Hyperoxic Exposures Research Articles

Hyperbaric oxygen rapidly produces intracellular bioenergetics dysfunction in human pulmonary cells

Hyperoxic exposure lasting days alters mitochondrial bioenergetic and dynamic functions in pulmonary cells as indices of oxygen toxicity. The aim of this study was to examine effects of short duration hyperbaric and hyperoxic exposures to induce oxygen toxicity similarly. Cultured human lung microvascular endothelial cells, human pulmonary artery endothelial cells and A549 cells were exposed to hyperoxia (∼5 % CO2 equivalent, balance O2) under hyperbaric conditions (4.8 ATA) for 1 and 4 h. Measures of mitochondrial dynamics, inner membrane potential, mitochondrial respiration, the intracellular distribution of bioenergetic capacity and respiration complex protein levels were then quantified. Exposures resulted in altered mitochondrial motility, presence of inhomogeneities in respiration parameters, loss of inner membrane potential, and changes in intracellular partitioning of ATP-linked respiration. Changes in the levels of respiration complex protein levels were also found. The combination of hyperoxic exposure with hyperbaric conditions rapidly produced changes in mitochondrial dynamics and bioenergetics in pulmonary cells. These changes are consistent with the onset of pulmonary oxygen toxicity previously known to result from long duration exposure to hyperoxia alone. These findings suggest health caution is warranted in environmental settings in which both hyperoxic and hyperbaric conditions are present. The synergism of hyperoxia and hyperbaria for rapid induction of oxygen toxicity in cellular models has utility for the study of mechanistic determinants of oxygen toxicity, testing of putative therapeutics, and associated investigations of mitochondrial dysfunction.

Exhaled Nitric Oxide and Pulmonary Oxygen Toxicity Susceptibility.

Individual susceptibility to pulmonary oxygen toxicity (PO2tox) is highly variable and currently lacks a reliable biomarker for predicting pulmonary hyperoxic stress. As nitric oxide (NO) is involved in many respiratory system processes and functions, we aimed to determine if expired nitric oxide (FENO) levels can provide an indication of PO2tox susceptibility in humans. Eight U.S. Navy-trained divers volunteered as subjects. The hyperoxic exposures consisted of six- and eight-hour hyperbaric chamber dives conducted on consecutive days in which subjects breathed 100% oxygen at 202.65 kPa. Subjects' individual variability in pulmonary function and FENO was measured twice daily over five days and compared with their post-dive values to assess susceptibility to PO2tox. Only subjects who showed no decrements in pulmonary function following the six-hour exposure conducted the eight-hour dive. FENO decreased by 55% immediately following the six-hour oxygen exposure (n = 8, p < 0.0001) and by 63% following the eight-hour exposure (n = 4, p < 0.0001). Four subjects showed significant decreases in pulmonary function immediately following the six-hour exposure. These subjects had the lowest baseline FENO, had the lowest post-dive FENO, and had clinical symptoms of PO2tox. Individuals with low FENO were the first to develop PO2tox symptoms and deficits in pulmonary function from the hyperoxic exposures. These data suggest that endogenous levels of NO in the lungs may protect against the development of PO2tox.

Hyperoxic exposure alters intracellular bioenergetics distribution in human pulmonary cells

AimsPulmonary oxygen toxicity is caused by exposure to a high fraction of inspired oxygen, which damages multiple cell types within the lung. The cellular basis for pulmonary oxygen toxicity includes mitochondrial dysfunction. The aim of this study was to identify the effects of hyperoxic exposure on mitochondrial bioenergetic and dynamic functions in pulmonary cells. Main methodsMitochondrial respiration, inner membrane potential, dynamics (including motility), and distribution of mitochondrial bioenergetic capacity in two intracellular regions were quantified using cultured human lung microvascular endothelial cells, human pulmonary artery endothelial cells and A549 cells. Hyperoxic (95 % O2) exposures lasted 24, 48 and 72 h, durations relevant to mechanical ventilation in intensive care settings. Key findingsMitochondrial motility was altered following all hyperoxic exposures utilized in experiments. Inhomogeneities in inner membrane potential and respiration parameters were present in each cell type following hyperoxia. The partitioning of ATP-linked respiration was also hyperoxia-duration and cell type dependent. Hyperoxic exposure lasting 48 h or longer provoked the largest alterations in mitochondrial motility and the greatest decreases in ATP-linked respiration, with a suggestion of decreases in respiration complex protein levels. SignificanceHyperoxic exposures of different durations produce intracellular inhomogeneities in mitochondrial dynamics and bioenergetics in pulmonary cells. Oxygen therapy is utilized commonly in clinical care and can induce undesirable decrements in bioenergy function needed to maintain pulmonary cell function and viability. There may be adjunctive or prophylactic measures that can be employed during hyperoxic exposures to prevent the mitochondrial dysfunction that signals the presence of oxygen toxcity.

