Prenatal thyroid releasing hormone and thyroid releasing hormone plus dexamethasone lessen the survival of newborn rats during prolonged high O2 exposure.

Newborn rats prenatally treated with TRH or the combination of TRH + DEX have lower lung antioxidant enzyme activities at birth than control newborns but are able to induce an adaptive antioxidant enzyme response to hyperoxic exposure of similar or even greater magnitude compared to O2 control offspring. Because of this greater antioxidant enzyme response, we hypothesized that the hormonally pretreated newborns might demonstrate superior tolerance to prolonged high O2 exposure. However, when placed in greater than 95% O2 at birth, the survival rates were consistently lower in the TRH- and TRH + DEX-treated pups at all time periods in hyperoxia from 9 d [control = 74 of 92 (80%); TRH + DEX = 32 of 47 (68%); TRH = 29 of 48 (60%); p less than 0.05] to 14 d [control = 43 of 92 (47%); TRH + DEX = 11 of 47 (23%); TRH = nine of 48 (19%); (p less than 0.05)]. Other evidence of poorer O2 tolerance in the prenatal hormone-treated pups included a greater incidence of intraalveolar edema and elevated lung conjugated dienes, an index of lipid peroxidation, at 3, 5, and 7 d of O2 exposure. There was also a persistent elevation in 3,5,3'-triiodo-L-thyronine and thyroxine serum levels in the 10-d-old TRH-treated offspring. We conclude that prenatal TRH treatment, possibly working through the secretion of 3,5,3'-triiodo-L-thyronine and thyroxine, has some important lasting postnatal effect (not completely reversed by dexamethasone) that predisposes newborn rats to greater O2 radical-induced lung sequelae of prolonged hyperoxic exposure.