Lee Frank

Preparation for birth into an O2-rich environment: the antioxidant enzymes in the developing rabbit lung.

Summary: To determine if some specific “preparation for birth” occurs in the developing lung to help assure its successful adaptation to a comparatively O2-rich world at birth, we measured the activities of the antioxidant enzymes in the developing lungs of rabbit fetuses from 10 d before parturition to several days after birth. Superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GP) activities showed similar maturational patterns with significant increases in activity, compared with earlier gestational levels, during the last 3-5 d before birth. During the final days in utero, SOD and CAT activities increased by ~110% and lung GP activity by ~200%. There were no parallel changes in lung O2 consumption demonstrable over this same prenatal period.

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 formation of alveoli in rat lung during the third and fourth postnatal weeks: effect of hyperoxia, dexamethasone, and deferoxamine.

ABSTRACT: Terminal gas-exchange units in the lung of many species are, at birth, relatively large structures termed saccules. Saccules septate postnatally forming smaller units that constitute the final alveoli. In the rat, septation occurs intensively during the first 2 postnatal wk after which it has been considered to stop. Treatment with dexamethasone or exposure to hyperoxia during the first 2 postnatal wk markedly inhibits septation as evidenced by the formation of fewer and bigger alveoli than in normally developed rats. Deferoxamine, an iron chelator, has been reported to protect the lung from the effects of exposure to hyperoxia in early postnatal life. In this study, we investigated the effects of these treatments during the 3rd and 4th postnatal wk, that is, after the early period of rapid alveolarization. Our results show that treatment with dexamethasone no longer had any inhibitory effect on alveoli formation; that exposure to hyperoxia continued to inhibit the formation of new alveoli, resulting in bigger and less numerous alveoli; that treatment of animals exposed to hyperoxia with deferoxamine still protected their lungs against hyperoxic inhibition; and that elastin fiber length density in the lung was significantly reduced in hyperoxic-exposed animals. These results suggest that septation continues beyond the 2nd postnatal wk and does not stop abruptly at age 14 d in air-breathing rats and that hyperoxic inhibition of alveolarization during the 3rd and 4th postnatal wk is due to the inhibition of septation of existing or newly generated gas-exchange units during that period of lung development.

Menhaden Fish Oil, n-3 Polyunsaturated Fatty Acids, and Protection of Newborn Rats from Oxygen Toxicity

ABSTRACT: We have previously reported that newborn rats born to mothers fed a high n-6 polyunsaturated fatty acid (PUFA) (safflower oil) diet demonstrated increased n-6 PUFA in lung lipids and superior tolerance to high oxygen exposure. In the present study, we explored whether high n-3 PUFA might also protect against hyperoxic damage and by what mechanism. Adult female rats were fed either regular rat chow, a high n-3 (menhaden fish oil-based) diet, or a high n-6 (safflower oil-based) diet for 6 wk before and then throughout pregnancy and lactation. Newborn offspring of the high n-3 (fish oil) dams demonstrated increased n-3 PUFA {i.e. eicosapentaenoic and docosahexaenoic acid) and decreased n-6 PUFA {i.e. linoleic and arachidonic acid) in their lung lipids compared to the other two diet groups. The high n-6 (safflower oil) offspring had the opposite PUFA lung lipid pattern (with increases in total n-6 fatty acids and decreases in total n-3 fatty acids). The high n-3 offspring demonstrated markedly decreased lung levels of prostaglandin E2, vja and thromboxane B2, whereas the high n-6 offspring had higher eicosanoid levels than the regular diet offspring. Offspring of both high n-6 and high n-3 diet dams demonstrated essentially the same superior hypperoxic tolerance compared to regular diet offspring [7-d (>95% O2) survival rates of 110/115 and 99/109, respectively, versus 70/91, p<0.01]. These studies lend further support to the speculation that increasing lung PUFA content may provide the newborn lung with increased ability to scavenge oxygenfree radicals and thus may serve to protect against oxygen toxicity.

Effects of low-dose prenatal corticosteroid administration on the premature rat.

Administration of dexamethasone (0.2 mg/kg/day) to the pregnant rat on days 19 and 20 of gestation has been found to have marked effects on the body and organ weight of fetuses delivered 1 day prematurely on day 21 of gestation. Longer-term dexamethasone treatment earlier in gestation (days 14-18) had even more pronounced effects on body and organ weights in rat fetuses delivered three days prematurely. Dexamethasone treatment for 2 days prior to delivery had no effect on the survival of 1-, 2- and 3-day premature rats. The marked inhibitory effect on organ and body weight observed in the premature rats resulted from prenatal steroid therapy approximating that used in human pregnancies (dexamethasone 0.2 mg/kg/day for 48 h prior to premature delivery).

