428 THE ONTOOENY OF THE RESPONSE TO GROWTH HORMONE

Abstract

We defined the ontogeny of five hepatic mRNA sequences which are responsive to growth hormone (GH) in juvenile (35 d) hypophysectomized rats ay in vitro translation of mRNA sequences and two-dimensional gel electrophoresis of the 35[S]-methionine labeled products. Each of the five had a well defined ontogeny which did not parallel the ontogeny of hepatic GH receptors or serum GH. Major periods for transition of this family of mRNA sequences were weaning and puberty. The pattern of ontogeny for two of the products was similar to that observed for GH responsive serum proteins. Induction of relative GH deficiency in rat pups by administration of propylthiouracil resulted in alterations in the mRNA sequences suggestive of GH deficiency; however, in twelve day old animals GH was much less effective in reversing these alterations than in older animals. Augmentation of normal pups with pituitary dependent hormones (thyroxine corticosterone and GH) resulted in changes consistent with an overall increase in maturation of the liver. This effect was also observed in non-hormonally responsive products, suggesting the change is not specific to the GH responsive mRNA family. From these observations we conclude that: 1). factors other than GH or GH receptor content are of importance in the expression of GH responsive products in the 12 day old animal; 2) the ontogeny of GH responsive serum proteins may reflect pretranslational events; 3) GH and the other pituitary dependent hormones modify the cadence of expression of hepatic gene products;4) and that the ability of the liver to respond to GH is an age dependent phenomenon.