Breathing Mice Research Articles

Ozone affects breathing and pulmonary surfactant function in mice

The effect on breathing of BALB/c mice immediately following ozone exposure (2 ppm) for 0, 2, 4, 6, and 8 h was studied with a whole body plethysmograph. Whether such exposure affected the normal function of pulmonary surfactant of maintaining airway patency was evaluated with a capillary surfactometer. Respiratory rate in mice that were not exposed was 358±16 (mean±S.E.) breaths/min and decreased to 202±10 after 6 h exposure. The mean pressure change caused by breathing diminished significantly, indicating a reduced tidal volume. BAL fluid from controls maintained patency for 88±2% of the study time, 120 s, implying a good surfactant function, but the ozone exposure caused the surfactant to lose its capability of maintaining patency ( P<0.0001).This decaying surfactant function of the BAL fluid coincided with an increasing protein concentration in the fluid of exposed animals (1.46±0.14 mg/ml in the 8-h group) as compared to controls (0.44±0.04 mg/ml, P<0.0001). It is concluded that leakage of plasma proteins into the airway lumen was probably the main reason for the surfactant dysfunction, which may have contributed to the altered breathing pattern.

Repeated measurement of respiratory function and bronchoconstriction in unanesthetized mice.

A noninvasive forced oscillation technique was used to determine respiratory function in unanesthetized and spontaneously breathing mice. Pseudorandom noise pressure variations in a frequency range of 16-208 Hz were applied to the body surface, and the flow response was measured at the nose. From the pressure-flow relationship, respiratory transfer impedance was calculated. Study of intra-animal variability on a short- and a long-term basis revealed that the real part of respiratory transfer impedance was reproducible within 9%. The imaginary part appeared less reproducible (within 22%). Furthermore, bronchoconstrictive responses were investigated and analyzed by evaluation of respiratory resistance as measured at 16 Hz (Rrs16). During the first 15 min after ovalbumin challenge in ovalbumin-sensitized mice, Rrs16 was significantly increased [49 +/- 7% (SE)]. Inhalation of methacholine in untreated mice induced an increase in Rrs16 of 75 +/- 16% (SE). In saline-challenged animals, no significant changes were observed. This method enables evaluation of long-term respiratory function in mice and appeared to be a sensitive measure for bronchoconstriction.

PO 2 in irradiated versus nonirradiated tumors of mice breathing oxygen at normal and elevated pressure

To determine if prior tumor irradiation influences tumor pO2 changes in mice breathing oxygen (100%) at normal and elevated pressure. Single-point pO2 measurements were performed in nonirradiated and previously irradiated (72 h) isotransplanted MCaIV tumors in C3H/Sed mice. Continuous recordings were performed at the same tumor locus under air breathing, followed by 100% oxygen and oxygen at three atmospheres pressure. Following decompression and induction of pentobarbital anesthesia, the procedure was repeated at the same locus. Six nonirradiated and five irradiated tumors were evaluated under the three gas breathing conditions +/- anesthesia. The mean, median, and range of pO2 values did not differ under air-breathing conditions in the nonirradiated vs. previously irradiated tumors. However, prior irradiation substantially enhanced the tumor pO2 increase when the inspired gas phase was switched from air to 100% oxygen at 1 or 3 atmospheres pressure. In four of six nonirradiated tumors, 100% oxygen breathing resulted in a pO2 increase of < 4 mmHg; in the irradiated tumors, the minimum increase was 16 mmHg. Pentobarbital anesthesia did not significantly influence the results obtained. These data indicate that the efficacy of oxygen breathing increases during tumor treatment, and suggests that oxygen breathing is a simple nontoxic method for reducing or eliminating radiobiologic hypoxia during therapy.

Maturational changes in the respiratory rhythm generator of the mouse.

