Effect of dimethyl fumarate on the radiation sensitivity of mammalian cells in vitro

Dimethylfumarate (DMF) depletes intracellular glutathione (GSH) by covalent bond formation in a reaction which may be mediated by GSH-S-transferase. In Chinese hamster ovary cells this depletion is rapid; e.g., 0.5 mM DMF depletes GSH to less than 10% of control in 5 min at room temperature. DMF is a very effective hypoxic cell radiosensitizer, with an enhancement ratio (ER) of about 3 obtained by a 5-min exposure of cells at room temperature to 5 mM DMF, without significant toxicity. At this same concentration of drug, there is a small enhancement of aerobic cells (ER = 1.3), but the 5 mM DMF in hypoxia results in nearly a complete collapse of the hypoxic dose-response curve to the same level as seen in air with DMF. It has been suggested previously that DMF sensitizes cells via electron affinic mechanisms. However, this appears not to be the case in this study, as shown by the fact that cells pretreated with DMF and then washed free of the drug remained equally radiosensitive as cells irradiated in the presence of the drug. This large enhancement of radiation sensitivity appears to be related to the drug's ability to deplete thiols; i.e., thiols appear to be a major factor responsible for radioresistance of hypoxic cells.     

Hormone pretreatment enhances recovery of spermatogenesis in rats after neutron irradiation.

Previous studies showed that a 6-week pretreatment of rats with testosterone plus estradiol enhanced the recovery of spermatogenesis 9 weeks after gamma irradiation, resulting in a dose-modifying factor (DMF) of about 2. To test whether the effect of the hormone treatment was mediated through changes in oxygen tension, thiol levels or DNA repair, we irradiated the testes of rats with neutrons, which depend less on these factors than does low-LET radiation for their cytotoxic action. Control rats and rats treated with testosterone plus estradiol were irradiated with 0.7-2.7 Gy of cyclotron-generated high-energy neutrons. The recovery of spermatogenesis was assessed 9 weeks after irradiation by testis weights, sperm counts and the tubule repopulation indices. Greater recovery of spermatogenesis was observed for all end points, with a DMF of about 2 for rats treated with testosterone plus estradiol compared to the irradiated, cholesterol-treated rats. The equal protection factors for neutrons and gamma rays indicate that oxygen, thiols and repair of DNA damage are unlikely to be involved in the protective effect of the hormone treatment.     

Maximum oxygen uptake and arterial blood oxygenation during hypoxic exercise in rats.

The objectives of these experiments were 1) to describe the effect of maximum treadmill exercise on gas exchange, arterial blood gases, and arterial blood oxygenation in rats acclimated for 3 wk to simulated altitude (SA, barometric pressure 370-380 Torr) and 2) to determine the contribution of acid-base changes to the changes in arterial blood oxygenation of hypoxic exercise. Maximum O2 uptake (VO2max) was measured in four groups of rats: 1) normoxic controls run in normoxia (Nx), 2) normoxic controls run in acute hypoxia [AHx inspiratory PO2 (PIO2) approximately 70 Torr], 3) SA rats run in hypoxia (3WHx, PIO2 approximately 70 Torr), and 4) SA rats run in normoxia (ANx). VO2max (ml STPD.min-1.kg-1) was 70.8 +/- 0.9 in Nx, 46.4 +/- 1.9 in AHx, 52.6 +/- 1.1 in 3WHx, and 70.0 +/- 2.4 in ANx. Exercise resulted in acidosis, hypocapnia, and elevated blood lactate in all groups. Although blood lactate increased less in 3WHx and ANx, pH was the same or lower than in Nx and AHx, reflecting the low buffer capacity of SA. In AHx and 3WHx, arterial PO2 increased with exercise; however, O2 saturation of hemoglobin in arterial blood (SaO2) decreased. In vitro measurements of the Bohr shift suggest that SaO2 decreased as a result of a decrease in hemoglobin O2 affinity. The data indicate that several features of hypoxic exercise in this model are similar to those seen in humans, with the exception of the mechanism of decrease in SaO2, which, in humans, appears to be due to incomplete alveolar-capillary equilibration.     

