EFFECT OF PORCINE SOMATOTROPIN ON NUMBER OF GRANULOSA-CELL LUTEINIZING-HORMONE HUMAN CHORIONIC-GONADOTROPIN RECEPTORS, OOCYTE VIABILITY, AND CONCENTRATIONS OF STEROIDS AND INSULIN-LIKE GROWTH FACTOR-I AND FACTOR-II IN FOLLICULAR-FLUID OF LEAN AND OBESE GILTS

Prepubertal gilts of obese (n = 24) or lean (n = 24) genetic lines were injected (s.c.) daily with 0, 2, or 4 mg of porcine somatotropin (pST) for 6 wk starting at 160 d of age to determine whether pST affects follicular function. Blood and ovaries were collected at slaughter 24 h after the last injection. Surface follicles greater-than-or-equal-to 1.0 mm in diameter were counted, and pools of follicular fluid (FFL) and granulosa cells were collected from 1.0- to 3.9-mm (small) and 4.0- to 6.9-mm (medium) follicles. Oocytes were collected from small and medium follicles and evaluated for maturational stage and viability. Porcine somatotropin increased (P < .08) the numbers of small but not the numbers of medium follicles per gilt (P > .10). Oocyte maturation and viability were not affected by pST or genetic line. Porcine somatotropin increased (P < .05) concentrations of insulin-like growth factor I (IGF-I) in serum and FFL of both obese and lean gilts; IGF-I was lower (P < .01) in lean gilts. Treatment with pST decreased (P < .05) IGF-II in FFL of lean but not in that of obese gilts. Dose of pST and line had no effect on concentrations of progesterone in FFL of small or medium follicles or on concentrations of estradiol in FFL of small follicles. Concentrations of estradiol in FFL of medium follicles were decreased (P < .05) by pST (2 and 4 mg/d) in obese gilts, but estradiol was lower (P < .01) and unaffected (P > .10) by pST in lean gilts. The numbers of LH/hCG binding sites in granulosa cells were decreased (P < .05) by pST (4 mg/d) in medium follicles. The 4 mg/d dose of pST increased (P < .05) IGF-I binding protein activity only in serum of obese gilts. We conclude that 2 and(or) 4 mg/d of pST can increase concentrations of IGF-I in serum and FFL and stimulate growth of small follicles in two genetically divergent lines of gilts without affecting oocyte viability.