Robert J. Roberts

OXYGEN TOXICITY: COMPARISON OF LUNG BIOCHEMICAL RESPONSES IN NEONATAL AND ADULT RATS

Summary: Neonatal rats (4–7 days old) and adult rats (approximately 80 days old) were continuously exposed to either 96–98% oxygen or air. Examination of the lungs of neonatal rats, who survived 5 days of oxygen exposure with no evidence of respiratory distress, showed significant increases in the pulmonary superoxide dismutase (SOD) activity (peak value: 144% of air-exposed controls), glutathione peroxidase (GP) activity (126%), glutathione reductase (GR) activity (122%), reduced glutathione (GSH) level (176%), and glucose-6-phosphate dehydrogenase activity (151%). Adult rats, most of whom succumbed within 3 days of oxygen exposure, did not show any significant increase in the activities of pulmonary SOD, GP, GR, and the level of GSH as compared to the air-exposed adult animals. Glucose-6-phosphate dehydrogenase was significantly elevated in the 72-hr oxygen-exposed adult rats. It. is concluded that increases in the lung complement of SOD, GR, GP, and GSH in the neonatal rat during oxygen challenge may provide the mechanism(s) for their increased tolerance to hyperoxia-induced lung injury as compared to the adults.Speculation: Further knowledge about the role of the antioxidant defense mechanism(s) of the neonatal lung may have important implications in the pursuit of the etiology of neonatal respiratory disease and in the identification of agent(s) that may minimize pulmonary toxicity associated with oxygen therapy.

Developmental Characteristics of Pulmonary Superoxide Dismutase: Relationship to Idiopathic Respiratory Distress Syndrome

Extract: Pulmonary superoxide dismutase (SOD) activity was determined for various groups of human fetuses, infants, and adults. Enzyme activity was found to increase with age from a low of 17 ± 1 units/nig DNA in fetal lung to 49 ± 6 units/nig DNA in infant lung and finally to 110.2 ± 14.8 units/mg DNA in adult lung (P < 0.05). No difference in lung SOD activity was demonstrated between normal infants and those with idiopathic respiratory distress/hyaline membrane disease (IRDS/HMD). No significant differences in SOD activity were found among all the samples of infant blood. Adult blood samples, however, contained significantly greater SOD activity both in terms of heme concentration and volume of whole blood (P < 0.05). SOD activity in lung tissue from both rats and rabbits was also found to increase with age from a low value in fetal animals to a maximum activity in adults (P < 0.05). Exposure of New Zealand White rabbits, prematurely delivered by caesarian section, to 80% oxygen for 24 hr resulted in a 42% increase in lung SOD activity. Similarly, 7-day-old Sprague-Dawley rats exposed to 85% oxygen for 24 hr showed a 43% increase in pulmonary SOD activity. No increase in pulmonary SOD was observed when adult rats were exposed to 85% oxygen for 24 hr. The effect of hyperoxia on SOD activity in excised lung was investigated. Rat lung, incubated in either heparinized whole blood or in plasma and exposed to 100% oxygen, showed a 30% increase in SOD activity after 2 hr. This capacity of lung tissue to respond to hyperoxia in vitro with increased SOD activity was age dependent. The maximum increase in SOD activity was seen with lungs from 10–12-day-old rats. The oxygen-stimulated increase in lung SOD activity disappeared at about 19–20 days of age.Speculation: Superoxide dismutase which catalyzes the dismutation of the oxygen free radical may well be a primary lung protectant against the depredations of environmental oxygen. SOD appears to be a maturationally important enzyme since the activity of this enzyme increases with development in both lung tissue and blood of animals and humans. The premature infant may be compromised when exposed to the relative hyperoxia of the extrauterine environment by a reduced complement of the enzyme or a reduced ability to increase pulmonary SOD activity in response to hyperoxia. Lung damage resulting from deficient endogenous protection may be a factor in the clinical picture of IRDS/HMD. Treatment of the immature lung with high concentrations of oxygen may further compromise a lung already deficient in SOD protection. Because prolonged expisure to hyperoxia precedes diagnosis of bronchopulmonary dysplasia, an SOD deficiency may also be important in the etiology of this condition.

