THE GROWTH-HORMONE (GH)-INDEPENDENT GROWTH OF THE OBESE ZUCKER RAT ISNOT DUE TO INCREASED LEVELS OF GH RECEPTOR MESSENGER-RNA IN THE LIVER

Obese Zucker rats maintain normal rates of linear growth and circulati ng concentrations of insulin-like growth factor-I (IGF-I) and of IGF-b inding protein-3 (IGFBP-3) in spite of low GH secretion. The mechanism s underlying this GH-independent growth in obesity are unknown. To ass ess whether the liver expression of the GH receptor (GHR) messenger RN A (mRNA) is increased and/or if the liver expression of IGFBP-3 mRNA i s maintained in the obese, Zucker rats of both genders and phenotypes (four groups, n = 6/group) were studied at 12 weeks of age. By Norther n analysis, mRNA levels for GHR and GHBP were not increased in obese r ats compared to their sex-matched lean littermates; the expression of these two transcripts was sexually dimorphic and the changes in GHBP m RNA/GHR mRNA ratios associated with obesity were sex-specific. In both genders, IGFBP-1 and IGFBP-3 mRNAs were decreased in the obese. We co nclude that the GH-independent growth of obese Zucker rats is not due to increased GHR mRNA or to maintained IGFBP-3 mRNA levels in the live r.