From UPTD to ESOT: Monitoring hyperoxic exposure in surface-oriented diving

A recent review suggested that the measure K = t2 x pO24.57 (t is exposure time in h, pO2 in Atm) should replace UPTD as an exposure index for pulmonary oxygen toxicity (POT) in surface-oriented diving. K would better predict reduction in vital capacity (VC) during exposure and allow prediction of recovery. Although K is more accurate estimating VC changes than UPTD, the calculation of K is more extensive, particularly when estimating hyperoxic exposure for dives with multiple pO2 segments. Furthermore, and in contrast with UPTD, K is difficult to interpret on its own given its non-linear dimension of time. We suggest that a new metric: ESOT (Equivalent Surface Oxygen Time) should be used to replace UPTD. ESOT = t x pO22.285 (t is exposure time in min, pO2 in Atm). ESOT=1 is thus the hyperoxic exposure reached after 1 min breathing of 100% O2 at surface pressure. Hyperoxic monitoring by ESOT is more practical than K to apply in an operational environment, with no loss of accuracy in POT prediction. In addition, it intuitively allows interpreting hyperoxic exposures on its own, analogous to UPTD. The daily hyperoxic threshold limits suggested by Risberg and van Ooij for two, five and an unlimited number of successive diving days would translate to ESOTs of 650, 500 and 420 respectively.

Intermittent Hypoxia-Hyperoxia and Oxidative Stress in Developing Human Airway Smooth Muscle.

Premature infants are frequently and intermittently administered supplemental oxygen during hypoxic episodes, resulting in cycles of intermittent hypoxia and hyperoxia. The relatively hypoxic in utero environment is important for lung development while hyperoxia during the neonatal period is recognized as detrimental towards the development of diseases such as bronchopulmonary dysplasia and bronchial asthma. Understanding early mechanisms that link hypoxic, hyperoxic, and intermittent hypoxic-hyperoxic exposures to altered airway structure and function are key to developing advanced therapeutic approaches in the clinic. Changes in oxygen availability can be detrimental to cellular function and contribute to oxidative damage. Here, we sought to determine the effect of oxygen on mitochondria in human fetal airway smooth muscle cells exposed to either 5% O2, 21% O2, 40% O2, or cycles of 5% and 40% O2 (intermittent hypoxia-hyperoxia). Reactive oxygen species production, altered mitochondrial morphology, and changes in mitochondrial respiration were assessed in the context of the antioxidant N-acetylcysteine. Our findings show developing airway smooth muscle is differentially responsive to hypoxic, hyperoxic, or intermittent hypoxic-hyperoxic exposure in terms of mitochondrial structure and function. Cycling O2 decreased mitochondrial branching and branch length similar to hypoxia and hyperoxia in the presence of antioxidants. Additionally, hypoxia decreased overall mitochondrial respiration while the addition of antioxidants increased respiration in normoxic and O2-cycling conditions. These studies show the necessity of balancing oxidative damage and antioxidant defense systems in the developing airway.

Effects of Perinatal Hyperoxia on Breathing.

