Skeletal muscle mitochondrial fragmentation and impaired bioenergetics from nutrient overload are prevented by carbon monoxide.

Nutrient excess increases skeletal muscle oxidant production and mitochondrial fragmentation that may result in impaired mitochondrial function, a hallmark of skeletal muscle insulin resistance. This led us to explore whether an endogenous gas molecule, carbon monoxide (CO), which is thought to prevent weight gain and metabolic dysfunction in mice consuming high-fat diets, alters mitochondrial morphology and respiration in C2C12 myoblasts exposed to high glucose (15.6 mM) and high fat (250 µM BSA-palmitate) (HGHF). Also, skeletal muscle mitochondrial morphology, distribution, respiration, and energy expenditure were examined in obese resistant (OR) and obese prone (OP) rats that consumed a high-fat and high-sucrose diet for 10 wk with or without intermittent low-dose inhaled CO and/or exercise training. In cells exposed to HGHF, superoxide production, mitochondrial membrane potential (ΔΨm), mitochondrial fission regulatory protein dynamin-related protein 1 (Drp1) and mitochondrial fragmentation increased, while mitochondrial respiratory capacity was reduced. CO decreased HGHF-induced superoxide production, Drp1 protein levels and mitochondrial fragmentation, maintained ΔΨm, and increased mitochondrial respiratory capacity. In comparison with lean OR rats, OP rats had smaller skeletal muscle mitochondria that contained disorganized cristae, a normal mitochondrial distribution, but reduced citrate synthase protein expression, normal respiratory responses, and a lower energy expenditure. The combination of inhaled CO and exercise produced the greatest effect on mitochondrial morphology, increasing ADP-stimulated respiration in the presence of pyruvate, and preventing a decline in resting energy expenditure. These data support a therapeutic role for CO and exercise in preserving mitochondrial morphology and respiration during metabolic overload.

Increased Antiseizure Effectiveness with Tiagabine Combined with Sodium Channel Antagonists in Mice Exposed to Hyperbaric Oxygen.

Hyperbaric oxygen (HBO2) is acutely toxic to the central nervous system, culminating in EEG spikes and tonic-clonic convulsions. GABA enhancers and sodium channel antagonists improve seizure latencies in HBO2 when administered individually, while combining antiepileptic drugs from different functional classes can provide greater seizure latency. We examined the combined effectiveness of GABA enhancers (tiagabine and gabapentin) with sodium channel antagonists (carbamazepine and lamotrigine) in delaying HBO2-induced seizures. A series of experiments in C57BL/6 mice exposed to 100% oxygen at 5 atmospheres absolute (ATA) were performed. We predicted equally effective doses from individual drug-dose response curves, and the combinations of tiagabine + carbamazepine or lamotrigine were tested to determine the maximally effective combined doses to be used in subsequent experiments designed to identify the type of pharmacodynamic interaction for three fixed-ratio combinations (1:3, 1:1, and 3:1) using isobolographic analysis. For both combinations, the maximally effective combined doses increased seizure latency over controls > 5-fold and were determined to interact synergistically for fixed ratios 1:1 and 3:1, additive for 1:3. These results led us to explore whether the benefits of these drug combinations could be extended to the lungs, since a centrally mediated mechanism is believed to mediate hyperoxic-induced cardiogenic lung injury. Indeed, both combinations attenuated bronchoalveolar lavage protein content by ~ 50%. Combining tiagabine with carbamazepine or lamotrigine not only affords greater antiseizure protection in HBO2 but also allows for lower doses to be used, minimizing side effects, and attenuating acute lung injury.

S-nitrosylation of GAD65 is implicated in decreased GAD activity and oxygen-induced seizures.

