It is well known that prostaglandin F2α (PGF2α) is a physiological luteolysine, and that its pulsatile release from the endometrium is a luteolytic signal in many species. There is now clear evidence that the vasoactive peptides endothelin-1 (ET-1) and angiotensin II (Ang II) interact with PGF2α in the luteolytic cascade during PGF2α-induced luteolysis in the cow. Thus, we investigated the local secretion of PGF2α, ET-1, and Ang II in the corpus luteum (CL) and their real-time relationships during spontaneous luteolysis in the cow. For this purpose, an in vivo microdialysis system (MDS) implanted in the CL was utilized to observe local secretion changes within the CL microenvironment. Each CL of cyclic Holstein cows (n = 6) was surgically implanted with MDS capillary membranes (18 lines/6 cows) on Day 15 (estrus = Day 0) of the estrous cycle. The concentrations of PGF2α, ET-1, Ang II, and progesterone (P) in the MDS samples were determined by enzyme immunoassays. The intraluteal PGF2α secretion slightly increased from 12 h after the onset of luteolysis (0 h) and drastically increased (by about 300%) from 24 h. Intraluteal ET-1 secretion increased from 12 h. Intraluteal Ang II secretion was elevated from 0 h and was maintained at high levels (about 180%) toward estrus. In each MDS lines (in the same microenvironment) within the regressing CL, the local releasing profiles of PGF2α, ET-1, and Ang II CL positively correlated with each other (P < 0.05) at high proportions in 18 MDS lines (PGF2α vs. ET-1, 44.4%; PGF2α vs. Ang II, 55.6%; ET-1 vs. Ang II, 38.9%). In contrast, there was no clear relationship among these substances released into different MDS lines implanted in the same CL (with different microenvironments). In conclusion, we propose that the increase of PGF2α, ET-1, and Ang II within the CL during luteolysis is a common phenomenon for both PGF2α-induced and spontaneous luteolysis. Moreover, this study illustrated the in vivo relationships in intraluteal release among PGF2α, ET-1, and Ang II during spontaneous luteolysis in the cow. The data suggest that these vasoactive substances may interact with each other in a local positive feedback manner to activate their secretion in the regressing CL, thus accelerating and completing luteolysis.
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