Air-breathing animals do not experience hyperoxia (inspired O2 > 21%) in nature, but preterm and full-term infants often experience hyperoxia/hyperoxemia in clinical settings. This article focuses on the effects of normobaric hyperoxia during the perinatal period on breathing in humans and other mammals, with an emphasis on the neural control of breathing during hyperoxia, after return to normoxia, and in response to subsequent hypoxic and hypercapnic challenges. Acute hyperoxia typically evokes an immediate ventilatory depression that is often, but not always, followed by hyperpnea. The hypoxic ventilatory response (HVR) is enhanced by brief periods of hyperoxia in adult mammals, but the limited data available suggest that this may not be the case for newborns. Chronic exposure to mild-to-moderate levels of hyperoxia (e.g., 30-60% O2 for several days to a few weeks) elicits several changes in breathing in nonhuman animals, some of which are unique to perinatal exposures (i.e., developmental plasticity). Examples of this developmental plasticity include hypoventilation after return to normoxia and long-lasting attenuation of the HVR. Although both peripheral and CNS mechanisms are implicated in hyperoxia-induced plasticity, it is particularly clear that perinatal hyperoxia affects carotid body development. Some of these effects may be transient (e.g., decreased O2 sensitivity of carotid body glomus cells) while others may be permanent (e.g., carotid body hypoplasia, loss of chemoafferent neurons). Whether the hyperoxic exposures routinely experienced by human infants in clinical settings are sufficient to alter respiratory control development remains an open question and requires further research. © 2020 American Physiological Society. Compr Physiol 10:597-636, 2020.

Consecutive, Resting, Long-Duration Hyperoxic Exposures Alter Neuromuscular Responses During Maximal Strength Exercises in Trained Men.

Purpose: The main objective of this study was to investigate the physiological effects of repetitive diving-induced hyperoxic conditions at 1.35 atmospheres absolute (ATA) on neuromuscular strength performance. We hypothesized that following five days of consecutive, resting, long-duration (6 h or more) hyperoxic water immersions (WIs) neuromuscular strength performance would be reduced with a longer recovery time in comparison to previously reported normoxic WIs.Methods: Thirteen (n = 13) active male divers [31.3 ± 1.7 (24–43) years, mean ± years] completed five consecutive days of 6-h resting WIs with 18-h surface intervals while breathing 100% O2 (n = 13) at 1.35 ATA. Skeletal muscle performance assessments occurred immediately before and after each WI and 24 and 72 h after the final WI. Performance assessments included maximum voluntary isometric contraction (MVIC) and maximal isokinetic (IK) knee extensions and elbow flexions, and isometric maximum handgrip (MHG) strength. Neuromuscular activation was also measured on the quadriceps, biceps brachii, and brachioradialis via surface electromyography (sEMG).Results: MHG declined by 7.8% (p < 0.001) by WI 5 with performance returning to baseline by 24-h post-WI. Brachioradialis neuromuscular activation increased by 42% on WI 5. MVIC knee extension performance dropped by 4% (p = 0.001) on WI 3 with a 11% overall decrease in quadriceps neuromuscular activation. Maximal IK knee extension dropped by 3.3% on WI 5 with 9% drop in overall quadriceps activation during the same period. MVIC elbow flexion declined by 5.1% on WI 5 but returned to baseline by 72-h post-WI. Maximal IK elbow flexion performance dropped by 8.6% on WI 5 with a continual decline in biceps brachii neuromuscular activation of 24% on WI 5.Conclusion: Consecutive, resting, long-duration hyperoxic WIs reduce muscular performance in multiple muscle groups and alter neuromuscular activation after 3 days of WI with performance adaptations recovering toward baseline by the end of the WI 5. However, neuromuscular activation remains decreased and appears to last beyond the 72-h post-WI recovery period.

Antiepileptic drugs prevent seizures in hyperbaric oxygen: A novel model of epileptiform activity

Breathing oxygen at sufficiently elevated pressures can trigger epileptiform seizures. Therefore, we tested the hypothesis that pre-treatment with FDA-approved antiepileptic drugs could prevent seizure onset in hyperoxia at 5 atmospheres absolute. We selected drugs from two putative functional categories, Na+-channel antagonists and GABA enhancers, each administered intraperitoneally at four doses in separate groups of C57BL/6 mice. The drugs varied in efficacy at the doses used. Of the five tested Na+-channel antagonists, carbamazepine and lamotrigine more than tripled seizure latency compared to values seen in vehicle controls. Primidone, zonisamide and oxcarbazepine were less effective. Of the four GABA reuptake inhibitors, tiagabine and vigabatrin also increased seizure latency by more than three times control values; valproic acid was less effective, and the GABA synthesis promoter gabapentin was intermediate in effectiveness. We infer that Na+-channel function and GABA neurotransmission may be critical targets in the pathophysiology of CNS O2 toxicity. Because these essential components of neuronal excitation and inhibition are also implicated in the pathogenesis of other seizure disorders, including generalized epilepsy, we propose that, at some level, common pathways are involved in these pathologies, although the initiating insults differ. Furthermore, hyperoxic exposures are not known to cause the spontaneously-recurring seizures that characterize true clinical epilepsy. Nonetheless, experimental studies of hyperbaric oxygen toxicity could provide new insights into molecular mechanisms of seizure disorders of various etiologies. In addition, the neuropathology of hyperbaric oxygen is particularly relevant to the hypothesis held by some investigators that oxidative stress is an etiological factor in clinical epilepsies.

