A homologous radioimmunoassay for ovine insulin-like growth factor-binding protein-2: ontogenesis 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 yet clearly understood. To address this question in sheep, we purified ovine IGFBP-2 and developed a homologous radioimmunoassay. We have studied 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 gestation (550 +/- 15 micrograms/l) but fell after birth (P < 0.05) and plateaued after 1 year of age (340 +/- 20 micrograms/l). In lactating ewes, bGH treatment for 7 days significantly reduced (21%; P < 0.05) IGFBP-2 relative to the saline-treated group. Similarly, in neonatal lambs, bGH treatment from day 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 IGFBP-2 in the ewe or the neonatal lamb. In well-fed yearling lambs, treatment with IGF-I reduced IGFBP-2 values by 27% (P < 0.05) relative to control animals. In yearling lambs, reduced nutrition increased plasma IGFBP-2 (41%; P < 0.05). However this increase was abolished by IGF-I treatment. The changes in plasma levels of IGFBP-2 were positively related to changes in IGF-II 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.