Administration of porcine somatotropin by a sustained-release implant: effect 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.

Abstract

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.