Breathing oxygen at partial pressures ≥2.5 atmospheres absolute, which can occur in diving and hyperbaric oxygen (HBO2) therapy, can rapidly become toxic to the central nervous system (CNS). This neurotoxicity culminates in generalized EEG epileptiform discharges, tonic-clonic convulsions and ultimately death. Increased production of neuronal nitric oxide (NO) has been implicated in eliciting hyperoxic seizures by altering the equilibrium between glutamatergic and GABAergic synaptic transmission. Inhibition of glutamic acid decarboxylase (GAD) activity in HBO2 promotes this imbalance; however, the mechanisms by which this occurs is unknown. Therefore, we conducted a series of experiments using mice, a species that is highly susceptible to CNS oxygen toxicity, to explore the possibility that NO modulates GABA metabolism. Mice were exposed to 100% oxygen at 4 ATA for various durations, and brain GAD and GABA transaminase (GABA-T) activity, as well as S-nitrosylation of GAD65 and GAD67 were determined. HBO2 inhibited GAD activity by 50% and this was negatively correlated with S-nitrosylation of GAD65, whereas GABA-T activity and S-nitrosylation of GAD67 were unaltered. These results suggest a new mechanism by which NO alters GABA metabolism, leading to neuroexcitation and seizures in HBO2.

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.

Safety of hyperbaric oxygen therapy for management of central airway stenosis after lung transplant.

BACKGROUND: Central airway stenosis (CAS) is common after lung transplantation and causes significant post-transplant morbidity. It is often preceded by extensive airway necrosis, related to airway ischemia. Hyperbaric oxygen therapy (HBOT) is useful for ischemic grafts and may reduce the development of CAS. METHODS: The purpose of this study was to determine whether HBOT could be safely administered to lung transplant patients with extensive necrotic airway plaques. Secondarily, we assessed any effects of HBOT on the incidence and severity of CAS. Patients with extensive necrotic airway plaques within 1-2 months after lung transplantation were treated with HBOT along with standard care. These patients were compared with a contemporaneous reference group with similar plaques who did not receive HBOT. RESULTS: Ten patients received HBOT for 18.5 (interquartile range, IQR 11-20) sessions, starting at 40.5 (IQR 34-54) days after transplantation. HBOT was well tolerated. Incidence of CAS was similar between HBOT-treated patients and reference patients (70% vs 87%, respectively; P=.34), but fewer stents were required in HBOT patients (10% vs 56%, respectively; P=.03). CONCLUSIONS: This pilot study is the first to demonstrate HBOT safety in patients who develop necrotic airway plaques after lung transplantation. HBOT may reduce the need for airway stent placement in patients with CAS.

Effects of striatal nitric oxide production on regional cerebral blood flow and seizure development in rats exposed to extreme hyperoxia.

The endogenous vasodilator and signaling molecule nitric oxide has been implicated in cerebral hyperemia, sympathoexcitation, and seizures induced by hyperbaric oxygen (HBO2) at or above 3 atmospheres absolute (ATA). It is unknown whether these events in the onset of central nervous system oxygen toxicity originate within specific brain structures and whether blood flow is diverted to the brain from peripheral organs with high basal flow, such as the kidney. To explore these questions, total and regional cerebral blood flow (CBF) were measured in brain structures of the central autonomic network in anesthetized rats in HBO2 at 6 ATA. Electroencephalogram (EEG) recordings, cardiovascular hemodynamics, and renal blood flow (RBF) were also monitored. As expected, mean arterial blood pressure and total and regional CBF increased preceding EEG spikes while RBF was unaltered. Of the brain structures examined, the earliest rise in CBF occurred in the striatum, suggesting increased neuronal activation. Continuous unilateral or bilateral striatal infusion of the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester attenuated CBF responses in that structure, but global EEG discharges persisted and did not differ from controls. Our novel findings indicate that: 1) cerebral hyperemia in extreme HBO2 in rats does not occur at the expense of renal perfusion, highlighting the remarkable autoregulatory capability of the kidney, and 2) in spite of a sentinel increase in striatal blood flow, additional brain structure(s) likely govern the pathogenesis of HBO2-induced seizures because EEG discharge latency was unchanged by local blockade of striatal nitric oxide production and concomitant hyperemia.

The Regulation of Proresolving Lipid Mediator Profiles in Baboon Pneumonia by Inhaled Carbon Monoxide.

