The insulin-like growth factors (IGF) are mitogenic peptides that are critical in fetal growth and development and are modulated by IGF binding proteins (IGFBP). The aim of this study was to evaluate the role of the IGF axis in the fetal rabbit and to determine if in the rabbit, fetal birth weight correlated with changes in the IGF axis.

Fetal rabbit serum was collected and assessed for circulating IGF concentration and the molecular distribution of the IGFBPs determined by chromatography and WLB analysis. To determine the tissue source of IGFBP-1 and IGFBP-3 mRNA expression, fetal liver and kidney, as well as, placenta were evaluated by Northern analysis.

The fetal weights at term ranged from 38 - 65 gms. In fetal rabbit serum concentrations of IGF-I varied from 29 - 67 ng/ml and the concentrations of IGF-II varied from 815 - 2157 ng/ml yet no significant correlation with fetal weight was observed. Evaluation of the serum for IGFBPs by WLB detected a faint 45-40 kDa IGFBP-3 band with predominant 29 and 24 kDa bands. Densitometric analysis revealed no correlation between the 45-40 kDa doublet and fetal size, although there was an inverse correlation between the predominant 29 kDa IGFBP and fetal size. Fractionation of the fetal serum revealed two predominant binding protein regions, a 150 kDa and a 44 kDa region. The 45 - 40 kDa IGFBP-3 eluted within the 150 kDa (ternary complex) IGFBP region of the chromatograph while smaller IGFBPs eluted in the 44 kDa(binary complex) region. Thus, the serum of fetal rabbits contains the ternary complex, unlike human fetal serum. However, no differences were seen in the molecular distribution of the IGFBPs in the rabbit serum based on birth size. Northern analysis revealed that the level of IGFBP-3 mRNA expression, within the various tissues studied, was unaffected by fetal size. IGFBP-1mRNA was not observed in any fetal tissue assessed nor in the placenta.

Thus, these data show there is no correlation in IGFBP-3, IGF-I or IGF-II and fetal weight.