ADMINISTRATION OF PORCINE SOMATOTROPIN BY A SUSTAINED-RELEASE IMPLANT - EFFECTS ON FOLLICULAR-GROWTH, CONCENTRATIONS OF STEROIDS AND INSULIN-LIKE GROWTH-FACTOR-I, AND INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN ACTIVITY IN FOLLICULAR-FLUID OF CONTROL, LEAN, AND OBESE GILTS

Prepubertal gilts of control (n = 30), obese (n = 30), or lean (n = 29) genetic lines were implanted with no, one, or two implants of porcine somatotropin (pST, each delivers 2 mg/d) for 6 wk starting at 160 d of age to determine whether pST affects ovarian function. At 4 mg/d, PST increased (P <.01) numbers of 4.0- to 6.9-mm (medium) follicles but not (P >.10) numbers of 1.0- to 3.9-mm (small) follicles per gilt. Both doses of pST increased (P <.01) serum and follicular fluid (FFL) concentrations of IGF-I and activity of IGF binding protein (IGFBP)-3 and 36-kDa IGFBP in all three lines; IGFBP-3 was the predominant IGFBP. In comparison, binding activity of IGFBP-2 was decreased (P <.01) in serum by 4 mg of PST but increased (P <.05) in FFL by 4 mg of PST. Lean gilts had lower (P <.05) serum concentrations of IGF-I and less (P <.05) total binding activity of IGFBP than control and obese gilts. Concentrations of estradiol in FFL of small and medium follicles tended (P <.08) to be increased by 2 mg/d of pST, whereas FFL concentrations of progesterone were unaffected by pST. Obese and control gilts had twofold greater (P <.05) FFL progesterone concentrations than lean gilts. We conclude that sustained-release implants of PST can stimulate follicular growth, increase concentrations of IGF-I in serum and FFL, and increase IGFBP activity in serum of genetically divergent lines of gilts without an adverse effect on ovarian function.