The Development of the Newborn Rat Lung in Hyperoxia: A Dose-Response Study of Lung Growth, Maturation, and Changes in Antioxidant Enzyme Activities

Summary: To examine the dose-response relationships of oxygen-induced lung changes, newborn rats were exposed to various patterns of concentrations of hyperoxia (0.4, 0.8, and >0.95 FiO2) for up to 12 days. Prominent findings included microscopic evidence of lung injury and retarded alveolar development (secondary septal development delayed by as much as 88%), lower whole lung DNA (50% of control), lung-to-body-weight ratios (by as much as 18%), and significantly less compliance in the lungs after exposures of 6− or 12-day durations to all concentrations of hyperoxia. Significant increases in the activities of the lung protective enzymes superoxide dismutase (129 to 160% of control), catalase (112 to 274% of control), and glutathione peroxidase (118 to 256% of control) were noted when activity was expressed on a DNA basis after 12-day exposures to the various patterns of hyperoxia.Lung changes noted after a 7-day recovery period in air included interstitial thickening (117% of control), persistance of the microscopic injury, and retarded alveolar development seen immediately after initial 6-day hyperoxic exposures. At the conclusion of a second wk of recovery in air, the lungs of hyperoxic exposed animals resembled controls in most respects, but a significantly altered compliance was exhibited by the lungs of animals initially exposed to 6 days of 0.4 or >0.95 FiO2.The dose dependency of oxygen-induced lung injury is complex. Straightforward, stepwise dose-response adequately describes the evolution of microscopic injury and slowing of alveolar development in hyperoxia, but the dose dependency is not as clearly identified in the oxygen-induced retardation of lung growth including DNA content and in changes in antioxidant enzyme activities. Changes in lung compliance clearly do not follow expected dose-response relationships.Speculation: The toxic influence of oxygen on the lung and on lung develop ment varies depending upon the concentration and duration of oxygen exposure. In addition, individual vulnerability to the toxic effects of oxygen on the lung may vary depending upon the stage of maturation of the lung at the time of oxygen exposure. The variety of clinical presentations and morphologic features of chronic lung disease in premature infants (e.g., Wilson Mikity, chronic pulmonary insufficiency of the premature, and bronchopulmonary dysplasia) may represent a spectrum of oxygen effects on the lung rather than diseases with distinctly different pathogenesis.

Effect of phenobarbital on the excretion of an exogenous bilirubin load

Abstract Treatment of mice with phenobarbital significantly enhanced the disappearance of exogenously administered bilirubin from the plasma. Not only did this enhanced disappearance occur in the absence of biliary excretion but it was accompanied by liver bilirubin concentrations exceeding those found in controls. Both observations suggest enhanced uptake of free bilirubin by the liver as one cause of the faster rate of disappearance of bilirubin from plasma. In rats, the maximal rate of bilirubin excretion was enhanced by treatment with phenobarbital. The bile volume was also greater in phenobarbital-treated animals than in controls. The concentration of bilirubin in the bile was not significantly different, indicating that the increase in bilirubin excretion was probably due to the increase in bile volume. Indirectly, some of the data suggest that increased bilirubin conjugation may play a role in the enhanced uptake and excretion of bilirubin after phenobarbital treatment.

Maximum Biliary Excretion of Bilirubin and Sulfobromophthalein During Anesthesia-Induced Alteration of Rectal Temperature.

Summary The influence of temperature on the maximum biliary excretion of bilirubin and sulfobromophthalein was studied in anesthetized rats and mice. Experimental techniques commonly employed in maximum biliary excretion studies were shown to significantly alter normal thermoregulatory mechanisms. A decrease in body temperature significantly decreased the bilirubin transport maximum in both rats and mice. A loss of body temperature in the rat also produced a significant decrease in the BSP transport maximum. In both rats and mice bile flow showed a diminution corresponding with the decrease in rectal temperature. The results emphasize the importance of monitoring body temperature during the course of experiments employing maximum biliary excretion as an endpoint. The authors wish to acknowledge with gratitude the excellent assistance of Mrs. Sharon Shriver.

Oxygen-Induced Alterations in Lung Vascular Development in the Newborn Rat

Summary: Newborn rats were exposed to air or hyperoxic conditions for the first 6 days of life. Resulting effects on the pulmonary vascular bed were determined by analysis of barium angiograms, scanning electron microscopy of methylmethacrylate corrosion casts and whole lung, morphometric estimations of pulmonary arteries/area and capillary number/area, and arterial blood gas measurements. Similar studies were also performed on the lungs of animals allowed to recover in air for 1 and 2 wk. Although the general pattern of the pulmonary arterial bed by barium angiograms appeared similar, diminished branching or underfilling of the distal arterial segments was more frequently encountered in hyperoxic-exposed animals. Morphometric examination and corrosion casts revealed differences in vascular pattern and density between hyperoxia and air-exposed animals. The number of capillaries/mm2 of lung tissue was less in hyperoxic-exposed pups than controls after 6 days of exposure to hyperoxia but markedly increased to slightly above control levels by 2 wk of air recovery. The number of 20–50 μm size vessels/mm2 followed a similar pattern of change. Corrosion casts of lungs exposed to 6 days of hyperoxia revealed less microvascular density compared to air controls, but after 1 wk recovery in air, hyperoxic-exposed animals had a more extensive network of microvessels. Maximum Pao2 attained by animals in the various groups closely resembled the patterns of change in microvessel density. These findings support the thesis that a major alteration of lung vascular growth and development occurs subsequent to exposure of the newborn to hyperoxia.

