaken as soon as possible before irreversible cellular damage has occurred.     

Steady-state measurements of respiration rate with an oxygen electrode.

An oxygen electrode was developed which measures steady-state respiration rates in a volume of 0.25 ml and at oxygen concentrations as low as 0.1 μm. The steady state was achieved by pumping air-equilibrated buffer into the respirometer at various rates. The method is most suitable for tissue slices.     

Effect of oxygen concentration on the formation of molondialdehyde-like material in a model of tissue ischemia and reoxygenation

This study was conducted to explore the functional relationship between oxygen concentration during tissue reoxygenation after ischemia and the extent of postischemic lipid peroxidation, an indicator of reoxygenation injury. Excised rat liver or kidney tissue was rendered ischemic for 1 h at 37°C, minced into 1 mm³ fragments, and then reoxygenated for 1 h in flasks of buffered salt solution containing various amounts of oxygen. Production of malondialdehyde-like material (MDA) was measured to indicate lipid peroxidation. MDA production was minimal at oxygen tensions less than 10 mmHg, increased sharply from 10 to 50 mmHg, and plateaued at approximately 100 mmHg. A similar functional relationship was produced by a simple mathematical model of free radical mediated lipid peroxidation in biological membranes, suggesting that MDA production is indeed caudes by free radical oxidation of membrane phospholipids and that the oxygen effect is governed by simple competition between chain propagation and chain termination reactions within the membrane. These experimental and analytical results confirm that relatively low concentrations of oxygen are sufficient to produce oxidative damage in post-ischemic tissues.     

Perivascular oxygen tensions in a transplantable mammary tumor growing in a dorsal flap window chamber.

Fischer 344 rats with R3230 Ac mammary carcinomas implanted in dorsal flap window chambers served as a model to obtain measurements of perivascular and stromal oxygen tension in normal and tumor tissues using Whalen recessed-tip microelectrodes (3- to 6-microns tip). Perivascular measurements were made adjacent to vessels with continuous blood flow. Thus the measurements and models provided are reflective of conditions leading to chronic hypoxia. Perivascular oxygen tensions averaged 72 +/- 13 mmHg in normal tissue vessels adjacent to tumor, 26 +/- 5 mmHg in tumor periphery, and 12 +/- 3 mmHg in tumor central vessels. There was a significant trend toward lower perivascular oxygen tensions in the tumor center (Kruskal-Wallis test, P = 0.002). A similar tendency was seen with a limited number of stromal measurements. Krogh cylinder models, which incorporate these data for perivascular oxygen tension, along with morphometric data obtained from the same tumor model suggest that hypoxic regions will exist between tumor vessels in the tumor center unless O2 consumption rates are well below 0.6 ml/100 g/min. The low perivascular measurements observed near the tumor center combined with the theoretical considerations suggest, for this model at least, that tissue oxygenation may best be improved by increasing red cell velocity and input pO2 and reducing oxygen consumption. The low perivascular oxygen tensions observed near the center also suggest that conditions conducive to increased red cell rigidity exist, that drugs which can decrease red cell rigidity could improve tumor blood flow and oxygenation, and that the endothelium of those vessels may be susceptible to hypoxia-reoxygenation injury.     

New insights into the mechanisms of embryonic stem cell self-renewal under hypoxia: a multifactorial analysis approach

Previous reports have shown that culturing mouse embryonic stem (mES) cells at different oxygen tensions originated different cell proliferation patterns and commitment stages depending on which signaling pathways are activated or inhibited to support the pluripotency state. Herein we provide new insights into the mechanisms by which oxygen is influencing mES cell self-renewal and pluripotency. A multifactorial approach was developed to rationally evaluate the singular and interactive control of MEK/ERK pathway, GSK-3 inhibition, and LIF/STAT3 signaling at physiological and non-physiological oxygen tensions. Collectively, our methodology revealed a significant role of GSK-3-mediated signaling towards maintenance of mES cell pluripotency at lower O(2) tensions. Given the central role of this signaling pathway, future studies will need to focus on the downstream mechanisms involved in ES cell self-renewal under such conditions, and ultimately how these findings impact human models of pluripotency.     

Uniform tissues engineered by seeding and culturing cells in 3D scaffolds under perfusion at defined oxygen tensions

In this work, we assessed whether culture of uniformly seeded chondrocytes under direct perfusion, which supplies the cells with normoxic oxygen levels, can maintain a uniform distribution of viable cells throughout porous scaffolds several milimeters in thickness, and support the development of uniform tissue grafts. An integrated bioreactor system was first developed to streamline the steps of perfusion cell seeding of porous scaffolds and perfusion culture of the cell-seeded scaffolds. Oxygen tensions in perfused constructs were monitored by in-line oxygen sensors incorporated at the construct inlet and outlet. Adult human articular chondrocytes were perfusion-seeded into 4.5 mm thick foam scaffolds at a rate of 1 mm/s. Cell-seeded foams were then either cultured statically in dishes or further cultured under perfusion at a rate of 100 microm/s for 2 weeks. Following perfusion seeding, viable cells were uniformly distributed throughout the foams. Constructs subsequently cultured statically were highly heterogeneous, with cells and matrix concentrated at the construct periphery. In contrast, constructs cultured under perfusion were highly homogeneous, with uniform distributions of cells and matrix. Oxygen tensions of the perfused medium were maintained near normoxic levels (inlet congruent with 20%, outlet > 15%) at all times of culture. We have demonstrated that perfusion culture of cells seeded uniformly within porous scaffolds, at a flow rate maintaining a homogeneous oxygen supply, supports the development of uniform engineering tissue grafts of clinically relevant thicknesses.     