Strategies for the treatment of bacterial pneumonia beyond traditional antimicrobial therapy have been limited. The recently discovered novel genus of lipid mediators, coined "specialized proresolving mediators" (SPMs), which orchestrate clearance of recruited leukocytes and restore epithelial barrier integrity, have offered new insight into the resolution of inflammation. We performed lipid mediator (LM) metabololipidomic profiling and identification of LMs on peripheral blood leukocytes and plasma from a baboon model of Streptococcus pneumoniae pneumonia. Leukocytes and plasma were isolated from whole blood of S. pneumoniae-infected (n = 5-6 per time point) and control, uninfected baboons (n = 4 per time point) at 0, 24, 48, and 168 hours. In a subset of baboons with pneumonia (n = 3), we administered inhaled carbon monoxide (CO) at 48 hours (200-300 ppm for 60-90 min). Unstimulated leukocytes from control animals produced a proresolving LM signature with elevated resolvins and lipoxins. In contrast, serum-treated, zymosan-stimulated leukocytes and leukocytes from baboons with S. pneumoniae pneumonia produced a proinflammatory LM signature profile with elevated leukotriene B4 and prostaglandins. Plasma from baboons with S. pneumoniae pneumonia also displayed significantly reduced LM-SPM levels, including eicosapentaenoic acid-derived E-series resolvins (RvE) and lipoxins. CO inhalation increased levels of plasma RvE and lipoxins relative to preexposure levels. These results establish the leukocyte and plasma LM profiles biosynthesized during S. pneumoniae pneumonia in baboons and provide evidence for pneumonia-induced dysregulation of these proresolution programs. Moreover, these SPM profiles are partially restored with inhaled low-dose CO and SPM, which may shorten the time to pneumonia resolution.

Baroreceptor afferents modulate brain excitation and influence susceptibility to toxic effects of hyperbaric oxygen.

Unexplained adjustments in baroreflex sensitivity occur in conjunction with exposures to potentially toxic levels of hyperbaric oxygen. To investigate this, we monitored central nervous system, autonomic and cardiovascular responses in conscious and anesthetized rats exposed to hyperbaric oxygen at 5 and 6 atmospheres absolute, respectively. We observed two contrasting phases associated with time-dependent alterations in the functional state of the arterial baroreflex. The first phase, which conferred protection against potentially neurotoxic doses of oxygen, was concurrent with an increase in baroreflex sensitivity and included decreases in cerebral blood flow, heart rate, cardiac output, and sympathetic drive. The second phase was characterized by baroreflex impairment, cerebral hyperemia, spiking on the electroencephalogram, increased sympathetic drive, parasympatholysis, and pulmonary injury. Complete arterial baroreceptor deafferentation abolished the initial protective response, whereas electrical stimulation of intact arterial baroreceptor afferents prolonged it. We concluded that increased afferent traffic attributable to arterial baroreflex activation delays the development of excessive central excitation and seizures. Baroreflex inactivation or impairment removes this protection, and seizures may follow. Finally, electrical stimulation of intact baroreceptor afferents extends the normal delay in seizure development. These findings reveal that the autonomic nervous system is a powerful determinant of susceptibility to sympathetic hyperactivation and seizures in hyperbaric oxygen and the ensuing neurogenic pulmonary injury.

Baroreflex-mediated cardiovascular responses to hyperbaric oxygen.

The cardiovascular system responds to hyperbaric hyperoxia (HBO2) with vasoconstriction, hypertension, bradycardia, and reduced cardiac output (CO). We tested the hypothesis that these responses are linked by a common mechanism-activation of the arterial baroreflex. Baroreflex function in HBO2 was assessed in anesthetized and conscious rats after deafferentation of aortic or carotid baroreceptors or both. Cardiovascular and autonomic responses to HBO2 in these animals were compared with those in intact animals at 2.5 ATA for conscious rats and at 3 ATA for anesthetized rats. During O2 compression, hypertension was greater after aortic or carotid baroreceptor deafferentation and was significantly more severe if these procedures were combined. Similarly, the hyperoxic bradycardia observed in intact animals was diminished after aortic or carotid baroreceptor deafferentation and replaced by a slight tachycardia after complete baroreceptor deafferentation. We found that hypertension, bradycardia, and reduced CO--the initial cardiovascular responses to moderate levels of HBO2--are coordinated through a baroreflex-mediated mechanism initiated by HBO2-induced vasoconstriction. Furthermore, we have shown that baroreceptor activation in HBO2 inhibits sympathetic outflow and can partially reverse an O2-dependent increase in arterial pressure.

