Role of the ovary in controlling luteinizing hormone, follicle stimulating hormone, and prolactin secretion during and after lactation in pigs

Concentrations of LH, FSH, prolactin, estrogen, and progesterone were measured in blood samples collected at various intervals during and immediately following lactation to determine the role of the ovary in controlling gonadotropin and prolactin secretion and initiation of estrous cycles in the pig. Ten primiparous sows were ovariectomized (ovex, n=5) or left intact (n=5) 2 to 4 days after farrowing. Serum progesterone and estrogens in samples collected daily during lactation were not different between intact and ovex sows. Serum FSH increased immediately after ovariectomy and remained elevated during lactation, but LH concentrations were similar between ovex and intact sows. Both LH and FSH were higher during the last 2 weeks of lactation than during the first 3 weeks. On Days 10 and 20 of lactation, all sows received (i.v.) two challenges of gonadotropin releasing hormone (GnRH; 400 μg each) at 135 min intervals. Peak change (maximum value minus baseline) in FSH after GnRH was higher in ovex in intact sows on both Days 10 and 20. Peak change in FSH in ovex sows was higher on Day 20 than on Day 10, but peak change in FSH in intact sows was similar between Days 10 and 20. Peak change in LH after GnRH was similar between ovex and intact sows. Peak change in LH after the second GnRH challenge on Day 10 was higher than that after each of the other three challenges. Although serum prolactin concentrations in selected samples collected on Days 10 and 20 were not significantly different between treatment groups, intact sows had consistently higher concentrations compared with ovex sows. Separation of the four sows from their litters for 4 h during the last week of lactation resulted in a decline in serum prolactin, but prolactin increased fivefold within 15 min and tenfold during the second hour after piglets were replaced. In contrast, LH did not change prior to or after piglets were allowed to nurse. After weaning, concentrations of estrogens peaked during 24 h around onset of estrus prior to the preovulatory surge of LH and FSH. In ovex sows treated with estradiol-17β on Days 8, 9, and 10 after weaning, estrous behavior and LH and FSH responses were indistinguishable from those observed in intact sows at the postweaning estrus. Serum LH, FSH, and prolactin increased in response to estrogen, and duration of estrus was similar to that observed in intact sows. Results from these experiments indicate that 1) LH is low during lactation in both intact and ovex sows; 2) FSH increases following ovariectomy, presumably because of removing some ovarian inhibitor of FSH secretion; 3) prolactin, but not LH, changes acutely with suckling or weaning; and 4) surges in LH, FSH, and prolactin at estrus are induced by estrogen whether of endogenous (intact sows) or exogenous (ovex sows) origin.