Recent findings suggest that the ovarian renin-angiotensin system may regulate ovarian function through the paracrine/autocrine actions of angiotensin II (Ang II). In this study, we have examined and characterized the local effects of Ang II as a luteolytic factor and its interaction with prostaglandin F2α (PGF2α) and endothelin-1 (ET-1) in the bovine corpus luteum (CL) of the mid-luteal phase, by using an in vitro microdialysis system (MDS). Ang II was detected in the MDS perfusate (4 pg/ml), and infusion of PGF2α (10−6 M) for 2 h increased the Ang II release by 50–100% during the following experimental period, in addition to its stimulation of ET-1 release. Two 2-h infusions of Ang II (10−7-10−5 M) separated by a 2-h interval induced a dose- and time-dependent decrease of progesterone (P4) release by 41–66%. When the luteal explants were pre-perfused with PGF2α (10−6 M) for 2 h, two consecutive perfusions of Ang II (10−6 M) at a 2-h interval rapidly reduced the P4 release (by 50%). This reduction occurred 6 h earlier than those of infusions of PGF2α or Ang II alone. The simultaneous infusion of either 1) Ang II (10−6 M) with PGF2α (10−6 M), 2) ET-1 (10−7 M) with PGF2α, or 3) Ang II + ET-1 with PGF2α (10−6 M) for 2 h also induced a rapid and pronounced (60%) decrease in P4 release. Perfusion with the Ang II antagonist blocked the P4-suppressing activity of Ang II alone or PGF2α + Ang II infusion. Ang II stimulated the release of ET-1 and oxytocin during infusion but inhibited them after infusion.
These results show that Ang II is released in the bovine midcycle CL in vitro, and this peptide, either alone or together with PGF2α, can suppress the release of P4. As PGF2α directly stimulated Ang II release, Ang II may influence the critical period for starting the cascade of functional luteolysis in vivo and might lead to structural luteolysis with ET-1 as a major vasoconstrictor. The overall results suggest that Ang II may have an important role at luteolysis in the bovine CL.