Practice recommendations in the diagnosis, management, and prevention of carbon monoxide poisoning.

Carbon monoxide (CO) poisoning is common in modern society, resulting in significant morbidity and mortality in the United States annually. Over the past two decades, sufficient information has been published about carbon monoxide poisoning in the medical literature to draw firm conclusions about many aspects of the pathophysiology, diagnosis, and clinical management of the syndrome, along with evidence-based recommendations for optimal clinical practice. This article provides clinical practice guidance to the pulmonary and critical care community regarding the diagnosis, management, and prevention of acute CO poisoning. The article represents the consensus opinion of four recognized content experts in the field. Supporting data were drawn from the published, peer-reviewed literature on CO poisoning, placing emphasis on selecting studies that most closely mirror clinical practice.

Safe administration of hyperbaric oxygen after bleomycin: a case series of 15 patients.

INTRODUCTION: Supplemental oxygen has been reported to cause pulmonary complications after bleomycin. We describe the safe administration of hyperbaric oxygen (HBO2) after bleomycin in 15 patients. METHODS: Paper and electronic records were reviewed for bleomycin-exposed patients at the Duke Center for Hyperbaric Medicine and Environmental Physiology from 1979 to 2010. RESULTS: Fourteen bleomycin-exposed patients received HBO2 at Duke under a special-precautions protocol. One was treated for DCS elsewhere. The protocol included: pretreatment evaluation; chest radiograph; spirometry; blood gases; a single, 2-atmospheres absolute (atm abs), 120-minute HBO2 treatment; and a gradual acceleration over one week to a twice-daily schedule contingent on clinical and laboratory findings. Bleomycin indications were: head-and-neck squamous cell carcinomas (11), Hodgkin's lymphoma (2), other carcinomas (2). HBO2 indications were: osteoradionecrosis (10), soft-tissue radionecrosis (3), DCS (1) and a provocative oxygen toxicity test for a military aviator (1). Total bleomycin doses ranged from 40 to 225u/m2 (mean +/- SD, 105 +/- 57) given in conjunction with other chemotherapies and/or radiation. Radiation was 63.3 +/- 31.72 Gy (mean +/- SD), none to the chest with the exception of one patient treated for DCS elsewhere. Other chemotherapies included: vinblastine (11), methotrexate (11), CCNU (6) cisplatinum (7), dacarbazin (2), Adriamycin (1), and vincristine (1). Median age at time of HBO2 was 52 years (range 22-77). Median bleomycin-to-HBO2 latency was 34 months (range 1-279). Three patients received HBO2 within six months, and seven patients received HBO2 within two years of their last bleomycin exposure. There were no adverse pre-to-post HBO2 changes in: arterial blood gases, spirometry, chest radiograph findings or clinical reports. There were no persistent post-HBO2 pulmonary complications on follow-up. Post-HBO2 data were available for 40%, 53%, 87% and 100% of these parameters respectively. DISCUSSION: Bleomycin and oxygen can individually cause acute pulmonary toxicity. However, evidence for increased long-term susceptibility based on their synergy may be overstated.

What is the role of hyperbaric oxygen in the management of bisphosphonate-related osteonecrosis of the jaw: a randomized controlled trial of hyperbaric oxygen as an adjunct to surgery and antibiotics.

