Prostaglandin F2(alpha)-induced luteolysis of aging corpora lutea in hysterectomized pigs

Prostaglandins primarily of uterine origin play an important role in parturition. Hysterectomy of nongravid pigs early in the luteal phase maintains luteal function until about Day 150, whereas the duration of normal pregnancy is about 114 days. A precisely timed peak release of relaxin and coincident decrease in progesterone secretion in unmated hysterectomized gilts are similar to hormonal changes that occur a few hours before parturition. It is hypothesized that prostaglandin F2(alpha) (PGF2(alpha)) in hysterectomized pigs mimics abrupt changes in ovarian and pituitary hormone secretion seen before normal parturition and in early lactation. Unmated Yorkshire gilts were hysterectomized on Days 6- 8 of a normal estrous cycle, and at 1200 h on Day 113, they were given an i.m. injection of 30 mg PGF2(alpha)-trihydroxymethylaminomethane (THAM) salt or PBS. None of these gilts expressed behavioral estrus immediately after PGF2(alpha) or vehicle treatment. On Day 113, PGF2(alpha) increased peak relaxin (60 ng/ml) compared with that of controls (34 ng/ml; p < 0.01), whereas progesterone decreased abruptly (4 vs. 16 ng/ml in PGF2(alpha) and PBS; p < 0.01). Prolactin remained at < 5 ng/ml from Day 98 to 120 in controls but peaked at 33 ng/ml immediately after PGF2(alpha) treatment on Day 113, and then decreased to levels similar to those of controls on Day 120. Sequential bleeding revealed an acute growth hormone release (4.5 ng/ml) immediately after PGF2(alpha) injection and return to basal levels (< 0.6 ng/ml) on Days 114-120. PGF2(alpha) induced abrupt shifts in progesterone, relaxin, prolactin, and growth hormone secretion in hysterectomized gilts that mimicked hormone changes seen in late pregnancy, parturition, and early lactation. These findings provide new insight into the role of PGF2(alpha) in abruptly changing hormone secretions by aging corpora lutea and the pituitary gland even in the absence of conceptuses or the uterus in the pig.