Profile of serum immunoreactive insulin-like growth factor I during gestation in Wistar rats

It has been reported that there is a striking homology between the basic insulin-like growth factors (IGF)-somatomedins (SM) found in humans and rats. The radioimmunoassay (RIA) developed for the human hormone IGF-I (basic somatomedin, B-SM) can measure immunoreactive IGF-I in rat serum (IrIGF-I). Using this RIA, the profile of serum IrIGF-I was measured at each day of gestation in groups of inbred and Charles River Wistar rats. In each case IrIGF-I showed a gradual increase in early and mid gestation followed by a sharp decrease that occurred late in gestation to values 20-40% of control. The profile obtained from Charles River Wistars was shifted in time compared with the inbred group. Neither ovariectomy nor progesterone administration to ovariectomized nonpregnant animals altered serum IrIGF-I levels. Thus, although rat IGF-I and human IGF-I can be measured using the same assay, the changes that occur in gestation are in opposite directions.