The changes in motor activity of the respiratory rhythm generator were quantitatively analysed in mice (from birth to at least 56 days old) in both awake and anaesthetized preparations, as well as in vitro to define the age at which the respiratory network is mature. In awake and anaesthetized spontaneously breathing mice respiratory-related thoracic movements were recorded and revealed an age-dependent increase in both inspiratory time (45%) and cycle length (22%) over the first 15 days of life. Similarly, the pattern of phrenic nerve activity recorded from anesthetized animals also changed from a short, rapid onset and offset burst, without a post-inspiratory phase (0-10 days old), to a discharge of longer duration which included both ramp and post-inspiratory components (> 15 days). This pattern was comparable to that seen in adult mice (> 56 days old). A recently developed tilted-sagittal brainstem slice preparation containing an isolated, but functionally intact, medullary respiratory network was employed in our in vitro studies. Since this preparation generates respiratory rhythmic activity spontaneously in both neonatal and mature mice (> 56 days old) it has permitted a direct comparison of the respiratory motor output pattern, recorded from the hypoglossal (XII) motor nucleus, during post-natal development in similar preparations. Consistent with our in vivo findings there was an age-dependent change in the motor pattern. The rhythmic burst of XII neurones recorded from slices of neonates (0-10 days old) was short in duration and decremented whereas a longer discharge (increase of 625% compared to neonate) containing a plateu component was seen in animals more than 15 days old. In addition, the cycle length of rhythmic XII neurones increased (143%) and, together with the changes in burst duration, reached a steady-state value over a similar time course to the maturational changes in phrenic nerve activity recorded in vivo.

The role of alpha 1-antitrypsin in the control of extracellular surfactant metabolism

Alveolar surfactant exists in several structural forms that are believed to be secular products of the secreted form, lamellar bodies. The conversion of tubular myelin to the small vesicular form appears to require the action of a novel serine protease, surfactant convertase. As the in vitro activity of this enzyme is quite sensitive to inhibition by the serine protease inhibitor alpha 1-antitrypsin, a normal constituent of the alveolar fluid lining layer, we explored the possibility that the alveolar level of alpha 1-antitrypsin might affect the rate of subtype conversion in vivo. When the alveolar alpha 1-antitrypsin level was augmented by tracheal instillation of exogenous alpha 1-antitrypsin, the newly synthesized surfactant phospholipids of spontaneously breathing mice accumulated in the heavy subtype, and virtually none was found in the light subtype form up to 72 h later. An in vivo turnover study suggested that when the alveolar alpha 1-antitrypsin level was raised by the same means, the flux of surfactant through the light (small vesicular) compartment was virtually shut off, despite the availability of abundant amounts of its precursor, heavy subtype (tubular myelin). Finally, mouse lungs that were lavaged to remove resident surfactant and mechanically ventilated for 1-5 h released surfactant that progressed through heavy and light subtype compartments as in vivo. But in mice whose lung alpha 1-antitrypsin might affect the metabolism of alveolar surfactant in addition to its well-known role in lung defense.

Discrepancies between measured changes of radiobiological hypoxic fraction and oxygen tension monitoring using two assay systems

Purpose : This study was conducted to assess the ability of computerized pO 2 histography to measure changes in tumor oxygenation produced by low oxygen breathing. Methods and Materials : Female syngeneic C3H/Km mice bearing SCC VII/St carcinomas were used in these experiments. Changes in tumor oxygenation produced by the mice breathing 10% oxygen were assessed with computerized pO2 histography, 3H-misonidazole binding, and the paired survival curve assay of radiosensitivity. Results : The hypoxic cell fraction of the tumors in mice breathing 10% oxygen was 3.1 times higher than that of tumors in mice breathing normal air determined by an in vivo-in vitro clonogenic assay. Binding of radiolabeled misonidazole to the tumors in mice breathing 10% oxygen was also significantly higher than that to tumors in mice breathing normal air ( p < 0.05). In addition, oxygen tension histograms for normal tissue showed a dramatic shift to a lower oxygen tension when the mice were breathing 10% oxygen. However, under identical conditions, there was only a minimal shift in the oxygen tension of tumor tissue. Although the number of oxygen tension readings in the relatively oxic class in tumor tissue was lower when the mice were breathing 10% oxygen than when breathing normal air, there was not a significant decrease in the median pO 2 value for the tumor. The number of pO 2 readings lower than 5 mmHg in the tumor was not affected by the 10% oxygen breathing. Conclusion : These findings indicate that increases in radiobiological hypoxic fraction produced by lower blood oxygen levels may not correlate well with the results of polarographic measurements of tumor pO 2 levels.

Nicotinamide and carbogen: major effect on the radiosensitivity of EMT6 and HRT 18 tumours

The effect of nicotinamide and/or carbogen breathing on the response of EMT6 and HRTl8 tumours, irradiated in vivo, was evaluated using an in vitro colony assay. In the single dose schedules, the most efficient treatment was carbogen plus nicotinamide compared to air breathing mice. In a fractionated regime carbogen plus nicotinamide increased also response to radiation.