Mechanisms of the radioprotective effect of cysteamine in Escherichia coli

The values of the oxygen effect (m) and the maximal protective effect of cysteamine (DMF*) were estimated for four Escherichia coli strains: AB1157 (wild type), AB1886 (uvrA), AB2463 (recA), and p3478 (polA). A correlation made between DMF* and m as well as the kinetics of the increase of DMF with oxygen depletion showed that the protective effect of cysteamine is realized by three mechanisms: (i) anoxia achieved by oxygen reduction, with the DMF varying from 2.2 to 4.2 for different E. coli strains (this protection is the major contribution to the entire mechanism); (ii) lowering of the indirect radiation effect; i.e., for 50 mM cysteamine DMF does not exceed 1.1; and (iii) increase of the efficiency of enzymatic repair. The latter effect of cysteamine is registered only with the wild-type E. coli, the DMF being not less than 1.4.     

The influence of haemoglobin on behavioural thermoregulation and oxygen consumption in Daphnia carinata

When placed in a temperature gradient, most ectotherms have a strict thermal preference that is lowered on exposure to hypoxia. Branchiopods, small aquatic crustaceans, are known to synthesise haemoglobin (Hb) when exposed to hypoxia; hypoxia can occur diurnally and seasonally in ponds. The effect of Hb on behavioural thermoregulation in the branchiopod Daphnia carinata following exposure to both normoxia and hypoxia was examined. Control animals raised in normoxia (Po2=150 mmHg, [Hb]=0.026+/-0.007 mg g dry wt-1) and Hb-rich animals raised in hypoxia (Po2=70 mmHg, [Hb]=0.080+/-0.017 mg g dry wt-1) were placed (N=30) in a tube (length=500 mm, diameter=8 mm) filled with pond water. In the absence of a thermal gradient, control and Hb-rich animals in normoxic water were uniformly distributed along the tube. The presence of a thermal gradient (13 degrees -28 degrees C) elicited clustering at a preferred temperature, T approximately 23 degrees C for both groups. Exposure to hypoxic water in a thermal gradient resulted in a behavioural shift: T approximately 16 degrees C for controls and T approximately 19 degrees C for Hb-rich animals. Measurements of oxygen consumption (V&d2;o2) at fixed temperatures revealed that Hb is associated with a metabolic acclimation to hypoxia.     

Hypoxic modulation of Ca2+ signaling in human venous endothelial cells. Multiple roles for reactive oxygen species

The effects of hypoxia (pO2 approximately 25 mm Hg) on Ca2+ signaling stimulated by extracellular ATP in human saphenous vein endothelial cells were investigated using fluorimetric recordings from Fura-2 loaded cells. In the absence of extracellular Ca2+, ATP-evoked rises of cytosolic Ca2+ concentration ([Ca2+]i) because of mobilization from the endoplasmic reticulum (ER). These responses were reduced by prior exposure to hypoxia but potentiated during hypoxia. Hypoxia itself liberated Ca2+ from the ER, but unlike the effects of ATP this effect was not inhibited by blockade of the inositol trisphosphate receptor. By contrast, ryanodine blocked the effects of hypoxia but not those of ATP. Antioxidants abolished the effects of hypoxia but potentiated the effects of ATP. Inhibition of NADPH oxidase also augmented ATP-evoked responses but was without effect on hypoxia-evoked rises of [Ca2+]i. However, either uncoupling mitochondrial electron transport or inhibiting complex I markedly suppressed the actions of hypoxia yet exerted only small inhibitory effects on ATP-evoked rises of [Ca2+]i. Both hypoxia and ATP were able to activate capacitative Ca2+ entry. Our results indicate that hypoxia regulates intracellular Ca2+ signaling via two distinct pathways. First, it modulates agonist-evoked liberation of Ca2+ from the ER primarily through regulation of reactive oxygen species generation from NADPH oxidase. Second, it liberates Ca2+ from the ER via ryanodine receptors, an effect requiring mitochondrial reactive oxygen species generation. These findings suggest that local O2 tension is a major determinant of Ca2+ signaling in the vascular endothelium, a finding that is likely to be of both physiological and pathophysiological importance.     