Oxygen modulates the growth of skin fibroblasts

Elevated oxygen tensions are inhibitory to the growth of skin fibroblasts. Skin fibroblasts grow better at oxygen tensions below 137 mm Hg regardless of seeding density. A wide range of oxygen tensions, including those in the physiological range, strongly modulate the growth of human skin fibroblasts. There were no significant differences between the responses of fetal and postnatal cell lines to changes in ambient oxygen tension. In all cases, higher oxygen tensions significantly impeded cell growth. Seeding cells at 10(4) cells/cm(2) afforded some protection from the deleterious effects of hyperoxia. Oxygen tensions exceeding the amount present in ambient room air also impeded cell growth at this higher seeding density, but the effect did not become significant until the oxygen partial pressure reached 241 mm Hg. At lower oxygen tensions, cells seeded at 10(3) cells/cm(2) grew more rapidly than did cells seeded at 10(4) cells/cm(2). These findings may have implications for the treatment of poorly healing wounds with hyperbaric oxygen.     

The secretion of inert gas into the swim-bladder of fish

The composition of the gas mixture secreted into the swim-bladders of several species of fish has been determined in the mass spectrometer. The secreted gas differed greatly from the gas mixture breathed by the fish in the relative proportions of the chemically inert gases, argon, neon, helium, and nitrogen. Relative to nitrogen the proportion of the very soluble argon was increased and the proportions of the much less soluble neon and helium decreased. The composition of the secreted gas approaches the composition of the gas mixture dissolved in the tissue fluid. A theory of inert gas secretion is proposed. It is suggested that oxygen gas is actively secreted and evolved in the form of minute bubbles, that inert gases diffuse into these bubbles, and that the bubbles are passed into the swim-bladder carrying with them inert gases. Coupled to a preferential reabsorption of oxygen from the swim-bladder this mechanism can achieve high tensions of inert gas in the swim-bladder. The accumulation of nearly pure nitrogen in the swim-bladder of goldfish (Carassius auratus) is accomplished by the secretion of an oxygen-rich gas mixture followed by the reabsorption of oxygen.     

Effect of culture PO2 on macrophage (RAW 264.7) nitric oxide production

Macrophages are commonly cultured at a PO2 of 149 Torr, but tissue macrophages in vivo live in an environment of much lower oxygen tension. Despite the many potential mechanisms for changes in oxygen tension to influence nitric oxide (NO) synthesis, there have been few reports investigating the effect of PO2 on macrophage NO production. With the use of a culture chamber designed to rigorously control oxygen tension, we investigated the effects of culture PO2 on macrophage NO production, inducible nitric oxide synthase (iNOS) activity, iNOS protein, and tumor necrosis factor production. NO production and iNOS activity were linearly related in the range of 39.4 to 677 Torr, but not in the range of 1.03 to 39.4 Torr. Therefore, results obtained in vitro for the high oxygen tensions commonly used in cell culture were quantitatively and qualitatively different from results obtained in cells cultured at the lower oxygen tensions that more accurately reflect the in vivo environment. The influence of oxygen tension on NO production has implications for cell culture methodology and for the relationship between microcirculatory dysfunction and inflammatory responses in rodent models of sepsis.     

Eggplant tissue homogenate-based bioselective membrane electrode for determination of catechol.

A novel eggplant tissue homogenate-based membrane electrode with high selective response to catechol (5 x 10(-6)-2.5 x 10(-5) M concentration) has been constructed by immobilizing tissue of eggplant (Solanum melangena L.) at dissolved oxygen probe. In order to optimize the stability of the electrode, general immobilization techniques are used to secure the eggplant tissue section physically in a gelatin-glutaraldehyde cross-linking matrix. The electrode response was maximum when 50 mM phosphate buffer was used at pH 7.0 and 35 degrees C. The sensor is stable for more than 3 months.     