Transient hyperoxia does not affect regional cerebral tissue oxygen saturation in moderately preterm or term newborns.

Even short periods of hyperoxia may induce prolonged cerebral vasoconstriction in newborn infants, and this could theoretically lead to cerebral ischaemia even once normoxia is re-established. This study aimed to investigate the effect of brief hyperoxic exposures on regional cerebral tissue oxygen saturation (rStO2 ) and to evaluate whether any observed prolonged cerebral vasoconstriction was related to maturity. The study included 30 infants with a postmenstrual age of more than 32 weeks, who were treated with nasal continuous positive airway pressure and a fraction of inspired oxygen of ≤0.3. The INVOS 5100C oximeter was used to measure rStO2 before, during and after two hyperoxic exposures. If hyperoxia induced a prolonged cerebral vasoconstriction, posthyperoxic rStO2 would be expected to decrease. rStO2 increased slightly after the first hyperoxic exposure, with a mean difference of 1.37% (95% CI 0.15, 2.6). After the second oxygen exposure, rStO2 remained unchanged with a mean difference of -0.4% (95% CI -1.6, 0.78). Differences in rStO2 were not related to gestational age in either of the two hyperoxic episodes. We found no evidence to support the theory that transient hyperoxia induces prolonged cerebral vasoconstriction in infants with a postmenstrual age above 32 weeks.

Absolute cerebral blood flow quantification with pulsed arterial spin labeling during hyperoxia corrected with the simultaneous measurement of the longitudinal relaxation time of arterial blood

Quantitative arterial spin labeling (ASL) estimates of cerebral blood flow (CBF) during oxygen inhalation are important in several contexts, including functional experiments calibrated with hyperoxia and studies investigating the effect of hyperoxia on regional CBF. However, ASL measurements of CBF during hyperoxia are confounded by the reduction in the longitudinal relaxation time of arterial blood (T(1a) ) from paramagnetic molecular oxygen dissolved in blood plasma. The aim of this study is to accurately quantify the effect of arbitrary levels of hyperoxia on T(1a) and correct ASL measurements of CBF during hyperoxia on a per-subject basis. To mitigate artifacts, including the inflow of fresh spins, partial voluming, pulsatility, and motion, a pulsed ASL approach was implemented for in vivo measurements of T(1a) in the rat brain at 3 Tesla. After accounting for the effect of deoxyhemoglobin dilution, the relaxivity of oxygen on blood was found to closely match phantom measurements. The results of this study suggest that the measured ASL signal changes are dominated by reductions in T(1a) for brief hyperoxic inhalation epochs, while the physiologic effects of oxygen on the vasculature account for most of the measured reduction in CBF for longer hyperoxic exposures.

One day of hyperoxia augments the hypoxic ventilatory response in neonatal rats

The O2 sensitivity of single-unit carotid body chemoreceptor cells is increased in neonatal rats after a one-day exposure to moderate hyperoxia (60% O2) (Donnelly et al., Respir. Physiol. Neurobiol. 168: 189–197, 2009). To determine whether the hypoxic ventilatory response (HVR) is similarly enhanced by this exposure, we measured the acute HVR of neonatal rats (P8) by head-body plethysmography immediately after 23–28 h in 60% O2. The HVR was increased by ~50% in hyperoxia-treated rats due to a larger increase in tidal volume. This effect was not blocked by administration of the antioxidant drug MnTMPyP, suggesting that this plasticity is not mediated by increased production of the superoxide anion. Moreover, a shorter exposure to 60% O2 (30 min) did not alter the HVR. In contrast to prolonged hyperoxic exposures that diminish the HVR, we conclude that short periods of hyperoxia (~ 1 d) augment the HVR of neonatal rats. The mechanisms underlying this plasticity will require further investigation. Supported by NIH grant HL-083972.