Enhancement of tumor radiosensitivity and reduced hypoxiadependent binding of a 2-nitroimidazole with normobaric oxygen and carbogen: A therapeutic comparison with skin and kidneys

To evaluate the therapeutic potential of normobaric oxygen and carbogen as hypoxic-cell sensitizers, both radiosensitization in a mouse mammary carcinoma, mouse skin and kidneys, and the reduction in the proportion of hypoxic tumor cells were quantified in mice breathing air, oxygen, or carbogen. Local tumor control, acute skin reactions, reduced renal clearance, and hematocrit were used as assays. X rays as 10 fractions in 5 days were given to skin and tumors and 10F/12 days to kidneys. In the tumor study, the pre-irradiation breathing time was varied from 2 to 20 min. Hypoxic cells, before and during a 10F/5 day schedule, were quantified using a 2-nitroimidazole with a theophylline side chain. Bioreductively reduced metabolites of this probe were localized in hypoxic cells that were then stained using an immunofluorescent technique and analyzed by flow cytometry. The fraction of cells with high fluorescence intensity was 19% in air, 9% in oxygen, and 3% in carbogen-breathing mice. For all three gases, hypoxia-dependent binding was similar in non-irradiated tumors and those treated with four or nine fractions. Both gases significantly enhanced tumor radiosensitivity (ER = 1.3 to 1.6) and carbogen was slightly more effective than oxygen. With carbogen, maximum sensitization was observed with a 5 min pre-irradiation breathing interval. With oxygen, pre-irradiation breathing times of 2–20 min gave similar sensitization. In skin an enhancement ratio of 1.2 was observed, whereas enhancement ratios for both renal endpoints were significantly lower (1.0 to 1.07). Relative to both tissues, there was therefore a substantial therapeutic gain by irradiating CaNT tumors under both gases, especially with carbogen.

Influence of carboxyhemoglobin level on tumor growth, blood flow, and radiation response in an experimental model

Carboxyhemoglobin (HbCO) is formed when carbon monoxide is bound to hemoglobin. High levels of HbCO are known to reduce the amount of oxygen that can be carried to the tissues. This experimental study focuses on the influence of HbCO on the growth, blood flow, and radiation response of an experimental mouse tumor. The study was designed to mimic the clinical situation where heavy smokers are undergoing radiotherapy while having a high HbCO level. The tumor was a C3H mammary carcinoma grown in the feet of CDF 1 mice. Chronic exposure to carbon monoxide, resulting in 10% HbCO, increased the tumor volume doubling time from 2.5 to 3.5 days ( p < 0.05). The acute exposure to carbon monoxide prior to and during irradiation significantly raised the radiation dose required to control the tumor locally. The TCD 50 increased from 54 Gy in air breathing mice (HbCO 0–2%) to 57 Gy (HbCO 7–9%) and 61 Gy (HbCO 20–23%). This increase corresponded to an increase in the fraction of clonogenic hypoxic tumor cells from 0.12 in air breathing animals to 0.21 and 0.41, respectively. The tumor blood flow, determined by the 86RbCl extraction technique, decreased to 63% (n.s.) and 50% ( p < 0.05) for low and high HbCO levels, respectively.

Superoxide dismutase activity in lung from copper- and manganese-deficient mice exposed to ozone

Nutritional manganese (Mn) or copper (Cu) deficiency was investigated in Swiss-Webster mice exposed to ozone (O 3). Mice rendered Mn-deficient were first reared from Mn-deficient dams and then fed a Mn-deficient (1 μg/g) diet. Mice rendered Cu-deficient were fed a diet containing 0.2 μg Cu/g diet. Control mice were fed a diet containing Mn at 45 μg/g and Cu at 8 μg/g. During the last week of the experiment (week 7, post-weanling), mice in each group were exposed continuously to 1.2 ppm O 3 or filtered air for 7 days. Superoxide dismutase (SOD) activity in lung was then estimated. In mice breathing filtered air, neither lung Cu,Zn- nor Mn-SOD activity (U/g) was affected by diet. In O 3-exposed mice, howver, Mn-SOD activity was lower in the Mn-deficient group and Cu,Zn-SOD activity was lower in the Cu-deficient group. Moreover, total lung Cu,Zn-SOD activity was elevated in the Mn-deficient mice, whereas total Mn-SOD activity was elevated in the Cu-deficient mice in response to O 3. These data indicate that under normal circumstances lung Cu,Zn-SOD and Mn-SOD are not affected by Cu or Mn deficiency. However, when an oxidant stress is superimposed on the Cu- or Mn-deficient condition, Cu,Zn- and Mn-SOD activities are impaired.

