A HOMOLOGOUS RADIOIMMUNOASSAY FOR OVINE INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-2 - ONTOGENY AND THE RESPONSE TO GROWTH-HORMONE, PLACENTAL-LACTOGEN AND INSULIN-LIKE GROWTH-FACTOR-I TREATMENT IN SHEEP

Although insulin-like growth factor-binding protein-2 (IGFBP-2) is an abundant IGFBP in fetal and postnatal plasma, its regulation is not ye t clearly understood. To address this question in sheep, we purified o vine IGFBP-2 and developed a homologous radioimmunoassay. We have stud ied its ontogenesis and measured serum concentrations of ovine IGFBP-2 after bovine growth hormone (bGH), ovine placental lactogen (oPL) and IGF-I treatment. Concentrations of IGFBP-2 were high at 125 days of g estation (550 +/- 15 mu g/l) but fell after birth (P<0.05) and plateau ed after 1 year of age (340 +/- 20 mu g/l). In lactating ewes, bGH tre atment for 7 days significantly reduced (21%; P<0.05) IGFBP-2 relative to the saline-treated group. Similarly, in neonatal lambs, bGH treatm ent from 3 to day 23 of life reduced (P<0.05) IGFBP-2 by 23% relative to the saline-treated group. oPL had no effect on serum levels of IGFB P-2 in the ewe or the neonatal lamb. In well-fed yearling lambs, treat ment with IGF-I reduced IGFBP-2 values by 27% (P<0.05) relative to con trol animals. In yearling lambs, reduced nutrition increased plasma IG FBP-2 (41%; P<0.05). However this increase was abolished by IGF-I trea tment. The changes in plasma levels of IGFBP-2 were positively related to changes in IGF-IT while there was a negative relationship between circulating IGF-I and IGFBP-2 such that both IGF-I and IGF-II may play a role in the regulation of IGFBP-2 in serum.