Age-related differences in salicylamide and acetaminophen conjugation in man†

Following a concomitant oral dose of salicylamide and acetaminophen (5 mg/kg of each) the urinary excretion of glucuronide and sulfate conjugates of the drugs were followed in children (ages seven to ten years) and adults. No significant difference were observed between the two age groups in the half-lives for appearance of salicylamide conjugates in urine. Age-related changes in the metabolic pathways, however, were observed. The mean percentage of dose excreted as salicylamide sulfate was significantly higher in children (78%) than in adults (36%). In contrast, salicylamide glucuronide was the major excretory product in adults. Similar age-related differences were observed for acetaminophen conjugation. Pharmacokinetic analysis indicated that the deficiency in glucuronide conjugation of these drugs in children is accompanied by a higher rate of sulfate formation.

Use of hemoperfusion for treatment of theophylline intoxication

We review our experience in the management of patients with plasma theophylline concentrations of 30 micrograms/ml or greater. Over a two-and-a-half-year period, 22 patients (Group 1) had plasma theophylline concentrations of 37 +/- 1 micrograms/ml (mean +/- SE) and experienced no severe toxicity (i.e., ventricular extrasystoles or tachycardia, seizures, cardiovascular collapse, or death). Six patients (Group 2) took overdoses of theophylline (92 +/- 12 micrograms/ml) and one died. Eight patients (Group 3) were iatrogenically intoxicated (48 +/- 6 micrograms/ml) and three died. Six patients from Groups 2 and 3 underwent hemoperfusion and did well, except for one patient, in whom seizures developed before hemoperfusion was initiated. We conclude from this experience that charcoal hemoperfusion is a useful procedure for the treatment of theophylline intoxication because of: (1) the serious morbidity and mortality of theophylline intoxication, (2) the prevention of complications with hemoperfusion, and (3) the relative safety of the procedure. We provide tentative guidelines for the initiation of hemoperfusion for the treatment of theophylline intoxication.

Pharmacological alteration of oxygen-induced lung toxicity

Abstract The effect of eight selected drugs on oxygen-induced pulmonary injury was evaluated in the rat. Several drug treatments, including meclofenamate (5 mg/kg/day), aminophylline (15 mg/kg/day), and vitamin C (20 and 50 mg/kg/every 12 hr) were found not to alter the survival of rats in 96 to 98% oxygen. Although vitamin E deficiency has repeatedly been shown to aggravate oxygen toxicity, pharmacologic doses of vitamin E (20 and 50 mg/kg/every 12 hr) in animals maintained on a normal diet did not offer protection against oxygen-induced lung toxicity. For the remaining drug treatments, the activity of pulmonary antioxidant defense systems [superoxide dimutase (SOD), catalase (CA), glutathione peroxidase (GP), and reduced glutathione (GSH)] were analyzed to explore the possible mechanism of pharmacological alteration of oxygen toxicity. Lungs from rats treated with dexamethasone (0.4 mg/kg/day) were found to have greater oxygen-induced lung damage and significantly lower pulmonary antioxidant activity. Rats pretreated with propylthiouracil (10 mg/kg/day) showed less oxygen-induced lung damage and greater pulmonary GSH levels and CA activity. Indomethacin pretreatment did not affect the course of oxygen toxicity or the activity of pulmonary antioxidant defense systems. GSH levels were lower in lungs of rats pretreated with levothyroxine (16.7 mg/kg/day), which produced an accelerated development of pulmonary oxygen toxicity. It was concluded that modification of the activity of the pulmonary antioxidant defense systems provides a plausible mechanism in explaining pharmacological alteration of oxygen-induced pulmonary injury.

Therapeutic problems arising from the use of the intravenous route for drug administration

The kinetics of drug delivery were studied under conditions of varying iv flow rates and varying sites of drug administration into an iv system. The iv sites and rates were selected on the basis of questionnaire data obtained from nursing personnel. The rate of drug delivery from the iv system was dependent upon the iv flow rate and site of injection of the drug into the iv system. Under conditions of slow iv flow rates (3 ml/hour) there was the expected time delay required for the drug to begin to be infused but an unanticipated protracted time required for actual completion of the drug infusion. Calculation of theoretical drug blood levels based on the results of these iv studies revealed very different blood levels of drug at respective times, including different peak concentrations, depending on the iv flow rate and site at which the drug was injected into the system. Failure to appreciate the effects of slow iv rates and distal iv injection sites on actual rates of drug administration can adversely affect pharmacokinetic decisions and conceivably influence therapeutic success or failure.