Fluorocarbon-enhanced mutagenesis of polyaromatic hydrocarbons

The widely used fluorocarbon refrigerant and cleaning solvent 1,1,2-trichloro-1,2,2-trifluoroethane (Freon TF), though generally considered biologically inert, enhances the metabolic activation of chemical carcinogens. Liver microsomal extracts from mice given single intraperitoneal injections of this fluorocarbon showed significant increases in their ability to activate carcinogenic polyaromatic hydrocarbons to form mutagens, compared to control mice injected with saline. Polyaromatic hydrocarbons aminofluorene and acetylaminofluorene were activated in this way. Mutagenicity was measured by a microbial assay. Both commercial grade and redistilled fluorocarbons gave similar results, that is, more highly active liver extracts after administration of the fluorocarbon preparation to mice. Neither industrial grade nor redistilled preparation was itself mutagenic. A combined liver microsomal extract from mice breathing Freon TF at 20,000 ppm in air for 8 hr also had enhanced ability to activate aminofluorene as a mutagen. Exposing mice to Freon TF by inhalation more closely matches the normal route of human exposure to fluorocarbons. The results of this study imply that low-molecular-weight fluorocarbons may pose a carcinogenic risk by acting as cocarcinogenic enhancers of carcinogen activation. The possibility that fluorocarbons are cocarcinogens in this way has apparently not been heretofore considered.

Metabolic mapping of functional activity in the olfactory system of normal and hypogonadal (hpg) mice.

The hypogonadal mouse, which lacks gonadotropin-releasing hormone, has been suggested as an animal model of Kallmann's syndrome, one symptom of which is hyposmia. We have determined the metabolic activity of the olfactory system, in normal and hypogonadal mice, using [14C]-2-deoxyglucose quantitative autoradiography. In the olfactory lobes, deoxyglucose uptake was greatest in the glomerular and granule cell layers and low in the olfactory nerve layer and bulb core. The pattern of uptake was similar in both hypogonadal and normal mice breathing filtered air. Exposure of normal mice to ethyl acetoacetate significantly increased deoxyglucose uptake in the olfactory nerve layer and glomerular layer, but not in the granule cell layer. Several foci of intense metabolic activity were produced, apparently corresponding to small groups of activated glomeruli. There were no changes in the secondary or tertiary projections of the olfactory system. In hypogonadal mice, ethyl acetoacetate failed to increase the number of foci and the density of labelling in the olfactory nerve layer and glomerular layer. These data show that the functional activity of the olfactory system in hypogonadal mice breathing air is apparently normal. However, the olfactory response to ethyl acetoacetate is significantly less in hypogonadal mice. Whether this is due to their lack of gonadotropin-releasing hormone requires further experimentation.

The influence of haemoglobin affinity for oxygen on tumour radiosensitivity

Appropriate control of the affinity of haemoglobin for oxygen is fundamental to the efficient oxygenation of our tissues. Important modifiers of this relationship are pH, CO2 concentration and the intraerythrocytic level of 2,3-diphosphoglycerate (2,3-DPG). We have studied the influence of haemoglobin affinity on the radiosensitivity of the RIF-1 sarcoma in the mouse. Changes in haemoglobin affinity were induced by exposing donor mice to either 10% oxygen, normal air, or 100% oxygen for 48 h. Blood was drawn from these animals and exchanged transfused into tumour-bearing mice immediately before irradiation. Transfusion of blood from mice breathing 10% oxygen carried a lowered haemoglobin affinity and produced marked radiosensitization of the tumours in the recipients; transfusion with normal blood had no significant effect and transfusions from mice breathing 100% oxygen caused a small increase in radioresistance. Measurements of the level of 2,3-DPG in the blood of these groups showed higher concentrations in the oxygen-deprived animals than in controls but no significant change in animals exposed to 100% oxygen. These results demonstrate that alterations in haemoglobin affinity, probably resulting from changes in 2,3-DPG levels, can have a powerful influence on tumour radiosensitivity. We feel that this mechanism could have considerable clinical importance.

