Progesterone secretion and prostaglandin F release in vitro by endometrial and luteal tissue of cyclic pigs

Blood and tissue were collected from 30 pigs on various days of the oestrous cycle to compare progesterone in peripheral plasma and progesterone release in vitro with prostaglandin (PG) F release in vitro by endometrial and luteal tissue (approximately 100 mg in 5 ml Krebs-Ringer-bicarbonate buffer, pH 7.4, for 2 h with three different incubation treatments). Plasma progesterone increased from Days 8 to 12 of the oestrous cycle (25.1 to 42.8 ng/ml, P < 0.05) and decreased from Day 14 to 16 (36.3 to 5.9 ng/ml, P < 0.05). Progesterone release in vitro at 37 degrees C decreased from 18.0 to 2.7 ng/mg tissue from Days 8 to 16 (P < 0.05). Endometrial PGF release in vitro at 37 degrees C increased slightly from Day 8 to 14, then increased significantly from 43.7 to 103.5 ng/100 mg tissue between Days 14 and 16 (P < 0.05). The rapid increase in endometrial PGF release coincided with the decrease in plasma progesterone. A small, non-significant increase in mean luteal PGF release was detected between Days 14 and 16. The results of this experiment did not suggest a critical role for luteal PGF-2 alpha in the process of luteolysis in the pig, but do provide indirect evidence for the involvement of endometrial PGF-2 alpha in luteolysis in the pig.