PURPOSE: This study tested hyperbaric oxygen (HBO) as an adjunct to surgery and antibiotics in the treatment of bisphosphonate-related osteonecrosis of the jaw (ONJ) and evaluated its effects on gingival healing, pain, and quality of life. MATERIALS AND METHODS: The investigators implemented a randomized controlled trial and enrolled a sample composed of patients with ONJ, where the predictor variable was HBO administered at 2 atm twice a day for 40 treatments as an adjunct to conventional therapy of surgery and antibiotics versus conventional therapy alone. Over the next 24 months, oral lesion size and number, pain, and quality of life were assessed. RESULTS: Forty-six patients (mean age, 66 yrs; 57% women) contributed data to the trial. There were no statistically significant differences in the distribution of variables used to assess randomization success between the HBO and standard treatment groups. Seventeen of 25 HBO-treated patients (68%) improved versus 8 of 21 controls (38.1%; P = .043, χ(2) test). Mean time to improvement was 39.7 weeks (95% confidence interval [CI], 22.4 to 57.0 weeks) for HBO-treated patients versus 67.9 weeks (95 CI, 48.4 to 87.5 weeks) for controls (P = .03, log-rank test). However, complete gingival healing occurred in only 14 of 25 HBO-treated patients (52%) versus 7 of 21 controls (33.3%; P = .203, χ(2) test), and time to healing was 59 weeks (95% CI, 42.8% to 75.8%) for HBO-treated patients versus 70 weeks (95 CI, 52.2% to 88.36%) for controls (P = .32, log-rank test). Pain decreased faster for HBO-treated subjects (P < .01, linear regression). Quality-of-life scores for physical health (P = .002) and perceived health (P = .043) decreased at 6 months for control group but for not the HBO group. CONCLUSIONS: ONJ is multifactorial and no single treatment modality is likely to reverse it; however, it is treatable and even advanced presentations can improve with intensive multimodal therapy. Clinically, HBO appears to be a useful adjunct to ONJ treatment, particularly for more severe cases, although this study was underpowered to fully support this claim.

Nitric oxide-mediated central sympathetic excitation promotes CNS and pulmonary O2 toxicity.

In hyperbaric oxygen (HBO(2)) at or above 3 atmospheres absolute (ATA), autonomic pathways link central nervous system (CNS) oxygen toxicity to pulmonary damage, possibly through a paradoxical and poorly characterized relationship between central nitric oxide production and sympathetic outflow. To investigate this possibility, we assessed sympathetic discharges, catecholamine release, cardiopulmonary hemodynamics, and lung damage in rats exposed to oxygen at 5 or 6 ATA. Before HBO(2) exposure, either a selective inhibitor of neuronal nitric oxide synthase (NOS) or a nonselective NOS inhibitor was injected directly into the cerebral ventricles to minimize effects on the lung, heart, and peripheral circulation. Experiments were performed on both anesthetized and conscious rats to differentiate responses to HBO(2) from the effects of anesthesia. EEG spikes, markers of CNS toxicity in anesthetized animals, were approximately four times as likely to develop in control rats than in animals with central NOS inhibition. In inhibitor-treated animals, autonomic discharges, cardiovascular pressures, catecholamine release, and cerebral blood flow all remained below baseline throughout exposure to HBO(2). In control animals, however, initial declines in these parameters were followed by significant increases above their baselines. In awake animals, central NOS inhibition significantly decreased the incidence of clonic-tonic convulsions or delayed their onset, compared with controls. The novel findings of this study are that NO produced by nNOS in the periventricular regions of the brain plays a critical role in the events leading to both CNS toxicity in HBO(2) and to the associated sympathetic hyperactivation involved in pulmonary injury.

Autonomic activation links CNS oxygen toxicity to acute cardiogenic pulmonary injury.

Breathing hyperbaric oxygen (HBO2), particularly at pressures above 3 atmospheres absolute, can cause acute pulmonary injury that is more severe if signs of central nervous system toxicity occur. This is consistent with the activation of an autonomic link between the brain and the lung, leading to acute pulmonary oxygen toxicity. This pulmonary damage is characterized by leakage of fluid, protein, and red blood cells into the alveoli, compatible with hydrostatic injury due to pulmonary hypertension, left atrial hypertension, or both. Until now, however, central hemodynamic parameters and autonomic activity have not been studied concurrently in HBO2, so any hypothetical connections between the two have remained untested. Therefore, we performed experiments using rats in which cerebral blood flow, electroencephalographic activity, cardiopulmonary hemodynamics, and autonomic traffic were measured in HBO2 at 5 and 6 atmospheres absolute. In some animals, autonomic pathways were disrupted pharmacologically or surgically. Our findings indicate that pulmonary damage in HBO2 is caused by an abrupt and significant increase in pulmonary vascular pressure, sufficient to produce barotrauma in capillaries. Specifically, extreme HBO2 exposures produce massive sympathetic outflow from the central nervous system that depresses left ventricular function, resulting in acute left atrial and pulmonary hypertension. We attribute these effects on the heart and on the pulmonary vasculature to HBO2-mediated central sympathetic excitation and catecholamine release that disturbs the normal equilibrium between excitatory and inhibitory activity in the autonomic nervous system.

