Ontogeny of ICF-I responsiveness to bGH in young lambs

The ontogeny of hepatic growth hormone (GH) receptors (GHR), as measured by responses of both plasma insulin-like growth factor-I (IGF-I) and hepatic GHR to an exogenous bGH stimulus, was examined using sheep of different age s (Days 1-7, 14-21, 28-35, and 56-63 of life, and yearlings). The IGF-I res ponse to bGH was first examined in yearling sheep using two doses of bGH (0 .1 and 0.2 mg/kg LW/d). Based on these results, lambs in four groups up to Day 63 of life were treated for 5 d with bGH (n = 10) at a dose of 0.15 mg/ kg LW/d or with saline (n = 10). Jugular blood samples were taken once dail y on Days -1, 4, and 5 of treatment. bGH treatment in lambs up to Day 63 of life had little effect on plasma concentrations of GH, insulin, glucose or urea, but significantly (P < 0.05) increased circulating concentrations of IGF-I at all ages and of NEFA at Day 62/63 of life. In contrast, bGH treat ment at either dose in yearlings significantly increased these parameters, except for plasma urea concentrations which were decreased in bGH-treated y earlings. However, the responses of plasma IGF-I concentration to bGH stimu lus in lambs up to Day 63 of Life were small compared to those in yearling sheep. Consistent with this, bGH treatment failed to affect hepatic GH bind ing in young lambs, but up-regulated it in yearling sheep. Furthermore, bas al (unstimulated) GH binding did not differ between sheep of 7 vs. 63 vs. 3 65 d of age, despite the greater IGF-I responses to bGH in the latter group . It is suggested that hepatic GHR in lambs up to Day 63 of life are not fu lly functional compared to the situation in yearlings. (C) 1999 Elsevier Sc ience Inc. AU rights reserved.