Relationships between concentrations of ovarian steroids, insulin-like growth factor-1 and IGF-binding proteins during follicular development in the ewe

The objective of the present study was to determine how insulin-like growth factor-1 (IGF-1) and IGF-binding proteins (IGFBPs) are related to in vivo follicular development in the sheep. Oestrus was synchronised in 20 cyclic ewes and the animals were slaughtered 44 h after the second injection, just before the start of preovulatory luteinising hormone (LH) surge. Normal growing follicles were dissected from the ovaries of each ewe and their diameters measured. The follicular fluid was aspirated and assayed for oestradiol, testosterone and total IGF-1 content. The follicles were classified as either non-oestrogenic or oestrogenic if the follicular fluid content of oestradiol was less than 60 ng ml−1 or more than 60 ng ml−1, respectively. The mean diameter of oestrogenic follicles was significantly (P < 0.001) higher than that of non-oestrogenic ones, but testosterone concentrations did not differ. IGF-1 concentrations in oestrogenic follicles were significantly (P < 0.01) lower than those in non-oestrogenic ones, with a significant (P < 0.01) negative correlation between follicular oestradiol content and IGF-1 concentration. IGFBPs were identified by Western ligand blot analysis using 12% sodium dodecyl sulphate polyacrylamide gel electrophoresis under non-reducing conditions and band intensities on autoradiographs were quantified by scanning densitometry. The intensity of the doublet of IGFBP at 42–44 kDa was significantly (P < 0.02) higher in follicular fluid from oestrogenic follicles, whereas the intensity of the band at 35 kDa was significantly (P < 0.001) higher in follicular fluid from non-oestrogenic follicles. Some of the non-oestrogenic follicles also exhibited bands at 32.0-28.5 kDa with variable intensities, but such bands were totally absent in oestrogenic follicles. The results of this study suggest an involvement of both IGF-1 and IGFBPs in ovine follicular development.