Age-Related Development of Pulmonary Antioxidant Enzymes in the Rat

Summary The age-related development of the activity levels of the pulmonary an-tioxidant defense systems: superoxide dismutase, reduced glutathione, glutathione peroxidase, glutathione reductase and catalase, were examined in the rat. All the pulmonary antioxidant enzymes studied showed a lower activity at two days before term than at birth. From day 12 to 70, with the exception of catalase, these enzymes and reduced glutathione generally showed a gradual increase in activity levels with age. Pulmonary catalase activity, however, was lower in the older rats compared to the younger animals. No significant differences were found in the various pulmonary enzyme activities and glutathione levels between male and female rats.

Prenatal dexamethasone administration to premature rats exposed to prolonged hyperoxia: A new rat model of pulmonary fibrosis (bronchopulmonary dysplasia)☆☆☆★★★

OBJECTIVE To evaluate the postnatal effects of prenatal dexamethasone treatment of preterm rats and to test the hypothesis that prenatal dexamethasone treatment projects against pulmonary oxygen toxicity in the preterm rats and stimulates lung antioxidant enzyme levels in response to hyperoxia. STUDY DESIGN We administered dexamethasone (0.4 mg/kg, intraperitoneally), or equivolume saline solution to pregnant rats at 48 and 24 hours before premature delivery at gestation day 21. Both groups of prematurely delivered rat pups were randomly assigned to other > 95% O2 or room air immediately after birth and brief resuscitation. RESULTS The hyperoxic survival rates from day 1 through day 14 were similar in both dexamethasone-treated and control preterm O2 groups. At 7 days of hyperoxia, the preterm pups demonstrated similar lung antioxidant enzyme activity and sufactant content responses to high O2 in the dexamethasone-treated and control groups. Lung quantitative morphometry changes were similar (equal degree of inhibition of normal alveolar development) in both groups. Unexpectedly, the lungs of the preterm O2 control rats showed evidence of septal fibrosis and the pups that received dexamethasone-O2 showed even greater severity of septal fibrosis and a greater increase (+50%) of lung hydroxyproline compared with the O2 groups control rats. CONCLUSIONS In preterm animals, prenatal dexamethasone administration does not show any of the hypothesized protective effects against hyperoxia or protective biochemical lung changes during prolonged O2 exposure. However, prenatal dexamethasone administration with prolonged exposure of the preterm rat to hyperoxia results in a pulmonary pathologic picture quite similar to bronchopulmonary dysplasia.

Oxygen therapy and hyaline membrane disease: the effect of hyperoxia on pulmonary superoxide dismutase activity and the mediating role of plasma or serum.

In vitro and in vivo hyperoxic exposure of the lungs of normal immature animals resulted in a rapid increase of pulmonary superoxide dismutase activity. The increase of pulmonary SOD activity with in vitro hyperoxic exposure requires the presence of plasma or serum in the incubation medium. Twenty-three out of 26 plasma samples from premature infants without hyaline membrane disease were found to support the hyperoxic increaase of pulmonary SOD activity, whereas only five of 15 plasma samples from infants with HMD were effective. A defective plasma-lung interaction in infants with HMD may result in an inability to increase pulmonary levels of this presumed protective enzyme during hyperoxic therapy.

Comparative responses of premature versus full-term newborn rats to prolonged hyperoxia.

ABSTRACT: Because fetal rat lungs have lower baseline levels of both surfactant and antioxidant enzymes than full-term newborn rats, we questioned whether prematurely delivered rats might be more susceptible to O2 toxicity than those born at term. In the present studies, prematurely delivered rats (gestational d 21 of 22) and full-term rat pups were simultaneously put in >95% O2 after birth. Surprisingly, we found that the preterm rats were not more susceptible to O2-induced lung damage and lethality than full-term newborns, but, in fact, the composite percentage of survival was even greater in the preterm pups from 7 to 9 d in hyperoxia and were similar thereafter up to 14 d in high O2. In addition, the preterm rats showed significantly decreased lung wet/dry weight ratios and consistently less severe pathologic evidence of pulmonary edema compared with term rats at 6 and 8 d of O2 exposure. The premature pups demonstrated the capability of inducing pulmonary antioxidant enzyme responses to hyperoxia by 3 d, and had significantly elevated copper-zinc superoxide dismutase, catalase, and glutathione peroxidase activities (and lung surfactant contents) at 6 d of O2 exposure compared with the term rats in O2. The rates of lung total O2 consumption and cyanide-resistant O2 consumption at d 6 in hyperoxia were not different for preterm versus term pups. Although the basis for the transiently improved survival and decreased evidence of pulmonary O2 toxicity in the preterm rats is presently unknown, these findings clearly indicate that premature animals of at least one species are equally able to induce protective lung antioxidant enzymes and surfactant responses to hyperoxia as full-term newborn animals.