Ovarian Response Of The Hypophyseal Stalk-Transected Pig To Pregnant Mare Serum Gonadotropin

Hypophysial stalk transection (HST) or sham operation (S-HST) was performed on 14 prepuberal gilts, 169 ± 3 days of age and 72.8 ± 3.4 kg body weight (day of surgery = Day 0). Gilts received 1,000 IU of pregnant mare's serum gonadotropin (PMSG) or saline vehicle (V) intramuscularly ( im ) on Day 2 resulting in the following groups: S-HST + V (n=3), S-HST + PMSG (n=4), HST + V (n=3) and HST + PMSG (n=4). Ovarian morphology and weights were recorded after ovariectomy on Day 6. Gilts were weighed on Day 59 ± 3. Sequential blood samples were collected via jugular vein cannula from 6 S-HST and 6 HST gilts on Day 60 ± 3, and all gilts were necropsied on Day 86 ± 7. Body weight gain and whole pituitary gland weight were greater (P<0.05) for S-HST than HST gilts. Mean serum LH concentration, basal serum LH concentration, frequency of LH peaks and LH peak amplitude were greater (P<0.005) for S-HST than HST gilts. Serum PRL and GH, were similar for both groups. Total ovarian and follicular fluid weights were greater (P<0.05) in S-HST gilts given PMSG than those of the other three groups which did not differ. Therefore, PMSG stimulated follicular growth in S-HST gilts, but failed to stimulate follicular growth in HST gilts. We suggest that a critical basal serum LH and/or FSH concentration must be maintained to support and promote follicular growth and a pulsatile delivery of GnRH to the anterior pituitary gland by an intact brain-pituitary unit may be required to provide this basal serum gonadotropin concentration.