Prostaglandin F2α Induced Luteolysis of Aging Corpora Lutea in Pigs
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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. Precisely timed peak release of relaxin and coincident decrease in progesterone secretion occur in unmated hysterectomized gilts similar to that found a few hours preceding parturition. It is hypothesized that prostaglandin F 2 α (PGF 2 α) in hysterectomized pigs mimics abrupt changes in ovarian and pituitary hormone secretion seen preceding normal parturition and in early lactation. Unmated Yorkshire gilts were hysterectomized on days 6–8 of a normal estrous cycle, and at 1200 hours on day 113, they were given an i.m. injection of 30 mg PGF 2 α -THAM salt or phosphate buffer saline (PBS). None of these gilts expressed behavioral estrus immediately after PGF 2 α or vehicle treatment. On day 113, PGF 2 α increased peak relaxin (60 ng/ml) compared with controls (34 ng/ml; P<.01), whereas progesterone decreased abruptly (4 vs. 16 ng/ml in PGF 2 α and PBS; P<.01). The prolactin (PRL) remained at <5 ng/ml from day 98 to 120 in controls but peaked at 33 ng/ml immediately after PGF 2 α treatment on day 113, and then decreased to levels similar to controls on day 120. Sequential bleeding revealed an acute growth hormone (GH) release (4.5 ng/ml) immediately after PGF 2 α injection and return to basal levels (<.6 ng/ml) on days 114 to 120. The PGF 2 α induced abrupt shifts in progesterone, relaxin, PRL, and GH secretion in hysterectomized gilts that mimic hormone changes seen in late pregnancy, parturition, and early lactation. These findings provide new insight into the role of PGF 2 α in abruptly altering hormone secretion by aging corpora lutea and pituitary gland even in the absence of the conceptuses or uterus in the pig.