Multimodality surgical and hyperbaric management of mandibular osteoradionecrosis.

PURPOSE: To elucidate long-term outcomes in 65 consecutive patients meeting a uniform definition of mandibular osteoradionecrosis (ORN) treated with multimodality therapy including hyperbaric oxygen (HBO). METHODS AND MATERIALS: Pretreatment, post-treatment and long-term follow-up of mandibular lesions with exposed bone were ranked by a systematic review of medical records and patient telephone calls. The ranking system was based on lesion diameter and number plus disease progression. Changes from pretreatment to post-treatment and follow-up were analyzed by Wilcoxon signed-rank tests. Improved wound survival, measured by time to relapse, defined as any less favorable rank after HBO treatment, was assessed by Kaplan-Meier analysis. RESULTS: In all, 57 cases (88%) resolved or improved by lesion grade or progression and evolution criteria after HBO (p < 0.001). Four patients healed before surgery after HBO alone. Of 57 patients who experienced improvement, 41 had failed previous nonmultimodality therapy for 3 months and 26 for 6 months or more. A total of 43 patients were eligible for time-to-relapse survival analysis. Healing or improvement lasted a mean duration of 86.1 months (95% confidence interval [95% CI], 64.0-108.2) in nonsmokers (n = 20) vs. 15.8 months (95% CI, 8.4-23.2) in smokers (n = 14) versus 24.2 months (95% CI, 15.2-33.2) in patients with recurrent cancer (n = 9) (p = 0.002 by the log-rank method). CONCLUSIONS: Multimodality therapy using HBO is effective for ORN when less intensive therapies have failed. Although the healing rate in similarly affected patients not treated with HBO is unknown, the improvements seen with peri-operative HBO were durable provided that the patients remained cancer free and abstained from smoking.

Phosphodiesterase-5 inhibitors oppose hyperoxic vasoconstriction and accelerate seizure development in rats exposed to hyperbaric oxygen.

Oxygen is a potent cerebral vasoconstrictor, but excessive exposure to hyperbaric oxygen (HBO(2)) can reverse this vasoconstriction by stimulating brain nitric oxide (NO) production, which increases cerebral blood flow (CBF)-a predictor of O(2) convulsions. We tested the hypothesis that phosphodiesterase (PDE)-5 blockers, specifically sildenafil and tadalafil, increase CBF in HBO(2) and accelerate seizure development. To estimate changes in cerebrovascular responses to hyperoxia, CBF was measured by hydrogen clearance in anesthetized rats, either control animals or those pretreated with one of these blockers, with the NO inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME), with the NO donor S-nitroso-N-acetylpenicillamine (SNAP), or with a blocker combined with l-NAME. Animals were exposed to 30% O(2) at 1 atm absolute (ATA) ("air") or to 100% O(2) at 4 or 6 ATA. EEG spikes indicated central nervous system CNS O(2) toxicity. The effects of PDE-5 blockade varied as a positive function of ambient Po(2). In air, CBF did not increase significantly, except after pretreatment with SNAP. However, at 6 ATA O(2), mean values for CBF increased and values for seizure latency decreased, both significantly; pretreatment with l-NAME abolished these effects. Conscious rats treated with sildenafil before HBO(2) were also more susceptible to CNS O(2) toxicity, as demonstrated by significantly shortened convulsive latency. Decreases in regional CBF reflect net vasoconstriction in the brain regions studied, since mean arterial pressures remained constant or increased throughout. Thus PDE-5 blockers oppose the protective vasoconstriction that is the initial response to hyperbaric hyperoxia, decreasing the safety of HBO(2) by hastening onset of CNS O(2) toxicity.

Two faces of nitric oxide: implications for cellular mechanisms of oxygen toxicity.

