Relationships among concentrations of steroids, inhibin, insulin-like growth factor-1 (IGF-1), and IGF-binding proteins during follicular development in weaned sows

The experimental objective was to determine how insulin-like growth factor binding proteins (IGFBP), as examined by Western ligand blot procedures, related to porcine follicular steroidogenesis. Weaned sows were ovariectomized at various times after litter removal in three experiments. In experiments 1 and 2, sows were ovariectomized at 48-120 h after weaning. In experiment 1, pools of all small (1-3 mm), medium ( 3-6 mm), or large ( 6-9 mm) follicles were made for each sow; in experiment 2, fluid was collected individually from the 10 largest follicles per ovary. A third experiment was conducted to examine changes after an ovulatory dose of hCG, but prior to ovulation. in this experiment, sows were treated with eCG at weaning, given hCG 72 h later, and ovariectomized 0-36 h after the ovulatory dose of hCG. Follicular fluid was collected from the 10 largest follicles per sow. In experiments 1 and 2, IGFBP-3 in follicular fluid remained constant over follicle diameters and stages of development, and IGFBP-2 decreased with advancing follicular development as concentrations of estradiol, androstenedione, and progesterone increased. In experiment 1, after the presumed LH surge when the concentration of all steroids was low, there was a sharp increase in band intensity for IGFBP-2. Similarly, estradiol and androstenedione were low in preovulatory sows in experiment 2, though progesterone increased and IGFBP-2 decreased with follicle diameter. In experiment 3, progesterone remained elevated from 0 to 36 h after hCG, even though IGFBP-2 did not increase until after 24 h post-hCG. Thus, it appeared that preovulatory sows in experiment 2 with reduced estradiol had only recently experienced an LH surge before ovariectomy, and IGFBP-2 in the largest follicles was yet to increase. Although eCG stimulated follicular insulin-like growth factor-1 in experiment 3, there was a negative association (experiment 1 ) or a weak, positive relationship (experiment 2) between IGF-1 and follicular development. We conclude that changes in IGFBP-2 are potentially, more important in modulating local actions of IGF-1 within the ovary than the secretion of IGF-1 itself