Effect of inhalation anesthetics on antipyrine pharmacokinetics of mice

The effects of the volatile anesthetics, enflurane, isoflurane and halothane, on the pharmacokinetics of antipyrine were examined in mice. The administration of 0.75% isoflurane or 1.0% enflurane in air resulted in a 173 and a 206% increase, respectively, in antipyrine plasma half-life and a 29.1 and a 41.2% decrease in antipyrine total body clearance. There was also an almost 2-fold increase in the volume of distribution of antipyrine. Halothane, at 0.5% in air, had no significant effect upon antipyrine plasma half-life or its volume of distribution. There was no significant change in antipyrine total body clearance and volume of distribution 4 hr after exposure to the volatile agents, but there was a small increase in half-life. The exposures to the volatile anesthetics were also carried out in an atmosphere of 8% oxygen. Antipyrine plasma half-life was increased significantly by 48% in mice breathing 8% oxygen, compared to mice breathing air. Isoflurane in 8% oxygen increased the plasma half-life of antipyrine by 296% compared to mice breathing 8% oxygen. This increase was greater than the effect of isoflurane seen in mice breathing air. Mice breathing halothane in 8% oxygen exhibited a 21% increase in antipyrine plasma half-life and mice breathing enflurane in 8% oxygen, a 117% increase in antipyrine plasma half-life, although the changes were not markedly different from those seen in mice breathing air. Enflurane and isoflurane produced a significant increase in the volume of distribution for antipyrine in the mice breathing 8% oxygen. Total body clearance of antipyrine was decreased markedly in mice breathing isoflurane and enflurane but showed a lesser decrease in mice breathing halothane in 8% oxygen. In vitro in mouse microsomes, halothane, enflurane and isoflurane were all inhibitors of aminopyrine metabolism. Possible mechanisms for these results are discussed.

Correction

In “Murine Lung Carcinogenesis Following Exposure to Ambient Ozone Concentrations,” by C. Hassett, M. G. Mustafa, W. F. Coulson, and R. M. Elashoff, pages 771–777, the chi-square analysis in experiments 1 and 2 were incorrectly computed and, in fact, was not the most appropriate test for assessing statistical significance. Analysis of the relevant data by means of a test of ordered differences, such as the Mann-WhitneyU test corrected for ties, is more appropriate. We have reanalyzed these data accordingly, and the results are as follows: In experiment 1 (0.31 ppm ozone), the lung-tumor response for ozone-exposed animals was not statistically different from that for mice breathing clean air (P< 09); and in experiment 2 (0.50 ppm ozone), there was a greater number of lung tumors in the ozone-exposed group as contrasted with the number in clean-air-breathing mice, and this difference was statistically significant (P<05). Our interpretation of the data regarding synergism between ozone and urethane effects and our overall conclusion that exposure to relatively high ambient concentrations of ozone can increase the incidence of lung adenomas in a sensitive animal model remain unchanged.

Reactions of olfactory bulb neurons to different stimulus intensities in laboratory mice.

The activity of 219 cells in the olfactory bulb of waking, freely breathing mice was analysed, and it was found that their spontaneous discharge rate varied between 0.3 and 33 AP/s. Both butyric acid and geraniol elicited responses of four types: 38% of the 98 responses were of type 1 (excitation), 43% of type 2 (inhibition), 10% of type 3 (complex structure), and 8% were characterized by a change in the temporal pattern of the activity. Response duration varied from less than 500 ms to more than 1 min. 52 secondary neurons were stimulated with four different concentrations of the odor substances. All of the responsive cells showed a clear ability to discriminate concentration. That is, response magnitude varied with intensity, producing non-monotonic curves. Most of the neurons responded only in a region of a more or less limited concentration, and in no case was a saturation curve observed. Approx. 40% of the neurons responded to the lowest concentration tested (10(-8) vol.% of butyric acid or geraniol). The strongest stimuli, 10(-2) vol.%, were relatively ineffective.

Acute Effects of a Perfluorochemical Oxygen Carrier on Normal Tissues of the Mouse

A single iv dose of 15 ml/kg fluosol DA (20%), a perfluorochemical oxygen carrier, caused hepatomegaly and splenomegaly which persisted for at least 3 weeks after drug injection. The peak increase in weight was at 3 days in the spleen (1.7x) and at 14 days in the liver (1.5x). Lung and kidney weights were not altered 1-21 days after administration of fluosol DA. The slopes of the single-dose radiation survival curves for intestinal epithelial cells and spermatogenic stem cells in mice breathing air or oxygen were not significantly altered by the administration of fluosol DA 10 min before irradiation, and the doses to achieve an isoeffect were altered by 1.03 or less. When mice were challenged with iv injected FSa tumor cells 24 h after treatment with fluosol DA, no increase in the number of artificial pulmonary metastases was observed.

