Effect of changes in thyroid status on tissue respiration in fetal and newborn sheep

The effect of both exogenous and endogenous changes in thyroid status on in vitro tissue respiration and thermogenic enzymes (sodium-potassium-adenosine triphosphatase and alpha-glycerophosphate dehydrogenase) was studied in fetal and newborn sheep. Oxygen consumption of liver and brain increased from 25 +/- 4.1 and 58.5 +/- 2.8 microliters O2 X 100 mg-1 X h-1, respectively, in tissues from unthyroidectomized fetal animals at 136-140 days gestation to 60 +/- 4.2 and 72 +/- 1.5 microliters O2 X 100 mg-1 X h-1 in tissues from unthyroidectomized newborn lambs between birth and 7 days of age. The physiological changes in thyroid function that normally occur at birth resulted in a mean (+/- SE) plasma triiodothyronine (T3) concentration of 563 +/- 39 ng/dl in the newborn lambs compared with 39 +/- 8 ng/dl in the fetal animals. Kidney respiration and thermogenic enzyme activities in the several tissues studied did not change. Liver, kidney, and brain respiration and thermogenic enzymes from T3-treated thyroidectomized fetal and newborn lambs were not increased (compared with untreated thyroidectomized animals) despite a marked increase in plasma T3 concentrations. Conclusions are that 1) liver, kidney cortex, and frontal brain cortex in the fetal and newborn lamb are relatively insensitive to the calorigenic effect of thyroid hormones, and 2) a perinatal increase in hepatic and cerebral respiration occurs in newborn animals (compared with fetal animals) but is probably not due solely to perinatal increases in thyroid hormones.