We investigated pituitary regulation of late-gestation fetal growth in the spontaneous dwarf rat, a strain with an autosomal recessive mutation (gene symbol dr) in the growth hormone (GH) gene resulting in complete isolated GH deficiency. GH-normal/GH-deficient (Dr/dr) females were crossed with Dr/dr or dr/dr males, producing both GH-deficient and GH-normal fetuses within the same litter. Pups were killed within 3 h after birth to approximate the developmental state of a late-gestation fetus. The body weight of GH-deficient fetuses was inhibited by 14% in comparison to GH-normal animals, but tail length remained unaffected. The brain and lungs were the only organs whose growth appeared to be pituitary-independent. Other organs showed moderate pituitary dependence in proportion to body weight. Serum IGF-I and IGF-II were reduced by 73% and 52%, respectively, in the absence of GH. The major IGF-binding proteins (IGFBP) were analyzed by Western ligand blot. The predominant 26- to 30-kD IGFBP band normally seen in neonatal rat serum was greatly increased in GH-deficient sera, to 250% of GH-normal sera as measured by densitometry. However, addition of a-Hec I antibody to IGFBP-2, which has been used to identify IGFBP-2 as the major neonatal IGFBP, resulted in immunoprecipitation of only a small amount of the 26- to 30-kD band from the GH-deficient fetuses, suggesting the presence of an additional IGFBP. Northern analysis of GH-deficient livers did not reveal any visible increase in IGFBP-1, IGFBP-2, or IGFBP-4 mRNA. We conclude that pituitary GH exerts a modest, but significant, selective effect on fetal body weight and organ growth. Serum levels of both IGF-I and IGF-II, as well as their binding proteins, were shown to be pituitary-GH-dependent in the fetal period. The increase of low-molecular-weight binding proteins in the GH-deficient fetus, which we were unable to attribute to IGFBP-1, -2, -3, or -4, may indicate the presence of unique fetal binding protein(s). The spontaneous dwarf rat may be an important model for further investigation of the development of pituitary dependence in fetal growth.
ASJC Scopus subject areas
- Pediatrics, Perinatology, and Child Health