Energetic Efficiency in Trained and Sedentary Rats After Exposure to Normobaric and Hyperbaric Oxygen

Contradictory results have been obtained regarding the beneficial effect of hyperbaric oxygen (HBO) on exercise performance. The purpose of this study was to investigate the effect of different combinations of pressure and time in hyperoxia on the energetic efficiency of trained and sedentary rats. At the end of the training period, rats were exposed to one of three protocols: 1) 100% normobaric oxygen for 24 h; 2) HBO at 2 ATA for 4 h; 3) HBO at 2.5 ATA for 6 h. After the hyperoxic exposures, V(O2max) was evaluated and compared with preexposure values. The slope of the linear section of the oxygen consumption-velocity curve in the trained rats was significantly steeper after exposure to either 100% normobaric oxygen for 24 h or HBO at 2 ATA for 4 h than before the exposure. The opposite was found for the sedentary rats. After exposure to HBO at 2.5 ATA for 6 h, the slope of the oxygen consumption-velocity curve in the trained rats did not differ from the pre-exposure slope. However, the highest velocity these rats reached was lower than their maximum velocity before this exposure. In the sedentary rats, the slope of the oxygen consumption-velocity curve was found to be steeper after the 2.5 ATA exposure compared with the preexposure slope. Our results suggest that exposure to 100% normobaric oxygen for 24 h and HBO at 2 ATA for 4 h induces a reduction in the energetic efficiency of trained rats, but improves energetic efficiency in sedentary rats.

Oxygen gradients for open well cellular cultures via microfluidic substrates

Controlling oxygen concentration at a microscale level can benefit experimental investigations involving oxidative stress, ischemia, and reactive oxygen species (ROS) mediated cellular pathways. Here, we report the application of microfluidic gradient generation in an open-well culture model, in which a gradient of gas is delivered via diffusion through a gas permeable substrate that separates cells from the gas microchannels below. By using diffusion to localize oxygen delivery, microgradients of oxygen concentrations can be rapidly and controllably applied without exposing cells to mechanical stresses or reducing culture volumes inside microfluidic culture chambers. Furthermore, we demonstrate the modulation of intracellular ROS levels in Madin-Darby Canine Kidney (MDCK) cells by applying these oxygen microgradients. Increases in ROS levels consistent with both oxidative stress and hypoxic exposures were observed in MDCK cells. The measured ROS increases were comparable to 100 microM hydrogen peroxide exposure in a control comparison, which is within the range of standard ROS induction methods. Incubation with 200 microM vitamin C was able to demodulate the ROS response at both hypoxic and hyperoxic exposures. By providing microfluidic controlled gradients, constant ROS exposure, and a shear-free open well design, the devices introduced here greatly improve upon standard oxygen-based culturing methods.

Arterial Compliance in Divers Exposed to Repeated Hyperoxia Using Rebreather Equipment

Acute hyperoxic exposure is known to modify cardiovascular parameters like a decrease in cardiac output, arterial vasoconstriction, and autonomic nervous system changes. We hypothesized that repeated hyperbaric hyperoxic exposures, as experienced by military oxygen divers, lead to long-term arterial alterations. Arterial blood pressure measurements and pulse wave velocity (PWV) recordings were performed during basal conditions in 15 elite military oxygen divers, and compared to 15 non-diver controls. The two groups were matched appropriately for physical characteristics (age: 35 +/- 5 yr, weight: 77 +/- 8 kg, height: 177 +/- 6 cm, body mass index: 24.6 +/- 2.0 kg x m(-2)), and aerobic capacity (VO2max : 52 +/- 7 ml x min(-1) x kg(-1)). No significant difference was found in systolic blood pressure (120 +/- 11 mmHg), diastolic blood pressure (70 +/- 8 mmHg), or pulse pressure (50 +/- 7 mmHg). Furthermore, there was no significant difference in the carotid-femoral PWV (6.7 +/- 0.9 m x s(-1)), the carotid-radial PWV (8.7 +/- 1.7 m x s(-1)), or the carotid-pedal PWV (8.1 +/- 1.1 m x s(-1)) between divers and controls. No difference in arterial compliance was observed in physically well-trained military oxygen divers in comparison with matched controls.

