Pulmonary Superoxide Dismutase Activity in the Euthyroid and Hypothyroid Ovine Fetus

Abstract

SummaryPulmonary superoxide dismutase (SOD) activity was measured in 11 euthyroid and 15 hypothyroid ovine fetuses at 130 days gestation. In the euthyroid fetus, the mean pulmonary SOD activity was similar in saline-and thyroxine-infused groups. Compared to the euthyroid fetus, the mean pulmonary SOD activity was significantly lower in the noninfused or saline-infused athyrotic fetus. Administration of thyroxine to the hypothyroid fetus results in a normalization of pulmonary SOD activity to control values. Thug, thyroxine appears to influence the maturation of pulmonary SOD activity in the ovine fetus during the third trimester. SpeculationThe influence of thyroxine on pulmonary SOD activity was studied in the euthyroid and hypothyroid sheep fetus during the last trimester of gestation.Superoxide dismutase (SOD), an enzyme which catalyzes the reaction: 2 O2 + 2 H+ + HzOz + 02, is found in all cells that metabolize oxygen (9,14). Despite numerous studies investigating the role of SOD activity in oxygen-induced lung injury, there is little information regarding the mechanisms responsible for endogenous regulation of pulmonary SOD activity (14). It is known that pulmonary SOD activity is lowest in the fetus, with an increase in activity seen in the neonatal and adult lung (2,23,26). In contrast to the adult lung, the neonatal lung exposed to a hyperoxic environment will increase pulmonary SOD activity, a response mediated at least in part by a factor(s) in plasma or serum and exposure to oxygen (2,13,21,24).It has been shown previously that pulmonary oxygen toxicity can be influenced by thyroid function. In the adult, pulmonary oxygen toxicity is enhanced by hyperthyroidism and delayed by hypothyroidism (6,20,25). Iodothyronines have also been shown to influence the maturation of the fetal lung (12,17,22). If thyroid hormones are used to pharmacologically accelerate fetal lung maturation, the fetus would be, in theory, at greater risk for the development of pulmonary oxygen toxicity. The purpose of this study was to investigate the relationship of iodothyronines and pulmonary SOD activity in the preterm ovine fetus. MATERIALS AND METHODSAnimal model. Date bred ewes were obtained from a local source, maintained at the University of Iowa vivarium and given free access to alfalfa and water. In fifteen singleton ewes, a surgical thyroidectomy was performed on the fetus at 95-99 days gestation. Term is 147 to 150 days. In nine of the hypothyroid fetuses, catheters were inserted into a fetal artery and vein at 122-124 days gestation. After a 3-5 day stabilization period, five fetuses received a 50 pg bolus intravenous injection of thyroxine (T4) followed by a continuous intravenous infusion of 100 &day of T4 for 72 h before sacrifice. Four fetuses received a constant infusion of an equivalent volume of saline. These nine fetuses and six noninfused fetuses were sacrificed at 130 days gestation.Eleven euthyroid ovine fetuses had catheters inserted into a fetal artery and vein at 122-124 days gestation. After a three-five ...

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