Comparative effects of insulin and porcine somatotropin on postweaning follicular development in primiparous sows

We compared the effects of exogenous insulin and porcine ST (pST) on follicular development after weaning. Crossbred primiparous sows received saline (1.5 mL i.m.; n = 9), insulin (.4 IU/kg BW s.c.; Eli Lilly Lente Iletin II; n = 10), or pST (40 μg/kg BW i.m.; n = 10) from d 1 to 5 after weaning (d 0). Ovaries were collected, the diameter of each follicle ≥ 2 mm was measured, and fluid from the 20 largest follicles was assessed for IGF-I, IGF binding proteins (IGFBP), estradiol, progesterone, and testosterone. The total number (27.7, 25.3, and 29.1 for saline, insulin, and pST, respectively; SEM = 3.2) and average diameter (4.7, 5.2, and 5.5 mm for saline, insulin, and pST treatments, respectively; SEM =.3 mm) of ovarian follicles were not affected by insulin or pST treatment. The pST and insulin increased follicular fluid estradiol and testosterone in medium and large follicles compared to fluid from saline-treated sows, but the increase was greater for insulin than for pST treatment (treatment x size interaction, P <.01). Similarly, progesterone concentrations in follicular fluid were higher in medium and large follicles after insulin treatment, and pST treatment induced higher progesterone concentrations in small follicles and increasingly lower concentrations of progesterone in medium and large follicles (treatment x size interaction, P <.0007) compared to saline treatment. Follicular fluid IGF-I was greater (treatment x health interaction, P <.0001) in atretic and nonatretic follicles from pST-treated sows than in those from insulin- and saline-treated sows. Follicular fluid IGFBP-2 (tendency, P <.07) and IGFBP, possibly representing IGFBP-5 (30 kDa) and IGFBP-4 (22 kDa), were higher in atretic follicles than in nonatretic follicles (P <.05), whereas IGFBP-3 was not influenced by health status. The 30- and 22-kDa IGFBP were also influenced by treatment, increasing due to pST compared with saline or insulin treatments (P <.008). Follicular fluid IGFBP-2 and IGFBP-3 were not influenced by treatment. In conclusion, pST and insulin positively influenced follicular steroidogenesis and possibly follicular development, although through different mechanisms.