Blunted hypoxic ventilatory response in quail after developmental hyperoxia

Perinatal hyperoxia (30–60% O2) impairs carotid body and ventilatory responses to hypoxia in rats. We hypothesized that hyperoxia would elicit similar respiratory plasticity in birds. Japanese quail (Coturnix japonica) were exposed to 60% O2 throughout incubation and/or for the first 2 weeks after hatching. Ventilatory responses to 11% O2 were attenuated in adult quail (7–11 wks) exposed to hyperoxia throughout development (13±6% increase from baseline vs. 38±10% in controls, P=0.05), primarily due to a smaller increase in respiratory frequency (P<0.05). However, ventilatory responses were unaffected by hyperoxic exposures restricted to incubation or after hatching (P>0.05), suggesting that exposures must be longer (>2 wks) or must span pre- and post-hatch periods. We conclude that respiratory control development is susceptible to perinatal hyperoxia in birds, as in mammals. Therefore, precocial birds may offer a convenient model for studying developmental plasticity without confounding maternal effects. Supported by NCRR grant P20 RR-016463.

Necrotic Cell Death in Response to Oxidant Stress Involves the Activation of the Apoptogenic Caspase-8/Bid Pathway

Human epithelial (A549) cells exposed to hyperoxia die by cellular necrosis. In the current study, we demonstrated the involvement of apoptogenic factors in epithelial cell necrosis in response to hyperoxia, including the formation of the Fas-related death-inducing signaling complex and initiation of mitochondria-dependent apoptotic pathways. We showed increased activation of both Bid and Bax in A549 cells subjected to hyperoxia. Bax activation involved a Bid-assisted conformational change. We discovered that the response to hyperoxia in vivo predominantly involved the activation of the Bid/caspase-8 pathway without apparent increases in Bax expression. Disruption of the Bid pathway by gene deletion protected against cell death in vivo and in vitro. Likewise, inhibition of caspase-8 by Flip also protected against cell death. Taken together, we have demonstrated the involvement of apoptogenic factors in epithelial cell responses to hyperoxia, despite a final outcome of cellular necrosis. We have, for the first time, identified a predominant role for the caspase-8/Bid pathway in signaling associated with hyperoxic lung injury and cell death in vivo and in vitro.

Enhanced binding of Sp1/Sp3 transcription factors mediates the hyperoxia-induced increased expression of the lung type I cell gene T1alpha.

The transcription factor Sp1 plays an important regulatory role in transactivation of the lung type I cell differentiation gene T1alpha. Like other lung cells, type I cells may encounter changes in oxygen concentration during the lifetime of the organism. We found that exposure of mice to hyperoxia rapidly increases expression of T1alpha and other type I cell genes, and that returning the mice to normoxia quickly decreases expression. Likewise hyperoxia increases both endogenous T1alpha expression in lung epithelial cell lines and transcription of luciferase (Luc) from T1alpha promoter deletion constructs. Using wild-type promoter fragments and gel shift assays, we determined that Sp1/Sp3 and a key Sp cis-element in the proximal promoter mediate the hyperoxic response. Mutations of this element and inhibition of Sp-DNA binding by mithramycin block the hyperoxic response. Western analyses of cell homogenates show that the overall abundance of Sp1 and Sp3 proteins is not altered by hyperoxia. However, the abundance of nuclear Sp1 increases after short hyperoxic exposures, suggesting that signaling pathways activated by hyperoxia lead to Sp protein translocation, perhaps as a result of increased Sp phosphorylation.

Level and duration of developmental hyperoxia influence impairment of hypoxic phrenic responses in rats.