Applicability of Henry's law to hydrogen, helium, and nitrogen solubilities in water and olive oil at 37 °C and pressures up to 300 atmospheres

The solubilities of pure hydrogen, helium, and nitrogen in water and olive oil were measured at 37 °C at gas-saturation pressures from 25 to 300 atmospheres. Rigorous thermodynamic criteria were used to assess the applicability of Henry's law to the pressure dependence of the gas solubility in each system. The solubilities of the three gases in water and helium in olive oil followed Henry's law as given by the Krichevsky-Kasarnovsky equation. In contrast, hydrogen and nitrogen in olive oil each attained concentrations high enough to cause significant concentration-dependent variations of the dissolved gas activity coefficient and/or partial molal volume. The consequent deviations from Henry's law were greatest in the nitrogen-oil system, where mole fraction nitrogen solubilities calculated from the Krichevsky-Kasarnovsky equation exceeded measured values by 8, 14, and 23% at 50, 100, and 250 atm, respectively. Incorporation of results into the critical volume model of nitrogen anesthesia, using olive oil as a model of the physiological anesthetic site and literature data for the anesthetic potency of nitrogen in mice breathing high-pressure He-N 2-O 2 atmospheres, shows that nonideal solution behavior may become important for gases dissolved in physiological hydrophobic regions at biologically active concentrations, even if dissolved gas binding to proteins or other macromolecules is not involved.

Tumor radiosensitization with concomitant vone marrow radioprotection: A study in mice using diethyldithiocarbamate (DDC) under oxygenated and hypoxic conditions

We have established, both in vitro and in vivo, that Diethyldithiocarbamate (DDC) protects mammalian cells from radiation. The in vivo protection, when non-toxic concentrations of DDC are present one-half hour before irradiation, is reflected by a dose modification factor (DMF) of 1.9 based on LD 50 30 and 1.5 using survival of CFU s as an endpoint. Further experiments in vivo have extended our knowledge to the differential radioprotective effects of DDC on the vone marrow of animals breathing room air compared to a 5.5% oxygen in nitrogen mixture. The DMF ( LD 50 30 ) for DDC in air breathing animals, previously established as 1.9, can be contrasted with a DMF, obtained in the present study, of 1.2 for animals irradiated in the hypoxic state. Moreover the DMF (CFU s survival) previously established at 1.5 for air breathing animals, can be compared to a value of 1.3, obtained in the present study, for mice irradiated under hypoxic conditions. Modification of the dose response by DDC, for both vone marrow and tumor, was also examined in animals bearing a RIF sarcoma. Although protection of the vone marrow was confirmed (DMF = 2.1), the striking finding was that the tumor cells were sensitized, in both air breathing and nitrogen breathing animals, by the addition of DDC one-half hour before the radiation exposure. Moreover, the tumor radiosensitization, a factor greater than 2, in air breathing animals, appeared to be independent of dose (D 0 = 200 rad, with or without DDC). The tumor radiosensitization was even more marked in the nitrogen breathing mice, in which a factor of 10 difference in survival was noted, together with a tendency towards greater sensitization at radiation doses in the clinical range. The results, demonstrating vone marrow radioprotection by DDC (aerobic > hypoxic) with concomitant tumor radiosensitization (hypoxic > aerobic) strongly suggest a large therapeutic gain factor (TGF) which could be further explored in a clinical setting.

The effect of breathing 100% oxygen on lung response to radiation in mice

The response of the lung after single doses of radiation was measured in mice breathing air, or 100% oxygen, or in air-breathing mice given the hypoxic cell sensitizer misonidazole 30 min before irradiation. There was a clear enhancement of only the pneumonitis response in the mice breathing oxygen when breathing rate or lethality was used to assess injury. Less enhancement of the late fibrotic reaction was observed in these animals. No enhancement of either phase of lung response was observed in the misonidazole-treated mice. Dose reduction factors (DRF) were estimated from these data and used to calculate oxygen concentration in the lung, giving values ranging between 187 and 250 micron oxygen.