Radioprotective effect of cysteamine in glutathione synthetase-deficient cells

The radioprotective role of endogenous and exogenous thiols was investigated, with survival as the end-point, after radiation exposure of cells under oxic and hypoxic conditions. Human cell strains originating from a 5-oxoprolinuria patient and from a related control were used. Due to a genetic deficiency in glutathione synthetase, the level of free SH groups, and in particular that of glutathione, is decreased in 5-oxoprolinuria cells. The glutathione synthetase deficient cells have a reduced oxygen enhancement ratio (1.5) compared to control cells (2.7). The radiosensitivity was assessed for both cell strains in the presence of different concentrations of an exogenous radioprotector:cysteamine. At concentrations varying between 0.1 and 20 mM, cysteamine protected the two cell strains to the same extent when irradiated under oxic and hypoxic conditions. The protective effect of cysteamine was lower under hypoxia than under oxic conditions for both cell strains. Consequently, the oxygen enhancement ratio decreased for both cell strains when cysteamine concentration increased. These results suggest that cysteamine cannot replace endogenous thiols as far as they are implicated in the radiobiological oxygen effect.     

Hypoxia enhances S-nitrosylation-mediated NMDA receptor inhibition via a thiol oxygen sensor motif

Under ambient air conditions, NO inhibits NMDAR activity by reacting with the NR2A subunit C399 along with two additional cysteine pairs if their disulfide bonds are reduced to free thiol groups [NR1(C744,C798); NR2(C87,C320)]. Here we demonstrate that relative hypoxia enhances S-nitrosylation of NMDARs by a unique mechanism involving an "NO-reactive oxygen sensor motif" whose determinants include C744 and C798 of the NR1 subunit. Redox reactions involving these two thiol groups sensitize other NMDAR sites to S-nitrosylation and consequent receptor inhibition, while their own nitrosylation has little effect on NMDAR activity. The crystal structure of the ligand-binding domain of NR1 reveals a flexible disulfide bond (C744-C798), which may account for its susceptibility to reduction and subsequent reaction with NO that is observed with biochemical techniques. These thiols may be nitrosylated preferentially during increasing hypoxia or stroke conditions, thus preventing excessive activity associated with cytotoxicity while avoiding blockade of physiologically active NMDARs.     

Radiosensitization in multifraction schedules. II. Greater sensitization by 2-nitroimidazoles than by oxygen

The objective of this study was to characterize the extent of and mechanisms involved in radiosensitization by 2-nitroimidazoles in multifraction schedules using low doses per fraction. For this purpose, contact-inhibited monolayers of C3H 10T1/2 cells were given 1.7 Gy every 12 h and plated 12 h after the last dose received to allow full repair of potentially lethal damage (PLD). Severe hypoxia was obtained by a 1-h gassing procedure at room temperature immediately before each irradiation. No toxicity occurred as a consequence of multiple exposures to 5 mM misonidazole (MISO) or SR 2508 (2508) during the deoxygenation procedure. Experimental conditions during the pregassing and irradiation (presence of drug and gas mixture) were appropriately manipulated to test for the different mechanisms of radiosensitization demonstrated by nitroimidazoles. A very low oxygen enhancement ratio (OER) results under these conditions (1.34). Exposure to 5 mM MISO or 2508 during the deoxygenation and irradiation of hypoxic cells resulted in greater radiosensitization than could be accounted for by oxygen-mimetic sensitization alone (MISO and 2508 enhancement ratios were greater than the OER). Pregassing cells with N2 in the presence of 5 mM drug sensitized cells which were subsequently irradiated under aerobic conditions (drug free), indicating the occurrence of the "preincubation effect" (which does not require hypoxia or the drug's presence during the irradiation). Thus, for the hypoxic irradiations, the preincubation effect could account for the greater sensitization by nitroimidazoles than by oxygen. The presence of 5 mM drug only during the irradiation of aerobic cells produced radiosensitization in both multifraction and single-dose experiments with delayed plating. This sensitization has been previously shown to involve reduced PLD repair. Finally, maximum radiosensitization occurred in the multifraction schedule when a transient period of hypoxia with drug preceded an aerobic irradiation with drug present, thus combining the benefits of both the preincubation effect and PLD repair inhibition. This work demonstrates the possibility that effects other than oxygen-mimetic radiosensitization could be largely responsible for the sensitization seen in multifraction schedules, particularly when the OER is already low and only transient periods of hypoxia occur.     

