Role of intraluteal prostaglandin F_2α, progesterone and oxytocin in basal and pulsatile progesterone release from developing bovine corpus luteum

The present study examined the role of intra-luteal prostaglandin (PG) F_2α, progesterone (P4) and oxytocin (OT) on the corpus luteum function by using specific hormone antagonists. Luteal cells from the developing CL (days 5-7 of the estrous cycle) were exposed to P4 antagonist (onapristone, OP, 10^-4 M), OT antagonist (atosiban, AT; 10^-6 M) or indomethacin (INDO; 10^-4 M), for 12 h and then stimulated with PGF_2α (10^-8 M) for 4 h. Pre-treatment of the cells with OP, AT or INDO resulted in an increase in P4 secretion in response to PGF_2α. To examine the temporal effects of P4, OT and PGs on P4 secretion, dispersed luteal cells were pre-exposed to OP, AT or INDO for 1, 2, 4, 6 or 12 h. Prostaglandin F_2α stimulated P4 secretion (P < 0.05) after 2 h of pre-exposition. In the microdyalisis study, the spontaneous release of P4 from developing CL tissue was of pulsatile nature with irregular peaks at 1-2 h intervals. Treatment with OP increased the number of P4 peaks (P < 0.05), whereas AT and INDO significantly reduced the number of P4 peaks detected (P < 0.05). Interestingly, INDO completely blocked the pulsatile nature in the release of P4, but it secretion remained stable troughout the experimental period. These results demonstrate that luteal PGF_2α, OT, and P4 are components of an autocrine/paracrine intra-ovarian regulatory system responsible for the episodic (pulsatile) release of P4 from the bovine CL during the early luteal phase.