HORMONAL-REGULATION OF THE FEMALE ENRICHED GH RECEPTOR-BINDING PROTEIN MESSENGER-RNA IN RAT-LIVER

At least two classes of mRNA for the GH receptor (GHR) and GH binding protein (GH BP) with different 5' untranslated first exons exist in th e rat. One such class, the GHR(1) is predominantly expressed in the li ver of female rats. The hepatic expression of the GHR(1) mRNA in norma l and hypophysectomized rats of both sexes was studied by employing an RNase protection/solution hybridization assay. Normal females express ed 10-fold more GHR(1) mRNA than males, hypophysectomy of female rats decreased the GHR(1) level to that observed in male rats. Continuous G H treatment of hypophysectomized male and female rats for 6 days incre ased the expression of GHR(1) mRNA to levels found in normal females, whereas intermittent GH treatment was without effect. Bovine GH (bGH) induced the GHR(1) expression in a time- and dose-dependent manner in primary cultures of adult rat hepatocytes as determined by solution hy bridization. Maximal induction was achieved after 72 h of treatment wi th 50 ng bGH/ml medium. Female enriched expression of receptor and bin ding protein mRNAs raises the possibility that they participate in det ermining the ability of the liver to respond differently to the male a nd female GH secretory patterns. Our in vitro model utilizing cultures of primary adult rat hepatocytes could be used to address this issue as well as explore a hormonal interplay in regulation of GHR(1) expres sion.