Classes of thiols that influence the activity of the skeletal muscle calcium release channel

The skeletal muscle Ca(2+) release channel/ryanodine receptor (RyR1) is a prototypic redox-responsive ion channel. Nearly half of the 101 cysteines per RyR1 subunit are kept in a reduced (free thiol) state under conditions comparable with resting muscle. Here we assessed the effects of physiological determinants of cellular redox state (oxygen tension, reduced (GSH) or oxidized (GSSG) glutathione, and NO/O(2) (released by 3-morpholinosydnonimine)) on RyR1 redox state and activity. Oxidation of approximately 10 RyR1 thiols (from approximately 48 to approximately 38 thiols/RyR1 subunit) had little effect on channel activity. Channel activity increased reversibly as the number of thiols was further reduced to approximately 23/subunit, whereas more extensive oxidation (to approximately 13 thiols/subunit) inactivated the channel irreversibly. Neither S-nitrosylation nor tyrosine nitration contributed to these effects. The results identify at least three functional classes of RyR1 thiols and suggest that 1) the channel may be protected from oxidation by a large reservoir of functionally inert thiols, 2) the channel may be designed to respond to moderate oxidative stress by a change in activation setpoint, and 3) the channel is susceptible to oxidative injury under more extensive conditions.     

Radioprotection of DNA in isolated nuclei by naturally occurring thiols at intermediate oxygen tension.

Incubation of isolated Chinese hamster ovary cell nuclei, equilibrated in an atmosphere containing 2% O2, with glutathione, cysteine, or cysteamine resulted in a decrease in the number of X-ray-induced DNA double-strand breaks (DSBs), determined by pH 9.0 filter elution. In the absence of exogenous thiol, no sensitization was observed with the addition of N-ethylmaleimide, indicating that endogenous thiols were not present at significant levels. Protection by 0.3 mM glutathione was not enhanced by the addition of exogenous glutathione S-transferases or by glutathione peroxidase. The data were analyzed according to a simple competition model with various hypotheses. Cysteamine was more than an order of magnitude more effective than the other thiols tested, on a molar basis, in preventing DSB formation. Depending on the hypothesis used to evaluate the data, glutathione was either much less effective, on a molar basis, in preventing the bulk of the DSBs or was capable of preventing only approximately 55% of the damage, regardless of concentration. These data suggest that natural thiols other than glutathione may contribute to cellular radioprotection even if their concentration is much lower than that of glutathione. The data also suggest that despite the relative inefficiency of glutathione as a radioprotector, some areas of oxygenated tissues--where the oxygen tension falls below 2%--may be protected by glutathione concentrations in the physiological range of 3-20 mM.     

Hypoxia-induced reactive oxygen species downregulate ETB receptor-mediated contraction of rat pulmonary arteries

Production of reactive oxygen species (ROS) may be increased during hypoxia in pulmonary arteries. In this study, the role of ROS in the effect of hypoxia on endothelin (ET) type B (ETB) receptor-mediated vasocontraction in lungs was determined. In rat intrapulmonary (approximately 0.63 mm ID) arteries, contraction induced by IRL-1620 (a selective ETB receptor agonist) was significantly attenuated after 4 h of hypoxia (30 mmHg Po2) compared with normoxic control (140 mmHg Po2). The effect was abolished by tiron, a scavenger of superoxide anions, but not by polyethylene glycol (PEG)-conjugated catalase, which scavenges H2O2. The hypoxic effect on ETB receptor-mediated vasoconstriction was also abolished by endothelium denudation but not by nitro-L-arginine and indomethacin. Exposure for 4 h to exogenous superoxide anions, but not H2O2, attenuated the vasoconstriction induced by IRL-1620. Confocal study showed that hypoxia increased ROS production in pulmonary arteries that were scavenged by PEG-conjugated SOD. In endothelium-intact pulmonary arteries, the ETB receptor protein was reduced after 4 h of exposure to hypoxia, exogenous superoxide anions, or ET-1. BQ-788, a selective ETB receptor antagonist, prevented these effects. ET-1 production was stimulated in endothelium-intact arteries after 4 h of exposure to hypoxia or exogenous superoxide anions. This effect was blunted by PEG-conjugated SOD. These results demonstrate that exposure to hypoxia attenuates ETB receptor-mediated contraction of rat pulmonary arteries. A hypoxia-induced production of superoxide anions may increase ET-1 release from the endothelium and result in downregulation of ETB receptors on smooth muscle.     