Recent investigations have elucidated some of the diverse roles played by reactive oxygen and nitrogen species in events that lead to oxygen toxicity and defend against it. The focus of this review is on toxic and protective mechanisms in hyperoxia that have been investigated in our laboratories, with an emphasis on interactions of nitric oxide (NO) with other endogenous chemical species and with different physiological systems. It is now emerging from these studies that the anatomical localization of NO release, which depends, in part, on whether the oxygen exposure is normobaric or hyperbaric, strongly influences whether toxicity emerges and what form it takes, for example, acute lung injury, central nervous system excitation, or both. Spatial effects also contribute to differences in the susceptibility of different cells in organs at risk from hyperoxia, especially in the brain and lungs. As additional nodes are identified in this interactive network of toxic and protective responses, future advances may open up the possibility of novel pharmacological interventions to extend both the time and partial pressures of oxygen exposures that can be safely tolerated. The implications of a better understanding of the mechanisms by which NO contributes to central nervous system oxygen toxicity may include new insights into the pathogenesis of seizures of diverse etiologies. Likewise, improved knowledge of NO-based mechanisms of pulmonary oxygen toxicity may enhance our understanding of other types of lung injury associated with oxidative or nitrosative stress.

Contributions of nitric oxide synthase isoforms to pulmonary oxygen toxicity, local vs. mediated effects.

Reactive species of oxygen and nitrogen have been collectively implicated in pulmonary oxygen toxicity, but the contributions of specific molecules are unknown. Therefore, we assessed the roles of several reactive species, particularly nitric oxide, in pulmonary injury by exposing wild-type mice and seven groups of genetically altered mice to >98% O2 at 1, 3, or 4 atmospheres absolute. Genetically altered animals included knockouts lacking either neuronal nitric oxide synthase (nNOS(-/-)), endothelial nitric oxide synthase (eNOS(-/-)), inducible nitric oxide synthase (iNOS(-/-)), extracellular superoxide dismutase (SOD3(-/-)), or glutathione peroxidase 1 (GPx1(-/-)), as well as two transgenic variants (S1179A and S1179D) having altered eNOS activities. We confirmed our earlier finding that normobaric hyperoxia (NBO2) and hyperbaric hyperoxia (HBO2) result in at least two distinct but overlapping patterns of pulmonary injury. Our new findings are that the role of nitric oxide in the pulmonary pathophysiology of hyperoxia depends both on the specific NOS isozyme that is its source and on the level of hyperoxia. Thus, iNOS predominates in the etiology of lung injury in NBO2, and SOD3 provides an important defense. But in HBO2, nNOS is a major contributor to pulmonary injury, whereas eNOS is protective. In addition, we demonstrated that nitric oxide derived from nNOS is involved in a neurogenic mechanism of HBO2-induced lung injury that is linked to central nervous system oxygen toxicity through adrenergic/cholinergic pathways.

A comparison of hyperbaric oxygen versus hypoxic cerebral preconditioning in neonatal rats.

The potency of hyperbaric preconditioning (HBO-PC) is uncertain compared to well-validated ischemic or hypoxic models and no studies have directly compared HBO-PC to hypoxic preconditioning (HPC). We subjected rat pups to unilateral carotid cauterization followed by 90 min (min) of hypoxia using 8% O(2). Three HBO-PC regimes (maximum 2.5 atmospheres for 150 min) were compared to HPC (150 min of 8% O(2)) for changes in mortality and brain weight. Preconditioning-induced oxidative stress was assessed using aconitase activity and manganese superoxide dismutase (MnSOD) transcript levels. Initial brain weight data revealed a large coefficient of variation and compelled an examination of the temperature sensitivity of the model that revealed a narrow optimal range of 35 to 37 degrees C of variability in brain injury and mortality. With rigorous temperature control, high dose HBO-PC and HPC showed comparable anatomic (mean hemispheric weight decrease: control 42%, HPC 25% (P=0.01), HBO-PC 26% (P=0.01) and mortality protection (control 14.7%, HPC 5.9% HBO-PC 5.7%, P=0.001). High dose HBO-PC, but not HPC, suppressed aconitase activity by 65% at 24 h after the preconditioning stimulus (P=0.001). In contrast, MnSOD mRNA increased 2.5-fold at 24 h after HPC (P=0.007) but not after high dose HBO-PC. Thus, when temperature variability is eliminated, HBO-PC and HPC elicit similar preconditioning efficacy in neonatal brain but invoke different defenses against oxidative stress.