We investigated pituitary regulation of growth during two critical stages of development in the rat, using hypophysectomy(Hx) with replacement of GH and/or T4. In the neonatal period (Hx on day 6), body weight and tail length were inhibited by 60% and 50%, respectively, while these parameters were inhibited by 80% and 85% by Hx in the juvenile period (Hx on day 45). Administration of T4alone significantly increased skeletal growth (tail length) in neonatal Hx rats, while T alone proved ineffective in promoting somatic growth in juvenile Hx rats. GH effects were greater on body weight than on tail length at both stages of development. Replacement of both GH and T4restored somatic growth to normal values during both time periods. The brain was the sole organ whose growth appeared to be independent of the pituitary.Hx reduced serum insulin-like growth factor-I (IGF-I) and -II in both age groups, and GH alone restored IGF-I and -II levels to the control range. The major IGF-binding proteins (IGFBPs) were analyzed by Western ligand blots. The effect of Hx on the predominant IGFBP was greater in the juvenile rat. T4replacement in the neonate and GH replacement in the juvenile rat restored IGFBPs to control levels.We conclude that somatic growth in the rat is less pituitary dependent in the neonatal, period. There are also important age- specific differences in organ response to GH and T4. Serum IGFs and their binding proteins are pituitary dependent even in infancy, and GH is their primary regulator. The neonatal Hx rat is an important model for the study of the dynamicdevelopment of the pituitary-dopendent growth.
ASJC Scopus subject areas