Exogenous growth hormone induces somatotrophic gene expression in neonatalliver and skeletal muscle

The extent to which the local somatotrophic axis is functional in extrahepa tic tissues in the neonate is unclear. We therefore determined the expressi on of growth hormone (GH) receptor (GHR), and insulin-like growth factors I and II (IGF-I and IGF-II) mRNA in liver and skeletal muscle (longissimus) of neonatal pigs given daily intramuscular injections of either recombinant porcine GH 1 mg/kg body wt; (n = 6) or saline (n = 5) for 7 days. Exogenou s GH increased plasma concentrations of GH 30-fold and IGF-I threefold. Abu ndances of specific mRNA in liver and muscle were measured by RNase protect ion assays (values are arbitrary density units). In liver, GH treatment inc reased GHR (6.0 vs. 9.7; P < 0.01) and IGF-I (5.2 vs. 49.0; P < 0.001) but not IGF-II (19.5 vs. 17.2) mRNA. In muscle, GH treatment increased IGF-I mR NA(13.3 vs. 22.8; P < 0.05) but not GHR (8.3 vs. 9.5) or IGF-II (16.1 vs. 1 6.9). These results demonstrate that exogenous GH can induce local somatotr ophic function predominantly in liver but also in muscle of newborn pigs. O ur novel finding on the selective increase in muscle IGF-I but not GHR gene expression suggests differences in posttranscriptional regulation and/or i ntracellular signaling mechanisms.