The radioprotective effect of a hypoxic respiratory mixture (8% O2) for intestinal epithelial stem cells during single and fractionated irradiation of mice]

The method of intestinal "microcolonies" was used to study the radioprotective effect of a gas mixture, containing 8% of O2, on mice subjected to single and fractionated (5 fractions for 30 min) irradiation. The protective effect was indicated by a decreased slope of dose curves of the stem cell injury; the extrapolation number decreased simultaneously. So the values of dose modifying factors (DMF) were higher, when calculated by D0 ratio (where they amounted to 1.76 and 1.39 for single and fractionated exposure respectively), than those determined by equally effective doses (1.19 and 1.26 for single and fractionated effects respectively, which corresponded to LD50/4 when calculated at lg N = 1.9). It is suggested that the radiation response of certain stem cell populations of intestinal epithelium are different: this is attributed to different degrees of hypoxia in cells and to different directions of the hypoxia effects on the injury and the ability of postirradiation repair.     

Hemoglobin synthesis in murine virus-induced leukemic cells in vitro. 3. Effects of 5-bromo-2'-deoxyuridine, dimethylformamide and dimethylsulfoxide

Erythroid differentiation of Friend leukemia cells is enhanced when the cells are grown for four days in the presence of dimethylsulfoxide (DMSO). Dimethylformamide (DMF) has a similar though less marked effect. 5-Bromo-2′-deoxyuridine (BUdR) (10^?5M) inhibits both DMF- and DMSO-stimulated differentiation. For maximum inhibition, BUdR must be present during the first two days of growth, during which time DNA synthesis is maximal. The addition of BUdR after the third day has no effect. Since BUdR is incorporated into DNA and thymidine prevents BUdR inhibition of DMSO-stimulated differentiation, it is likely that BUdR acts by virtue of its incorporation into DNA. Although BUdR alone had little effect upon cell multiplication, in combination with DMSO, cell growth was inhibited up to 40%. Since the BUdR-inhibition of the DMSO effect was approximately 70%, it is unlikely that its effect on differentiation is due to selective killing of those cells which are stimulated to differentiate.     

Facilitation of dopamine and acetylcholine release by intermittent hypoxia in PC12 cells: involvement of calcium and reactive oxygen species.

We have investigated the effects of preconditioning pheochromocytoma (PC12) cells with intermittent hypoxia (IH) on transmitter release during acute hypoxia. Cell cultures were exposed to either alternating cycles of hypoxia (1% O(2) + 5% CO(2); 30 s/cycle) and normoxia (21% O(2) + 5% CO(2); 3 min/cycle) for 15 or 60 cycles or normoxia alone (control) for similar durations. Control and IH cells were challenged with either hyperoxia (basal release) or acute hypoxia (Po(2) of approximately 35 Torr) for 5 min, and the amounts of dopamine (DA) and acetylcholine (ACh) released in the medium were determined by HPLC combined with electrochemical detection. Hypoxia augmented DA (approximately 80%) but not ACh release in naive cells, whereas, in IH-conditioned cells, it further enhanced DA release (ranging from 120 to approximately 145%) and facilitated ACh release (approximately 30%). Hypoxia-evoked augmentation of transmitter release was not seen in cells conditioned with sustained hypoxia. IH-induced increase in DA but not IH-induced ACh release during hypoxia was partially inhibited by cadmium chloride (100 microM), a voltage-gated Ca(2+) channel blocker. By contrast, 2-aminoethoxydiphenylborate (75 microM), a blocker of inositol 1,4,5-trisphosphate (IP(3)) receptors, and N-acetyl-L-cysteine (300 microM), a potent scavenger of reactive oxygen species, either attenuated or abolished IH-evoked augmentation of transmitter release during hypoxia. Together, the above results demonstrate that IH conditioning increases hypoxia-evoked neurotransmitter release from PC12 cells via mechanisms involving mobilization of Ca(2+) from intracellular stores through activation of IP(3) receptors. Our findings also suggest that oxidative stress plays a central role in IH-induced augmentation of transmitter release from PC12 cells during acute hypoxia.     

