IGFBP-2 EXPRESSION IN LIVER AND MAMMARY TISSUE IN LACTATING AND PREGNANT EWES

Binding proteins for the insulin-like growth factors (IGFBPs) modulate the actions of IGF I and IGF II. IGFBP-2 is particularly high in plas ma of pregnant and fetal animals and in milk. We investigated the peri -lactational control of IGFBP-2 expression and secretion. Fifteen sing leton-bearing pregnant ewes at day 101 of gestation were injected sc t wice daily for 8 days with bovine growth hormone (bGH) or ovine placen tal lactogen (oPL) both at 0.15 mg.kg-1.d-1 or saline. A further fifte en ewes at day 17 of lactation were injected sc twice daily for 5 days with bGH or oPL at 0.1 mg.kg-1.d-1 or saline. On the last day of inje ction blood samples were taken and the animals were sacrificed. Liver and mammary tissue samples were immediately frozen and subsequently ex tracted to provide total RNA for evaluation by Northern blot analysis using a rat IGFBP-2 cDNA probe. Plasma samples were analysed by Wester n ligand blotting for IGFBP-2. The comparison of the two saline-treate d groups (pregnant vs lactating ewe) revealed no difference in the pla sma concentrations of IGFBP-2. IGFBP-2 mRNA expression in the liver of the lactating ewes was markedly increased compared to that in the pre gnant ewes. In contrast, in mammary tissue the expression was signific antly lower in lactating than in pregnant sheep. In pregnant animals t reatment with bGH, but not oPL, decreased the expression of IGFBP-2 in liver. There was a similar trend in the lactating ewe. GH treatment. but not PL treatment, moderately reduced IGFBP-2 levels in the lactati ng but not the pregnant ewes. bGH but not oPL induced hyperinsulinaemi a. We conclude that GH has actions in pregnancy and lactation which ar e not mimicked by oPL. The regulation of hepatic IGFBP-2 expression by GH is similar in pregnant and postpartum animals. However, there are distinct differences in the tissue-specific regulation of IGFBP-2 betw een pregnancy and lactation. The liver might be the major source of ci rculating IGFBP-2.