Engineering select physical properties of cross-linked red blood cells and a simple a priori estimation of their efficacy as an oxygen delivery vehicle within the context of a hepatic hollow fiber bioreactor

Bovine red blood cells (bRBCs) can potentially provide a simplistic and economic means of improving oxygenation within hollow fiber (HF) bioreactor cell cultures. Bovine RBCs are also interesting since many of their physical properties can be altered as a result of glutaraldehyde (G) cross-linking. Cross-linking bRBCs produces an oxygen carrier that is expected to be beneficial under specific circumstances (i.e., delivery of oxygen to cells that are sensitive to free hemoglobin (Hb) and cells that require low inlet oxygen tensions). We have examined the osmotic stability and electrophoretic mobility of cross-linked bRBCs and observed that cross-linking improves osmotic stability while minimally impacting electrophoretic mobility. The oxygen binding/dissociation properties (P(50) and n) of cross-linked bRBCs were also measured, and under the reported reaction conditions, cross-linking increased the oxygen affinity and reduced the cooperativity of bRBCs. A basic Krogh tissue cylinder model was then utilized to provide a quick a priori estimate of oxygen delivery and release to hepatocytes housed within a HF bioreactor in order to demonstrate potential oxygenation benefits arising with both normal and cross-linked bRBC media supplementation. This model showed that bRBCs generally improved oxygen delivery and release to HF cell cultures and that cross-linked bRBCs are particularly beneficial in specifically targeting oxygen delivery to cells maintained at low inlet oxygen tensions. Additionally, the model showed that bRBC supplementation can significantly improve oxygen delivery without requiring extreme bRBC concentrations.     

A bypass of sucrose synthase leads to low internal oxygen and impaired metabolic performance in growing potato tubers

Plants possess two alternative biochemical pathways for sucrose (Suc) degradation. One involves hydrolysis by invertase followed by phosphorylation via hexokinase and fructokinase, and the other route-which is unique to plants-involves a UDP-dependent cleavage of Suc that is catalyzed by Suc synthase (SuSy). In the present work, we tested directly whether a bypass of the endogenous SuSy route by ectopic overexpression of invertase or Suc phosphorylase affects internal oxygen levels in growing tubers and whether this is responsible for their decreased starch content. (a) Oxygen tensions were lower within transgenic tubers than in wild-type tubers. Oxygen tensions decreased within the first 10 mm of tuber tissue, and this gradient was steeper in transgenic tubers. (b) Invertase-overexpressing tubers had higher activities of glyceraldehyde-3-phosphate dehydrogenase, lactate dehydrogenase, and alcohol dehydrogenase, and (c) higher levels of lactate. (d) Expression of a low-oxygen-sensitive Adh1-beta-glucuronidase reporter gene construct was more strongly induced in the invertase-overexpressing background compared with wild-type background. (e) Intact transgenic tubers had lower ATP to ADP ratios than the wild type. ATP to ADP ratio was restored to wild type, when discs of transgenic tubers were incubated at 21% (v/v) oxygen. (f) Starch decreased from the periphery to the center of the tuber. This decrease was much steeper in the transgenic lines, leading to lower starch content especially near the center of the tuber. (g) Metabolic fluxes (based on redistribution of (14)C-glucose) and ATP to ADP ratios were analyzed in more detail, comparing discs incubated at various external oxygen tensions (0%, 1%, 4%, 8%, 12%, and 21% [v/v]) with intact tubers. Discs of Suc phosphorylase-expressing lines had similar ATP to ADP ratios and made starch as fast as wild type in high oxygen but had lower ATP to ADP ratios and lower rates of starch synthesis than wild type at low-oxygen tensions typical to those found inside an intact tuber. (h) In discs of wild-type tubers, subambient oxygen concentrations led to a selective increase in the mRNA levels of specific SuSy genes, whereas the mRNA levels of genes encoding vacuolar and apoplastic invertases decreased. (i) These results imply that repression of invertase and mobilization of Suc via the energetically less costly route provided by SuSy is important in growing tubers because it conserves oxygen and allows higher internal oxygen tensions to be maintained than would otherwise be possible.     

Noninvasive oxygen measurements and mass transfer considerations in tissue culture flasks

Murine hybridomas were cultivated in tissue culture flasks. Dissolved oxygen tensions in the gas and liquid phases during cell growth were monitored. Oxygen levels were measured noninvasively by interrogating an oxygen-sensitive patch mounted on the interior surface of the tissue culture flask with an optrode from outside the tissue culture flask. Readings were made in tissue culture flasks with caps both cracked open and completely closed. Although the oxygen in the gas phase remained near atmospheric oxygen levels in both flasks, over time the liquid-phase oxygen tension at the bottom of the flasks reached zero during cell growth in both the open and closed tissue culture flasks. These results suggest that the widespread practice of cracking open tissue culture flask caps during cell growth with a view to supplying adequate oxygen to cells is ineffective and probably unnecessary.The mass transfer characteristics of the tissue culture flask were also studied. The dominant resistance to oxygen mass transfer to the sensor and the cells was through the liquid media. The mass transfer rates through the liquid layer under standard laboratory conditions were found to be greater than those predicted by diffusion alone. This suggests that mixing at a microscale occurs. Volumetric and specific oxygen consumption rates were also calculated from the sensor data. These consumption rates were comparable with values published elsewhere. (c) 1996 John Wiley & Sons, Inc.