Antioxidant effect of simvastatin is not enhanced by its association with alpha-tocopherol in hypercholesterolemic patients

Among the pleiotropic effects of statins, their antioxidant action may be involved in their protective effects. Thus, we investigated the antioxidant effect of simvastatin, associated or not with alpha-tocopherol, on levels of electronegative low-density lipoprotein (LDL-), nitrotyrosine, thiols (homocysteine, glutathione, cysteine, methionine), and lipid-soluble antioxidants in blood plasma of hypercholesterolemic subjects. In this study, 25 hypercholesterolemic subjects were treated for 2 months with simvastatin (20 mg/day) and with simvastatin (20 mg/day) + alpha-tocopherol (400 IU/day). Concentrations of thiols were determined by high-performance capillary electrophoresis-laser-induced fluorescene. Lipid-soluble antioxidants were determined by HPLC, and LDL-, and nitrotyrosine by ELISA. Simvastatin, independent of its association with alpha-tocopherol, reduced plasma concentrations of LDL-, nitrotyrosine, total cholesterol, and LDL cholesterol and the LDL cholesterol/HDL cholesterol ratio. Neither simvastatin nor simvastatin plus alpha-tocopherol altered plasma levels of the thiols analyzed. alpha-Tocopherol did not change the antioxidant effect of simvastatin on the levels of LDL- and nitrotyrosine in hypercholesterolemic subjects. The reduction of LDL- and nitrotyrosine by simvastatin seems to be related to the pleiotropic effects of this statin, and it may have an important protective effect against endothelial dysfunction and atherosclerosis.     

The protective effect of atrial natriuretic peptide (ANP) on cells damaged by oxygen radicals is mediated through elevated CGMP-levels, reduction of calcium-inflow and probably G-proteins.

ANP increases cellular cGMP content in cultured hepatocytes and decreases Ca2(+)-inflow in a concentration- and time-dependent manner which explains a beneficial effect on hypoxia cell injury (25). Both observations are mimicked by SNP and 8-Br-cGMP and blocked by Ly 83583 indicating a cGMP-mediated mechanism. The protective effect was also inhibited by Pertussis Toxin (PT) without lowering the elevated cGMP-level. But PT in combination with ANP leads to a higher Ca2(+)-inflow. Stimulated Na(+)-inflows are also be lowered by ANP. Here, neither SNP can mimick nor PT can inhibit this effect. Our results now indicate that the beneficial effect by ANP at the cellular level is mediated through cGMP which decreases calcium-inflow. ANP seems to control Ca2(+)-channels direct via a PT-sensitive G-protein and indirect by a cGMP-mediated mechanism and Na(+)-channels cGMP-independent through a PT-insensitive G-protein, thus preventing cells on hypoxia and oxygen radicals.     

Effect of alkalosis on maximum oxygen uptake in rats acclimated to simulated altitude.

