Are Luteal PGF2a Secretions Similar During Spontaneous and PGF2a-Induced Luteolysis in Ewes?

Luteolysis is initiated by uterine secretion of PGF2alpha, which is delivered locally from the uterine vein to the adjacent ovarian artery to the luteal-containing ovary. Some conclude that uterine secretion of PGF2alpha induces luteal PGF2alpha secretion to complete luteolysis. Most studies have elucidated mechanisms involved in luteolysis within the corpus luteum (CL) by first inducing luteolysis with an exogenous luteolytic dose of PGF2alpha and then quantifying changes in the CL in vivo or in vitro to describe mechanisms involved in luteolysis. Circulating progesterone declines slowly to less than 1 ng/ml during spontaneous luteolysis at the end of the estrous cycle; however, circulating progesterone decreased precipitously to less than 1 ng/ml within 4 hours after manual removal of the corpus luteum or when luteolysis was induced with a luteolytic dose of PGF2alpha in cows, indicating that mechanisms of luteolysis during a spontaneous luteolysis and PGF2alpha-induced luteolysis may not be similar. These experiments were conducted to examine luteal secretion of progesterone and PGF2alpha in vitro during spontaneous or PGF2alpha-induced luteolysis in ewes. In Experiment 1, jugular venous blood was collected for progesterone analysis and day-12 or day-16 ovine corpora lutea were removed and weighed. Luteal slices were incubated in vitro in M199 for 4 and 8 hours and concentrations of progesterone and PGF2alpha were quantified by RIA. In Experiment 2, ewes received Vehicle or a luteolytic dose of PGF2alpha on day 9, jugular venous plasma was collected daily, and corpora lutea were collected 48 hours (day-11) after treatments were given, weighed, and luteal slices were incubated in vitro in M199 for 4 and 8 hours and progesterone and PGF2alpha in media and progesterone in jugular venous plasma were quantified by RIA. In Experiment 1, jugular venous progesterone on days 12 and 16 and luteal weights were analyzed by a One Way ANOVA and data for progesterone and PGF2alpha in culture media were analyzed by a 2X2 Factorial Design for ANOVA. In Experiment 2, jugular venous progesterone was analyzed by a 2X3 Factorial Design for ANOVA and progesterone and PGF2alpha in culture media were analyzed by a 2X2 Factorial Design for ANOVA. In Experiment 1, jugular venous progesterone and luteal weights were greater (P<0.05) in day-12 than day-16 ewes and progesterone secretion in media from day-12 ewes was greater (P<0.05) than day-16 ewes, while luteal PGF2alpha secretion in vitro by day-12 and day-16 did not differ (P>0.05). In Experiment 2, jugular venous progesterone and luteal weights in PGF2alpha-treated ewes over the daily sampling period were lower (P<0.05) than in Vehicle controls, and progesterone secretion in media from Vehicle-treated ewes was greater (P<0.05) than PGF2alpha-treated ewes, while luteal PGF2alpha secretion in vitro by PGF2alpha-treated ewes was greater (P<0.05) than Vehicle-treated ewes. In conclusion, at least one mechanism of spontaneous luteolysis differs from PGF2alpha-indused luteolysis in ewes. (poster)