Effect of Hypophysectomy with and without Thyroxine Replacement on Growth and Circulating Concentrations of Insulin-Like Growth Factors I and II in the Fetal Lamb*

The role of the pituitary gland in the regulation of skeletal growth and plasma insulin-like growth factor (IGF)-I and IGF-II concentrations in the late gestation sheep fetus has been studied. Singleton fetuses were either hypophysectomized (n = 14) or sham operated (n = 8) between days 110 and 125. Fetal and maternal blood samples were collected three times weekly through the remainder of gestation. In some hypophysectomized fetuses (HXT4 group, n = 4), T4 was administered (100 micrograms L-T4/day) to overcome the hypothyroidism caused by hypophysectomy. The other hypophysectomized fetuses received no replacement therapy (HXNR group, n = 10). Six HXNR fetuses were allowed to remain in utero post term and were killed at day 163 of gestation. All other animals were killed at day 147. All values are group means +/- SE. Hypophysectomized fetuses had significantly shorter limbs and long bones and delayed osseous maturation at term compared to sham controls. Plasma free T4 concentrations in HXT4 fetuses were not significantly different from those measured in sham fetuses (P greater than 0.05). Bone maturation at term was normal in HXT4 fetuses although there was no improvement in limb or bone elongation. Retention of hypophysectomized fetuses in utero until 16 days past term yielded fetuses which were heavier than controls but whose limb and bone lengths were no greater than hypophysectomized fetuses killed at term. Osseous maturation was appropriate for term in five of the six postterm hypophysectomized fetuses. The plasma IGF-I and IGF-II concentrations were not significantly affected by hypophysectomy, hypophysectomy with T4 replacement, gestational age, or prolonged gestation. The plasma IGF-I concentrations in the sham, HXNR, and HXT4 fetuses were 35.4 +/- 6.6, 28.2 +/- 3.0, and 34.4 +/- 1.7 ngeq human (h)IGF-I/ml, and the IGF-II concentrations were 656.3 +/- 59.2, 635.3 +/- 56.3, and 645.5 +/- 71.9 ngeq hIGF-II/ml, respectively, and remained within these ranges throughout the experiment. Fetal IGF-I concentrations were significantly lower than mean maternal IGF-I concentrations (88.0 +/- 6.8 ngeq hIGF-I/ml, P less than 0.05), and fetal IGF-II concentrations were significantly higher than mean maternal IGF-II concentrations (362.4 +/- 24.0 ngeq hIGF-II/ml, P less than 0.05). We conclude that in the late gestation fetal sheep, elongation of the appendicular skeleton is under some direct pituitary control whereas appendicular maturation exhibits some dependence on circulating T4 concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)