The objective of the present experiments was to determine whether prevention or moderation of exercise acidosis would influence arterial blood oxygenation and exercise capacity in hypoxia. The effect of administration of 0.3 M NaHCO3 (3 ml/100 g) on maximum O2 uptake (VO2max) and arterial blood oxygenation was determined in rats acclimated to simulated altitude (370-380 Torr barometric pressure) for 3 wk (HxBic) and in normoxic littermates (NxBic). Controls were simulated-altitude (HxNaCl) and normoxic rats (NxNaCl) given 0.3 M NaCl. Inspiratory PO2 during treadmill exercise was approximately 70 Torr for hypoxic rats and 140-145 Torr for normoxic rats. VO2max was 18% higher in HxBic than in HxNaCl (62.8 + 1.6 vs. 53.1 + 1.0 ml STPD.min-1.kg-1, respectively, P less than 0.05) and only 8% higher in NxBic than in NxNaCl (74.0 + 1.1 vs. 68.7 + 1.5 ml STPD.min-1.kg-1, respectively, P less than 0.05). Exercise in HxNaCl resulted in a decrease in arterial O2 concentration (CaO2), which was largely due to a pH-induced decrease in O2 saturation of arterial blood, and occurred despite an increase in arterial PO2. NaHCO3 moderated the acidosis of exercise and largely attenuated the decrease in CaO2. The effects of acidosis and bicarbonate on CaO2 were much less evident in the normoxic controls. There was an almost linear relationship between VO2max and the corresponding CaO2 for all four groups, suggesting that the effect of NaHCO3 on VO2max may be related to moderation of the decrease in CaO2.     

Radiosensitivity during the irradiation of animals in an altered gaseous environment. A comparative study on mice and rats of the radiation-protective effect of oxygen-deficient gas mixtures

The radioprotective effect of gas hypoxic mixtures containing 5, 7, 8, 10 and 15% of oxygen on mice and rats was comparatively studied. The dependence of DMF upon oxygen concentration in the mixture was approximated by a hyperbolic function similar to the dependence of the radiomodifying effect of circulatory hypoxia caused by radioprotective agents of the indolylalkylamine series.     

The reflex ventilatory responses of conscious, newborn rats to alternations of inspiratory oxygen concentration.

We examined the effect of a dynamic, hypoxic stimulus upon the reflex respiratory responses of 15, conscious rat pups on post-natal days 5-7 in order to ascertain the influence of a non-adapting peripheral chemoreceptor discharge upon respiratory control during hypoxia in the newborn. Respiration was measured as integrated airflow into and out of a body plethysmograph. The respiratory response to 6 minutes of a 16-breath cycle (approximately 5 s) in FiO2 between 0.21 and 0.10 (alternating hypoxia) was compared with the response to 6 min of a constant FiO2 of 0.12 (non-alternating hypoxia). Ventilation increased significantly from a control level of 0.12 +/- 0.02 ml/s (mean +/- SEM) to 0.18 +/- 0.02 and 0.17 +/- 0.02 ml/s in non-alternating and alternating hypoxia runs respectively during the first minute (phase 1) of each run, after which ventilation in both run types fell progressively and significantly back towards control levels to reach, by the sixth minute (phase 2), 0.13 +/- 0.01 and 0.12 +/- 0.02 ml/s respectively. No significant difference was found between the levels of ventilation in non-alternating hypoxia and alternating hypoxia during either phase 1 (P greater than 0.10) or phase 2 (P greater than 0.60). No significant alternation was found in any respiratory variable at the frequency of the 16-breath hypoxic cycle during either phase 1 or phase 2 of non-alternating hypoxia. However, a significant alternation, at this frequency, of 37 +/- 6% (P less than 0.05 compared to control) was found in ventilation during phase 1 of alternating hypoxia which was further increased to 62 +/- 8% (P less than 0.05 compared to phase 1) during phase 2. In phase 1 the alternation was due primarily to significant alternation in inspiratory time whilst in phase 2 significant alternation also occurred in tidal volume, expiratory time and mean inspiratory flow. Our results show that the magnitude of hypoxic ventilatory depression (HVD) in the newborn is not affected by an alternating hypoxic stimulus and that, during phase 2, ventilation can still be stimulated by peripheral chemoreceptors. We suggest that peripheral chemoreceptor adaptation is unlikely to be a major cause of HVD in the newborn rat and that the magnitude of HVD is, in part, the result of a competitive interaction between peripheral chemoreceptor stimulation and a centrally-mediated inhibitory action of hypoxia.