Prostaglandin F2alpha-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 F2alpha (PGF2alpha) 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 PGF2alpha-trihydroxymethylaminomethane (THAM) salt or PBS. None of these gilts expressed behavioral estrus immediately after PGF2alpha or vehicle treatment. On Day 113, PGF2alpha 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 PGF2alpha 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 PGF2alpha 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 PGF2alpha injection and return to basal levels (< 0.6 ng/ml) on Days 114-120. PGF2alpha 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 PGF2alpha 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.