INFLUENCE OF INSULIN ON FOLLICULAR DEVELOPMENT AND THE INTRAFOLLICULAR INSULIN-LIKE-GROWTH-FACTOR-I (IGF-I) SYSTEM IN SOWS AFTER WEANING

Twenty-four crossbred primiparous sows were used to investigate the in fluence of insulin administration after weaning on the intrafollicular insulin-like growth factor I (IGF-I) system. Sows received 0.4 iu ins ulin kg(- 1) bodyweight or an equivalent volume of saline for 3 days ( n = 5 insulin; n = 4 saline) or 5 days (n = 5 insulin; n = 6 saline) a fter weaning or served as untreated controls on day I (n = 4). The num ber and diameters of ovarian follicles were recorded, and fluid was as pirated from the 20 largest follicles for determination of oestradiol and IGF-I by radioimmunoassay and of insulin-like growth factor-bindin g proteins (IGFBPs) by western ligand blotting. The walls of the folli cles were collected for mRNA analysis by RNase protection assay or gra nulosa cells were collected for estimation of apoptosis by flow cytome try. Insulin treatment resulted in smaller diameters of all follicles (P < 0.05) and tended (P < 0.07) to increase the number of follicles a vailable on day 5 compared with saline-treated animals (19.8 versus 17 .8). The concentration of oestradiol in follicular fluid from large (7 -10 mm) follicles on days 3 and 5 was reduced (treatment by size class interaction; P < 0.05) by insulin treatment. Insulin also reduced int rafollicular concentrations of IGF-I-at days 3 and 5 after weaning (tr eatment by day interaction; P < 0.02) while the amounts of IGFBP-3 and IGFBPs of molecular mass 30 and 22 kDa decreased from day 3 to day 5 in saline-treated animals only (treatment by day interaction; P < 0.05 ). Gene expression for IGF-I increased in saline-treated animals but d ecreased fourfold in insulin-treated sows from day 3 to day 5 (treatme nt by day interaction; P < 0.002). Gene expression for IGFBP-3 decreas ed (P < 0.04) from day 3 to day 5, while expression of IGFBP-2 was una ffected by treatment or day. Overall, insulin influenced the IGF-I sys tem in a manner consistent with slowing follicular growth and possibly allowed more follicles to become available for ovulation.