Developmental hyperoxia (1-4 wk of 60% O2) causes long-lasting impairment of hypoxic phrenic responses in rats. We hypothesized that shorter or less severe hyperoxic exposures would produce similar changes. Hypoxic phrenic responses were measured in 3- to 5-mo-old, urethane-anesthetized rats exposed to 60% O2 for postnatal day 1 or week 1 or to 30% O2 for postnatal week 1. Whereas 1 day of 60% O2 had no lasting effects (P > 0.05 vs. control), both 1 wk of 60% O2 and 1 wk of 30% O2 decreased adult hypoxic phrenic responses (P < 0.05 vs. control), although the effects of 30% O2 were smaller. Hypoxic ventilatory responses (expressed as the ratio of minute ventilation to metabolic CO2 production) were also reduced in unanesthetized rats (5-10 mo old) exposed to 1 wk of 60% O2 during development (P < 0.05). An age-dependent increase toward normal hypoxic phrenic responses was observed in rats exposed to 1 wk of 60% O2 (P < 0.05), suggesting a degree of spontaneous recovery not observed after 1 mo of 60% O2. These data indicate that long-lasting effects of developmental hyperoxia depend on the level and duration of hyperoxic exposure.

Oxygen-induced metabolic changes and transdifferentiation in immature fetal rat lung lipofibroblasts

Preterm infants lack adequate surfactant production and often require oxygen support for adequate oxygenation. Prolonged oxygen treatment leads to the development of bronchopulmonary dysplasia (BPD), a disease process characterized by the blunting of alveolarization and proliferation of myofibroblasts. In the present study, we investigated metabolic adaptive changes in cultured fibroblasts isolated from immature (d18) and near-term (d21), fetal rat lungs in response to normoxic (21%) and hyperoxic (95%) exposures. We used the [1,2- 13C 2] d-glucose tracer and gas chromatography/mass spectrometry to characterize glucose carbon redistribution between the nucleic acid ribose, lactate, and palmitate synthetic pathways, and reverse transcriptase-polymerase chain reaction to assess adipose differentiation related protein (ADRP) mRNA expression in response to hyperoxic exposure. Exposure to hyperoxia at each passage caused decrease (*, p<0.05 vs. 21% O 2) in ADRP mRNA expression in the d18 fibroblasts. This passage-dependent transdifferentiation is accompanied by a moderate (9–20%) increase in the synthesis of nucleic acid ribose from glucose through the non-oxidative steps of the pentose cycle. In contrast, d18 fibroblasts showed over an 85% decrease in the de novo synthesis of palmitate from glucose, while d21 fibroblasts showed a less pronounced 32–38% decrease in de novo lipid synthesis in hyperoxia-exposed cultures. It can be concluded from these studies that: (1) there is a maturation dependent sensitivity to hyperoxia; (2) transdifferentiation of flbroblast as demonstrated by changes in ADRP expression is accompanied by metabolic enzymes changes affecting ribose acid synthesis from glucose, and (3) hyperoxia specifically inhibits lipogenesis from glucose. Hyperoxia-induced metabolic changes thus play a key role in the transdifferentiation of lung fibroblasts to myofibroblasts and the pathogenesis of BPD.

Dopaminergic modulation of ventilation in obese Zucker rats.

To investigate the hypothesis that the impaired respiratory drive noted in morbid obesity was attributable to altered dopaminergic mechanisms acting on peripheral and/or central chemoreflex sensitivity, seven obese and seven lean Zucker rats were studied at 11 wk of age. Ventilation (VE) was measured by the barometric technique during hyperoxic (100% O(2)), normoxic (21% O(2)), hypoxic (10% O(2)), and hypercapnic (7% CO(2)) exposures after the administration of vehicle (control), haloperidol [Hal, 1 mg/kg, a central and peripheral dopamine (Da) receptor antagonist], or domperidone (Dom, 0.5 mg/kg, a peripheral Da receptor antagonist). In both lean and obese rats, Hal increased tidal volume and decreased respiratory frequency during hyperoxia or normoxia, resulting in an unchanged VE. In contrast, Dom did not affect tidal volume, frequency, or VE during hyperoxia or normoxia. During hypoxia, however, VE significantly increased from 1,132 +/- 136 to 1,348 +/- 98 ml. kg(-1). min(-1) (P < 0.01) after the administration of Dom in obese rats, whereas no change was observed in lean rats. Hal significantly decreased VE during hypoxia compared with control in lean but not obese rats. In both lean and obese rats, Hal decreased VE in response to hypercapnia, whereas Dom had no effect. Our major findings suggest that peripheral chemosensitivity to hypoxia in obese Zucker rats is reduced as a result of an increased dopaminergic receptor modulation in the carotid body.